What is "r-phase"

The R-phase is a phase found in nitinol. It is a martensitic phase in nature, but is not the martensite that is responsible for the shape memory and superelastic effect. When one uses the word martensite in connection with nitinol, one is invariably referring to the B19' monoclinic martensite phase, not the R-phase. The R-phase competes with martensite, often completely absent, and often appearing during cooling before martensite then giving way to martensite upon further cooling. Similarly, 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 degrees C). 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 degrees C in most binary nitinol alloys.

The R-phase was observed during the 1970s but generally was not correctly identified until Ling and Kaplow's landmark paper of 1981. The crystallography and thermodynamics of the R-phase are now well understood, but it still creates many complexities in device engineering, leading to the well-worn phrase, "It must be the R-phase" whenever a device fails to perform as expected.