Strain hardening of Titanium alloy Ti6Al4V sheets with prior heat treatment and cold working

Abstract

Strain hardening of Titanium alloy Ti6Al4V has been studied at four different strain rates (10−4 to 10−1s−1) under tensile loading at four different heat treatment conditions viz. mill annealing (MA), recrystallization annealing (RA), solution treatment (ST) and solution treatment and aging (STA). Effect of prior cold working before tensile testing has also been studied. Strain hardening exponent (n) is found to be decreasing with increasing strain rates for samples in all the four heat treatment conditions, whereas trend is found to be opposite in case, of cold worked (~15%) samples prior to tensile testing. This is due to rearrangement of dislocations generated during the prior cold working. Strength coefficient (K) is also found to follow similar trend.Optical microstructure clearly showed two phase (α+β) structure in all the samples and completely fragmented grain boundaries in cold worked condition. Non-basal oriented grains in as-annealed samples and higher volume fraction of basal (0001) grains in annealed and cold worked samples were observed along with fragmented grain boundaries/regions of void coalescence. Low angle misorientation was found to be lower in sample subjected to STA heat treatment as compared to other heat treated conditions. Twinning was observed in ST as well as in STA conditions, which was absent in annealed as well as annealed and cold worked samples.Image quality maps revealed regions of low image quality in deformed samples indicating presence of high dislocation density. Regions of void coalescence were confirmed by KAM (Kernel average misorientation) maps, GOS (grain orientation spread) maps and unique grain colour maps. Analysis of fractographs showed ductile mode of fracture in all heat treated conditions and mixed mode of fracture with fragmented grain boundaries in all samples subjected to heat treatment followed by tensile testing with prior cold working.