FORMATION AND STABILITY OF A NITRIDE WITH THE STRUCTURE OF BETA-MANGANESE IN NI-CR-N TERNARY-SYSTEM

A nitride with the metal-atom arrangement of beta-manganese, which is designated as pi-phase, was confirmed to exist as an equilibrium phase in the ternary Cr-Ni-N system. A Cr-40 mass pct Ni binary alloy was nitrided under a pure nitrogen atmosphere at 1523 K to prepare a Cr40Ni-5N alloy consisting of a nickel-rich face-centered cubic (fcc) gamma-phase and a dichromium nitride, Cr2N, designated as epsilon-phase. Upon aging the alloy at 1273 K, the pi-phase was formed through a peritectoid reaction between the gamma and epsilon-phases. A volume fraction of the pi-phase reached 95 pct after 3.6 x 10(5) s, and no more change of the volume fraction was observed, even after 3.6 x 10(7) s aging. The chemical composition of the pi-phase was determined to be Cr13Ni7N4, whose lattice parameter was a = 0.6323 nm. The pi-phase became unstable above 1473 K, which explains a previous unsuccessful attempt to produce the Cr-Ni-N ternary pi-phase by the replacement of molybdenum in Mo12Ni8N4 by chromium at 1473 K.