Argon—Nitrogen Phase Diagram

Abstract

The phases and phase transformations in the argon—nitrogen system have been determined by x‐ray diffraction with results very different from previous published phase diagrams. The solid phase in equilibrium with the liquid between 1% and 100% N₂ is close‐packed hexagonal in spite of the fact that pure argon is face‐centered cubic. The argon‐rich solid solutions with 1% to 55% N₂ undergo a strain‐induced martensitic transformation from cph (stable at high temperatures) to fcc at low temperatures. The highest temperatures at which this transition becomes possible are those at which the combination of thermal expansion plus solid solution expansion gives a lattice constant for the fcc phase of a₀=5.486 Å, which corresponds to that obtained by extrapolating the curve of a₀ for pure argon to a point 5° above the melting point. The c/a value for the cph argon‐rich phase is 1.633 within experimental uncertainty at all compositions and temperatures. Stacking faults are infrequent in the as‐frozen samples but are very numerous in the deformed samples. At temperatures and compositions where fcc and cph have nearly equal free energies it is possible to destroy the crystalline diffraction pattern by cold work, leaving only an amorphous pattern. The α‐N₂ solid solution forms spontaneously and isothermally from the β‐N₂ solid solution at N₂ contents above 77.5% N₂. The transformation appears to be that known in metallic systems as a ``massive'' type, and presumably involves only short‐range diffusion in the phase interface. The curve of distance between nearest molecular centers in the β phase is not an extension of the curve for the argon‐rich phase. The α phase, which is cubic, is not continuous with the cubic argon‐rich solid solution; samples with 55%—75% N₂ do not transform from the hexagonal form even when cold worked at 4.2°K.