TWIN BOUNDARY MOBILITY IN SHAPE MEMORY ALLOYS

Magnetic Shape Memory Alloys (MSMA) have great potential for small size, large stroke, highly energy efficient, high speed actuator applications, thus facing a huge market. To enter this market, MSMAs must perform reliably over long life times, i.e. they must be resistant against fatigue. This fundamental study will provide (i) a quantitative experimental and theoretical understanding of fracture mechanisms under dynamical loading and (ii) a mechanism-based methods for improving fatigue resistance.

The goal of this project is to develop a fundamental understanding of damage accumulation mechanisms and fracture mechanisms of MSMAs under high-cycle magneto-mechanical loading. The overarching hypothesis rests on the assumption that fracture and failure in high-cycle actuation of MSMAs is caused by the interaction of course, orthogonal (i.e. ‘intersecting’) twins. To avoid fracture and failure, twins must not intersect.

Ni-Mn-Ga single crystals were produced with various twin microstructures ranging from densely twinned self-accommodated martensite over thin twins expanding across the entire sample, coarse twins, to single domain crystals. High-cycle magneto-mechanical experiments were performed in closed-loop control at frequencies up to 800 Hz. Furthermore, we developed a testing method utilizing magnetic-field actuation capable of testing up to 60 single crystal samples simultaneously. Crack nucleation was studied at crystals with and without notches and crack propagation was measured.

First results show that crack formation is controlled by concentrations of normal stress components on {100} planes. In contrast, the modulation of shear stresses on the {110} drives crack propagation. We will complete a series of systematic fatigue experiments with Ni-Mn-Ga single crystals to substantiate these results quantitatively.