New Insights into the Intermartensitic Transformation and Over 11% Magnetic Field-Induced Strain in 14 m Ni-Mn-Ga Martensite

Stress-induced intermartensitic phase transformation from 10 m to 14 m modulated martensite structure is characterized in Ni50.5-Mn28.9-Ga20.6 at% single crystals, with particular focus on periodic atomic shuffling by in situ synchrotron radiation diffraction. Under compression, along the [001] direction, at 6 MPa, the self-accommodated 10 m single crystal is found to transform to a nearly single-variant state. Further stress increase up to 20 MPa triggers intermartensitic transformation to an intermediate, mixed martensite state. Intermartensitic transformation is hereby invoked at a considerably lower stress than previously reported for Ni-Mn-Ga alloys. However, the stress level applied along the [001] direction does not promote the complete 10 m -> 14 m martensite transformation. The 14 m structure is then fully achievable, when a consecutive compression along the [100] direction is applied. Such mechanically promoted 14 m single crystals are stable at room temperature and yield about 11.4% magnetic-field induced strain-being the highest noted up to date for the Ni-Mn-Ga system.