R-Phase

The R-phase is a phase found in nitinol, a shape-memory alloy. It is a martensitic phase in nature, but is not the martensite that is responsible for the shape memory and superelastic effect.

In connection with nitinol, "martensite" normally refers to the B19' monoclinic martensite phase, rather than the R-phase. The R-phase competes with martensite, is often completely absent, and often appears during cooling before martensite, then giving way to it upon further cooling. In the same way, it can be observed during heating prior to reversion to austenite, or may be completely absent.

The R-phase to austenite transformation (A-R) is reversible, with a very small hysteresis (typically 2–5 °C (36–41 °F)). It also exhibits a very small shape memory effect, and within a very narrow temperature range, superelasticity. The R-phase transformation (from austenite) occurs between 20 and 40 °C (68 and 104 °F) in most binary nitinol alloys.