The martensitic phase transformation temperature, T-m and the Curie tempera
ture, T-C, of polycrystalline samples Ni2+xMn1-xGa(x = -0.1,0,0.08,0.13,0.1
8,0.2) and Ni2-xMn1+x/2Ga1+x/2(x = -0.1,0,0.04,0.06,0.1) have been investig
ated by measuring the temperature dependence of alternating-current suscept
ibility. It was found that, with the increase of Ni content, T-C decreases
and T-m increases for Ni2+xMn1-xGa, and both T-C and T-m increase first and
decrease afterwards for Ni2-xMn1+x/2Ga1+x/2 Stress-free and two-way thermo
elastic shape memory, with 1.2% strain, has been found in single crystal Ni
52Mn24Ga24 The deformation can be enhanced more than three times, up to 4.0
% shrinkage with a bias field 1.2T applied along the measurement direction.
The origin of large recoverable strain should be attributed to the low lev
el of internal stress and the field-induced rearrangement of martensitic va
riants through twin boundary motion rather than the phase boundary motion.