A problem central to designing the next generation of adva nced multifunctional materials for various applications is that the time required for development of commercial product and their components far outpaces the time required for new material design. Thus, a grand challenge in materials engineering is to drastically reduce the development time for new materials and processes, while preserving or exceeding current safety benchmarks. Indeed, accelerating the pace of discovery, development, manufacturing, and deployment of advanced materials systems is the principal objective of the “Materials Genome Initiative.” In this regard, integrated computational materials engineering (ICME) provides a conceptual roadmap to new design materials; however, critical knowledge gaps currently preclude the application of ICME for the development of advanced materials such as nanocrystalline (NC) alloys. To address this need, our group in collaboration with the ARL, and George Mason University, are using a multiscale modeling approach along with detailed experimental and advanced manufacturing processes to push the design boundary closer to the theoretical limit.
Extreme creep resistance in a microstructurally stable nanocrystalline alloy
Nanocrystalline metals, with a mean grain size of less than 100 nanometres, have greater room-temperature strength than their coarse-grained equivalents, in part owing to a large reduction in grain size1. However, this high strength generally comes with substantial losses in other mechanical properties, such as creep resistance, which limits their practical utility; for example, creep rates in nanocrystalline copper are about four orders of magnitude higher than those in typical coarse-grained copper. The degradation of creep resistance in nanocrystalline materials is in part due to an increase in the volume fraction of grain boundaries, which lack long-range crystalline order and lead to processes such as diffusional creep, sliding and rotation3. Here we show that nanocrystalline copper–tantalum alloys possess an unprecedented combination of properties: high strength combined with extremely high-temperature creep resistance, while maintaining mechanical and thermal stability. Precursory work on this family of immiscible alloys has previously highlighted their thermo-mechanical stability and strength, which has motivated their study under more extreme conditions, such as creep. We find a steady-state creep rate of less than 10−6 per second—six to eight orders of magnitude lower than most nanocrystalline metals—at various temperatures between 0.5 and 0.64 times the melting temperature of the matrix (1,356 kelvin) under an applied stress ranging from 0.85 per cent to 1.2 per cent of the shear modulus. The unusual combination of properties in our nanocrystalline alloy is achieved via a processing route that creates distinct nanoclusters of atoms that pin grain boundaries within the alloy. This pinning improves the kinetic stability of the grains by increasing the energy barrier for grain-boundary sliding and rotation and by inhibiting grain coarsening, under extremely long-term creep conditions. Our processing approach should enable the development of microstructurally stable structural alloys with high strength and creep resistance for various high-temperature applications, including in the aerospace, naval, civilian infrastructure and energy sectors. see
Compressive creep response of nanocrystalline Cu–10 at% Ta. (a) Conventional creep strain versus time for various applied temperatures and constant-stress conditions and (b)Map of the theoretical creep deformation mechanisms including values for Cu-10 at% Ta.
Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions
Fundamentally, material flow-stress increases exponentially at deformation rates exceeding, typically, ~103 s-1, resulting in brittle failure. The origin of such behavior derives from the dislocation-motion causing non-Arrhenius deformation at higher-strain-rates due to drag-forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain-rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high temperature strength maintained up to 80% of the melting point (~1356 K). also, see
Yield strength and flow strength for NC-Cu-10at.%Ta’s data along with NC and UFG data reported in the literature at (a) quasi-static and (b) high strain rates
Cryogenic behavior of nanocrystalline binary alloys
The work looks to investigate the mechanisms at play in a stable NC alloy by mechanically testing stabilized NC-Cu-3Ta (at.%) over a range of cryogenic-temperatures from 273 to 113 K. The NC-Cu-3Ta represents an optimized Ta concentration; consequently, this work will focus on this alloy composition. The tested specimens will then have their microstructures characterized via transmission electron microscopy (TEM) to elucidate which deformation mechanisms are operating at different temperatures and address the microstructural stability.
Normalized yield stress for Cu-3Ta (at.%) plotted as a function of Zener-Hollomon parameter at various cryogenic and high temperatures and compared with that of pure Cu |
