Effect of grain size on high temperature creep properties of a Ni-20Cr-20W alloy

The effect of grain size on the creep behavior of a Ni-20Cr-20W alloy has been studied at 1173 and 1273 K by using the specimens with different grain diameters; 56, 246, and 620 μm. It has been found that creep resistance essentially increases with the increase in grain size and that the grain size dependence of rupture life varies with the testing conditions. In a short-time creep test, the specimen with a grain diameter of 246 μm has a slightly longer rupture life than the others. When the testing time was prolonged above about 100 h, the grain size dependence of the rupture life at 1273 K was very different from that at 1173 K, i.e., in the former condition, the rupture life increased with the increase in the grain size, in analogy to the creep resistance, however at 1173 K, the remarkable decrease in rupture life was detected for the specimen with a grain diameter of 620 μm. From the investigation of the microstructures of the ruptured specimens, it was speculated that for a coarse grained specimen crept at 1273 K, copious α2-W rich phase precipitated on grain boundaries retards both the deformation of the matrix adjacent to the grain boundaries and the initiation of grain boundary crackings, while, at 1173 K, α2-phase within grains in the shape of cellular structure does not seem to be effective in preventing the initiation and propagation of cracks at grain boundaries. It was concluded that the grain size dependence of the creep behavior of a Ni-20Cr-20W alloy is closely related to the changes in the volume fraction and the shape of α2-phase on the grain boundaries. (author)