SOLIDIFICATION OF PERITECTIC ALLOYS

The solidification of peritectic alloys is reviewed, including both nu cleation and growth. The introduction defines various compositions ass ociated with binary peritectic alloys, their 'equilibrium' solidificat ion, multicomponent peritectic systems, and metastable phases in perit ectic systems. Nucleation studies are reviewed according to the rate o f cooling and the type of specimen. Some comparisons are made with obs ervations in eutectic alloys. At fast cooling rates metastable events can occur, as reviewed for alloys based on the AI-Ti and other systems . Growth studies in peritectic alloys have identified three stages: th e 'reaction' (th ree phases in contact); the 'transformation' (the per itectic phase thickens by solid state diffusion); and 'direct solidifi cation' of the peritectic phase. Models of the stages are outlined and compared with experiment for various binary alloys, followed by a sum mary of observations of peritectic solidification in multicomponent Fe -Cr-Ni and Fe-C-X alloys. Recent advances of single phase solidificati on are reviewed and applied to phase selection with increased velocity , in peritectic alloys such as in Fe-Cr-Ni, Co-Sm-Cu, and Fe-Nd-B allo ys. independent growth of the two solid phases also is discussed, as i llustrated by superconducting Y-Ba-Cu-O alloys. Attempts to produce co upled structures in peritectic alloys are reviewed, followed by a disc ussion of low velocity banding. Limitations in current experimental or theoretical work are pointed out to aid future work in this important area.