Fatigue crack growth mechanism of Ni-based weld metal in a 9% Ni steel joint

This research focused on the impact of temperature and driving force ΔK on the fatigue crack growth (FCG) mechanism of C-276 Ni-based weld metal in a 9% Ni steel joint. On one hand, the FCG resistance increased due to higher yield strength at 77 K than at room temperature (RT), and the decrease of FCG rate can be well explained by Weertman's model. On the other hand, critical ΔK values (37.5 MPa m^1/2 at 77 K and 30.2 MPa m^1/2 at RT) existed in the process of FCG, accompanied by the transition of FCG mechanism at both 77 K and RT. In detail, the fatigue crack mainly propagated in a quasi-cleavage mode at a lower ΔK level at 77 K, in a single-slip mode at a higher ΔK level at 77 K or a lower ΔK level at RT, and in a multi-slip mode at a higher ΔK level at RT. Furthermore, the dislocation-precipitate interaction was enhanced and even provided favourable locations for FCG at a higher ΔK level at RT, leading crack to propagate via linking after the formation of micro-cracks and micro-voids near precipitates.