Nucleation of Grain Boundary Phases

Phase Pattern Formation in Grain Boundaries

In this Letter we derive conditions that predict the existence of two-phase periodic-pattern grain boundary structures that are stable against coarsening. While previous research has established that elastic effects can lead to phase pattern formation on crystal surfaces, the possibility of stable grain boundary structures composed of alternating grain boundary phases has not been previously analyzed. Our theory identifies the specific combination of grain boundary and materials properties that enable the emergence of patterned grain boundary states and shows that the dislocation content of grain boundary phase junctions, absent in surface phenomena, weakens the stability of the patterned structures. The predictions of the theory are tested using a model copper grain boundary that exhibits multiple phases and two-phase pattern formation. We discuss how, similarly to surfaces, elastic effects associated with grain boundary phase junctions have profound implications for how grain boundary phases transform.

Crystallization mechanism of the Pt50Au50 alloy with grain boundary segregation during the solidification process

The crystalline mechanism of the Pt50Au50 alloy with grain boundary (GB) segregation during the rapid solidification process is investigated using molecular dynamics simulations. The cluster evolution and phase transformation processes during the GB segregation are analyzed by means of the energy temperature (E-T) curve, the pair distribution function (g(r)) curves, common neighborhood analysis (CNA), cluster-type index method (CTIM) and three-dimensional visualizing analyses. It is found that the GB segregation phenomenon of the Pt50Au50 alloy comes from various solidification temperatures of Pt- and Au-centered clusters. Four critical temperatures T1 (1153 K), T2 (1073 K), T3 (853 K) and T4 (753 K) are discovered during the liquid-solid transition, corresponding to the supercooled liquid, Pt-centered atom nucleation, Pt-centered cluster growth, Au-centered atom nucleation and grain growth process, respectively, which is observably different to the solidification process of other alloys. The Pt atoms begin to gather together in the high-temperature liquid before the liquid-solid transition. It is also found that the CTIM proposed by us would provide an effective tool to investigate the GB segregation process.

Dual phase patterning during a congruent grain boundary phase transition in elemental copper

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemental copper by atomic resolution imaging. The same grain boundary phases are found by computational grain boundary structure search indicating a first-order transformation. Finite temperature atomistic simulations reveal a congruent, diffusionless transition between these phases under ambient pressure. The patterning of the grain boundary at room temperature is dominated by the grain boundary phase junctions separating the phase segments. Our analysis suggests that the reduced mobility of the phase junctions at low temperatures kinetically limits the transformation, but repulsive elastic interactions between them and disconnections could additionally stabilize the pattern formation.