The effect of hydrogen as a temporary alloying element on the microstructure and tensile properties of Ti-6Al-4V
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I.
INTRODUCTION
TITANIUM and titanium alloys have a large affinity for hydrogen. Pure titanium will absorb more than 60 at. pct (3 wt pct) hydrogen at one atmosphere pressure and 640 ~ Still greater concentrations are absorbed at lower temperatures.~'2 The most obvious and most studied effect of hydrogen on titanium alloys is embrittlement at ambient temperature. 3'4'5 Unlike other gases, the absorption of hydrogen is reversible. The diffusivity of hydrogen is sufficiently high, 6 and its equilibrium concentration at easily achieved vacuum pressures is sufficiently low, so that hydrogen can be removed by vacuum annealing. The ease with which hydrogen can be added to and removed from titanium has led to the utilization of hydrogen as a temporary alloying element. Room temperature embrittlement as a result of sufficiently high hydrogen contents provides an economical method for producing titanium powder by attrition. 7 Hydrogen is removed from the powder and ductility is restored by vacuum annealing. Several studies have shown improvements in the hot workability of titanium alloys charged with high concentrations of hydrogen.7 ~2 increases in plasticity were attributed to enhanced self-diffusion of titanium in the presence of hydrogen, and decreases in forging loads were related with hydrogen stabilization of the more hot-workable bcc beta phase. Hydrogen in titanium alloys may also be used to effect a phase transformation which results in unique, fine microstructures. Above 815 ~ Ti-6A1-4V containing 0.4 to 1.35 wt pct hydrogen transforms completely to beta. When specimens of this alloy containing 0.46, 0.93, and 1.35 wt pct hydrogen were water quenched from the beta phase field (870 ~ the phases identified were, respectively: primary alphaplus orthorhombic martensite (c~ .+ a~); orthorhombic martensite plus beta saturated with hydrogen (a~ + B.); and beta saturated with hydrogen (BH). No hydrides were
WILLIAM R. KERR is Metallurgist with High Temperature Materials Branch, Metals and Ceramics Division, Air Force Wright Aeronautical Laboratories/Materials Laboratory, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted December 8, 1981.
METALLURGICAL TRANSACTIONS A
detected either by optical microscopy or by X-ray diffraction. When heated to the beta phase field then step cooled to 650 ~ and held for various times, the 0.93 wt pct hydrogen specimen underwent a transformation which resulted in a finely divided mixture of three phases identified as ~,/3, and hydride. ~3 The transformation suggests a three phase eutectoid in which/3H ~ a + hydride. This reaction has been observed in Ti-6AI-4V containing 0.53 to 1.35 wt pct hydrogen and for isothermal transformation temperature in the range 540 ~ to 700 ~ Dehydrogenation, at appropriate temperatures, of the finely divided eutectoid transformation structure results in unique microstructures consisting of essentially equiaxed particles of alpha and beta. Transformation kinetics as determined by optical microscopy were reported in Reference 12. However, the
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