Effects of Nb addition on the carbides, grain boundary characteristics and tensile properties of a Ni–26W–6Cr-based superalloy

In this work, the potential of Nb addition to improve the mechanical performance of a new Ni–26W–6Cr-based superalloy designed for molten salt reactor (MSR) was studied. To this purpose, the evolution of the microstructure and the tensile properties of the Ni–26W–6Cr alloy with different Nb contents have been systematically investigated, with special attention to the solidification process. Nb addition in the Ni–26W–6Cr alloy causes constitutional undercooling and refines the grains of the as-cast alloys. In the following hot rolling and annealing treatment, the average grain size of Nb-bearing alloys remains smaller than that of the Ni–26W–6Cr alloy. Nb addition in the Ni–26W–6Cr alloy does not change the type of M6C carbides but increases the number of them, refining the grains of the alloys further by hindering grain boundary migration. When tensiled at room temperature (RT), the strength of the Ni–26W–6Cr-xNb alloys increases with Nb content due to the effects of solid solution strengthening, grain refinement and increased carbides number. Conversely, the elongation of the Ni–26W–6Cr-xNb alloys maintains a semblance of uniformity, as the deleterious effect engendered by the amplified number of carbides counterbalances the enhancement in plasticity, facilitated by a reduction in grain size and an elevated proportion of twin boundaries. The effect of Nb addition on the strength and EL of the alloys at 850 °C is similar to that at RT. The strength of the Ni–26W–6Cr-xNb alloys increases with Nb content, and the EL is roughly equal, except for the decreased EL of the alloy with 4 wt% Nb addition. Nb addition slows down dynamic recrystallization in the plastic deformation stage of the Ni–26W–6Cr-xNb alloys, and it is completely inhibited in the alloy with 4 wt% Nb content.