Interphase boundary precipitation in a Ti-1.7 at. pct Er alloy
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I.
INTRODUCTION
A. Rapid Solidification of Titanium-Erbium Alloys
TITANIUM–RARE earth alloys have been considered promising candidates for rapid solidification processing because of the low solid-state solubilities of rare earth elements in titanium and the large negative heat of formation of rare earth oxides.[1,2,3] The high cooling rates imposed by rapid solidification can result in a martensitic transformation from the primary b Ti phase to an a' Ti phase that is supersaturated with rare earth atoms. Subsequent annealing of the a' Ti yields a fine distribution of rare earth sesquioxide (RE2O3) particles within the titanium matrix.[2–7] A recently calculated equilibrium phase diagram[8] for the binary titanium-erbium system is shown in Figure 1. The phase diagram predicts that slowly cooling a hypoeutectic alloy from the liquid state first results in solidification of proeutectic b Ti. The eutectic reaction at 1320 7C (L → b Ti 1 Er) is followed by a peritectoid transformation (b Ti 1 Er → a Ti) at approximately 890 7C. If the solid-state cooling rate is greater than 103 7C s21, the b → a transformation can be diffusionless, yielding martensitic a' Ti.[9] If the cooling rate is below the critical rate for martensite formation, the transformation occurs by diffusional nucleation and growth of a Ti. Of the titanium–rare earth alloys, rapid solidification of Ti-Er received the most attention because of the relatively large solid solubility extension of erbium in titanium and the thermal stability of erbium oxide precipitates.[10] Processing by electron-beam melting/splat quenching[3] or laser surface melting[11] results in a single-phase a' Ti microstructure for erbium concentrations up to 1.8 at. pct. Annealing the a' Ti produces erbium oxide particles with diameters ranging from 5 to 200 nm, depending on the temperature M.V. KRAL, formerly Graduate Student, Department of Materials Science and Engineering, Vanderbilt University, is Postdoctoral Fellow, Naval Research Laboratory, Physical Metallurgy Branch, Washington, DC 20375. W.H. HOFMEISTER, Research Associate Professor, Department of Chemical Engineering, and J.E. WITTIG, Associate Professor, Department of Applied and Engineering Sciences, are with Vanderbilt University, Nashville, TN 37235. Manuscript submitted August 1, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
and duration of annealing. Annealing temperatures are typically maintained below the b:a transus to avoid rapid coarsening of precipitates in the b Ti phase. Konitzer et al.[11] studied precipitate morphology and crystallography in a laser surface melted and annealed Ti-0.7 at. pct Er alloy and identified two precipitate types. Type I particles were faceted, were identified as cubic Er2O3 (I a 3) with a0 5 ˚ , and had an orientation relationship with the ma10.15 A trix, where {1 1 1} \ (0 0 0 1)Ti ,1 1 0. \ ,1 1 2 0.Ti Type II particles were more common than type I, were also ˚ , and identified as cubic Er2O3 (I a3) with a0 5 10.547 A had an orientation relationship with the matrix, where {
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