Local elasticity and mobility of twin boundaries in martensitic films studied by atomic force acoustic microscopy

Abstract

Nanoscale elastic properties of twinned martensite NiMnGa films were characterized by means of atomic force acoustic microscopy using cantilever contact-resonance spectra to measure the local contact stiffness k and the local damping Q(-1), which contains information on the crystallographic anisotropy of martensitic twin variants and the dissipative motion of twin boundaries (TBs). Images of k and indentation modulus maps were obtained. Similar to topography images measured by conventional atomic force microscopy in contact mode, they show the nature of the twin structure and thus a regular variation in local elastic modulus. A correlation between k and Q(-1) was observed and mirrors the motion of the TB accompanied by a viscoelastic procedure. The k -image and the topography image measured are opposite in contrast, which likely arises from mobile and immobile TBs depending on the geometry of twinning. Multi-resonance spectra were measured, which can be related to martensitic multivariants and are explainable as different types of nanotwins. A critical stress, defined as the starting point of softening due to TB movement was determined to be about 0.5 GPa for a thick film (1 mu m) and 0.75 GPa for a thin film (0.15 mu m), respectively. The values are much larger than that measured for bulk materials, but reasonable due to a large internal stress in the films.