Influence of hot-rolling on the microstructure and mechanical properties of a near β-type Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy

Abstract

In this work, the 90° clock rolling and the uni-directionally rolling processes at high temperature were carried out on the near β-type Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr titanium alloy cutting from an ingot, respectively. The corresponding microstructures were quantitatively characterized, and its effect on the dynamic mechanical properties and fracture mechanism were emphatically investigated. It was found that after 90° clock rolling, the microstructure composed of equiaxed primary α phase(αp) with an average size of about 2 ​μm and the β transformed regions containing the acicular secondary α phase(αs) with an average thickness of about 50 ​nm and the separated β phase was obtained. However, in the uni-directionally rolled titanium alloy, no acicular αs was observed, and the corresponding microstructure consisted elongated lamellar α phase (average thickness: about 1.3 ​μm), few equiaxed α phase (average grain size: about 300 ​nm) and the inlaid β phase. The microstructural difference of the hot-rolled titanium alloys was closely related to the deformation process. Moreover, a great number of αp and αs in the 90° clock rolled titanium alloy effectively enhanced the strength, and the dynamic compressive strength reached to 1730 ​MPa. Furthermore, equiaxed αp was conducive to the homogeneous deformation, which counteracted the localized deformation caused by acicular αs to a certain extent and made the 90° clock rolled titanium alloy exhibit an acceptable critical fracture strain of about 10.5%. Moreover, the fracture microstructures showed that the main failure mode of the 90° clock rolled and the uni-directionally rolled titanium alloy were ductile fracture and brittle fracture, respectively.