Enhance Fatigue Resistance of Nanocrystalline NiTi by Laser Shock Peening

A novel Pre-Strain Laser Shock Peening (LSP) method was proposed to fabricate grain size gradient nanocrystalline NiTi material with enhanced fatigue resistance. Grain size gradient surface layer of about 100 μm thick was fabricated on 1.5 mm thick bulk nanocrystalline NiTi plate. The grain size distribution of this layer was obtained by TEM and XRD. Residual stress profile of the LSPed plate was measured by a novel Focous Ion Beam-Digital Image Correlation (FIB-DIC) method. Tensile stress–strain curves, cyclic stress-time curves are obtained with MTS 858 universal testing machine with the temperature and strain fields synchronized by infrared thermography and digital image correlation. It is shown that, compared with the untreated plate the maximum surface average temperature decreased and there is no obvious strain and phase transition localization after LSP treatment. Displacement controlled bending fatigue test shows that the fatigue life is enhanced significantly with LSP treatment. The work demonstrates that Pre-Strain Laser Shock Peening without surface coating is an effective method to enhance fatigue resistance of nanocrystalline NiTi by surface grain refinement and residual compressive stress.