טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZreihan Noam
SubjectBarriers, Mechanisms of Motion, and Kinetic Laws for Twin
Boundary Dynamics in Ferroic Materials
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Doron Shilo
Dr. Eilon Faran
Full Thesis textFull thesis text - English Version


Abstract

Ferroic materials, including ferroelastics, ferroelectrics, and ferromagnetics typically exhibit a twinned microstructure, in which a grain or a single crystal is divided into different twin variants or domains. When subjected to an external driving force (i.e. magnetic\electric field, mechanical stress) one twin variant may expand at the cost of the other in a process called twinning reorientation. The twinning transformation is associated with a significant strain change, which is used in a variety of application such actuation, sensing. The basic mechanism responsible for the twinning reorientation is the propagation of individual twin boundaries, which are planar defects separating adjacent twins. The motion of individual twin boundary is controlled by several barriers and may exhibit different kinetic behaviors.

In the current work, I will present new aspects of twin boundary dynamics in a typical ferromagnetic shape memory alloy Ni-Mn-Ga.  The motion of a twin boundary is studied under various loading conditions, rates and at different temperatures. Velocity values of individual boundaries are extracted from high-resolution force measurements taken during displacement-driven mechanical tests, as well as from force-driven pulsed magnetic tests, and cover an overall range of six orders of magnitude. In addition, acoustic emission (AE) generated by the motion of twin boundaries is measured during mechanical tests. Temperetaure dependent velcoity meausrements provide a direct correaltion between the lattice barrier for twin boundary motion and the twinning strain. A statistical analysis of the the measured velcoties revealedthe coexistence of scale-invariant power-law distributions and a normal distribution at the same physical processes. We suggest that this coexistence reflects the complex nature of twin boundary motion. An analysis of the relationship between stress drops and dominant AE signals, on the level of individual events, revealed a good correlation between the stress drop and a lower bound for the energy of the AE signal.  We suggest that the lower bound AE energy is generated by the macroscopic stress changes. An additional contribution to the AE energy is generated by local microscopic events, such as the overcoming of energy barriers for twin boundary motion.

Those results will provide new insights on mechanism and dynamic behaviour of more complex physical phenomena such as geophysics, material mechanics.