Study on the hot deformation behavior of TC4-DT alloy with equiaxed a+β starting structure based on processing map

摘要

Hot deformation behavior of TC4-DT alloy with an initial equiaxed a+β structure was investigated by conducting isothermal hot compression test in the deformation temperature range of 930-1020 ℃ and strain rate range of 0.001-10 s~(-1) with the maximum height reduction of 60%. The hot deformation behavior of TC4-DT alloy was characterized based on the analysis of the stress-strain behaviors, kinetics and processing map for obtaining optimum processing parameters and achieving desired microstructure during hot working. A constitutive equation by which the flow stress was expressed as a function of strain rate and deformation temperature was established and the apparent activation energy of deformation was calculated to be 644.642 kjmol~(-1) in a+β phase region and 592.054 kJ mol~(-1) in β phase region. A processing map was constructed at the true strain of 0.9 based on dynamic materials model (DMM) to delineate the safe and unsafe regions in the axes of temperature and strain rate. Two regions were marked as unsafe with one in a+β phase field and the other in p phase field and both were in strain rate higher than 1s~(-1). These instability domains exhibited localized plastic flow and cracking along grain boundaries which should be taken care of during hot processing. The stable domain covered through the whole temperatures under strain rate of 1 s~(-1) with peaks in power dissipation of 41% and 39% which indicated superplasticity (SP) and dynamic recrystallization (DRX). Microstructure observation of the deformed specimens validated the applicability of the processing map at obtaining the optimum processing parameters of TC4-DT alloy.