Complexion time-temperature-transformation (TTT) diagrams: Opportunities and challenges

Abstract

a b s t r a c tGrain boundaries and other interfaces can undergo complexion transitions from one thermodynamic state to another, resulting in discontinuous changes in interface properties such as diffusivity, mobility, and cohesive strength. The kinetics of such complexion transitions has been largely overlooked until recently. Just as with bulk phase transformations, complexion transition kinetics can be represented on time-temperature-transformation (TTT) diagrams. An experimental complexion TTT diagram is presented here for polycrystalline Eu-doped spinel annealed at 1400-1800°C. This material developed a microstructure with a bimodal grain size distribution, indicating that a complexion transition occurs within this temperature range. The time and temperature dependence of this complexion transition was analyzed and used to produce a grain-boundary complexion TTT diagram for this system. Complexion TTT diagrams have the potential to be remarkably useful tools for manipulating the properties of internal interfaces in polycrystalline metals and ceramics. The development of experimental complexion TTT diagrams is likely to have an important impact on the field of grain-boundary engineering, and hence the development of these experimental diagrams should be an intense area of focus in the coming years.

Review of grain boundary complexion engineering: Know your boundaries

“…Complexion nucleation and growth are essential for predicting the temporal effects of grain boundary complexion transitions during the dynamic process of grain growth. Therefore, time‐temperature‐transformation (TTT) curves were introduced as a processing tool to induce or avoid complexion transitions . Figure shows a hypothetical complexion TTT‐curve, like those introduced by Cantwell et al .…”

“…Such a TTT‐curve is useful in predicting the microstructure evolution and can be used by design engineers to create sophisticated heat treatments that avoid or purposefully induce a set of complexions as those described in Refs. . To achieve a dense body with small grain size, a sample can be heated to high temperatures for short times to improve densification without nucleating the high temperature complexion and then final densification can be conducted below the complexion transformation temperature.…”

“…While still under development, the TTT diagram provides many processing opportunities and can illustrate what factors influence the kinetic mechanisms of complexion transitions. Readers are referred to Schumacher et al for detailed demonstrations of the empirically derived complexion TTT diagram.…”

“…In many instances, grain boundary complexion transitions can be used to enhance the processing, properties, and performance of engineering materials. In Table , we provide a summary of recent applications and examples of complexion transitions, some of which can be considered as examples of grain boundary complexion engineering (GBCE). The idea of engineering grain boundaries to enhance the bulk properties of engineering materials is not new.…”

“…Finally, we highlight a few innovative developments in GBCE. One step toward making GBCE an accessible and useful tool is the development of time‐temperature‐transformation (TTT) complexion diagrams . TTT complexion diagrams are analogous to their counterparts for bulk phases, and offer a visual representation of the conditions under which complexion transitions occur.…”