Effect of Microstructure and Alloy Chemistry on Hydrogen Embrittlement of Precipitation-Hardened Ni-Based Alloys
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PRECIPITATION-HARDENED (PH) nickel-based superalloys combine high strength, ductility with good formability, weldability, and long-time microstructural stability at elevated temperatures. They derive their strength from the combination of solid-solution and precipitation strengthening with the latter through the
G.C. OBASI, Z. ZHANG, and D. SAMPATH are with the BP International Centre for Advanced Materials, School of Materials, The University of Manchester, Manchester, M13 9PL, UK. Contact e-mail: [email protected] ROBERTO MORANA is with the BP Exploration Operating Company Limited, Sunbury-on-Thames, TW16 7L, UK. N.R. AKID and M. PREUSS are with the School of Materials, The University of Manchester, Manchester, M13 9PL, UK. Manuscript submitted August 8, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
formation of L12-ordered c¢ (Ni3(Ti, Al) and/or DO22ordered c¢¢ (Ni3Nb) in a face-centered-cubic (fcc) c matrix. In addition, precipitation of grain boundary carbides contributes to creep strengthening. Importantly, c¢¢ is a metastable phase and a precursor for the more stable orthorhombic d phase, which has the same stoichiometry as c¢¢, but tends to be significantly coarser and incoherent with the matrix.[1] Alloy 718 (UNS N07718) was specifically developed for high-strength and high-temperature aeroengine applications by the formation of fine c¢¢ when heat treated appropriately. It also displays good resistance to stress corrosion cracking.[2] Today, Alloy 718 is used far more widely, now also including applications in the oil and gas sectors for high-pressure, high-temperature (HPHT) applications. However, corrosion processes can result in hydrogen ‘pick-up’ during service in these environments that may cause hydrogen enrichment in
Table I. Alloying Elements 718 945X
Chemical Compositions of Ni Alloys 718 and 945X (In Wt Pct)
Ni
Cr
Mo
Cu
Nb
Ti
Al
C
Fe
53.5 53.5
18.2 20.6
3 3.2
2.2
5 4.1
1 1.52
0.5 0.12
0.08 0.011
bal. bal.
near-surface regions and, consequently, local embrittlement of the material.[3] To date, hydrogen embrittlement (HE) in PH nickel-based superalloys is mitigated by limiting the hardness to less than 40 HRC or 388HV.[4] This is achieved in Alloy 718 by overaging the material, which results in the reduction of c¢¢ volume fraction and the formation of d phase. However, study by Liu et al. has highlighted potential deleterious effects of d formation as the d/matrix interface can initiate transgranular fractures and microcracks, as well as promoting susceptibility of the alloy to HE by a mechanism that remains to be identified.[5] HE has been strongly linked with hydrogen segregation toward dislocations and hydrogen transport in the form of a Cottrell atmosphere around these dislocations, thereby reducing the stress field associated with a dislocation.[5,6] Hence, by the presence of hydrogen, the elastic interaction between dislocations is reduced resulting in a lower critical stress level at which dislocations become glissile.[7,8] Such behavior, in conjunction wit
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