Propagation Of Ledges Research Articles

Modeling stress–strain response of shape memory alloys during reorientation of self-accommodated martensites with different morphologies

The shape memory effect observed in many alloys arises due to stress induced transformation between variants of the martensitic phase. It is difficult to study this process in detail using continuum approaches and particle based methods are eminently more suitable. In this work, we study detwinning, which is the transformation between martensite variants due to applied stress using a novel discrete particle model. The approach uses a novel multibody interparticle interaction defined directly using a non-convex free energy potential pertinent to such material behavior. This model is able to describe simultaneously occurring multiple microscale events during the detwinning process: nucleation and propagation of ledges along twin boundary. Due to these underlying microscale events the plateau region of stress–strain response shows a jerky nature. The effect of temperature and morphology on the stress–strain behavior of the self-accommodated martensite microstructure is studied in detail. From the simulations, we identify the morphological features affecting the transformation stress for detwinning. The critical parameters are found to be the length and number of twin and macro-twin boundaries and the number of mobile ledges.

Recordings and Analysis of Atomic Ledge and Dislocation Movements in InGaAs to Nickelide Nanowire Phase Transformation.

The formation of low resistance and self-aligned contacts with thermally stable alloyed phases is a prerequisite for realizing reliable functionality in ultrascaled semiconductor transistors. Detailed structural analysis of the phase transformation accompanying contact alloying can facilitate contact engineering as transistor channels approach a few atoms across. Original in situ heating transmission electron microscopy studies are carried out to record and analyze the atomic scale dynamics of contact alloy formation between Ni and In0.53 Ga0.47 As nanowire channels. It is observed that the nickelide reacts on the In0.53 Ga0.47 As (111) || Ni2 In0.53 Ga0.47 As (0001) interface with atomic ledge propagation along the Ni2 In0.53 Ga0.47 As [101¯0] direction. Ledges nucleate as a train of strained single-bilayers and propagate in-plane as double-bilayers that are associated with a misfit dislocation of b→=2c3[0001]. The atomic structure is reconstructed to explain this phase transformation that involves collective gliding of three Shockley partials in In0.53 Ga0.47 As lattice to cancel out shear stress and the formation of misfit dislocations to compensate the large lattice mismatch in the newly formed nickelide phase and the In0.53 Ga0.47 As layers. This work demonstrates the applicability of interfacial disconnection (ledge + dislocation) theory in a nanowire channel during thermally induced phase transformation that is typical in metal/III-V semiconductor reactions.

In situ TEM investigation on the precipitation behavior of μ phase in Ni-base single crystal superalloys

The precipitation behavior of μ phase in Ni-base single crystal superalloys was investigated by in situ transmission electron microscopy (TEM). A layer-by-layer growth process with a ledge propagation mechanism was first observed during in situ precipitation. Three types of μ phase with different morphologies were found, which grow along [001]μ with (001)μ planar defects, [-111]μ with (1–12)μ planar defects, as well as both directions with mixed planar defects. High-resolution TEM image and established atomic models reveal a basic growth mechanism of μ phase by stacking on (001)μ plane and randomly forming coherent planar defects, while the nucleation of incoherent (1–12)μ planar defects at the early stage of precipitation plays an important role in affecting the basic growth mechanism. The frequent faults during the stacking process of the sub-unit layers within μ lattice should be responsible for the defect formation.

The Use of Size Distributions in Determining Growth Mechanisms: The Growth of Grain Boundary Precipitates in Cobalt-20 Iron

Accurate prediction of microstructural stability in an alloy depends not only on a sound knowledge of the thermodynamics of the system but also of the kinetics of the phase changes involved. Conventionally, precipitate growth mechanisms have been inferred from the variation with aging time of various single parameters such as the mean, mode or maximum of the precipitate size distribution, which has then been compared to theoretical models of growth of an individual precipitate. In the present study, the development, with aging time at 1003 K (730 °C), of the size and shape distributions of grain boundary precipitates in Co-20Fe has been examined to determine the rate-controlling processes, and the conclusions compared to those from conventional analysis. The growth of the precipitates was well described by the grain boundary-dependent collector plate mechanism of Brailsford and Aaron. As the precipitates grew, low-energy facets were formed, which could move only by the propagation of ledges, and thickening was inhibited. The precipitates’ diffusion fields in the grain boundary overlapped and the size distributions of the longest aged specimens showed that local coarsening occurred under partial interface control.

The Bainite Controversy

There has long existed a controversy regarding the formation of the bainite phase in steels. Nonetheless, following the clear identification of bainite in 1930, several characteristic features associated with the bainite transformation became well established. These features include: the classification into upper and lower bainite; the existence of a bainite start temperature; and the incomplete reaction phenomenon. Accordingly, two competing theories have been developed to explain the bainite transformation. However, no overriding consensus has been reached as to which is correct. The first theory invokes a diffusion controlled transformation, describing the growth of bainite via the propagation of ledges – a series of steps on the transformation interface. The second interpretation favours a displacive, diffusionless transformation. Bainite growth occurs via the autocatalytic nucleation and growth of successive subunits or platelets. Over time, a wealth of techniques has been implemented in order to increase the understanding of the mechanism by which bainite forms. From the early approaches that involved thermodynamic and kinetic considerations; through detailed work on the crystallography of the transformation; to studies involving advanced characterisation techniques that focused on the distribution of atoms, etc. Some of the theories have been progressively adapted to new evidence and new concepts as they emerge. For example, the concepts of the T0 curve and of paraequilibrium transformation. This work presents a chronological summary of the two theories of the bainite transformation, charting their progression and noting the variety of evidence both in support of and in contradiction to each viewpoint. The bainite controversy is still debated by steel metallurgists today, although it has mostly been reduced to the question of whether or not bainitic ferrite initially forms with a supersaturation of carbon. This review gathers and evaluates some of the accumulated work in the hope that this question may finally be answered.

Structure, Growth Kinetics, and Ledge Flow during Vapor−Solid−Solid Growth of Copper-Catalyzed Silicon Nanowires

We use real-time observations of the growth of copper-catalyzed silicon nanowires to determine the nanowire growth mechanism directly and to quantify the growth kinetics of individual wires. Nanowires were grown in a transmission electron microscope using chemical vapor deposition on a copper-coated Si substrate. We show that the initial reaction is the formation of a silicide, eta'-Cu(3)Si, and that this solid silicide remains on the wire tips during growth so that growth is by the vapor-solid-solid mechanism. Individual wire directions and growth rates are related to the details of orientation relation and catalyst shape, leading to a rich morphology compared to vapor-liquid-solid grown nanowires. Furthermore, growth occurs by ledge propagation at the silicide/silicon interface, and the ledge propagation kinetics suggest that the solubility of precursor atoms in the catalyst is small, which is relevant to the fabrication of abrupt heterojunctions in nanowires.

Ledge-flow-controlled catalyst interface dynamics during Si nanowire growth

Self-assembled nanowires offer the prospect of accurate and scalable device engineering at an atomistic scale for applications in electronics, photonics and biology. However, deterministic nanowire growth and the control of dopant profiles and heterostructures are limited by an incomplete understanding of the role of commonly used catalysts and specifically of their interface dynamics. Although catalytic chemical vapour deposition of nanowires below the eutectic temperature has been demonstrated in many semiconductor-catalyst systems, growth from solid catalysts is still disputed and the overall mechanism is largely unresolved. Here, we present a video-rate environmental transmission electron microscopy study of Si nanowire formation from Pd silicide crystals under disilane exposure. A Si crystal nucleus forms by phase separation, as observed for the liquid Au-Si system, which we use as a comparative benchmark. The dominant coherent Pd silicide/Si growth interface subsequently advances by lateral propagation of ledges, driven by catalytic dissociation of disilane and coupled Pd and Si diffusion. Our results establish an atomistic framework for nanowire assembly from solid catalysts, relevant also to their contact formation.

An atomistic investigation of the kinetics of detwinning

This paper presents a “first principles” atomistic study of the dynamics of detwinning in a shape-memory alloy. In order to describe the macroscopic motion of twin boundaries, the continuum theory of twinning must be provided with a “kinetic relation”, i.e. a relation between the driving force and the propagation speed. This kinetic relation is a macroscopic characterization of the underlying atomistic processes. The goal of the present atomistic study is to provide the continuum theory with this kinetic relation by extracting the essential macroscopic features of the dynamics of the atoms. It also aims to elucidate the mechanism underlying the process of detwinning. The material studied is stoichiometric nickel–manganese, and interatomic interactions are described using three physically motivated Lennard-Jones potentials. The effect of temperature and shear stress on detwinning — specifically on the rate of transformation from one variant of martensite to the other — is examined using molecular dynamics. An explicit formula for this (kinetic) relation is obtained by fitting an analytic expression to the simulation results. The numerical experiments also verify that transverse ledge propagation is the mechanism underlying twin-boundary motion. All calculations are carried out in a two-dimensional setting.

Dynamics of grain boundary motion at the atomic level

AbstractGrain boundaries (GBs) in polycrystalline materials play a pivotal role in controlling their mechanical and physical behavior. High-resolution electron microscopy (HREM) was used to study thermally activated GB migration in thin films of Al and Au at elevated temperatures (T > 0.5 Tm). Grain boundary engineering via epitaxial templating allowed the manufacture of well-defined grain and interfacial geometries. These techniques enabled the observation of tilt, but also twist and general GBs at atomic resolution in-situ at high temperatures. Surface-energy driven GB migration occurred in general GBs, whereas tilt GB motion was curvature driven. Digital analysis of HREM video recordings have given considerable insight in the dynamics of GB motion at elevated temperatures. It is not surprising that the complex and diverse migration mechanisms depend on GB geometry as well as on interatomic interactions. The results provide, among others, direct evidence for collective effects by concerted atomic shuffles, ledge propagation in (113) symmetric tilt GBs, and motions of triple junctions at elevated temperatures.

Optimization of TEm specimen preparation by double-sided ion beam thinning under low angles

We have developed special specimen holders for double-sided low angle ion beam thinning and have systematically studied possible improvements of the resulting TEM specimens with respect to surface roughness, radiation-induced damage and contamination. The specimen holders are entirely made from graphite and were built for two different ion beam thinners. The design makes it possible to work at angles of incidence of 6° and lower and to precisely position the centre of rotation. The resulting specimen geometry was characterized with respect to a number of critical parameters. The surface topography was measured with atomic force microscopy, defect-induced localized strain was revealed by HRTEM and the thicknesses of contamination layers, carbon coating and amorphous oxide layers were quantitatively determined by electron spectroscopic imaging in an energy filtering TEM. It was found that the surface roughness not only decreases with decreasing angle of incidence, but also when the ion energy is lowered. In the lowangle regime, the thinning mechanism is based on the formation and propagation of ledges. Simultaneous ion bombardment from both sides has been found to be one of the most critical parameters.