Effects of texture on the functional and structural fatigue of a NiTi shape memory alloy

The effects of crystallographic texture on the fatigue performance of superelastic NiTi sheet were investigated to assess the evolution of plastic deformation and transformation stress (functional fatigue) and the resistance to crack growth (structural fatigue). During uniaxial cycling, about twice as much plasticity accumulated for tensile specimens aligned in the rolling direction (RD) of the NiTi sheet compared to those in the transverse direction (TD). Fatigue experiments on compact tension specimens also showed a 25% lower fatigue threshold and twice the fatigue crack growth rate when loaded in the RD than in the TD. These macroscopic findings correlated with the microscopic observations of crack-tip displacements, where the crack tip opened at a lower stress intensity (K approximate to 0.9 MPa root m ) when loaded in the RD than in the TD (approximate to 1.5 MPa root m). The dissimilar crack growth rates highlight the strong orientation-dependent mechanisms of plasticity, transformation, and twinning on both functional and structural fatigue. Although the RD had a larger transformation strain that might suggest an enhanced deformation accommodation, under stress-controlled cycling the TD was more resistant to plastic slip, leading to more reversible transformation. Assuming other microstructural aspects are held fixed (composition, grain size, and precipitates), these results suggest that tension along the textures akin to that of the TD (i.e., with a higher density of [100] and lower density of [111] and [110]) is beneficial to fatigue performance during stress-limited cycling. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study revealed that crystallographic texture significantly affects the fatigue performance of NiTi sheet, with strong orientation-dependent mechanisms of plasticity, transformation, and twinning on both functional and structural fatigue. Under stress-controlled cycling, tension along textures similar to that of the TD demonstrated better fatigue performance.