Gradient hardening of Ni-based superalloy K403 for enhanced thermal fatigue resistance

Polycrystalline nickel-based superalloys are widely used in the aerospace industry. However, their prolonged exposure to extreme high and low temperature service environments frequently results in thermal fatigue cracks, adversely affecting their lifespan. Herein, a gradient reinforced layer is successfully fabricated on the nickel-based high-temperature alloy K403 using ultrasonic shot peening (USP). This treatment notably enhanced the surface hardness from 374 HV to 544 HV and increased the residual compressive stress from −117 MPa to −543 MPa. Thermal fatigue tests show that this reinforced layer effectively hinder surface oxidation and microstructural degradation, extending the initiation of thermal fatigue cracks from 50 to 100 cycles. Experimental observations and detailed microstructural evolution analysis reveal that USP creates a gradient structure of closely intermingled γ and γ′ phases on the surface, along with residual stress, which inhibits γ′ phase degradation, carbide precipitation, and diffusion during thermal fatigue cycles. These findings underscore the potential of USP as a surface treatment technique to markedly improve the thermal fatigue performance of high-temperature alloys.