Grain boundary segregation in immiscible nanocrystalline alloys

Abstract

Grain boundary (GB) solute segregation has been proposed as a route to mitigate grain growth in nanocrystalline (NC) metals and stabilize their grain structures. An interesting effect emerges in immiscible NC alloys due to the intertwined roles of GB segregation and bulk alloy phase separation. Based on a diffuse interface model, we examine grain growth dynamics in immiscible NC alloys, where both GB solute segregation and bulk phase separation act in conjunction. Analytical treatments identify regimes, where the reduction in GB energy due to segregation is significant. Simulation results reveal that microstructural evolution and enhanced stability are a manifestation of the competing effects of GB heat of segregation and that of bulk heat of mixing. More specifically, in systems with low GB segregation precipitation of soluterich domains and associated GB (Zener) pinning effects lead to sluggish grain growth rates. In contrast, GB solute segregation plays a more dominant role as the heat of segregation increases in comparison with the bulk heat of mixing. On the whole, this modeling framework provides an avenue to explore the role of bulk alloy and interfacial effects on the microstructural evolution of NC metals. mobility [32]. In regular solutions, solute-solute interactions within the GB region lead to an activation energy of GB migration that is concentration-dependent [31]. The second effect stems from the Gibbs adsorption equation. Accumulation of solute atoms at a GB when the chemical potential is increased at a fixed pressure and temperature leads to a reduction in GB energy, and thus the driving force for GB migration [17][18][19]. Both of the aforementioned effects are highly intertwined and act in conjunction; therefore, detangling the role of each on the observed thermal stabilization of NC metals is of interest from a scientific and application standpoint.Theoretical and computational frameworks provide tools by which each of the solute segregation effects can be examined in isolation. With the aid of the regular solution approximation for binary NC alloys, several analytical treatments were developed to examine the reduction in GB energy due to solute segregation [17,19,21,23,25,28]. Mesoscale computational studies of GB solute segregation have been developed with the aim of examining, in dilute alloys, GB break-away events [33,34], inhomogeneity in diffusion pathways [35], abnormal grain growth [36] and role of strain energy [37], while others examined the thermodynamic aspect of stabilization [38][39][40]. It was found that the reduction in GB energy, due to segregation, is most significant in alloys with concentrations away from the dilute limit [22,39]. In such regimes, however, a wide range of nanocrystalline binary alloys, such as Cu-Zr [41], Fe-Zr [42], Ni-P [43], Pt-Au [44], Fe-Cu [45] and Ni-W [46], are not soluble across the whole concentration range and several exhibit miscibility gaps. Therefore, in a GB segregating alloy the increase in the global solute concentration may lead to an im...

Materials selection rules for amorphous complexion formation in binary metallic alloys

“…On the other hand, the Cu-Mo system is stabilized by a combination of grain boundary segregation as well as the presence of small precipitates, meaning both thermodynamic and kinetic stabilization are active. The kinetic contribution comes from Zener pinning caused by the dopant clusters [59,[76][77][78]. Clustering of Mo and the eventual precipitation of a second phase in a Curich alloy has been previously reported due to the immiscibility of the added dopant [59,60,62,79].…”

The evolving landscape for alloy design

“…This has enabled, for example, rigorous studies of segregation to grain boundaries and stacking faults. [57][58][59][60][61][62][63][64][65][66][67][68] Refractory element segregation has been observed at superlattice intrinsic stacking faults in a new class of cobalt-based alloys containing L1 2 precipitates, 57,58,69,70 and is revealed by both local electrode atom probe tomography and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) (see Figure 3a). This provides a new pathway for control of fault energies and higher length scale mechanical properties sensitive to this property.…”

A phase-field approach for modeling equilibrium solute segregation at the interphase boundary in binary alloys

“…1b. The parallel tangent construction is a standard equilibrium condition between a bulk phase and an interface phase in thermodynamic models [48], which has been widely used in the context of free surfaces and GBs [41,45,49].…”

“…This allows properties of the phase-field models to be described in relation to traditional statistical thermodynamic treatments of solute segregation to interfaces [42]. Such approaches are consistent with classical segregation isotherms and satisfy Gibbs adsorption [40,41]. In the context of IBs, however, such descriptions have not been formulated to the best of our knowledge.…”