Structural Relationships in the Pseudo - Binary System

Structural relationships were studied among ternary Laves phases in the system ZrFe2 -ZrCr2 . Among the phenomena observed was the allotropy in ZrCr 2 , which was shown to exhibit a high temperature modification of the hexagonal C14 structure type close to the melting point and a low temperature modification of the C15 cubic type. In connection with the latter observation was the fact that at 8900 C Fe was soluble in C15 ZrCr2 up to " 8 percent followed by a narrow two phase region and then a single phase region extending from -20 percent Fe to v 80 percent Fe of the C14 structure type. After a very narrow two phase region, the C15 cubic structure appeared again at 90 percent Fe and at ZrFe2 . At the Fe rich end the cubic structure appeared to be stable at 8900 C, 10000 C and 14000 C to the same degree of solubility. At the Cr rich end the C15 structure seemed to decrease in solubility with increasing temperature. On the basis of these observations a proposed pseudo-binary phase diagram was drawn enclosing the C15 ZrCr2 with a gamma loop and allowing the C14 high temperature phase to extend its solubility over to ~ 80 per cent Fe. A peritectic reaction was employed to account for the cubic to hexagonal transformation at the iron rich end. From X-ray data, the lattice constant of both the cubic and hexagonal phases were solved and size effects were plotted against composition. It was seen that A-A (Zr-Zr) atom distances were most distorted at the Cr rich end and that B-B distances (FeCr-FeCr) were distorted abruptly due to the transformation near the Fe rich end. The transformations observed were partially explained on the basis of these size factors. Thesis Supervisor: R. F. Ogilvie, Assistant Professor of Metallurgy