Crystal Structure and Internal Defects of Thermoelastic Martensite in a Ag-47 at%Cd Alloy

The thermoelastic martensite formed upon subzero cooling a Ag-47 at%Cd alloy has been studied by means of low temperature optical and electron microscopy observations, and electron and X-ray diffractions. The martensite has a modified 2H structure; namely, in the orthorhombic coordinate the second layer parallel to the basal plane is stacked at the (a⁄2.60, 0, c⁄2) position instead of the normal (a⁄3, 0, c⁄2) one, and lattice parameters of the martensite are a=4.914 Å, b=3.111 Å and c=4.782 Å. The modified 2H martensite turns out to be almost identical to that found by Masson and Barrett in the same alloy subjected to subzero cooling. Internal defects of the modified 2H martensite are twins on the {111} plane, which are alternatively expressed as {1011} twins in the hexagonal coordinate.The modified 2H martensite is discussed in connection with the current model of the transformation mechanism from the B2 type parent lattice, and the shifted stacking position of the second layer is well explained by a hard sphere model of atoms.