JPS648066B2 - - Google Patents

Description

Claim 1 Essentially the compositional formula of Al _ba1 Zr _a Li _b X _c (X is Cu,
Mg, Si, Sc, Ti, U, Hf, Be, Cr, V, Mn,
at least one element selected from the group consisting of Fe, Co and Ni, a is 0.15-2% by weight,
Al-Li consisting of (b is 2.5-5% by weight, c is 0-5% by weight, and the balance is aluminum)
forming the alloy into a microstructure in which a high concentration of slip deformation-resistant dispersoids is sufficiently uniformly distributed;
A treatment method for increasing the strength and ductility of an aluminum-based low-density alloy, the treatment method comprising a multi-stage aging treatment step of the Al-Li alloy. 2. Process according to claim 1, wherein said alloy is characterized by precipitates of a composite Al ₃ (Li, Zr) phase in an aluminum matrix. 3. A treatment method according to claim 1, in which the number of times of prescription treatment is in the range of 2 to 10. 4. The treatment method according to claim 1, wherein the number of times of aging treatment is in the range of 2 to 5. 5. A method for producing a high-strength, high-ductility, low-density aluminum-lithium alloy, characterized by including the following steps: Essentially the compositional formula Al _ba1 Zr _a Li _b X _c (X is Cu, Mg,
V, Si, Sc, Ti, U, Hf, Be, Cr, Mn, Fe,
At least one element selected from the group consisting of Co and Ni, a is 0.15-2% by weight, b
is 2.5-5% by weight, c is 0-5% by weight, and the balance is aluminum) is heated to a temperature T ₁ for a time sufficient to melt the intermetallic substances therein. cooling the alloy to ambient temperature at a rate sufficient to retain the components in supersaturated solid solution; cooling the alloy at a rate sufficient to activate the nucleation of complex Al ₃ (Li, Zr) precipitates; heating the alloy to a temperature T ₂ for an hour; cooling the alloy to ambient temperature; subjecting the alloy to additional growth of complex Al ₃ (Li, Zr) precipitates and their nucleation by Zr heating to a temperature T ₃ for a time sufficient to cause unpromoted melting of the δ′ precipitates; and cooling the alloy to ambient temperature for controlled precipitation of the composite Al ₃ (Li, Zr) phase in the aluminum matrix. to give rise to 6. A method according to claim 1, further comprising the step of stretching said molten alloy. 7. A method according to claim 5, further comprising the step of stretching said alloy. 8 T ₁ ranges from 500℃ to 555℃, T ₂ from 100℃
180°C and _T3 is in the range 120°C to 200°C. Technical field The present invention is a high-strength, aluminum-based
The present invention relates to an alloy with high ductility, low density, and in particular an alloy characterized by a homogeneous distribution of complex precipitates in its aluminum matrix. The microstructure is developed by a heat treatment process consisting of an initial solutionizing treatment followed by multiple aging treatments. BACKGROUND OF THE INVENTION In aerospace applications, there is an increasing demand for structural alloys with improved specific strength in order to achieve significant weight savings. Aluminum-lithium alloys offer the possibility of meeting weight savings due to the significant effect of lithium on the mechanical and physical properties of aluminum alloys. Addition of 1% by weight lithium (approximately 3.5 atomic %) reduces density by approximately 3% and increases modulus by approximately 6%. This therefore results in a significant increase in the specific modulus (E/ρ). Furthermore, heat treatment of the alloy induces the precipitation of a uniform metastable phase, δ(Al ₃ Li),
Provides considerable reinforcement. However, the development and widespread application of aluminum-lithium alloys
The main obstacle was the inherent brittleness of the system. In the Al-Li system, it has been shown that the low toughness of the alloy is due to brittle cracks along grain boundaries or grain boundaries. The two main microstructural features contributing to its brittleness appear to be precipitation of intermetallic phases along grain and/or grain boundaries and significant planar misalignment in the alloy. The latter causes stress concentration at grain boundaries. These intergranular precipitates tend to cause embrittlement of the boundaries and at the same time pull Li out of the boundary regions, creating precipitate-free regions that act as stress localization points. Planar slip is mostly caused by the sliding deformability (shearing property) of δ′ precipitates.
This results in a decrease in the resistance to dislocation sliding on planes containing slip-deformed δ′ precipitates. Several metallurgical methods have been taken to eliminate problems such as these. PFZ (precipitation-free zone) and intergranular cracks induced by precipitation are stable.
It has been found that it can be reduced by controlled treatments to avoid intergranular precipitation of Al-Li, Al-Cu-Li, Al-Mg-Li phases. The problem of planar slip can be solved by adding elements that form slippery dispersoids,
Al−Cu−Li, Al−Cu−Mg, or Al−Li−
It can be partially alleviated by promoting the dispersion of slip by controlling co-precipitation of Mg intermetallic substances. Elements that form slip dispersion include Mn, Fe, and Co. Co-precipitation of intermetallic materials containing Cu and/or Mg appears to be relatively effective in dispersing deformation motion. However, these intermetallic substances form slowly, so drawing operations and multi-stage aging treatments (PJ Gregson and MMFlower, Acta
Metallurgica, Vol. 33, pp. 527-537, 1985) or a high content of Cu (B), which has an adverse effect on the density of the alloy.
van der Brandt, PJvon den Brink, H.F.
de Jong, L. Katgerman, and H. Kleinjan, “Aluminum-Lithium Alloys” AIME Metallurgical Society,
433-446, 1984). Moreover, the properties of alloys treated in this way are still unsatisfactory. In recent years, with the development of Zr-modified δ′ precipitates, Al
A new method to modify the deformation behavior of −Li alloy systems is suggested. This method is based on the observation that the metastable Al ₃ Zr phase in the Al-Zr alloy system is highly stable against sliding deformation and has a crystal structure (Ll ₂ ) similar to δ'. In this regard,
Composite Al ₃ (Li, Zr) with ternary order in aluminum matrix using Al-2.34Li-1.07Zr alloy
Efforts have been made to create a phase (FWGayle
and JBVander Sande, Scripta
Metallurgica, vol. 18, pp. 473-478, 1984). However, the process of creating such a uniform distribution of phases requires long-term solution treatment or aging, as well as strict control of parameters during thermomechanical processing. From a practical perspective, this process is completely undesirable and can also lead to undesirable microstructural features such as recrystallization and large precipitate-free areas. Furthermore, the process cannot be effectively applied to alloys with low contents of Zr (e.g. 0.2 wt% Zr), which create a small volume fraction of coarse-grained composite precipitates that are non-uniformly distributed (PLMakin
and B. Ralph, Journal of Materials Science,
Volume 19, pp. 3835-3843, 1984; PJ Gregson and
HMFlower, Journal of Materials Science
Letters, vol. 3, pp. 829-834, 1984; PLMakin,
DJ Lloyd, and WMStobbs, Philosophical
Magazine A, Volume 51, Pages L41-L47, 1985). Despite considerable efforts to develop low-density aluminum alloys, conventional techniques such as those described above have not been able to provide low-density aluminum alloys with the desired combination of high strength, high ductility, and low density. was impossible. As a result, conventional aluminum-lithium alloy systems have not been completely satisfactory for applications such as aircraft structural parts that require high strength, high ductility, and low density. SUMMARY OF THE INVENTION The present invention provides a method for producing aluminum-lithium alloys containing high concentrations of sufficiently uniformly distributed slip-resistant dispersoids that significantly improve strength and ductility. The aluminum-based low density alloy of the present invention essentially consists of Al _ba1 Zr _a Li _b X _c , where X is Cu,
Mg, Si, Sc, Ti, U, Hf, Cr, V, Mn, Fe,
at least one element selected from the group including Co and Ni, where "a" is approximately 0.15-2% by weight;
"b" is approximately 2.5-5% by weight, "c" is approximately 0
-5% by weight, with the remainder being aluminum. The microstructure of these alloys is characterized by the precipitation of a complex Al ₃ (Li, Zr) phase in their aluminum matrix. This fine structure is
It is developed by the manufacturing method of the present invention in which an alloy having the above-mentioned compositional formula is subjected to solution treatment and subsequent multi-stage aging treatment. About it,
An improved method for making high-strength, high-ductility, low-density aluminum-based alloys is presented, whereby the aluminum-based alloys produced have a combination of strength and ductility (at the same density).
It has been improved. The aluminum-based, high-strength, high-ductility, low-density alloys prepared by the present invention utilize controlled composite Al ₃ (Li, Zr) precipitation that advantageously provides a wide range of strength and ductility combinations. contains things.

[Brief explanation of drawings]

The present invention may be more fully understood, and other advantages thereof, apparent from the following detailed description and accompanying drawings. About this attached drawing: Figure 1 is a dark-field transmission electron microscopy image of an alloy with the composition Al-3.1Li-2Cu-1Mg-0.5Zr, which contains complex precipitates in its aluminum matrix. A two-stage aging treatment (4 hours at 170°C followed by 16 hours at 190°C) was performed to develop the alloy; Figure 2 shows the weak beam of the alloy with the composition Al-3.7Li-0.5Zr. Dark-field microscopy images showing the resistance of composite precipitates to dislocation sliding during deformation; Figure 3a shows conventional aging treatment (180°C for 16 hours)
Figure 3b shows a micrograph showing the planar slip observed in an alloy with composition Al-3.7Li-0.5Zr subjected to a treatment process according to the present invention (160°C 4 hours, then
Figure 4 is a photomicrograph showing the beneficial effect of aging treatment (16 hours at 180°C), which promotes uniform deformation of the alloy; )
FIG. 5 is a micrograph showing slidingly deformed δ' precipitates observed in an alloy having a composition of Al-3.1Li-2Cu-1Mg-0.5Zr; and FIG. Composite precipitates when an alloy with composition Al-3.2Li-3Cu-1.5Mg-0.2Zr is treated according to the claimed treatment method (4 hours at 170℃, followed by 16 hours at 190℃) This is a micrograph showing the development of. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention generally relates to a method of making Al-Li-Zr-X alloys that are high strength, high ductility, and low density. The low density alloy mainly composed of aluminum in the present invention essentially consists of Al _ba1 Zr _a Li _b X _c , where X is
Cu, Mg, Si, Sc, Ti, U, Hf, Cr, V, Mn,
At least one element selected from the group including Fe, Co, and Ni, where "a" is approximately 0.15-2% by weight, "b" is approximately 2.5-5% by weight, and "c" is 0%.
- in the range of about 5% by weight, the balance being aluminum. The content of Zr is set to 0.15 to 2 wt% so that slip-resistant dispersoids are formed at a high concentration. When the Zr content is less than 0.15wt%, the formation of slip-resistant dispersoids is not sufficient to cause improvement in mechanical properties;
If the amount exceeds 2 wt%, most of the Zr forms cellular Al ₃ Zr, which is undesirable in terms of mechanical properties. The lower limit value of 2.5wt% for Li content is set to achieve the predetermined minimum density, and if the Li content is less than 2.5wt%, the density of the alloy is higher than the predetermined density. Become. When the Li content exceeds 5wt%, Li is contained together with stable AlLi phase or alloying elements such as Cu or Mg at grain boundaries.
Stable phases such as Al ₆ CuLi ₃ , Al ₅ CuLi ₃ or Al ₂ MgLi are formed, and the occurrence of these phases is unfavorable for the mechanical properties of the alloy. The upper limit value of X (5wt%) is also set for the same reason as above. When the content of X exceeds 5 wt%, stable phases as described above are formed, which is disadvantageous for the mechanical properties of the alloy. The method of the present invention includes the use of multi-stage aging treatments during heat treatment of the alloy. This alloy is developed by heat treatment, as described below, into a characteristic microstructure consisting essentially of "complex" Al ₃ (Li, Zr) precipitates (Figure 1) in an aluminum matrix. It can be characterized accordingly. The alloy may also contain other Li-, Cu-, and/or Mg-containing precipitates, provided that the precipitates do not significantly degrade the mechanical and physical properties of the alloy. The factors governing the properties of Al-Li-Zr-X alloy are:
Basically its Li content and microstructure, followed by the remaining alloy constituent elements. The microstructure is primarily determined by the composition and the final thermo-mechanical treatment, such as extrusion, forging and/or heat treatment parameters. Typically, alloys in their as-processed state (just cast, extruded, or forged) contain large intermetallic particles. Further processing is required to develop any microstructural features for any property. The alloy is subjected to an initial solution treatment. That is, heating to a constant temperature (T ₁ ) for a sufficient time to fully melt most of the intermetallic particles that were present during the forging or extrusion process, followed by heating the alloy constituents in the solution. Cool to ambient temperature at a rate fast enough to maintain Generally, the time at which temperature T ₁ is maintained depends on the alloy composition and manufacturing method (eg, ingot casting, powder metallurgy), and typically ranges from about 0.1 to 10 hours. The alloy is reheated to the aging temperature T ₂ for a sufficient time to activate the nucleation of the composite Al ₃ (Li, Zr) precipitates. It is then cooled to atmospheric temperature and subjected to a second aging treatment at a temperature of _T3 . This is done for a time sufficient to allow the growth of the composite Al ₃ (Li, Zr) precipitates and melting of the δ′ precipitates whose nucleation is not assisted by Zr. The alloy at this point is
It is characterized by a characteristic microstructure consisting essentially of composite Al ₃ (Li, Zr) precipitates. This composite Al ₃ (Li, Zr) precipitate has resistance to dislocation sliding and is extremely effective in dispersing deformation motion (Figure 2). As a result, an optimal amount of composite Al ₃ (Li,
The alloy containing Zr) precipitates deforms in a uniform deformation manner and its mechanical properties are improved. Figure 3b clearly shows the uniform deformation behavior in the alloy treated with the method claimed in the invention. On the other hand, FIG. 3a shows the severe degree of planar slippage observed in conventionally processed alloys due to dislocation slippage of the δ' precipitates due to deformation (see FIG. 4). The combination of ductility and high strength is most conveniently achieved according to the invention when the concentration of slip-resistant dispersoids is in the range of approximately 10-60% by volume, preferably approximately 20-40% by volume. be done. The exact temperature at which the alloy is heated during the solution treatment stage
T ₁ is not critical as long as the intermetallic particles at this temperature are molten. A typical range for temperature T ₁ is approximately 500°C to 555°C. composite
The exact temperature T ₂ during the first aging step at which nucleation of the Al ₃ (Li, Zr) precipitates is promoted depends on the alloying constituents present and the last aging step. The optimum temperature range for T ₂ is approximately 100°C to 180°C
It is ℃. The range of T ₃ is from 120℃ to 200℃,
The exact temperature T ₃ depends on the alloying constituents present and the mechanical properties desired. In general, the temperature T ₂
The holding time at _T3 varies depending on the alloy composition and thermal-mechanical processing history. A typical range is approximately 0.1 to 100 hours. Example 1 The ability of composite Al ₃ (Li, Zr) precipitates to modify the deformation behavior of Al-Li-Zr alloys is demonstrated as follows: Figure 2 shows that the deformed alloy (Al-3.7Li-0.5 Zr)
This is a weak-beam dark-field transmission electron microscopy image showing the fine structure of . The alloy was solution treated at 540°C for 4 hours, followed by aging at 160°C for 4 hours, followed by a final aging at 180°C for 16 hours. The concentration of slip-resistant dispersoids in the alloy shown in FIG. 2 is approximately 30% by volume. Such heat treatment produces composite Al ₃ (Li, Li,
Zr) Promotes precipitation of precipitates. FIG. 3a is a bright field electron microscopy image showing the microstructure of a deformed alloy (Al-3.7Li-0.5Zr) that has not been treated according to the present invention. The alloy is
After solution treatment at 540℃ for 4 hours, 16
Time-aged. The concentration of slip-resistant dispersoids in this alloy after one aging treatment is approximately 5
It is volume %. This alloy is a characteristic and common deformation of brittle alloys. It showed remarkable planar slip. In contrast, Figure 3b shows Al−3.7Li−0.5Zr
1 shows the beneficial effects of the treatment according to the invention on the deformation behavior of alloys with the composition. After solution treatment at 540°C for 4 hours, the alloy was subjected to two-stage aging treatment at 160°C for 4 hours and at 180°C for 16 hours. The concentration of slip-resistant dispersoids in this alloy is approximately 30% by volume. The state of deformation of this alloy is very uniform, indicating high ductility. Example 2 An alloy having the composition Al-3.1Li-2Cu-1Mg-0.5Zr was investigated for medium strength applications as shown in the table. The alloy has a temperature of 2.5 at 540°C.
The conventional 1
One was subjected to a stage aging treatment, and the other was subjected to a two-stage aging treatment according to the present invention. The concentration of slip-resistant dispersoids in this alloy treated according to the invention is approximately 30% by volume. Hereinafter, in Examples 3 to 5, the concentration of slip-resistant dispersoids in the alloy subjected to the multi-stage aging treatment of the present invention is also approximately 30% by volume.

[Table] Effect treatment
Conventional aging treatment (16 hours at 190°C) shows poor ductility (3.6%) due to slippage of δ' precipitates (Figure 4). On the other hand, the composite precipitates developed by the two-stage aging treatment (Figure 1) have improved strength and ductility (6.1% elongation). Example 3 A high strength Al-Li alloy was prepared to meet the requirements of high strength applications for aerospace structural materials. Al
542 alloy with −3.2Li−2Cu−2Mg−0.5Zr composition
Solution treatment was carried out at ℃ for 4 hours. As shown in the table, conventional aging treatment (190°C for 16 hours) results in low strength (yield strength 521 MPa) and low ductility (3.6%). However, in a two-stage aging treatment of the alloy (4 hours at 160°C, followed by 16 hours at 180°C),
Considerably high strength (yield strength 554 MPa) and high ductility (5.5%) were obtained, which meet the required properties of high-strength alloys needed for aerospace structural applications. be.

[Table] Effect treatment
Example 4 This example uses a simple ternary alloy Al-3.7Li-0.5Zr.
2 shows the beneficial effects of the method according to the invention on the mechanical properties of. The alloys were solution treated at 540° C. for 4 hours and subsequently aged as indicated in the table. The results show that the tensile properties show that the method according to the invention gives improved results in terms of strength and ductility compared to conventional methods.

Table: Example 5 A wide range of mechanical properties can be achieved by using multi-stage aging conditions. For example, 3-stage aging treatment (4 hours at 120℃, 16 hours at 140℃ and
(at 160℃ for 4 hours) yield strength of 446MPa,
A tensile strength of 464 MPa and an elongation of 4.6% were obtained.
Aging treatment is performed 2 to 5 times, but in some cases 2 to 5 times.
Do ~10 times. As a result, different heat treatments of the alloy according to the invention can be used to produce alloys with different mechanical properties. Example 6 This example demonstrates the potential of the process according to the invention for developing complex precipitates in Al-Li alloys with low Zr content. Figure 5 shows
Solution treatment was carried out at 540°C for 4 hours, reheated at 170°C for 4 hours, followed by a final aging at 190°C for 16 hours.
This is a dark field electron microscope image of a typical Al-3.2Li-3Cu-1.5Mg-0.2Zr alloy. The concentration of slip-resistant dispersoids in this alloy is approximately 15% by volume. The large volume fraction of composite Al ₃ (Li, Zr) precipitates found in such alloys is due to the treatment method according to the present invention.
It is also shown to be very effective in Al-Li alloys with Zr content as low as 0.2 wt%. Although the invention has thus been described in near complete detail, such details are not to be construed as strictly adhered to, and further changes and modifications may themselves suggest themselves to those skilled in the art, all of which may be found in the attached appendix. It will be understood that it is included within the scope of the invention as defined in the claims.