Influence of dwell times on microstructure, deformation and damage behavior of NiCr22Co12Mo9 under thermomechanical fatigue

Thermomechanical fatigue (TMF) tests under in-phase (IP) and out-of-phase (OP) conditions with continuous and dwell time cycles were conducted on wrought Ni-base alloy NiCr22Col2Mo9 (comparable to Inconel Alloy 617). The temperature range was T = 100-850 degrees C. In dwell time tests, dwells of 5 min were introduced at T-max = 850 degrees C. In continuous cycle tests, the material shows significant cyclic hardening, mainly because finely dispersed carbides of type M23C6 form inside the grains. Introducing dwell times leads to coarsening of these carbides, which reduces cyclic hardening leading to lower stress amplitudes and higher plastic strain amplitudes than in continuous cycle tests. Serrated yielding indicating dynamic strain ageing (DSA) effects occur prominently in dwell time tests and less pronounced in continuous cycle tests. The DSA effects appear to be closely related to formation and growth of the M23C6 carbides. IP and OP loading lead to comparable microstructural evolution and deformation behavior. Damage in form of cracks arises under IP loading predominantly intergranularly and under OP loading mainly transgranularly. Dwell times do not affect the dominant damage form. For both continuous cycle and dwell time tests, IP lifetime is shorter than OP lifetime indicating that intergranular cracks propagate faster than transgranular cracks. Introducing dwells unexpectedly increases the lifetime under IP loading, while OP lifetime is only slightly affected. The lifetime behavior is discussed based on the observed microstructural and damage evolution.