• PDF / 2,865,188 Bytes
• 18 Pages / 593.972 x 792 pts Page_size
• 50 Downloads / 190 Views

DOWNLOAD

REPORT

---

THE mechanical properties of precipitation-hardened alloys depend on the precipitate size, shape and their arrangement in the matrix.[1–6] Of particular interest are precipitate reactions where multiple solidphases precipitate from the matrix as here we find interesting examples of self-organized patterns/configurations. For coherent interfaces, there exist elastic interactions between the precipitates which influence the eventual configuration of the precipitate cluster as well as the individual shapes of the precipitates that arise out of a precipitation reaction. Hence, it is important to determine these attributes of the precipitates or the cluster of the precipitates that evolve as a function of different parameters such as the anisotropy in the elastic energy and the misfit-strain, the characteristic size of the precipitates, inhomogeneity of the elastic stiffness matrices etc. There are several possibilities for equilibrium precipitate configurations that could arise out of a precipitate reaction. For example, in certain multi-component alloy systems, such as Al-based alloys,[7–18] given a certain heat treatment, the matrix yields a microstructure where the precipitate morphologies are characterized by the formation of a core–shell type structure. The

BHALCHANDRA BHADAK, RAVI KUMAR SINGH, and ABHIK CHOUDHURY are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] Manuscript submitted January 1, 2020.

METALLURGICAL AND MATERIALS TRANSACTIONS A

precipitates that form the core–shell microstructures are not homogeneous in composition, where the central precipitate is surrounded by another precipitate phase with a different composition. As the interface between the phases is coherent in nature, eventually due to anisotropy in the elastic energy the core–shell type structure gives rise to cubic shapes which is described by Tolley and coworkers.[7] Research is also focused on the evolution of such core–shell type microstructures, their coarsening behavior and subsequent mechanical properties obtained at the room as well as elevated temperatures.[8–11,15–18] With regards to the criteria for the formation of such structures, there is however some lack of clarity. Wolverton and coworkers[19] have performed first-principles calculations for evaluating the bulk thermodynamics, elastic misfit and stiffness and interfacial energies for two precipitate phases in the Al-Li-Sc alloy system. Using the calculations, the authors have determined the conditions for nucleation and the stability of formation of such core–shell microstructures between the Al3Li and Al3Sc phases in the Al-matrix. Based on the calculations of the interfacial energies and their dependence on the compositions, the authors have concluded that it is only possible for the Al3Li phase to wet/engulf the Al3Sc precipitates as the interfacial energies of the corresponding interfaces satisfy the wetting condition, which is supported by experimental observations in this alloy. These co