Surface hardening of Ti-6AI-4V alloy by electrochemical hydrogenation
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I.
INTRODUCTION
THE absorption o f hydrogen causes serious damage in titanium alloys, but proper usage o f hydrogen as a temporary alloying element can bring surprisingly beneficial effects f o r certain materials. By adding an adequate amount o f hydrogen to some titanium alloys, the forging and superplastic forming temperatures o f these alloys can be lowered.t].2] Also, the microstructure o f the alloys can be modified by hydrogenation, I~-9] w h i c h has been adopted by the aerospace industry to refine coarse and dendritic microstructures, thus improving their mechanical properties. W e note that the microstructures o f titanium net-shape products are very difficult to change by using conventional thermomechanical treatments. Thermal charging is not the only way to hydrogenate Ti-6A1-4V alloy. In this study, hydrogenation was carried out by various aqueous cathodic polarizations to improve surface hardness. A comparison o f microstructure and hardness changes between various processing parameters is reported. II.
EXPERIMENTAL PROCEDURES
grade abrasive paper, ultrasonically cleaned, and cathodically charged with 1 m A / c m2 at 15 °C in 1 N NaOH and 1 N n 2 s o 4 for 24 and 48 hours with o r without 1 g / L thiourea, respectively; then they were removed from the electrolyte, cleaned with deionized w a t e r and acetone, and dried with pressurized air. The specimens were subsequently dehydrogenated in a tubular furnace in an argon atmosphere to 760 °C. The furnace was subsequently p u m p e d to a vacuum o f 2 x 10-4 Pa, and the temperature was held at 760 °C f o r 1 hour. Then the p o w e r was turned off and the v a c u u m system was kept running during furnace-cooling to room temperature. To compare the effects of low-temperature annealing on grain refinement, specimens with 48 hours charging ($9 to S12) were subjected in an air furnace at 200 °C for 3 hours followed by forced air convection; then they were placed in a vacuum furnace, and the same dehydrogenation treatment was performed. C. Microscopy Specimens prepared for metallographic observations were finely ground by 1000 grade SiC paper, polished with l/xm A1203 powder, and etched with Kroll's reagent.
A . Material The material used in this study was received in the mill-annealed condition. The chemical composition is analyzed and given in Table I. Round bar stocks 13 mm in diameter were first/3-solution-treated at 1000 °C in a vacuum o f 2 x l 0 - 4 Pa and then furnace-cooled to r o o m temperature. Specimens were cut from the treated bar to 3 mm in thickness for various processing treatments.
Table I. Chemical Composition of the As-Received Alloy (Weight Percent)
A1 6.48
V C Fe O N H Ti 4.27 0.044 0.204 0.16 0.012 0.0079 balance
~-solution
B . Thermochemical Processing Thermochemical treatment and processing parameters are shown in Figure 1 and Table II. The processing parameters were grouped from S1 to S 12. S 13 to S 16 were performed earlier at our laboratory, and the experimental results have been published, [1°'HI these results are li
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