We present a phase field model of solidification which includes the effects of the crystalline orientation in the solid phase. This model describes grain boundaries as well as solid-liquid boundaries within a unified framework. With an appropriate choice of coupling of the phase field variable to the gradient of the crystalline orientation variable in the free energy, we find that high angle boundaries undergo a premelting transition. As the melting temperature is approached from below, low angle grain boundaries remain narrow. The width of the liquid layer at high angle grain boundaries diverges logarithmically.

Fully amorphous ceramics in the system silicon-carbon-nitrogen were produced with the polymer precursor route using the commercially available polysilazane CerasetTM. Besides their high temperature thermal stability, these ceramics show excellent high temperature creep resistance. Not many investigations have been dedicated to the fracture mechanics of these materials. This paper provides data on toughness measurements utilizing bulk and indentation techniques. The double cantilever beam method (DCB) was used to study crack propagation. To determine the intrinsic toughness, the crack opening displacements (COD) of indentation cracks were determined.

The reaction of grain boundaries to a wide spectrum of forces is reviewed. Curvature, volume energy and mechanical forces are considered. The boundary mobility is strongly dependent on misorientation, which is attributed to both grain boundary structure and segregation. In magnetically anisotropic materials grain boundaries can be moved by magnetic forces. For the first time a directionality of boundary mobility isreported. Flat boundaries can also be moved by mechanical forces, which sheds new light on microstructure evolution during elevated temperature deformation. Curvature driven and mechanically moved boundaries can behave differently. A sharp transition between the small and large angle boundary regime is observed. It is shown that grain boundary triple junctions have a finite mobility and thus, may have a serious impact on grain growth in fine grained materials. The various dependencies can be utilized to influence grain boundary motionand thus, microstructure evolution during recrystallization and grain growth.

It has been known for many years that the presence of a distribution of domain boundaries in crystals with ordered superlattice structures can affect both the separation of glissile superpartial dislocations and the yield stress. Based on the details of the interactions between the domain boundaries and glissile extended superdislocations, we incorporate the effects of the thin band of perfect crystal that arises whencoupled superpartial dislocations straddle a domain boundary, and the additional domain boundary area createdonce the whole superdislocation has passed through. If these effects are included then, in an ordered phase containing many domains, we would expect the equilibrium separation of the coupled superpartial dislocations to vary more significantly with domain size than is predicted by previous models. These effects also change the way in which yield stress is predicted to vary with domain size. This variation gives a much better match to published experimental data than existing models.

The influence of crystal interfaces related to stacking faults on the properties of >110< superdislocations and hence on the temperature dependences of the yield stress in intermetallic compounds is investigated. This is achieved by a combination of the ab-initio density functional electron theory (which describes the electronic scale) with the generalized Peierls-Nabarro model (which describes the atomistic scale). For Pt3Al and doped Al3Ti our data do not support the hypothesis that the strong increase of the yield stress with decreasing T at low T results from sessile SISF-bound dislocation dissociations. Alternative explanations are suggested. For Ni3Al our results do not rule out the idea that Kear-Wilsdorf locks are responsible for the experimentally observed anomalous temperature dependence of the yield stress.

A method for construction of “processing windows” to avoid negative strain rate sensitivity and associated serrated flow in some aluminum alloys is described. The method is based on the amplitude Ginzburg-Landau (GL) equations and analysis of bifurcation diagrams. The mathematical technique developed in the present work was applied to a specific aluminum alloy, Al-0.4%Mg-0.2%Si considered earlier in the literature [1-3], and yielded good results in terms of predicting the negative strain rate sensitivity regions in the “strain rate “temperature” parameter space. Using the GL-analysis it was demonstrated that even though the instability area is located in the region of intermediate strain rates, a qualitative difference exists between the areas of (relatively) fast and (relatively) slow strain rates. In the first case the dynamic behavior of the system is supercritical, in the second case it is subcritical. The second case is highly undesirable because it causes a sudden onset of stable stress serrations that are difficult to suppress, while in the first case the development of instability is gradual and, consequently, more easily controllable.

Defects produced in massively transformed L10-ordered τ-MnAl have been characterized by detailed TEM studies. The defect population in massive τ-MnAl comprises arrays of overlapping stacking faults, {111}-conjugated microtwins, thermal antiphase boundaries and dislocations. The genesis of these defects has been attributed to atomic attachment faulting on {111}- and {020}-type facets of the essentially incoherent growth interface between the parent and product phases. The features of the defect genesis in τ-MnAl are discussed with respect to the role of atomic level processes at solid-state transformation interfaces in general and growth interfaces in massively transforming materials systems in particular.

We are studying heterophase interfaces between molybdenum nitride precipitates and an α-iron matrix on a nanometer scale by electron and atom-probe field-ion microscopies. Fe-2 at.% Mo-0.5 at.% Sn and Fe-2 at.% Mo-0.4 at.% Sb are internally nitrided by annealing in an NH3/2 atmosphere. Nitridation at 550°C produces coherent thin platelet-shaped molybdenum nitride precipitates, while at 600°C semicoherent coarse plates and spheres also form. Segrega-tion of Sn and Sb is observed at the heterophase interfaces of only the semicoherent precipitates. The loss of coherency, that is the formation of misfit dislocations, is essential for substantial interfacial solute segregation to occur at heterophase interfaces.

A phase-field model for predicting the domain structure evolution in constrained ferroelectric thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. In particular, the model is applied to the domain structure evolution during a cubic→tetragonal proper ferro- electric phase transition. The effect of substrate constraint on the volume fractions of domain variants, domain-wall orientations, and domain shapes is studied. It is shown that the predicted results agree very well with existing experimental observations in ferroelectric thin films.

The simulation of curvature driven growth in grain boundary systems is becoming an important tool in understanding the behavior of microstructure evolution and there is much distinguished work in this subject. Here we address the mesoscale simulation of large systems of grain boundaries subject to the Mullins equation of curvature driven growth with the Herring force balance equation imposed at triple junctions. We discuss several novel features of our approach which we anticipate will render it a flexible, scalable, and robust tool to aid in microstructural prediction. What is the result of the simulation? We discuss what such a simulation is capable of predicting, taking as a prototype the histogram of relative area population as it changes through the simulation. We do not use this data to seek the best distribution, like Hillert, Rayleigh, or lognormal. Instead we treat the set of distributions as the solution of an inverse problem for a time varying function and determine the equation they satisfy. This results in a coarse graining of the complex simulation to simpler system governed by a Fokker-Planck Equation. Even so, fundamental questions concerning the predictability of simulations of large metastable systems arise from these considerations.

Transmission electron microscopy was employed to study the microstructure of GaAs films grown by molecular-beam epitaxy at low temperature and delta-doped with Sb. Thus obtained material contained 0.5 at.% of excess arsenic that precipitates upon post-growth anneals. The Sb doping was found to strongly affect the microstructure of arsenic clusters and their ripening rate upon annealing. Segregation of Sb impurities in the As clusters was revealed. In contrast to the well known pure As clusters, the As-Sb clusters induced strong local deformations in the surrounding GaAs matrix. Relaxation of these deformations resulted in formation of dislocation loops, which was studied both experimentally and theoretically.

Grain boundary and crystallographic orientation information of an Al-foil with a columnar grain structure is characterized by Electron Backscattered Diffraction (EBSD) technique. The starting microstructure and grain boundary properties are implemented as an input for the three- dimensional grain growth simulation. In the computational model, minimization of the interface energy is the driving force for the grain boundary motion. The computed evolved microstructure is compared with the final experimental microstructure, after annealing at 550 °C. Good agreement is observed between the experimentally obtained microstructure and the simulated microstructure. The constitutive description of the grain boundary properties was based on a 1- parameter characterization of the variation in mobility with misorientation angle.

We present a general formalism for incorporating dislocations in Phas Field methods. This formalism is based on the elastic equiva;ence betweem a dislocation loop and a platelet inclusion of specific stress-free strain related to the loop Burgers vector and normal. Dislocations are thus treated as platelet inclusions and may be coupled dynamically to any other field such as a concentration field. The method is illustrated through the simulation of a Frank-Read source and of the shrinkage of a loop in presence of a concentration field.

The effect of dislocations on deuteride formation in the Pd-D system has been investigated with small-angle neutron scattering (SANS). Deuteride precipitation morphologies in well-annealed single crystal Pd and deformed single crystal Pd have been determined with SANS. High densities of dislocations were introduced into the deformed sample by cycling across the deuteride miscibility gap. The characteristic response from small, 20-30 Angstrom thick, plate-like particles has been observed in the SANS measurement of both sample types. We believe these small plates act as precursors to the formation of much larger, tens-of-microns thick incoherent plate-like particles. These much larger deuteride plates have a preferred habit plane that depends on sample type; (100) for the well-annealed matrix and (110) for the deformed matrix.

Laminated composites with polycrystalline layers typically break down at high temperatures through grain boundary grooving and the pinch-off of individual layers. Such materials, when exposed to high temperatures, develop grooves where grain boundaries meet the interfaces between layers. The depths of the grooves are controlled by the ratios of grain boundary and interfacial free energies, γgb/γint. Depending on the dimensions of the grains, these grooves can extend through the entire layer, causing pinch-off at the grain boundary. This pinch-off destroys the layering and eventually leads to a gross coarsening of the microstructure. Because microstructural stability is critical to performance for most applications, the ability to understand and predict the stability of microlaminates is a necessary tool. An existing model of this capillarity-driven breakdown requires the interfacial free energies, γgb and γint, as input parameters. Both biaxial and uniaxial zero creep tests have been used in conjunction with transmission electron microscopy to measure these interfacial energies in Ag/Ni and Nb/Nb5Si3 microlaminates.

The partially filled skutterudite structure is a candidate thermoelectric material with the capacity for phonon scattering by the decoupled rattling of filling ions. In this transmission electron microscopy investigation of a 1.6%Ce, 1.6%Ni, 4.9%Ge, 22.8%Co, and 69.1%Sb alloy, the structure is confirmed to be of a partially-filled skutterudite, but with a number of defects. Isolated dislocation dipoles, arrays of dislocation dipoles, and a subgrain boundary are observed and characterized. An atomic structure of a dislocation dipole is also proposed, and the effect of such dislocations on thermoelectric properties is discussed.

Diffusion and phase formation were monitored in solder joints consisting of flip chips with Ni(V) under-bump metallizations bumped with Ag3.8Cu1.85Sn94.35 (atomic percentage) solder reflowed on printed circuit boards with Cu/Ni/Au metallizations. A ternary alloy, (CuNi)6Sn5, was observed to form during reflow at solder/Ni interfaces in these Ag3.8Cu1.85Sn94.35/Au/Ni solder joints. After extended thermal aging at 150 oC, a second ternary compound, (CuNi)3Sn4 forms at the Ni/(CuNi)6Sn5 interface. The growth of these alloys depletes the solder of essentially all Cu, changing its mechanical properties and melting temperature.

We 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.

Fully relaxed SiC grain boundaries were produced by the sublimation re-crystallization method. Several kinds of grain boundaries of bi- and tri-crystals were investigated applying an atomic resolution high-voltage transmission electron microscopy (ARHVTEM). The rotation axis and the rotation angle were common tofrequently observed grain boundaries, which were, respectively, 70.5 degrees and <1120>. But boundary planes differed from each other. It was shown that the atomic bonding direction was continuous across the stable grain boundary.

In situ straining experiments in a high-voltage electron microscope allow the observation of the dynamic behaviour of individual dislocations. Such experiments have been performed on the intermetallic alloys NiAl, NiAl containing 0.2 at% Ta, α-TiAl, and MoSi2, and the oxide dispersion strengthened (ODS) alloys INCOLOY MA956 and INCONEL MA754 in a wide range of temperatures. There are many similarities in the dynamic behaviour of dislocations in the different materials. In the intermetallic alloys, a transition occurs between low temperature mechanisms and a viscous motion in the temperature range of the flow stress anomaly. The viscous motion at high temperatures can be explained by diffusion processes in the dislocation cores, whichcan be described by the theory of the Cottrell effect. The diffusing species can be quite different, alloying components or intrinsic point defects like vacancies and antisite defects existing in the lattice or only in the dislocation cores. If the dislocations are straight and crystallographically oriented during their motion, they may be dissociated and move by a succession of glide and conservative climb between the partial dislocations. In the ODS alloys, the dislocations move between the oxide particles again in a viscous way. The relation between the dislocation dynamics and the strain rate sensitivity of the flow stress is discussed for thedifferent materials.