Deformation mechanisms during low-and high-temperature superplasticity in 5083 Al-Mg alloy
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SUPERPLASTIC forming of commercial low-priced aluminum, magnesium, and titanium sheets could be developed into one of the important future fabrication means for the automobile, train, architecture, and electronic appliance industries. There are at least three main factors that need to be considered: (1) the alloy itself is commercially widely available and cheap; (2) the forming rate is sufficiently high; and (3) the forming temperature is as low as possible. A higher forming rate to a strain rate greater than 10⫺2 s⫺1 would satisfy the current fabrication speed,[1] while a lower forming temperature would save fabrication energy and prevent severe grain growth, cavitation, and solute-loss from surface layers, as well as maintain superior postform properties.[2] Following these guidelines, the development of superplastic Al-Mg, Al-Mg-Si, or Mg-Al-Zn base alloys has attracted attention lately, including experimental alloys such as Al-3Mg or Al-10Mg and commercial alloys such as AA5052, 5083, 6061, 6011, AZ91, AZ31, etc. There have been numerous efforts in processing aluminum materials to exhibit high strain rate superplasticity (HSRSP) or low-temperature superplasticity (LTSP) by using thermomechanical treatments (TMT), equal channel angular extrusion (ECAE), multiple forging, cyclic extrusion, torsion under compression, or accumulative roll bonding (ARB)[3–14] on commercial or experimental alloys. In the beginning of the development of LTSP, the strain rate was typically less than 10⫺3 s⫺1. Until the work by Valiev et al.[15] in 1997, the Al alloy exhibited not only low-temperature superplasticity but also high strain rate (10⫺2 to 10⫺1 s⫺1) superplasticity. Selected LTSP reports are summarized in Table I, where it can be seen that LTSP has been developed in Al, Mg, Zn, Ti, Ni, and Cu and intermetallic alloys over the past 10 years.[9,15,22–35] In our previous studies,[26,36,37] a simple rolling-type TMT I.C. HSIAO, Postdoctoral Student, and J.C. HUANG, Professor and Chairman, are with the Institute of Materials Science and Engineering, National Sun Yat-Sen University, Taiwan 804, Republic of China. Contact e-mail: [email protected] Manuscript submitted September 18, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
was applied to process the low-priced commercial 5083 alloy, resulting in low-temperature superplasticity at around 230 ⬚C to 270 ⬚C and 2 ⫻ 10⫺4 to 1 ⫻ 10⫺2 s⫺1, with an optimum tensile elongation of 511 pct. The TMT processed thin sheet contained grains and subgrains measuring around 0.5 m. At temperatures lower than 270 ⬚C, the grains grew limitedly and maintained LTSP, with failure by cavitation coalescence plus partial necking. The flow stress of the LTSP specimens at 250 ⬚C dropped to nearly one-half as compared with the as-received non-LTSP samples, and the strain rate sensitivity m increased from 0.15 for the as-received specimens to around 0.3 to 0.35 of the LTSP ones. The grainstructure, texture, and grain-orientation evolutions as a function of strain, strain rate, and temperatu
