Investigation of microhardness evolution in an ultrafine grained NiTi alloy formed via high speed high pressure torsion (HSHPT)

¹ Faculty of Engineering ”Dunarea de jos” University of Galati, Domneasca Street, No. 111, 800201, Galati, Romania
² Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai - 600 036, India

^a Corresponding author: carmela.gurau@ugal.ro

Abstract

An investigation was conducted so as to evaluate the homogeneity of microstructure and mechanical properties of an NiTi shape memory alloy subjected to high speed high pressure torsion (HSHPT). The alloy chosen for the study had 50.3 at.% of Ni in its composition and was supplied in the form of hot extruded rods of 9 mm diameter. The final shape achieved after severe plastic deformation was a disc, whose diameter and thickness measured 40 mm and 0.18 mm, respectively. The TEM studies and the microhardness determination using a Vickers hardness tester revealed the effect of dynamic recrystallisation, which is very unique to severe plastic deformation involving HSHPT. An analysis of the microhardness data shows that the disc-shaped element is essentially homogeneous. In addition, the microhardness increases with the increase in accumulated strain. The results are presented and discussed in detail in the paper..