Compression Deformation Behaviors Of CP-Ti Processed By ECAP At Room Temperature Using A90°Die

In this paper, ultrafine-grained (grain size is about170nm) commercially pure(CP)titanium (TA1, after4passes of ECAP via route C) and coarse-grained (grain size isabout26μm) CP-Ti were used as the experimental materials. Thermal compressionexperiments were done on a Gleeble-3500thermal simulation testing machine at strainrate ranging from10-5s-1to101s-1with deformation temperature from298K to723K.The effect of strain rate and deformation temperature on true stress-true strain curve, themicrostructure evolution of ultrafine-grained (UFG) CP-Ti and coarse-grained (CG)CP-Ti, hot deformation behaviors and the high temperature deformation softeningmechanism were investigated with metallographic observation and transmissionelectron microscopy (TEM) analysis. And also a preliminary study of strain ratesensitivity of the two materials was studied. The main results were as follows:(1)The flow stress of CG CP-Ti decreases with increasing deformation temperatureand decreasing strain rate. The flow stresses of true stress-true strain curvescontinuously increases with increasing strain. The reason is that, the softening effect ofdynamic recovery and recrystallization was not enough to offset the effect of workhardening during the compression process. But when the strain rate is10-4s-1and thedeformation temperature is723K, the flow stress of the curves present steady state.(2)The flow stress of UFG CP-Ti increases with increasing strain rate anddecreasing deformation temperature. The overall shape of the stress-strain curves is thatthe flow stress increases significantly with increasing strain at initial deformation. Thenthe flow stress keeps increasing but the increasing rate is much slower. At last, the flowstress basically remains a relatively stable value. This is due to a dynamic equilibrium between the work hardening and the dynamic recovery and recrystallization.(3)The strain rate sensitivity index m of the UFG CP-Ti is higher than that of CGCP-Ti in the same hot compression deformation conditions. The value m (0.015~0.125)of UFG CP-Ti distributes wider than that of CG CP-Ti (0.015~0.04) in the differents hotcompression deformation conditions.(4)The steady-state deformation stage of UFG CP-Ti in hot compression true stress-true strain curves relates to the softening of high-angle grain boundaries: One isdislocation annihilation at grain boundaries; The other is grain boundaries sliding anddiffusion.