Mutual effects of hydrogenation and deformation in Ti-Nb alloys
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11/9/03
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Mutual Effects of Hydrogenation and Deformation in Ti-Nb Alloys D. ZANDER, D.L. OLSON, and D. ELIEZER Alloying of Ti-based alloys with hydrogen is used to modify the microstructure and improve mechanical properties. In this study, hydrogen charging was performed electrochemically in a 2:1 glycerinphosphoric acid electrolyte at high fugacities. This research investigated in detail, by means of X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal desorption spectroscopy (TDS), and microhardness tests, the influence of hydrogen at high fugacities on the phase stability, desorption behavior, and microhardness in Ti-Nb (20 to 45 wt pct Nb) alloys before and after deformation. Hydrogenation of Ti-Nb was found to exhibit a significant effect on the phase stability as well as the microhardness of Ti-Nb alloys. Hydrogenation of Ti-20 wt pct Nb led to precipitation of (Ti,Nb)Hx in the metastable matrix. In Ti-Nb alloys with 40 or 45 wt pct niobium, hydrogen stabilized the bcc phase, but destabilized the hcp phase. With increasing hydrogen content, an expansion of the lattice constant of the phase occurred, followed by the formation of (Ti,Nb)Hx . The influence of hydrogen on the microhardness of Ti-40 wt pct Nb and Ti-45 wt pct Nb shows only a minor effect, whereas Ti-20 wt pct Nb exhibits significant softening in response to hydrogen charging up to 3000 ppm. The TDS showed that deuterium desorption strongly depends on the niobium content and the deformation treatment prior to or after charging. The observed results should provide further insight on the mutual effects and the resulting micromechanism of hydrogenation and deformation in Ti-Nb alloys.
I. INTRODUCTION
THE wide spectrum for application of titanium and its alloys has necessitated adjusting their mechanical properties to the appropriate structural needs. The microstructure of -and -type titanium alloys has a dominating influence on the tensile strength, fatigue properties, and fracture toughness. Historically, thermomechanical processing and heat treatment were used to control microstructural properties in titanium alloys,[1] but more recently, thermochemical processing (TCP)[2–5] has become the more important technique in improving processing, as well as the mechanical properties of these alloys. The design of Ti-based alloys can be made more efficient through the use of hydrogen as a temporary alloying element, since both manufacturing temperatures and stresses can be decreased. Early work using hydrogen-assisted processing in Ti-based alloys demonstrated that hydrogen could lead to improved workability.[6,7,8] Titanium and conventional Ti-based alloys have a high solubility for hydrogen. The Ti-H system is of a eutectoid type and consists of several hydride phases of the hcp phase and the bcc phase: two interstitial solid solutions of hydrogen based on the allotropic and forms of pure titanium.[9] The -titanium alloys have D. ZANDER, Assistant Professor, is with
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