Abstract
We discuss the suitability of laser-based resonant ultrasound spectroscopy (RUS) for the characterization of soft shearing modes in single crystals of shape memory alloys that are close to the transition temperatures. We show, using a numerical simulation, that the RUS method enables the accurate determination of the c′ shear elastic coefficient, even for very strong anisotropy, and without being sensitive to misorientations of the used single crystal. Subsequently, we apply the RUS method to single crystals of three typical examples of shape memory alloys (Cu-Al-Ni, Ni-Mn-Ga, and NiTi), and discuss the advantages of using the laser-based contactless RUS arrangement for temperature-resolved measurements of elastic constants.
Highlights
Athermal martensitic transitions are known to be mediated by soft acoustic phonons [1,2,3]
For a Cu-Al-Ni single crystal, we utilize the ability of resonant ultrasound spectroscopy (RUS) to determine accurately the c0 coefficient in a narrow temperature interval close to the transition temperature and discuss the correlation between the elastic softening and the internal friction; for Ni-Mn-Ga single crystals with two different chemical compositions, we show the advantage of high-temperature RUS measurements for materials with extremely strong anisotropy, and for a Ni-Ti single crystal we demonstrate the ability of RUS to simultaneously monitor the evolutions of c0 and c44
Using three typical examples of shape memory alloys (SMAs) (Cu-Al-Ni, Ni-Mn-Ga and Ni-Ti), we demonstrated that the laser-based RUS is a very suitable tool for the characterization of elastic precursors of martensitic transitions
Summary
Athermal martensitic transitions are known to be mediated by soft acoustic phonons [1,2,3]. In the shape memory alloys (SMAs), the relation between the structural change and acoustic phonons can be most apparently observed for the forward transition, i.e., the transition from the high temperature phase (austenite) to either the low temperature phase (martensite) or some intermediate phases (R-phase in NiTi [4], premartensite in stoichiometric Ni2 MnGa [5], and similar). Prior to this transition itself, the austenite crystal lattice anomalously softens with respect to specific shearing modes when approaching the transition temperature [6,7,8], which indicates acoustic phonon condensation.
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