Chemical heterogeneities in tungsten containing TiAl alloys processed by powder metallurgy

International audienceDuring the last decade, it has been shown that the W-containing IRIS-TiAl alloy displays promising properties for structural applications at high temperatures. The manufacturing process of this alloy is divided into three successive steps: electrode production, powder atomisation and spark plasma sintering (SPS) densification. However, an IRIS alloy densified by using pre-alloyed powders atomized by EIGA process (Electrode Induction melting Gas Atomisation) has recently been found to exhibit chemical heterogeneities. The aim of the present work is to look for the origin of such heterogeneities all along this manufacturing process. The microstructures and the chemical compositions of the material obtained after these different steps are thus investigated at intermediate and local scales by using various experimental tools. A particular attention is paid to the distribution of tungsten atoms in correlation to the constituent phases. Effects of these heterogeneities on mechanical properties are measured by performing tensile tests at room and high temperatures. It will be demonstrated that these heterogeneities are issued from tungsten segregation occurring during the first stage of the initial solidification of the electrode, thus prior to atomisation

The effect of zirconium on the Ti-(42-46 at.%)Al system

In recent years, Zr has emerged as a promising alloying element for intermetallic γ-TiAl based alloys to improve their mechanical properties. The present work focuses on the influence of this element on the microstructure and the thermodynamic phase equilibria in the ternary Ti-(42-46)Al-(2-4)Zr (at.%) system. Alloying with Zr was found to increase the amount of the γphase in the microstructure of cast material densified by hot-isostatic pressing. Simultaneously, the material’s hardness increased due to solid solu- tion strengthening as well as the refinement of lamellae in the α2 / γcolonies. With respect to the phase transformation behaviour, a significant decrease of the solidus temperature was observed in the high Zr alloyed material variants. In combination with the stabilization of the γphase, this essentially results in a narrowing of the single αphase field region in the Ti-Al-Zr phase diagram derived in this work. In situ high-energy X-ray diffraction was performed on Ti-46Al-2Zr and Ti-46Al-4Zr (at.%) specimens to investigate the phase transitions above and below the solidus temperature by utilizing two different ex- perimental setups. These experiments showed that upon heating, small amounts of βphase are formed in both alloys prior to the transition into the peritectic α+ β+ L phase field region. Furthermore, an ad- ditional heat treatment study was conducted to determine the influence of Zr and temperature on the resulting microstructure. The combination of X-ray diffraction techniques with ab-initio calculations re- vealed a significant asymmetric influence of Zr on the lattice parameter of the γphase, resulting in a decreasing c/a ratio