Microstructural stability of rapidly-solidified CuB alloys

Abstract

The ability to retain refined microstructures at high temperatures is a necessity for those materials which will have a high service temperature or will experience a high-temperature stage during processing, for example during extrusion or forming. It is therefore essential to understand the factors controlling stability and coarsening of the refined and metastable structures typical of rapid solidification. The present study examines the stability of CuB alloys prepared by melt spinning. As-cast ribbons exhibit the typical two-zone structure, namely a fine, microcrystalline wheel side with a certain boron solubility, and a columnar-grained free side with boron-rich particles outlining the cell walls. This microstructure demonstrates a remarkable resistance to change on initial heat treatment. However, after a critical time and temperature state is reached, the microstructure becomes locally unstable and then changes rapidly. The grain boundaries remain immobile as long as particle coarsening is negligible. The boron particles are initially amorphous and highly resistant to coarsening: an interface-controlled process appears to be the cause. Following the crystallization of the boron particles, these coarsen very rapidly because of enhanced solute diffusion along the high-density region of the grain boundaries.