Influence of dislocation and twin structures on the mechanical characteristics of Ni–Mn–Ga alloys at ultrasonic frequencies

Magnetic shape memory alloys are a specific subtype of shape memory materials. The magnetic deformation phenomenon causes the high research interest in these alloys. Thus, in one of the most promising alloys based on Ni–Mn–Ga, using a magnetic field, it is possible to achieve changes in a single crystal size by up to 10 % due to the reorientation of the magnetic field in magnetic domains. The high magnetic deformation is directly related to the high mobility of twin boundaries separating two domains. In this work, the authors used a composite piezoelectric oscillator at a frequency of about 100 kHz to determine the influence of such defects as dislocations and twin boundaries on the mechanical characteristics of Ni₄₉Mn₃₀Ga₂₁. The authors investigated the features of temperature dependences of internal friction in the samples before and after deformation and provided the amplitude dependences of these characteristics. In the studied single-crystal martensitic phase, the transition from the tetragonal phase to the orthorhombic phase was detected at 235 K. In the Ni–Mn–Ga tetragonal phase, the formation of new defects contributes to the more pronounced and early onset of amplitude-dependent internal friction. At lower loads, the successive stages occur associated with the processes of dislocations and twin boundaries movements inside the Cottrell clouds, dislocations and twin boundaries movement outside the Cottrell clouds, and supposedly, the slowdown of dislocations and twin boundaries movement due to their interaction. As well as internal friction, the authors studied the change in Young’s modulus. Its decrease at all temperatures is most pronounced in the samples with the defective structures. The study identified that in the orthorhombic phase, it is possible to observe the internal friction dependence on the deformation amplitude at a lower load due to an increase in the twin boundaries mobility with increasing temperature.