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

In this work, the 90 degrees clock rolling and the uni-directionally rolling processes at high temperature were carried out on the near beta-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 degrees clock rolling, the microstructure composed of equiaxed primary alpha phase(alpha(p)) with an average size of about 2 mu m and the beta transformed regions containing the acicular secondary alpha phase(alpha(s)) with an average thickness of about 50 nm and the separated beta phase was obtained. However, in the uni-directionally rolled titanium alloy, no acicular alpha(s) was observed, and the corresponding microstructure consisted elongated lamellar alpha phase (average thickness: about 1.3 mu m), few equiaxed alpha phase (average grain size: about 300 nm) and the inlaid beta phase. The microstructural difference of the hot-rolled titanium alloys was closely related to the deformation process. Moreover, a great number of alpha(p) and alpha(s) in the 90 degrees clock rolled titanium alloy effectively enhanced the strength, and the dynamic compressive strength reached to 1730 MPa. Furthermore, equiaxed alpha(p) was conducive to the homogeneous deformation, which counteracted the localized deformation caused by acicular alpha(s) to a certain extent and made the 90 degrees 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 degrees clock rolled and the uni-directionally rolled titanium alloy were ductile fracture and brittle fracture, respectively.