The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+<i>x</i>Ga_{25−<i>x</i>}magnetic shape memory alloys

Abstract

In search for the origins of the extraordinary low twinning stress of Ni-Mn-Ga 10M martensite, we studied the temperature induced changes in lattice parameters of Ni 50 Mn 25+x Ga 25−x (x = 2.7–3.9) single crystal samples and compared them with twinning stress dependences. The alloys exhibited transformation to five-layered (10M) martensite structure (cubic to monoclinic) between 297 to 328 K and exhibited magnetic shape memory effect in martensite. The structural changes were monitored using X-ray diffraction in the temperature range 200–343 K. The 10M structure was approximated by monoclinic lattice, a = b > c, γ > 90 ◦ with the coordinates derived from the cubic unit cell of the parent L2 1 phase. The lattice parameters γ and c/a correlate well with the universal linear increase of twinning stress of Type 1 twins with decreasing temperature. On the contrary, the twinning stress is not affected by differences between a and b.