TOPOLOGICALLY CLOSE-PACKED PRECIPITATIONS AND PHASE-DIAGRAMS OF NI-MO-CR AND NI-MO-FE AND OF NI-MO-FE WITH CONSTANT ADDITIONS OF CHROMIUM

The microstructure and morphology of the topologically close-packed (TCP) precipitates in the alloy systems Ni-Mo-Cr and Ni-Fe-Mo, as well as Ni-Fe-Mo with constant additions of chromium, were investigated. Isothermal sections of these systems at 700, 850, 1000 and 1200-degrees-C were constructed. In these systems the following TCP phases exist: sigma, mu, delta, beta and P. The ternary phase P, which formed in a peritectic reaction from the sigma-phase, was detected. In the system Ni-Fe-Mo the sigma-phase exists only at temperatures above 1200-degrees-C, whereas in the systems Ni-Cr-Mo and Ni-Fe-Mo-Cr it was also found at lower temperatures. The progressive addition of chromium (10, 20 and 30 wt.%) to the Ni-Fe-Mo system causes the appearance of a larger amount of sigma-phase, so that at 30 wt.% Cr only sigma-exists.