Impact of block-loading on the high cycle fatigue strength of superalloy 617M at 973 K

The study demonstrates the positive influence of coaxing on the fatigue strength of alloy 617M under high cycle fatigue (HCF) loading at stress ratio (R) - 1, frequency approximate to 85 Hz, and temperature of 973 K. Tests were performed employing resonance excitation in two patterns viz., constant amplitude loading and block loading using a 'lowto-high' pattern. The fatigue limit for the constant amplitude loading pattern was determined to be 320 MPa for a runout criterion of 108 cycles. Upon under-stressing the alloy by application of a block loading pattern, the limit increased to 335 MPa. The increase of stiffness and associated resonance frequency during cycling indicated an initial hardening regime brought about by work hardening followed by a period of secondary hardening owing to precipitations occurring in the matrix. In the later stage, the frequency response also indicated the influence of dynamic strain ageing (DSA) on the alloy behaviour. Comparative microstructural characterisation of specimens is conducted to extend the understanding of coaxing and the resulting changes occurring in the work-hardening behaviour of the alloy.