Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium

Definitions

• the invention relates to a process for making high strength, high ductility, low density aluminum-based alloys, and, in particular, to the alloys that are characterized by a homogeneous distribution of composite
• the microstructure is developed by heat treatment method consisting of initial solutionizing treatment followed by multiple aging treatments,
• 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.
• _. appear to be the precipitation of intermetallic phases along the grain and/or subgrain boundaries and the marked planar slip in the alloys, which create stress concentrations at the grain boundaries.
• the inter- granular 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 ⁇ ' precipitates which result in decreased resistance to dislocation slip on planes containing the sheared ⁇ 1 precipitates.
• 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, Al-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.
• the sluggish formation of these intermetallics requires the thermomechanical treatments involving stretching operations and multiple aging
• the present invention provides a process for making aluminium-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 A lbal Zr a L b x c wherein X is at least one element selected from the group consisting of Cu, Mg, Si, Sc, Ti, U, Hf, Cr, V, Mn, Fe, Co and Ni, "a” ranges from about 0.15-2 wt%, "b” ranges from about 2.5-5 wt%, "c” . ranges from about 0-5 wt% and the balance is aluminum.
• the icrostructure of these alloys is characterized by the precipitation of composite Al ⁇ Li, Zr) phase in the aluminum matrix thereof. This microstructure is developed in accordance with the process of the present
• the high strength, high ductility, low density aluminum-based alloy produced in accordance with the present invention has a controlled composite Alo(Li, Zr) precipitate which, advantageously, offers a wide range of strength and ductility combinations.
• Fig. 1 is a dark field transmission electron micro ⁇ graph of an alloy having the composition Al-3.lLi-2Cu-
• Fig. 2 is a weak beam dark field micrograph of an 5 alloy having the composition Al-3.7Li-0.5Zr, illustrating the resistance of the composite precipitate to dislocation shear during deformation;
• 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 hrs. followed by 180°C for 16 hrs.), thereby promoting the homogeneous deformation thereof;
• Fig. 4 shows the sheared ⁇ ' precipitates observed
• Fig. 5 shows the development of composite precipi ⁇ tates in an alloy having the composition Al-3.2Li-3Cu- 1.5Mg-0.2Zr, the alloy having been subjected to
• the present invention relates to the process of making high strength, high ductility, and low
• the process involves the use of multiple aging steps during heat treatment of the alloy.
• the alloy is characterized by a unique micro ⁇ structure consisting essentially of "composite" __ Al (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 0 properties of the alloy.
• the factors governing the properties of the Al-Li- Zr-X alloys are primarily its Li content and micro ⁇ structure and secondarily the residual alloying elements.
• the microstructure is determined largely by the composition and the final thermomechanical 5 treatments such as extusion, forging and/or heat treatment parameters.
• an 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 treat- 5 ment, that is, heating at a temperature ( ⁇ _) 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 l ⁇ retain alloying elements in said solution.
• T- j _ the time at temperature T- j _, 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 about 0.1 to 10 hours.
• the alloy at this point is characterized by a unique m i crostructure which con ⁇ sists essentially of composite Al (Li, Zr) precipi-
• 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 ⁇ 1 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 about 10 to 60 percent by volume, and preferably from about 20-40 percent by volume.
• the exact temperature, T ⁇ _, 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 AI3 (Li, Zr) precipitate is promoted depends upon the alloying elements present and upon the final aging step.
• the optimum temperature range for T2 is from about 100°C to 180°C.
• 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 about 0.1 to 100 hours.
• 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 hrs. and subsequently ayed at 160°C for 4 hrs. followed by final aging at 180°C for 16 hrs. Such heat treatment promotes the precipitation of. composite Al 3 (Li, Zr) which is highly resistant to dislocation shear and is quite effective in dispersing the dislocation movement.
• Al-3.7Li-0.5Zr Al-3.7Li-0.5Zr
• Fig. 3(a) shows a bright field electron micrograph showing microstructure of a deformed alloy (Al-3.7Li- 0.5Zr) which has not been given the claimed process.
• the alloy had been aged, for 16 hrs. at 180°C after solutionizing at 540° for 4 hrs.
• This alloy showed the pronounced planar slip which is the common deformation characteristic of brittle alloy.
• Fig. 3(b) illustrates the beneficial effect of the claimed process on the deformation behavior of an alloy having the composition Al-3.7Li- 0.5Zr.
• the alloy had been subjected to the double aging treatment of 160°C for 4 hrs. and 180°C for 16 hrs.
• the deformation mode of this alloy is quite homogeneous indicating high ductility.
• An alloy having a composition of Al-3.lLi-2Cu-lMg- 0.5Zr was developed for medium strength applications as shown in Table I.
• the alloy was solutionized at 540°C for 2.5 hrs., quenched into water at about 20°C and given conventional single aging and the claimed double aging treatments.
• Example 3 A high strength Al-Li alloy was made to satisfy the requirements for high strength applications for aero ⁇ space structure.
• An alloy having a composition of Al- 3.2Li-2Cu-2Mg-0.5Zr was solutionized at 542°C for 4 hrs.
• conventional aging treatment 190°C for 16 hrs.
• showed lower strength yield strength of 521 MPa
• ductility 3.6%)
• double aging of the alloy 160°C for 4 hrs. followed by 180°C for 16 hrs.
• 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 hrs., and subsequently aged as shown in Table IIJ.
• the resulting tensile properties show that the claimed process results in improved strength and ductility compared to the conventional process.
• a wide range of mechanical properties can be achieved by using multiple aging conditions.
• Fo r - example, a triple aging treatment (120°C, 4 hrs. + 140°C, 16 hrs. + 160°C, 4 hrs.) produced yield strength of 446 MPa and ultimate tensile strength of 464 MPa with 4.6% elongation.
• a variety of heat treatments of the alloys according to the claims can be employed to produce alloys having a variety of mechanical properties.
• FIG. 5 shows the dark field electron micrograph of a typical alloy Al-3.2Li-3Cu-l.5Mg-0.2Zr which had been solutionized at 540°C for 4 hrs., reheated to 170°C for 4 hrs. followed by final aging at 190°C for 16 hrs.
• the large volume fraction of composite Al 3 (Li, Zr) precipi ⁇ tate observed in suc an alloy indicates that the claimed process is also quite effective in Al-Li alloys having low Zr content of 0.2%.

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• Chemical & Material Sciences (AREA)
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• Organic Chemistry (AREA)
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• Manufacture Of Alloys Or Alloy Compounds (AREA)

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• 1986
  • 1986-04-11 AU AU57749/86A patent/AU578828B2/en not_active Ceased
  • 1986-04-11 JP JP61502355A patent/JPS62502295A/ja active Granted
  • 1986-04-11 EP EP19860902711 patent/EP0229075B1/en not_active Expired
  • 1986-04-11 DE DE8686902711T patent/DE3665884D1/de not_active Expired
  • 1986-04-11 WO PCT/US1986/000757 patent/WO1987000206A1/en active IP Right Grant
  • 1986-07-08 CA CA000513291A patent/CA1280342C/en not_active Expired - Lifetime

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