Contrasting Effects of Laser Shock Peening on Austenite and Martensite Phase Distribution and Hardness of Nitinol

Laser shock peening of cold rolled Nitinol was carried out at high power density (7 and 9 GW/cm(2)) and high overlap ratio (90%). Tensile surface residual stresses were generated in the peened material. An enhancement in surface microhardness from 351 for unpeened material to 375 and 394 VHN for the 7 and 9 GW/cm(2) samples, respectively, was also observed. However, at a depth of 50 mu m, the hardness of the peened material was lower than that of the as-received material. These contrasting observations were attributed to the change in the austenitic phase fraction brought about by laser interactions.