A Fatigue Life Model for Predicting Crack Nucleation at Inclusions in Ni-Based Superalloys
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I.
INTRODUCTION
STRUCTURAL alloys such as steels,[1] Al-alloys,[2–6] and Ni-based superalloys[7–21] often contain non-metallic inclusions in the microstructure. These inclusions can serve as fatigue crack nucleation sites during cyclic loading conditions, thereby promoting the onset of fatigue failure and reducing the cycles-to-failure or fatigue life. Powder-metallurgy (PM)[7–9,14–22] and additively manufactured (AM) superalloy[23,24] components are particularly disposed to exhibiting inclusions in the microstructure since they are processed using powder particles that can be mixed with small amounts of non-metallic inclusions. As a result, there has been considerable interest in characterizing and studying the influence of inclusions on the fatigue life of forged,[10–13] PM[14–21] and AM[21,22] superalloy materials. For examples, carbide-induced fatigue fracture was studied for forged IN-718 by Spa¨th et al.,[11] Ono et al.,[12] and Bhowal et al.[13] Spa¨th et al.[11] showed that the fatigue life of IN 718 increased with increasing ASTM grain size number (i.e., decreasing grain size). In addition, fatigue lives were lowered when crack nucleation switched from
KWAI S. CHAN is with the Southwest Research Institute, San Antonio, TX 78238 and also now with the MESI Technologies LLC, San Antonio, TX 78250. Contact e-mail: [email protected] Manuscript submitted August 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
slipbands to small carbides.[11] Inclusion-initiated fatigue fracture was investigated for PM Astroloy,[20,21] Rene 95,[14–16] Rene 88 DT,[17,18] and ME3,[22] as well as for AM 718Plus.[23,24] Inclusions that have been identified as crack nucleation sites in Ni-based superalloys include Al2O3,[16,20,21] MgO,[21] SiO2,[21] carbides,[10–13] and nitrides.[10] These non-metallic inclusions (NMI) can manifest as hard particles or granular agglomerates.[14,15,20] Most of the inclusion-induced nucleation occur in interior grains but some may lie near the surfaces, while slipband-induced crack nucleation also can occur in interior and surface grains.[7–10,17–22,25,26] The transition of interior nucleation at inclusions to surface nucleation at matrix grains was also reported in a number of investigations.[7–10,17–22,25,26] Furthermore, the concurrent occurrence of slipband facets, which are evidence of slipband-induced nucleation, and inclusion-induced facets, which are evidence of inclusion-induced nucleation, in individual fatigue specimens has been reported, but the reason for its occurrence is not well understood. Early studies of correlations of oxide inclusion and fatigue fracture were reviewed by Lankford,[1] who reported an empirical relation between a fatigue strength reduction factor and inclusion size to a power of 1/3. Micromechanics-based fatigue life models for slipband nucleation and inclusion nucleation were proposed by Tanaka and Mura[27,28] and extended by Chan.[29,30] Statistical models of crack nucleation at inclusions were
proposed by Morris and James[6] for Al-alloys on the basis
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