The effect of Cr and Zr on the structure and phase composition of TNM gamma titanium aluminide alloy

Quantitative analysis of the zirconium and chromium cooperative effect on the structure and phase composition, after solidification and heat treatment, of the TNM type alloy, was performed using both experimental techniques and thermodynamic calculations. It is shown that the joint alloying by chromium and zirconium up to 1 at.% does not change the sequence of phase transformations during solidification and subsequent cooling of the TNM alloy. Rather, it significantly decreases the alpha(2)-phase amount and increases beta-phase amount. Increasing the amount of beta-phase is associated with a high beta-stabilizing effect of chromium, whose partition coefficients k(beta/gamma) and k(beta/alpha) are significantly higher than 1 (ranging from 1.67 to 3.8). Zirconium is distributed quite uniformly with partition coefficients k(beta/gamma) and k(beta/alpha) close to 1. HIP treatment of the alloy with chromium and zirconium up to 1 at.% at 170 MPa and 1250 degrees C leads to a significant reduction of the alpha(gamma/alpha(2))-phase fraction, and increases the amount of gamma-globular in comparison to vacuum annealing at 1250 degrees C. The higher content of the softest gamma-globular decrease the hardness from 365 HV to 333 HV, increases the total elongation of the alloy from 0.5% to 0.8% at room temperature and substantially decreases the ultimate tensile strength from 750 MPa to 715 MPa. (C) 2017 Elsevier Ltd. All rights reserved.