High strength aluminum alloy and process

Abstract

An improved dispersion strengthened aluminum-base alloy and an improved method for producing the alloy are provided. A preferred alloy comprises, by weight, about 3 to 5% Mg, about 0.2-2.5% C, and about 0.3 to 4% O and the balance essentially aluminum.

Description

The present invention relates to powder metallurgy, and more particularly to a method for controlling and/or optimizing strength and workability of dispersion-strengthened aluminum-magnesium alloys by variations in thermomechanical processing of mechanically alloyed powders.

In recent years considerable research efforts have been expended to develop high strength aluminum which would satisfy the demands of advanced design in aircraft, automotive, and electrical industries. It is known to increase the strength of aluminum by the use of certain additives which will form, for example, oxide dispersion-strengthened, age hardened and solid solution hardened alloys. The use of any particular additives or combinations of them depend on desired properties in addition to strength, such as corrosion resistance, ductility, electrical conductivity and hardness. It will be appreciated that the property requirements depend on ultimate use of the aluminum. The processing of aluminum alloys may be through the formation of ingot melts or various powder metallurgy techniques. Using either the ingot melt or powder metallurgy route the incorporation of additives which strengthen aluminum usually decreases its workability. Workability takes into account ductility at the working temperature and the load necessary to form the material.

The production of shaped high strength aluminum forms from powders is known to have advantages over traditional ingot metallurgy processes. Oxide dispersion-strengthening is, in general, more easily accomplished by powder metallurgy techniques than by forming oxides in an ingot. A fine dispersion of insoluble alloying additives is made possible by powder metallurgy. A fine grain size can often be easily obtained by powder metallurgy by restricting powder particle size, and strengthening is easily accomplished by dispersion strengthening. Parts may be pressed to shape from the powder, eliminating the need for costly working operations required after billet formation made via the ingot route. Powder metallurgy techniques generally offer a way to produce homogeneous material and to control chemical composition. Also, difficult to handle alloying elements can at times be more easily introduced via powder metallurgy than by ingot melt techniques.

U.S. Pat. Nos. 3,740,210 and 3,816,080 (incorporated herein by reference) disclose a process for preparing and consolidating mechanically alloyed dispersion-strengthened aluminum. These patents further disclose a means for applying the concept of U.S. Pat. No. 3,591,362 (also incorporated herein by reference) to oxide dispersion-strengthened aluminum. The oxide dispersion-strengthened mechanically alloyed powder is different from the sintered aluminum product commonly referred to as SAP, which is produced by a complex process including flaking of the aluminum particles in the presence of a high amount of grinding agent, e.g. stearic acid or isopropyl alcohol, to form an oxide surface on the flakes, and then removing the agent before the particles are consolidated. For most uses, a powder must be fabricated into a final product, which is ultimately a metal forming operation, e.g. by hot pressing, hot die compacting, or cold isopressing followed by extrusion, forging or rolling. In contrast to the SAP-type preparation, the mechanical alloying route, which does not use a high amount of grinding agent, can produce a material with a lower level of dispersoid while achieving the same level of strength with greater ductility. Thus, there is increased potential for producing materials with greater strength and/or higher workability with mechanically alloyed powders than with conventional aluminum powders such as SAP. Further, the use of the mechanical alloying technique enables the production of aluminum alloys of high strength without resorting to age hardening additives. Age hardening in conventional aluminum alloys may produce internal composition differences at the grain boundaries, which may be associated with high susceptibility to stress corrosion cracking. Also, age hardened alloys can soften upon elevated temperature exposure as strengthening precipitates coarsen. Thus, mechanically alloyed aluminum, which can be strengthened sufficiently without the use of age hardening, has a potential for certain high corrosion resistance applications, e.g. aircraft skins without cladding, aircraft interior structural members, rifle parts, lightweight automotive parts, etc.

The method disclosed in the aforementioned U.S. Pat. Nos. 3,740,210 and 3,816,080 for producing mechanically alloyed powders also discloses examples of consolidated products of dispersion-strengthened essentially pure aluminum extruded under various conditions. In general, the extrusion is carried out at about 850.degree. to 900.degree. F. at extrusion ratios of 45:1 and 28:1, and they are shown to have room temperature UTS (ultimate tensile strength) of about 45 to 66 ksi. In the absence of data on the effects of variations in powder processing, it could be assumed that the properties would vary with changes in the thermomechanical treatments consistent with reported responses of aluminum alloys. For example, a study of extrusion-consolidation processing variables on 7075 aluminum powder reported by F. J. Gurney et al in POWDER MET., 17 (33), pp. 46-69, shows that increasing the extrusion temperature above about 600.degree. F. causes an increase in strength. J. H. Swartzwelder (INT. J. POWDER MET. 3 (3) 1967) reports the behavior of extruded 14 wt. % oxide dispersoid SAP aluminum rod at extrusion ratios varying from 2:1 to 64:1 and 8 wt. % oxide dispersoid SAP aluminum rod at ratios of 2:1 to 76:1. At both dispersoid levels the SAP materials showed a rapid increase in tensile strength as extrusion ratios increased up to about 8:1. The more extensive data obtained for the 8 wt. % dispersoid alloy show a leveling out or slight increase in tensile strength after the initial rapid increase. A. S. Bufferd et al (TRANS. ASM, Vol. 60, 1967) extruded SAP aluminum alloys containing up to about 5% Mg. In FIG. 2 they report the tensile stress of alloys at levels of about 7 and 12 vol. % oxide. At 12 vol. % the maximum UTS room temperature strength (at about 4 wt. % Mg) of roughly 66 ksi. At a level of about 7 vol. % oxide and about 4.5 wt. % Mg the maximum UTS shown is slightly less than 65 ksi. There is no indication of decrease in UTS during processing.

It has now been found that a dispersion-strengthened aluminum-magnesium alloy of the present invention characterized by improved high strength and by corrosion resistance can be prepared by mechanical alloying. Further it has been found that contrary to the behavior expected, oxide dispersion-strengthened mechanically alloyed aluminum-magnesium has an unconventional response to thermomechanical processing. The knowledge of this unexpected behavior of the mechanically alloyed aluminum can be used to control properties when the material is hot worked into useful form, making it possible to process the material with optimization of the properties of workability and strength. Optimization may involve selection of processing conditions to obtain the highest possible strength or sacrificing strength for workability, depending on the requirements of the end product.

The unconventional response of mechanically alloyed oxide dispersion-strengthened aluminum-magnesium to thermomechanical processing is illustrated in the accompanying figures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a working temperature strength profile of an oxide dispersion-strengthened mechanically alloyed aluminum-magnesium alloy of the present invention.

FIG. 2 is a graph showing the effect of extrusion ratio at an extrusion temperature of 650.degree. F. (343.degree. C.) on room temperature tensile strength (UTS) of an alloy of the present invention (Curve A) and a comparison with the effect on a prior art aluminum alloy, viz. SAP (Curves B and C) containing substantially higher dispersoid levels than the alloy of Curve A.

FIG. 3 is a graph showing the direct relationship between Brinell hardness (BHN) of compacted billets and room temperature tensile strength (UTS) of rods extruded from each given billet of a dispersion-strengthened mechanically alloyed aluminum of the present invention. The alloys have different dispersoid levels, varying from about 1.5 to 4.5 vol. %, and varying strength, but are all extruded at an extrusion ratio of 33.6:1 at two temperature levels, at the lower temperature (Curve D) and a higher temperature level (Curve E).

SUMMARY OF THE INVENTION

Generally speaking the present invention is directed to an improved dispersion-strengthened mechanically alloyed aluminum and to an improved method for processing it to optimize its properties. In accordance with a particular aspect of the present invention, an oxide dispersion-strengthened mechanically alloyed aluminum consisting essentially, by weight, of a small but effective amount for increased strength up to about 7% or 8% magnesium, up to about 2.5% carbon, a small but effective amount for increased strength up to about 4% oxygen, and the balance essentially aluminum. Preferably, for high corrosion resistance, the material will contain about 2% or 3% up to about 5% magnesium, and more preferably about 4 to 5% magnesium. Preferably, the alloy contains at least about 0.3% oxygen and about 0.2% carbon.

One aspect of processing the alloy in accordance with this invention resides in the selection of a composition which has in compact form suitable strength so that it is potentially possible to obtain a product of a desired strength. Another aspect of the invention resides in controlling the thermomechanical processing conditions to achieve predictably a desired strength of the material relative to the workability required for a given application. The appropriate choice of composition and selection of processing conditions are made possible through the recognition of the different response of dispersion-strengthened mechanically alloyed aluminum to hot working compared with prior art aluminum alloys. Thus, in accordance with the present invention an oxide dispersion-strengthened mechanically alloyed aluminum-magnesium alloy is worked at an elevated temperature to form a product having a required strength by a method comprising:

(a) selecting as the initial charge material a dispersion-strengthened mechanically alloyed aluminum-magnesium alloy having in compacted form prior to working at elevated temperature a room temperature tensile strength at least equal to the room temperature tensile strength of the desired worked product, said charge material also having the property in a temperature range up to incipient melting of increased workability with increasing working temperature; (b) determining the working temperature-strength profile of the selected material, said profile being characterized by an overall decrease in strength relative to the working temperature; and (c) working the charge material at an elevated temperature selected with reference to said profile to optimize the workability of the charge material and the strength of the worked product.

In accordance with another aspect of this invention the working temperature-strength profile includes a critical-working temperature-strength transition zone which is characterized by a sharp lowering of room temperature strength relative to increased working temperature, as illustrated in FIG. 1. For optimized workability of the charge material and strength of the worked product, the working temperature is selected with reference to this transition zone.

In a preferred embodiment of the present invention, the working temperature-strength profile shows a pattern of behavior which includes a strength-temperature plateau, shown as "P" in FIG. 1, in which region an increase in working temperature has substantially no affect on strength. In the embodiment of FIG. 1, the maximum temperature of the plateau is between about 700.degree. F. and about 750.degree. F. Above the maximum there is a critical working temperature-strength transition zone, shown as "TZ" in FIG. 1. In accordance with this pattern, the use of working temperatures below those of the "TZ" zone permits processing of the alloys at temperatures for optimum workability without sacrifice of strength. Also in keeping with the pattern, if greater workability is required and lower strength permissible, the processing may be carried out at a higher temperature than that permitted for maximum strength. Alternatively, if because of workability considerations it is necessary to process a material at temperatures in or above the critical transition zone, compensating changes in prior processing can be applied to assure that the required strength can be achieved. FIG. 2, which shows the difference in the effect of extrusion ratio on strength of a material of the present invention (Curve A) from the effect on two samples of prior art aluminum alloys having different oxide dispersoid levels, illustrates that for material of the present invention, unexpectedly, its initial compacted strength, i.e., before thermomechanical treatment, must be greater than the strength required for a particular product. In other words, in materials of the present invention, strength of the product will not increase with thermomechanical working in the range studies, as would be expected under certain conditions from the reported behavior of other aluminum alloys.

Bearing in mind that the processing conditions for the present materials shown in the accompanying figures are developed in particular equipment with a specific composition which has been processed to obtain a given initial strength, a dispersion-strengthened mechanically alloyed aluminum containing about 2% up to about 5% magnesium, up to about 21/2% carbon, a small but effective amount for increased strength up to about 4% oxygen can be extruded optimally for highest workability and highest room temperature strength in the product at a temperature-strength profile equivalent to that shown in FIGS. 1 and 2. For example, for the composition and equipment used, for highest strength hot working is carried out at a temperature in the range of about 650.degree. F. (340.degree. C.) up to below about 750.degree. F. (400.degree. C.), the critical transition temperature zone being in the range of about 750.degree. F. to about 800.degree.-850.degree. F. For greater workability, processing may be carried out at a higher temperature than in the maximum plateau temperatures, but there will be a sacrifice in strength.

In accordance with another aspect of the present invention the ultimate tensile strength of an extruded dispersion-strengthened mechanically alloyed aluminum consisting essentially of about 2 to about 7% Mg, up to about 21/2% C, up to about 4% oxygen and the balance essentially aluminum, and containing a small but effective amount for improved strength e.g. about 1 volume % up to about 81/2 volume % dispersoid can be optimized by employing processing conditions in the interrelationship set forth by the following formula:

UTS=-0.059T.sub.1 -0.014T.sub.2 -0.034T.sub.3 -0.055E.sub.R+ 11.5(wt. % O)+20.1(wt. % C)-0.18.epsilon.-3t+214.6 where UTS=Ultimate Tensile Strength in ksi (at room temperature) T.sub.1 =Degas Temperature T.sub.2 =Compaction Temperature T.sub.3 =Extrusion Temperature E.sub.R =Extrusion Ratio, which is the ratio of the cross sectional area of the extruded billet to the cross sectional of the extruded rod. .epsilon.=Strain Rate (sec.sup.-1) t=Time at highest degassing temperature (hours) and all temperatures are in degrees Rankine. The use of the formula permits the selection of composition and consolidation conditions which mutually satisfy the strength requirement and the permissible extrusion conditions for a particular extrusion. By particular extrusion is meant the extrusion variables which are selected by cost considerations and/or equipment availability. The remaining variables can be controlled by use of the equation to obtain a desired strength level.

Using the method of this invention, dispersion-strengthened mechanically alloyed aluminum-magnesium with excellent corrosion resistance can be processed to products having an ultimate room temperature tensile strength of greater than 66.3 ksi and up to 90-110 ksi, and even higher. Alloys can be prepared having tensile strength in the range of about 69 to 88 ksi with % elongation of 6 to 8.

DESCRIPTION OF PREFERRED EMBODIMENTS

Composition

The dispersion-strengthened mechanically alloyed aluminum of the present invention is composed principally of aluminum, magnesium, carbon, oxygen, and at least a part of the oxygen and carbon are present as dispersoid material. The magnesium is present advantageously in an amount of about 2% up to about 7 or 8%, preferably about 2% to about 5%, e.g. about 3% to about 5%. The oxygen level is advantageously about 0.2 to about 4%, preferably about 0.3 or 0.4% up to about 2%, and the carbon level is advantageously about 0.1% to about 2.5%, preferably about 0.2 to about 2%. It may also contain various additives in addition to magnesium which may, for example, solid solution harden or age harden the aluminum and provide certain specific properties so long as they do not interfere with the desired properties of the Al-Mg alloy for the ultimate purpose. The magnesium, which in the amounts present forms solid solution with aluminum, provides strength, corrosion resistance, good fatigue resistance and low density. Other additives for additional strength are, for example, Li, Cr, Si, Zn, Ni, Ti, Zr, Co, Cu and Mn. The use of various additives to aluminum alloys are well known in the art.

The dispersoid comprises an oxide, and it may also contain carbon, silicon, a carbide, a silicide, aluminide, an insoluble metal or intermetallic which is stable in the aluminum matrix at the ultimate temperature of service. Examples of dispersoids are alumina, magnesia, thoria, yttria, rare earth metal oxides, aluminum carbide graphite, iron aluminide. The dispersoid such as Al.sub.2 O.sub.3, MgO, C may be added to the composition in dispersoid form, e.g., as a powder, or they may be formed in-situ. Preferably the dispersoid is formed in-situ during the production of the mechanically alloyed powder. The dispersoids may be present in the range of a small but effective amount for increased strength up to about 5 volume % (v/o) or even as high as 81/2 v/o. Preferably the dispersoid level is as low as possible consistent with desired strength. Typically alloys having strength greater than 66.3 ksi contain about 1 up to but less than 7 v/o dispersoid, and preferably with a minimum of about 2-3 v/o. In a preferred embodiment the oxide dispersoid is present in an amount of less than 5 v/o, e.g. .about.1 to <5 v/o.

PREPARATION PRIOR TO THERMOMECHANICAL TREATMENT

Mechanical Alloying

Powder compositions treated in accordance with the present invention are all prepared by a mechanical alloying technique. This technique is a high energy milling process, which is described in the aforementioned patents incorporated herein by reference. Briefly, aluminum powder is prepared by subjecting a powder charge to dry, high energy milling in the presence of a grinding media, e.g. balls, and a weld-retarding amount of a surfactive agent or a carbon-contributing agent, e.g. graphite or an asymmetric organic compound under conditions sufficient to comminute the powder particles of the charge, and through a combination of comminution and welding actions caused repeatedly by the milling, to create new, dense composite particles containing fragments of the initial powder materials intimately associated and uniformly interdispersed. The surfactive agent is preferably an organic material such as organic acids, alcohols, heptanes, aldehydes and ethers. In a preferred embodiment an oxalic acid-stearic acid mixture can be used as the surfactive agent. The formation of dispersion-strengthened mechanically alloyed aluminum is given in detail in U.S. Pat. Nos. 3,740,210 and 3,816,080, mentioned above. Suitably the powder is prepared in an attritor using a ball-to-powder ratio of 15:1 to 60:1. Preferably the carbon-contributing agents are methanol, stearic acid, and graphite. Carbon from these organic compounds is incorporated in the powder, and it contributes to the total dispersoid content.

Degassing

Before the dispersion-strengthened mechanically alloyed powder is consolidated by a thermomechanical treatment, it must be degassed. A compaction step may or may not be used.

In the mechanical alloying processing step, various gases such as H.sub.2 or H.sub.2 O, may be picked up by the powder particles, and if they are not removed before hot working, the material may blister. Degassing must be carried out at a high temperature, e.g., in the range of 550.degree. to 1050.degree. F. (287.degree. to 565.degree. C.). Degassing may be accomplished before compacting the powder, e.g. by placing the powder in a metal can and evacuating the can under vacuum at an elevated temperature. After degassing the can may be sealed and hot compacted against a blank die in an extrusion press. The can material may be subsequently removed by machining, leaving a fully dense billet for further working. Alternatively, the material may be degassed as a loose powder under an inert cover gas at an elevated temperature. In another alternative method a billet compacted at room temperature to less than theoretical density, e.g. 85% theoretical density, may be annealed under argon to remove gasses. In any degassing process a time-temperature interrelationship is involved. Preferably, the time-temperature combination is chosen to minimize loss of strength in the powder and for reasons for cost it is preferred to work materials at the lowest temperature possible consistent with other factors.

THERMOMECHANICAL TREATMENT

As indicated above, FIGS. 1 and 2 disclose a pattern of behavior of materials of the present invention during thermomechanical processing. While the invention is disclosed herein mainly with reference to dispersion-strengthened mechanically alloyed aluminum containing, by weight, about 4 to 5% magnesium, about 0.2 to 21/2% carbon, and about 0.3 to 4% oxygen, prepared under given conditions for extrusion, it will be understood that the trends of behavior disclosed can be applied more generally to dispersion-strengthened mechanically alloyed aluminum-magnesium. Thus powders of various compositions and prior conditioning can be used and worked at elevated temperatures in a manner other than extrusion. As indicated above, as a practical matter there will be conditions fixed by commercial processing equipment available or on hand and by considerations of cost. However, on the basis of the unexpected behavior disclosed herein, fixed conditions can be taken into account and variables such as composition and treatment of powders, and consolidation conditions can be adjusted to optimize workability during processing and strength in the product for a particular end use, as explained in further detail below.

As indicated above, certain processing conditions such as extrusion ratio will be, or are more likely to be fixed, e.g. by the equipment on hand. Variable conditions are more likely to be temperature and extrusion rate. As indicated above, dispersoid content may be varied. Generally speaking, to process the material in accordance with the present invention, one might proceed as follows: (1) determine processing variables that are fixed by outside factors. (Assume, for example, the extrusion ratio is fixed at 30:1 and strain rate is no greater than 1 inch per second.), (2) select a dispersoid content which has the potential to meet strength/ductility requirements and use additives if indicated, for specific properties, (3) select a degas temperature to provide a sufficient gas evolution so that the integrity of the material is maintained during thermomechanical processing or service, (4) select a compaction temperature. (For convenience, the compaction temperature is often the same as the degassing temperature to enable compaction to be done immediately after degassing is complete, thereby eliminating an additional powder heat-up.) and (5) the strength of the finished product can be estimated from a Brinell hardness indentation made on the compacted billet which with other factors held constant correlates linearly to the ultimate tensile strength (UTS), of the finished product (extruded rod) as shown in FIG. 3. The desired strength-workability combination can be obtained by selecting the extrusion temperature according to a working temperature-strength pattern such as shown in FIG. 1. It is important to note that the invention offers other degrees of freedom, for example, alterations in degassing time or extrusion speed can also be used to tune properties to the desired level.

The following examples illustrate processing variations on dispersion-strengthened mechanically alloyed aluminum compositions in accordance with the present invention. Samples of dispersion-strengthened mechanically alloyed aluminum were prepared by high energy milling in a 4, 30 or 100 gallon attritor for 6 to 16 hours at a ball-to-powder ratio of about 20:1 or 24:1 by weight in a nitrogen or air atmosphere, in the presence of either methanol or stearic acid. Samples were prepared having the nominal compositions shown in TABLE I. Compositions given above and in the examples are in weight percent except for dispersoid levels which are given in volume percent. (Oxide dispersoid is based on 1 wt. % O=1.92 vol. % Al.sub.2 O.sub.3. Carbide dispersoid is calculated based on 1 wt. % C corresponds to 3.71 vol. % Al.sub.4 C.sub.3.)

TABLE I______________________________________ Composition (Wt. %) Dispersoid (Vol. %)Powder Sample Mg C O Al Oxide Carbide______________________________________A 4 .54 1.5 Bal. 2.9 2.0B 5 .27 1.2 Bal. 2.3 1.0C 4 .55 1.79 Bal. 3.4 2.0D 4 1.25 .89 Bal. 1.7 4.6E 5 1.22 1.00 Bal. 1.9 4.5F 5 .27 1.1 Bal. 2.1 1.0______________________________________

EXAMPLE 1

This example illustrates the effect degassing temperature has on room temperature strength and ductility of extruded rod. Two cans of powder Sample A were compacted and degassed, one at 950.degree. F. (510.degree. C.) and the other at 800.degree. F. (427.degree. C.) for a time of 3 hours each. Both cans were extruded to 5/8" diameter rod at 800.degree. F. at an extrusion ratio (E/R) of 33.6:1. Two cans of powder Sample B were degassed for 3 hours, one at 1050.degree. F. (566.degree. C.) and the other at 950.degree. F. (510.degree. C.). After degassing the second two samples were rolled to 0.80" diameter plate at 800.degree. F. Room temperature tensile and ductility tests were performed on the resultant plates. Results are shown in TABLE II.

TABLE II______________________________________ DegasTest Powder T Compaction T YS UTS El. R.A.No. Type (.degree.F.) (.degree.F.) ksi ksi % %______________________________________1 A 950 950 75.6 82.4 7 29.52 A 800 800 80.8 87.9 6 253 B 1050 800 66.3 69.7 8 294 B 950 800 74.2 77.3 6 23______________________________________

The data for Powder Type A show that there was an increase in strength with either or both decrease in degas and compaction temperatures. The data for Powder Type B indicate that decrease degassing temperature appears to be the controlling factor.

EXAMPLE 2

This example illustrates the effect of temperature of thermomechanical treatment on strength of dispersion-strengthened mechanically alloyed aluminum samples having the nominal composition and the powder processing conditions of powder Type B.

Six identical cans of powder type B were canned and degassed for 3 hours at 950.degree. F. (510.degree. C.). Each can was compacted and extruded at temperature T.sub.i, where T.sub.i took the values 950.degree., 850.degree., 800.degree., 750.degree., 650.degree., 550.degree. F. The extrusion ratio was held constant at 13.6. Tensile specimens were taken from the middle of each extruded rod to determine the effect of extrusion temperature on tensile properties. The results are given in FIG. 1.

FIG. 1 shows the unexpected effect of extrusion temperature on the room temperature ultimate tensile strength (UTS) of a dispersion-strengthened mechanically alloyed aluminum. The pattern of behavior includes a strength-temperature plateau "P", which illustrates that an increase in working temperature from 550.degree. F. (285.degree. C.) up to a maximum temperature which is roughly 750.degree. F. (400.degree. C.) has substantially no affect on strength. The sharp transition to lower strength relative to the working temperature referred to above as the critical working temperature-strength zone, "TZ", occurs in the region between about 750.degree. F. and 800.degree. F. (400.degree. C. and 425.degree. C.). In subsequent tests on comparable materials a mean increase of 5.8 ksi in tensile strength occurred in lowering the extrusion temperature from 800.degree. F. to 650.degree. F. (425.degree. C. to 340.degree. C.) on 14 experimental samples. An increase in strength for at least one sample was found to be as high as 20 ksi.

EXAMPLE 3

This example illustrates the effect of extrusion ratio on strength of dispersion-strengthened mechanically alloyed aluminum samples of this invention, and it shows a comparision with prior art materials.

Six cans of powder type C were degassed for 3 hours at 950.degree. F. (510.degree. C.). Five cans were extruded at 650.degree. F. (340.degree. F.) at a ratio of 13.1, 23.4, 33.6, 52.6, and 93.4, respectively. The sixth can remained as compacted, which corresponds to an extrusion ratio of 1. It is noted that the cans were extruded at a temperature well into the higher strength region to avoid excursions into the transition region (i.e., the critical working temperature-strength transition zone) by a slight temperature fluctuation. Longitudinal tensile properties were determined and the data plotted as Curve A of FIG. 2.

Unexpectedly the tensile strength decreases with increasing the extrusion ratio for extrusion ratios up to about 50. This is contrary to behavior encountered with conventional alloys. Curves B and C of FIG. 2, for example, which are based on the aforementioned study by Swartzwelder in the INT. J. POWDER MET., show that strength does not decrease with extrusion ratio. The reference gives the oxide dispersoid levels as 8% and 14%, but is is ambiguous on whether this is volume or weight %. It is believed to be weight %. In any event both alloys have a higher volume percent dispersoid than the present alloy of Curve A having a total oxide+carbide dispersoid level of about 5.4 volume %; which shows a marked difference in strength.

FIGS. 1 and 2 illustrate the unexpected strengththermomechanical processing interrelationship of alloys of this invention, the understanding of which constitutes a useful means of controlling the properties of dispersion-strengthened mechanically alloyed aluminum.

EXAMPLE 4

This example illustrates the use of the formula given above to select the composition and consolidation conditions which mutually satisfy the strength requirement and permissible extrusion conditions for a particular extrusion.

Seventy-eight samples of dispersion-strengthened mechanically alloyed aluminum-4-5 wt. % magnesium were prepared essentially comparable to powder samples A, B and C, but containing various amounts of oxygen and carbon. Degassing temperature was 1410.degree. R (510.degree. C.) unless otherwise indicated. Compaction temperatures were varied from about 550.degree. to 1050.degree. F. (285.degree. to 565.degree. C.), the compacted powders were extruded to 1" to 0.375" rod at extrusion temperatures varying from 550.degree. to 950.degree. F. and extrusion ratios from 13.1:1, to 93.4:1. The compositions contained, in addition to aluminum and magnesium, about 0.8 to 2 wt. % oxygen, and 0.2 to 1.9 wt. % carbon. The oxide dispersoid varied from about 1.7 to about 3.4 vol. %, based on about 1 wt. % O corresponding to 1.92 vol. % oxide dispersoid. The carbide dispersoid varied from about 0.8 to about 5.8 vol. % based on about 1 wt. % C corresponding to about 3.71 vol. % aluminum carbide dispersoid. The data are tabulated in TABLE III, which shows actual room temperature tensile strength of samples. It was found that the actual room temperature tensile strength varied from theoretical calculated from the equation given above by approximately +6.2 to -7.3 ksi.

TABLE III__________________________________________________________________________Compaction ExtrusionTemp. Temp. Extrusion O C UTS (ksi)Sample .degree.R .degree.R Ratio Wt. % Wt. % Actual Calculated__________________________________________________________________________1 1360 1360 13.1 1.2 .27 72.4 70.22 1310 1310 13.1 1.2 .27 73.7 73.03 1260 1260 13.1 1.2 .27 74.7 75.24 1210 1210 13.1 1.2 .27 81.8 78.15 1110 1110 13.1 1.2 .27 83.7 83.16 1010 1010 13.1 1.2 .27 81.3 88.17 1410 1210 13.1 1.2 .27 81.2 75.28 1335 1210 13.1 1.2 .27 79.0 76.09 1260 1210 13.1 1.2 .27 77.5 77.110 1185 1210 13.1 1.2 .27 77.2 78.211 1110 1210 13.1 1.2 .27 76.8 77.512 1410 1260 33.6 1.1 .27 65.7 71.513 1410 1260 33.6 1.1 .30 68.5 71.914 1410 1260 33.6 1.3 .34 78.5 74.415 1410 1260 33.6 75.4 74.416 1410 1260 33.6 1.79 .55 84.1 83.717 1410 1110 33.6 1.79 .55 87.5 90.218 1410 1110 13.1 1.79 .55 95.1 91.319 1410 1110 23.4 1.79 .55 88.8 90.720 1410 1110 52.6 1.79 .55 86.5 89.121 1410 1110 93.4 1.79 .55 89.3 86.822 1410 1260 93.4 1.79 .55 83.0 81.023 1410 1110 33.6 1.5 .54 85.3 81.224 1410 1260 33.6 1.5 .54 82.4 81.425* 1260 1260 33.6 1.5 .54 87.9 91.726 1410 1110 33.6 1.6 .52 88.1 87.427 1410 1260 33.6 1.6 .52 83.2 82.128 1410 1260 33.6 1.98 .7 86.9 88.929 1410 1110 33.6 1.98 .7 94.4 95.330 1410 1110 33.6 1.2 1.4 94.5 100.431 1410 1110 33.6 1.2 1.4 99.9 100.432 1410 1260 33.6 1.2 1.4 101.1 94.833 1410 1110 33.6 1.35 1.3 100.0 100.234 1410 1110 33.6 1.35 1.3 88.2 94.235 1410 1260 33.6 1.35 1.3 95.2 94.236 1410 1260 33.6 1.35 1.3 98.8 98.737 1410 1110 33.6 1.5 1.81 110.6 112.238 1410 1260 33.6 .91 1.81 78.8 83.839 1410 1110 33.6 .91 1.05 88.7 90.040 1410 1260 33.6 .99 1.01 79.7 84.041 1410 1260 33.6 1.6 .77 84.2 86.442 1410 1260 33.6 1.32 .95 88.4 86.843 1410 1110 33.6 1.32 .95 92.5 92.744 1410 1260 33.6 1.3 1.16 86.6 90.345 1410 1260 33.6 1.1 1.29 84.0 91.146 1410 1260 33.6 1.0 1.23 90.0 89.347 1410 1110 33.6 1.0 1.23 97.5 94.748 1410 1260 33.6 1.08 1.34 93.3 91.449 1410 1110 33.6 1.08 1.34 99.5 97.950 1410 1260 33.6 1.27 .60 81.2 79.651 1410 1110 33.6 1.27 .60 86.7 85.152 1410 1260 33.6 .90 1.25 84.0 88.153 1410 1110 33.6 .90 1.25 87.8 94.454 1410 1260 33.6 1.1 1.22 82.5 89.755 1410 1260 33.6 1.08 1.24 94.2 90.056 1410 1110 33.6 1.08 1.24 97.6 95.857 1410 1110 33.6 .92 1.25 93.7 94.258 1410 1260 33.6 .86 1.20 87.7 86.759 1410 1110 33.6 1.02 1.56 103.6 101.060 1410 1110 33.6 .99 1.55 106.2 101.061 1410 1110 33.6 .98 1.35 102.7 96.962 1410 1110 33.6 .89 1.25 95.0 93.863 1410 1110 33.6 .96 1.23 95.5 94.464 1410 1110 33.6 .83 1.08 87.6 89.765 1410 1110 33.6 .87 1.02 92.4 89.066 1410 1110 33.6 .89 1.14 92.6 91.667 1410 1110 33.6 1.23 1.24 102.4 97.668 1410 1110 33.6 .89 1.25 92.7 91.069 1410 1110 33.6 .89 1.25 93.3 93.970 910 1110 33.6 .89 1.25 111.9 107.171 1110 1110 33.6 1.02 .28 81.0 79.872 1110 1110 33.6 1.02 .28 84.4 79.773 1110 1110 33.6 1.02 .28 80.4 85.074 1110 1110 33.6 1.02 .28 82.9 84.775 1110 1110 33.6 1.02 .28 81.3 81.576 1110 1110 33.6 1.02 .28 80.5 76.677 1110 1110 33.6 1.02 .28 75.9 78.878 1110 1110 33.6 .91 .22 74.9 79.2__________________________________________________________________________ *Degas Temperature = 1260.degree. F.

EXAMPLE 5

The following example shows how the knowledge of the effect of degassing time on tensile properties can be used to control properties of the final product.

Two billets of powder type D were formed in the following degassing sequences:

Billet 1: Degas for 3 hours at 950.degree. F. in can and compact at 950.degree. F. (510.degree. C.). Billet 2: Degas for 1 hour at 950.degree. F. in open tray under an argon blanket, can, degas for 1-1/2 hours at 450.degree. F. compact at 450.degree. F. (230.degree. C.).

The two billets were extruded to rod at a ratio of 33.6:1 at 650.degree. F. (340.degree. C.). Data obtained on tensile strength and ductility of the samples are given in TABLE IV.

TABLE IV______________________________________ Hrs.Billet at Highest UTS YSNo. Degassing T (ksi) (ksi) % El. % R.A.______________________________________1 3 93.3 85.5 3 2.8/15.6*2 1 111.9 108.3 <1 <1______________________________________ *The data in the table are the mean of two similar values. If values differ significantly, both values are included.

It can be seen from the data in TABLE IV that the shorter time at the higher degassing temperature (Billet 2) is responsible for a substantial increase in tensile strength, viz. over 18 ksi, of the finished product.

EXAMPLE 6

This example illustrates the use of processing information in accordance with the present invention.

If powder Type D is to be used in a very high strength condition, e.g. for lightweight parts which are to be machined out of aluminum, it may be processed as follows:

To insure complete degassing, a 3 hour 950.degree. F. vacuum degas is used followed by 950.degree. F. compaction. Because the pieces are to be machined and service conditions warrant extremely high strength, the finished product is the compacted billet. Mechanical properties of the compacted material are:

______________________________________UTS (ksi) YS (ksi) % El. % R.A.______________________________________122.2 111.3 2 4______________________________________

If powder Type D is to be used for high strength aircraft extrusions with properties including greater than 90 ksi room temperature tensile strength and a sufficient elongation so as to permit stretch straightening after extrusion, the information of FIGS. 1 and 2 is used as follows:

The powder is degassed at 950.degree. F. to insure that all detectable hydrogen is removed and degassing is continued for 4 hours. The additional hour of degassing causes sufficient softening to occur so that extrusions of a 33.6:1 ratio will not be overly high in strength. The hardness of the compacted billet (176 BHN 500 kg load) indicates that strength will be greater than 90 ksi if extruded at 650.degree. F. at a ratio of 33.6:1. The extrusion is carried out at 650.degree. F. and properties are as follows:

______________________________________UTS (ksi) YS (ksi) % El. % R.A.______________________________________92.7 86.4 4 12.6______________________________________

These results demonstrate that the processing information of the present invention can be used to obtain the proper conditions for each specific application by utilization of the strength-workability trade-off associated with metal processing of dispersion-strengthened mechanically alloyed aluminum.

EXAMPLE 7

This example illustrates the increased workability with increased working temperature of aluminum alloys of the present invention.

Several heats of dispersion-strengthened mechanically alloyed aluminum powder containing about 4% magnesium were prepared. The powder was degassed at 950.degree. F. for 3 hours, compacted at 950.degree. F. and extruded at an extrusion ratio of 33.6:1. Two extrusion temperatures for each heat were used in sets, at 650.degree. F. and at 800.degree. F. Breakthrough pressure in ksi for extrusion at each temperature for typical samples are shown in Table V.

TABLE V______________________________________ BreakthroughHeat Pressure (ksi)No. 650.degree. F. 800.degree. F.______________________________________1 134 872 118 893 145 914 124 805 132 1126 116 837 130 75______________________________________

The data in TABLE V show that the breakthrough pressure is lower or workability is greater at higher temperature. Further experiments showed that breakthrough pressure is greater with increased extrusion ratio. FIG. 2 shows that strength is greater at lower extrusion ratios. Thus, at lower extrusion ratios workability is easier and higher strength material can be obtained.

EXAMPLE 8

This sample illustrates the preparation of an alloy of the present invention in the form of sheet.

Two samples of mechanically alloyed powder of Types E and F were degassed at 800.degree. F., compacted at 750.degree. F. and rolled at 750.degree. F. to 0.8" plate. The sample of Type F (F-1) was then hot rolled to 0.3" plate and then cold rolled to 0.08" sheet. Another mechanically alloyed powder having the composition of type F (F-2) was degassed at 950.degree. F., compacted at 800.degree. F., rolled at 800.degree. F. to 0.3" plate and annealed for 1 hour at 900.degree. F. The properties of the samples were as follows:

______________________________________Type UTS YS El. RASample (ksi) (ksi) (%) (%)______________________________________E 100.4 91.5 5 24F-1 95.3 88.9 5F-2 74.2 71.2 7 3.5______________________________________

EXAMPLE 9

This example illustrates the high corrosion resistance of mechanicalloy alloyed aluminum-magnesium alloys of the present invention.

A mechanically alloyed aluminum-magnesium alloy having the composition of Powder Type F degassed at 800.degree. F. and compacted at 750.degree. F. was rolled to 0.8" plate. The sample was exposed to 90-days of alternate immersion in a 3.5% NaCl solution. One sample of commercial alloy 7050-T-7651 and one sample of 5083-H-1112 were subjected to the same alternate immersion test. In general aluminum alloys of the 7000 series have relatively high strength, poor corrosion resistance and the aluminum alloys of the 5000 series have low strength but excellent corrosion resistance. On comparing strength and corrosion resistance of the alloy of the present invention with the commercial alloys of the 7000 and 5000 series, it was found that the present alloy had corrosion resistance at least as good as the alloy of the 5000 series and strength approaching that of the 7000 series alloy.

EXAMPLE 10

This example shows the effect of Mg content on stress corrosion cracking (SCC) resistance of mechanically alloyed aluminum-magnesium alloys of this invention, when exposed to an alternate immersion test.

Eleven laboratory-prepared materials of this invention having Mg contents ranging from 2 to 8% were evaluated. The test specimens were in the form of C-rings machined so that stressing was oriented with the short transverse direction. The specimens were exposed for up to 90 or 120 days in an alternate immersion test which consisted of a 10-minute immersion in a neutral 3.5% NaCl solution at ambient temperature and a 50-minute drying cycle each hour. Ten liters of solution were used. During the drying period a fan was used to provide a constant flow of air across the samples.

Specimen dimensions were recorded and deflection values calculated according to ASTM STP 425, page 165 (1967).

Data are summarized in TABLE VI.

TABLE VI__________________________________________________________________________ Applied Stress Cracking TimeSample and Comp. (Wt. %) Y.S. (% Y.S.) (Days)Condition* Mg O C (ksi) Ring #1 Ring #2 Ring #1 Ring #2__________________________________________________________________________1A 2 .83 .62 55 14 87 OK/120 OK/120B 57 96 96 OK/120 OK/1202A 3 1.2 .99 58 97 93 OK/120 OK/120B 60 93 97 OK/ 93 OK/ 933A 4 .92 .26 58 90 97 OK/120 OK/120B 58 97 97 OK/ 93 OK/ 934A 4 1.2 .51 71 97 97 OK/120 OK/120B 71 97 97 OK/120 OK/1205A 5 1.1 .28 58 97 93 OK/120 OK/120B 60 93 93 OK/ 80 OK/ 936A 5 .87 .21 63 94 97 OK/120 OK/120B 62 97 97 OK/ 93 OK/93**7A 5 .77 .10 50 92 104 OK/120 OK/120B 50 104 116 OK/120 C/238A 6 -- -- 62 97 97 OK/120 OK/120B 62 97 103 C/47 C/109A 6 -- -- 53 100 111 OK/120 OK/120B 53 134 96 C/<4 C/2710A 7 -- -- 58 97 121 OK/120 OK/120B 57 100 97 C/<1 C/<111A 8 -- -- 63 110 94 OK/120 OK/120B 62 65 97 C/<4 C/<1__________________________________________________________________________ *A = 1 h/482.degree. C. (900.degree. F.)/WQ; B = A + 7 days/94.degree. C. (200.degree. F.)/AC. **Specimen cracked during subsequent storage 93-120 days. C = Cracked; OK = No evidence of cracking. -- = Not determined.

Some evidence of pitting corrosion was found on some test samples. It is not certain if these two forms of corrosion was interrelated during the exposure of these materials at the indicated conditions. Further testing would be necessary for such determination.

With respect to the SCC resistance, regardless of Mg content or applied stress level, all of the eleven materials when tested in the annealed (A) condition were resistant to stress corrosion cracking. Cracking was detected, however, in the C-ring specimens of aged materials having Mg contents of 5% or greater. Although all of the aged specimens of the 6, 7 and 8% Mg containing alloys cracked, only one aged specimen from each of three 5% Mg containing alloys cracked.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

Claims

1. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy having a composition consisting essentially, by weight, of magnesium in a small but effective amount for increased strength up to about 7%, about 0.3% up to about 4% oxygen, up to about 21/2% carbon, and the balance essentially aluminum, and characterized by a tensile strength (UTS) at room temperature above 66.3 ksi.

2. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the alloy has a tensile strength (UTS) at room temperature of at least 69.7 ksi.

3. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the magnesium content is at least about 2%.

4. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 3, wherein the magnesium content is up to about 5%.

5. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the magnesium content is about 4% up to about 5%.

6. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the carbon content is at least 0.2%.

7. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 3, wherein the oxygen content is about 0.4% to about 2%.

8. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 3, wherein the carbon content is about 0.2% to 2%.

9. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the alloy is characterized by a room temperature tensile strength (UTS) of about 69.7 ksi up to about 87.9 ksi and an elongation of about 6% to about 8%.

10. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the alloy is characterized in the extruded condition by a tensile strength (UTS) at room temperature of at least about 93.3 ksi.

11. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 1, wherein the dispersoids are present in a small but effective amount for improved strength up to about 81/2 volume %.

12. An oxide dispersion-strengthened mechanically alloyed aluminum-base alloy according to claim 11, wherein the oxide dispersoid is present in an amount of about 1% up to less than 5%.

13. A process for working an oxide dispersion-strengthened mechanically alloyed aluminum-magnesium powder at elevated temperature to produce a consolidated worked product, said aluminum-magnesium power having a composition consisting essentially, by weight, of magnesium in a small but effective amount for increased strength up to about 7%, about 0.3% up to about 4% oxygen, up to about 21/2% carbon, and the balance essentially aluminum, and characterized by a tensile strength (UTS) at room temperature of above 66.3 ksi and said powder having in compacted form a working temperature-strength profile which includes a critical working temperature-strength transition zone characterized by a sharp lowering of room temperature strength relative to increased working temperature, comprising:

(a) determining the working temperature-strength profile of the selected charge material, said profile being characterized by an overall decrease in strength relative to the working temperature; and

(b) working the charge material at a temperature selected with reference to the working temperature-strength profile to optimize the workability of the charge material and the strength of the worked product.

14. A process according to claim 13, wherein the said critical transition zone is preceded by a plateau region in which the strength of the product is substantially unaffected by increased temperature.

15. A process according to claim 13, wherein working of the charge material is carried out at a temperature selected in the plateau region for maximum strength.

16. A process according to claim 13, wherein working of the charge material is carried out at a temperature selected above the maximum temperature of the plateau region to achieve optimum workability of the charge material with sacrifice in strength of the worked product.

17. A process according to claim 13, wherein the working step comprises extruding the charge material.

18. A process according to claim 17, wherein for maximum strength the extrusion is carried out at a minimum ratio.

19. A process according to claim 13, wherein the dispersoids of the dispersion-strengthened mechanically alloyed aluminum is present in a small but effective amount of dispersoid for improved strength up to about 81/2 volume %.

20. A process according to claim 19, wherein the charge material consists essentially of about 2% up to about 5% Mg, up to about 21/2% C, about 0.3% up to about 4% O, and the extrusion is carried out at a temperature below the critical working temperature-strength transition zone to obtain optimum strength in the worked product.

21. A process according to claim 20, wherein the extrusion is caried out at a temperature up to the equivalent of about 750.degree. F.

22. A process according to claim 20, wherein the extrusion is carried out at a maximum extrusion ratio.

23. A process for treating a dispersion-strengthened mechanically alloyed aluminum containing, by weight, from about 2% up to about 7% Mg, up to about 21/2% C, and from 0.3 up to about 4% O, by a method including steps comprising hot working said aluminum to form a consolidated product, the improvement of optimizing the strength of the consolidated product and workability during hot working by employing processing conditions in the interrelationship set forth by the following formula:

UTS=0.059T.sub.1 -0.014T.sub.2 -0.034T.sub.3 -0.55E.sub.R +11.5(wt. % O)+20.1(wt. % C)-0.18.epsilon.-3t+214.6

where

UTS=Ultimate Tensile Strength in ksi (at room temperature)

T.sub.1 =Degas Temperature

T.sub.2 =Compaction Temperature

T.sub.3 =Extrusion Temperature

E.sub.R =Extrusion Ratio, which is the ratio of the cross sectional area of the extruded billet to the cross sectional of the extruded rod.

.epsilon.=Strain Rate (sec.sup.-1)

t=Time at highest degassing temperature (hours).