Microstructural Development Under the Influence of Elastic Energy in Ni-Base Alloys Containing γ' Precipitates

Abstract

The changes in the size and the distribution of y' precipitate particles during ageing of some Ni-base alloys were investigated by means of transmission electron microscopy (TEM). In the alloy system which has smaller lattice misfit and hence smaller elastic energy (e.g. Ni-Cr-Al or Ni-Si-Al), the yt particles coarsen steadily and the mean particle size r at an ageing time t is proportional to t l/3 . The size distribution of y1 particles does not change during the ageing, and the standard deviation c of size distribution is practically constant. In the alloy system which has larger elastic energy (e.g. Ni-Cu-Si), the deceleration of the coarsening of y1 particles occurs during ageing. At the same time, the size distribution of y1 particles becomes sharper gradually, that is, the c decreases in the course of coarsening. When the elastic energy is small (e.g. Ni-Cr-Al or Ni-Si-Al), the y' particles are uniformly (i.e. homogeneously) distributed in the y matrix. However, when the elastic energy is large (e.g. Ni-Al or Ni-Al-Ti), the y' particles have a tendency to exhibit non-uniform (i.e. inhomogeneous) distribution in the y matrix. When the volume fraction of y' is higher, this tendency is less obvious, that is, the y' distribution appears uniform, even if the elastic energy is large. The deceleration of coarsening, the decrease in and the formation of non-uniform distribution are the results of elastic interaction energy. When understanding the microstructure of Ni-base superalloys strengthened by yt particles, we should always take account of the important role of elastic interaction energy. SuperaUoys1988 Edited by S. Reichman, D.N. Duhl, G. Maurer, S. Antolovich and C. Lund The Metallurgical Society, 1988