Thermoelastic Martensitic Transformation and Shape Memory Effect in Sr2(Si, Ge)O4.

Abstract

Crystals of Sr2(Si1-xGex)O4 with 0≤x≤1 were prepared and examined by powder XRD, optical microscopy, and AFM. The crystals with 0.3≤x≤0.6 were composed of both the α′ (orthorhombic)- and twinned β (monoclinic)-phases. On the basis of the lattice correspondence between the two phases and their cell parameters, the phenomenological crystallographic theory has been applied to determine the habit planes and shape deformations upon α′-to-β martensitic transformation. The habit planes, which define the coherent interphase boundaries between α′ and β, were nearly parallel to either (100) or (001). Because the transformation was accompanied by a small volumetric shrinkage of -0.3%, the parent α′-phase would elastically accommodate the strains upon the polysynthetic twin formation of the β-phase. At 293K, the crystals with x=0.6 were composed of -92.5mass%α′ and -7.5mass%β. During further cooling in liquid nitrogen, the α′-to-β transformation proceeded which increases the phase composition of β up to -56.3%. The transformation is accompanied by the formation of plate-like surface reliefs. The surface relief angles have been determined from both observations (7.4±0.2°) and calculations based on a phenomenological analysis (7.55°). The fair agreement of these values indicates that the transformation is martensitic and mainly governed by a shear mechanism. The shape memory effect has been demonstrated by the reproducibility of the surface reliefs. The coherency at the interface boundaries between the α′ and β-phases as well as the effective strain accommodation substantially account for the thermoelasticity of the solid solutions.