Rapid solidification processing of titanium-copper alloys containing rare earth additions
Ohls, Mark Anthony
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https://hdl.handle.net/2142/20507
Description
Title
Rapid solidification processing of titanium-copper alloys containing rare earth additions
Author(s)
Ohls, Mark Anthony
Issue Date
1990
Doctoral Committee Chair(s)
Rigsbee, J. Michael
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Language
eng
Abstract
Rapid solidification (RS) studies have been performed on various Ti-Cu (2-10 wt.%) alloys with and without 2.5 wt.% Er additions. Rapid solidification was effected by laser surface melting and/or centrifugal atomization. Prior to RS, careful characterization of the starting (arc-melted) materials was performed. Macrostructures of arc-melted buttons consisted of a 'classical' chill zone at the bottom, a columnar region in the middle and an equiaxed zone at the top. Microstructures of arc-melted hypoeutectoid binary Ti-Cu alloys consisted primarily of proeutectoid $\alpha$-Ti, which formed as overlapping plates (crisscross structure). Eutectoid was present between the plates. Microstructures of hypereutectoid alloys were almost entirely eutectoid ($\alpha$-Ti + Ti$\sb2$Cu).
Microstructures of laser treated binary Ti-Cu alloys consisted entirely of martensitic $\alpha\sp\prime$-Ti. Microstructures of laser treated Ti-Cu-Er alloys consisted of a supersatured solid solution of Cu, Er and O in $\alpha\sp\prime$-Ti together with discrete, globular particles. It was shown by analytical electron microscopy that the particles contained Er, O and a trace of Cu. Erbia (Er$\sb2$O$\sb3$) and $\alpha\sp\prime$-Ti were identified by x-ray diffraction analysis. A mechanism for the formation of the microstructure was related to a metastable liquid miscibility gap in the binary Ti-Er system.
Centrifugally atomized Ti-10Cu and Ti-10Cu-2.5Er alloys were found to consist of a supersaturated solution of Cu, Er (and oxygen) in $\alpha\sp\prime$-Ti and $\beta$-Ti. There were few second phase particles present in thin foils of Ti-10Cu-2.5Er powder indicating that no apparent liquid phase separation or extensive segregation occurred. It was shown quantitatively that the true fine particle density was significantly higher in finer powders.
Consolidation of the centrifugally atomized powders by hot isostatic pressing produced a microstructure with a fine (diameter $\sim$2 $\mu$m) grain size. The Ti-10Cu-2.5Er alloy exhibited a less refined dispersion of erbia. It was concluded that the coarse dispersion of erbia did not contribute to alloy strengthening. The 0.2% yield strengths of the HIP$\sp\prime$ed Ti-10Cu-2.5Er alloys were significantly higher than those of an annealed Ti-10Cu alloy. This is attributed to the fine grain size in the consolidated materials. (Abstract shortened with permission of author.)
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