Reversibility of Martensitic Transformation Using B2-NiAl Nanoprecipitates

Martensitic transformation can be classified into two types: thermoelastic, characterized by glissile interfaces and small hysteresis, and non-thermoelastic. In the thermoelastic martensitic transformation, the transformation is reversible and good shape memory properties can be obtained. In this type of transformation, the strain due to lattice deformation associated with martensitic transformation is elastically accommodated. The elastic accommodation occurs when lattice deformation is small, when local shape strain is reduced by refining twinning, or when the strength of the parent phase is sufficiently high. These conditions have often been satisfied in ordered structures.<br/>The present authors found that Fe-Mn-Al alloy exhibits non-thermoelastic martensitic transformation from the BCC phase to the FCC phase and that the transformation mode changes to thermoelastic by precipitation of the B2-NiAl in the A2 matrix. The NiAl particles with about 10 nm are coherent with the martensite matrix, and the NiAl lattice is distorted by the martensitic transformation. On the other hand, the martensite has a cubic structure (FCC) but nano-twins are frequently introduced, which play a key role for strain accommodation and reversible transformation in Fe-Mn-Al.<br/>Another example is Cu-Al-Mn-Ni alloy with high functional fatigue resistance. Functional fatigue is one of the most significant problems to be solved for applications of superelastic alloys. Highly ordered alloys tend to show less degradation of superelasticity by cycling, but it is not easy to achieve both ductility and functional fatigue resistance. Cu-Al-Mn superelastic alloys show good ductility and good superelasticity until about 100 cycles. Cu-Al-Mn-Ni alloy has the B2-NiAl nanoprecipitates in the A2 matrix. The Cu-Al-Mn-Ni single crystal exhibits better functional fatigue resistance (over 1000 cycles) compared to Cu-Al-Mn single crystal without NiAl precipitates. This improvement of the reversibility of martensitic transformation for mechanical cycles can be attributed to the particle dispersion strengthening due to the NiAl precipitation.