Matrix-Dispersoid Mechanical Interaction in Microstructurally Stable Hierarchical and Nanocrystalline Alloys—A SAXS/WAXS Study

There is a strong interest in increasing the high temperature performance of metallic alloys used in extreme conditions, such as those found in many energy generations and propulsion systems. Microstructure refinement has long been considered a potential route for improvement; however, nanocrystalline (NC) metals and alloys (with grain size less than 100 nm) exhibit significant microstructure instability at high temperatures and even at room temperatures under, e.g., cyclic loads. Thermodynamic (solute segregation to reduce grain boundary energy) and/or kinetic approaches (solute drag, Zener pinning, etc., to pin grain boundaries) can be used to suppress grain growth through the addition of alloying elements and second phases. Some NC alloys such as Cu-10at%Ta fabricated using far from equilibrium processing techniques have displayed stability of their microstructures mostly through kinetic mechanisms at various thermo-mechanical loading conditions. However, the low melting point of Cu makes Cu alloys ill-suited for advanced applications compared to Ni and Ti. Hence, we plan to investigate a new class of NC Ni-Y alloys with stable microstructures under thermomechanical loads.<br/>In particular, the interplay between matrix and dispersoids in alloys with NC and hierarchical, i.e., multiple characteristic length scales, microstructures, is of significant importance to understand the mechanical behavior of these materials under thermomechanical loading. High voltage X-ray diffraction measurements performed in-situ during thermomechanical loading of samples at beamline 1-ID of the Advanced Photon Source (APS) were used to characterize the evolution of both matrix and dispersoids as functions of macroscopic stress and strain in the samples. Powder consolidation at high temperatures was used to produce NC Ni-based samples alloys with intermetallic precipitates (the precipitates are expected to be ~ 4 nm in size with ~ 10 nm spacing). The NC alloys were then compared to hierarchical CG alloys with similar compositions with an ultrafine eutectic phase (~150-300 nm average lamellar spacing) synthesized using conventional arc-melting techniques in an inert atmosphere. Additionally, analysis on Cu-10Ta alloys (with precipitates of sizes 3-32 nm and grain sizes of 170 nm) synthesized by powder consolidation through equal channel angular extrusion (ECAE) processing provided a basis for comparison of properties between Ni and Cu based alloys.<br/>Small-angle X-ray scattering (SAXS) analysis was performed to characterize dispersoid size and spacing distributions while wide-angle X-ray scattering (WAXS) was performed to quantify the evolution of lattice strains in both dispersoid and matrix as functions of macroscopic stresses and strains applied to the samples, so that their mechanical interactions, e.g., stress partitioning and its evolution with plastic strain, could be quantified and used to elucidate basic deformation mechanisms in these novel materials. This research is based upon work supported by the National Science Foundation under Grant DMR/MMN # 1810431, resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357, and guidance and help of the beamline scientist, Dr. Jun-Sang Park.