Martensitic transformations in NiMnAl β phase alloys

Martensitic transformation characteristics in a wide range of alloy compositions in the NiMnAl system have been investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results indicate that while the NiMnAl alloys with relatively high Al and Mn contents transform martensitically to either the 14M(7R) or the 10M phase with a long-period stacking-order structure, those with low Al or Mn contents transform to the L1₀(3R:2M) structure. The M_s temperature and the c/a ratio of the L1₀ martensite increase with decreasing Al or Mn content. In the Ni-30Mn-20Al alloy, typical premartensite microstructures, such as needlelike, modulated, and tweed structures, are observed and it is also found that a distorted B2 structure is formed as an intermediate phase prior to transformation to the 10M martensite.