Dimension changes induced by phase transformation behaviors in Ni-Mn-Cu alloys

The phase transformation behaviors, microstructural evolutions and dimension changes of Ni50-xCuxMn50 (x=8, 10, 12) (at%) were investigated. The results show that the phase transformation sequence of Ni50-xCuxMn50 alloys obtained by quenching from 900 °C during heating and cooling is: γ(fcc) → non-martensite L10 phase→14M martensite↔B2↔γ(fcc). As these alloys are annealed under a variety of temperatures between 500 °C and 900 °C, different microstructural evolutions occur. The interesting changes that occurred during the heating and cooling of the above phase transformation are observed in Ni50-xCuxMn50 alloys. Due to the transformations of non-martensite L10 phase→14M martensite→B2, two consecutive dimension expansions are observed during heating for the alloys quenched from 900 °C. The largest dimension expansion can reach to 2.2% in Ni40Cu10Mn50 alloy. The reversible transformation behaviors of 14M martensite↔B2 and B2↔γ result in the reversible dimension changes. During cycling or as the alloys quench from 700 °C, the irreversible phase transformation behaviors and the corresponding dimension changes gradually recede and even disappear.