Influence of residual stress distribution and microstructural characteristics on fatigue failure mechanism in Ni-based Superalloy

Abstract

The influence of residual compressive stress (RCS) depth and magnitude generated through surface treatments such as shot peening (SP), deep cold rolling (DCR), and vibro-peening (VP) on fatigue crack mechanisms of Ni-based superalloy is investigated. The fatigue performance with associated failure mechanisms is measured under strain-controlled fatigue testing upto 104 cycles with total strain in the range of 0.9%–1.4% at an R ratio of 0.1 and 400°C followed by load controlled fatigue until failure. In-depth understanding of the failure mechanism is obtained through fractography analysis, cyclic stress–strain plot, and microstructural features. A pronounced improvement in fatigue life tested at low strain range (0.9%–1.1%) is achieved after inducing RCS up to 400 μm depth. However, the fatigue life is reduced when RCS increased to 800–1000 μm depth. Failure is primarily driven by micro-cracks formed due to balancing tensile stresses and high intensity stress concentration generated as the result of dislocation pile-ups and slip bands. Results are discussed in detail through the evidence of grain refinement, addition of low angle grain boundaries (LAGBs), strain accumulation, and intragranular deformation in the sub-surface.