Composition, Structure and Magneto-Mechanical Properties of Ni-MN-Ga Magnetic Shape-Memory Alloys

Magnetic shape-memory alloys (MSMAs) are smart materials which show in single crystalline form a magnetic field induced plastic and recoverable deformation of up to 10 %. Ni-Mn-Ga is the as most prominent representative. The shape change of MSMAs is based on the motion of twin boundaries driven by a magneto-stress due to an applied magnetic field. The plastic deformation takes place in the martensite phase and does not require a phase change as needed in shape-memory alloys (SMAs). The combination of high strain of SMAs and high actuation frequencies positions MSMAs as attractive smart actuator materials. Several aspects influence the magneto-mechanical properties. To identify the influence of composition, surface deformation, and constraints separately, this dissertation consists of three parts: first, the characterization of composition, structure, transformation temperatures, magnetic and mechanical properties as a study on position within an ingot. Second, the influence of surface polishing and surface deformation on the twinning stress. Third, the influence of training and constraints on magneto-mechanical properties. It can be shown in this work that each of the investigated aspects has strong influences on i.e. martensite structure and transformation temperatures, twinning stresses, twin microstructure, and magneto-mechanical properties of MSMAs.