Controlling Gradient Microstructure and Mechanical Behavior by Surface Mechanical Attrition Treatment

A surface nano-crystallization process, surface mechanical attrition treatment (SMAT), is used to tailer the microstructure of alloys and achieve ideal properties. To evaluate the effect of strengthening and fatigue resistance enhancement, two widely used alloy systems, NiTi shape memory alloy (SMA) and CoCrNi medium-entropy alloy are chosen to investigate.

Many structures like medical stents made of superelastic NiTi SMA are subject to cyclic bending loads, where the material shows a limited fatigue life due to crack nucleation and growth in the surface layers under local tensile stress. In this thesis, the bending fatigue life of NiTi plates is enhanced by pre-strain warm surface mechanical attrition treatment (pw-SMAT) where the austenite phase is directly subject to severe plastic deformation and grain refinement without inducing phase transformation. Amorphous and grain size gradient (5-100 nm) microstructures as well as a maximum compressive residual stress of 1093 MPa are produced in the surface of the NiTi plates via the pw-SMAT. The compressive residual stress notably reduces the surface tensile stress from bending. The grain size gradient layers with improved hardness and reduced hysteresis have high fatigue crack nucleation resistance, whereas the middle large-grained layer has high fatigue crack growth resistance. The combined effects of the gradient nanostructure and the compressive residual stress substantially increase the bending fatigue life of the NiTi plates from an original 10³ cycles to over 1.3×10⁴ cycles. The results open up a new route to improve the bending fatigue life of NiTi plates by heterogenous nanostructures.

SMAT was used to produce a grain-size gradient layer on the surface of spark- plasma-sintered medium-entropy alloy CoCrNi. In comparison with the as-sintered fine-grained CoCrNi, the SMAT alloy shows a 46% increase in yield strength and over three times higher strain hardening rate. The ultimate tensile strength and the elongation reach 1420 MPa and 61.3%, respectively. The excellent strength-ductility synergy should be attributed to the Hall-Petch strengthening and the formation of high density of dislocations and nanotwins.

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