Hydrogen tolerance of a Ti 3 AI-based alloy
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INTRODUCTION
Two o f the materials that had been considered for possible applications in the National Aero-Space Plane (NASP) were the Ti3A1- and TiAl-based intermetallic alloys. For these applications, the candidate materials will encounter high-temperature, high-pressure gaseous hydrogen. [m~ Since hydrogen is known to cause embrittlement in many alloys and intermetallics, [3,4,5j compatibility between material and high-temperature, high-pressure gaseous hydrogen must therefore be examined before either o f the two titanium aluminide alloys can be used in the NASP. Recent studies have shown that Ti3Al-based alloys are susceptible to hydrogen embrittlement. In the second NASA Hydrogen Workshop,TM it was reported that Ti3A1 was attacked by hydrogen.J6] Hydride formation was observed in stoichiometric Ti3AI, Ti-24Al-llNb, and Ti25A1-10Nb-3V-1Mo charged with H2.~6-~3~ The hydrides were observed, via transmission electron microscopy, to lie on the p r i s m planes in the Ti-24-11 alloy and on the b a s a l planes in the stoichiometric Ti3A1 alloy.[7~ The presence o f hydrides in Ti3Al-base alloys generally led to degradation in mechanical properties, t3-7,9-ua4-z°l. The fracture strength and ductility o f H2-charged Ti-24-11 were reduced in tension, but the yield strength increased under compression.[~4J Zero ductility was observed in this alloy when the H2 content reached 4000 wppm. t61 In a 13.8 M P a gaseous H2 environment, Ti-24-11 specimens subject to a constant stress o f 75 pc[ o f the ultimate tensile strength failed in 50 hours at 93 °C but failed in only 4 hours at 204 °C.[~sJ The Ti-25Al-10Nb-3V-1Mo alloy (Super Alpha-Two) showed significant decreases in both elongation and reduction in area upon exposure to hydrogen, t15'~6] The fracture mechanisms were predominantly cracking in the a 2 phase and at the a2/[3 interface. In contrast, fracture occurred in the fl phase when the KWAI S. CHAN, Staff Scientist, is with Southwest Research Institute (SwRI), San Antonio, T X 78228-0510. Manuscript submitted April 9 , 1992. METALLURGICAL TRANSACTIONS A
alloy was tested in a He environment at the same pressure. [x7] In a series o f recent articles, tt~,t9,2°l Thompson and coworkers reported the detrimental effects o f hydrogen on the tensile behavior, os,x91 fracture toughness, t~8,19] and sustained-load cracking~2°1 in Ti-24-11 charged with various amounts o f hydrogen. Their tensile results indicated that the ultimate tensile strength, tensile ductility, fracture strength, and fracture toughness (Klc values) all decreased with increasing hydrogen contents. [18,~9] Like uncharged materials, fracture in hydrogen-charged Ti24-11 occurred by formation o f microcracks ahead o f the c r a c k tip and the subsequent linkage o f microcracks with the main crack. ]~8,~9,2°] This fracture process led to stable c r a c k growth and a resistance-curve behavior; the magnitude o f the resistance curve, however, was lowered by an increasing hydrogen content.[~8a9,2°] Despite exhibiting a resistance-curve behavior, the fracture
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