Phase transformations in iron-ruthenium alloys under high pressure

Abstract

At atmospheric pressure, iron-ruthenium alloys containing less than 12 atom percent ruthenium exhibit a diffusionless α(b.c.c.) ⇄ γ(f.c.c.) transformation on heating and cooling, while alloys containing 12–36 atom percent ruthenium exhibit a diffusionless ϵ(h.c.p.) ⇄ γ(f.c.c.) transformation. High pressure displaces the α → γ reaction to lower temperatures, but shifts the ϵ ⇄ γ reaction to higher temperatures. These effects appear to be governed by the pressure dependence of the thermodynamic properties. The application of pressure also produces a new transformation, α ⇄ ϵ, at room temperature in alloys which contain the α-phase at atmospheric pressure. Pressure-temperature diagrams at constant composition thus contain triple points involving α(b.c.c.), γ(f.c.c.) and ϵ(h.c.p.). As the ruthenium content is increased, the triple-point pressure decreases significantly. Comparison of the alloy pressure-temperature diagrams with the high-pressure behavior of iron demonstrates that the pure metal also exhibits a triple point, at which the high-pressure, low-temperature phase is ϵ. A thermodynamic analysis has yielded values of Δ Fα → ϵ[ T] for pure iron, thus establishing the relative stability of the ϵ-phase in iron.