A problem central to designing the next generation of advanced 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 is 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
Gradient nanocomposite layer improved corrosion resistance of aluminum alloy
Gradient nano-grained structures have been a promising technique to evade the strength-ductility trade-off in metals and alloys. Therefore, in this work, the effect of surface mechanical attrition treatment (SMAT) on the microstructure and corrosion behavior of the high-strength aluminum alloy was investigated. SMAT was performed at room temperature and liquid-nitrogen (LN2) flow conditions to generate two distinctly different initial gradient microstructures. Potentiodynamic polarization, electrochemical impedance spectroscopy, and intergranular corrosion tests were performed. Surface film characterization of untreated and treated samples was performed using X-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy techniques. Result reveals significant microstructural changes in SMAT processed samples such as the formation of precipitates and dissolution of inherent phases. In addition, a reduced anodic dissolution rate was observed with the SMAT processed samples. Furthermore, the surface film characterization revealed a thicker oxide film with Cu and SiO2 enrichment in SMAT samples.