Patent Grant
11111562
The invention relates to aluminum-copper-lithium alloy products, and more specifically, such products and processes for production and use thereof, in particular in the field of aeronautical and aerospace construction.
Products, in particular thick rolled, forged or extruded aluminum alloy products, are developed in order to produce, by cutting, surface milling or machining from the solids, high-strength parts intended in particular for the aeronautical industry, the aerospace industry or mechanical construction.
Aluminum alloys comprising lithium are very beneficial in this regard because lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each weight percent of lithium added. In order for these alloys to be selected in airplanes, their performance with respect to other usage properties must be as good as that of commonly used alloys, in particular in terms of the compromise between the static mechanical strength properties (yield strength, ultimate tensile strength) and the damage tolerance properties (fracture toughness, resistance to fatigue crack propagation), these properties generally being contradictory. For thick products, these properties must particularly be obtained at the quarter-and half-thickness, and the products therefore must have low quench sensitivity. It is said that a product is quench sensitive if these static mechanical properties, such as the yield strength, decrease when the quenching rate decreases. The quenching rate is the average cooling rate of the product during quenching.
These mechanical properties must also preferably be stable over time and not be significantly modified by aging at the working temperature. Thus, prolonged use of products in civil aviation applications requires good stability of the mechanical properties, which is simulated for example by thermal exposure for 1000 hours at 85° C.
These alloys must also have sufficient corrosion resistance, be capable of being formed according to usual processes and have low residual stress so as to be capable of being integrally machined.
U.S. Pat. No. 5,032,359 describes a very large family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, in particular between 0.3 and 0.5 percent by weight, enables the mechanical strength to be increased.
U.S. Pat. No. 5,234,662 describes alloys with the following composition (in weight percent): Cu: 2.60-3.30, Mn: 0.0-0.50, Li: 1.30-1.65, Mg: 0.0-1.8, and elements controlling the granular structure chosen from Zr and Cr: 0.0-1.5.
U.S. Pat. No. 5,455,003 describes a process for producing Al—Cu—Li alloys that have improved mechanical strength and toughness at cryogenic temperature, in particular owing to suitable strain hardening and aging. This patent recommends in particular the following composition, in weight percentages: Cu: 3.0-4.5, Li: 0.7-1.1, Ag: 0-0.6, Mg: 0.3-0.6 and Zn: 0-0.75. The problem of thermal stability for civil aeronautics applications is not mentioned in said document because the intended applications are essentially cryogenic storages for rocket launchers or space shuttles.
U.S. Pat. No. 7,438,772 describes alloys comprising, in weight percentages: Cu: 3-5, Mg: 0.5-2, Li: 0.01-0.9 and discourages the use of higher lithium contents due to degradation of the compromise between toughness and mechanical strength.
U.S. Pat. No. 7,229,509 describes an alloy comprising (weight %): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn, 0.4 max Zr or other grain-refining agents such as Cr, Ti, Hf, Sc or V, in particular having a toughness K1C(L)>37.4 MPa√m for a yield strength of Rp0.2(L)>448.2 MPa (products with a thickness above 76.2 mm) and in particular a toughness K1C(L)>38.5 MPa√m for a yield strength of Rp0.2(L)>489.5 MPa (products with a thickness below 76.2 mm). US Patent Application No 2009/142222 A1 describes alloys comprising (in weight %), 3.4 to 4.2% Cu, 0.9 to 1.4% Li, 0.3 to 0.7% Ag, 0.1 to 0.6% Mg, 0.2 to 0.8% Zn, 0.1 to 0.6% Mn and 0.01 to 0.6% of at least one element for controlling the granular structure.
Also known are alloy AA2050, which includes (weight %): (3.2-3.9) Cu, (0.7-1.3) Li, (0.20-0.6) Mg, (0.20-0.7) Ag, 0.25 max. Zn, (0.20-0.50) Mn, (0.06-0.14) Zr and alloy AA2095 (3.7-4.3) Cu, (0.7-1.5) Li, (0.25-0.8) Mg, (0.25-0.6) Ag, 0.25 max. Zn, 0.25 max. Mn, (0.04-0.18) Zr. Products of alloy AA2050 are known for their quality in terms of static mechanical strength and toughness.
There is a need for products, in particular thick products made of an aluminum-copper-lithium alloy having improved properties over those of known products, in particular in terms of compromise between properties of static mechanical strength and properties of damage tolerance, thermal stability, corrosion resistance and machinability, while having a low density.
The invention first relates to a wrought product such as an extruded, rolled and/or forged aluminum alloy-based product, comprising, in weight %:
Cu: 3.0-3.9;
Li: 0.8-1.3;
Mg: 0.6-1.0;
Zr: 0.05-0.18;
Ag: 0.0-0.5;
Mn: 0.0-0.5;
Fe+Si≤0.20;
Zn≤0.15;
at least one element among:
Ti: 0.01-0.15;
Sc: 0.05-0.3;
Cr: 0.05-0.3;
Hf: 0.05-0.5;
other elements ≤0.05 each and ≤0.15 total, remainder aluminum.
The invention secondly relates to a method to manufacture an extruded, rolled and/or forged aluminum alloy-based product in which:
a) an aluminum-based liquid metal bath is prepared, comprising 3.0 to 3.9% by weight Cu, 0.8 to 1.3% by weight Li, 0.6 to 1.0% by weight Mg, 0.05 à 0.18% by weight Zr, 0.0 to 0.5% by weight Ag, 0.0 to 0.5% by weight Mn, at most 0.20% by weight Fe+Si, at most 0.15% by weight Zn, at least one element chosen from among Cr, Sc, Hf and Ti, the amount of said element, if chosen, being from 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and 0.01 to 0.15% by weight for Ti, the other elements at most 0.05% by weight each, and 0.15% by weight in total, remainder aluminum;
b) an unwrought shape is cast from said liquid metal bath;
c) said unwrought shape is homogenized at a temperature of between 450° C. and 550° and preferably between 480° C. and 530° C. for a period of between 5 and 60 hours;
d) said unwrought shape is hot and optionally cold worked into an extruded, rolled and/or forged product;
e) said product is solution heat treated at between 490 and 530° C. for 15 min at 8 h and quenched;
f) said product is stretched in a controlled manner with a permanent set of 1 to 6% and preferably at least 2%;
g) said product is aged artificially, by heating at a temperature of from 130 to 170° C. for 5 to 100 hours and preferably from 10 to 40 h so as to obtain a yield strength close to the peak.
The invention also relates to a structural element comprising a product according to the invention.
The invention also relates to the use of a structural element according to the invention for aeronautical construction.
Unless otherwise indicated, all of the indications relating to the chemical composition of the alloys are expressed as a weight percentage based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed in weight % is multiplied by 1.4. The alloys are designated according to the regulations of The Aluminum Association, known to a person skilled in the art. The density is dependent on the composition and is determined by calculation rather than by a weight measurement method. The values are calculated according to the procedure of The Aluminum Association, which is described on pages 2-12 and 2-13 of “Aluminum Standards and Data”. The definitions of metallurgical tempers are indicated in the European standard EN 515.
Unless otherwise indicated, the static mechanical properties, in other words the ultimate tensile strength Rm, the conventional yield strength at 0.2% elongation Rp0.2 (“yield strength”) and the elongation at rupture A %, are determined by a tensile test according to standard EN 10002-1, with the sample and the direction of the test being defined by standard EN 485-1.
The stress intensity factor (KQ) is determined according to standard ASTM E 399. Standard ASTM E 399 gives criteria making it possible to determine whether KQ is a valid value of K10. For a given test piece shape, the values of KQ obtained for different materials are comparable to one another insofar as the yield strengths of the materials are on the same order of magnitude.
Unless otherwise indicated, the definitions of standard EN 12258 apply. The thickness of the profiles is defined according to standard EN 2066:2001: the transverse cross-section is divided into basic rectangles with dimensions A and B; A is always the largest dimension of the basic rectangle and B can be considered to be the thickness of the basic rectangle. The die holder is the basic rectangle having the largest dimension A.
The MASTMAASIS (Modified ASTM Acetic Acid Salt Intermittent Spray) is performed according to standard ASTM G85.
In this document, the term “structure element” or “structural element” of a mechanical construction will refer to a mechanical part for which the static and/or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural calculation is normally prescribed or performed. This typically involves elements of which the failure is likely to endanger said construction, users thereof or others. For an airplane, these structural elements include in particular the fuselage (such as the fuselage skin), the stringers, the bulkheads, the circumferential frames, the wing skins, the stringers or stiffeners, the ribs and spars and the tail unit comprised in particular of horizontal and vertical stabilizers, as well as floor beams, seat tracks and doors.
According to the present invention, it has been discovered that by using a selected class of aluminum alloys that contain specific and important amounts of lithium, copper and magnesium and zirconium, wrought products are able to be prepared with an improved compromise between toughness and mechanical strength, and good corrosion resistance. In addition, these products, when they are subjected to an aging process chosen so as to obtain a yield strength Rp0.2 close to the peak yield strength Rp0.2, have excellent thermal stability.
The present inventors have surprisingly noted that according to embodiments of the present invention, it is possible to improve the compromise between the static mechanical resistance properties and the damage tolerance properties, in particular of thick aluminum-copper-lithium alloy products such as, in particular, alloy AA2050 by increasing the magnesium content. In particular, for thick products having been subjected to near-peak aging, the choice of copper, magnesium and lithium contents enables a favorable compromise of properties to be achieved, and satisfactory thermal stability of the product to be obtained.
The copper content of the products according to the invention is advantageously from 3.0 to 3.9% by weight. In an advantageous embodiment of the invention, the copper content is from 3.2 to 3.7% by weight. When the copper content is too high, the toughness may be insufficient, in particular for near-peak aging processes, and, moreover, the density of the alloy may not be advantageous. When the copper content is too low, the minimum static mechanical properties may not be capable of being achieved.
The lithium content of the products according to the present invention is advantageously from 0.8 to 1.3% by weight. Advantageously, the lithium content is from 0.9 to 1.2% by weight. Preferably, the lithium content is at least 0.93% by weight or even at least 0.94% by weight. When the lithium content is too low, the density reduction associated with the addition of lithium may be insufficient.
The magnesium content of the products according to the present invention is advantageously from 0.6 to 1.2% by weight and preferably from 0.65 or 0.67 to 1.0% by weight. In an advantageous embodiment of the present invention, the magnesium content is at most 0.9% by weight and preferably at most 0.8% by weight. For certain applications, it may be advantageous for the magnesium content to be at least 0.7%.
The zirconium content is advantageously from 0.05 to 0.18% by weight and preferably between 0.08 and 0.14% by weight so as to preferably obtain a fibrous or slightly recrystallized grain structure.
The manganese content is advantageously from 0.0 and 0.5% by weight. In particular in the production of thick sheets, the manganese content is preferably from 0.2 to 0.4% by weight which typically enables the toughness to be improved without compromising mechanical strength.
The silver content is advantageously from 0.0 to 0.5% by weight. The present inventors have noted that, although the presence of silver is advantageous, in the presence of a magnesium amount according to the present invention, a large amount of silver may not be necessary for obtaining an improvement desired in the compromise between the mechanical strength and the damage tolerance. The limitation of the amount of silver is generally economically highly favorable. In an advantageous embodiment of the invention, the silver content is from 0.15 to 0.35% by weight. In an embodiment of the present invention, which has the advantage of typically minimizing density, the silver content is preferably not more than 0.25% by weight.
The sum of the iron content and the silicon content is preferably not more than 0.20% by weight. Preferably, the iron and silicon contents are each not more than 0.08% by weight. In an advantageous embodiment of the present invention, the iron and silicon contents are at most 0.06 and 0.04% by weight, respectively. A controlled and limited iron and silicon content can contribute to an improvement in the compromise between mechanical strength and damage tolerance.
The alloy also advantageously contains at least one element capable of contributing to the control of the grain size selected from among Cr, Sc, Hf and Ti, with the amount of the element, if chosen, being between 0.05 and 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf 0.01 to 0.15% by weight for Ti. Preferably, titanium is chosen in an amount of 0.02 and 0.10% by weight.
Zinc is an undesirable impurity. The zinc content is preferably Zn≤0.15% by weight and preferably Zn≤0.05% by weight. Zinc content is advantageously not more than 0.04% by weight.
The density of products according to the present invention is advantageously not more than 2.72 g/cm3. To reduce the density of products, it may be advantageous to select the composition so as to obtain a density of not more than 2.71 g/cm3 and preferably not more than 2.70 g/cm3.
An advantageous alloy according to the present invention is particularly intended for producing thick, extruded, rolled and/or forged products. By thick products, in the context of the present invention, is intended products of which the thickness is at least 30 mm and preferably at least 50 mm. Indeed, an advantageous alloy according to the present invention preferably has a low quenching sensitivity, which is particularly advantageous for thick products.
Rolled products according to the present invention preferably have a thickness of from 30 to 200 mm and more preferably from 50 to 170 mm.
The thick products according to the present invention have a particularly advantageous compromise between mechanical strength and toughness.
A product according to the present invention, in a rolled state, solution treated, quenched and aged so as to reach near-peak yield strength, advantageously has, at half-thickness at least one of the following pairs of properties for thicknesses from 30 to 100 mm:
(i) for thicknesses of 30 to 60 mm, at half-thickness, a yield strength Rp0.2(L)≥525 MPa and preferably Rp0.2(L)≥545 MPa and a toughness K1C (L−T)≥38 MPa√m and preferably K1C (L−T)≥43 MPa√m,
(ii) for thicknesses of 60 to 100 mm, at half-thickness, a yield strength Rp0.2(L)≥515 MPa and preferably Rp0.2(L)≥535 MPa and a toughness K1C (L−T)≥35 MPa√m and preferably K1C (L−T)≥40 MPa√m,
(iii) for thicknesses of 100 to 130 mm, at half-thickness, a yield strength Rp0.2(L)≥505 MPa and preferably Rp0.2(L)≥525 MPa and a toughness K1C (L−T)≥32 MPa√m and preferably K1C (L−T)≥37 MPa√m,
(iv) for thicknesses of 30 to 100 mm, at half-thickness, a yield strength Rp0.2(L) expressed in MPa and a toughness K1C (L−T) expressed in MPa√m so that K1C (L−T)≥−0.217 Rp0.2(L)+157 and preferably K1C (L−T)≥−0.217 Rp0.2(L)+163 and greater than 35 MPa
(v) after thermal exposure for 1000 hours at 85° C., a tensile yield strength Rp0.2(L) and an elongation at rupture A % (L) having a difference with a tensile yield strength Rp0.2(L) and an elongation at rupture A % (L) before thermal exposure of less than 10%, and preferably less than 5%.
In another embodiment, thinner products, with a thickness comprised from 10 to 30 mm, typically around 20 mm, are however preferred because the compromise between mechanical strength and toughness in these conditions is particularly advantageous.
A product according to the present invention, in a rolled state, solution treated, quenched and aged so as to reach near-peak yield strength, advantageously has, at least one of the following pairs of properties at half-thickness for thicknesses from 10 to 30 mm:
(i) a yield strength Rp0.2(L)≥525 MPa and preferably Rp0.2(L) 545 MPa and a toughness K1C (L−T)≥40 MPa√m and preferably K1C (L−T)≥45 MPa√m,
(ii) a yield strength Rp0.2(L) expressed in MPa and a toughness K1C (L−T) expressed in MPa√m so that K1C (L−T)≥−0.4 Rp0.2(L)+265 and preferably K1C (L−T)≥−0.4 Rp0.2(L)+270 and greater than 45 MPa√m,
(iii) after thermal exposure for 1000 hours at 85° C., a tensile yield strength Rp0.2(L) and an elongation at rupture A % (L) having a difference with a tensile yield strength Rp0.2(L) and an elongation at rupture A % (L) before thermal exposure of less than 10%, and preferably less than 5%.
Products according to embodiments of the present invention also have advantageous properties in terms of fatigue behavior with regard to both crack initiation (S/N) and propagation rate (da/dN).
The corrosion resistance of the products of the present invention is generally high; thus, the MASTMAASIS test result (standards ASTMG85 & G34) is at least EA and preferably P for the products according to the present invention.
A suitable process for producing products according to the present invention includes steps of development, casting, hot working, solution treating, quenching and aging. A suitable process is described below.
In a first step, a liquid metal bath is prepared so as to obtain an aluminum alloy with a composition according to the invention.
The liquid metal bath is then cast as an unwrought shape, such as a billet, a rolling ingot or a forging stock.
The unwrought shape is then homogenized at a temperature of between 450° C. and 550° and preferably between 480° C. and 530° C. for a period of between 5 and 60 hours.
After homogenization, the unwrought shape is generally cooled to room temperature before being preheated so as to be hot worked. The preheating is intended to reach a temperature preferably between 400 and 500° C. and more preferably on the order of 450° C., enabling the raw product to be worked.
The hot working and optionally cold working is typically performed by extruding, rolling and/or forging, so as to obtain an extruded, rolled and/or forged product of which the thickness is preferably at least 30 mm. The product thus obtained is then solution heat treated by solution heat treatment at between 490 and 530° C. for 15 min to 8 hours, then typically quenched with water at room temperature or preferably with cold water. The product is then subjected to a controlled stretching with a permanent set of to 6% and preferably at least 2%. The rolled products are preferably subjected to controlled stretching with a permanent set of above 3%. In an advantageous embodiment of the invention, the controlled stretching is performed with a permanent set of between 3 and 5%. A preferred metallurgical temper is T84. Known steps such as rolling, flattening, straightening and forming can optionally be performed after solution heat treating and quenching and before or after the controlled stretching. In an embodiment of the invention, a step of cold rolling of at least 7% and preferably at least 9% is carried out before performing a controlled stretching with a permanent set of 1 to 3%.
Artificial aging is carried out, by heating at a temperature of between 130 and 170° C. and preferably between 150 and 160° C. for 5 to 100 hours and preferably for 10 to 40 hours so as to achieve a yield strength of Rp0.2 near the peak yield strength of Rp0.2.
It is known that, for alloys with age hardening such as Al—Cu—Li alloys, the yield strength increases with the artificial aging time at a given temperature to a maximum value called the hardening peak or “peak”, then decreases with the aging time. In the context of this invention, the term aging curve will refer to the change in the yield strength as a function of the equivalent aging time at 155° C. An example of an aging curve is provided in
To obtain a value close to PN, for a point N on the curve in an under-aged temper, the slope of the line passing through point N and through the preceding point N−1, obtained for a period tN-1<tN and having a yield strength Rp0.2 (N-1), can be determined; we thus have PN≈(Rp0.2 (N)−Rp0.2 (N-1))/(tN−tN-1). In theory, the exact value of PN is obtained when tN-1 tends toward tN. However, if the difference tN−tN-1 is small, the variation in the yield strength risks being insignificant and the value imprecise. The present inventors have noted that a satisfactory approximation of PN is generally obtained when the difference tN−tN-1 is between 2 and 15 hours and is preferably on the order of 3 hours.
The equivalent time ti at 155° C. is defined by the formula:
where T (in Kelvin) is the instantaneous metal treatment temperature, which changes with time t (in hours), and Tref is a reference temperature set at 428 K. ti is expressed in hours. The constant Q/R=16400 K is derived from the activation energy for the diffusion of Cu, for which the value Q=136100 J/mol has been used.
The yield strength close to the peak yield strength is typically equal to at least 90%, generally even equal to at least 95% and frequently at least 97% of the peak yield strength Rp0.2. The maximum peak yield strength can be obtained by varying the time and temperature parameters of the aging. The peak yield strength is generally considered to be satisfactory when the aging time is varied between 10 and 70 h for a temperature of 155° C. after a stretching of 3.5%.
In general, for Al—Cu—Li alloys, the clearly under-aged tempers correspond to compromises between the static mechanical strength (Rp0.2, Rm) and the damage tolerance (toughness, fatigue crack propagation resistance) that are better than at the peak and especially beyond the peak. However, the present inventors have noted that a near-peak under-aged temper may enable a beneficial damage tolerance to be obtained, while also improving the performance in terms of corrosion resistance and thermal stability.
In addition, the use of a near-peak under-aged temper can enable the robustness of the industrial process to be improved: a variation in the aging conditions leads to a low variation in the properties obtained.
It is thus advantageous to carry out a near-peak under-aging, i.e. an under-aging with time and temperature conditions equivalent to those of a point N on the aging curve at 155° C. so that the tangent to the aging curve at this point has a slope PN, expressed in MPa/h, so that 0<PN≤3 and preferably 0.2<PN≤2.5.
Products according to the present invention can advantageously be used for example, in structural elements, in particular for airplanes. The use of a structural element incorporating at least one product according to the present invention and/or manufactured from such a product is advantageous, in particular for aeronautical construction. Products according to the present invention are particularly advantageous in the production of products machined from solids, such as in particular underwing or upper wing elements of which the skin and stringers are obtained from the same starting material, spars and ribs, as well as any other use in which these properties might be advantageous.
These aspects, as well as others of the invention, are explained in greater detail in the following illustrative and non-limiting examples.
In this example, a plurality of slabs with dimensions 2000×380×120 mm of which the composition is provided in table 1 were cast.
The slabs were homogenized at around 500° C. for around 12 hours, then cut and scalped so as to obtain parts with dimensions of 400×335×90 mm. The parts were hot rolled to obtain plates with a thickness of 20 mm. The plates were solution treated at 505+/−2° C. for 1 h, quenched with water at 75° C. so as to obtain a cooling rate of around 18° C./s and thus simulate the properties obtained at half-thickness in a plate with a thickness of 80 mm. The plates were then stretched with a permanent elongation of 3.5%.
The plates were subjected to artificial aging for between 10 h and 50 h at 155° C. Samples were taken at half-thickness in order to measure the static mechanical tensile properties as well as the toughness KQ. The test pieces used for measuring toughness had a width W=25 mm and a thickness B=12.5 mm. In general, the values of KQ obtained from this type of test piece are smaller than those obtained from test pieces having a greater thickness and width. Two measurements, obtained from test pieces with a width W=40 mm and a thickness B=20 mm, confirm this tendency. It may be believed that measurements obtained from even wider test pieces enabling valid measurements of K1C to be obtained would also be higher than the measurements obtained with the test pieces with a width W=25 mm and a thickness B=12.5 mm.
The results obtained are presented in table 2.
In this example, a plurality of slabs with a thickness of 406 mm of which the composition is provided in table 3 were cast.
The slabs were homogenized, then scalped. After homogenization, the slabs were hot rolled in order to obtain plates with a thickness of 50 mm. The plates were solution treated, quenched with cold water and stretched with a permanent elongation of between 3.5% and 4.5%
The plates were then subjected to aging for between 10 h and 50 h at 155° C. Samples were obtained at half-thickness in order to measure the static mechanical tensile properties as well as the toughness KQ. The test pieces used to measure the toughness had a width W=80 mm and a thickness B=40 mm. The validity criteria of K1C were satisfied for certain samples. The results obtained are presented in table 4.
In
In this example, a plurality of slabs with dimensions 2000×380×120 mm of which the composition is provided in table 5 were cast.
The slabs were homogenized at around 500° C. for around 12 hours, then cut and scalped so as to obtain parts with dimensions of 400×335×90 mm. The parts were hot rolled to obtain plates with a thickness of 20 mm. The plates were solution treated at 505+/−2° C. for 1 h, and quenched with cold water. The plates were then stretched with a permanent elongation of 3.5%.
The plates were subjected to artificial aging for between 18 h and 72 h at 155° C. Samples were taken at half-thickness in order to measure the static mechanical tensile properties as well as the toughness KQ. The test pieces used for measuring toughness had a width W=25 mm and a thickness B=12.5 mm.
The results obtained are presented in table 6.
In this example, thermal stability of products made of alloy 12 were compared for different aging conditions.
Plates made of alloy 12 and manufactured according to the method described in example 3 until the artificial aging step excluded underwent artificial aging at 155° C. or at 143° C. for the increasing durations indicated in Table 7. Plates which were artificially aged 34 h at 143° C. or 40 h at 155° C. were subsequently thermally tested for 1000 hours at 85° C. Samples were taken at half-thickness in order to measure the static mechanical tensile properties before and after thermal exposure. Results are presented in Table 7. After aging 34 hours at 143° C., for which the slope PN was evaluated to 7.1 the plate does not exhibit satisfactory thermal stability. Thus after thermal exposure the tensile yield strength has increased 15% and elongation has decreased 13%. To the contrary, after aging 40 hours at 155° C., for which the slope PN is evaluated to 1.9 the plate exhibit a satisfactory thermal stability, with an evolution of those properties less than 5%.
This application claims priority to U.S. Provisional Application 61/220,249 filed Jun. 25, 2009 and FR 09/03096 filed Jun. 25, 2009, the contents of which are incorporated herein by reference in their entireties.
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