Enhance fatigue resistance of NiTi by grain size gradient (INVITED)

Abstract eng:
Recent research shows that NiTi with GS=10nm can significantly suppress the crack nucleation and enhance the fatigue life but at the expenses of reduced ductility and crack growth resistance. In this paper, surface mechanical attrition treatment (SMAT) is used to create a nanocrystallined Grain-Size-Gradient (GSG) surface layer with smooth GS distribution to enhance fatigue life of NiTi. The key idea is to suppress the surface crack nucleation and growth by the Hall-Petch strengthening and the high residual compressive stress, both due to the severe plastic deformation of the GSG layer. Fatigue experiment under cyclic tension is performed and the intrinsic and extrinsic fatigue and toughening mechanisms and their GSG dependence is discussed. The importance of hybridizing the size dependent material properties and developing the next generation fatigue resistant NiTi alloy by grain size engineering is demonstrated.

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