Martensitic transformation of Ni-Mn-Ga (C, Si, Ge) Heusler alloys

The effect of the IV elements C, Si, Ge on the martensitic transformation of Ni-Mn-Ga Heusler alloys has been examined by the measurement of resistivity and thermal expansion. There are two kinds of martensitic transformations for Ni-Mn-Ga ternary Heusler alloys below the Tc. The first one shows typical cubic-tetragonal martensitic transformation characteristic. The second one may correspond to a change in the periodicity of commensurate modulated martensite lattice. The Si and Ge additions obviously decrease the martensitic transformation temperature of the alloys and depress the second phase transformation completely above the 77K. An increasing of C and lowering of Ga content increases the second martensitic transformation temperatures more quickly than that of the first transformation.