Grain size dependence of Young's modulus and hardness for nanocrystalline NiTi shape memory alloy

In this paper, grain size (GS) dependence of Young's modulus and hardness for nanocrystalline NiTi shape memory alloy is investigated by experiments. Amorphous NiTi with nanocrystalline debris is fabricated via cold-rolling and polycrystalline NiTi with average GS from 10 nm to 120 nm is obtained by subsequent annealing. Young's modulus and hardness of nanocrystalline NiTi are quantified by macroscopic isothermal tension and microscopic nanoindentation. It is shown that Young's modulus of nanocrystalline NiTi first decreases (for GS < 62 nm) and then increases (for GS > 62 nm) with GS in the nano-scale region. The non-monotonic GS dependence of Young's modulus originates from the combined effects of grain size and volume fractions of austenite, martensite and amorphous phase in the material. It is also shown that with the increase of GS up to 120 nm, hardness of nanocrystalline NiTi monotonically decreases due to the reduced nominal phase transition stress and plastic yielding stress. Such GS dependence of hardness can be utilized for rapid determination of GS in nanocrystalline NiTi via nanoindentation hardness test.