Oxide Dispersion Strengthened Research Articles

Enhanced thermal stability in additive friction stir deposited ODS IN9052 Al alloy

Mechanically alloyed (MA) oxide dispersion strengthened (ODS) IN9052 Al alloy exhibits excellent strength both at ambient and high temperatures (up to 300 °C). The limited ductility at ambient temperatures, stemming primarily from the inherent microstructure, impede its practical appeal in structural applications. The present study signifies a maiden effort to additively manufacture MA-ODS IN9052 alloy using solid-state additive friction stir deposition (AFSD) process which has added advantages of uniform particle dispersion, severe particle breakdown, and a recrystallized microstructure. This study indicates a remarkable transition from the initial strain softening behavior in the as received alloy to significant work hardening in the AFSD condition, while maintaining satisfactory yield strength of ∼380 MPa at ambient temperature highlighting a favorable strength-ductility synergy. Importantly, exceptional thermal stability has been achieved in the AFSD alloy by systematically optimizing the process parameters to engineer microstructural heterogeneity. The AFSD alloy demonstrated an impressive high temperature strength, with ∼22 % and ∼49 % increase in yield strength as compared to forged alloy at 240 °C and 340 °C, respectively. Enhancing high temperature strength in aluminum alloys has been a long standing goal and the current results outperform other commercially available high temperature Al alloys. Excellent thermal stability in the AFSD processed alloy is attributed to the nano-scale carbon-rich cluster formation around oxides and carbides in the matrix. These particles restrict grain boundary sliding and are more effective in dislocation-particle interaction. The findings are supported by detailed TEM and atom probe tomography analysis.

Interfacial characterization and bonding mechanism of W/ODS-316 L steel multi-material structure fabricated by laser powder bed fusion

Interfacial characterization and bonding mechanism of W/ODS-316 L steel multi-material structure fabricated by laser powder bed fusion

The phase and morphology evolution of Y-Zr-O complex oxide dispersion-strengthened (ODS) Mo powders during mechanical alloying and annealing

Nanostructural Y-Zr-O complex oxide dispersion-strengthened (ODS) Mo alloys are designed for accident-tolerant fuel cladding. The phase and morphology evolution of ODS Mo powders during mechanical alloying (MA) and annealing were studied. The dissolution and reprecipitation mechanisms were revealed by quantitative calculation of dissolved Y and Zr amounts, thermodynamics and dynamics analysis, and phase analysis. The results reveal MA induces Mo grain refinement and a high density of dislocations in powders, facilitating the dissolution of Y2O3 and Mo2Zr and the formation of Mo supersaturated solid solution, rather than the negative mixing heat. By subtracting vacancies and oxygen effects, the dissolved amount is estimated as 2.2 at.% Y and 1.8 at.% Zr after 96 h-milling. During annealing, the Y-Zr-O complex oxide is precipitated in Mo-10 wt%Y2O3 powders above 900 °C since the introduced ZrO2 contamination during MA, and then cubic-Y2O3 beyond 1100 °C. The findings provide a scientific basis for controlling Y-Zr-O complex oxide nanoprecipitates and optimizing the service performance of Mo alloys.

Creep behavior of a novel ODS ferrite steel reinforced with ultra-fine Y2(Zr0.6, Ti0.4)2O7 particles

Oxide dispersion strengthened (ODS) ferrite steel is a competitive candidate structural material for the Generation IV fission reactors. In this work, a novel ODS ferrite steel reinforced with ultra-fine Y2(Zr0.6, Ti0.4)2O7 particles was prepared by powder metallurgy method, which exhibit superior mechanical properties at elevated temperatures. Creep tests of the ODS ferrite steel at 800 °C was conducted to reveal its high-temperature creep failure mechanism. This ODS steel has an average grain size of 1.31 μm, and the mean size and number density of the ultra-fine oxide particles are 5.48 nm and 4.85 × 1023 m−3, respectively. When the applied creep stress is 100 MPa, the creep rupture time of the ODS steel is 2448.5 h, and the minimum creep rate is only 1.05 × 10−7 s−1. Under the similar creep stress at 800 °C, the creep rate of this ODS steel is at least one order of magnitude lower than that of other typical ODS steels reported in the literatures, showing excellent creep properties. By fitting the data of minimum creep rate, applied stress and effective stress normalized by shear modulus, the creep threshold stress of the ODS steel was determined as 56.2 MPa, and the true stress exponent was calculated as 4.5 ± 1.0, which indicates that the creep mechanism is dominated by dislocation. Orowan bypass mechanism and dislocation climb over the particle mechanism play a major role in this dislocation mediated creep behavior. Due to the strong hindering effect of dislocation motion caused by the ultra-fine and dispersed Y2(Zr0.6, Ti0.4)2O7 particles, the novel ODS ferrite steel thereby achieved excellent creep properties.

Effects of Hf and/or Ti addition on the morphology, crystal, and metal/oxide interface structures of nanoparticles in FeCrAl-ODS steels

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials due to its exceptional properties. For 3.8Al–0.46Hf (Fe–15Cr–2W–3.8Al–0.46Hf–0.32Y2O3) and 3Al–0.1Ti–0.6Hf (Fe–15Cr–2W–3Al–0.1Ti–0.6Hf–0.35Y2O3) ODS steels, the morphologies of matrix grains & nanoparticles, and the crystal structures of oxides & metal/oxide interface structures were studied by scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). For 3.8Al–0.46Hf and 3Al–0.1Ti–0.6Hf ODS steels, the mean grain size is 1.26 μm and 0.90 μm, respectively, and the dispersion morphology of nanoparticles in the latter is better than that of the former. Crystal & interface structures of 313 and 222 oxides in 3.8Al–0.46Hf and 3Al–0.13Ti–0.6Hf ODS steels were characterized, respectively. The proportions of Y–Hf–O and Y–Al–O particles in the former are 63.3 % and 29.1 %, respectively. However, the Y–Hf–O, Y–Al–O, and Y–Ti–O particles in the latter account for 74.8 %, 16.7 %, and 8.5 %, respectively. The much better dispersion morphology of oxides in 3Al–0.1Ti–0.6Hf ODS steel, relative to 3.8Al–0.46Hf ODS steel, is due to the much higher proportion of Y–Hf–O, the considerably lower proportion of Y–Al–O and the fairly high number fraction of Y–Ti–O. The study delved into the formation mechanisms of various oxides and the strengthening mechanisms in both ODS alloys.

Low-cycle fatigue behavior and microstructure evolution of ODS steel pipes at high temperatures

Oxide-dispersion-strengthened (ODS) steels are candidate materials for application in advanced nuclear reactors. In this study, the low-cycle fatigue performances of 13Cr-ODS ferritic steel pipes were investigated at 600, 700, and 800 °C. Cyclic softening was observed at high strain amplitudes with an increase in the number of fatigue cycles. However, cyclic hardening appeared first, and then cyclic softening occurred at a low strain amplitude with the increase in the number of fatigue cycles. By comparing the cyclic stress–strain curves and the monotonic stress–strain curves, it was found that cyclic softening occurred regardless of the strain amplitude. The Coffin–Manson and Basquin equations were used to predict the fatigue of the pipes. Microstructure analysis indicated that cyclic softening was induced by the dynamic recovery and recrystallization, which reduced the number of low-angle grain boundaries in the deformed grains by promoting dislocation annihilation and reorganization. A complex multi-layer core–shell structure with a large size (∼500 nm) was observed.

Fully Consolidated Deposits from Oxide Dispersion Strengthened and Silicon Steel Powders via Friction Surfacing

Abstract The objective of this work is to study the ability of friction surfacing to deposit metal alloys that are difficult to process with traditional methods. Creep and neutron irradiation resistant oxide dispersion strengthened (ODS) materials cannot be produced via conventional casting route due to insolubility of the oxidic and metallic alloy constituents, causing unintended inhomogeneous oxide dispersion and material behavior. Increasing silicon content of Iron-Silicon (Fe-Si) improves electromagnetic properties but embrittles the material significantly and the fusion based manufacturing methods unable to process this steel. The solid-state nature of the friction surfacing process offers a potential alternative processing route to enable wider usage of difficult to process alloy systems. Both ODS and Fe-Si materials are available in powder forms. While the existing literature in friction surfacing focuses on depositing composites by incorporating small quantities powders through holes in consumable rod, this is a first study that shows a large charge of powder can be converted to a homogeneous fully consolidated deposit in friction surfacing. A novel methodology is used that incorporates the high portion of powder feedstock into hollow consumable friction surfacing rods (up to 35% volume fraction). It was found that fully consolidated deposits can be produced with powder feedstocks using proposed methodology. A recrystallized, homogeneous, equiaxed microstructure was observed in Fe-Si 6.8 wt% and a new generation FeAlOY ODS alloy deposits processed with hollow stainless-steel friction surfacing rods. Both powder and rod material plasticize and deposit without bulk intermixing.

Effects of Zr, Al, and Al+Zr Addition on Phase Evolution Characteristics and Strengthening Mechanisms of Fe‐42Ni‐2Y2O3 Oxide Dispersion Strengthening Steels Developed by Mechanical Alloying and Spark Plasma Sintering

In the present study, Fe‐42 wt% Ni‐2 wt% Y2O3 super‐invar‐based steels have been explored to study the effects of Zr, Al, and Zr+Al addition on phase evolution, strengthening mechanisms, and mechanical behavior. Three different compositions, such as Fe‐42Ni‐2Y2O3‐2Zr (2YZr), Fe‐42Ni‐2Y2O3‐2Al (2YAl), and Fe‐42Ni‐2Y2O3‐2Zr‐2Al (2YAlZr), are developed by mechanical alloying (MA) followed by spark plasma sintering (SPS) at 1100 °C with an applied pressure of 60 MPa. The 2YZr alloy after SPS shows the evolution of a uniform grain size of ≈1.7 μm, whereas 2YAl reveals the formation of bimodal grain structure sintered under the same condition. Conversely, the combined effect of Al and Zr addition results in the evolution of ultrafine grained structure (≈380 nm) after SPS at the same condition (i.e., at 1100 °C). The superior mechanical strength of the 2YAlZr alloy (5.3 GPa/1415 MPa) is attributed to the evolution of uniformly dispersed extremely fine (60 nm) high‐density Al‐ and Zr‐rich oxide complexes in the ultrafine grained matrix. The analysis of strengthening contributions to the yield strength is found to be correlated well with the experimental results and attributed to the morphology of microstructural constituents, their stability, and volume fraction of dispersoids in the matrix.

A Microchannel Heat Exchanger Produced From a Metal Matrix Composite by Hybrid Laser Powder Bed Fusion and Inkjet Printing

Abstract This paper explores the production of an oxide dispersion strengthened (ODS) 304L stainless steel microchannel heat exchanger (HX) using a hybrid additive manufacturing process of laser powder bed fusion and inkjet printing. The study investigates the capabilities and economics of the hybrid inkjet-laser powder bed fusion (LPBF) process and evaluates the dimensional accuracy, functionality, and mechanical properties of the resulting ODS alloy. The effectiveness and pressure drop of the ODS heat exchangers produced by the hybrid LPBF tool are also determined. Results show that the inkjet-doped samples have a lower mean channel height with higher standard deviation than samples produced by LPBF alone. This is attributed to greater absorption of laser energy for the powder coated with the oxide precursor. The economic analysis shows that the hybrid process has a potential for reducing the unit cost of the heat exchanger based on cost modeling assumptions.

Multiscale oxide dispersion strengthened refractory high entropy alloys with superior mechanical properties

Refractory high-entropy alloy (RHEA) is one of the most promising materials for high-temperature applications. However, their application is inhibited by insufficient strength at elevated temperatures and brittleness at ambient temperatures. Herein, we proposed a multiscale oxide dispersion strengthened (ODS) NbTaTiV RHEA with an excellent combination of strength and plasticity at both room and elevated temperatures. The multilevel microstructures are composed of a fine-grained BCC phase matrix (∼1.2 μm), submicron Ti-(O, N) particles (∼330 nm), and nanoscale Y2Ti2O7 particles (∼18 nm). Owing to the strengthening of combinatorial multilevel microstructures, the yield strength of the RHEA was found as high as 1626 MPa, which exhibits a significant improvement (∼46%) in comparison with the NbTaTiV RHEA, without significant loss of ductility. The ODS NbTaTiV RHEA also shows superior thermal-softening resistance, with yield strengths of 945 MPa at 700 °C and 482 MPa at 1000 °C. The enhanced strength is mainly attributed to the multilevel microstructures that mediate the collaborative strengthening mechanism among dispersed strengthening, precipitation strengthening, and grain boundary strengthening. This work presents a new insight to develop multiscale microstructures in RHEAs for high-temperature structural materials.

The Irradiation Effects in Ferritic, Ferritic-Martensitic and Austenitic Oxide Dispersion Strengthened Alloys: A Review.

High-performance structural materials (HPSMs) are needed for the successful and safe design of fission and fusion reactors. Their operation is associated with unprecedented fluxes of high-energy neutrons and thermomechanical loadings. In fission reactors, HPSMs are used, e.g., for fuel claddings, core internal structural components and reactor pressure vessels. Even stronger requirements are expected for fourth-generation supercritical water fission reactors, with a particular focus on the HPSM's corrosion resistance. The first wall and blanket structural materials in fusion reactors are subjected not only to high energy neutron irradiation, but also to strong mechanical, heat and electromagnetic loadings. This paper presents a historical and state-of-the-art summary focused on the properties and application potential of irradiation-resistant alloys predominantly strengthened by an oxide dispersion. These alloys are categorized according to their matrix as ferritic, ferritic-martensitic and austenitic. Low void swelling, high-temperature He embrittlement, thermal and irradiation hardening and creep are typical phenomena most usually studied in ferritic and ferritic martensitic oxide dispersion strengthened (ODS) alloys. In contrast, austenitic ODS alloys exhibit an increased corrosion and oxidation resistance and a higher creep resistance at elevated temperatures. This is why the advantages and drawbacks of each matrix-type ODS are discussed in this paper.

Anisotropic creep property related to non-spherical shape of mechanically alloyed powder of oxide dispersion strengthened F82H

Nanometric oxide particles dispersed within the matrix play an important role in improving the creep property of Oxide Dispersion Strengthened (ODS) steels. Conventional studies have primarily focused on investigating the distribution of the particles to control the creep property of ODS steels. However, achieving complete control over the creep property still poses challenges, as there are potentially other microstructural features like different matrices, micrometric precipitates, and inclusions derived from powder metallurgical processes that can influence it.In our previous research, we examined one such microstructural feature known as prior particle boundary (PPB). PPB refers to the surface of mechanically alloyed (MA) powder before consolidation. We revealed that the ODS steel with fine PPBs produced from smaller MA powder, exhibited shorter creep rupture times, compared to that with coarse PPBs produced from larger MA powder. This result was attributed to the increased formation of creep cavities on the PPBs in the ODS steel produced from smaller MA powder. Therefore, the size of MA powder affected the formation of creep cavity and had an impact on the creep property.In this study, we shifted our focus to the shape of MA powder with non-spherical shapes containing flake-like shape. Such shapes have the potential to induce anisotropy in the formation of creep cavities and/or the creep crack initiation and growth along anisotropic prior particle boundaries (PPBs), ultimately leading to anisotropic creep behavior.We conducted small punch creep tests on specimens with two different orientations to study the possible anisotropy. The results revealed that the creep rupture times varied depending on the orientation of specimen, thus indicating anisotropic creep property. This anisotropic creep property was related to the non-spherical shape of MA powder. This research highlighted the significance of managing the shape of MA powder in controlling the creep property of ODS steels.

Control of the grain structure and wear behavior of a Y2O3 nanoparticle dispersed Stellite 6 alloy fabricated by laser-directed energy deposition

A nuclear reactor is operated with load following that generates more wear than the base load; therefore, the wear resistance of the Stellite 6 alloy should be further improved. This study fabricated a wear-resistant oxide dispersion-strengthened (ODS) Co-based Stellite 6 alloy by varying the amount of Y2O3 nanoparticles (0, 0.6, and 1.5 wt%). The feedstocks for the ODS Stellite 6 alloy samples were prepared via mechanical mixing and radio frequency plasma spheroidization. The samples were then prepared by laser-directed energy deposition to obtain a finer grain structure with superior wear properties. In the ODS Stellite 6 alloy samples, the addition of nanoparticles changed the grain structure from columnar to equiaxed and refined the maximum grain size by 68.9 % compared with that of the Stellite 6 alloy sample. In addition, the nanoparticles were dispersed in the interdendritic region of the ODS Stellite 6 alloy samples. The nanoparticles formed a layer in front of dendritic grains and inhibited the movement of the grains and diffusion of solute atoms. As a result, the ODS Stellite 6 alloy samples had a small equiaxed grain structure. Furthermore, a high density of nanoparticles had an Orowan dispersion-strengthening effect. Therefore, the ODS Stellite 6 alloy samples exhibited a maximum increased Vickers hardness value of 15.9 % and maximum reduced specific wear rate of 58.2 % compared with those of the Stellite 6 alloy sample. Moreover, the ODS alloys exhibited superior wear behavior than Stellite 6 alloy because Y2O3 reinforcement prevents the ODS alloys from being easily abraded or plowed from counterpart material. This study indicates that the ODS Stellite 6 alloys with the optimal amount of Y2O3 nanoparticles dispersed control the grain structure and exhibit superior wear behavior compared with other reported Co-based alloys.

Oxide particles in oxide dispersion strengthened steel neutron-irradiated up to 158 dpa at Joyo

We investigated the stability of oxide nano particles in oxide dispersion-strengthened (ODS) steel, which is a promising candidate material for next-generation reactors, under neutron irradiation at high temperature to high doses. MA957, a 14Cr-ODS steel, was irradiated with Joyo in Japan Atomic Energy Agency under irradiation conditions of 130 dpa at 502 ºC, 154 dpa at 589 ºC, and 158 dpa at 709 ºC. Three-dimensional atom probe (3D-AP) and transmission electron microscope (TEM) observation were performed to characterize the oxide particles in the ODS steels. A high number density of Y-Ti-O particle was observed in the unirradiated and irradiated samples. Almost no change in the morphology of the oxide particles, i.e. average diameter, number density, and chemical composition, has been observed in the samples irradiated to 130 dpa at 502 ºC and to 154 dpa at 589 ºC. A slight decrease in number density was observed in the sample irradiated to 158 dpa at 709 ºC. The hardness of any of the irradiated samples was almost unchanged from that of the unirradiated sample. It was revealed that the oxide particles existed stable, and the strength of the material was sufficiently maintained even after being neutron irradiated to high dose of ∼160 dpa at high temperature up to 700 ºC.

Microstructural Characteristics of Modified ODS Ni-Based Superalloys Fabricated using Laser-Powder Bed Fusion

Oxide dispersion-strengthened (ODS) superalloys are usually manufactured by a hot isostatic pressing after mechanical alloying (MA-HIP). However, this process cannot produce complex-shaped parts. Here, we used an additive manufacturing product (powders produced by laser powder bed fusion, L-PBF) to address this issue. Modified ODS Ni-based superalloy powders with a similar composition to the mechanical alloy 6000 (MA6000) were manufactured using a gas atomization process. The obtained powders and L-PBF synthesized samples were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), LPSA, and EBSD analyses.

A microstructure-based constitutive model for creep rate and damage of oxide dispersion strengthened high-entropy alloy

Oxide dispersion strengthened (ODS) high-entropy alloys (HEAs) exhibit a great resistance to creep, which has a great application potential on the nuclear energy field. However, the classical creep model hardly considers the coupling effect of the cutting and Orowan bypassing mechanism as well as severe atomic lattice distortion in ODS HEAs, leading to low prediction accuracy. Here, we propose a dislocation theory-based constitutive model for strength and creep rate considering the climbing mechanism, shearing mechanism, and Orowan bypassing mechanism in ODS HEAs. In addition, the effects of the size and spatial distribution of the oxide particle on creep rate and strength are studied. The creep rate and yield strength obtained from the experiments and developed model are compared to verify the rationality and accuracy. The proposed model has demonstrated a high prediction accuracy, with the creep rate and yield strength achieving levels as high as 98.2 % and 98.1 %, respectively. The incorporation of oxide particles within a specific size range and volume fraction has been proven to significantly improve creep resistance. Interestingly, the fracture time has the potential to exceed 41 years, significantly surpassing that Zr-based alloys under the same conditions. This model would be utilized to predict the creep rate in response to a complex applied stress, offering theoretical guidance for the development of ODS HEAs with superior creep resistance for nuclear reactor cladding materials.

Microstructure evolution and oxidation resistance properties of oxide dispersion strengthened FeCrAl alloys at 1100 °C

Oxide dispersion strengthened (ODS) FeCrAl alloys were prepared by mechanical alloying (MA) and spark plasma sintering (SPS) technology. To reveal the high temperature oxidation properties, ODS FeCrAl alloys were exposed to dry air for various times (1h, 4h, 10h, 20h, 45h, 70h, and 100h) at 1100 °C to investigate the microstructure evolution. The results illustrated that a flat and adherent alumina scale was gradually thickened on the alloy substrate with the increase of the oxidation time. In particular, it was observed that large-size reactive element (RE) oxides were inclined to be associated with cavities close to the gas interface and these cavities were progressive evolution to be larger-size pores with longer exposure time, while tiny RE oxides were not dispersed with cavities after 100h oxidation. In addition, α-Al2O3 with a double-scale structure were mainly composed of inner columnar grains and outer equiaxed grains, as well, the growth of α-Al2O3 scales illustrated that the oxidation rate decreased with oxidation reaction proceeding. For one thing, Ti segregation along grain boundaries (GBs) near the gas interface decreased the outward diffusion of the Al ions to weaken the oxidation rate. Furthermore, the gradual coarsening of columnar grains near the alloy substrate implied a longer diffusion path of O atoms, thereby accounting for slower oxidation process. To sum up, ODS FeCrAl alloys with thinner oxide thickness demonstrated the superior oxidation resistance than other materials, which would make it one of the promising candidate structural materials applicated in accident-tolerant fuel (ATF) cladding materials.

Effect of Y2Ti2O7 on microstructure and tensile properties of ODS-CLAM steel

Nano-Y2Ti2O7 is one of the phases used for improving the strength of oxide dispersion strengthened (ODS) steels, which is usually precipitated with Ti and Y2O3 added. The effect of synthesized Y2Ti2O7 on China low activation martensitic (CLAM) steel was studied. Two kinds of ODS-CLAM samples with nano-Y2Ti2O7 and amorphous Y2Ti2O7 additions were prepared using mechanical alloying (MA) and spark plasma sintering (SPS). The results indicates that amorphous Y2Ti2O7 powders had both amorphous and crystal oxides after milling with CLAM powders. Y2Ti2O7 powders were refined to below 10 nm in size with almost no structural change, and randomly distributed in the matrix. The initial size of added Y2Ti2O7 had a smaller impact on the size of precipitates, and the initial structure or the transform of the oxides during milling process were more important. The added Y2Ti2O7 particles were refined from ∼32 nm to 9.2 nm in size, while the size of amorphous Y2Ti2O7 particles was reduced from micron size to 6.9 nm in the sintered sample. The structure of Y2Ti2O7 in ODS powders affected the microstructure and strength of the sample after sintering. The sample with Y2Ti2O7 had higher strength and lower plasticity at both room temperature and 650 °C because of the higher dislocation density, while the sample with amorphous Y2Ti2O7 had higher plasticity with almost the same yield strength due to its higher dispersion strengthening. This study reveals the changes in Y2Ti2O7 structure during preparation process and suggests that the expected properties of ODS steels can be obtained by controlling the oxide structure.

Improving mechanical properties of W-Ni3Al alloy through Zr element addition: Solid-solution and oxide particle strengthening

In this study, fine Ni3Al flakes facilitated the W-Ni3Al alloy liquid phase sintering, while the added 0.9 wt% Zr element enhanced its mechanical properties, finally obtaining the 1200 °C sintered W-Ni3Al-Zr alloy (WNZ-1200 alloy) with excellent comprehensive properties. Specifically, the relative density, tungsten grain size, bending strength, and macro hardness of WNZ-1200 alloy were 98.26 ± 0.31 %, 3.06 ± 0.65 µm, 1472.56 ± 61.90 MPa, and 76.10 ± 0.82 HRA, respectively. By analyzing the alloy’s phase and microstructure, its excellent mechanical properties were originated from the formed Ni3(Al, W, Zr) solid solution, unique nano-Al2O3 ‘rivet’, the semi-coherent interface of W/t-ZrO2, oxide dispersion strengthening, and the t-ZrO2 phase transformation strengthening.

Laser powder bed fusion of oxide dispersion-strengthened IN718 alloys: A complementary study on microstructure and mechanical properties

In this study, two new grades of oxide dispersion strengthened (ODS) Inconel 718 (IN718) alloys were designed by the thermochemical CALPHAD method and produced by laser powder bed fusion (LPBF) technique. Alloys designated as IN718-YF and IN718-YFH, that consist Y2O3–FeO and Y2O3–FeO–Hf, respectively, were fabricated with >99.9 % densification using optimized process parameters. CALPHAD calculations were highly consistent with experimental findings, highlighting the formation of Al-containing Y–Ti–O and Y–Hf–O nano-oxides in both alloy types. Texture analyzes revealed no significant texture development in as-built (AB) or heat-treated (HT) alloys. Heat treatment was applied at 1050 °C for 1 h to enhance nano-oxide density. The nano-oxide number density remained similar in IN718-YF while it decreased in IN718-YFH alloy as a result of carbide formation after the heat treatment. Besides, formation of secondary γ′ particles was observed in the IN718-YFH/HT alloy. Even though the yield strengths of IN718-YF and IN718-YFH alloys in both AB and HT conditions were similar, the ductility of IN718-YFH was ∼50 % less in almost all conditions compared to the ductility of IN718-YF. This has been shown to be as a result of irregular shaped micron-sized Y-Hf-O oxides, martensite formation in AB condition, increased amount of carbides and existence of secondary γ′ particles in HT condition in IN718-YFH. High density of stacking faults (SF) forming at the interface of the nano-oxides have been detected in IN718-YF alloys. Besides dislocation/nanoparticle interactions, SFs which are responsible for the delocalization of the deformation improve the ductility of IN718-YF alloys. Overall, high temperature mechanical tests exhibit that both alloys have higher strength with improved ductility compared to the standard IN718 alloys, indicating the contribution of the nano-oxides.