Effects Of Grain Size On Fatigue And Wear Behaviors Of NiTi Shape Memory Alloy

The effects of grain size (GS) on the tensile fatigue life and the wear behaviors of nanostructured NiTi superelastic shape memory alloys (SMAs) with GS = 10 nm, 42 nm and 80 nm are investigated. Results show that low-cycle fatigue life (under σmax = 450 MPa) of nanostructured NiTi polycrystalline SMA increases significantly when GS decreases from 80 nm to 10 nm. However, there is no significant effect of GS on the higher cycle fatigue life (under σmax = 300 MPa). During the test, we found that the accumulated acoustic energy can be used to distinguish the three stages of fatigue: slow crack propagation, fast crack propagation and final fracture. For the wear behaviors of the nanostructured NiTi superelastic SMA, although the results show that the hardness increases when the grain size decrease, the grain refinement may not bring any improvement in wear resistance. This could due to the phase transition is suppressed when the grain size decreases, which decreases the contact area during wear, and cause higher contact pressure during the wear process. As a result, the wear performance of the nanostructured NiTi superelastic SMA is the result of the competition between the hardness and the contact pressure. Results indicate that grain refinement down to nanoscale has potential in developing high fatigue resistance SMAs, but may not give high wear resistance SMAs.