STRESS-ASSISTED TRANSFORMATION IN TI-60 WT PCT TA ALLOYS

An investigation of the influence of processing variables on mechanica l properties and phase development for a Ti-60 wt pet Ta (Ti-28.5 at. pet Ta) alloy was conducted. The alloy was hot-rolled, subjected to he at-treatment temperatures above the beta (bcc) transus (I hour at 700 degrees C, 800 degrees C, or 900 degrees C), and water quenched. All h eat treatments produced a combination of metastable beta (bcc) and met astable alpha '' (orthorhombic martensite), with the amount of retaine d beta essentially independent of heat treatment, ranging from 20 to 3 3 vol pet. 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 2 theta diffractometry and texture analysis) on both deformed and undeformed m aterial determined that the leveling of the stress-strain curve was a result of the beta --> alpha '' martensitic transformation. The stress required to initiate the transformation increased with pre-quench tem perature. This was determined to be due to the presence of athermal om ega. Grain growth kinetics have been determined in the course of this work.