EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Reducing the emissions of CO2 and NOx in aviation and automotive industry requires the usage of innovative high temperature materials. An important class of materials meeting the specific requirements are intermetallic γ-TiAl based alloys. Their properties include a low density compared to other high temperature materials, a high specific strength and a good creep resistance up to 750 °C. A promising alloying element for a further improvement of the creep behaviour of γ-TiAl alloys is Si. In this study the effect of Si on the microstructure evolution and phase transformation behaviour is adressed in four TNM-based alloys, nominal composition Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), with different Si contents up to 0.75 at.%. A Si-rich eutectic phase was found in the as-cast microstructure of alloys with a Si content higher than 0.5 at.%. A heat treatment simulating the hot isostatic pressing process resulted in a microstructure consisting of α₂/γ colonies surrounded by β_o phase and globular γ phase. Alloying with Si yielded an increased amount of γ phase at the expense of α₂ phase, as well as the formation of silicides within the microstructure. These silicides were found to have a significant impact on the grain coarsening of the α phase during heat treatments at high temperatures. Due to the pinning of grain boundaries, the silicides are capable of suppressing grain growth of the α grains in the alloys with a sufficient number of silicides. In situ high energy X-ray diffraction heating experiments as well as differential scanning calorimetry measurements were conducted to study the effect of Si on the phase transition temperatures. Additionally, the amount of the occurring phases and their lattice parameters in dependence of temperature and Si content were determined from the in situ heating experiments.