Magneto-mechanical properties and fracture of a mechanically constrained Ni-Mn-Ga single crystal after extended magnetic cycling

In a previous study, magnetic-field-induced strain (MFIS) had been measured for a Ni-Mn-Ga single crystal in a rotating magnetic field of 0.97 T for a total of 100 million cycles. The MFIS increased from below 0,5% during the first 20,000 magneto-mechanical cycles to above 1.2% at 250,000 cycles. After a maximum MFIS of 2,1 % at 0,6 million cycles was reached, the MFIS decreased slowly and stayed nearly constant for the final 30 million cycles. After the test was stopped, cracks were found on the surface of the sample. In the present work, these cracks were examined using x-ray micro computer tomography and optical microscopy. The magneto-mechanical properties of the sample were then tested in a static magnetomechanical experiment with linear magnetic field showing a recoverable strain increasing from less than 1% at 0,3 T to 5,9% at 1 T. Subsequently, the sample was forcibly broken, The parts of the fracture surfaces cracked forcibly show plastic deformation while the cracks grown slowly during magneto-mechanical cycling follow crystallographic planes. The results confirm that cracks nucleate where coarse twins interact, while densely twinned microstructures are fracture resistant,.