A Grain Boundary Fracture Model for Predicting Dynamic Embrittlement and Oxidation-Induced Cracking in Superalloys
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INTRODUCTION
IT is well-established that the interaction of oxygen with metallic alloys such as iron-,[1] nickel-,[2–4] and cobalt-based superalloys[5] at elevated temperatures can lead to two closely related environmental degradation mechanisms, which have been coined as (1) oxygen[1,4] or dynamic embrittlement,[6–9] and (2) stress-accelerated grain boundary oxidation (SAGBO) and fracture of the oxidized grain boundaries.[1,4] Both degradation mechanisms are time-dependent fracture processes that proceed along grain boundaries at elevated temperatures [‡923 K (650 C)] where intake of oxygen by grain boundary diffusion can occur at relatively short times. The pioneering work by Bricknell and Woodford[1–5] has firmly established that oxygen embrittlement,[1,4] also referred to as dynamic embrittlement,[6–9] is likely the operative degradation mechanism in iron-based superalloy IN 903A, cobalt-based superalloy, pure Ni, and Ni-based superalloys.[4] Dynamic embrittlement was first used to describe the loss of tensile ductility and fracture toughness in Ni3Al due to oxygen-induced intergranular fracture at both ambient and elevated temperatures.[6] Under sustained loading in air, the fracture process proceeds as a timedependent grain boundary decohesion process instigated by the ingress of oxygen from the external surface to the interior grain boundaries in polycrystalline Ni3Al. Dynamic embrittlement of Ni3Al can be reduced by the microalloying of boron to strengthen the grain boundaries[6] and by Cr addition to promote the formation of KWAI S. CHAN, Institute Scientist, is with the Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238. Contact e-mail: [email protected] Manuscript submitted July 31, 2014. Article published online March 31, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
a protective oxide layer to suppress or delay the ingress of oxygen to interior grain boundaries.[10,11] As pointed out by several investigators,[7,9,12,13] dynamic embrittlement of Ni alloys due to the segregation of oxygen to grain boundaries is similar to temper-embrittlement induced as the result of segregation of trace elements such as S, P, and Sn to grain boundaries.[14,15] These impurities lower the grain boundary cohesion, promote intergranular fracture, and reduce both tensile ductility and fracture resistance. The time-dependent crack growth response of IN 718 polycrystalline material and bicrystals under sustained loading at 923 K (650 C) were reported by Pfaendtaner and McMahon[7] and by Krupp.[8,9] Rapid crack growth rates, da/dt, were observed as a function of the stress intensity factor, K, once a growth threshold was exceeded. The fracture surfaces appeared to show an absence of oxides on grain boundary facets and the embrittlement mechanism was identified to be oxygen-induced intergranular cracking or dynamic embrittlement.[7–9] On the other hand, the crack growth threshold, Kth, of IN 718 was relatively high in the dynamic embrittlement regime. Figure 1 shows a comparison of the da/dt data of IN 718 t
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