Review of environmental effects in intermetallics
- Oak Ridge National Lab., TN (United States). Metals and Ceramics Div.
The phenomenology and mechanisms of environmental embrittlement in ordered intermetallics is reviewed by considering two model alloy systems of the L1{sub 2} and B2 crystal classes (Ni{sub 3}Al and FeAl). Both H{sub 2}O and H{sub 2}, at levels found in ordinary ambient air, are found to cause environmental embrittlement, with the former usually more deleterious. In the case of H{sub 2}O, the micromechanism involves reaction with the intermetallic to form an oxide (or hydroxide) and simultaneous generation of atomic hydrogen which then enters the metal and causes embrittlement. In the case of H{sub 2}, on the other hand, atomic hydrogen is generated as a result of the dissociation of physisorbed hydrogen molecules on the intermetallic surfaces. Consistent with the proposed embrittlement mechanism, ductility is found to increase with decreasing amounts of H{sub 2}O (or H{sub 2}) in the test environment, increasing strain rate, and decreasing (or increasing) temperature. Environmental embrittlement in Ni{sub 3}Al (and other L1{sub 2} alloys) occurs predominantly intergranularly, whereas in FeAl (and other B2 alloys) it can also occur transgranularly--presumably because diffusion of hydrogen is fast enough through the bulk in the more open B2 structure but only so along grain boundaries in the L1{sub 2} structure. Microalloying with B, which segregates strongly to the grain boundaries, can overcome environmental embrittlement in L1{sub 2} alloys, but not in B2 alloys; in the latter, alloying additions probably have to be added at significantly higher (macroalloy) levels to affect the bulk properties. In neither alloy is environmental embrittlement the sole source of brittleness: depending on the alloy stoichiometry, and grain boundary character, a given grain boundary may be intrinsically weaker (or stronger) than the bulk, thereby influencing overall ductility.
- Research Organization:
- Oak Ridge National Lab., TN (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 52838
- Report Number(s):
- CONF-941144--131; ON: DE95010120
- Country of Publication:
- United States
- Language:
- English
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