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Incorporating strengthening phase with a higher thermal stability than traditional L12-ordered Ni3Al phase offers a promising avenue for developing the next-generation Ni-based superalloys. In this study, a powder metallurgy superalloy featuring addition of high-melting-point δ-Nb3Al was fabricated to investigate the high-temperature oxidation behavior at 900 °C. Heterogeneous and accelerated oxidation resulting from the various phase transition products in the alloy were observed. Notably, the preferential oxidation of δ-Nb3Al to Nb2O5 causes obvious surface bulges and cracks. The matrix and Laves phase form oxide layers containing a discontinuous Al2O3. The oxidation mechanism was elucidated through the formation oxidation diagrams based on Gibbs formation energy and total valence of Al2O3. The low Al content in matrix and the high Nb content in Laves phase were identified to cause the discontinuity of surface Al2O3 layer. Additionally, Nb could impede oxygen diffusion within Al2O3 by reducing oxygen vacancy concentration. The findings will provide optimization guidance for the composition design and oxidation protection of novel δ-Nb3Al-strengthened Ni-based superalloys. [ABSTRACT FROM AUTHOR]

This research investigates the microstructure and defects of powder metallurgy (PM) nickel-based superalloys prepared by spark plasma sintering (SPS). The densification, microstructural evolution, and precipitate phase evolution processes of FGH96 superalloy after powder heat treatment (PHT) and sintering via SPS are specifically analyzed. Experimental results demonstrate that SPS technology, when applied to sinter at the sub-solidus temperature of the γ' phase, effectively mitigates the formation of a prior particle boundary (PPB). Based on experimental and computational findings, it has been determined that the presence of elemental segregation and Al2O3 oxides on the surface of pre-alloyed powders leads to the preferential precipitation of MC-type carbides and Al2O3 and ZrO2 oxides in the sintering necks during the hot consolidation process, resulting in the formation of PPB. This study contributes to the understanding of microstructural modifications achieved through SPS technology, providing crucial information for optimizing sintering conditions and reducing the widespread occurrence of PPB, ultimately enhancing the material performance of PM nickel-based superalloys. [ABSTRACT FROM AUTHOR]

Thermo-mechanical process of nickel-based oxide dispersion strengthened (ODS) superalloys is critical to produce desired components. In this study, an efficient method of consolidating powder is introduced to optimize the preparation process, microstructure and properties of nickel-based ODS superalloys. The influences of consolidation temperature, strain rate and ball milling time on the hardness of nickel-based superalloys were studied. The relationship among process, microstructure and hardness was established, the nanoparticles strengthening and grain boundary strengthening in nickel-based ODS superalloys were discussed. The results indicate that long ball milling time, moderately low consolidation temperature and high strain rates are beneficial to improving properties of nickel-based superalloys. Moreover, dispersion strengthening of nanoparticles and grain boundary strengthening play important roles in enhancing nickel-based ODS superalloys. [ABSTRACT FROM AUTHOR]

With excellent radiation resistance and high temperature strength, the oxide dispersion strengthened (ODS) nickel-based superalloys are deemed as candidate materials for advanced fission and fusion reactor. Since conventional manufacturing processes are difficult to produce components with complex shapes, additive manufacturing becomes an alternative method. In this work, ODS nickel-based superalloy was manufactured by laser metal deposition (LMD) technology using mechanically alloyed powder. The microstructure and high temperature mechanical properties of the as-deposited alloy were analysed. The results indicated that the as-deposited alloys prepared by this method owned anisotropic structure and uniformly distributed high-density oxides, contributing to superior high-temperature performance, in comparison with the samples produced by LMD using pre-alloyed powder and traditional hot extrusion. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of oxide-dispersion-strengthened nickel-based superalloy (ODSN) are related to its thermal stability and pose an essential challenge in modeling. This study focuses on the relationship between the microstructure and yield strength of heat-treated ODSN. Through a comprehensive description of the microstructure as investigated by electron-backscatter diffraction analysis and transmission electron microscopy, the contributions to the yield strength arising from various strengthening mechanisms in heat-treated ODSN specimens are addressed. The yield strength calculated using the characterized microstructure parameters in combination with different strengthening models are compared with the measured yield strength. This comparison reveals that grain-boundary strengthening, followed by dispersion and dislocation strengthening, is the significant contributor, while a root-sum-square superposition model reproduces the measured yield strength of heat-treated ODSN within the experimental scatter. [ABSTRACT FROM AUTHOR]

As a result of their excellent creep strength at high temperatures, oxide dispersion strengthened alloys are attractive candidates as structural materials for high temperature applications. However, traditional production processes encounter several challenges when producing complex shape. This study employed a single/multi-track process optimization approach for the preparation of oxide dispersion strengthened nickel-based superalloy by laser metal deposition. A multi-objective optimization method (power, scanning speed, powder feed rate) was adopted in the preparation process to improve the dilution, wetting angle, powder capture efficiency and hardness of the single-track. The Taguchi method-Principal component analysis-Gray relational analysis-Response surface methodology was used to optimize the characteristics of single-track. Thermocapillary convection drives the motion of gas bubbles in single-track depositions, while surface pores typically cluster along the track edge. The microstructure and crack of multi-track are optimized by overlapping ratio. In order to investigate the effect of the transition from columnar to equiaxed, the microstructure of the laser-deposited bulk specimen was studied. The results verify the feasibility of multi-objective process optimization in track and present experience for the application of microstructure and mechanical properties of track process optimization laser-deposited sample. • The multi-objective process optimization was applied to optimize single track. • The microstructure of as-deposited bulk was optimized by track process optimization. • The introduced nanoparticles can promote columnar to equiaxed transition. [ABSTRACT FROM AUTHOR]

The effects of Y addition on the microstructural evolution and room/high temperature mechanical properties of SLM Ni-based superalloy were investigated. Y-free and Y-added SLM Ni-based superalloy as two alloys for comparison. In the Y-added superalloy, the main reasons for the denser microstructure are a flat melt pool and epitaxial growth of columnar dendrites with <001> orientation. Moreover, the Y-added SLM specimens have superior hot tensile performance due to high density dislocation and the formation of Y 2 O 3 -oxides. Analysis of microstructural evolution under elevated temperatures showed that the interaction between dislocation and oxide could pin dislocation at the grain boundary and increase the annihilation temperature of dislocation. We also examined the high temperature fracture morphology of Y-added specimens, and found that their excellent performance is primarily because of tenacity nests and serpentine sliding originating from the oxide and dislocation, as well as their impediment to microvoid extension. • The Y-added SLM sample decrease defects though complete melting. • Oxide strengthened Ni-based superalloy is in-situ prepared by selective laser melting. • The oxide formation mechanism of Y-added Ni-based superalloy in SLM is revealed. • Y 2 O 3 -oxides can elevate annihilation temperature of dislocation. • The Y-added sample has superior tensile strength than Y-free sample. [ABSTRACT FROM AUTHOR]

The determination of mechanical properties of oxide dispersion strengthened (ODS) nickel-based superalloys has certain challenges because of the irradiation and high-temperatures in nuclear industry. In this study, the microstructure and mechanical properties of ODS nickel-based superalloy with different grain size distributions were investigated using electron backscatter diffraction, transmission electron microscopy, tensile strength, and Vickers hardness. Besides, the correlation of hardness and yield strength was obtained using thermal-exposed samples. The results showed that the relationship between the hardness and yield strength mainly came from microstructure and anisotropy, especially the grain size distributions, the texture, and the tested plane. This allowed Vickers hardness tests rather than the tensile tests to evaluate the changes in strength, which is especially useful for the study of thermal stability of ODS nickel-based superalloys in nuclear industry. • Obtain the correlation of hardness and strength of ODS nickel-based superalloy. • The hardness/yield strength changed significantly before and after thermal exposure. • The relationship of hardness and strength is sensitive to the anisotropy and texture. • The bimodal grain has a higher ratio of hardness/yield strength. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened nickel-based superalloy (ODSN) is widely used in structural materials such as aerospace and nuclear industry, but its uniform dispersion distribution of particles makes it only necessary to manufacture using conventional powder metallurgy technique. Selective laser melting (SLM) is first selected as a novel method for manufacturing ODSN. The influence of SLM process parameters (laser power, scanning speed, hatch spacing) on the microstructure and mechanical property in ODSN has been investigated, using Taguchi method (TM) and Response surface methodology (RSM). Orthogonal array and Box-Behnken design were utilized to design the experiments. Analysis of variance, signal-to-noise ratio, main influence diagrams, 3D response surface, and the corresponding contour plots were measured to evaluate the effects of factors on the tensile strength. By comparing the results of the experiment showed that optimum parametric combination from TM and RSM induced better tensile strength. The optimum process parameters were predicted, validated, and subsequently employed to build samples to assess mechanical properties during 25–1000 °C. The samples produced using SLM showed approximate tensile strength, compared to hot extruded ODSN of similar composition. Fracture morphology results good agreement with mechanical property at room and elevated temperature. Unlabelled Image • Selective laser melting is first employed to manufacture oxide dispersion strengthened nickel-based superalloy. • The taguchi-response surface methodology was first applied to optimize the process parameters of selective laser melting. • The influence of power, scanning speed and hatch spacing on the tensile strength was investigated. • The tensile strength of the optimized samples is almost the same as that of the traditional hot extrusion. [ABSTRACT FROM AUTHOR]

During the creep deformation, damage mainly occurs in the tertiary creep stage. Thus, creep life is essentially controlled by the tertiary stage. To find out the impact of the dislocation microstructure on the creep rate in this stage, and in particular its thickness-dependence, creep tests of Ni-based single crystal superalloy are carried out at 1100 °C/130 MPa with sheet samples of varying thickness. The creep rate in tertiary creep clearly increases and rises faster in thinner sample sheets. The observed microstructures demonstrate that the γ/γ' phase distribution shows no topological inversion. It is found that the variety of dislocations in γ' rafts decrease with decreasing sample thickness. The γ' rafts in thin samples are sheared by only two types of superdislocation involving individual screw superdislocations and double Lomer-Cottrell structures, while the γ' rafts in thick samples are sheared by three types of superdislocations, again individual screw superdislocations and double Lomer-Cottrell structures, but also Kear-Wilsdorf structures. Also the density of disclocations varies significantly. A higher density of individual screw superdislocations and of mobile dislocation pairs, but lower density of locked non-planar structures promotes the creep of thinner samples. [Display omitted] • γ' rafts are sheared by individual screw superdislocations in both thin and thick samples during tertiary creep. • Only one type of non-planar structure, namely the double Lomer-Cottrell structure is detected in γ' rafts of thin samples. • Two types of non-planar superdislocation structures are found in γ' rafts of thick samples. • Higher density individual screw superdislocations and less variants non-planar ones promotes the creep of thinner samples. [ABSTRACT FROM AUTHOR]