Optimizing Nanoscale Coherent Precipitates in Aluminum Alloys by Microalloying with Tungsten

We propose a fundamental physical-metallurgy study of microstructures and resulting mechanical properties - as they evolve during various heat-treatments - for aluminum micro-alloyed with tungsten, an element barely studied in the archival aluminum literature. This research will connect to an existing ONR-funded project, where we study the effects of four other transition metals TMs [Ni (Group 10), Zn (Group 12), Mo (Group 6) and Hf (Group 4)] on precipitation and coarsening of L12, coherent Al3(Er,Zr,TM) nanoprecipitates during aging of dilute Al-Er-Zr-TM alloys. New research on W (Group 6, like Mo) additions will bring synergy to our ongoing research on the effects of Group 4, 6, 10, 12 TMs on precipitation in Al-Er-Zr-TM alloys. Our ongoing studies indicate that Zn, Mo, Hf have sizeable solubility in the precipitates, which bodes well for their coarsening and creep resistance. With the proposed W-addition study, we will focus on the nanostructure of Al3(Er,Zr,W) nanoprecipitates, and test our hypothesis that they consist of a W-rich outer shell (formed last during aging step, as W has a very small diffusivity in Al, which increases coarsening resistance, and an Er-rich core (formed first, as Er has a very high diffusivity in Al and is a well known nucleant), which provides creep resistance via a lattice parameter mismatch. Given the paucity of data on W additions to Al, we will first examine dilute ternary Al-Er-W and Al-Zr-W alloys; this will shed light on the effect of W (in equilibrium and supersaturated solid-solutions) upon precipitation of Al3(Er,W) and Al3(Zr,W), respectively. We will then proceed to the quaternary Al-Er-Zr-W alloy where Al3(Er,Zr,W) nanoprecipitates will be studied. For all alloys, the effect of Si micro-additions will also be examined, since Si is a known inoculant for precipitation of Al3(Er,Zr). Finally, we will investigate the simultaneous additions of Mo and W to these alloys. The proposed research on W additions to Al-Er-Zr alloys involves the same three tasks, as in our ongoing study on Ni, Zn, Mo and Hf additions: “(1) Alloy fabrication and thermo-mechanical heat-treatments; (2) Microstructural characterization of the temporal evolution of the nanoprecipitates, in collaboration with Dr. K.E. Knipling, NRL. We will elucidate the kinetics of nanoprecipitate nucleation, growth, transformation and coarsening, relying principally on three-dimensional (3-D) atom-probe tomography (APT), which provides nanostructural and chemical information on a sub-nanoscale. Additionally, we will use optical microscopy, scanning electron microscopy and transmission electron microscopy to study the complete hierarchy of length scales, in parallel with electrical conductivity and microhardness measurements. This task will be complemented utilizing first-principles calculations concerning the role of Si in accelerating the kinetics of precipitation utilizing the Vienna ab initio simulation package; and (3) Investigation of the strength of the newly developed alloys, both at ambient and elevated temperatures, in conjunction with models for calculating yield stress at ambient temperature and creep threshold stress at elevated temperature. Specifically, the critical microstructural information (volumetric number density, mean radius, volume fraction of nanoprecipitates, mean edge-to-edge distance between nanoprecipitates, lattice parameter mismatch and dendrite dimensions) will be used to both predict and optimize alloy strength at high temperatures.” As for our ongoing study, the above tasks will be integra