ON THE NATURE OF PERITECTIC REACTION AND THE MECHANISM OF THE GRAIN REFINEMENT RESULTING THEREFROM,

Alpha alloys rich in Cu the size of the crystal grain become very fine when an adequate amount of Fe or Co is added to cause peritectic reaction. Similar phenomena was found in the case of peritectic alloys other than alpha alloys rich in Cu though the degrees of the refinement are less than in the case of the latter. The nature of the peritectic reaction was studied. The contact between the primary crystal and the melt should be increased in the course of the peritectic reaction in order that the system should change so as to conform to the equilibrium diagram as possible. This requirement is fulfilled when the disintegration of the primary crystal takes place in the course of the reaction. The disintegrated fragments of the primary crystal may have different orientations with each other, though they are produced from its mother dendrite, and behave as separate nuclei for further peritectic reaction. The enveloping phenomenon of the primary cyrstal by the secondary is a result of an establishment of an apparent equilibrium. Enveloping phenomenon takes place to a less extent in the ease of ternary alloys than in that of binary alloys. These conclusions were satisfactorily proved to be correct by experiments on Sb-Sn-Pb alloys.