Stress-assisted transformation in Ti-60 wt pct Ta alloys

An investigation of the influence of processing variables on mechanical properties and phase development for a Ti-60 wt pct Ta (Ti-28.5 at. pct Ta) alloy was conducted. The alloy was hot-rolled, subjected to heat-treatment temperatures above the β (bcc) transus (1 hour at 700 °C, 800 °C, or 900 °C), and water quenched. All heat treatment produced a combination of metastable β (bcc) and metastable α″ (orthorhombic martensite), with the amount of retained β essentially independent of heat treatment, ranging from 20 to 33 vol pct. Deformation of as-rolled and heat-treated tension specimens showed an anomalous leveling of the stress-strain curve in the stress-strain curves at low strains. X-ray diffraction (both simple 2ϑ diffractometry and texture analysis) on both deformed and undeformed material determined that the leveling of the stress-strain curve was a result of the β→α″ martensitic transformation. The stress required to initiate the transformation increased with prequench temperature. This was determined to be due to the presence of athermal ω. Grain growth kinetics have been determined in the course of this work.