Heat Treatment To Refine The Microstructure Of Cast Tial-based Alloy

Microstructure refinement of a cast TiAl based alloy with a nominal composition of Ti-48Al-2Cr-0.5Mo (mol%) has been investigated by means of the optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In this study, the hot water was used to substitute for other quenching media and the simple process of (quenching +tempering) was determined by the detailed study of such process parameters as the quenching and tempering temperatures, tempering times and cooling technique during solution treatment. Then the best process route was determined to be 1400Cx5min,50CWQ - 1250Cx 3h,AC - 1320C X2min FC 1250C,AC, by which fine and homogeneous duplex and lamellar microstructures with the grain size of about 30 u m can be obtained from the as-cast TiAl based alloy with the initial colony of about 1000u m. Combining with the experimental results, the author brought forward an evolution model of microstructure refinement.It was found that tempering the quenching microstructures in the two-phase field was a good method to obtain the fine and homogeneous microstructures. The quenching is absolutely necessary to destroy the coarse microstructure of cast alloys firstly. Increasing the quenched temperature is beneficial to the massive transformation while decreasing the temperature is against the fine and homogeneous duplex or fully lamellar structures, because at a given cooling rate, the higher quenching temperature provides larger drive force for massive transformation, making it transform almost completely. At the tempering stage, the tempering time and temperature are also important factors for microstructure formation to control the rate of atomic diffusion and microstructure growth. They need control strictly. At the stage of solution treatment, the (FC+AC) two-step cooling technique is an effective method for homogeneous lamellar structures, but it is difficult to obtain fine and homogeneous lamellar if the microstructures before solution treatment are coarse or inhomogeneous. The reason for fine and homogeneous microstructures obtained by the (quenching +tempering) process with the 50C hot water as the quenching liquid, is that the fine lamellar in the massive microstructures can transform to gamma grainsduring tempering, which contain as rich microcosmic defects such as dislocations, stacking faults, APBs as massive microstructures. These microcosmic defects offer a lot of nucleating sites to form the fine and homogeneous structures.