|Ph.D Student||Faran Eilon|
|Subject||Dynamics of Twinning Processes in Active Materials|
|Department||Department of Mechanical Engineering||Supervisor||Professor Doron Shilo|
|Full Thesis text|
Twinning is a shear dominated material transition that serves as the basic mechanism in actuation of active materials such as shape memory alloys and ferroelectrics. The kinetic relations of twinning sub-processes are essential for understanding the dynamics of twinning transformation, and for modeling the overall response of a macroscopic sample. In this work we employ combined theoretical and experimental approaches for studying the twinning processes at the microscopic scale of individual twins.
First, we present a theoretical description of the main twinning sub-processes and show that they are all governed by several atomistic and mesoscale properties of the twin wall. Sidewise twin wall motion is identified as the rate limiting sub-process of the overall twinning transformation, and its kinetics is analyzed in detail. Our theoretical analysis predicts that sidewise twin motion follows different kinetics at different ranges of the driving force.
Next, we present a unique experimental study of the dynamics of twinning sub-processes in two prototypical active material systems: ferromagnetic shape memory alloy (FSMA) NiMnGa and ferroelectric BaTiO3. The experimental investigation enables the detection and evaluation of all twinning sub-process in the two representative material systems. In particular, sidewise velocities of twin walls are calculated based on the distance a wall passed during the application of a tunable magnetic / electric pulse. The measured velocities are plotted as a function of the driving force to obtain an experimental kinetic relation.
In the case of sidewise twin wall motion in NiMnGa, the formulated kinetic relations are validated experimentally, leading to quantitative extraction of all governing material parameters. In addition, the explicit kinetic relations are used to correlate basic material properties to the frequency response of NiMnGa actuators. It is shown that in the frequency range relevant for applications, the response of the actuator is determined by properties evaluated in our study rather than by the commonly used quasi-static twinning stress. These new observations are extremely important for the development of FSMA crystals for fast magnetically induced actuation applications.
In the case of ferroelectric BaTiO3, our results indicate that sidewise twin wall motion is controlled by the presence of defects. In addition, intersonic forward growth of needle twins with a velocity exceeding the shear wave speed is detected, and its observation is correlated to similar phenomena in crystalline solids, and in particular to super-sonic dislocation motion.