Abstract: The inelastic deformations of shape memory alloys (SMAs) always show poor controllability due to the avalanche-like martensite transformation, and the effective control for the deformation of precision devices has been not yet mature. In this work, the phase field method was used to investigate the shape memory effects (SMEs) of NiTi SMAs undergoing grain size (GS) engineering, to obtain tunable one-way and stress-assisted two-way SMEs (OWSME and SATWSME). The OWSME and SATWSME of the systems with various gradient-nanograin structures and bimodal grain structure, as well as that with geometric gradients were simulated. The simulated results indicate that due to the GS dependences of martensite transformation and reorientation, the occurrence and expansion of martensite reorientation, martensite transformation and its reverse can be efficaciously controlled via the GS engineering. When combining the GS engineering and geometric gradient design, since the effects of GS and stress gradient can be superimposed or competing, and the responses of martensite reorientation, martensite transformation and its reverse to this are different, the OWSME and SATWSME of the geometrically graded systems with various nanograin structures can exhibit different improvements in controllability. In short, the reorientation hardening modulus during OWSME is increased and the transformation temperature window during SATWSME is widened by GS engineering, indicating the improved controllability of SMEs. The optimal GS engineering schemes revealed in this work provide the basic reference and guidance for designing tunable SMEs and producing NiTi-based driving devices catering to desired functional performance in various engineering fields.

Key words: Phase field, NiTi, Shape memory effect, Grain size engineering, Geometric gradient