R-phase

The R-phase is essentially a rhombohedral distortion of the cubic austenite phase.

The R-phase can be stress-induced as well as thermally-induced.

One often-encountered complication regarding nitinol is the so-called R-phase.

While the electrical resistivity of austenite and martensite are similar, the R-phase has a very high resistance.

The R-phase is another martensitic phase that competes with the martensite phase mentioned above.

For most of Nitinol applications, the R-phase is an annoyance and engineers try to suppress its appearance.

The R-phase was observed during the 1970s but generally was not correctly identified until Ling and Kaplow's landmark paper of 1981.

The R-phase to austenite transformation (A-R) is reversible, with a very small hysteresis (typically 2-5 degrees C).

Direct transformation, with no evidence of R-phase during the forward or reverse transformation (cooling or heating), occurs in titanium-rich alloys and fully annealed conditions.

Year groups 7-9 are dubbed the "r-phase" and study "Learning to Learn" lessons until the end of Year 8 focussing on reflectiveness, resourcefulness and resilience.

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.

When austenite transforms to the R-phase, its energy is reduced and its propensity to transform to martensite is lessened, leading to a larger austenite-martensite hysteresis.

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.

A large amount of heat is given off when austenite transforms to the R-phase, and thus it gives rise to a well-defined differential scanning calorimetry (DSC) peak.

The "symmetric R-phase transformation" occurs when the R-phase intervenes between austenite and martensite on both heating and cooling (see Figure 3).

The R-phase becomes more pronounced through additions of Fe, Co, and Cr, and is suppressed by additions of Cu, Pt and Pd.

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.

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.

Here the R-phase occurs during cooling, but not upon heating, due to the large hysteresis of the austenite-martensite transformation-by the time one reaches a sufficiently high temperature to revert martensite, the R-phase is no longer more stable than austenite, and thus the martensite reverts directly to austenite.