Martensitic transformation of a Ni-Al alloy. I. Experimental results and approximate structure of the seven-layered phase

Neutron-diffraction experiments were performed in order to investigate the structure of the thermoelastic martensite of the ${\mathrm{Ni}}_{62.5}$ ${\mathrm{Al}}_{37.5}$ alloy. The following results were obtained: (i) the low-temperature phase has a seven-layered monoclinic unit cell (7M martensite). (ii) Anomalous shifts of the superlattice peaks from the commensurate position were observed, which could not be explained by an ordinary incommensurate structure. At the same time, anomalous broadenings of the peak profiles were also observed. The overall feature of the incommensurability and the broadening was tentatively understood by considering that there are two slightly different monoclinic lattices coexisting in the crystal. (iii) Ignoring these anomalous features, the approximate structure was investigated following three different procedures: (a) Analysis on the (5,2¯) stacking-sequence model; (b) analysis on the pseudospin model; and (c) model-free analysis based on the postulated space group P2/m.