Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy
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THE Al 7075 alloys are widely used in the structural components of aircraft and automotive industries due to their exceptionally high strength to weight ratio.[1–3] The mechanical properties of these Al 7075 alloys can be enhanced further by refining the grain structure to ultrafine regime. It is reported that about 46 pct of aluminum alloys used in various applications are in the form of sheet.[4] Cryorolling is identified as a promising and novel route to produce ultrafine-grained sheets of pure metal and alloys from its bulk counterpart by deforming them at cryogenic temperature.[5–18] Many researchers have investigated and compared the mechanical behaviors of cryorolled (CR) and roomtemperature rolled materials such as Cu,[5] Al 2024 alloy,[7] Al 6063 alloy,[11] Al 6061 alloy,[16] and Al 7075 alloy.[17] They found that the CR materials exhibit superior mechanical properties to those of room-temperature rolled materials. The enhancement in mechanical properties of the CR materials is due to the effective suppression of dynamic recovery and accumulation of higher dislocation density, during low-temperature rolling. However, the ductility and formability of the CR Al SUSHANTA KUMAR PANIGRAHI, formerly Research Associate, with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India, is now Postdoctoral Fellow, with the Department of Material Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409. R. JAYAGANTHAN, Associate Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee. Contact e-mail: rjayafmt@iitr. ernet.in Manuscript submitted November 28, 2010. Article published online May 10, 2011 3208—VOLUME 42A, OCTOBER 2011
alloy sheets are reported to be less due to (1) higher alloy compositions, (2) reduced dislocation activity, (3) plastic instability,[19] (4) instability of crack nucleation and growth,[20] and (5) lesser dislocation accumulation capacity due to smaller grains.[21] In order to improve ductility and stabilize the microstructure of the CR Al 7075 alloys, a proper annealing treatment is required. Superplastic forming is an attractive manufacturing process to produce complex sheet metal components.[22–25] Development of a superplastic Al 7075 alloy would be useful for applications in fuselage skins and other thinwalled components of aircraft structures as well as structural components of automotive industries. The superplasticity of aluminum alloys is critically dependent upon stability of fine grains at high temperatures. The basic requirements for achieving superplasticity[26–28] are (1) small and stable grain size, typically as small as possible; and (2) the material should sustain high testing temperature (‡0.5 Tm, where Tm represents the melting point of the material). If the ultrafine grains are stable at high temperatures, where the diffusion rate is rapid, there is the possibility of achieving superplasticity at high strain rates.[29,30] T
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