Mechanism-Informed Design of Complex Nanocrystalline Nickel Alloys for Enhanced Hardness

Abstract

In this work, a multifactorial, mechanism-informed approach is used to design a novel class of complex multiphase nanocrystalline Ni-based alloys with excellent microstructural stability and mechanical properties. Nanocrystalline materials present promising candidates for advanced engineering applications due to their unique properties, including high strength due to large amounts of Hall-Petch strengthening, which would benefit critical mechanical applications. However, nanocrystalline systems naturally experience a high driving force for grain growth and must be carefully designed to limit grain boundary motion. Simple model nanocrystalline materials have been successfully stabilized against grain growth, but they are often binary and contain only one or two phases. The mechanisms relating to grain boundary behavior are the primary focus of many of these studies, rather than those which enhance properties beyond the grain boundary/size contribution.

In the present system, thermal stability against grain growth is not the primary goal. Instead, it is leveraged to provide significant and stable Hall-Petch strengthening to support enhanced high strength in combination with additional strengthening mechanisms from secondary phases and solutes. Hardness testing and advanced characterization techniques, including scanning transmission electron microscopy, were used to elucidate composition-processing-microstructure-property relationships.

The primary objective was to use a mechanism-informed design approach to create a complex nanocrystalline alloy which exhibits high strength and long-term thermal stability against coarsening. Ni - 11 at% W - 3 at% Ta - 2 at% Y (Ni-11W-3Ta-2Y) was designed to form nanoscale precipitates, which aid in maintaining a stable nanocrystalline structure to exploit Hall-Petch hardening and strengthening due to a dispersion hardening effect. Solutes were selected to promote a large amount of solid solution strengthening and to decrease the stacking fault energy of the Ni-based matrix for high temperature strength. This mechanically alloyed metal maintained nanoscale grains following anneals up to 67% Tm for 100 hours due to the formation of grain boundary pinning Y₂O₃ particles and argon bubbles, and it also exhibited hardness in the range of 6.6 – 9.7 GPa. Hall-Petch strengthening contributes roughly 35 – 40% of the total hardness, and dispersion hardening contribution from Orowan looping around fine ceramic particles constitutes another 30%.

In support of this primary aim, two other objectives were introduced. The first was to identify the effects of two important milling parameters on the alloy’s microstructure: milling time and environment inside of the milling vial, and the second was to elucidate the roles of individual components based on selective removal or variation of added components in the successful quaternary alloy. Milling time was found to strongly impact the contaminant phase content and distribution in the annealed state as well as the quaternary alloy’s microstructural evolution and its bulk properties in a surprising way. Rather than improving homogeneity, an increase in milling time by 50% was found to exacerbate the effects of localized inhomogeneities in deformation and composition. This ultimately led to an uneven distribution of grain boundary pinning phases and the formation of abnormally large grains among normal, nanoscale grains following a long-term anneal. Additionally, the presence of Ar bubbles in an annealed cryomilled alloy was able to be mitigated by loading powders in a vacuum glovebox rather than an Ar-filled glovebox. Direct comparison of the annealed microstructures of these vacuum glovebox samples to those loaded in an Ar-filled glovebox prior to milling revealed that the Y₂O₃ particle – Ar bubble clusters are more effective at pinning grain boundaries than Y₂O₃ particles with no Ar bubbles.

The strategic creation of four new alloys was found to be effective in highlighting the phase and mechanistic roles of individual components. First, the removal of Y to create the Ni-11W-3Ta ternary alloy, produced highly stable Ta₂O₅ nanoparticles surrounded by a layer of Ta instead of Y₂O₃. The Ta₂O₅ particles were found to be more effective at pinning against grain growth than the Y₂O₃, resulting in finer grain sizes following both short- and long-term anneals. These particles also enhance the alloy’s hardness via a dispersion hardening effect. Meanwhile, in the Y-containing ternary alloys (Ni-11W-2Y and Ni-3Ta-2Y), an uneven distribution of pinning phases was found, giving rise to the growth of abnormally large, unpinned grains following long-term anneals. Here, the inhomogeneous distribution of pinning phases was attributed to the lower total solute content enabling the formation of two different Ni – Y intermetallic phases depending on local depletions or enrichments of W/Ta solute. Similarly, a quaternary alloy enriched in W was also produced using the same process, and it was found to contain non-uniform distributions of secondary phases after annealing, but this was attributed to an exacerbation of inhomogeneities from milling due to the increased powder density, and therefore increased energy input. Despite the observation of an undesirable, non-uniform microstructure and loss of strength in some of the milling parameter and compositional variations, these findings inform future process and alloy design for nanocrystalline materials. They also provide insight into the promise of Ta-based oxides over Y-oxides for improved stability in Ni-11W-3Ta and future alloys. The successful mechanism-informed design of two nanocrystalline alloys which are thermally stable against grain growth and exhibits enhanced hardness provides a framework for design of complex, multiphase nanocrystalline alloys, and the exploration of altered milling parameters and alloy compositions provides further insight into the system’s composition-processing-structure-properties relationship. Overall, this project supports the future development of advanced engineering materials through an increasingly well-informed design approach.