History and development of γ-TiAl alloys and the effect of alloying elements on their phase transformations

γ-TiAl alloys, as the high temperature and lightweight structural material, have been applied in aerospace and automotive industry, and will remain the focus of research worldwide. After more than 40 years of development, γ-TiAl alloys not only completed the entire process from theoretical research to industrial application, but also accumulated a large amount of data. In this review, a new perspective, Al-equivalent, is selected to summarize the development trend of γ-TiAl alloys, and then the intrinsic nature of this trend is discussed in detail from the aspects of two fundamental phase transformations and solidification microstructures. Furthermore, the role of common alloying elements in γ-TiAl alloys has also been summarized from the perspective of phase transformations. There are two main purposes of this work, one is to introduce Al-equivalent based on the summary of the development process of TiAl alloys in order to reveal the inherent laws of alloy development; the other is to combine phase diagrams to investigate the effects of alloying elements on the phase transformation of TiAl alloys.