Triple Junction Motion Research Articles

A microstructure evolution model including dislocation plasticity

ABSTRACTWe present a stochastic microstructure evolution model applicable to grain growth and its recent extensions, in particular relative to dislocation plasticity. The model is implemented by means of numerical simulations based on the velocity Monte Carlo algorithm. It describes the evolution of a two-dimensional microstructure by tracking the motion of triple junctions, i.e. the vertices where three grain boundaries meet. Grain boundaries can be modeled as straight or curved segments; the misorientation dependence of both grain-boundary energies and mobilities can be included, as well as grain rotation. We show simple examples of normal, abnormal and oriented grain growth. The model is already capable of dealing with a two-phase (liquid-solid) system, to simulate both grain growth and grain dissolution in the liquid. Finally, we report preliminary results of a recent extension of the model to include mechanical deformation from dislocation plasticity.

The Effect of Triple Junctions on Grain Boundary Motion and Grain Microstructure Evolution

The theory of steady state motion of grain boundary sytems with triple junctions and the main features of such systems are considered. A special technique of in-situ observations and recording of triple junction motion is introduced, and the results of experimental measurements on Zn tricrystals are discussed. It is shown, in particular, that the described method makes it possible to measure the triple junction mobility. It was found that the measured shape of a moving half-loop with a triple junction agrees with theoretical predictions. A transition from triple junction kinetics to grain boundary kinetics was observed. This means that triple junctions can drag boundary motion. It is demonstrated that the microstructural (granular) evolution is slowed down by triple junction drag for any n-sided grain. The second consequence pertains to six-sided grains. For a boundary system with dragging triple junctions there is no unique dividing line between vanishing and growing grains with respect to their topological class anymore, like n = 6 in the Von Neumann-Mullins relation.

A Variational Approach to Crystalline Triple-Junction Motion

A variational description is given for curves with triple junctions for the motion with normal velocity v=M(C+κΦ), where κΦ stands for the crystalline curvature as determined by the curves and by the crystalline (polygonal Wulff shape) surface free energy functions Φ for each interface, C is constant on each interface, and M is a compatible normal-dependent mobility function for each interface. This variational formulation is based on the idea that the motion should be gradient flow, in the L2 inner product, for the sum of the surface free energy and the bulk free energy. If the surface free energy functions Φ are identically zero, the motion is that given by Taylor (1995). If the surface free energy functions are positive and crystalline, then the motion is that given by Taylor (1993). Finally, if the surface free energy functions are written as Φ=eΦ0, then the limiting motion as e↓0 is in general different from the motion for e=0 [and hence different from that given by Taylor (1993); the limiting motion is presumably that given by Reitich and Mete Soner (1996)].

The Role of the Triple Junctions During Grain Growth

Experimental data on triple junction (TJ) motion in Al and Zn are presented. There is experimental evidence that during TJ motion the pre-exponential factor changes linearly with activation enthalpy. The TJs impact on the kinetics of microstructure evolution during grain growth is outlined.

Grain Structure Evolution in 1-d Rods and 2-d Strips of Polycrystalline Aluminium

The paper presents data on the motion of grain boundary system with individual triple junction in aluminium and data on the grain structure evolution in one-dimensional (1-D) rods (bamboo structure without triple junctions) and two-dimensional (2-D) strips (columnar structure) of polycrystalline aluminium. Influence of triple junction motion on the grain growth in polycrystalline aluminium is discussed.

Influence of triple junctions on grain boundary motion

The paper is dedicated to the steady state motion of the grain boundary systems with the triple junctions. The main features of one of the systems where the steady state motion is possible are considered.In the experimental part the special technique of in-situ observations and recording of the triple junction motion and the results of the experiments on the tricrystals of Zn are described comprehensively. It was shown, in particular, that the described method makes it possible to study the motion of a grain boundary system with a triple junction and, what is of importance to measure its mobility. The shape of the moving half-loop in the tricrystal fits the theoretically calculated. The transition from the motion controlled by the triple junction kinetics to the boundary kinetics is observed. By this is meant that the triple junction along with the other structural defects can drag the boundary motion, or, conversely, and their role and properties should be taken into consideration in theories of grain growth.

Grain Boundary Migration in Metals: Recent Developments

Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.

Microstructural characterization of high strength and high conductivity nanocomposite wires

Bulk Cu/Ag multilayered composites with controlled individual layer thickness (h) varying from several hundred micrometers down to 20 nm were fabricated via cross accumulative roll bonding (CARB). The well-defined, continuous Cu/Ag multilayer structure with flat, planar, and sharp interfaces was found to remain stable when the layer thickness was reduced from several hundred micrometers down to 20 nm. A preferential interface character of {110}Cu[111]//{110}Ag[111] was formed when the layer thickness was reduced to 20 nm. Specifically, high strength, high electrical conductivity and excellent thermal stability were obtained simultaneously in bulk Cu/Ag nanolayered composite with h = 20 nm. Ultimate tensile strength of 938.1 MPa was achieved, corresponding to 2.9 times higher than the rule-of-mixtures estimate based on the strength of the heavily deformed Cu and Ag samples with 95% rolling reduction. Furthermore, a high electrical conductivity higher than that of pure copper was obtained, while high hardness (3.74 GPa) was maintained up to an annealing temperature of 500 °C. Nevertheless, degradation of the mechanical hardness and nanolayered structure occurred once the temperature exceeded 500 °C. Two major mechanisms are responsible for driving the onset of the thermal instability in this CARB-processed Cu/Ag nanolayered composites, namely triple junction motion and Rayleigh instability mechanisms.

Evolution of Triple Junctions

THE boundaries between the rigid plates which make up the Earth's crust are recognized as being of three distinct kinds: ridges, from which new crustal material spreads symmetrically ; trenches, at which plates descend back into the mantle ; transform faults, along which plates may slip relative to each other1–5. An important concept is the triple junction6, which is the point at which three plate boundaries meet. Discussions of the development of oceanic and continental crustal features almost inevitably contain considerations of motion, stability and instability of triple junctions. McKenzie and Morgan6 showed there are sixteen possible triple junctions and set out rules for examining their stability. They concluded that fourteen of them were stable under certain conditions. Here I point out that fifteen of the triple junctions are stable in the appropriate conditions.