Temperature/stress dependence of stress rupture behavior and deformation microstructure of an advanced superalloy for additive manufacturing

Abstract

A self-developed crack-free advanced superalloy ZGH451 fabricated by direct energy deposition (DED) was applied to investigate the microstructure evolution, stress rupture behavior, and deformation mechanisms at moderate-high temperatures and high-low stress conditions. The high Ta/Al ratio induces large misfit lattice stress and low stacking fault energy of alloy, resulting in approximate cubic γ′ phases in dendrites and the formation of initial dislocation tangles. After the stress rupture test at 760°C/780 MPa, high content cubic γ′ phases, small size of voids as well as preserved dislocation tangles are observed, showing stable structures with high-stress rupture resistance. High content and suitable size of cubic γ′ phases, initial dislocation tangles, and L-C locks hinder the dislocation motion, which decreases the minimum strain rate and prolongs life significantly, forming four stress rupture stages. Hence, the deformation mechanism is determined by dislocation piled-up on γ/γ′ interface, formation of stacking faults in γ′ phases, and dislocations shearing γ′ phases. However, the microstructure exhibits uneven structures composed of large sizes of rafted γ′ phases and voids at 980°C/260 MPa. The rafted structure and high temperature provide continuous channels and enough energy for dislocation motion, resulting in the increase of minimum strain rate, decline of life, and typic three stress rupture stages, even though there are obstacles to dislocation movement caused by dislocation networks. The deformation mechanism transforms to form dislocation networks on γ/γ′ interface and dislocations shearing γ′ phases. Besides, the decomposition of carbides on GBs also depends on temperature, which decomposes into harmful chain-like M23C6 carbides at moderate temperatures and reinforced granular-shaped M6C carbides at high temperatures. The applied stress always decreases mechanical properties due to its degradation of microstructure induced by elongating the precipitates and defects.