1. Field of Invention

The invention relates to rapidly solidified aluminum-lithium-copper-magnesium-zirconium powder metallurgy components having a combination of high ductility and high tensile strength; and more particularly to a process wherein the components are subjected to thermal treatment which improves yield and ultimate strengths thereof with minimal loss in tensile ductility.

2. Brief Description of the Prior Art The need for structural aerospace alloys of improved specific strength and specific modulus has long been present. It is known that the elements lithium, beryllium, boron, and magnesium can be added to an aluminum alloy to decrease its density. Conventional methods for producing aluminum alloys, such as direct chill (DC), continuous and semi-continuous casting, yield aluminum alloys having up to 5 wt% magnesium or beryllium; but such alloys are inadequate for use in applications requiring a combination of high strength and low density. Lithium contents of about 2.5 wt% have been satisfactorily incorporated into the lithium-copper-magnesium family of aluminum alloys, including those alloys designated 8090, 8091, 2090 and 2091. These alloys have copper and magnesium additions in the 1 to 3 wt% and 0.25 to 1.5 wt% range, respectively. In addition, zirconium is also added at levels up to 0.16 wt%.

The above alloys derive strength and toughness through the formation of several precipitate phases, which are described in detail in the Conference Proceedings of Aluminum-Lithium V, edited by T.H.

Sanders and E.A. Starke, pub. MCE, (1989). An important strengthening precipitate in . aluminum-lithium alloys is the metastable δ phase which has a well defined solvus line. Thus, aluminum-lithium alloys are heat treatable, their strength increasing as δ' homogeneously nucleates from the supersaturated aluminum matrix.

The δ* phase consists of the ordered Ll ₂ crystal structure and the composition Al ₃ Li. The phase has a very small lattice misfit with the surrounding aluminum matrix and thus a coherent interface with the matrix. Dislocations easily shear the precipitates during deformation, resulting in the buildup of planar slip bands. This, in turn, reduces the toughness of aluminum lithium alloys. In binary aluminum-lithium alloys where this is the only strengthening phase employed, the slip planarity results in reduced toughness.

The addition of copper and magnesium to aluminum-lithium alloys has two beneficial effects. First, the elements reduce the solubility of lithium in aluminum, increasing the amount of strengthening precipitates available. More importantly, however, the copper and magnesium allow the formation of additional precipitate phases, most importantly the orthorhombic S' phase (Al ₂ MgLi) and the hexagonal T ₁ phase (Al ₂ CuLi) . Unlike δ' , these phases are resistant to shearing by dislocations and are effective in minimizing slip planarity. The resulting homogeneity of the deformation results in improved toughness, increasing the applicability of these alloys over binary aluminum-lithium. Unfortunately, these phases form sluggishly, precipitating primarily on heterogeneous nucleation sites such as dislocations. In order to generate these nucleations sites, the alloys must be cold worked prior to aging.

Zirconium, at levels under approximately 0.15 wt%, is typically added to the alloys to form the etastable Al ₃ Zr phase for grain size control and to retard recrystallization. Metastable Al ₃ Zr consists of an Ll ₂ crystal structure which is essentially isostructural with δ' (Al ₃ Li) . Additions of zirconium to aluminum beyond 0.15 wt% using conventional casting practice result in the formation of relatively large dispersoids of equilibrium Al ₃ Zr having the tetragonal D0 ₂₃ structure which are detrimental to toughness.

Much work has been done to develop the aforementioned alloys, which are currently near commercialization. However, the processing constraint imposed by the need for cold deformation has limited the application of these alloys to thin, low dimensional shapes such as sheet and plate. Complex, shaped components such as forgings are not amenable to such processing. Hence, conventional aluminum-lithium alloy forgings lack the combination of strength, ductility, and low density required for aerospace structural applications. RIIIHIHΓY of the Invention The invention provides a method for increasing the tensile strength of a component composed of a rapidly solidified aluminum-lithium-copper-magnesium-zirconium alloy by subjecting the component to a multi-step aging treatment. Generally stated, the component is a consolidated article, formed from an alloy that is rapidly solidified and consists essentially of the formula Al _bal Li _a Cu _tr Mg _c Zr _d wherein "a" ranges from about 2.1 to 3.4 wt%, "b" ranges from 0.5 to 2.0 wt%, "c" ranges from 0.2 to 2.0 wt%, and "d" ranges from about 0.4 to 1.8 wt%, the balance being aluminum. The aging treatment to which the component is subjected comprises the steps of subjecting the

component to a preliminary aging treatment at a temperature of about 400°C-500°C for a time period ranging from about 0.5 to 10 hours; quenching the component in a fluid bath; and subjecting the component to a final aging treatment at a temperature of about 100°C-250°C for a time period ranging up to about 40 hours.

In addition, the invention provides a component consolidated from a rapidly solidified aluminum-lithium alloy of the type delineated, which component has been subjected to the multi-step aging treatment specified hereinabove.

It has been found that when specific components consolidated from rapidly solidified alloys of the composition delineated are subjected to the multi-step aging treatment specified , they exhibit increased strength and elongation, as compared with components that are thermally processed in a conventional manner. The improved combination of properties afforded by components of the invention renders them especially suited for lightweight structural parts used in automobile, aircraft or spacecraft applications.

Brief Description of the Drawings The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiment of the invention and the accompanying drawings in which: FIG. 1 is a graph depicting the heat evolution/absorption vs. temperature as measured by differential scanning calorimetry for an Al-2.6Li-l.0Cu-0.5Mg-l.0Zr alloy aged at 590 ^β C for 2 hours and ice water quenched; FIG. 2 is a graph of the yield strength vs. aging temperature of a transverse specimen cut from an extruded bar aged for 2 hrs. followed by an ice water quench and subsequent aging for 16 hrs. at

135°C, the open rectangle providing data for a transverse specimen cut from an Al-2.34Li-l.07Zr extruded bar; the specimen being aged at 500°C for 1 hr. was water quenched and subsequently aged at 190°C for 2 hours;

FIG. 3 is a graph of the ultimate tensile strength vs. aging temperature for specimens aged in the manner of the specimens of Fig. 2;

FIG. 4 is a graph of the tensile elongation vs. aging temperature for specimens aged in the manner of the specimens of Fig. 2; and

FIG. 5 is a graph depicting the ultimate strength vs. elongation for the alloys of Fiα. 2 illustrating the improvement in properties extant along the diagonal away from the origin.

Description of the Preferred Kmh flimen The invention provides a thermal treatment that increases the tensile strength of a low density rapidly solidified aluminum-base alloy, consisting essentially of the formula wherein "a" ranges from 2.1 to 3.4 wt%, "b" ranges from about 0.5 to 2.0 wt%, "c" ranges from 0.2 to 2.0 wt%, "d ^M ranges from about 0.4 to 1.8 wt% and the balance is aluminum. In accordance with the invention, the compacted alloy or component is subjected to a preliminary thermal treatment at temperatures ranging from about 400°C to 500 ^β C for a period of approximately 0.5 to 10 hours. While not being bound by theory, it is believed that this treatment dissolves elements such as Cu, Mg, and Li which may be microsegregated in precipitated phases such as δ' , δ, T, and S. In addition, the thermal treatment produces an optimized distribution of cubic Ll ₂ particles ranging from about 5 to 50 nanometers in size. The alloy article is then quenched in a fluid bath, preferably held between 0° and 60 ^β C. As used

hereinafter in the specification and claims, the term "preliminary aging" is intended to define the thermal treatment described in the first sentence of this paragraph. The compacted article is then aged at a temperature ranging from about 100°C to 250°C. for a time period ranging up to about 40 hours to provide selected strength/toughness tempers. No cold deformation step is required during this thermal processing, with the result that complex shaped components such as forgings produced from the aged component have excellent mechanical properties.

Preliminary aging below approximately 400°C results in a deleterious drop in tensile properties due to the formation of undesirable phases such as the δ (AlLi) phase. Preliminary aging above approximately 500 ^β C results in an acceptable combination of tensile properties but does not result in the attainment of the optimum tensile strength since the volume fraction of precipitates is reduced. Grain coarsening may also occur at temperature beyond 550 ^β C, further reducing strength. Consolidated articles aged in accordance with the invention exhibit tensile yield strength ranging from about 400 MPa (58 ksi) to 545 MPa (79 ksi) , ultimate tensile strength ranging from about 510 MPa (74ksi) to MPa (83 ksi) and elongation to fracture ranging from about 4 to 9 % when measured at room temperature (20°C) .

The following examples are presented to provide a more complete understanding of the invention. The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles and practice of the invention are exemplary and should not be construed as limiting the scope of the invention.

KTAMPT. R 1 -T

Thermal processing in accordance with the invention was carried out on extruded bar made from rapidly solidified alloys having compositions (in wt ) listed in Table I.

TABLE I

l.Al-2.1Li-l.OCu-0.5Mg-0.2Zr

2.Al-2.6Li-l.OCu-0.5Mg-0.6Zr 3.Al-2.6Li-l.OCu-0.5Mg-1.0Zr

EXAMPLE 4

Al-2.6Li-l.0Cu-0.5Mg-l.0Zr, made via rapid solidification and formed into an extrusion, was given a preliminary age at 590°C for 2 hours and ice water quenched. The heat evolution/absorption as a function of temperature was then measured using the technique of differential scanning calorimetry (DSC) , shown in Figure 1. The peaks in Figure 1 represent the dissolution of precipitate phases during heating while the troughs represent precipitation. A precipitation reaction is represented by the trough centered at 450°C. It is this precipitation reaction which is responsible for the enhanced strength resulting from the preliminary aging treatment.

The tensile properties of consolidated articles formed by extrusion of the alloys listed in Table I and thermally processed in accordance with the method of the invention are listed in Table II. The extruded bars were given a preliminary age for 2 hours at temperatures between 400°C and 600°C and quenched into an ice water bath; subsequently, they were aged at 135°C for 16 hours. Transverse specimens were then cut and machined into round tensile specimens having a gauge diameter of 3/8

- 8 - inches and a gauge length of 3/4 inches. Tensile testing was performed at room temperature at a strain rate of 5.5x10 -4 _sec ^_1

TABLE II

Composition ( U) 0.2% YS UTS Elong. to Aαlno Temp. (MPa) (MPa) fract. (1)

Figures 2, 3, and 4 are graphs of the data listed in Table II. The graphs illustrate that the peak ultimate tensile strength (UTS) is a function of both zirconium content and temperature of the first aging treatment. For example, a peak UTS of 570 MPa is obtained for 440°C preliminary aged Al-2.6Li-l.Cu-0.5Mg-l.0Zr while a peak UTS of 540 MPa is obtained for a 490°C preliminary aged Al-2.6Li-l.0Mg-0.6Zr.

Also included for reference in the Figures 2, 3, and 4 is the transverse tensile data for an

Al-2.34Li-l.07Zr extrude bar processed in the manner disclosed by U.S.P. 4,747,884 to Gayle et al. It is clear that the tensile properties of materials processed in accordance with the present invention are superior to those of conventionally processed alloy chemistries. Gayle et al. have also included data on rod produced from this alloy which displays, qualitatively, a similar strength behavior with preliminary aging temperature. The rod, due to enhanced deformation compared with bar, displays a strength somewhat higher than the bar stock, as shown in the data of Gayle et al. It will be appreciated by those skilled in the art that similar comparative gains in strength induced by enhanced deformation will be achieved in rod made from Al-Li-Cu-Mg-Zr alloys. Data is also included for the ingot aluminum-lithium alloy 8090, is taken from Damerval et. al., "4th International Aluminum-Lithium Conference", J. de Physique, pg. C3-661, (1987), for transverse sections of extruded bar. The bar was solutionized at 540°C-1 hr; cold water quenched, given a 2% cold stretch, and peak aged at 190°C for 12 hours. It will be seen that the properties of S Al-Li-Cu-Mg-Zr bar stock given a preliminary age in accordance with the invention are superior to those of the alloy 8090, without the cold deformation commonly employed on ingot aluminum-lithium alloys. g∑ftMP g 6 This example illustrates that the enhanced strength resulting from control of the preliminary age is greater than and thus distinct from merely extending the aging time of the second low temperature aging treatment. The tensile yield strengths for an Al-2.6Li01.0Cu-0.5Mg-0.6Zr extrusion measured in the manner set forth in Example 5 are listed in Table III. Reducing the preliminary aging temperature from 540°C to 400°C results in a 14%

increase in tensile strength compared with only 4% increase in strength when a 540"C preliminary aged specimen is aged for double the time 135°C.

Table III

Thermal Treatment YS(MPa>

400°C-2hr ice WQ;135 ^β C-16hr 500

540°C-2hr ice WQ;135°C-16hr 440 540°C-2hr ice WQ;135°C-32hr 460

Having thus described the invention in rather full detail, it will be understood that these details need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims.