Boundary-Phase Plasticity

Abstract

Chromium is an essential addition to some nickel-base alloys, since it increases their corrosion-resistance. In particular, the resistance of nickel-chromium alloys to corrosion by residual fuel-oil ash, which contains vanadium, sodium, and sulphur, is markedly improved by increasing the chromium content to 50%. Alloys with < 35% Cr are single-phase and can be easily hot worked but materials with higher contents contain the α-Cr phase and generally cannot be worked. This is attributed to the presence of nitrogen, the acicular form of the chromium-rich phase in the cast structure, and the large as-cast grain size of the alloys. The structure is modified when an eutectic is formed with some of the nickel by the addition of zirconium, cerium, yttrium, or hafnium. The addition of, e.g., zirconium influences the alloy structure in a number of ways. Zir. onium combines with nitrogen which reduces the hardness of the α-Cr phase. The formation of the Ni–Cr–Zr ternary eutectic lowers the solidus temperature of the alloy, which in turn changes the way in which the α-Cr phase precipitates; presumably by increasing diffusion and lowering the surface tension of the melt constituents. Lastly, the as-cast grain size is reduced. These modifications combine to render the alloy forgeable.