Alloy design and process optimisation of gamma-TiAl-based alloys for improved mechanical properties

Abstract

In the current research, the influence of powder characteristics blending, ingot melting parameters and the effects of Si, Mn, and Sn additions on the microstructures of Ti-48Al-2Nb-0.7Cr-0.3Si, Ti-48Al-2Nb-0.7Cr-0.3Mn and Ti-48Al-2Nb-0.7Cr-0.3Si-1Sn (at.%) alloys were investigated. The aim was to improve the microstructures and hence improve the mechanical properties for efficient hot deformation simulation and optimum hot-pack rolling without the use of the intermediate thermo-mechanical processes. The alloys were successfully fabricated from blended and uniaxial cold-pressed precursor powders and subsequently arc re-melted under vacuum and heat treated prior to the hardness and tensile tests at room temperature. The alloy with the best mechanical properties was further tensile tested at 750 °C and fatigue tested at room temperature. Its hot workability was tested in a Gleeble 1500DTM thermo-mechanical simulator by hot compression tests in the temperature and strain rate ranges of 1050-1200 °C and 0.001-0.1 s^(-1) respectively. Finally, the alloy was hot-pack rolled in a conventional two-high rolling mill by employing optimum parameters for hot-working of the alloy to produce a 4 mm -TiAl-based sheet. After the final rolling pass, the specimen was rapidly cooled in the air to room temperature and then heat treated in the α+ phase field. From the obtained results, irregular, spherical and angular-shaped powder particles of a mean size range of 8.12 to 101.24 m improved the density of the compacts. The melting of dense compacts using optimised arc voltage, current and vacuum level in the vacuum arc re-melting (VAR) process, improved the densities and homogeneity of the ingots. Unlike 0.3 at.%Si and 0.3 at.%Mn additions, 1 at.%Sn improved the castability of the alloy. The Si significantly improved the mechanical properties whereas Sn deteriorated the properties after heat treatment by hindering the nucleation of α_2 and Ti_5 Si_3 phases. 1050 to 1080 °C and 0.0087 to 0.071 s^(-1) were identified as optimum parameters for the hot-working of the Ti-48Al-2Nb-0.7Cr-0.3Si alloy. When hot-rolling was carried out within the optimum window with subsequent rapid cooling and heat treatment, a sheet with fine and equiaxed grains and with weak cube textured microstructure and improved mechanical properties was produced.