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 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. History. 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 . According to awell-worn phrase, "It must be the R-phase" whenever a device fails to perform as expected. Crystallographic structure and transformation. The R-phase is essentially a rhombohedral distortion of the cubic austenite phase. Figure 1 shows the general structure, though there are shifts in atomic position that repeat after every three austenitic cells. Thus the actual unit cell of the actual R-phase structure is shown in Figure 2. The R-phase can be easily detected via x-ray diffraction or neutron diffraction, most clearly evidenced by a splitting of the (1 1 0) austenitic peak. While  the R-phase transformation is a first order transformation and the R-phase is distinct and separate from martensite and austenite, it is followed by a second order transformation: a gradual shrinking of the rhombohedral angle and concomitant increased transformational strain. By suppressing martensite formation and allowing the second order transformation to continue, the transformational strain can be maximized. Such measures have shown memory and superelastic effects of nearly 1%. In commercially available superelastic alloys, however, the R-phase transformational strain is only 0.25 to 0.50 percent. There are three ways Nitinol can transform between the austenite and martensite phases : The R-phase can be stress-induced as well as thermally-induced. The stress rate (Clausius–Clapeyron constant, formula_1) is very large compared to the austenite–martensite transformation (very large stresses are required to drive the transformation). Practical implications. While an essentially hysteresis-free shape memory effect sounds exciting, the strains produced by the austenite-R transformation are too small for most applications. Because of the very small hysteresis, and the tremendous cyclic stability of the A-R transformation, some effort has been made to commercialize thermal actuators based on the effect. Such applications have been of limited success at  best. For most of Nitinol applications, the R-phase is an annoyance and engineers try to suppress its appearance. Some of the difficulties it induces are as follows: The R-phase becomes more pronounced through additions of iron, cobalt, and chromium, and is suppressed by additions of copper, platinum and palladium. Cold working and aging also tend to exaggerate the R-phase presence.