Peculiarities of Superelastic Tensile Behavior of Ni-Mn-Ga and Ni(Co)-Fe-Ga Single Crystals

Heusler ferromagnetic shape memory alloys in the martensitic state demonstrate extraordinary actuation ability in a magnetic field. Whereas many stress-strain studies in compression mode have shown that they also exhibit an excellent superelasticity, the mechanical behavior of these alloys in tensile experiments has been rarely addressed due to the enhanced brittleness of the austenite.

In this work, we present the original results of the tensile tests carried out using <001>_A -oriented Ni-Mn-Ga (S1) and Ni(Co)-Fe-Ga (S2) single crystals with transformation temperatures of about 300 K  and 220 K, respectively. The measurements were performed with a DMA Q800 analyzer which allowed recording, in the same sample, the temperature dependence of the elastic modulus and internal friction in the dynamic tensile mode, as well as the “stress versus strain” curves at different constant temperatures, T_exp, and “strain versus temperature” curves at zero stress in the static tensile mode. The temperature dependence of the elastic modulus demonstrates both a deep softening in the cubic phase and typical anomalies at the martensitic and intermartensitic transformations. Particularly, multistep superelastic strains of about 10% and superelastic strains of 7% have been obtained in the alloys S1 and S2, respectively.  The quasi-equilibrium stress – temperature phase diagrams of L2₁-5M, 5M-7M and 7M – 2M phase transitions in the alloy S1, and L2₁ – 2M in alloy S2 are in agreement with thermodynamic estimations and the deductions of ab initio calculations. These results are compared with those obtained previously in the compression mode.