Patent Application
20090028743
This invention generally relates to processed magnesium alloy compositions exhibiting improved ductility at room temperature. More specifically, magnesium alloyed with cerium is subjected to high temperature deformation to improve the alloy's formability at room temperature.
Magnesium is the lightest structural metal. In engineering applications it is alloyed with one or more elements, for example, aluminum, manganese, rare earth metals, lithium, zinc, and silver. Magnesium usually constitutes eighty-five percent by weight or more of these alloys.
The cost of magnesium has decreased dramatically in recent years and magnesium and its alloys have become attractive structural materials for a wide range of applications due in part to desirable physical properties such as light weight, high specific strength and stiffness, machinability, and the ability to be easily recycled. However, the use of magnesium in wrought products like sheet and extrusions has been limited due to the poor workability of magnesium castings and the lower formability and ductility of magnesium in the primary fabricated stage. At room temperature, pure magnesium is generally characterized by limited ductility as a result of its hexagonal close-packed crystal structure and resulting limited number of active slip systems. This inherent limitation often discourages widespread use of magnesium in wrought products made from sheets and extrusions because it is difficult and expensive to process the poorly workable metal into useable finished shapes.
It is generally understood by those skilled in the art that metal manufacturing techniques which promote grain refinement may help to improve certain tensile properties such as work hardening and ductility in magnesium metals. One non-conventional metal working process, known as equal channel angular extrusion (ECAE), operates by enforcing simple shear on a metal material at the intersection of two identically sized channels (i.e., no significant reduction in cross-section) that together form a processing route containing a sharp bend (usually 90°) through which the alloy material may make multiple passes. ECAE processes have been shown to improve elongation values to greater than 25% in certain magnesium alloys. However, the high operating expenses associated with ECAE often render the process economically unattractive and cause those in industry to utilize other alloys more susceptible to cheaper processing techniques, such as steel and aluminum alloys.
It is also generally recognized among those skilled in the art that mechanical workability of magnesium metals may be improved by alloying magnesium with particular metal additives. The metal additives have been credited with lowering the critical resolved shear stress (CRSS) in various slip systems, thereby resulting in changes to the magnesium material's microstructure during hot forming. This phenomenon may enhance formability of the metal by increasing the number of independent slip modes available for general deformation while at the same time minimizing the effect of twinning so as to delay the onset of fracture. Unfortunately, knowledge in this area is scarce because only a limited amount of attention has been devoted to identifying alloy additions capable of favorably manipulating formability of wrought magnesium materials at room temperature.
Thus, there is a general need to provide magnesium alloys in a primary fabrication stage having improved ductility for fabrication into wrought magnesium metal products.
In an embodiment of the invention, a melt containing, by weight 0.2 percent cerium and the balance magnesium was cast into a round cylindrical billet for in-line extrusion. The billet was preheated to 400° C. for two hours and pushed along a straight axis (not ECAE) through a circular die with an extrusion ratio of 25:1 to produce a solid rod about fifteen millimeters in diameter. A like extruded billet was produced consisting of magnesium with 0.5 weight percent cerium. For purposes of comparison of resulting properties, a billet of pure magnesium was cast and extruded in the same way.
It was observed that the grain size of the extruded cerium-containing magnesium billets was smaller than the grain size of the extruded pure magnesium billet. The room temperature tensile strength of the cerium-containing billet material was decreased and the percent elongation was significantly increased. Elongation at ultimate load in tensile testing is a measure of ductility. As will be seen in data presented below in this specification, the ductility of the hot deformed cerium-containing magnesium alloys was surprisingly increased compared with magnesium and other known magnesium alloys.
The addition of cerium in amounts up to about one percent by weight is found to enhance the room temperature ductility and workability of magnesium alloys following suitable hot deformation processing. In a specific embodiment, the hot deformation is accomplished by extrusion at billet temperatures of about 350° C. to about 475° C. with extrusion ratios in the range of about 10:1 to about 60:1 at suitable extrusion speeds. During the hot deformation the billets were suitably lubricated with graphite based lubricants or boron nitride, although this is not necessarily required.
The presence of the small amount of cerium favors the formation of recrystallized grains with their basal planes oriented at 40-50 degrees to the extrusion axis. The magnesium-cerium alloy provides easier basal slippage during subsequent straining along the extrusion axis which is effectively precluded in the extruded rods of pure magnesium. But whatever the straining mechanism, the presence of about 0.2 to about 0.5 weight percent cerium in the hot worked magnesium matrix markedly increased the ability to further shape the extruded bar material at room temperature. And the improvement was realized whether the hot worked material was solid in cross-section or hollow. Thus, hot extruded magnesium alloy bars or tubes, for example, may then be subjected to bending or hydroforming steps, for example, at an ambient temperature to more easily form more complex shapes for automotive vehicle structures or parts, or the like.
Significant increases in room temperature ductility have been demonstrated in binary magnesium-cerium alloys containing up to about 0.5 percent by weight cerium and the process may be practiced using cerium in amounts up to about one percent of the magnesium alloy. The cost of the cerium addition may be reduced by using cerium-containing mischmetal which typically comprises fifty percent by weight or more cerium with smaller amounts of lanthanum and other lanthanum-group elements. In this embodiment, the magnesium-based alloy may contain around one half to one percent cerium together with other rare earth elements in the mischmetal.
Relatively small amounts of other alloying elements are sometimes added to magnesium for physical properties other than room-temperature ductility. For example, aluminum in amounts up to about nine percent by weight, and/or zinc in amounts up to about three percent by weight, and/or manganese in amounts up to about one percent by weight have been used in commercial magnesium based alloys. And small amounts of titanium have been added for grain-refinement of magnesium alloys. Cerium additions in an amount up to about one weight percent may be used to improve the room-temperature ductility of these many different magnesium alloys but the room temperature ductility may not be as high as in the magnesium-0.2-1 cerium binary alloys. In most embodiments, magnesium will constitute at least eighty-five percent by weight of the magnesium-cerium alloy when these other alloying constituents are used for other properties of the resulting alloy.
Other exemplary embodiments will become apparent from the following description of illustrative embodiments. It should be understood that the detailed description and specific examples, while providing exemplary embodiments of the invention, are intended for illustrative purposes only and are not intended to limit the scope of the invention.
The disclosure will now be described, by way of example, and not limitation, with reference to the accompanying drawings. The following is a brief description of the drawings.
The description of the following embodiment(s) is merely exemplary in nature and is in no way intended to limit the claimed invention, its application, or its uses.
Magnesium alloys comprising primarily magnesium with small additions of cerium may be formed by a hot deformation process into a wrought article that exhibits improved ductility at room temperature. Here room temperature means a typical in-door ambient temperature of, for example, about fifteen to about thirty degrees Celsius. The wrought article may be in a final product shape. However, the room temperature ductility of the wrought article makes it useful for further deformation processing into a desired different shape. The unexpected ductility of the hot deformed magnesium body is attributable to its cerium content and hot deformation processing that contribute to an alteration in slip distribution, a decrease in yield strength, an increase in work hardening, a reduction in grain size, and a recrystallized texture that favors basal dislocation activity.
Cerium is a preferred rare earth element for addition to magnesium for improved ductility of the magnesium-cerium combination. It may be preferred to use cerium in the form of mischmetal. Cerium mischmetal is an alloy of rare earth elements in various naturally occurring proportions. It is sometimes called cerium mischmetal or rare earth mischmetal. A representative mischmetal composition includes approximately, by weight, fifty percent cerium, forty-five percent lanthanum, with small amounts of neodymium and praseodymium. Sometimes, the alloys contain more or less cerium. An embodiment of the invention will be illustrated using cerium as the sole rare earth element additive in magnesium for markedly improving the ductility at room temperature of certain exemplary binary magnesium-cerium alloys.
In one embodiment, a magnesium alloy comprising a small amount up to about one weight percent cerium may undergo a hot deformation process to fabricate a wrought metal object that exhibits enhanced room temperature ductility as compared to that of magnesium and conventional magnesium alloys. The solubility of cerium in magnesium is approximately 0.1% at 500° C. Any excess cerium ultimately forms intermetallics with magnesium and oxide particles within the alloy.
A hot deformation technique suitable for improving ductility in a magnesium-cerium alloy may be a conventional in-line hot extrusion process. In one embodiment, a magnesium alloy comprising up to about one weight percent cerium may be cast as a billet. The initial cast billet is suitably round in cross-section with a diameter of, for example, about 50 millimeters to typically about 300 millimeters, although larger billets are also extruded. The cast billet is preheated to a deformation temperature in the range of about 300° C. to 475° C. Precautions may be taken to ensure that the magnesium-cerium alloy billet is sufficiently lubricated during extrusion by any known metal lubricant such as, for example, graphite or boron nitride. The magnesium alloy billet may be direct extruded through a conventional circular or conical extrusion die possessing an extrusion ratio in the range of 10:1 to 60:1 at a speed in the range of 10 mm per second to 1000 mm per second of extrudate. Depending on the expected use of the extruded article and/or the particular configuration of the eventual final product, the magnesium-cerium alloy may be hot extruded into any one of a number of sizes and shapes known to those of ordinary skill in the art, such as, but not limited to, solid or hollow rods, I-beams, or other achievable extruded shapes. The enhanced ductility of these shapes may then be utilized by further working of the shapes (for example by bending or hydroforming) at a room temperature.
In one embodiment, a magnesium alloy comprising 0.2 weight percent cerium and the balance magnesium that underwent a hot direct extrusion process in accordance with the specifications outlined above achieved an elongation value of approximately 30% during subsequent room temperature deformation. A similarly extruded alloy comprising 0.5 weight percent cerium achieved a slightly smaller elongation value of approximately 25%. For comparison, a billet of substantially pure magnesium was hot extruded by the same method and extrusion die. The pure magnesium extrusion had a room temperature elongation of 9% at ultimate tensile load along the extrusion axis. These test values are presented in
Tensile strength values for the three extrusions are presented in
The relatively high ductility observed in these magnesium-cerium alloys may be at least partly attributable to the dominant role attributed to slip during subsequent tensile deformation of the hot worked alloy along the extrusion axis. Another factor that may be at least partly attributable to high ductility is the intense shear banding that occurs parallel to the extrusion axis which ultimately leads to a redistribution of deformation away from the shear bands and towards the matrix during continuous recrystallization at room temperature. It is contemplated that these phenomena, as well as a reduction in yield strength and grain size, are attributable to the small amount of cerium present in the magnesium-cerium alloy.
In fact, the alloying of small amounts of cerium with magnesium and subjecting the magnesium alloy to hot deformation has been shown to promote grain refinement in comparison to a pure magnesium material. The reduced grain size observed in the magnesium alloy is likely the result of cerium-based intermetallic particles influencing the nucleation and growth of recrystallized grains.
The presence of small amounts of cerium in a hot deformed magnesium alloy has also been shown to decrease the anisotropy of CRSS (critical resolved shear stress) for different slip systems and also increase the amount of dislocations in the alloy during hot deformation. This increase in stored work in the magnesium-cerium alloy favors nucleation and the growth of grains during recrystallization that are not predisposed to forming basal planes aligned parallel to the extrusion axis. For instance, during recrystallization, the formation of grains with their basal planes oriented 40° to 50° away from the extrusion axis has been observed in magnesium alloys comprising small amounts of cerium. This orientation allows slip to play a dominant role in alloy deformation while the significance of twinning is significantly reduced.
It is theorized that small amounts of cerium may alter the electronic charge distribution within the magnesium lattice and modify the CRSS by reducing barriers to dislocation glide on all the slip systems associated with the hexagonal lattice. Thus, slip activity in magnesium-cerium alloys may be at least partly enhanced by the formation of favorable grain orientations as a result of hot deformation, as well as the capacity of the alloy to store work within shear bands to promote continuous recrystallization at room temperature.
The ease of basal slip in smaller, more favorably oriented grains of magnesium-cerium hot deformed alloys may contribute to a reduction in the alloys' tensile yield stress and an increase in work hardening. Normally, grain refinement results in an increase in an alloy's yield strength due to the Hall-Petch relationship. This effect, however, has been shown to be relatively weak in materials defined by a hexagonal closed packed crystal structure where texture anisotropy plays a dominant role in defining yield strength. Thus, in the case of magnesium alloyed with small amounts of cerium, a decrease in yield strength is observed to some extent because of reorientation effects which carry more significance and slightly outweigh the competing Hall-Petch effect. Likewise, the improved strain hardening coefficient attained by a magnesium-cerium alloy is partly attributable to the ease of dislocation cross slip from basal to pyramidal planes.
In view of the surprising increase in room temperature ductility of the hot deformed magnesium-cerium billets, a further description of the extrusion process is provided with respect to the cast billet of pure magnesium (pure Mg) and a billet of magnesium-cerium alloy comprising about 0.2 weight percent cerium (Mg-0.2Ce). Both billets were approximately 75 mm in diameter and extruded along a straight axis in a Wellman Enefco™ 500-ton multipurpose vertical hydraulic press to form solid rods approximately 15 mm in diameter. To accomplish this hot extrusion, the billets were each preheated and maintained at a temperature of about 400° C. for a period of two hours and subsequently forced through a circular die with an extrusion ration of 25:1 at a speed of 10 mm/sec. Boron nitride was used as a lubricant to facilitate the hot metal extrusion. Afterwards, the mechanical properties and microstructure of the pure Mg and Mg-0.2Ce solid rods were analyzed and compared.
To analyze room temperature mechanical properties, samples of the solid extruded rods were tested to evaluate yield strength, compressibility, and elongation. First, tensile specimens having a 25 mm gauge length and a 6.25 mm gauge diameter were tested with an Instron Universal Testing Machine at an average strain rate of 0.66×10−3 s−1. Three specimens were taken from different locations along the steady state portion of the extruded rods and the average values were reported. At room temperature, tensile tests on the pure Mg sample revealed a yield strength of 106 MPa, an ultimate tensile strength of 170 MPa, and an elongation value 9.1%. Corresponding tests performed on the Mg-0.2Ce sample revealed a yield strength of 68.6 MPa, an ultimate tensile strength of 155 MPa, and an elongation value greater than 30%. Clearly, the Mg-0.2Ce alloy extruded rod sample has a lower yield strength and is significantly more ductile than the pure Mg metal rod sample. Additional details pertaining to the fracture surfaces of the tensile specimens will be set out below in conjunction with the microstructure analysis of the extruded rods.
Second, uniaxial compression tests were performed on a United Testing FRM60™ machine equipped with a 27 ton load cell on a 12.7 mm compressometer gauge. Samples of the extruded rods having a length of 37.4 mm and a diameter of 12.8 mm that satisfy the ASTM E9 testing procedure requiring a length to diameter ratio of 3:1 were compressed to a maximum value of 10% at a strain rate of 0.005 min−1. The compressive yield strengths of the two samples were nearly identical with the strength of pure Mg measured at 53.5 MPa and the strength of Mg-0.2Ce measured at of 55.8 MPa. The similarity in compressive yield strengths arises from the fact that the threshold deformation required to nucleate the extension twins are equivalent in each sample. A noted difference, however, was observed in the way each sample transitioned from elastic behavior to plastic behavior. More specifically, the pure Mg sample abruptly transitioned from elastic to plastic mode which signals that deformation occurs almost exclusively by twinning. In the Mg-0.2Ce sample, a gradual transition between the elastic and plastic mode was observed, which suggests that deformation occurs by a combination of twinning and slip mechanisms.
To analyze the microstructure characteristics of the extruded rods, polished samples sectioned parallel and normal to the extrusion axis were prepared by first scrapping 0.15 m off the leading end of the extruded rod to ensure that the material being examined represents a portion of the rod formed by way of steady state extrusion. Next, metallographic samples of the type needed were prepared and polished by standard methods. The samples were then etched in a solution containing 20 mL glacial acetic acid, 50 mL picric acid, 10 mL methanol, and 10 mL de-ionized water.
Polished samples cut parallel and normal to the extrusion axis were fabricated from both extruded rods and examined with a Nikon™ optical microscope interfaced with a Leco™ image analyzer to inspect the microstructure in both the longitudinal and transverse directions. The optical micrographs show no anisotropy in grain morphology along either direction and indicate a fully recrystallized, nearly equi-axed grain structure with an average grain size of approximately 60 μm for the pure Mg sample and 45 μm for the Mg-0.2Ce sample. There was also some evidence of twinning in the Mg-0.2Ce sample sectioned parallel to the extrusion axis that was not present in the corresponding pure Mg sample.
Furthermore, as alluded to above, optical micrographs showing the microstructure characteristics of the fracture surfaces of the pure Mg and Mg-0.2Ce tensile specimens were examined. In both samples, nearly equal amounts of twinned grains were observed in optical micrographs taken from the mid-section of the deformed samples normal to the fracture surface. It was also found that in both specimens the fracture was intragranular; that is, crack propagation occurred along grain boundaries. Nevertheless, microstructure disparities between the fracture surfaces of the two samples were evident. For instance, an examination of the pure Mg sample revealed the existence of cleavage planes found predominantly within grains, evidence of void formation, and very little necking (when compared to Mg-0.2Ce). These observations suggest that crack initiation in the pure Mg sample may occur at twin intersections and subsequently propagate to generate the observed fracture structure. Conversely, in the Mg-0.2Ce sample, significant necking was observed in comparison to the pure Mg sample as well as an absence of cleavage planes. The dimples on the fracture surface of the Mg-0.2Ce sample were demarcated by intense slip at the boundary region and small particles situated inside the voids suggest particle initiated fracture in the alloy. The examination of the Mg-0.2Ce fracture surface sample also identified the existence of void formation confined solely to nonintersecting shear bands situated parallel to the tensile axis. This observation is evidence of possible continuous recrystallization in the shear bands ahead of the crack tip and strongly conveys that slip activity and deformation are redistributed to regions away from the shear band. It is theorized that this phenomenon is partly responsible for the enhancement in ductility and the existence of necking in the Mg-0.2Ce extrusion sample, as well as the ability of the sample to inhibit crack growth.
The samples were also examined by electron backscattered diffraction (EBSD) in a LEO™ 1450 scanning electron microscope placed 18 mm from the samples and fitted with a TSL™ EBSD camera operating at 20 KV. The EBSD data maps, generated with TSL data analysis software from samples cut parallel to the extrusion axis indicate that both the pure Mg and Mg-0.2Ce metals underwent a fully recrystallized extrusion. For example, the data maps show that both the pure Mg metal and the Mg-0.2Ce metal contain grains having equi-axed shapes and straight grain boundaries. The data maps also provided grain size measurements adjusted for twinning of approximately 34 μm for the pure Mg sample, which exhibited very little twinning, and approximately 26 μm for the Mg-0.2Ce sample, which exhibited twinning in about 2% of its grains.
Furthermore, differences in the pure Mg and Mg-0.2Ce grains were also observed with respect to basal plane orientation. In the pure Mg sample, the basal planes of nearly all the grains were oriented parallel to the extrusion axis to form a circumferential ring texture about the axis. This texture translates to a Schmid factor of over 0.4 in approximately 36% of the grains and a Taylor factor of less than 1 for over 80% of the grains for the pure Mg sample. The grains in the Mg-0.2Ce sample, on the other hand, possessed basal plane orientations at an angle roughly between 40° and 50° to the extrusion axis. Because of this texture, approximately 56% of the grains in the Mg-0.2Ce sample exhibit a Schmid factor greater than 0.4 and over 80% of the grains have a Taylor factor less than 1. Based on the above findings, the Mg-0.2Ce sample, and accordingly the Mg-0.2Ce extruded rod, comprises a grain structure with a basal slip orientation more acquiescent to formability than the pure Mg extruded rod. The addition of small amounts of cerium evidently alters the texture of the magnesium material during the recrystallization phase that follows hot extrusion.
In addition to the above microstructure analyses, a detailed chemical mapping of cerium, oxygen, and magnesium was obtained from the surface of a polished Mg-0.2Ce sample using a Cameca™ SX100 electron probe micro analyzer operating at 20 KV. The electron microprobe maps show cerium particles associated with oxygen particles in many instances, which suggests that cerium particles form a mixture of Mg—Ce intermetallic particles and dispersed oxide particles within the microstructure of the alloy. The larger intermetallic Mg—Ce particles accounted for about 1 volume percent of the alloy and were routinely observed within grains and on grain boundaries.
The hot extrusion of cast billets of pure magnesium, magnesium-0.2 cerium, and 0.5 cerium each at 400° C. and at an extrusion reduction ratio of 25:1 is described above together with the resulting room temperature tensile test properties. Billets of magnesium-0.2 cerium-3 aluminum and magnesium-0.2 cerium-5 aluminum were hot extruded in the same way. Tensile elongation at ultimate load for the extruded Mg-0.2 Ce-3 Al billet was 18% and for the Mg-0.2 Ce-3 Al billet was 16%.
When Mg-0.2 Ce billets were extruded at 350° C. (with an extrusion ratio of 25:1) their tensile elongation along the extrusion axis was about 21%. And when Mg-0.2 Ce billets were extruded at 450° C. (with an extrusion ratio of 25:1) their tensile elongation along the extrusion axis was about 28%.
Extrusion ratio may also have an effect on the ductility of the hot extruded magnesium-rare earth element billets. For example, when Mg-0.2 Ce billets were extruded at 400° C. at an extrusion ratio of 9:1, the tensile elongation was about 20%, and when Mg-0.2 Ce billets were extruded at 400° C. at an extrusion ratio of 36:1 the tensile elongation was about 23%.
Thus, the enhanced ductility of magnesium-rare earth alloys following an in-line directional hot deformation is attributed to altered metallurgical texture favoring basal slip activity, altered slip angle distribution, higher work hardening, and smaller grain size. Hot rolling with a significant reduction in cross-section may also be used as the directional hot deformation. The effect is obtained by finding a suitable preferred temperature (of at least about 300° C.) for a directional deformation of a suitably configured billet of the alloy. As described hot extrusion at an experimentally obtained preferred extrusion ratio is particularly effective. The improvement seems most pronounced in binary magnesium-cerium alloys, using cerium or a cerium mischmetal as the alloying constituent. But the effect has been demonstrated in magnesium-cerium alloys containing additional alloying constituents such as aluminum. Other alloying constituents such as manganese, zinc, and titanium may be used for other properties but the room temperature ductility may be reduced. The magnesium content of the alloy is suitably at least eighty-five percent of the alloy and preferably ninety percent by weight of the alloy or more.
Thus the practice of the invention is not limited to the specific illustrative embodiments used to illustrate its practices.
This application claims priority based on provisional application 60/952,018, titled “Forming Magnesium Alloys with Improved Ductility,” filed Jul. 26, 2007 and which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60952018 | Jul 2007 | US |
