Session Sheet - Superalloys 2024: Interactive Session E: Additive Manufacturing & Environmental Behavior

Superalloys 2024: Interactive Session E: Additive Manufacturing & Environmental Behavior
Program Organizers: Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346

Wednesday 7:45 PM
September 11, 2024
Room: Winterberry
Location: Seven Springs Mountain Resort

E-1: Beyond Hot Cracking: Impact of Minor Elements on a Novel Ni-based Superalloy for Additive Manufacturing: Kai Dorries¹; Christoph Haberland²; Juri Burow³; Joachim Rösler¹; Bodo Gehrmann³; Christina Somsen³; Sebastian Piegert²; Hĺkan Brodin²; ¹TU Braunschweig; ²Siemens Energy Global GmbH & Co. KG; ³VDM Metals International GmbH
    Minor elements such as boron, carbon, and zirconium have been used for many decades to improve the high-temperature properties of Ni-based superalloys. However, the advances in additive manufacturing technologies and the resulting popularity have put these elements in a bad light since they have been identified to be the major cause of hot cracking problems. This study covers the influence of these elements on hot cracking but its focus lays on their impact on strain-age cracking (SAC) and mechanical properties. The impact of these elements has been studied in four versions of a high tantalum-containing novel Ni-based superalloy that is being developed for Powder Bed Fusion-Laser Beam/Metals (PBF-LB/M). Increasing the boron content from 0.007 wt.% to 0.019 wt.% leads to severe hot cracking, but reduces SAC during the heat treatment. The addition of 0.022 wt.% zirconium does not increase the hot cracking susceptibility but increases the SAC susceptibility. The variation of minor elements does not affect room temperature tensile properties, but an increased zirconium and boron content increases the elongation at fracture at 850°C. The alloys with a low boron and medium boron content show a high notch-sensitivity during stress-rupture tests, which leads to failure in the fillet of the sample. Only the boron and zirconium alloys were able to achieve valid stress-rupture results.

E-2: Concurrent Improvement of Additive Manufacturing Processability and Creep Performance in a Legacy Polycrystalline Superalloy Using Grain Boundary Strengtheners: A Shaafi Shaikh¹; Eduard Hryha²; Mohammad Sattari²; Mattias Thuvander²; Kevin Minet-Lallemand³; ¹Chalmers University of Technology / EOS Metal Materials; ²Chalmers University of Technology; ³EOS Metal Materials
    Microcracking during processing and underperformance in creep have limited wider adoption of high γ'-fraction superalloys in additive manufacturing (AM). Certain processing issues are now understood to be related to solidification cracking caused by elements such as B and Zr, that are also essential for creep performance, particularly with fine-grained AM microstructures. A legacy γ'-strengthened polycrystalline superalloy 738LC was the subject of the current investigation. Printing trials conducted with the legacy composition and a modified version with 10 times the initial B content revealed extensive micro-cracking in the legacy composition, whereas the modified alloy produced a dense crack-free microstructure. CALPHAD-based solidification simulations and cracking susceptibility index calculations were performed to attempt to rationalise these findings. After hot isostatic pressing (1120 °C, 200 MPa, 4 hours) and ageing heat treatment (850 °C, 24 hrs), stress rupture tests showed an improvement in the rupture life of the modified alloy. Samples perpendicular to the AM building direction (typically the weaker orientation) showed a 50 % increase in rupture life compared to the conventional composition, and rupture ductility was also enhanced. Elevated temperature tensile ductility in the perpendicular direction increased to ≈ 11 El% for the modified alloy versus ≈ 6 El% for the conventional composition. These improvements are attributed to the presence of fine boride precipitates at grain boundaries of the modified alloy. The findings indicate that increasing the grain boundary strengthening element content may be a potential solution for both processing and mechanical performance issues in this superalloy.

E-3: Improving the Additive Manufacturing Processability of a γ/γ' Cobalt-based Superalloy Through Tailored Chemical Modifications Without Degrading Hot Mechanical Properties: Thibaut Froeliger¹; Didier Locq¹; Louise Toualbi¹; Thomas Elcrin²; Rémy Dendievel³; ¹ONERA; ²AddUp; ³CNRS, SIMaP
    This study aims to optimize the content of minor elements (C, B, Zr, Si, Hf) to enhance the manufacturability of γ/γ' cobalt-based superalloys by additive manufacturing without degrading hot mechanical properties or oxidation resistance. Results show that reducing the amount of minor elements improves resistance to liquid phase cracking during additive manufacturing. However, removing these elements leads to a degradation of grain boundary strength or a reduction in the efficiency of the protective oxide layer at high temperatures. To reconcile these different aspects, the contents of minor elements are adjusted in a γ/γ' cobalt-based superalloy following the study of its solidification path. The chemical modifications enable the development of various crack-free microstructures by directed energy deposition, which is unattainable before the chemical modifications. Creep and cyclic oxidation testing show that these chemical modifications do not affect the initial properties of the superalloy.

E-4: Influence of the γ/γ’ Misfit on the Strain-age Cracking Resistance of High-γ’ Ni and CoNi Superalloys for Additive Manufacturing: Stephane Forsik¹; Austin Dicus¹; Gian Colombo¹; Tao Wang¹; Mario Epler¹; Eamonn Connolly²; Jiraphant Srisuriyachot³; Alexander Lunt³; Ning Zhou¹; ¹Carpenter Technology Corporation; ²Diamond Light Source; ³University of Bath
    A series of new printable Ni and CoNi high ă’ superalloys designed for additive manufacturing have been evaluated for strain-age cracking (SAC) resistance. Printability studies and heat treatment experiments were conducted to identify processing windows and characterize the overall resistance to SAC. High-resolution synchrotron X-Ray diffraction experiments were performed to measure the ă and ă’ lattice parameters as a function of the temperature. All the superalloys tested were found to have a positive ă/ă’ misfit at room temperature which decreases as the temperature increases. The misfit of a SAC-prone alloy, 247LC, decreases rapidly and turns negative at about 600 °C, whereas the misfit of superalloys with intermediate resistance to strain-age cracking remains slightly positive. In the three most SAC-resistant superalloys, the misfit remains larger than 0.05 % until at least 883 °C. The results show that a critical aspect for designing SAC-resistant alloys is ensuring that the misfit between ă and ă’ remains positive at all temperatures to generate compressive stresses on grain boundaries. Furthermore, the work also highlights a critical positive misfit value of 0.05% to prevent cracking.

E-5: Microstructural Evolution and Thermal Stability of Additively Manufactured XH67 Nickel-based Superalloy: Nithin Baler¹; Indu Kollapalli¹; Subhradeep Chatterjee²; Dheepa Srinivasan¹; Rohit Kumar Yadav³; ¹PWRDC; ²IIT Hyderabad; ³Indian Institute of Technology Bombay
    A new high strength nickel-based superalloy, XH67, has been fabricated by the laser powder bed fusion process (LPBF). Novel heat treatments via direct aging, solutionizing and aging, were carried out and compared with the as printed alloy, to study the evolution of phase equilibria and bring out the optimum mechanical properties of high strength and good ductility. A strengthened alloy, XH67 shows a high strength of 1040 MPa with a 25 % ductility, after direct aging (size of 10-30 nm and a volume fraction of 25-30 %). The as-printed structure with a cellular-dentritic morphology was retained during direct aging heat treatment contributing to the strength by Hall-Petch strengthening along with precipitation hardening by γ' precipitates. Discontinuous precipitation along the cell boundaries was a unique microstructural feature that are observed in the present alloy. Annealing twins were seen after solution and aging along with coarsening of the precipitates up to 50 nm. Thermodynamic calculation (Thermo-Calc) was used to validate the observed phase evolution as a function of heat treatment.

E-6: Microstructure-mechanical Properties of Short-cycle Heat Treated Additively Manufactured Mar-M 509 Cobalt Superalloy: Naimish Shah¹; Rohit K. Yadav²; Dheepa Srinivasan¹; Nagamani Jaya Balila²; ¹Pratt & Whitney Research and Development Center; ²Indian Institute of Technology Bombay
    Additively manufactured Mar-M 509, a cobalt-based superalloy, was evaluated for its microstructure and tensile behavior (at room temperature and 650 °C) after short cycle heat treatments, along the two orientations, longitudinal (L) and transverse (T) to the build direction. The microstructural evolution after single step heat treatments at 950 °C, 1150 °C and 1250 °C for 3 h was characterized using transmission electron microscopy. The alloy comprises a columnar-cellular dendritic microstructure strengthened by MC carbides forming a network along the cell boundaries in the as-printed condition. On heat treatment, the microstructure was characterized by the precipitation of M23C6 along with MC carbides. The T orientation showed higher yield strength and lower elongation than L for all the conditions. Amongst these, the 1150 °C heat treatment showed the optimum combination of yield strength and elongation (850 MPa, 20 %), attributed to the presence of fine MC carbides along the cell boundaries and coarse M23C6 carbides at the grain boundaries, with a carbide fraction of nearly 18%. At the test temperature of 650 °C, the optimum yield strength of ~740 MPa and elongation of 21 % was seen in the 950 °C HT condition. This understanding of microstructure-mechanical property correlation for a palette of short cycle ageing treatments thus allows for choosing the right combination for the desired application.

E-7: Reinventing H230 Alloy Through Additive Manufacturing with Breakthrough Performance Gain: Youping Gao¹; ¹Castheon Inc
    By controlling additive manufacturing process, particularly Laser Powder Bed Fusion process, significantly different microstructure and constituents’ formation and distribution can be achieved robustly for superior materials properties gain without altering the bulk materials chemistry. Furthermore, a supersaturated solid solution structure can be obtained without solution treatment and subsequent quench operation to attain optimized properties by forming a high-volume fraction and uniformly distributed fine strengthening phase. In this study, fine and stable carbides for Carbide Dispersive Strengthening (CDS) to improve not only general materials properties but more critically to provide strengthening mechanisms above γ' solvus temperature for extreme environment applications such as hypersonic leading edge and combustion devices. Uniaxial tensile data were obtained for Laser Powder Bed Fusion of Haynes 230 at temperatures from 982 to 1177°C. The data is analyzed in terms of the strain rate sensitivity m and the stress dependence n. These two parameters are used to provide insight into the possible deformation mechanisms controlling plastic flow in this alloy over the temperature – strain rate range of interest. The values obtained suggest that under the present experimental conditions Haynes 230 deforms by a combination of dislocation slip and diffusion mediated recovery within the grain interior. Stress – strain curves exhibit oscillations suggesting the material is undergoing dynamic recrystallization during the tensile test. Optical imaging of the gage sections confirms the presence of dynamic recrystallization.

E-8: Solidification and Crack Defect Formation on the Single Melt-track Scale for High γ' CoNi-base Superalloy Variants: Evan Raeker¹; Kira Pusch¹; Kaitlyn Mullin¹; James Lamb¹; Ning Zhou²; Stephane Forsik²; Austin Dicus²; Michael Kirka³; Tresa Pollock¹; ¹University of California, Santa Barbara; ²Carpenter Technology Corporation; ³Oak Ridge National Laboratory
    Additive manufacturing enables the fabrication of complex part geometries, and is attractive for advanced aerospace components. Laser powder-bed fusion (LPBF), specifically, is being assessed for manufacturing structural components of gas turbine engines made from high-γ' volume fraction superalloys. However, the formation of crack defects during LPBF of nearly all superalloys within this class has undercut their mechanical performance greatly. This study builds on prior work examining the cracking susceptibility of high-γ' volume fraction superalloys during LPBF by simplifying the LPBF process down to single-track laser melting scans. The CoNi-base alloy GammaPrint-700 is utilized in this study, as the cracking resistance of the alloy can be controlled through the boron content. A means of improving the cracking resistance of the alloy through homogenization treatments prior to laser melting was identified. Characterization of the single-tracks reveals a possible mechanism of crack initiation via liquation cracking of grain boundaries in the substrate material, and propagation via solidification cracking along grain boundaries in the melt pool. Additionally, a protocol for assessing the cracking-resistance while developing new high-γ'volume fractionsuperalloys for additive manufacturing is discussed.

E-9: Damage of Thermal Barrier Coated Superalloy Under Thermal Gradient Mechanical Fatigue: Xin Zhan¹; Dong Wang¹; Guang Xie¹; Jian Zhang¹; ¹Institute of Metal Research
    Thermal gradient mechanical fatigue (TGMF) tests were conducted on thermal barrier-coated (TBC) tubular specimens to investigate the damage behavior of the TBC under close-to-service conditions. Special attention was paid to the cracking behavior of TBC under TGMF tests with different temperature ranges, mechanical strain ranges and phase angles including in-phase (IP) and out-of-phase (OP) loading. Crack initiation, propagation, and coalescence within the ceramic top coat (TC) caused TBC failure during IP-TGMF tests with a 300-1000 °C temperature range and a mechanical strain range of 0.45% to 0.65%. Few cracks extend to the bond coat (BC) but did not further propagate into the substrate. Increasing the maximum temperature and mechanical strain range significantly changed the cracking behavior of these TBCs. Cracks run straight through the TC and BC followed by partial penetration into the substrate. The OP-TGMF loading shortened the TBC cycles to failure and accentuated the TBC damage, with separation occurring between TC and BC as well as between BC and substrate. Severe delamination at the thermally grown oxide (TGO)/BC interface results in premature TBC failure when the TBC system reaches a certain critical thickness of TGO.

E-10: Effect of Free Surface, Oxide and Coating Layers on Rafting in γ-γ' Superalloys: Wajih Jbara¹; Vincent Maurel¹; Kais Ammar¹; Samuel Forest¹; ¹Mines Paris - PSL University
    Complex microstructure evolution has been observed both bare and coated Ni-based single crystal superalloys. Rafting and γ' depletion are investigated in this study through a brief experimental analysis and a detailed phase field model to account for mechanical-diffusion coupling. The proposed model has been implemented in a finite element code. As a main result, it is shown that rafting, γ' depletion close to free surface/oxide layer or γ' coalescence close to coating layer, and mechanical behavior are strongly coupled. The local additional flux of Al explains this coupling to a large extent. Finally, a discussion of strain localization and local flux of Al paves the way for clarification of these cases that degrade the performance of superalloys.

Cancelled
E-11: In situ Observation of Initial Oxidation in Different Generations of Nickel-based Single-crystal Superalloys: Yunsong Zhao¹; ¹Beijing Institute of Aeronautical Materials
    Due to the application of thermal barrier coatings, the concentrations of Cr in turbine blade alloys have been limited to low values (approximately 5 at.% or less) since the introduction of second-generation single-crystal superalloys. Thermal oxidation-induced rumpling and swelling of coating could lead to coating spallation and inner alloy failure, especially in advanced thin-wall turbine blades. The initial oxidized surface morphologies and elemental distributions were considered crucial to understanding the failure of superalloys. In this work, initial oxidation behavior in typical 1st- to 3rd-generation single-crystal superalloys was systematically studied in situ at nano-scale using an environmental transmission electron microscope from 20 - 800 oC. With increasing oxygen pressure, the oxide nucleated at the γ/γ' interface, expanded along the γ channel and grew into the γ' phase. In thin foil samples, oxidation prompted the diffusion of base elements from the inner γ and γ' phases to the γ/γ' interfaces in all alloys. With increasing Re content, the oxidation resistance decreased due to the evaporation of Re2O7 at the γ/γ' interface in the 3rd-generation superalloy. This study provided technical guidance for optimizing the compositions of advanced single-crystal superalloys to enhance their oxidation resistance.

E-12: Oxidation Behavior of Platinum-containing γ, γ' and TROPEA Superalloy: Louis Hunault¹; Fernando Pedraza¹; Jonathan Cormier²; Renaud Podor³; Stephane Mathieu⁴; ¹La Rochelle Université - LASIE UMR 7356 CNRS; ²Institut Pprime; ³ICSM; ⁴Institut Jean Lamour
    Platinum is used to form NiPtAl coatings and improve the oxidation/corrosion resistance of nickel-based superalloys operating under harsh conditions. The addition of 2 wt% (0.6 at%) Pt to the bulk composition of a single crystal nickel-based superalloy (TROPEA) markedly increased the mechanical properties at high temperature. However, the effects of Pt on the oxidation behavior of this new superalloy have never been studied, which is the main goal of this paper. TROPEA and model ă and ă’ single phase alloys were thus oxidized at 950 °C in synthetic air. TROPEA reaches a parabolic regime only after 70 h of exposure. The establishment of an á-Al₂O₃ protective layer is peculiarly delayed in comparison with the parent CMSX-4 superalloy according to literature. TROPEA forms a multilayer of oxides, including a continuous layer of (Ti_xTa_1-x)O₂ during the transient period. It appears that Pt promoted the formation of this Ta-rich oxide. The Ta-rich oxide layer may play a role hindering the lowering of the PO₂ needed to selectively develop Al₂O₃ scale or its presence just outlines the difficulty of TROPEA to develop the protective Al₂O₃ scale in presence of platinum in the substrate. The 0.6 at% of Pt addition thus hampers the formation of a continuous and protective layer of á-Al₂O₃ scale. In contrast, the model ă and ă’ phases are prompt to develop an adherent and even scale of duplex Cr₂O₃ + NiCr₂O₄ and of á-Al₂O₃ + NiO respectively, when oxidised at 950 °C in air for 100 hours.

E-13: Stress Relaxation Testing as a High-throughput Method for Assessing Creep Strength in Laser Powder Bed Fusion Processed Ni-based Superalloys: Daniel McConville¹; Ben Rafferty²; Jeremy Iten²; Kevin Eckes²; Stan Baldwin²; Amy Clarke¹; Jonah Klemm-Toole¹; ¹Colorado School of Mines; ²Elementum 3D
    Ni-based superalloys processed by laser beam powder bed fusion (PBF-LB) additive manufacturing (AM) are ideal for high temperature structural applications in the aerospace and power generation industries due to the increased component complexity afforded by AM. However, conventional creep testing limits the rate at which new materials can be produced with AM. To help accelerate the acceptance of AM Ni-based alloys for high temperature applications, methods for high-throughput creep evaluation are needed. The stress relaxation test has potential to hasten the development and validation of PBF-LB Ni-based structural alloys by assessing a wide range of creep rates relevant to service conditions with a single test. In this work, alloy Ni230, a gas atomized powder derivative of Haynes 230, and variant thereof containing added TiC are assessed. Each material was subjected to a limited subset of conventional creep tests accompanied by stress relaxation tests. Following the calculation methodology described herein, stress relaxation tests predict creep rates and rupture times that align well with conventional creep test results. Stress relaxation tests also reveal features of microstructural characteristics and evolution which are not readily apparent with other experiments. Several advantages and challenges with stress relaxation testing are discussed.

E-14: Surface-roughness Effects on Creep Performance In Ni-based Single-crystal Superalloys: Aidan O'Donnell¹; Pawan Chaugule¹; Jean Briac Le-Graverend¹; ¹Texas A&M University
    Ni-based single-crystal superalloys must endure high temperature applications where creep is the main cause of failure. Coatings to protect against oxidation are not always applicable and oxidation should, therefore, be factored in. Thermogravimetric analyses were performed at 1000°C on René N4 to obtain the oxidation rate depending on the surface roughness. It was observed a strong non-linearity attributed to a transition between chemisorption and physisorption. The evolution of the oxidation rate was then phenomenologically modelled combining Lennard-Jones-type and Gaussian-type equations. The obtained expression was then implemented in a crystal-plasticity model used to predict the lifetime variation depending on the initial value of the surface roughness. The predicted lifetimes were then compared to the creep experiments at 1000°C/200 MPa. The evolution of the lifetime depending on the surface roughness match the evolution of the oxidation rate depending on the roughness and, therefore, the model predictions. Clearly, the results point to the importance of considering the contribution of surface roughness not only for fatigue loading but also to creep, which will allow to better understand creep-fatigue interactions.

E-15: The Effect of Ti Replacement with Nb on the Long-term Oxidation Behaviour at High Temperatures of Ni-base Superalloys for Turbine Disc Applications: Ana Carolina Silva¹; Nabil Chaia²; Satoshi Utada³; Luciano Alkmin⁴; Gilberto Coelho¹; Yuanbo Tang³; Roger Reed³; Carlos Nunes³; Phani Karamched³; ¹University of Săo Paulo; ²Institut of Science & Technology, Federal University of Alfenas; ³University of Oxford; ⁴Federal Center of Technological Education Celso Suckow da Fonseca
    This work focuses on the effect of partial and total replacement of Ti with Nb on the long-term oxidation at 800 °C up to ~2000 h of Ni-based superalloys for turbine disc applications. The replacement of Ti with Nb leads to a lower mass gain of the specimens, in both pseudo and isothermal oxidation conditions. The alloys presented a relatively smooth oxide scale surface; however, the roughness of the Nb-free/high-Ti alloy is twice of that of the Ti-free/high-Nb alloy. From XRD data, the Nb-free/high-Ti alloy presented Cr₂O₃, TiO₂, Al₂O₃, (Ni,Co)Cr₂O₄ and γ+γ' (matrix) phases, while the Ti-free/high-Nb alloy presented Cr₂O₃, Al₂O₃, (Ni,Co)Cr₂O₄, γ+γ' (matrix) and δ-Ni₃Nb phases. The chromia layer thickness of the Ti-free/high-Nb alloy ranged from 1.0 to 1.5 µm, the thinnest among all the alloy for any given oxidation time and condition, with no mass change after approximately 280 h exposure. The exceptional oxidation behavior is attributed to the absence of Ti in this specific alloy composition. The semi-continuous alumina formation underneath the chromia scale as well as the formation of δ-Ni₃Nb phase filling up the spaces between the alumina intrusions are probably the reason of delayed outward Cr diffusion.

E-16: The Role of Silicon in the Protection Against Type I Hot Corrosion: Fernando Pedraza¹; D. Piel¹; T. Kepa¹; C. Gossart²; M. Mondet²; ¹Universite de La Rochelle; ²Safran Aircraft Engines
    Hot corrosion dramatically lowers the life of superalloy components by inducing pitting (Type II) or an extensive homogeneous attack (Type I), hence initiating cracks and decreasing the load bearing section. This degradation phenomena may occur in the coldest areas of high pressure turbine components but they are commonly found in the low pressure turbine parts like in the DS200+Hf nickel-based superalloy investigated in this work. Therefore, a Si-modified aluminide slurry coating was applied to the alloy, and its hot corrosion resistance was assessed under type I conditions with a Na₂SO₄ deposit at various temperatures and times. The comparison against simple slurry aluminide with and without Si and out-of-pack aluminide coatings revealed for the first time in the literature that Si ties up W in the coatings thereby impeding acidic dissolution, which greatly improves the corrosion resistance of DS200+Hf superalloy. The W-free out-of-pack aluminide coating forms a protective alumina scale that delays the initiation of hot corrosion unless the coating is damaged. In contrast, the simple slurry aluminide coating without Si does not offer any protection in spite of the Cr segregation close to the surface.