A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 5 7 FEATURE Like shape memory metals, SMCs exhibit a reversible martensitic transformation that involves significant strains, matched with a very high intrinsic strength as shown in Fig. 1. What is more, SMCs are dielectric, so the two phases—austenite and martensite—can exhibit mismatches in electrical properties of the kind shown in Table I. In the authors’ recent work[4] they studied the martensitic transformation of single crystal zirconia doped with 2 mol% yttria, between its low-temperature monoclinic phase (martensite) and its high-temperature tetragonal phase (austenite). Figure 2 shows hysteresis curves for this transformation, measured using in situ single-crystal x-ray diffraction during a heating and cooling cycle. As expected, upon heating the monoclinic phase reflections decline and the tetragonal phase reflections emerge, with the transformation being complete at about 600°C. This is a thermal martensite-to-austenite transformation enabled by an entropy difference; it is also accompanied by the shape (strain) mismatch between the phases as noted in Table 1. It is a reversible transformation as well, and can be realized thermally upon cooling, albeit at lower temperatures as shown in Fig. 2. Also shown in Fig. 2 is a new result for shape memory zirconia, namely, that the same martensite-to-austenite transformation can be driven directly with an electric field. The same single crystal specimen, in the form of a ~125 µm thick plate, was outfitted with electrodes and subjected to short ~1 s bursts of applied electric field. As shown by the yellow data series, voltage causes the transformation to occur at 550°C, below where temperature alone would trigger it. This result was also shown to be reversible (cooling reverted to martensite) and reproducible, and unlike shape memory alloys, did not involve significant current flow or joule heating. Rather, this is a demonstration of direct electrically triggered martensitic transformation, and in this specific crystal orientation gives rise to a ~1% linear strain. Fig. 2 — Experimental results on the tetragonal-monoclinic shape memory transformation in yttria-doped zirconia, measured by tracking single-crystal diffraction 111 peaks in situ during transformation. Gray hysteresis loops show the thermally triggered transformation during a full cycle. The data series in black and yellow show an experiment in which an applied electric field was used to directly trigger the monoclinic-to-tetragonal transformation. Reproduced from Lai[4]. Categorization of shape memory effects on the basis of the applied triggering effect (work input) and property mismatch it works upon. In addition to well-known intrinsic property mismatches ΔS, Δε, ΔM, and ΔP, it is possible to induce significant mismatches and cause phase transformations by virtue of a second-order (field-dependent) property such as electrical susceptibility. TABLE 1 − CATEGORIZATION OF SHAPE MEMORY EFFECTS Work input Property difference between phases Transformation type Temperature change Entropy, ∆S All phase transformations Mechanical stress Strain, ∆ε Shape memory and superelasticity Magnetic field Magnetization, ∆M Magnetic shape memory Electric field Polarization, ∆P Ferroelectric shape memory Electric susceptibility Paraelectric shape memory 9
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