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Application of a bi-stable chain model for the analysis of jerky twin boundary motion in NiMnGa

Itamar Benichou (Technion - IIT), Eilon Faran (Technion - IIT), Doron Shilo (Technion - IIT), Sefi Givli (Technion - IIT)

Mechanics of Phase Transforming and Multifunctional Materials

Wed 9:00 - 10:30

CIT 219

Discrete phase transformations can be recognized by a typical saw-tooth response under external activation (e.g. the stress-strain response in a hard device). In most cases discrete transformations take place at the nano scale. Classical examples are the discrete phase transformations exhibited by biological macro-molecules such in overstretching of DNA, opening RNA hairpins during protein production and in stretching titin. Titin is a structural protein in skeletal muscles that works as a nano shock absorber. Discrete phase transformations can also be found in micro specimens of materials, such as shape memory allows (SMA) and single-crystal metals. Interestingly, we observed a similar pattern in relatively large specimens size, up to few millimeters long, of single-crystal, ferromagnetic shape memory alloys (FSMA), NiMuGa. In the current work we employ a novel approach of a bi-stable chain model in order to interpret macroscopic stress-strain experiments and extract important micro-level properties. Our approach considers the collective behavior of an ensemble of bi-stable elements. The model proposes a simple representation of the bi-stable elements that consists from a linear elastic spring connected in series with an ideal snapping element. This method allows us to separate between the bulk and the tween boundary properties so we can learn about the nano-scale parameters of the material from a simple macro-scale experiment. The analysis reveals the existence of a periodic barrier for type I twin boundary motion with an average distance of 19 micro-meter and amplitude of 0.16 J/m^2. The model suggests a narrow localization of barrier potential traps which explains the “stick-slip” like motion of the twin boundary. Furthermore, we show that the macroscopic mechanical response depends on the length of the crystal and predict a significant decrease of the hysteresis in sub-mm length specimens.