STUDY OF COHERENCY STRAIN EFFECT ON THE COARSENING IN NI-AL-SI ALLOYS BY NEUTRON TECHNIQUES^* Haydn Chen, Department of Materials Science & Engineering, University of Illinois, Urbana, IL 61801, G. Muralidharan, MSD, Argonne National Laboratory, Argonne, IL 60439, J. Richardson, IPNS, Argonne National Laboratory, Argonne IL 60439

Superalloys are utilized at a higher proportion of their actual melting point than any other class of broadly commercial metallurgical materials. Ni-based alloys are the most complex, the most widely used for the hottest parts, and to many metallurgists, the most fascinating of all superalloys. Their excellent high temperature mechanical properties are mainly due to the fine precipitation of a variety of phases. Since the commercial alloys are usually complex multicomponent systems, understanding the precipitation phenomena in simple binary and ternary alloy systems is necessary before a full understanding of the commercial variants can be expected. A careful comparison between experimental data and existing precipitation theories is then possible, and more fundamental guidelines can be set to help superalloy design applications. The present study is thus aimed to investigate the kinetics and mechanism of the precipitation process of an ordered g' phase (L1₂ structure) in Ni-Al and Ni-Si binary alloys as well as in Ni-Si-Al ternary system employing time-resolved small-angle neutron scattering and neutron powder diffraction techniques. The objective of this work is first to provide a unified and comparative view of the precipitation phenomena in two binary systems with opposite signs of lattice mismatch, and secondly, to investigate the effect of lattice-mismatch strain on the kinetic behavior in properly-controlled ternary system. The strain effects on the spatial correlation of precipitates and on the coarsening rates are addressed. Anomalous coarsening rates were observed in the ternary system which is attributed to the coherency strain resulted from the lattice mismatch between the two phases. A plausible mode is described to explain this anomaly.

^*Work supported by the United States Department of Energy.