On the role of failure analysis investigations in alloy development illustrated with case study involving Ni-Mo alloy

During thermo-mechanical processing of a Ni-Mo alloy, three hot-rolled slabs at 1250 °C have been stacked on top of each other during air-cooling. Upon reaching ambient temperature, the middle slab is found to develop a mid-length transverse crack running through the entire thickness. However, no cracks have been detected in the other slabs. It is shown that the middle slab has been embrittled by ordering to Ni₄Mo due to comparative slower cooling rate. An investigation of the ordering behavior of stoichiometric Ni₄Mo alloy has shown that ordering occurs homogeneously during the earlier stages of thermal aging at elevated temperatures which results in nanosized structure of ordered Ni₄Mo domains capable of deforming by twinning and characterized by high strength and high ductility However, due to self-induced recrystallization by grain boundary migration during extended thermal exposure, the homogeneous nanosized structure is replaced by coarse platelets in the matrix and coarse lamellar structure at grain boundaries resulting in severe embrittlement. It is demonstrated that this problem can be combated by doping with either boron or yttrium which tend to segregate at grain boundaries and reduce their tendency toward migration and therefore self-induced recrystallization. Therefore, the desirable nanosized morphology of Ni₄Mo is restored in the matrix phase and the lamellar grain boundary morphology is replaced by nanosized layers of Ni-Mo solid-solutions with compositions not susceptible to Ni₄Mo ordering. As a result, the doped alloys become able to maintain high strength and high ductility after extended thermal aging at elevated temperatures.