Aluminum Beryllium Composites Produced by Rapid Solidification (Metastable Equilibrium, Modulus Hardening, Liquid Immiscibility)
Van Aken, David Carlton
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https://hdl.handle.net/2142/71839
Description
Title
Aluminum Beryllium Composites Produced by Rapid Solidification (Metastable Equilibrium, Modulus Hardening, Liquid Immiscibility)
Author(s)
Van Aken, David Carlton
Issue Date
1986
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Abstract
Al-Be alloys, 4.4-20 at.% Be in composition, have been rapidly solidified by laser surface remelting and melt-spinning. Microstructures of uniform dispersions of spherical Be particles (10-15 nm in diameter), randomly dispersed in an Al matrix were produced for compositions 5-8 at.% Be. Two crystal forms of Be were observed in these solidified microstructures; equilibrium cph and metastable bcc. A cubic lattice parameter of 0.25 (+OR-) 0.02 nm was experimentally determined by electron diffraction for the metastable bcc Be. A mechanism for the formation of the microstructure was described by a metastable phase diagram involving a liquid miscibility gap. Thermal decomposition of the rapidly solidified microstructure resulted in a prism-rod morphology of cph Be particles with 8 crystallographic orientations observed.
Mechanical properties for melt-spun (MS) Al-11Be were obtained for ribbon consolidated by dynamic compaction (DC) and by thermomechanical extrusion (Extr). The ribbon was extruded at 648 K (375(DEGREES)C) with a reduction in area of 18:1 and the resultant microstructure exhibited a dynamically recrystallized grain size of 1-2 microns dispersed with Be particles. A Young's modulus of 71.5 GPa was experimentally measured and this value was consistent with calculations based on a materials composite theory. Monotonic and cyclic properties for the extrusion were also reported. The yield stress for as-cast, MS-DC, and MS-Extr Al-11Be was 65,248, and 126 MPa, respectively. Mechanical strengths were related to the size and distribution of the Be particles and an Orowan hardening mechanism was proposed.
In addition, Al-Be microstructures were compared with results obtained for rapidly solidified Al-In monotectic alloys. Rapidly solidified Al-In alloys were produced by melt-spinning and electrohydrodynamic atomization. Alloys near the monotectic composition exhibited a dispersion of In-rich particles in an Al matrix. Each particle was facetted parallel to 111 and 100 Al planes forming the shape of a truncated octahedron. These In-rich particles also exhibited a metastable cubic (face centered) crystal structure, a = 0.47 (+OR-) 0.02 nm. These metastable In particles (designated as In') had the following orientation relationship with the matrix: 001 In'// 001 Al and (100)In'//(100)Al. The presence of the In' in the rapidly solidified microstructure was related to the liquid cavity shape and the small degree of tetragonality of In at the Eutectic solidification temperature.
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