Stability criteria for nanocrystalline alloys

Abstract

Alloying nanocrystalline materials to stabilize them against grain growth is proving a critical enabling strategy for the processing and usage of bulk nanocrystalline parts. Alloying elements that segregate strongly to grain boundaries can lead to a preference for nanocrystalline structure, and to be most stable the grain boundary segregated state would need to be preferred to forming any other phase or solute configuration, including a solid solution, ordered compounds, or solute precipitates. In this paper, a stability criterion is developed by comparing the enthalpy of the grain boundary segregated state against such stable bulk phases. This enthalpic criterion is also translated into a lattice model framework to enable the use of Monte Carlo simulations to incorporate entropic and geometric effects in assessing nanocrystalline stability. Monte Carlo simulations show that entropy can play a role in stabilizing nanocrystalline states, leading to duplex structures, and also in forming a grain boundary network preferentially over a disordered or amorphous-like bulk phase.

Materials selection rules for amorphous complexion formation in binary metallic alloys

“…By lowering the grain boundary energy through dopant segregation, the thermodynamic driving force for grain growth is mitigated, allowing these materials to retain their desirable nanocrystalline structure even when exposed to elevated temperatures [43]. The theoretical framework to predict stable nanocrystalline materials using a thermodynamic stabilization route has made considerable progress in recent years [42,[44][45][46][47][48][49][50]. Darling et al [51] also contributed to this field by calculating stability maps for the solute composition needed to minimize the excess grain boundary energy for a given grain size and temperature.…”

“…A dot indicates that the alloy has an atomic radius mismatch greater than 12%, as calculated using Equation 2. Other sources were also used to further confirm the enthalpy parameter values where applicable [41,50,78,[84][85][86]. It is worth noting that we do not explicitly treat any competition for dopants from second phase formation here, which can add an additional complication.…”

Amorphous complexions enable a new region of high temperature stability in nanocrystalline Ni-W

“…Based on the materials selection rules introduced above, Ni-W would be expected to form AIFs at sufficiently high temperatures, with ΔH seg = 10 kJ/mol and ΔH mix = -3 kJ/mol [30]. This alloy system has been studied extensively in the past, with the unique feature that the grain size can be tailored during electrodeposition by controlling the reverse pulse current and, therefore, the W content [48,49].…”

Solid-state dewetting instability in thermally-stable nanocrystalline binary alloys

“…Kinetic stabilization generally involves alteration of the grain boundary mobility through solute drag and Zener pinning caused by particle dispersions [32][33][34][35][36][37][38], which has been observed for Cu-Ta [38,39] and Cu-Nb [40]. Thermodynamic stabilization methods seek to reduce the root driving force for grain growth by reducing the grain boundary energy through dopant segregation to the grain boundary region [23,24,26,27,[41][42][43][44], which has been observed for a variety of systems including Ni-W [45], W-Ti [46], Fe-Zr [47], and Cu-Zr [48]. [49] recently reviewed how dopant segregation can be used to tailor thin film microstructures for a multitude of alloys, including Fe-Pt [50], Cu-Ni [51], Fe-Cr [52], and W-Ti [53].…”