Mn-deficient Ni-Mn-Ga alloys: phase transitions, magnetic properties, and disappearance of martensitic transformation
Composition gradients formed during directional solidification are generally considered undesirable in Ni-Mn-Ga magnetic shape-memory alloys. However, these gradients offer a highly effective platform for systematically investigating composition-property relationships under identical growth and heat-treatment conditions. To exploit this approach, one Mn-deficient and two near-stoichiometric Ni-Mn-Ga oligocrystals were grown by optical zone remelting and characterized by X-ray fluorescence mapping, optical microscopy, X-ray diffraction, low-field magnetic measurements, and vibrating-sample magnetometry. The ingots contained elongated grains and axial compositional gradients, primarily associated with the redistribution of Mn and Ga, whereas Ni remained relatively uniformly distributed. The martensitic and premartensitic transformation temperatures generally increased with increasing Mn content. Neither transformation was detected between 10 and 400 K in specimens containing 20.5-22.0 at.% Mn, all of which had $e/a < 7.40$. In contrast, both transformations were observed in the investigated specimens containing at least 22.1 at.% ($e/a \geq 7.46$). This abrupt change may indicate a possible electron-concentration threshold for the occurrence of these transformations. Saturation magnetization increased with Mn content up to 25.8 at.%, reaching 96.9 A m$^2$/kg at 10 K and 71.2 A m$^2$/kg at 300 K. Post-growth annealing increased the transition temperatures and changed the low-field magnetic response. The results indicate that composition-gradient oligocrystals serve as an effective platform for investigating the relationships between composition, structure, and magnetic properties in Ni-Mn-Ga alloys with the same processing history.