The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+xGa_25−xmagnetic shape memory alloys

Abstract

In search for the origins of the extraordinary low twinning stress of Ni-Mn-Ga magnetic shape memory alloys we studied the thermally induced changes of structure in 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 between 297 to 328 K. All samples exhibited magnetic shape memory effect. Just below the transformation temperature the samples had very low twinning stress of about 0.1-0.3 MPa, which increased with decreasing temperature. The structural changes were monitored using X-ray diffraction in the temperature range 173-343 K. The 10M structure was approximated by monoclinic lattice with the unit cell derived from the cubic unit cell of the parent L2 1 phase. With decreasing temperature, the lattice parameters a and γ increased, c decreased, while b was nearly constant. For x ≤ 3.5, sudden sharp changes in a and b parameters additionally occurred, resulting in a = b in some regions of the phase diagram, which might be related to the refinement of twin structure of 10M martensite on nanoscale. The temperature dependences of lattice parameter γ (and c or c/a) correlate well with the temperature dependences of twinning stress in agreement with the prediction by a microstructural model of twin boundary motion. On the contrary, there is no correlation between (a − b) and twinning stress. This indicates no significant role of a/b twins or laminate in twin boundary motion mechanism and low twinning stress.

Ni–Mn–Ga Single Crystal Exhibiting Multiple Magnetic Shape Memory Effects

“…In martensite measured thermal expansion was not linear and relatively large. This agrees with the large and nonlinear change of c lattice constant with temperature in the vicinity of martensitic transformation measured by X-ray diffraction [25]. Without magnetic field, the transformation to cubic austenite occurred at 320.5 K and reverse transformation to martensite started at 312.5 K. The sample transformed back to approximately single variant due to stress *1 MPa applied to upper moving plate in dilatometer device to fix the sample.…”

Highly mobile interfaces in shape memory alloys

“…The characteristic spacing between the obstacles (∼10−50μm) resulting from the analysis of mobility given in [25][26][27][28][29] seem to relate to the characteristic thickness of the a−b lamina observed by Chulist et al [41]. However, the differences between the Type 1 and Type 2 interfaces were observed also for alloys with a ≈ b [48], which proves that such a pining is not a dominant mechanism.…”

Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review

“…Nevertheless, even within the narrow topic of the review, some features were not considered that may play important roles in MIR. Most of these features, however, appear to be difficult to approach with any existing theoretical treatments, such as the interaction of a moving twin boundary with lattice defects (which has been documented by jerks on the stress-strain curve [255] and acoustic emissions [256] ), the effect of the lattice parameters on the twinning stress, [257] and the experimentally observed difference between the twinning stress of a single interface and that for a laminate of fine twins of the same type. [258] There are also several important properties of Ni-Mn-Gabased FSMAs that are out of the scope of this review, although they pose similar challenges to the calculations.…”