Improvement of Ti alloy fatigue properties by Pt ion plating
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HIGH temperature titanium alloys such as Ti-6A12Sn-4Zr-2Mo, l Ti-11, z IMI-6853 and Ti-5A1-5Sn-2Zr2Mo 4 were developed to operate under prolonged exposure at 500 ~ Due to increase of surface oxidation and decrease of creep strength, the use of these alloys is limited to this temperature for extended service periods. Creep experiments of Ti-6242, s IMI-6856 and Ti-115 conducted in vacuum or Ti-5522 in argon atmosphere, 4 showed that due to the lack of oxygen, the creep strength of these alloys can be substantially improved. It was also shown that the air creep strength of titanium alloy specimens improves when they are ion-plated with a ductile oxidation resistant coating like Au or Pt. The Pt was found to provide better oxidation resistance than Au at the 500 to 600 ~ range v due to its lower solubility and diffusivity in titanium at high temperatures. 8 It was also shown that the oxidation resistant coatings will improve the creep strength only in alloys containing both alpha and beta phases. 9 Several studies on protective coatings for titanium alloys have been performed in the past. t~ However, the use of either electroplating and subsequent diffusion or slurry process caused severe fatigue strength degradation. The objective of this work was to find out if the Pt ion plating, which was previously developed for improved creep and oxidation resistance, will not cause fatigue strength degradation which will restrict its usefulness for aerospace component applications. EXPERIMENTAL PROCEDURE Material and Specimens The material used in this work was taken from 30 mm diam bars of the Ti-6A1-2Sn-4Zr-2Mo-0.1 Si alloy solution-treated for one hour at 950 ~ and water quenched. The material was further annealed at 600 ~ for 8 h and air cooled. The resulting microstructures
consisted of 60 vol-pct primary alpha phase in a matrix of fine transformed beta phase (Fig. 3(d)). Round smooth fatigue specimens with 6 m m gage diam and 25 m m gage length were machined parallel to the long axis of the heat treated bars. The specimen geometry is shown in Fig. 1. The specimens were machined and polished to a minimum of 2 RMS surface finish to reduce surface defect effect on fatigue crack initiation. Tests were performed on a mechanical 6 ton axial loading Schenck fatigue machine at 33 Hz. A resistance heating furnace was used to heat the fatigue specimens to 4 5 _+ 3 ~ The load-time waveform was sinusoidal and tests were conducted in air at a stress ratio ( m i n i m u m / m a x i m u m ) R -- 0.1. To establish the tensile properties of the heat treated material, tensile tests were performed at room temperature and 455 ~ R o u n d specimens with 4 mm gage diam and 25 mm gage length were machined parallel to the long axis of the bars. Three tests were performed for each temperature at a crosshead rate of 0.4 m m / m i n which corresponds to a gage length strain rate of 0.01 m m / m m 9 min-i. The average test results are summarized in Table I. Coating Some of the round fatigue specimens were Pt ion plated in an ion-plating apparatus m
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