Date of Completion

5-6-2014

Embargo Period

5-6-2014

Keywords

phase stability, grain coarsening, electron microscopy, powder metallurgy, Al alloys

Major Advisor

Mark Aindow

Associate Advisor

Harold D. Brody

Associate Advisor

Rainer Hebert

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

There has been significant interest in Al-rich Al-Transition Metal-Rare Earth element alloys due to the variety of metastable phases that can be formed. Here a series of microstructural studies is presented on two powder-processed alloys consolidated by warm extrusion: Al-Ni-Co-Zr-Y and Al-Mn-Ce. The microstructures of the alloys in the as-extruded condition and after heat-treatment were evaluated by X-ray diffraction and electron microscopy techniques and these correlated with the mechanical responses. For the Al-Ni-Co-Zr-Y alloy, the microstructure in all samples contains 22% by volume of Al19(Ni,Co)5Y3 plates surrounded by grains of Al. The softening of the alloy is limited for heat-treatment temperatures of up to 400˚C, and the Al19(Ni,Co)5Y3 plates coarsen slowly. At higher temperatures abnormal coarsening is observed with the development of a secondary population of much larger Al19(Ni,Co)5Y3 plates. An analysis of the coarsening kinetics reveals that the particles coarsen by Ostwald-type ripening. There is a distinct transition in the activation energies suggesting that normal coarsening occurs at lower temperatures by short circuit diffusion, whereas abnormal coarsening develops at higher temperatures by lattice diffusion. The Al grain size is dictated by the inter-particle separation, and grain growth is limited by the extent of plate coarsening. For the Al-Mn-Ce alloy, the powder microstructure consists mainly of an amorphous phase, Al, and a previously unreported phase, Al20Mn2Ce. The extrudate is fully devitrified and contains a mixture of Al, Al20Mn2Ce and Al6Mn, with a small amount of Al12Mn and Al11Ce3. Upon heat-treatment at up to 450˚C the Al20Mn2Ce and Al6Mn phases decompose to give a hard stable phase mixture with 72-73% Al12Mn plus 13-14% each of Al11Ce3 and Al. Heat treatments at 500˚C give a much softer phase mixture consisting of 60% Al, 22% of an unknown Al3(Mn,Ce) phase, 9% Al12Mn, 8% Al­6Mn and 1% Al11Ce3. The formation of large volume fractions of Al12Mn for heat-treatments at up to 450˚C suggests that the presence of Ce may stabilize this phase. These two systems could form the basis of new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.

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