Coherent Ni2(Cr, Mo) precipitates in Ni–21Cr–17Mo superalloy

The microstructure of Ni2(Cr, Mo) precipitates in a Ni–21Cr–17Mo (wt.%) superalloy subjected to a two-step heat treatment has been investigated via high-resolution transmission electron microscopy and electron diffraction, combined with image and diffraction calculations. Various high-density nanoscale Ni2(Cr, Mo) precipitates were distributed homogeneously in the Ni–Cr–Mo matrix, and no other marked secondary phase was observed in the age-hardened alloys. The typical orientation relationship between the precipitates and the Ni–Cr–Mo matrix was determined as (2 0 0)matrix//(1 0 3) Ni2(Cr,Mo)Ni2(Cr,Mo), [0 0 1]matrix//[0 1 0] Ni2(Cr,Mo)Ni2(Cr,Mo). The interfaces between the precipitates and the surrounding matrix were coherent at the atomic scale, resulting in the remarkable improvement on the alloy strength after heat treatment. Moreover, the Ni2(Cr, Mo) particles exhibited a special oval shape, with the long axis perpendicular to the long-range-ordered plane of (Cr, Mo), that is, (2 2 0)Ni. A high spatial resolution chemical composition analysis revealed that the Ni2(Cr, Mo) precipitates were enriched in Mo, with an average composition of ∼19 wt.% Cr and ∼20 wt.% Mo, which probably caused the easy precipitation of the long-range-ordered Ni2(Cr, Mo) phase precipitated in Ni–Cr–Mo alloys with a high Mo content.

► The improvement on strength was derived from nanoscale Ni2(Cr, Mo) precipitates.
► The nanoscale coherent interface played a key role in strengthening materials.
► The long axis was perpendicular to the ordered plane of (Cr, Mo).
► The chemical analysis showed Ni2(Cr, Mo) precipitates were enriched in Mo a little.