Fabrication of 2 GPa multiphase core-shell nanocrystalline titanium alloys using martensite decomposition under thermomechanical coupling conditions

Grain refinement is a well-known technique that can be used to improve the strength of titanium alloys. However, when the grain size is refined to below 100 nm, even during the fabrication process, nanocrystalline titanium alloys lose their excellent mechanical properties as grain coarsening invariably occurs. In this work, we describe the fabrication of triple, quadruple, and quintuple phase core-shell nanocrystalline titanium alloys using martensite decomposition under thermomechanical coupling conditions. Experimental results indicate that increasing the number of phases present in the core-shell microstructure is beneficial for grain refinement. This is due to the pinning properties of the multiphase core-shell microstructure, which lead to increased microstructural stability of the nanocrystalline titanium alloys. The α grain size in the quintuple phase core-shell Ti6Al4V5Cu3Ni2Co0.7Si alloy is 80 ± 50 nm with an ultrahigh strength of 2 GPa. The results of the present study provide insight into the development of future ultrahigh strength titanium alloys.