Thermal stability of nanocrystalline microstructures.

摘要

The objective of the proposed research is to develop the experimental data and scientific basis that can optimize the thermodynamic stabilization of a nanoscale microstructure during consolidation of Fe powder particles through select solute diffusion to grain boundaries.;Fe based alloys were high energy ball milled to produce supersaturated solid solutions with a nominal grain size of ∼10nm. Solutes such as Y, W, Ta, Ni and Zr were selected based on their propensity to grain boundary segregated in Fe. Based on preliminary heat treatments Zr was selected as the solute of choice. Upon further heat treating experiments and microstructural analysis it was found that Zr solute additions of 4at% could stabilize a nanocrystalline microstructure of 100nm at temperatures in excess of 900°C. This is in stark comparison to pure nanocrystalline Fe which shows coarsening to the micron scale after annealing above 600°C. Reduction in grain boundary energy due to Zr segregation and solute drag are proposed as mechanism responsible for the observed thermal stability.;In addition to the work presented on Fe based Zr alloys supplementary research is presented on the following systems: Fe based Ni alloys, Pd 20at%Zr, Cu3Ge and CuGeO3. The addition of Ni to Fe was selected as a control. Since Ni and Fe have similar atomic radii, the elastic enthalpy of segregation of Ni in Fe is low (+1kJ/mol) and at high temperatures Ni has complete solid solubility in Fe; it is suggested that Ni will have a negligible influence in the thermal stability of nanocrystalline Fe. It was shown that at 700°C the addition of 1at% Ni produce a bimodal microstructure consisting of ∼70% abnormally grown grains and ∼30% nanocrystalline grains of 100-200nm. While these results are interesting extensive work is still needed to understand the mechanisms governing the thermal stability in this system. A presentation of the collected data is given.;Pd 20 at% Zr was high energy ball milled to produce an average grain size of ∼6nm. Fe contamination was high due to the extensive cold welding exhibited by this system. The as-milled alloy showed an increase in hardness from ∼6.5GPa to ∼10GPa after heat treating at 1000°C for 1 hour. Based on these hardness measurements this alloy exhibits high thermal stability up to 1000°C. However, after heat treating at 1273°C the formation of Zr oxides were detected. Occurring simultaneously with the secondary phase formation was a rapid decrease in Vickers hardness from ∼10GPa to ∼3GPa. Ion channeling contrast images reveled that a nanocrystalline microstructure was not maintained at 1273°C. While these results are in conflict with what was reported in literature additional work is needed to confirm the results, however a presentation of the collected data is presented.