Determination of Phase Equilibrium in the Ti-Rich Ti–Mo–Al Ternary System Using the X-Ray Microanalyser

Abstract

Isothermal sections from 700 to 1000°C of the Ti-rich corner of the Ti–Mo–Al ternary phase diagram were determined by analysing quantitatively the α and β phases within the two-phase field using the electron-probe X-ray microanalyser. High accuracy can be achieved with a small number of specimens and treatments, and the method is suggested as a standard procedure for ternary phase-diagram determination. The solubility of molybdenum in the α phase is 0·6–0·75 wt.- % at 700–800°C, and in α₂ ∼ 1·2 wt.-%Mo. Tie lines in the α + β field were determined, their average inclination being ∼−0·07 wt.-%Al/wt.-%Mo. This is also the average slope of the α + β/α+α₂ + β phase boundary, whose location was determined from the abrupt rotation of tie lines, without the limitatioll that arises in other methods. by the need to resolve the α₂ phase. The boundary appears at a low aluminium content, in agreement with recent reports on the evidence of ordering in Ti-AI alloys with 6 wt.-%Al or less. A method for checking whether specimens are close enough to equilibrium is suggested and utilized. It is based upon step-down and step-up heat treatments, which result in approach to the equilibrium concentration from two opposite directions.