Effects Of Regional Heating Treatment On The Microstructure Evolution Of The TiAi Alloy

TiAl alloy has low density, high elastic modulus, good high temperature strength and good creep resistance and oxidation resistance. It has been the best candidate for the aircraft engine and car heat-resistant parts. But. because of the fragile nature of the TiAl alloy, it performers the low plasticity at room temperature, and thus it is hard to generate plastic deformation at room temperature, which is currently the main obstacle to expand practical application of the alloy. Studies have shown that the added of Nb element, not only can improve the melting point and the ordering temperature of alloys, but also can significantly improve high strength and oxidation resistance of TiAl alloy. Making organization orientation is an effective method to improve the properties of material. The material which the tensile stress direction is parallel to the grain boundary orientation has excellent creep resistance, high fatigue resistance and good ability to resist crack propagation. Therefore, this article studied the influence of withdraw rates, hotspot temperature and alloy element of Nb on the evolution of microstructure on the basis of Ti-45Al-8Nb alloy, and making clear the change regularity of layer orientation and lamellar spacing, and last analyzed the change regularity of hardness properties of organization after different directional annealing.The experimental results show that the withdraw rates have significant influence on the evolution of organization. If the withdraw rates were too fast or too slow, columnar crystals with a large aspect ratio can not be obtained. And with the increase of withdraw rates, the lamellar spacing decreases at the same temperature. And at the same temperature, with the increase of withdrawal rates, the proportion of y-phase shows a little change, and the α2-phase increases slightly. The hardness of alloy also increases.With the increase of temperature, the withdraw rates which were required to obtain columnar crystals with a large aspect ratio will increase. Under the condition of the same withdraw rates, the higher the temperature, the columnar crystal was easier to obtain. In addition, the temperature to growth the columnar crystal exist a critical value, which is the phase transformation point. When the temperature is below the critical value, no matter how-to change the withdraw rates, the organization of the columnar crystal cannot be obtained. And with the increase of temperature, the lamellar spacing decrease slightly.With the increase of the content of Nb alloy, the columnar crystals which are in the area of directional growth zone of stable growth stage become more and more long, and the grain boundaries of columnar crystals are also become more and more straight. At the same time, the addition of Nb alloy expands the area of β phase and reduces the transition point of Ta. With the increase of the content of Nb alloy, the lamellar spacing of alloy decreases gradually on the same heat treatment process.