Microstructure and mechanical behaviour of a precipitation hardened Ni-25 wt.% Mo-8 wt.% Cr alloy

The paper gives a broad characteristic of a nickel-based superalloy contain ing 25wt.%Mo and 8wt% Cr with particular attention to morphology of the met astable, dispersed Ni-2(Mo, Cr) phase. The influence of different variants of heat treatment on the microstructure and mechanical properties was inves tigated. The applied heat treatments comparised soaking at temperature rang e 1065 degreesC to 1095 followed by conventional aging at 650 degreesC for 24 to 72 hours and intermediate heat treatment at 760 degreesC to 980 degre esC preceded the aging at 650 degreesC. The conventional aging leads to the formation of lenticular precipitates of the dispersed Ni-2(Mo,Cr) phase. T he intermediate heat treatment influenced the size of the ordered precipita tes and thus mechanical properties of the alloy. The largest precipitates w ere produced when the aging at 650 degreesC was preceded by annealing at 76 0 degreesC, the precipitates size decreased with the increase of intermedia te heat treatment temperature bringing about the decrease of the yield stre ngth. The influence of prolonged aging at 650 degreesC on the microstructure and properties was investigated. The aging for 4000 hours did not cause the dec omposition of the metastable Ni-2(Mo, Cr) phase. The plastic deformation pr eceded the aging accelerated the decomposition of this phase on the mixture of the Ni3Mo and Ni4Mo-based phases. It was shown that at relatively small deformation the presence of the order ed phase changes the mechanism of deformation from dislocation glide to the mechanical twinning. The development of microstructure and crystallographi c texture during deformation do not depend substantially on the precipitate s size. The deformation microstructure as well as texture was typical for t he f.c.c. metals with low stacking fault energy. The influence of temperature, strain rate and environment on mechanical pro perties was investigated. It was shown that the long-term exposure at 650 d egreesC led to the increase of the precipitates size and strength. The dram atic drop in 650 degreesC dictility was observed upon testing in air. The g ood dictility in vacuum was preserved.