Interphase Defects, Structures, and Phase Stability

Interface is defined as a common boundary of two spaces or phases. In solid-state materials, this usually means a two-dimensional defect that separates two regions of otherwise perfect bulk, and sometimes it goes to three dimensional, e.g., in the case of diffuse interface or disconnections. In general, the different bulk regions surrounding the interface can be different materials or crystal phases, so these interface boundaries can be called interphase boundaries. Solid-state materials with abundant internal interfaces may possess unusual physical properties or functionalities. This promising opportunity motivates many research studies, both experimental and theoretical in nature, geared toward incorporating knowledge of atomic-level structure and properties of interfaces into mechanistically informed interface design of materials. The unusual physical properties caused by interfaces include, for example, extraordinary radiation damage resistance and mechanical behaviors. The physical basis behind these extraordinary performances of materials is due to the fact that the interfaces can interact with point defects and dislocations in a strong manner. Understanding of the detailed mechanisms, including thermodynamic and kinetic factors at atomistic levels, is thus very important. The characterization of interface structures is a foundation step in the understanding. Additionally, how the stability of phases maybe affected by external driving forces is also of high importance because it is desired to keep the phases stable so the performance of materials maybe sustainable. Five articles are included in this issue representing some of the recent research efforts in understanding interphase defects, structures, and phase stabilities in different perspectives. The first article by X.M. Bai and B.P. Uberuaga gives a review of atomistic simulation works that examined the influence of grain boundaries, which are of the homophase interface boundary type, on the defects production during collision cascades. Collision cascade is a set of atomic collision events induced by energetic particles in material when the material is under irradiated conditions. Radiation damage is of great concern for materials in nuclear energy applications and is connected directly to the defect production in collision cascade processes. Atomistic simulation studies demonstrated that grain boundaries absorb more interstitials than vacancies during collision cascades. The absorbed interstitials can also spontaneously emit from interfaces to annihilate vacancies in the nearby bulk. The grain boundaries also disrupt the channeling effect in collision cascades, thus reducing cascade sizes. As a result, the number of defects produced is also lowered. Several different materials including face-centered cubic (fcc) Ag, Ni, Cu, bodycentered cubic (bcc) Fe, UO2, TiO2, and SiC were examined. And it was noted that the influences of grain boundaries are also sensitive to the type of materials. These atomistic simulation results provide an improved understanding of how interfaces may influence radiation damage evolution in materials. The second article by K. Kolluri et al. focuses on a research summary of the behavior of vacancies and interstitials at heterophase fcc-bcc semicoherent interfaces. Experimentally, nanolayered fcc-bcc composites such as Cu-Nb exhibit significantly enhanced radiation-damage tolerance as compared to monolithic metals. Knowledge of defects behavior at interfaces is essential for the understanding of such effects as well as other material properties such as creep and swelling. Using atomic-scale simulations, it is found that the formation, migration, and clustering of interfacial vacancies and interstitials at Cu-Nb KS interfaces is determined by the structure of these interfaces, which can be described with ordered arrays of misfit dislocations. In addition, the migration of defects at the interfaces is through a mechanism that depends on interfacial misfit dislocation properties and can be described using dislocation mechanics. To systematically test the effect of bonding and lattice parameter on point defect behavior, potentials were ‘‘tuned’’ by changing either the dilute of heats of mixing between Cu and Nb or the Xiang-Yang (Ben) Liu is the guest editor for the Chemistry and Physics of Materials Committee of the TMS Structural Materials Division, and coordinated the topic Interphase Defects, Structures, and Phase Stability in this issue. JOM, Vol. 65, No. 3, 2013