Mechanical twinning and microstructure refinement in metastable titanium alloys

Abstract

Metastable beta titanium alloys gain increasing interest due to their unique properties such as high strength, good hardenability, high fracture toughness and excellent corrosion resistance. In this study two different metastable beta titanium alloys, Ti‐15Mo and Ti‐6.8Mo‐4.5Fe‐1.5Al (TIMETAL LCB) were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT) technique. The main purpose of SPD methods is the insertion of large plastic deformation to the material without a change of the original shape of the material. The decreasing grain size and increasing the number of high‐angle grain boundaries results in changes of mechanical and other physical properties. The main aim of this work is to analyse the effect of deformation on the microstructure and lattice defects. The initial stages of microstructure refinement were examined by electron backscatter diffraction (EBSD). The inverse pole map figure of Ti‐15Mo alloy after N = ¼ HPT turns is depicted in the Figure 1. It was shown that initial stages of deformation include misorientation within grains (see changing ‘colour’ along black line) and formation of low‐angle sub‐grain boundaries (black arrows). Moreover, it was proven that multiple twinning occurs in the material (Figure 2) which notably contributes notably to the microstructure refinement. Further microstructure refinement by HPT observed by transmission electron microscopy (TEM) and automated crystal orientation mapping (ACOM‐TEM) will be also presented.