Low Temperature Physics: 28, 864 (2002); https://doi.org/10.1063/1.1528580 (11 pages)
Fizika Nizkikh Temperatur: Volume 28, Number 11 (November 2002), p. 1211-1226    ( to contents , go back )

Low temperature deformation and failure of bulk nanostructured titanium processed by severe plastic deformation through equal channel angular pressing

V. Z. Bengus, E. D. Tabachnikova, V. D. Natsik

B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukrain
E-mail: bengus@ilt.kharkov.ua

J. Miskuf, K. Csach

Institute experimental physics Academy of Sciences Vatsonova st. 47 Koshiche, 04353, Slovakia

V. V. Stolyarov, and R. Z. Valiev

Institute of physics news materials, Ufimskii state aviation technical university, K. Marksa st., 12, Ufa, 450000

Received June 21, 2002

Abstract

Low temperature plasticity and failure of polycrystals of coarse-grained (CG) and nanostructured (NS) titanium of commercial purity are studied. The NS titanium was prepared by equal channel angular pressing (ECAP) and subsequent thermomechanical treatment. Two structural modifications were prepared which had 0,3 and 0,1 mm grains. The measurements were made at 300, 77 and 4,2 K under uniaxial compression at the strain rate of 4×10-4 s-1. The work-hardening "stressstrain" curves were obtained; the macroscopic yield stress and the plasticity resource were measured for the samples compressed along and across the ECAP axis. The yield stress increased 1.5-2 times on changing from CG to NS titanium and on cooling from 300 to 4.2 K. Plasticity anisotropy was also observed in NS titanium when the compression axis orientation was changed from parallel to perpendicular - the yield stress increased 1.2-1.5 times. These changes in the sample structure and the experimental conditions reduced the plasticity resource systematically but the pre-failure strain was still above 4%. No cold brittleness was observed in NS titanium down to liquid helium temperature, though at 4.2 K the plastic flow became jump-like, as in CG titanium. It is found that under low temperature uniaxial compression, the failure of NS titanium is caused by unstable plastic shear leading to a local adiabatic heating-up of the material. This is not characteristic of CG titanium. Scanning electron microscopy of the shear failure surface morphology detected the characteristic "vein" pattern indicative of local heating to 800°C and over. The analysis of the experimental results makes it possible to suggest that at low temperatures the plastic deformation in NS titanium is a thermally activated process, the yield stress is influenced significantly by the microstructural internal stresses induced by thermal anisotropy and by probable microtwinning.

PACS:
62.20.Fe - Deformation and plasticity (including yield, ductility, and superplasticity) (see also 83.50.-v Deformation and flow in rheology)
62.20.Mk - Fatigue, brittleness, fracture, and cracks
81.05.Ys -
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