Stabilized nanocrystalline iron-based alloys: Guiding efforts in alloy selection

“Bulk” Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

“…In general, two approaches are used to reduce the velocity of a moving grain boundary (and stabilize the grain structure): 45 by kinetically hindering GB mobility or by thermodynamically lowering the GB energy through solute segregation (i.e., modifying either the kinetic parameter M or the thermodynamic driving force P, respectively). 24,40,[46][47][48][49][50][51][52][53][54][55][56][57][58] These two approaches are often referred to as the thermodynamic approach and kinetic approach. Because grain boundary mobility follows an Arrhenius behavior, kinetic approaches for reducing grain boundary mobility will eventually be overcome by temperature.…”

“…Recently, a number of models have been derived that look at these effects independently and the combined effect for systems in which both methods of stability could operate. [46][47][48][49][50][51][52][53][54][55][56][57][58][59] …”

Phase transitions in stable nanocrystalline alloys

“…Pure nanocrystalline materials undergo rapid grain growth into micron-scale grain sizes at relatively low homologous temperatures, [1][2][3][4] but with the addition of alloying elements grain growth can be greatly hindered. [5][6][7][8][9][10][11][12][13] Two mechanisms for suppressing grain growth by alloying have been proposed: alloying elements can either kinetically impede grain growth by increasing the activation energy for grain boundary motion (effectively decreasing grain boundary mobility) or decrease the grain boundary energy thermodynamically through grain boundary segregation (or both). The latter mechanism is particularly promising, as substantial decreases to the grain boundary energy from grain boundary segregation could stabilize nanoscale grain sizes to higher temperatures and for longer times.…”

“…The latter mechanism is particularly promising, as substantial decreases to the grain boundary energy from grain boundary segregation could stabilize nanoscale grain sizes to higher temperatures and for longer times. [8][9][10][11][12][13] There is an even more enticing prospect when alloying to reduce the grain boundary energy: if the excess energy of the grain boundary is eliminated by grain boundary segregation, grain growth can be entirely avoided and a thermodynamically stable grain size in the nanocrystalline regime could exist. This concept was put forth by Weissmüller,14,15 with an accompanying analytical model that revealed that a system with an enthalpy of grain boundary segregation large enough to offset the pure grain boundary energy should have such a stable nanocrystalline state.…”

“…Several of these alloy systems have shown considerable nanometer-scale grain size stability in experiments, including Ni-P, Ni-W, Fe-Zr, and W-Ti. [8][9][10][11][12][13] While these models are able to identify systems with potential stability through energetic considerations of grain boundary segregation, the accurate accounting of entropic effects has been limited by assumptions needed to make the analytical models tractable, i.e., dilute limit or regular solution assumptions. A number of analytical [20][21][22][23] and atomistic models 24,25 have been developed for determining the change in free energy associated with grain boundary segregation under a variety of conditions, but typically do not allow the grain size to change during disordering.…”