Influence of thermal exposure on the precipitates and mechanical properties of a newly developed Ni-21Cr-17Mo alloy

The microstructure and mechanical properties of a newly developed commercial Ni-21Cr-17Mo (wt%) alloy with excellent corrosion resistance and high strength after long-term exposures of up to 1000 h at 600, 625 and 650 °C were studied. The long range ordered Ni2(Mo,Cr) phases with an average size of the order of 10 nm as well as secondary M6C carbides of various sizes ranging from 100 nm to 2 μm were found in this age-hardened alloy and coarsened but did not changed their crystallographic and ordering characteristics during thermal exposure. Exposure at 650 °C for time beyond 500 h led to the dissolution of secondary M6C carbides and the precipitation of new secondary Cr23C6 carbides along grain boundaries. The 0.2% offset yield strength and ultimate tensile strength of the exposed alloys increase with the volume fraction of Ni2(Mo,Cr) phases but decrease with their increasing size, whereas the ductility and toughness show the opposite trend. The notable reductions in the tensile elongation and Charpy impact energy of the alloy exposed for 1000 h at 650 °C are due to the formation of nearly continuous or elongated secondary Cr23C6 carbides at grain boundaries. This newly developed alloy can be used in the intermediate temperature range below 650 °C.