Aqueous environmental crack propagation in high-strength beta titanium alloys
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INTRODUCTION
M E T A S T A B L E beta titanium alloys are being developed for structural applications that require formability, hardenability, fracture toughness, and aqueous corrosion resistance.l~-4) The environment-assisted cracking (EAC) behavior of these alloys is important for long-life components stressed in aggressive marine, oil or gas well, aerospace, and medical implant environments. 15'61While the stress-corrosion cracking, hydrogen embrittlement and corrosion fatigue of a and a/Btitanium alloys have been researched extensively. 17-~~]EAC in beta and a-precipitation-hardened B-titanium alloys is relatively unexplored.* *a is the hexagonal close-packed phase, while/3 is body-centered cubic.
Solution-treated and aged (STA) B-titanium alloys with c~ precipitates exhibit excellent yield strength (trvs) and fracture toughness (K~c) but can be susceptible to brittle cracking in ambient-temperature aqueous solutions with halide ions. (~~ Early experiments utilizing cracked specimens and fracture mechanics methods demonstrated that developmental B-titanium alloys LISA M. YOUNG, formerly Graduate Research Associate, University of Virginia, is Metallurgist, General Electric Corporate Research and Development Center, Schenectady, NY 12301. GEORGE A. YOUNG, Jr., formerly Graduate Research Associate, University of Virginia, is Engineer, Knolls Atomic Power Laboratory, Schenectady, NY 12301. JOHN R. SCULLY, Assistant Professor, and RICHARD P. GANGLOFF, Professor, are with the Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903. Manuscript submitted December 6, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
(Ti-11.5Mo-6Zr-4.5Sn and Ti-13V-I 1Cr-3A1; both STA)* *All compositions are in weight percent.
and model Ti-Mo compositions were prone to EAC in NaC1 and KCI solutions at static or quasi-static load threshold stress intensities (Ktscc) as low as 15 pct of K~c.18'13-~5iThe EAC in these high-strength fl-titanium alloys progressed by intergranular separation and transgranular cleavage or "quasi cleavage," depending on alloy composition and microstructure. 17Jl.~3i The EAC was exacerbated at intermediate loading rates, at intermediate applied electrode potentials (near - 6 0 0 mVscE), and by increased ~rvs;IsJ4-~6i however, data were limited. Studies o f EAC in modern B-titanium alloys, employing the slow strain-rate technique with smooth uniaxially loaded tensile specimens, demonstrated resistance to brittle cracking in aqueous NaCI at 25 ~ and near-free corrosion potentials,l~2.JT-22i For example, high-strength STA Ti-3AI-8V-6Cr-4Mo-4Zr (BETA-C*) was not susceptible to EAC in neutral chloride, with or without cathodic polarization; such conditions promoted cracking of a/B Ti-6A1-4V. 12~ Similar good EAC resistance was reported for STA Ti- 15Mo-3Nb-3AI (Beta-2 IS or TIMETAL-21S**), t221however, Ti-15V-3Cr-3AI-3Sn (Ti15-3) was susceptible to brittle cracking in neutral aqueous chloride, but only at intermediate loading r
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