抄録
The thermoelastic nature and crystal structure of the Cu-∼40 wt%Zn martensite (β1″), which is formed at temperatures below about −70°C exhibiting a parallelogram shape and may directly be related to the remarkable shape memory effect of the alloy, have been examined by means of low temperature optical and electron microscopy observations and electron and X-ray diffractions. The reverse transformation of β1″ martensite to β1 matrix is completed upon heating up to room temperature with a small hysteresis, that is, the martensite is thermoelastic in agreement with a previous observation, although it abruptly appears and disappears upon cooling and heating, respectively. The thermoelastic nature may closely be related to the shape memory effect of the Cu–Zn alloy as well as other marmem alloys.
The crystal structure of the β1″ martensite is identified as a monoclinic lattice slightly modified from the normal 9R type orthorhombic one (β1′) observed at room temperature, and lattice parameters of the modified 9R structure are determined to be a=4.412, b=2.678, c=19.19Å, and β=88.43°. The appearance of the modified 9R type β1″ martensite is attributed to the general tendency that more close packed structures are realized at lower temperature, since composite atoms of the β1″ martensite are more closely packed than those of the normal 9R type β1′ martensite. Internal defects of the β1″ martensite are not twin faults as observed in martensites of other marmem alloys, but stacking faults on the (001) basal plane.
