Evolution of texture and grain misorientation in an Al-Mg alloy exhibiting low-temperature superplasticity
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I. INTRODUCTION
SUPERPLASTIC forming of commercial low-priced aluminum sheets could be developed into one of the important future fabrication means for the automobile, train, architecture, or electric appliance industries. There are at least three main factors that need to be taken into account: (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 1022 s21, 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 or Al-Mg-Si 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, etc. There have been numerous efforts in processing aluminum materials to exhibit high-rate superplasticity (HRSP) and/or low temperature superplasticity (LTSP) by using thermomechanical treatments (TMTs), equal-channel angular (ECA) extrusion, multiple forging, cyclic extrusion, torsion under compression, or accumulative roll bonding[3–14] on commercial or experimental alloys. In some cases, modified alloy compositions with an addition of a small amount of Fe, Mn, Zr, Cr, or Sc[9,13–21] have resulted in either superior or detrimental effects. In our previous studies,[22,23] a simple rolling-type TMT was applied to process the low-priced 5083 alloy, resulting in LTSP at ,250 8C and 1 3 1023 s21, with an optimum tensile elongation to 400 pct. The TMT-processed thin sheet I.C. HSIAO and S.W. SU, Graduate Students, and J.C. HUANG, Professor, are with the Institute of Materials Science and Engineering, National Sun Yat-Sen University, Taiwan 80424, Republic of China. Manuscript submitted October 26, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
contained subgrains measuring 0.3 to 0.5 mm. At temperatures lower than 300 8C, the grains grew limitedly and the alloy maintained LTSP. The flow stress of the LTSP specimens dropped to nearly one half as compared with the asreceived (AR) non-LTSP samples, and the strain-rate sensitivity increased from ,0.17 in the AR specimens to above 0.4 in the LTSP ones at 250 8C and 1 3 1023 s21. Recently, the evolutions of microtexture and grain misorientation in superplastic or nonsuperplastic aluminum or copper alloys have been examined by X-ray, transmission electron microscopy (TEM), or electron backscattered diffraction (EBSD) associated with scanning electron microscopy.[24–31] Numerous interesting observations on the grainorientation evolution as a function of deformation strain level at elevated temperatures have provided important evidence for acting deformation mechanism (e.g., References 24, 25, 27, and 29). In this report, the evolution of texture and grain misorientation during TMT proce
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