Superalloy 718 and Derivatives: Microstructure & Properties
Program Organizers: Joel Andersson, University West; Chantal Sudbrack, National Energy Technology Laboratory; Eric Ott, GE Additive; Zhongnan Bi, Central Iron and Steel Research Institute
Monday 1:55 PM
May 15, 2023
Room: Admiral
Location: Sheraton Pittsburgh Hotel at Station Square
Session Chair: Eric Ott, GE Additive; Ian Dempster, Wyman Gordon /PPC
1:55 PM
Tailoring the γ- γ′-γ″ Dual Superlattice Microstructure of Ni-based Superalloy IN725 by High Temperature Aging and Nb/Ta Additions for Superior Creep Properties: Stoichko Antonov1; Chang-Yu Hung1; Jeffrey A. Hawk1; Paul D. Jablonski1; Martin Detrois1; 1National Energy Technology Laboratory
Next generation energy systems require superior resistance to creep deformation due to the considerably pro-longed exposure times at operating stress and temperature. To improve the elevated temperature properties of Inconel alloy 725 (IN725), a corrosion resistant alloy, several variants with different Ti/Al ratio and judicious amounts of Nb and Ta were made. Furthermore, a high temperature aging (HTA) heat treatment – where a higher temperature first step was used to promote precipitate phase formation - was explored. These adjustments allowed tailoring of the amount and type of precipitate strengthening which led to significant increases in time to failure. The Ti/Al ratio was used to design alloys with preferential formation of γ′ or γ″ precipitates. Compact morphology precipitates, which have superior coarsening resistance, were formed in alloys with a low Ti/Al ratio. The HTA increased the creep life of various alloy formulations up to a maximum improvement of 371% as compared to the standard aging heat treatment. The Nb and Ta additions had a similar effect on increasing creep life by promoting and stabilizing γ″ precipitation. The positive effect of the additions was even more pronounced when coupled with the HTA. Although the low Ti/Al ratio resulted in favorable microstructures, the effect on the creep life was less evident than that of the HTA. The findings of this study enable design of dual superlattice alloys through microstructural engineering that yields superior performance alloys and can be applied to a wide range of alloys in the 718 and derivative family.
2:15 PM
Investigating Deformation Mechanisms in a Creep-deformed 718-variant Superalloy: Semanti Mukhopadhyay1; Hariharan Sriram1; Richard DiDomizio2; Andrew Detor2; Robert Hayes3; Yunzhi Wang1; Michael Mills1; 1The Ohio State University; 2GE Global Research Center; 3Metals Technology Inc.
Improving the efficiency of a land-based industrial gas turbine ultimately relies on novel alloy development for the turbine wheel. However, this alloy development task is challenging because it necessitates higher temperature capabilities along with phase stability during low cooling rate processing of a full-scale wheel. These challenges make 718-based variant alloys an attractive choice because of their superior thermal stability. However, to develop these novel alloys, their deformation behavior must also be accounted for. Thus, in the present work, we investigate the microstructure and creep deformation behavior of a novel 718-variant alloy. Detailed microstructural characterization reveals that the phase fraction of the γ'' phase in the variant alloy is much lower than 718. In addition, the presence of Mo causes detrimental grain boundary precipitation which leads to final failure during tensile creep deformation. The variant alloy accumulates creep strain faster than 718, ultimately fracturing at 0.6% strain. Finally, a detailed characterization of the deformed variant alloy reveals extensive microtwinning.
2:35 PM
Effect of Pre-straining on the Tensile and Stress-rupture Properties of a Novel Ni-Co Based Superalloy: Bin Gan1; Zhongnan Bi1; Cheng Yang2; Hongyao Yu1; Rui Hu2; Jinhui Du1; 1Central Iron and Steel Research Institute; 2Northwestern Polytechnical University
Ni-base superalloys are well suited for aero engine and power generation system, as they possess an extraordinary combination of excellent high temperature mechanical properties, a long-term structural integrity and microstructural stability, and robustness to surface degradation in highly corrosive and oxidizing environment. For most Ni-base superalloys, high strength is mainly derived from precipitation hardening by intermetallic compounds. While the recent development of Ni-Co based superalloys demonstrates that with the addition of Co content, the stacking fault energy could be effectively reduced that will assist the formation of microtwins or nanotwins. In the present study, a Ni-Co based superalloy with a chemistry of Ni-20.0Co-16.5Cr-5.0W-2.5Al-2.5Ti-2.5Nb-0.02C (wt.%) was produced by a vacuum induction melting and electro slag remelting. The cast ingot was homogenized and then forged into a rod with a diameter of 150 mm. The rod was solution treated at 1080 °C for 4 h followed by water quenching to room temperature. The influences of pre-straining temperature and pre-straining amount on the tensile properties of Ni-Co based superalloy at different temperatures as well as the stress-rupture properties at elevated temperatures were investigated. Dislocations are commonly formed at room temperature, while stacking faults and microtwins or nanotwins could be formed at elevated temperatures. The tensile testing results reveal that the application of pre-straining could effectively increase the yield strength of Ni-Co based superalloys while retaining a decent level of stress-rupture properties. The assimilated body of knowledge will give insight to the further alloy development for the fastener applications.
2:55 PM Break
3:15 PM Invited
Preferential γ' Precipitation on Coherent Annealing Twin Boundaries in Alloy 718: Semanti Mukhopadhyay1; Fei Xue2; Hariharan Sriram1; Robert Hayes3; Emmanuelle Marquis2; Yunzhi Wang1; Michael Mills1; 1The Ohio State University; 2University of Michigan; 3Metals Technology Inc.
Early strain localization parallel and adjacent to annealing twin boundaries (ATBs) have been reported in several superalloys. While strain localization is generally attributed to local shear stresses developed near ATBs due to elastic anisotropy, it is presently unclear if local microstructural features near ATBs might also play an important role. Precipitate-free zones (PFZ) parallel to ATBs in a γ''–strengthened alloy 945X have been reported and were found to influence strain localization in the alloy greatly. However, it is unclear if such PFZs near ATBs occur in other superalloys, potentially influencing strain localization. The present work investigates microstructures near ATBs in a Ni-based superalloy - 718, strengthened by both γ' and γ'' phases. This work aims to characterize the precipitates present at the ATBs and to explore if a microstructural cause for strain localization exists in this alloy. Based on our characterization experiments, Alloy 718 shows a high density of herringbone-like γ^'-γ'' precipitates at ATBs. However, the ATBs exhibit a much higher fraction of γ' (25.3±2.8%) than γ'' (18.4±2.4%), ultimately causing the coprecipitate fraction at ATBs to be as high as 43%. We also report a local HCP-phase at the ATBs within γ' precipitates exhibiting Nb segregation and Al depletion. On the contrary, no appreciable change in the ATB composition was observed within γ''. Finally, our characterization experiments also reveal precipitate-free zones parallel and adjacent to the ATBs in alloy 718.
3:45 PM
Effect of Short-term Isothermal Exposure on the Ductility Signature of Waspaloy in the Temperature Range of 750-950°C – A Comparison with Haynes® 282®: Fabian Hanning1; Abdul Khan2; Olanrewaju Ojo2; Joel Andersson1; 1University West; 2University of Manitoba
The evolution of microstructure and ductility has been investigated for Waspaloy after isothermal exposure between 5 and 1800s at 750 to 950°C. Gamma prime (γ’) with 1.7 nm diameter is found in the mill-annealed condition, while precipitate-growth following a t1/3 relationship is observed for isothermal exposure. Grain boundary carbide networks are formed during isothermal exposure together with a rapid hardness increase. A drop in ductility is observed with the lowest values at 750 and 800°C. Further ductility reduction during isothermal exposure correlates with the rapid hardness increase of Waspaloy. While grain boundary strengthening can compensate for the moderate age hardening observed for Haynes® 282®, the more rapid hardness increase due to γ' precipitation appears to be the dominating effect on ductility in Waspaloy. Carbide precipitation and growth kinetics are slower than those of Haynes® 282®, further increasing the relative effect of age hardening reactions on the ductility of Waspaloy.
4:05 PM
(LBN - P1) Investigation of the Deformation Mechanisms for Bi-Modally Distributed γ’ Precipitates in Allvac 718Plus Superalloy: Geeta Kumari1; Carl Boehlert1; S Sankaran2; M Sundararaman2; 1Michigan State University; 2IIT Madras
Alloy Allvac 718Plus is a relatively new superalloy developed to improve the properties of the widely-used superalloy Inconel 718 (IN 718). The strength of IN 718 significantly decreases at temperatures greater than 650°C due to the transformation of the metastable γ’’ (tetragonal, D022 structure) phase into the δ phase (orthorhombic, D0a structure). Allvac 718Plus, which was designed to address this issue, exhibits service temperature up to 704°C (55°C more than that for IN 718 and close to that for Waspaloy) and its formability is similar to IN 718 and better than that for Waspaloy. Allvac 718Plus contains γ’ precipitates as a major strengthening phase along with some grain boundary δ phase. The volume fraction and size of the γ’ and the δ phases are controlled by solution treatment followed by aging.
4:25 PM
(LBN - P2) Phase Stability and Phase Transformation Related to Nb/Ta Additions in a Ni-based Superalloy: Chang-Yu Hung1; Stoichko Antonov2; Paul Jablonski2; Martin Detrois2; 1NETL Support Contractor; 2National Energy Technology Laboratory
Alloys based on Inconel 725, and with elevated levels of Nb and Ta, have been designed for application in environments requiring high strength and corrosion resistance at elevated temperatures. The present study further examines the phase stability and subsequent phase transformation in those alloys following aging and long-term exposure of up to 10,000 hours at 700℃. Changes in the microstructure were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two alloy bases with different Ti/Al ratio were considered. Following aging, the Nb/Ta additions favor the development of γ’/γ’’ co-precipitates in the low Ti/Al ratio alloy, whereas the high Ti/Al ratio counterpart only contained γ’’ precipitates. Upon 10,000 hours of exposure, complex geometrical closed-packed (GCP) phases formed from the γ’/γ’’ co-precipitates in a layer-by-layer manner, i.e., the plate-like δ phase precipitates became decorated by blocky α-Cr and enveloped by a wavy γ’ film. Increasing the concentration of Nb and/or Ta did not change the basic characteristic of this phase transformation; however, the precipitate number density increased in the grain interior. The underlying phase transformation behavior of the layer-by-layer structure is likely initiating from the γ’’-δ phase transition that ejects Cr, Al, and Ti into the neighboring phases, thus resulting in local phase separation into α-Cr and γ’ thin film. This phase transformation process had a significant impact on the phase morphologies with experimental results contradicting those from computational prediction.
4:45 PM
(LBN - P3) Development of Ni Based Superalloys with Medium Entropy Alloys Concept: Elyorjon Jumaev1; Amir Abidov2; Abdullo Khursanov2; 1Almalyk Mining And Metallurgical Combine JSC; 2Almalyk Mining and Metallurgical Combine JSC
The Ni-based Ni48(CrAlFe)15Ti7 and Ni48(CrCuFe)12.5Al7.5Ti7 medium entropy alloys were designed by Thermocalc software and fabricated using arc suction casting method having Ti-guttered in an argon atmosphere. The effect of Cu element on phase evolution, microstructure, and mechanical characteristics was investigated. Detailed characterization reveals that the alloys exhibit dual microstructures consisting of BCC dendrite and FCC interdendritic in Ni48(CrAlFe)15Ti7 alloy meanwhile, rectangular shape morphology (γ + γ՛ phases) in Cu addition Ni48(CrCuFe)12.5Al7.5Ti7 alloy. The result of the mechanical property test illustrates that alloys present outstanding strength at a high-temperature range compared to Inconel 713C and excellent ductility with the microstructure of γ + γ՛ phases.
5:05 PM
Characterization of γ′ Precipitation Behavior in Additively Manufactured IN738LC Superalloy via In-situ Small-angle Neutron Scattering: Hailong Qin1; Ying Tao2; Songyi Shi1; Hongyao Yu1; Jinli Xie1; Zhongnan Bi1; 1Central Iron and Steel Research Institute; 2University of Science and Technology Beijing
IN738LC is a typical high-end Ni-based superalloy developed to meet the needs for harsh working environment (~980 ℃). The total amount of Al and Ti in its composition is over 6.8 wt%, which cause the intensive susceptibility to strain age cracking (SAC). Particularly, γ′ phase precipitates rapidly during the subsequent heating process, resulting in the volume shrinkage and local stress concentration. In the present study, quantitative evaluation of the size and volume fraction of γ′ particles have been studied f by Small-Angle Neutron Scattering (SANS) and FE-SEM in additively manufactured IN738LC Superalloy employing isothermal heat treatment conditions of 850 ℃. The results shows that there is no observable γ′ precipitation in the as-deposited alloy, while a large number of small cellular and striated sub-structures can be characterized with widths ranging from 0.5 to 1.5 μm. During the 850 ℃ isothermal heat treatment, the γ′ phase rapidly precipitates in large quantities and shows a unimodal irregular sphere-like shape. The average size of the γ′ particles increase on prolonging the time: After isothermal heat treatment for 10 min, the average particle size is 72.5 nm; and it reaches 137.3 nm when ageing time is 120 min. However, the volume fraction of γ′ phase do not change with the extension of isothermal heat treatment time, which reaches about 40% after 10 minutes, which is quite different from that prepared by traditional casting process.Key words: Small-Angle Neutron Scattering, IN738LC; Precipitation behavior; Additive manufacturing.