Effect of shot peening and nitrogen ion implantation on the fatigue behavior of TA15 titanium alloys

•Ion implantation on the surface of shot peening at medium temperature.•The improvement of thermal stability of shot peening strengthening layer under 500 °C.•The median fatigue life of titanium alloy TA15 increased by 17 times at 500 °C. The titanium alloy TA15 was treated with SP (shot peening), followed by heat exposure at 500 °C and NII (nitrogen ion implantation) treatment at 500 °C for 5 h each. The surface topography, microstructure, micro-hardness, residual stress, and fatigue behavior were, thereafter, analyzed. The results showed that the SP and NII treatments changed the surface topography of the TA15 titanium alloy, producing a high density dislocation and a high amplitude residual compressive stress on its TA15 titanium surface. In addition, the local stress concentration on the surface was reduced, pushing the fatigue nucleation site below the surface. Also, the difficulty of crack initiation and propagation was increased. At 450 MPa, the fatigue life of the SP treated sample was increased by 23.5 times. A large amount of TiN was generated in the grains of the NII treated sample, and compared with a sample without any NII treatments, the median fatigue life was increased by 17 times. The results showed that SP can effectively improve the fatigue properties of a TA15 titanium alloy, and the NII treatment is an effective way to improve the thermal stability of the SP strengthening effect.