A new approach to strengthen grain boundaries for creep Materials Science & Engineering A 625 (2015) 164–168 A new approach to strengthen grain boundaries for creep improvement of a Ni-Cr-Co-Mo superalloy at 950 ℃ J.W. Lee a , D.J. Kim b , H.U. Hong a a Department of Materials Science and Engineering, Changwon National University, 20 Changwondaehak-ro, Changwon, Gyeongnam 641-773, Republic of Korea b Nuclear Materials Research Division, Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea Advisor：Tzu-Yao Tai Advisee：Pei-Yu Zhu Department of Mechanical Engineering Southern Taiwan University Date：2015/12/22

Outline Experimental - Solution-treated Standard specimen - Proprietary heat treatment Serrated GBs - Slow cooling process The GB began to be wavy - Tensile strain of 5% at room temperature Serrated GBs Results and Discussion Conclusions

Standard specimen containing straight GBs Solution-treated The investigated material was a commercially available Alloy 617 in a hot-rolled plate form of 19.5 mm thickness. The chemical composition of this plate was Ni-22Cr-12.3Co-9.5Mo-0.39Ti-0.99Al-0.08C (wt%). Few Cr-rich M 23 C 6 secondary carbides least 5% γ' volume fraction with an average precipitate size of around 20-30 nm The as-received Alloy 617 which was solution-treated from a manufacturer had a mean grain size of 80-100 μm with Mo-rich M 6 C primary carbide stringers parallel to rolling direction. Standard specimen containing straight GBs

Proprietary heat treatment Alloy 617 had serrated GBs decorated with thick planar M 23 C 6 . The samples were solution-annealed at 1200 ℃ for 10 min and slow-cooled to 800 ℃ at a cooling rate of 5 ℃/min, subsequently aged for 2 h at the same temperature. Serrated GBs

Slow cooling process When a specimen was cooled from 1200 ℃ to 1100 ℃ at 5 ℃/min. The GB began to be wavy

Tensile strain of 5% at room temperature A cylindrical specimen of 6 mm gauge diameter and 25 mm gauge length was employed for creep test with the condition of 960 ℃/30 MPa in air. Cold work treatment was carried out by loading the heat-treated specimen to a tensile strain of 5% at room temperature with a strain rate of 5× 10 −4 /s before creep tests.

Results and Discussion Alloy 617 950℃/30MPa/Air Alloy 617 950℃/30MPa/Air

Pre-strained/serrated specimens Pre-strained/serrated specimens Showing longitudinal sections near the fractured surfaces after creep at 950 ℃/30 MPa Fig a-c corresponds to Pt-passivated area for machining TEM foil by FIB system. Pre-strained/serrated specimens Pre-strained/serrated specimens Standard Standard Serrated Serrated

Ideal microstructures Pre-strained/serrated specimens Standard Distribution of dislocations and M 23 C 6 carbides in the subgrain structures after creep at 950 ℃ /30 MPa for the various microstructures Ideal microstructures Pre-strained/serrated specimens Standard Serrated

Conclusions In summary, the proprietary heat treatment led successfully to the transition of serrated GBs for the first time in a solid-solution strengthened Alloy 617. TEM analysis on the serrated GB without precipitates indicated that the fundamental driving force for the GB serration stems from lowering interfacial free energy of GB per unit area. The serrated GBs with fine/stable intragranular carbides ensured 2.8 times better creep resistance due to their significant contribution to GB strengthening.

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