Eilon Faran

Implications of twinning kinetics on the frequency response in NiMnGa actuators

The explicit kinetic relation for twin wall motion in NiMnGa is used to correlate basic material properties to magneto-mechanical actuation rates in these crystals. In particular, we identify two parameters: the Peierls energy barrier and the twin wall mobility, which directly determine the dynamic response of NiMnGa actuators at frequencies above 10 Hz. Comparison between the kinetics of type I and type II twin walls reveals a correlation between the Peierls energy barrier and the commonly used twinning stress property. However, it is shown that twinning stress dictates twin wall dynamics only at very slow frequencies, typically below 1 Hz.

Experimental study of the reaction zone at boron nitride ceramic-Ti metal interface

Abstract In the present work, solid state interaction between boron nitride (BN) ceramic and Ti metal was studied on BN plates immersed into a loose Ti powder and on BN particles embedded into a dense Ti matrix. The samples were vacuum annealed at 1000–1200°C, and the reaction zones formed at BN/Ti interfaces were characterized employing X-ray diffraction, scanning electron microscopy with electron probe microanalysis, high resolution SEM, and transmission electron microscopy with parallel electron energy loss spectroscopy. Similar but not identical multi-phase reaction zones were formed at both types of interfaces containing Ti borides (TiB and TiB 2 ), Ti nitride (TiN 1− x ) and solid solution of nitrogen in Ti (α-Ti(N)). In both cases, α-Ti(N) constituted the major part of the reaction zone, its grains containing fine Ti 2 N/α-Ti(N) precipitates with the lamellar structure formed during cooling from the annealing temperature. The phase sequences at both BN–Ti powder and BN–Ti matrix interfaces were in a good agreement with the equilibrium B–N–Ti ternary diagram and could be plotted as diffusion paths on the isothermal section of the diagram. The differences between the two reaction zones were explained based on the different conditions of mass transfer at BN interface with a loose Ti powder and with a dense Ti matrix.

Coating of BN via solid state reaction with Ti powder

Abstract Ti-based coatings were grown on boron nitride (BN) using an original Powder Immersion Reaction Assisted Coating (PIRAC) method, based on the interaction between ceramic substrates and metal powders. The microstructure of the coating was investigated employing X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive (EDS), and high resolution SEM (HRSEM). The coating was found to consist of the inner layer of fine Ti borides (TiB 2 , TiB) and the outer layer containing Ti and N. The coating growth was diffusion controlled with the activation energy of 215 kJ/mol.

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

The “jerky” motion of a twin boundary in the ferromagnetic shape memory alloy NiMnGa is studied experimentally and theoretically. We employ a bi-stable chain model in order to interpret macroscopic stress-strain experiments and extract important micro-level properties. The analysis reveals the existence of a periodic barrier for type I twin boundary motion with an average distance of 19 μm and amplitude of 0.16 J/m2. Further, 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.

The exploration of the effect of microstructure on crackling noise systems

A wide variety of physical systems respond to changing external conditions through discrete impulsive events called jerks, typically leading to collective “crackling noise” behaviour. Statistical distributions of jerky events often exhibit a universal scale-invariant power law, regardless of the specific mechanisms that are responsible for crackling noise processes and microstructural features that affect them. Here, we analyse uniaxial compression loading curves of two different physical systems that exhibit jerky behaviour: a martensitic NiMnGa single crystal and a stack of corrugated fiberboards. The jerky response is attributed to a non-uniform twin boundary motion along the NiMnGa crystal and to a local buckling of individual fiberboard layers. In both cases, our analysis reveals that different variables exhibit different statistical distributions. While the velocity of temporal processes within jerky events exhibits scale invariant distribution, the irreversible displacements induced throughout comp...

The effects of temperature on the lattice barrier for twin wall motion

The sideways motion of twin walls in ferroic materials requires overcoming an intrinsic energy barrier that originates from the periodicity of the crystal structure. Here, we measure the temperature dependence of the lattice barrier in a ferromagnetic Ni-Mn-Ga crystal using the pulsed magnetic field method. Our results reveal a monotonic decrease in the lattice barrier with increasing temperature. Yet, the barrier does not vanish as the temperature approaches the temperature of the martensite to austenite transformation. These findings enable the formulation of an analytical expression that correlates the lattice barrier to the physical properties of the twin wall, such as its thickness and the associated transformation strain. The derived relation provides a good quantitative description of the data measured in Ni-Mn-Ga.

The effects of microstructure on crackling noise during martensitic transformation in Cu-Al-Ni

Martensitic phase transformations often exhibit crackling noise response of the emitted energy. This type of behavior implies that the phase transformation proceeds through numerous events that do not exhibit any characteristic scale. On the other hand, the twinned microstructure along the phase boundary exhibits a characteristic size that is expected to affect the propagation of the phase boundary. Here, we present a statistical analysis of jerky events during martensitic transformation, induced by uniaxial compression of a single crystal Cu-Al-Ni. The results indicate that the characteristic length scale of the internal twinned microstructure dictates μm-scale displacement events of the phase boundary. At the same time, each of these μm-scale events proceeds through a multitude of smaller events that span several orders of magnitude and follow a scale-invariant power law distribution. The smaller events are associated with the local nucleation and propagation of twinning interfaces close to the phase bo...

A discrete twin-boundary approach for simulating the magneto-mechanical response of Ni–Mn–Ga

The design and optimization of ferromagnetic shape memory alloys (FSMA)-based devices require quantitative understanding of the dynamics of twin boundaries within these materials. Here, we present a discrete twin boundary modeling approach for simulating the behavior of an FSMA Ni–Mn–Ga crystal under combined magneto-mechanical loading conditions. The model is based on experimentally measured kinetic relations that describe the motion of individual twin boundaries over a wide range of velocities. The resulting calculations capture the dynamic response of Ni–Mn–Ga and reveal the relations between fundamental material parameters and actuation performance at different frequencies of the magnetic field. In particular, we show that at high field rates, the magnitude of the lattice barrier that resists twin boundary motion is the important property that determines the level of actuation strain, while the contribution of twinning stress property is minor. Consequently, type II twin boundaries, whose lattice barrier is smaller compared to type I, are expected to show better actuation performance at high rates, irrespective of the differences in the twinning stress property between the two boundary types. In addition, the simulation enables optimization of the actuation strain of a Ni–Mn–Ga crystal by adjusting the magnitude of the bias mechanical stress, thus providing direct guidelines for the design of actuating devices. Finally, we show that the use of a linear kinetic law for simulating the twinning-based response is inadequate and results in incorrect predictions.

The effects of magnetic and mechanical microstructures on the twinning stress in Ni-Mn-Ga

The ferromagnetic 10M Ni-Mn-Ga alloy exhibits complex magnetic and mechanical microstructures, which are expected to form barriers for motion of macro twin boundaries. Here, the contributions of both microstructures to the magnitude of the twinning stress property are investigated experimentally. A series of uniaxial loading-unloading curves are taken under different orientation angles of a constant magnetic field. The different 180° magnetic domains microstructures that are formed across the twin boundary in each case are visualised using a magneto optical film. Analysis of the different loading curves and the corresponding magnetic microstructures show that the latter does not contribute to the barriers for twin boundary motion. In accordance, the internal resisting stress for twin boundary motion under any magnetic field can be taken as the twinning stress measured in the absence of an external field. In addition, a statistical analysis of the fine features in the loading profiles reveals that the barr...

Microstructural Effects During Crackling Noise Phenomena

Crackling noise phenomena typically exhibit scale-free statistical distributions (e.g., power law) of the measured variables. Such a universal behavior reveals little information regarding the physical mechanisms and microstructures that are either responsible and/or affect crackling behavior. Here, we address this issue and show three physical systems in which the distributions of certain variables are centered around a most probable value, which is related to a characteristic size of the internal microstructure. These variables represent microstructural-related events. At the same time, each microstructural-related event proceeds through a multitude of smaller mesoscopic events that span several orders of magnitude. Statistical analyses of other variables, which are associated with the mesoscopic events, follow a scale-invariant power law distribution. The origins for the co-existence of events at different scales and their different statistical distributions are discussed in light of the physical characteristics of the investigated systems.