Formation mechanism and mechanical behavior of gradient nanograin structure in directional solidified Ti₃Al alloy: atomic-scale study

The formation mechanism of Ti₃Al alloy during a directional solidification process was systemically investigated by means of molecular dynamics (MD) simulations, and its mechanical behavior was explored by comparing with its nanograined (NG), coarse-grained (CG) and gradient nanograined (GNG) counterparts. It is found that the solidified front forms equiaxed crystals first, then they transform into columnar crystals, and the GNG structure is formed finally. Noticeably, the grains will grow preferentially in the direction parallel to the solidification direction. Besides, it is also found that the directional solidified alloy with the GNG structure has higher tensile strength and better ductility than its NG and CG counterparts. The GNG structure not only suppresses strain localization and grain growth in its small grain regions, but also promotes more cross dislocations in the large grain regions, resulting in a better mechanical performance.