Hydride structures in Ti-aluminides subjected to high temperature and hydrogen pressure charging conditions

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The distribution and chemistry of hydrides produced in single and dual phase alloys with a composition near TiAl have been investigated by using a combination of TEM and x-ray diffraction techniques. The alloys were exposed at 650 °C to 13.8 MPa of gaseous H2 for 100 h. In the single-phase gamma alloy, large hydrides preferentially nucleated on the grain boundaries and matrix dislocations and a population of small hydrides was distributed throughout the matrix. X-ray and electron diffraction patterns from these hydrides indicated that they have an fee structure with a lattice parameter of 0.45 nm. EDAX analysis of the hydrides showed that they were enriched in Ti. The hydrides were mostly removed by vacuum annealing at 800 °C for 24 h. On dissolution of the hydrides, the chemistry of hydride-free regions of the grain boundary returned to the matrix composition, suggesting that Ti segregation accompanied the hydride formation rather than Ti enrichment causing the formation of the hydride. The hydrogen content in the two-phase (7-0:2) alloy was approximately three times that of the single phase alloy, which was presumably a consequence of the presence of the a 2 -Ti3Al phase in the two-phase alloy. The hydrides in the two-phase material were shown by x-ray diffraction to have an fee structure and were removed on annealing in vacuum at 800 °C for 24 h.

I. INTRODUCTION In a comparison of the properties of materials, especially at elevated temperatures, the low density, high melting temperature, good elevated temperature strength and modulus retention, and the creep and oxidation resistance of the titanium aluminides make them a material of great interest for structural applications in advanced hypersonic flight vehicles.1'2 However, use of these materials is limited because of their poor ductility at room temperature which results in a low fracture toughness and a fast fatigue crack growth rate. These problems may be partially ameliorated by suitable alloying and processing conditions.1 An additional requirement imposed by the aerospace applications is that the material must not be susceptible to hydrogen embrittlement, as in some areas of use the material will be exposed to a wide variety of temperature/hydrogen gas pressure combinations. This requirement may be the most stringent of all and may limit or even exclude the employment of titanium aluminides. It has been shown that Ti3Al based systems are susceptible to hydrogen embrittlement.3'4 Chu et al? have demonstrated that in compression tests the yield strength increased, but the ultimate tensile strength, strain to failure, nominal fracture stress, cleavage fracture strength, and fracture toughness decreased with increasing hydride 1230 http://journals.cambridge.org

J. Mater. Res., Vol. 6, No. 6, Jun 1991

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content. The structure of the hydrides formed in Ti3Al under low temperature and low hydrogen pressure charging conditions has been investigated by Rudman et al.5 and Xiao et al.6 Rudman et al. identified the following phases: an hep