2, Universitat de Barcelona, Barcelona, Catalonia, Spain
It is generally claimed that physical processes which display scale-invariant power law distributions are subjected to a dynamic criticality that prohibits a well-defined kinetic law. In this paper, we demonstrate the coexistence of these two apparently contradicting behaviors during the same physical process – the motion of type II twin boundaries in martensite Ni-Mn-Ga. The process is investigated by combined measurements of the temporal twin boundary velocity and the acoustic emitted energy. Velocity values are extracted from high-resolution force measurements taken during displacement driven mechanical tests, as well as from force driven magnetic tests, and cover an overall range of six orders of magnitudes. Acoustic emission (AE) is measured during mechanical tests.
Velocity values follow a normal distribution whose characteristic value is determined by the material’s kinetic relation, and its width is scaled with the average velocity. In addition, it is observed that velocity distributions are characterized by a heavy tail at the right (i.e., faster) end that exhibits a power law over more than one and a half orders of magnitude. At the same time, the AE signals follow a scale-invariant power law distribution, which is not sensitive to the average twin boundary velocity. The coexistence of these two different statistical behaviors reflects the complex nature of twin boundary motion and suggests the possibility that the transformation proceeds through physical sub-processes that are close to criticality alongside other processes that are not.