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22 pages, 10382 KiB

Open AccessEditor’s ChoiceArticle

An Effective Strengthening Strategy of Nano Carbide Precipitation and Cellular Microstructure Refinement in a Superalloy Fabricated by Selective Laser Melting Process

by Kai-Chun Chang, Meng-Yun Lee, Tzu-Hou Hsu, Yao-Jen Chang, Kai-Chi Lo, Hyoung Seop Kim, Kuo-Kuang Jen and An-Chou Yeh

Metals 2021, 11(11), 1691; https://doi.org/10.3390/met11111691 - 23 Oct 2021

Cited by 8 | Viewed by 2417

Abstract

An effective strategy to strengthen a superalloy processed by selective laser melting (SLM) is proposed. The aim is to increase the yield strength of Inconel 718 fabricated by SLM to beyond 1400 MPa, which has never been achieved before. In this study, various [...] Read more.

An effective strategy to strengthen a superalloy processed by selective laser melting (SLM) is proposed. The aim is to increase the yield strength of Inconel 718 fabricated by SLM to beyond 1400 MPa, which has never been achieved before. In this study, various NbC additions (0.0%, 0.5%, 1.0%, and 5.0% by weight) were added in the powder bed of Inconel 718, and two types of post-SLM heat treatments were investigated, i.e., solution heat treated plus aging (STA) and direct aging (DA). With NbC addition, smaller depth of melt pool and finer dendritic cells were obtained. Both STA and DA promoted the precipitations of γ′ and γ″. STA eliminated the cellular dendrites and induced grain growth while DA retained the as-built cellular dendrites, grain size, and nano-carbide from NbC addition, rendering a significant 326.2 MPa increase in yield strength. In this work, 0.5% NbC addition exhibited a record-high yield strength of 1461 MPa and ultimate tensile strength of 1575 MPa for Inconel 718 processed by laser manufacturing process according to literature data to-date. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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17 pages, 5333 KiB

Open AccessFeature PaperArticle

Quantification of Solid Solution Strengthening and Internal Stresses through Creep Testing of Ni-Containing Single Crystals at 980 °C

by Uwe Glatzel, Felix Schleifer, Christian Gadelmeier, Fabian Krieg, Moritz Müller, Mike Mosbacher and Rainer Völkl

Metals 2021, 11(7), 1130; https://doi.org/10.3390/met11071130 - 16 Jul 2021

Cited by 8 | Viewed by 2862

Abstract

Various alloy compositions were cast as single crystals in a Bridgman vacuum induction furnace and creep tested at 980 °C: pure Ni, the equiatomic alloys CoCrNi and CrMnFeCoNi (Cantor alloy), single-phase fcc (Ni) solid solution alloys (with the composition of the matrix-phase of [...] Read more.

Various alloy compositions were cast as single crystals in a Bridgman vacuum induction furnace and creep tested at 980 °C: pure Ni, the equiatomic alloys CoCrNi and CrMnFeCoNi (Cantor alloy), single-phase fcc (Ni) solid solution alloys (with the composition of the matrix-phase of CMSX-3 and CMSX-4), and two-phase Ni-based superalloys CMSX-3 and CMSX-4. Due to the single-crystal state, grain size effects, grain boundary sliding, and grain boundary diffusion can be excluded. The results identify two major strengthening mechanisms: solid solution strengthening and other mechanisms summarized as precipitation hardening. Configurational entropy does not increase creep strength: The Cantor alloy, with the highest configurational entropy of all alloys tested, shows a weak and similar creep strength at 980 °C in comparison to pure Ni with zero configurational entropy. The element Re is a very effective strengthener, both in single-phase fcc (Ni) solid solution alloys as well as in two-phase superalloys. Quantitative estimations of different strengthening mechanisms: internal back stress, misfit stresses, Orowan bowing, and γ′-phase cutting (in the case of two-phase superalloys) are presented. Finite element simulations allow estimating the influence of solid solution strengthening of the matrix on the creep behavior of the two-phase superalloys. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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14 pages, 4850 KiB

Open AccessArticle

Laser Processing of Ni-Based Superalloy Surfaces Susceptible to Stress Concentration

by Boris Rajčić, Sanja Petronić, Katarina Čolić, Zoran Stević, Ana Petrović, Žarko Mišković and Dubravka Milovanović

Metals 2021, 11(5), 750; https://doi.org/10.3390/met11050750 - 01 May 2021

Cited by 8 | Viewed by 1834

Abstract

Reliable and resilient constructions are basic for ensuring the safety of various structures. The superalloys are used as constructive materials due to their superb mechanical properties and endurance. However, even these materials can have certain areas where the stress concentration is higher than [...] Read more.

Reliable and resilient constructions are basic for ensuring the safety of various structures. The superalloys are used as constructive materials due to their superb mechanical properties and endurance. However, even these materials can have certain areas where the stress concentration is higher than expected, such as drilling holes, which are common in various structures that need additional enhancement. Surface laser modifications of the areas surrounding the holes drilled in Nimonic 263 sheets are done by pulsed picosecond and nanosecond Nd:YAG laser irradiations with pulse durations of 170 ps and ≤8 ns, respectively. Following the laser surface treatment, the effectiveness of the enhancement was analyzed by the microhardness test and the deformation test. The results show that the deformation and stress values are decreased by 25−40 percent, showing the improvement in the resilience to deformation. The Vickers microhardness test results indicate an improvement in the Nimonic 263 microhardness. The dimensions of the microcracks are higher for the untreated material in comparison to the laser-treated material. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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17 pages, 7851 KiB

Open AccessFeature PaperArticle

Deformation Mechanisms in Ni-Based Superalloys at Room and Elevated Temperatures Studied by In Situ Neutron Diffraction and Electron Microscopy

by Frank Kümmel, Andreas Kirchmayer, Cecilia Solís, Michael Hofmann, Steffen Neumeier and Ralph Gilles

Metals 2021, 11(5), 719; https://doi.org/10.3390/met11050719 - 27 Apr 2021

Cited by 8 | Viewed by 2689

Abstract

Polycrystalline Ni-based superalloys are one of the most frequently used materials for high temperature load-bearing applications due to their superior mechanical strength and chemical resistance. In this paper, we presented an in situ diffraction study on the tensile deformation behavior of the polycrystalline [...] Read more.

Polycrystalline Ni-based superalloys are one of the most frequently used materials for high temperature load-bearing applications due to their superior mechanical strength and chemical resistance. In this paper, we presented an in situ diffraction study on the tensile deformation behavior of the polycrystalline Ni-based superalloy VDM^® Alloy 780 at temperatures up to 500 °C performed at the STRESS-SPEC neutron diffractometer at the Heinz Maier-Leibnitz Zentrum. A detailed microstructural investigation was carried out by electron microscopy before and after testing. The results of these studies allowed us to determine the deformation mechanism in the differently orientated grains. It is shown that the deformation behavior, which is mainly dislocation motion and shearing of the γ′-precipitates, does not change at this temperature range. The deformation is strongly anisotropic and depends on the grain orientation. The macroscopic hardening can mainly be attributed to plastic deformation in grains, where the (200) lattice planes were orientated perpendicular to the loading direction. Accordingly, a remaining lattice strain and high dislocation density were detected predominantly in these grains. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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16 pages, 13983 KiB

Open AccessArticle

The Effect of Recrystallization on Creep Properties of Alloy IN939 Fabricated by Selective Laser Melting Process

by Santhosh Banoth, Chen-Wei Li, Yo Hiratsuka and Koji Kakehi

Metals 2020, 10(8), 1016; https://doi.org/10.3390/met10081016 - 28 Jul 2020

Cited by 20 | Viewed by 4177

Abstract

In this research, we studied the creep properties of a selective laser melting (SLM)-processed γ′-strengthened IN939 superalloy along the building direction compared to a conventional cast alloy as a reference specimen. In the as-built condition, high-density dislocations were formed as a result of [...] Read more.

In this research, we studied the creep properties of a selective laser melting (SLM)-processed γ′-strengthened IN939 superalloy along the building direction compared to a conventional cast alloy as a reference specimen. In the as-built condition, high-density dislocations were formed as a result of the SLM process due to the generation of the larger thermal gradient. Post-heat treatment was necessary to obtain specific mechanical properties to match industrial requirements. Two heat treatment conditions were used: the first was lower temperature heat treatment (LTH: solution treatment at 1160 °C/4 h + aging at 850 °C/16 h). The second was higher temperature heat treatment (HTH: solution treatment at 1240 °C/6 h + aging at 850 °C/16 h). Creep tests were conducted at 816 °C/250 MPa. The first and second heat treatment conditions were used for the SLM specimens, but only the first condition was used for the cast alloy (cast-LTH). The SLM specimens in the as-built and LTH conditions showed very poor creep life but good elongation. The poor creep life of the as-built specimen was caused by high dislocation density and the small recrystallized grains formed during testing. In the LTH specimen, poor creep life was due to the formation of the undesirable η phase at the grain boundary, as well as the formation of small recrystallized grains during testing. The creep life of the HTH specimen was 2.7 times longer compared to the LTH specimen. This was because these specimens were covered with recrystallized grains that included low-density dislocations, columnar grain morphology with random orientation, improvement in γ′ precipitate size, and elimination of undesirable η phase. The cast LTH specimen showed longer creep life than SLM specimens because of coarser grains with low-density dislocations, γ′ precipitate coarsening during the creep, and the presence of carbides at grain boundaries. In addition, the cast LTH specimen exhibited lower creep strain rate than SLM specimens also helped in creep life improvement. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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14 pages, 7124 KiB

Open AccessFeature PaperArticle

Microstructural Investigations of Novel High Temperature Alloys Based on NiAl-(Cr,Mo)

by Camelia Gombola, Alexander Kauffmann, Golnar Geramifard, Malte Blankenburg and Martin Heilmaier

Metals 2020, 10(7), 961; https://doi.org/10.3390/met10070961 - 16 Jul 2020

Cited by 12 | Viewed by 2795

Abstract

Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae by adding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the [...] Read more.

Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae by adding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to 1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo)_ss reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trend predicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diffraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientation relationship between NiAl matrix and (Cr,Mo)_ss reinforcing phase was

(100)

_NiAl||

(100)

_Cr,Mo and

⟨ 100 ⟩

_NiAl||

⟨ 100 ⟩

_Cr,Mo. An orientation relationship described by a rotation of almost 60° about

⟨ 111 ⟩

was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observed. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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18 pages, 7106 KiB

Open AccessArticle

Characterization of Hot Deformation Behavior and Dislocation Structure Evolution of an Advanced Nickel-Based Superalloy

by Zhihao Yao, Hongying Wang, Jianxin Dong, Jinglin Wang, He Jiang and Biao Zhou

Metals 2020, 10(7), 920; https://doi.org/10.3390/met10070920 - 09 Jul 2020

Cited by 8 | Viewed by 2388

Abstract

The hot deformation behavior of an advanced nickel-based Haynes282 superalloy was systematically investigated employing isothermal compression tests in the sub-solvus and super-solvus temperature with various strain rates. The influence of deformation temperature and strain rate on the microstructure was studied by transmission electron [...] Read more.

The hot deformation behavior of an advanced nickel-based Haynes282 superalloy was systematically investigated employing isothermal compression tests in the sub-solvus and super-solvus temperature with various strain rates. The influence of deformation temperature and strain rate on the microstructure was studied by transmission electron microscope. The results reveal that the interaction between work hardening and dynamic softening did not reach equilibrium under lower deformation temperature and higher strain rate. The active energy of alloy is around 537.12 kJ/mol and its hot deformation constitutive relationship equation was expressed. According to the processing map and microstructure observations, two unsafe flow instability domains should be avoided. The optimum hot processing condition for homogeneous and fine dynamic recrystallization grains are obtained. TEM micrograph observations indicated that deformation temperature and strain rate affected recrystallization by affecting the evolution of dislocation substructures within the alloy. The nucleation and growth of DRX grains can be promoted by the relatively high deformation temperature and low strain rate. The main mechanism of dynamic recrystallization nucleation preferred to discontinuous dynamic recrystallization and the typical feature of discontinuous dynamic recrystallization showed grain boundary migration nucleation. The findings improve the understanding of hot deformation behavior and dislocation substructures evolution of the superalloy, which benefits the accurate control of microstructures of nickel-based superalloys, and tailors the properties of final components used in the land-based gas turbine. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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19 pages, 5227 KiB

Open AccessFeature PaperArticle

Aging of γ′ Precipitates at 750 °C in the Nickel-Based Superalloy AD730^TM: A Thermally or Thermo-Mechanically Controlled Process?

by Nicolas Mrozowski, Gilbert Hénaff, Florence Hamon, Anne-Laure Rouffié, Jean-Michel Franchet, Jonathan Cormier and Patrick Villechaise

Metals 2020, 10(4), 426; https://doi.org/10.3390/met10040426 - 25 Mar 2020

Cited by 9 | Viewed by 4239

Abstract

The microstructural stability during aging at 750 °C of the γ/γ′ nickel-based superalloy AD730^TM is investigated in this work. Strain-free aging and aging during fatigue tests are conducted, with a focus on the influence of the strain ratio, the maximum applied strain, [...] Read more.

The microstructural stability during aging at 750 °C of the γ/γ′ nickel-based superalloy AD730^TM is investigated in this work. Strain-free aging and aging during fatigue tests are conducted, with a focus on the influence of the strain ratio, the maximum applied strain, and the cycle waveform (with or without dwell). Two classical mechanisms of γ′ precipitates aging are identified at 750 °C: the coarsening of small spherical γ′ precipitates via the thermally-activated Ostwald ripening process and the coalescence of at least two precipitates into one. These mechanisms appeared to be concomitant during aging. It has been demonstrated that the coarsening kinetics of γ′ precipitates can be described by a classical Lifshitz-Slyozow-Wagner (LSW) equation. The introduction of a cyclic strain during aging at 750 °C increases the coarsening kinetics by means of changes in the volume diffusion of γ′-forming elements and of constraint misfit effects. More precisely, it is shown that the higher the maximum applied strain and/or the strain rate, the higher the coarsening rates. Finally, dwell-fatigue promotes the activation of the γ′ coalescence at 750 °C. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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12 pages, 4686 KiB

Open AccessFeature PaperArticle

Microstructure and Creep Properties of Ni-Base Superalloy IN718 Built up by Selective Laser Melting in a Vacuum Environment

by Toshiki Nagahari, Taigi Nagoya, Koji Kakehi, Naoko Sato and Shizuka Nakano

Metals 2020, 10(3), 362; https://doi.org/10.3390/met10030362 - 11 Mar 2020

Cited by 19 | Viewed by 4762

Abstract

Selective laser melting (SLM) in a vacuum environment is a relatively new process. Although the material is expected to undergo a gradual heat change, which will influence the microstructure and creep properties of IN718, little research has been conducted to date. Here, we [...] Read more.

Selective laser melting (SLM) in a vacuum environment is a relatively new process. Although the material is expected to undergo a gradual heat change, which will influence the microstructure and creep properties of IN718, little research has been conducted to date. Here, we compared specimens built in vacuum (V-SLM) to those built in an Ar atmosphere (Ar-SLM). We investigated the microstructure and creep properties of V-SLM, and compared the V-SLM specimen to a conventional Ar-SLM specimen. The V-SLM specimen had a larger-grained texture, and the quantity of the δ phase was much lower. In addition, the V-SLM specimen had lower amounts of aluminum and titanium oxides, which improved the stability of the γ″ phase. Thus, the V-SLM specimen showed better creep life than the Ar-SLM, due to prevention of brittle fractures along the interdendritic regions. Full article

(This article belongs to the Special Issue Advances in Superalloys and High Temperature Intermetallics)

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