Enhance Fatigue Resistance of Nanocrystalline NiTi by Laser Shock Peening

A pre-strain laser shock peening method is proposed to fabricate fatigue-resistant nanocrystalline NiTi with graded nanolayers and compressive residual stress layers. Grain size gradient surface layer of about 100 μm thick is fabricated on a 1.5-mm-thick bulk nanocrystalline NiTi plate. It is found that the nanostructure shows a gradient distribution from middle region of the plate to its laser-treated surface, and the grain size at the treated surface is about 5 nm. B2, B19′ phase and Ni₄Ti₃, Ni₃Ti precipitates are found at the treated surface. The nanohardness at the laser-treated surface reaches 10 GPa. Residual stress profile on the laser-treated plate cross-section is measured by a focused ion beam-digital image correlation technique. The measured maximum residual compressive stress is about 1.2 GPa at the laser-treated top surface, while there is residual tensile stress of about 200 MPa in the middle region. Four-point bending displacement-controlled experiments show that the fatigue life of the NiTi sample increases about seven times after laser treatment. The work demonstrates that pre-strain laser shock peening without surface coating is an effective method to fabricate fatigue-resistant nanocrystalline NiTi with gradient grain size and compressive residual stress layers.