The compressive deformation behavior and damage characteristics of ultrafine-grained (UFG) pure titanium produced by equal channel angular pressing (ECAP) and coarse-grained (CG) pure titanium were investigated and compared at temperatures ranging from 77 K (À196 C) to 873 K (600 C). It was found that the compressive stress-strain response and deformation damage morphologies exhibit quite different features for the UFG-Ti and CG-Ti, strongly depending upon the testing temperature. Different from the case of CG-Ti, the stress-strain curve of UFG-Ti exhibits a distinctive stage of strain softening, as the temperature is below recrystallization temperature (e.g. 77 K (À196 C) $ 423 K (150 C)). The yield stress and maximum flow stress of UFG-Ti are contrarily lower than those of CG-Ti, as the temperature is above recrystallization temperature. As compared to CG-Ti, UFG-Ti seems to exhibit more superior high-temperature deformation ability and a comparable lowtemperature deformation capacity. The corresponding microstructural changes after compressive deformation of UFG-Ti and CG-Ti were also examined by TEM observations to interpret their differences in deformation micromechanisms.

Tensile deformation and fracture behavior of a commercially pure (CP) titanium were investigated at different temperatures through mechanical tests, microstructural observations and fractal analyses. It was found that, with increasing temperature, the number and size of microvoids formed along shear bands (SBs) or at the intersections of SBs on the deformed specimen surface increase, and the fractal dimensions of the scanning profile at the surface near fracture increase correspondingly, and the ones measured perpendicular to the tensile direction is obviously larger than those parallel to the tensile direction, indicating an increased concentration of plastic deformation of CP Ti along the tensile axis. The diameter and depth of dimples on the fracture surfaces of CP Ti increase significantly with increasing temperature, giving rise to a higher fracture surface roughness reflected by a higher fractal dimension. TEM observations demonstrated that the plastic deformation of CP Ti is gradually occupied by dislocation slipping rather than twinning with increasing temperature. This is in good agreement with the fractal analyses of the deformation and fracture features.