EP0229075B1

EP0229075B1 - High strength, ductile, low density aluminum alloys and process for making same

Info

Publication number: EP0229075B1
Application number: EP19860902711
Authority: EP
Inventors: Nack Joon Kim; Colin Mclean Adam; Richard Lister Bye, Jr.; Santosh Kumar Das
Original assignee: AlliedSignal Inc
Current assignee: Honeywell International Inc
Priority date: 1985-07-08
Filing date: 1986-04-11
Publication date: 1989-09-27
Also published as: AU578828B2; JPS62502295A; WO1987000206A1; DE3665884D1; CA1280342C; EP0229075A1; AU5774986A; JPS648066B2

Abstract

A process for making high strengh, high ductility, low density aluminum-base alloys, consisting essentially of 2,5 to 5 wt % Li, 0,15 to 2 wt % Zr, 0 to 5 wt % of at least one element selected from the group consisting of Cu, Mg, Si, Se, Ti, U, Hf, Be, Cr, V, Mn, Fe, Co, Ni, balance Al. The alloy is given multiple aging treatments after being solutionized. The microstructure of the alloy is characterized by the precipitation of a composite phase in the aluminum matrix thereof.

Description

The invention relates to a process for making high strength, high ductility, low density aluminum-based alloys that, in particular are characterized by a homogeneous distribution of composite precipitates in the aluminum matrix thereof. The microstructure is developed by heat treatment method consisting of initial solutionizing treatment followed by multiple aging treatments.
There is a growing need for structural alloys with improved specific strength to achieve substantial weight savings in aerospace applications. Aluminum-lithium alloys offer the potential of meeting the weight savings due to the pronounced effects of lithium on the mechanical and physical properties of aluminum alloys. The addition of one weight percent lithium (-3.5 atom percent) decreases the density by ~ 3% and increases the elastic modulus by ~6%, hence giving a substantial increase in the specific modulus (E/p). Moreover, heat treatment of alloys results in the precipitation of a coherent, metastable phase, 8' (Al₃Li) which offers considerable strengthening. Nevertheless, development and widespread application of the Al-Li alloy system have been impeded mainly due to its inherent brittleness.
It has been shown that the poor toughness of alloys in the Al-Li system is due to brittle fracture along the grain or subgrain boundaries. The two dominant microstructural features responsible for their brittleness appear to be precipitation of intermetallic phases along the grain and/or boundaries and the marked planar slip in the alloys, which create stress concentrations at the grain boundaries. The intergranular precipitates tend to embrittle the boundary, and simultaneously extract Li from the boundary region to form precipitate free zones which act as sites of strain localization. The planar slip is largely due to the shearable nature of 8' precipitates which result in decreased resistance to dislocation slip on planes containing the sheared 8' precipitates.
Several metallurgical approaches have been undertaken to circumvent these problems. It has been found that the PFZ (precipitate free zone) and precipitate-induced intergranular fracture can be reduced by controlling processing to avoid the intergranular precipitation of stable AI-Li, Al-Cu-Li, Al-Mg-Li phases. The problem of planar slip can be partly alleviated by promoting slip dispersion through the addition of dispersoid forming elements and the controlled co-precipitation of Al-Cu-Li, AI-Cu-Mg and/or Al-Li-Mg intermetallics. The dispersoid forming elements include Mn, Fe, Co, etc. The co-precipitation of Cu and/or Mg containing intermetallics appears to be relatively effective in dispersing the dislocation movement. However, the sluggish formation of these intermetallics requires the thermomechanical treatments involving stretching operations and multiple aging treatments (P.J. Gregson and M.M. Flower, Acta Metallurgica, vol. 33, pp. 527-537, 1985), or a high Cu content which adversely affects the density of alloys (B van der Brandt, P.J. von den Brink, H.F. de Jong, L. Katgerman, and H. Kleinjan, in «Aluminum-Lithium Alloy II», Metallurgical Society of AIME, pp. 433-446, 1984). Moreover, the properties of alloys thus processed were less than satisfactory.
Recently, a new approach has been suggested to modify the deformation behavoir of Al-Li alloy system through the development of Zr modified 6' precipitates. This approach is based on the observation that the metastable A1₃Zr phase in the AI-Zr alloy system is highly resistant to dislocation shear and is of the same crystal structure (LI₂) as 6', in this regard, attempts have been made to produce a ternary ordered composite A1₃ (Li, Zr) phase in the aluminum matrix with an alloy of AI-2.34Li-1.07Zr (F.W. Gayle and J.B. Vander Sande, Scripta Metallurgica, vol. 18, pp. 473-478, 1984). However, the processed for developing a homogeneous distribution of such phase has required the strict control of processing parameters during the thermomechanical processing, as well as prolonged solutionizing and/or aging treatments. From the practical point of view, this process is quite undesirable and may also result in undesirable microstructural features such as recrystallization and wide precipitate free zones. Moreover, the process cannot be effectively applied to low Zr (e.g., 0.2 wt% Zr) containing alloys which produce a small volume fraction of heterogeneously distributed coarse composite precipitates (P.L. Makin and B. Ralph, Journal of Materials Science, vol. 19, pp. 3835-3843, 1984; P.J. Gregson and H.M. Flower, Journal of Materials Science Letters, vol. 3, pp. 829-834, 1984; P.L. Makin, D.J. Lloyd, and W.M. Stobbs, Philosophical Magazine A, vol. 51, pp. L41-L47, 1985).
A paper by Lin et al in Metallurgical Transactions A, vol. 13A, March 1982, pp. 401-410 discusses the effect on two aluminum alloys containing, as alloying elements, Li 2.7%, Cu 2.3%, Zr 0.17% and Li 2.7%, Cu 2.3%, Zr 0.19% and Cd 0.19%, of various solutionizing, quenching and aging treatments.
Despite considerable efforts to develop low density aluminum alloys, conventional techniques, such as those discussed above, have been unable to provide low density aluminum alloys having the sought for combination of high strength, high ductility and low density. As a result, conventional aluminum-lithium alloy systems have not been entirely satisfactory for applications such as aircraft structural components, wherein high strength, high ductility and low density are required.
The present invention provides a process for making aluminum-lithium alloys containing a high density of substantially uniformly distributed shear resistant dispersoids which markedly improve the strength and ductility thereof. The low density aluminum-base alloys, of the invention consist essentially of the formula AI_balZr_aLi_bX_c, wherein X is at least one element selected from Cu, Mg, Si, Sc, Ti, U, Hf, Be, Cr, V, Mn, Fe, Co and Ni, «a» is from 0.15-2 wt%, «b» is from 2.5-5 wt%, «c» is from 0-5 wt% and the balance is aluminum. The microstructure of these alloys is characterized by the precipitation of composite A1₃ (Li, Zr) phase in the aluminum matrix thereof. This microstructure is developed in accordance with the process of the present invention by subjecting an alloy having the formula delineated above to solutionizing treatment followed by multiple aging treatments. An improving process for making high strength, high ductility, low density aluminum-based alloy is thereby provided wherein the aluminum-based alloy produced has an improved combination of strength and ductility (at the same density).
The solutionizing and aging treatments according to the invention comprise the steps of:

heating the aluminum alloy having the constitution set out above, at a temperature, T₁ (preferably 500 to 555°C), for a period of time sufficient to substantially dissolve most of the intermetallic particles therein;
cooling said alloy to ambient temperature at a rate sufficient to retain its elements in supersaturated solid solution;
heating said alloy at a temperature, T₂ (preferably 100 to 180°C), for a period of time sufficient to activate nucleation of composite A1₃ (Li, Zr) precipitates;
cooling said alloy to ambient temperature;
heating said alloy at a temperature, T₃ (preferably 120 to 200°C), for a period of time sufficient to effect additional growth of composite A1₃ (Li, Zr) precipitates, and dissolution of 8' precipitates whose nucleation is not aided by Zr; and
cooling said alloy to ambient temperature to produce therein a controlled precipitation of composite Al₃ (Li, Zr) phase in said aluminum matrix.

According to one embodiment of the invention the number of aging treatments can be from 2 to 10, more preferably from 2 to 5.
According to another embodiment of the invention, the solutionized alloy can be stretched.
The high strength, high ductility, low density aluminum-based alloy produced in accordance with the present invention has a controlled composite A1₃ (Li, Zr) precipitate which, advantageously, offers a wide range of strength and ductility combinations.
The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description and the accompanying drawings, in which:

Fig. 1 is a dark field transmission electron micrograph of an alloy having the composition Al-3.1 Li-2 Cu-1Mg-0.5Zr, the alloy having been subjected to double aging treatments (170°C for 4 hours followed by 190°C for 16 hours) to develop a composite precipitate in the aluminum matrix thereof;
Fig. 2 is a weak beam dark field micrograph of an alloy having the composition AI-3.7Li-0.5Zr, illustrating the resistance of the composite precipitate to dislocation during the formation;
Fig. 3(a) shows the planar slip observed in an alloy having the composition Al-3.7Li-0.5Zr, the alloy having been subjected to a conventional aging treatment (180°C for 16 hours);
Fig.3(b) shows the beneficial effect of subjecting the alloy of Fig. 3(a) to treatment in accordance with the claimed process (160°C for 4 hours followed by 180°C for 16 hours), thereby promoting the homogeneous deformation thereof;
Fig. 4 shows sheared δ' precipitates observed in an alloy having the composition Al-3.1 Li-2Cu-1Mg-0.5Zr, the alloy having been subjected to a conventional aging treatment (190°C For 16 hours); and
Fig. 5 shows the development of composite precipitates in an alloy having the composition AI-3.2 Li-3Cu-1.5Mg-0.2Zr, the alloy having been subjected to treatment in accordance with the claimed process (170°C for 4 hours followed by 190°C for 16 hours).

In general, the present invention relates to the process of making high strength, high ductility, and low density AI-Li-Zr-X alloys. The process involves the use of multiple aging steps during heat treatment of the alloy. The alloy is characterized by a unique microstructure consisting essentially of «composite» A1₃ (Li, Zr) precipitate in an aluminum matrix (Fig. 1) due to the heat treatment as hereinafter described. The alloy may also contain other Li, Cu and/ or Mg containing precipitates provided such precipitates do not significantly deteriorate the mechanical and physical properties of the alloy.
The factors governing the properties of the Al-Li-Zr-X alloys are primarily its Li content and microstructure and secondarily the residual alloying elements. The microstructure is determined largely by the composition and the final thermomechanical treatments such as extrusion, forging and/or heat treatment parameters. Normally, and alloy in the as- processed condition (cast, extruded or forged) has large intermetallic particles. Further processing is required to develop certain microstructural features for certain characteristic properties.
The alloy is given an initial solutionizing treatment, that is, heating at a temperature (T_i) for a period of time sufficient to substantially dissolve most of the intermetallic particles present during the forging or extrusion process, followed by cooling to ambient temperature at a sufficiently high rate to retain alloying elements in said solution. Generally, the time at temperature T₁, will be dependent on the composition of the alloy and the method of fabrication (e.g., ingot cast, powder metallargy processed) and will typically range from 0.1 1 to 10 hours. The alloy is then reheated to an aging temperature, T₂, for a period of time sufficient to activate the nucleation of composite A1₃ (Li, Zr) precipitates, and cooled to ambient temperature, followed by a second aging treatment at temperature, T_s, for a period of time sufficient for the growth of the composite A1₃ (Li, Zr) precipitate and a dissolution of 8' precipitate whose nucleation is not aided by Zr. The alloy at this point is characterized by a unique microstructure which consists essentially of composite A1₃ (Li, Zr) precipitate. This composite Al₃ (Li, Zr) precipitate is resistant to dislocation shear and quite effective in dispersing dislocation motion (Fig. 2). The result is that the alloy containing an optimum amount of composite Al₃ (Li, Zr) precipitate deforms by a homogeneous mode of deformation resulting in improved mechanical properties. Fig. 3(b) clearly shows the homogeneous mode of deformation in an alloy subjected to the process claimed in this invention, while Fig. 3(a) shows the severe planar slip observed in a conventionally processed alloy due to the shearing of 8' precipitates by dislocations (see Fig. 4). The combination of ductility with high strength is best achieved in accordance with the invention when the density of the shear resistant dispersoids ranges from 10 to 60 percent by volume, and preferably from 20-40 percent by volume.
The exact temperature, T₁, to which the alloy is heated in the solutionizing step is not critical as long as there is a dissolution of intermetallic particles at this temperature. The exact temperature, T₂, in the first aging step where the nucleation of composite A1₃ (Li, Zr) precipitate is promoted, depends upon the alloying elements present and upon the final aging step. The optimum temperature range for T_z, is from 100°C to 180°C. The exact temperature, T₃, whose range is from 120°C to 200°C, depends on the alloying elements present and mechanical properties desired. Generally, the times at temperatures T₂ and T₃ are different depending upon the composition of the alloy and the thermomechanical processing history, and will typically range from 0.1 to 100 hours.

Example 1

The ability of composite A1₃ (Li, Zr) precipitates to modify the deformation behavior of AI-Li-Zr alloys is illustrated as follows:
Fig. 2 is a weak beam dark field transmission electron micrograph showing microstructure of a deformed alloy (Al-3.7Li-0.5Zr) which has been solutionized at 540°C for 4 hours and subsequently aged at 160°C for 4 hours followed by final aging at 180°C for 16 hours. Such heat treatment promotes the precipitation of composite Al₃ (Li, Zr) which is highly resistant to dislocation shear and is quite effective in dispersing the dislocation movement.
Fig. 3(a) shows a bright field electron micrograph showing microstructure of a deformed alloy (Al-3.7 Li-0.5Zr) which has not been given the claimed process. The alloy had been aged, for 16 hours at 180 ° C after solutionizing at 540°C for 4 hours. This alloy showed the pronounced planar slip which is the common deformation characteristic of brittle alloy.
In contrast, Fig. 3(b) illustrates the beneficial effect of the claimed process on the deformation behavior of an alloy having the composition AI-3.7Li-0.5Zr. After solutionizing at 540°C for 4 hours, the alloy had been subjected to the double aging treatment of 160°C for 4 hours and 180°C for 16 hours. The deformation mode of this alloy is quite homogeneous indicating high ductility.

Example 2

An alloy having a composition of Al-3.1Li-2Cu-1 Mg-0.5Zr was developed for medium strength applications as shown in Table I. The alloy was solutionized at 540°C for 2.5 hours, quenched into water at about 20°C and given conventional single aging and the claimed double aging treatments.
Conventional aging treatment (190°C for 16 hours) showed poor ductility (3.6%) due to the shearing of 8' precipitate (Fig. 4), while composite precipitate developed by double aging (Fig.1) improve both strength and ductility (6.1 % elongation).

Example 3

A high strength AI-Li alloy was made to satisfy the requirements for high strength applications for aerospace structure. An alloy having a composition of AI-3.2Li-2Cu-2Mg-0.5Zr was solutionized at 542°C for 4 hours. As shown in Table II, conventional aging treatment (190°C for 16 hours) showed lower strength (yield strength of 521 MPa) and ductility (3.6%). However, double aging of the alloy (160°C for 4 hours followed by 180°C for 16 hours) gave significantly higher strength (yield strength of 554 MPa) and ductility (5.5%), which meets property requirements for high strength alloys needed for aerospace structural applications.

Example 4

This example illustrates the beneficial effect of the claimed process on the mechanical properties of a simple ternary alloy Al-3.7Li-0.5Zr. The alloy was solutionized at 540°C for 4 hours, and subsequently aged as shown in Table III. The resulting tensile properties show that the claimed results in improved strength and ductility compared to the conventional process.

Example 5

A wide range of mechanical properties can be achieved by using multiple aging conditions. For example, a triple aging treatment (120°C, 4 hours + 140°C, 16 hours + 160°C, 4 hours) produced yield strength of 446 MPa and ultimate tensile strength of 464 MPa with 4.6% elongation. As a result, a variety of heat treatments of the alloys according to the claims can be employed to produce alloys having a variety of mechanical properties.

Example 6

This example illustrates the potential of the claimed process for de development of composite precipitate in low Zr containing AI-Li alloys. Fig. 5 shows the dark field electron micrograph of a typical alloy AI-3.2Li-3Cu-1.5Mg-0.2Zr which had been solutionized at 540°C for 4 hours, reheated to 170°C for 4 hours followed by final aging at 190°C for 16 hours. The large volume fraction of composite AI₃ (Li,Zr) precipitate observed in such an alloy indicates that the claimed process in also quite effective in Al-Li alloys having low Zr content of 0.2%.

Claims

1. A process for increasing the strength and ductility of a low density aluminum-base alloy comprising the steps of:

heating an aluminum alloy, having the formula AI- _balZr_aL_ibX_c, wherein X is at least one element selected from Cu, Mg, V, Si, Sc, Ti, U, Hf, Be, Cr, Mn, Fe, Co and Ni, «a» is from 0.15-2 wt%, «b» is from 2.5-5 wt%, «c» is from 0 to 5 wt% and balance is aluminum, at a temperature, T₁, for a period of time suffi- cientto substantially dissolve most of the intermetal- lici particles therein;

cooling said alloy to ambient temperature at a rate sufficient to retain its elements in supersaturated solid solution;

heating said alloy at a temperature, T₂, for a period of time sufficient to activate nucleation of composite A1₃ (Li, Zr) precipitates;

cooling said alloy to ambient temperature;

heating said alloy at a temperature, T₃, for a period of time sufficient to effect additional growth of composite A1₃ (Li, Zr) precipitates, and dissolution of δ' precipitates whose nucleation is not aided by Zr; and

cooling said alloy to ambient temperature to produce therein a controlled precipitation of composite Al₃ (Li, Zr) phase in said aluminum matrix.

2. A process according to claim 1, characterized in that the number of aging treatments ranges from 2 to 10.

3. A process according to claim 1, characterized in that the number of aging treatments ranges from 2 to 5.

4. A process according to any one of claims 1 to 3, characterized in that T is from 500 to 555°C, T₂ is from 100 to 180°C and T₃is from 120 to 200°C.