R-Phase
Encyclopedia
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.
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,
) is very large compared to the austenite-martensite transformation (very large stresses are required to drive the transformation).
-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 actuator
s 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 Fe, Co, and Cr, and is suppressed by additions of Cu, Pt and Pd. Cold working and aging also tend to exaggerate the R-phase presence.
Nickel titanium
Nickel titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages....
. It is a martensitic phase in nature, but is not "the" martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
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
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
, 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
Temperature range
Atmospheric temperature range is the numerical difference between the minimum and maximum values of temperature observed in a given location....
, 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.
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 cellCrystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
of the actual R-phase structure is shown in Figure 2. The R-phase can be easily detected via x-ray diffraction
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
or neutron diffraction
Neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material: A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of...
, 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:
- Direct transformation, with no evidence of R-phase during the forward or reverse transformation (cooling or heating), occurs in titaniumTitaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
-rich alloys and fully annealed conditions. - The "symmetric R-phase transformation" occurs when the R-phase intervenes between austenite and martensite on both heating and cooling (see Figure 3). Here, two peaks are observed on cooling, and two peaks upon heating, with the heating peaks much closer to one another due to the lower hysteresis of the A-R transformation.
- The "asymmetric R-phase transformation" is by far the more common transformational route (Figure 4). 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.
The R-phase can be stress-induced as well as thermally-induced. The stress rate (Clausius-Clapeyron constant,
) is very large compared to the austenite-martensite transformation (very large stresses are required to drive the transformation).Practical Implications
While an essentially hysteresisHysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
-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 actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s 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:
- 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. This, in turn reduces actuator efficiencyEfficient energy useEfficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...
and superelastic energy storageEnergy storageEnergy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time. A device that stores energy is sometimes called an accumulator....
capacity. - Stress-strain curves of the austenite often show a slight inflection during loading, making elastic limits and yield stresses difficult to pinpoint
- While a 0.25% strain is too small to take advantage of, it is more than enough to cause stress relaxationStress relaxationStress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion...
in many interference-fit applications such as pipe couplings. - 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 calorimetryDifferential scanning calorimetryDifferential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature...
(DSC) peak. This makes DSC curves difficult to interpret if one is not careful: the R-phase peak is often mistaken for a martensite peak, and errors are often made in determining transformation temperatures. - While the electrical resistivityResistivityElectrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
of austenite and martensite are similar, the R-phase has a very high resistance. This makes using electrical resistanceElectrical resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
all but useless in determining the transformation temperatures of Nitinol.
The R-phase becomes more pronounced through additions of Fe, Co, and Cr, and is suppressed by additions of Cu, Pt and Pd. Cold working and aging also tend to exaggerate the R-phase presence.