Low-temperature deformation and fracture of bulk nanostructural titanium obtained by intense plastic deformation using equal channel angular pressing

Abstract

The low-temperature plasticity and fracture of polycrystals of coarse-grained (CG) and nanostructural (NS) technical-grade titanium of two structural modifications with grain size 0.3 and 0.1 μm, which were prepared by equal channel angular pressing (ECAP) with additional thermomechanical treatment are studied. The measurements are performed at temperatures 300, 77, and 4.2 K with uniaxial compression at deformation rate 4×10−4 s−1. The “stress-plastic deformation” hardening curves are obtained, the macroscopic yield stress, and the ultimate plasticity are measured for samples with compression axis orientations parallel and transverse to the ECAP axis. It is found that the yield stress for NS titanium is 1.5–2 times higher than for CG titanium and the yield stress on cooling from 300 to 4.2 K. Plasticity anisotropy is also observed in NS titanium—the yield stress is 1.2–1.5 times greater when the compression axis is oriented perpendicular to the ESAP axis than for parallel orientation. The ultimate plasticity with such changes in the structure of samples and under the experimental conditions systematically decreases, but the deformation to fracture remains above 4%. Nanostructural titanium does not show cold-brittleness right down to liquid-helium temperatures, but at 4.2 K plastic flow becomes jumplike, just as in CG titanium. It is established that for low-temperature uniaxial compression NS titanium fractures as a result of unstable plastic shear accompanied by local adiabatic heating of the material. This phenomenon is not characteristic of CG titanium. A study of the morphology of the shear-fracture surfaces using a scanning electron microsope shows a characteristic “vein” pattern, attesting to local heating at temperatures ⩾800 °C. It is established that plastic deformation in NS titanium is thermally activated at low temperatures. It is shown that microstructural internal stresses due to thermal anisotropy and possible microtwinning affect the yield stress.