Siavash Zamani - Department of Materials and Metallurgical Engineering, Iran University of Science and Technology, Tehran, Iran
Hekmat Razavizadeh - Department of Materials and Metallurgical Engineering, Iran University of Science and Technology, Tehran, Iran
Mohammad Jahazi - Department of Mechanical Engineering, Ecole de Technologie Superieure, Montreal, Canada

In the present study, the homogenizing and post heat-treatment processes were employed to refine the as-cast microstructure of a TiAl alloy. In order to this reason, microstructure evolution, phase consistence and hardness of the alloy were characterized and studied using optical and scanning electron microscopies and Vickers micro-hardness measurements. The nominal composition of the cast alloy was Ti-48Al-2Cr at.% which produced using the vacuum arc re-melting (VAR) process. Initial microstructure of the cast ingot was a large size columnar structure and different post heat-treatment routs were designed so that the final structure was a fine duplex (DP) microstructure. Homogenizing process was performed at 1100°C for 40 hours in the controlled atmosphere. Also, in order to find the optimized heat-treatment cycle, the effects of variations of heating rate in range of 10°C/min to 10°C/sec, solution temperature in range of 1200 to 1400°C, solution time in range of 20 to 60 min and cooling rate in range of 20°C/min to water quench condition, on the characteristics of the microstructure evolution and different phases present in the structure were studied. Analysis of the results indicated that, a combination of heating and cooling rates increasing and selection of optimum solution temperature leads to achieve a fine duplex microstructure. In the best developed condition, the size of the duplex microstructure was 25 μm for the singles phase gamma grains and 42 μm for the lamellar colonies with hardness value of 430 HV. To achieve such microstructure and hardness level, a two-step cooling process was employed in the present investigation

TiAl, heat treatment, microstructure, refining, hardness