This project aims to design Ti-rich high entropy alloys (HEA) with TRIP effect suitable for laser powder bed fusion (LPBF). A set of design criteria have been proposed comprising three aspects: (A1) HEAs formation based on mixing entropy (∆G), atomic-size mismatch (δ), valence electron concentration (VEC), Interatomic spacing mismatch (Sm) and bulk modulus mismatch (Km)); (A2) Transformaiton induced plasticity (TRIP) effect occurrence based on the Fe equivalent (Feeq) and the temperature at which the Gibbs free energy of parent and prodcut phase become equal (T0); and (A3) printability, comprising values for the solidification temperature range (STR), the variation of partition coefficient (Vk), and the performance index (PI). An optimisation scheme, combiing genetic algorithms and thermochemical caluclations, is constructed to screen out the composition with maximum solid solution hardening (SSH).

Ti-rich HEAs typically display a hard and brittle BCC structure. The beta parent phase metastability has been exploited to induce in TaHfZrTi HEA the TRIP effct, enhancing ductility and workhardenability [1]. In this work, TiNbTaZr system was studied. A1—A3 criteria are implemented. After several iterations of optimization by the genetic algorithm, some alloy components were screened out. Some samples were cast for mirco-indentation test. The EBSD results show that HCP phase generated at the edge of indentation, while the martix is BCC phase. The TRIP effect is successfully verified.