Environmental embrittlement caused by hydrogen for intermetallic compounds: Preliminary model of ductility reduction
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I. INTRODUCTION
MANY intermetallic compounds are embrittled at room temperature in air with moisture, a hydrogen atmosphere, etc., and exhibit a reduction of elongation. For example, L12 intermetallics like Ni3Al-, Co3Ti-, Ni3Si-, and (Fe,Co,Ni)3Vbased polycrystalline alloys[1–4] are embrittled in air and exhibit intergranular fracture surfaces. A Co3Ti-based (Co74Ni3Ti23) single crystal is also embrittled and exhibits a dimpled fracture surface mixed with a small amount of cleavage facets in air.[2] Fe-Al alloys with B2 and D03 structures exhibit the reduction of fracture strain in air and hydrogen gas.[5,6] TiAl (L10 structure)–based alloys are embrittled and exhibit cleavage fracture and separation of lamellae in air and hydrogen gas.[7,8] They also exhibit slow crack growth, that is, delayed fracture-crack growth, in air.[9,10] Recently, it has been found that hydrogen is produced on the specimen surface of Ni3Al-,[11] Fe3Al-,[12] or FeAlbased[13] alloys, when the surface reacts with the air. It is also reported that a Ni3Al-based alloy exhibits an excellent room-temperature elongation in an ultrahigh vacuum, although it has poor elongation in air.[1] These facts indicate that poor room-temperature elongation of intermetallic alloys is caused partly by an extrinsic factor, although it cannot be explained for all intermetallic compounds. In this report, we propose a preliminary model which phenomenologically explains hydrogen-induced environmental embrittlement in tensile tests for intermetallic alloys.
MORIHIKO NAKAMURA, Supervising Researcher, is with the National Research Institute for Metals, Ibaraki 305-0047, Japan. TATSUO KUMAGAI, formerly Senior Researcher, National Research Institute for Metals, is Associate Professor, National Defense Academy, Kanagawa 2390811, Japan. Manuscript submitted June 15, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS A
II. ENVIRONMENTAL EMBRITTLEMENT OF INTERMETALLIC ALLOYS IN TENSILE TESTS It is well known that many intermetallic alloys, such as those based on Ni3Al, Ni3Si, Co3Ti, (Ni,Co,Fe)3V, Fe-Al, and TiAl, exhibit environmental embrittlement in tensile deformation in air at room temperature. The characteristics of environmental embrittlement are as follows. (1) In general, the test environment, such as air, vacuum, and hydrogen gas, hardly affects a tensile stress–strain relationship, including yield strength at room temperature, but a reduction of elongation is observed in air and hydrogen gas.[2] When a large amount of hydrogen is introduced by cathodically charging the specimen, the flow stress is changed.[14] (2) Ni3Al,[1] Fe-Al,[5] and (Co, Fe)3V[15] have good or excellent tensile elongation in high vacuum and oxygen gas. (3) Elongation increases with increasing strain rate in air.[14,16,17] (4) In cathodically hydrogen–charged specimens, baking recovers the elongation, which is decreased by hydrogen charging.[18] Room-temperature environmental embrittlement is considered to be caused by hydrogen, because hydrogen is produced on the specimen surface of Ni3Al-, N
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