High Temperature Creep Resistance Research Articles

Rapid screening of creep resistance in additive manufactured Ti-6Al-4V alloy

In this study, rapid screening of creep resistance for additively manufactured (AM) materials is enabled using an accelerated creep test (ACT) called the dynamic negative stepped test (DNST). Additively manufactured components are on the rise in aerospace systems, particularly turbine engine parts. It can be challenging to identify the AM process parameters that correspond to high-temperature strength and creep resistance. Tests are needed that rapidly screen and qualify creep resistance. In DNST, the temperature is held constant, and the specimen is taken through a series of load holds from high to low. The DNST contains two distinct characteristics: (1) the hold duration for each step is “dynamically” controlled to ensure the minimum-creep-strain-rate is achieved at every step, and (2) the “descending” profile maximizes damage accumulation while remaining on the edge of instability. The DNST assesses a material's ability to resist creep damage accumulated across multiple deformation mechanisms. In this study, electron beam melted Ti–6Al–4V, built at six (6) directions: 0°, 30°, 45°, 60°, 90°, and V, is tested using a four-step DNST at 650 °C. Analysis of the minimum-creep-strain-rate (at each step), creep ductility, and rupture were performed. A strain energy density (SED) benchmark metric was established to rapidly screen and classify creep resistance using an “Ashby-like” plot that balanced energy dissipated and energy dissipation rate. The creep resistance was found to be ranked from V, 60, 45, 30, 90, and 0 in descending order. Microstructural and crystallographic characterization revealed grain refinement progressing from the strain-free to the high-strain regions, indicative of dynamic recrystallization (DRX). Further observations revealed that the material's energy capacity increased with the alignment of the grains' long axis and tensile direction.

Influence of Oxide Inclusions on the Creep Properties of Electron Beam Welds in Ti-added Reduced Activation Ferritic/Martensitic Steel

In this study, the effects of reducing oxide inclusions on the microstructure, tensile properties at 550 ℃, and creep resistance of electron beam welded (EBW) joints in Ti-added reduced activation ferritic/martensitic (RAFM) steel were investigated. Two RAFM steels, designated as Steel A and Steel B, were prepared by adjusting the Al and Si contents to contain different fractions of oxide inclusions. The microstructures of the base metal and heat-affected zone (HAZ) were analyzed, and the mechanical properties of the welds were evaluated. Reducing the Al and Si content in Steel B resulted in a decrease in both the fraction and size of oxide inclusions compared to Steel A. Both steels exhibited tempered martensitic microstructures with M23C6 and MX precipitates. Hardness tests revealed that the over-tempered HAZ (OTHAZ) was the weakest region. However, both tensile test and creep tests at 550 ℃ showed fractures occurring in the base metal, indicating that the low heat input of EBW effectively minimized the weak HAZ regions. Steel B demonstrated improved tensile properties and creep resistance due to the reduced oxide inclusions. It was found that Steel B had a significantly longer creep life than Steel A at 550 ℃. SEM analysis revealed ductile fracture characterized by dimples, where oxide inclusions containing Al, Si, Ta, and Ti were predominant. The presence of these inclusions reduced the effectiveness of solid solution and precipitation strengthening by consuming Ta and Ti. Additionally, microvoids could easily nucleate at the interface between the hard oxide inclusions and the soft metal matrix during creep. Therefore, the reduction of oxide inclusions improved deformation resistance and high-temperature stability, resulting in better creep resistance in Steel B. In conclusion, the reduction of oxide inclusions in Ti-added RAFM steel enhances the high-temperature mechanical properties and creep resistance of EBW joints, underscoring the importance of oxide control in the development of RAFM steels for fusion reactor applications.

Atomic-scale compositional complexity ductilizes eutectic phase towards creep-resistant Al-Ce alloys with improved fracture toughness

Hierarchical microstructures spanning from micro-sized eutectic structure to nano-sized precipitates are promisingly engineered in lightweight Al alloys to improve the high-temperature creep resistance that is increasingly required for rapid industrial development. However, the intrinsically-brittle eutectic phase is ready to fracture upon applied loading, which, dramatically reducing room-temperature ductility and fracture toughness, greatly hampers practical applications of the creep-resistant Al alloys. Here, through the combination of Sc microalloying with sub-rapid solidification, we observe the ductilization of Al11Ce3 eutectic phase in cast heat-resistant Al-Ce-Sc alloys due to the formation of atomic-scale compositional complexity. High-concentration Sc atoms are frozen within the Al11Ce3 intermetallic phase by the sub-rapid solidification, which then assemble into unusual atomic-scale compositional dipoles with the Sc atoms enriched at one pole and the Al atoms at the opposite during subsequent heat treatment. The dispersed Sc-Al compositional dipoles induce local lattice distortions that stimulate dislocation activities, as temporally and spatially visualized by in-situ neutron diffraction tensile test and microstructural characterizations. The unexpected plastic deformation triggered in Al11Ce3 improves the deformation compatibility between the eutectic phases, enabling the sub-rapidly-solidified Al-Ce-Sc alloy to reach a room-temperature tensile elongation 3 times and fracture toughness over 8 times of its counterpart derived from traditional solidification. In addition, the sub-rapidly-solidified Al-Ce-Sc alloy exhibits an excellent creep resistance at 300 °C, achieving a tensile creep stress threshold of ∼ 70 MPa. These findings provide new perspectives on the design of ductile intermetallic phases and the development of creep-resistant Al alloys with application-level ductility.

The Microstructure and Mechanical Properties of Si3N4f/BN/SiBCN Microcomposites Fabricated by the PIP Process.

SiBCN ceramics based on SiC, BN and Si3N4 structures have good comprehensive properties such as high-temperature resistance, oxidation resistance, creep resistance and long life, which makes it one of the very promising ceramic material systems in military and aerospace fields, etc. In this study, SiBCN ceramics, as well as Si3N4f/BN/SiBCN microcomposites, were prepared by a polymer infiltration pyrolysis method using PBSZ as the polymer precursor. The PBSZ was completely ceramized by pyrolysis at 900 °C. The weight loss and elemental bonding forms of the products after the pyrolysis of the precursors hardly changed from 600 °C to 900 °C. After pyrolysis at 600 °C for 4 h and using the BN coating obtained from twice deposition as the interfacial phase, a more desirable weak interface of fiber/matrix with a binding strength of 21.96 ± 2.01 MPa can be obtained. Si3N4f/BN/SiBCN ceramic matrix microcomposites prepared under the same pyrolysis conditions have a relatively good tensile strength of 111.10 MPa while retaining a weak interface between the fibers and the matrix. The results of the study provide more theoretical and methodological support for the application of new composite structural ceramic material systems.

High compressive strong-ductile tungsten-rhenium alloy via B4C addition

Tungsten-rhenium alloys with high-temperature strength and creep resistance are widely used in the cutting-edge scientific and technological sectors. However, W-5Re alloy with Re content of 5 wt% shows brittleness at room temperature, limiting its aviation temperature measurement. This work provides W-5Re-xB4C (x = 0∼2.0 wt%) tungsten-rhenium alloys with high compressive performance by introducing B4C ceramic phase and regulating the B4C content. The alloys are composed of W(Re) solid solution, W2B phase and a few W2C. When increasing B4C addition, the grain size of the W(Re) matrix decreases from about 200 μm to 17 μm in W-5Re-2B4C and the content of intermetallic compounds increase to be about 12% in W-5Re-0.5B4C and 58% in W-5Re-2B4C. The W-5Re-0.5 wt% B4C alloy exhibits a high yield strength of 1.19 GPa and compressive plasticity with a plastic strain of 42.50%, while the W-5Re-0.75B4C alloy has a high yield strength of 1.52 GPa and good compressive plasticity of 22.52%, obviously higher than the reported W-5Re alloys. Fine-grain strengthening from the refinement of W(Re) and second-phase strengthening from intermetallic compounds simultaneously enhance the alloys, while good deformability of W(Re) metallic phase contributes to the room-temperature plasticity. This study provides necessary data accumulation and theoretical support for the application of tungsten-rhenium alloys.

Creep behavior and fracture mechanism of an additively manufactured 316L stainless steel with extraordinary creep resistance

The creep behaviors of laser powder bed fusion (LPBF) additively manufactured (AM) 316L stainless steel (SS) and its recrystallized (Re) counterpart were investigated via uniaxial constant-load creep tests at 600 °C with nominal stress levels ranging from 235 to 360 MPa. Anisotropic creep behavior was observed in AM 316L SS, with superior creep resistance but inferior creep ductility in the horizontal sample (loaded perpendicular to the build direction (BD)) compared to the vertical sample (loaded parallel to the BD). This superior creep resistance was likely resulted from shorter dislocation slip distance and the inferior creep ductility was due to faster propagation of cracks along the columnar grain boundaries. Compared with both the Re counterpart and conventional 316L SS, AM 316L SS in this study exhibited an extraordinary creep resistance at various stress levels, with the minimum creep rate being two to three orders of magnitude lower and much longer creep life. This exceptional creep resistance of AM 316L SS was attributed to the presence of dislocation cells that impeded the deformation-induced dislocations. This led to a remarkably low rate of creep deformation and delayed the creep crack initiation, ultimately resulting in a long creep life. The gradual development of the precipitate films enriched with Mo, Si and Cr along high-angle grain boundaries, following prolonged exposure to high temperatures, was found to restrict the creep ductility in AM 316L under low stress conditions. Nevertheless, the study demonstrates that the stable dislocation cells are beneficial in enhancing the high-temperature creep resistance of AM 316L SS.

The enhancement effects of multiple hydrogen bonds between bi-terminal groups and penta-alanine assemblies on creep resistance and mechanical strength of polyisoprene rubbers

Along with the continuously advanced requirements for high performance rubbers such as in aircraft tires, natural rubbers show limitations in achieving high temperature creep resistance and high mechanical strength simultaneously. Meanwhile, the bi-terminal interactions of natural rubber (NR) exhibit unique reinforcement effects, which are difficult to realize in synthetic rubbers. In this article, four different types of bi-terminal polyisoprenes with distinct groups were synthesized, and they were assembled with penta-alanine molecule (5 A) to obtain high-performance rubbers by terminal hydrogen bonding reinforcement. Infrared tests indicate that the strong hydrogen bonds can be formed between two terminals and turn stronger after adding 5 A. Thus, the network of rubbers remains stable under different strains and temperatures and the activation energy of networks is elevated. As a result, this biomimetic design endows polyisoprene with high tensile strength compared with Malaysian NR and greater creep resistance. Further Mooney fitting and DMA test suggests that these confined hydrogen bonds not only increased the number of crosslinking points and entanglements of network, but also are beneficial for the segmental chain motion. In contrast to our common sense on hydrogen bonds which present large relaxation and hysteresis, this study proposes a facile method for preparing high-temperature creep-resistant rubbers by introducing confined hydrogen bonds, which represents a great advance in the structural biomimicry of NR.

Study on microstructure, tensile performance and creep resistance of Al–Mg–Si-Sc-Zr alloy strengthened by Al3(Sc, Zr) nanoprecipitates

In this work, a newly designed Al–Mg–Si-Sc-Zr alloy with both excellent room-temperature tensile performances and high-temperature creep resistance was prepared. The microstructure of Al–Mg–Si matrix alloy was significantly modified by Sc, Zr micro-alloying. The average grain size of Al–Mg–Si-Sc-Zr alloy in as-cast and as-rolled states was 68.3 μm and 7.33 μm respectively, which was far smaller than those of Al–Mg–Si alloy. Numerous Ll2-ordered Al3(Sc, Zr) nanoprecipitates ranging from 20 to 60 nm were generated during heat treatment, and they had a strong coherent interface relation with α-Al matrix. During rolling deformation, a further DRX (dynamic recrystallization) progress happened in Al–Mg–Si-Sc-Zr alloy due to the severe dislocations pile-up caused by Al3(Sc, Zr) nanoprecipitates. The YS (yield strength), UTS (ultimate tensile strength) and EL (elongation) of as-rolled Al–Mg–Si-Sc-Zr alloy at room temperature were 315.2 MPa, 372.3 MPa and 20.1 % respectively, which were increased by 21.9 %, 17.1 % and 41.5 % compared to tensile performances of Al–Mg–Si matrix alloy. An enhanced high-temperature creep resistance was obtained by Al–Mg–Si-Sc-Zr alloy, which was mainly attributed to the outstanding thermal stability possessed by Al3(Sc, Zr) nanoprecipitates. The strengthening mechanisms including grain refinement strengthening and Orowan strengthening were also discussed.

Mechanical Response of Cu/Sn58Bi-xNi/Cu Micro Solder Joint with High Temperatures

Sn58Bi solder is considered a promising lead-free solder that meets the performance requirements, with the advantages of good wettability and low cost. However, the low melting point characteristic of Sn58Bi poses a serious threat to the high-temperature reliability of electronic products. In this study, Sn58Bi solder alloy based on nickel (Ni) functionalization was successfully synthesized, and the effect of a small amount of Ni on creep properties and hardness of Cu/Sn58Bi/Cu micro solder joints at different temperatures (25 °C, 50 °C, 75 °C, 100 °C) was investigated using a nanoindentation method. The results indicate that the nanoindentation depth of micro solder joints exhibits a non-monotonic trend with increasing Ni content at different temperatures, and the slope of the indentation stage curve decreases at 100 °C, showing that the micro solder joints undergo high levels of softening. According to the observation of indentation morphology, Ni doping can reduce the indentation area and accumulation around the indentation, especially at 75 °C and 100 °C. In addition, due to the severe creep phenomenon at 100 °C, the indentation hardness rapidly decreases. The indentation hardness values of micro solder joints of Cu/Sn58Bi/Cu, Cu/Sn58Bi-0.1Ni/Cu, and Cu/Sn58Bi-0.2Ni/Cu at 100 °C are 14.67 ± 2.00 MPa, 21.05 ± 2.00 MPa, and 20.13 ± 2.10 MPa, respectively. Nevertheless, under the same temperature test conditions, the addition of Ni elements can improve the high-temperature creep resistance and hardness of Cu/Sn58Bi/Cu micro solder joints.

High-Temperature Creep and Microstructure Evolution of Alloy 800H Weldments with Inconel 625 and Haynes 230 Filler Materials

Alloy 800H stands as one of the few code-qualified materials for fabricating in-core and out-of-core components operating in high-temperature reactors. Welding is a common practice for assembling these components; however, the selection of a suitable filler material is essential for enhancing the high-temperature creep resistance of Alloy 800H weldments in high-temperature applications. In this study, Inconel 625 and Haynes 230 filler materials were used to weld Alloy 800H plates by employing the gas tungsten arc welding technique. The high-temperature tensile and creep rupture properties, microstructural stability, and evolution of the weldments after high-temperature exposure were investigated and compared with those of Alloy 800H. The results show that both weldments exhibit enhanced tensile and creep behavior at 760 °C. The creep rupture times of the weldments with Inconel 625 filler and Haynes 230 filler materials were about two and three time longer, respectively, than those of Alloy 800H base metal when tested at 80 MPa and 760 °C. Carbides (MC and M23C6) were commonly observed in the microstructures of both the weld and base metals in the two weldments after high-temperature creep tests. However, the Inconel 625 filler weldment displayed detrimental δ and Laves phases in the fusion zone, and these precipitates could be potential sites for initiating cracks following prolonged high-temperature exposure. This study shows that the weldment with Haynes 230 filler material exhibit better phase stability and creep rupture properties than the one with Inconel 625, suggesting its potential for use as a candidate filler material for Alloy 800H for further investigation. This finding also emphasizes the critical consideration of microstructural evolutions and phase stability in evaluating high-temperature materials and their weldments in high-temperature reactor applications.

Overview of Surface Modification Techniques for Titanium Alloys in Modern Material Science: A Comprehensive Analysis

Titanium alloys are acclaimed for their remarkable biocompatibility, high specific strength, excellent corrosion resistance, and stable performance in high and low temperatures. These characteristics render them invaluable in a multitude of sectors, including biomedicine, shipbuilding, aerospace, and daily life. According to the different phases, the alloys can be broadly categorized into α-titanium and β-titanium, and these alloys demonstrate unique properties shaped by their respective phases. The hexagonal close-packed structure of α-titanium alloys is notably associated with superior high-temperature creep resistance but limited plasticity. Conversely, the body-centered cubic structure of β-titanium alloys contributes to enhanced slip and greater plasticity. To optimize these alloys for specific industrial applications, alloy strengthening is often necessary to meet diverse environmental and operational demands. The impact of various processing techniques on the microstructure and metal characteristics of titanium alloys is reviewed and discussed in this research. This article systematically analyzes the effects of machining, shot peening, and surface heat treatment methods, including surface quenching, carburizing, and nitriding, on the structure and characteristics of titanium alloys. This research is arranged and categorized into three categories based on the methods of processing and treatment: general heat treatment, thermochemical treatment, and machining. The results of a large number of studies show that surface treatment can significantly improve the hardness and friction mechanical properties of titanium alloys. At present, a single treatment method is often insufficient. Therefore, composite treatment methods combining multiple treatment techniques are expected to be more widely used in the future. The authors provide an overview of titanium alloy modification methods in recent years with the aim of assisting and promoting further research in the very important and promising direction of multi-technology composite treatment.

The Effect of Replacing Ni with Mn on the Microstructure and Properties of Al2O3-Forming Austenitic Stainless Steels: A Review.

Al2O3-forming austenitic steel (AFA steel) is an important candidate material for advanced reactor core components due to its excellent corrosion resistance and high temperature strength. Al is a strong ferrite-forming element. Therefore, it is necessary to increase the Ni content to stabilize austenite. Ni is expensive and highly active, and so increasing the Ni content not only increases the costs but also damages the radiation resistance. Mn is a low-cost austenitic stable element. Its substitution for Ni will not only help to improve the irradiation resistance of austenitic steel, but also reduce the cost. In order to explore the feasibility of Mn-substituted Ni-stabilized austenite in AFA steel, this paper summarized the research progress of Mn-added AFA steels, whilst the research status of traditional Mn-added austenitic steels are also referred to and compared herein. The effect of the addition of Mn on the microstructure and properties of AFA steel was analyzed. The results show that Mn can promote the precipitation of the M23C6 phase and inhibit the precipitation of the B2-NiAl phase and secondary NbC phase. With the increase in Mn content, the strength of AFA steel at room temperature and high temperature decreased slightly, the room temperature elongation increased slightly, while the high temperature elongation and creep resistance decreased obviously. In addition, for austenitic steel free of Al, the addition of Mn will destroy the oxide layer of Cr2O3, which will decrease the oxidation resistance of the steel. But the preliminary study shows that Mn has little effect on the Al2O3 oxide layer. It is worth studying the effect of Mn-substituted Ni on the oxidation resistance of AFA steel. In summary, more efforts are necessary to investigate the optimal Mn content to balance the advantages and disadvantages of introducing Mn instead of Ni.

Thermally stable strong texture in additively manufactured cobalt-based superalloys

Cobalt-based γ'-strengthened superalloy exhibits tremendous potential in numerous fields, such as power generation equipment and the aerospace industry. The characteristics of laser powder bed fusion technology have aligned well with the demands for superalloy's strong texture or single crystal microstructure. However, the post-heat treatment processes often disrupt the initial microstructure of the as-printed parts due to unavoidable recrystallization. In this study, we obtained a strong <101> texture in an as-printed Co-5Al-14V (at.%) ternary cobalt-based superalloy, which is difficult to achieve in nickel-based superalloys. Intriguingly, this <101> texture remains thermally stable even after aging treatment at a near γ' solvus temperature. Hence, a <101> textured microstructure with an evenly distributed γ'(L12) precipitates was attained, which benefits the improvement of high-temperature creep resistance. The unique microstructure characteristics revealed in this work can be beneficial for the development of cobalt-based superalloys and shed light on the microstructural engineering of other LPBF-manufactured superalloys.

Compressive creep behavior of powder metallurgy manufactured Y2O3-reinforced TNM-B1 TiAl alloy with equiaxed and lamellar microstructure

The high-temperature strength and compressive creep resistance of TNM B1-type TiAl alloy modified with Y2O3 nanoparticles are studied. The effect of equiaxed or lamellar microstructure on mechanical properties and deformation mechanisms is demonstrated. It is established that the lamellar microstructure provides higher strength compared to the equiaxed one at high temperatures. Microstructural studies of the samples, subjected to compressive creep, revealed that creep is governed by the dislocation mechanisms for the equiaxed alloy, while a combination of dislocation mechanisms, twinning, dynamic recrystallization, and dynamic recovery is typical for the lamellar alloy. In both types of alloys, Y2O3 nanoparticles successfully act as reinforcement at 800 °C, while being prone to failure and facilitating microcrack nucleation at the nanoparticle/matrix interface at 1100 °C.

Experimental analysis and response surface optimization of the influence of Zn/Al mass ratio on the creep resistance and tensile properties of As-cast Mg-Zn-Al alloy

Mg-Zn-Al (ZA) alloys are intended to exhibit creep resistance and high temperature mechanical capabilities due to appropriate Zn/Al mass ratio control. The effect of altering Al concentration (1, 2, 3, 4, 5, 6) on the tensile and creep resistance properties of Mg-9Zn-xAl alloy was determined experimentally and quantitatively optimized using response surface method (RSM). It was observed that 2 wt% Al improves both room and elevated temperature tensile strength, and ductility of the considered ZA alloy. The creep rate sharply increased when the mass fraction of aluminum rose above 4 wt%. For the purpose of forecasting the tensile and creep properties of the ZA alloy at any given aluminum mass percentage, quartic models were generated, with all model terms significant. Strong room and high-temperature tensile strength, %elongation, and creep resistance were all reached in the low Al-containing ZA alloys, with 1.597 wt% anticipated ideal Al mass concentration, according to RSM optimization.

Fracture toughness of Mo–Si–B-quartz particulate composites

Multiphase Mo–Si–B alloys are potential candidates for high temperature applications owing to their favorable high temperature strength, creep and acceptable oxidation resistance. The inherent brittle nature of these alloys coupled with their low fracture toughness (<8 MPa m) below 1000 °C, limits their application in industry. The addition of α-quartz as particulate reinforcement in various multiphase alloys containing MoSS, T2 (Mo5SiB2) and Mo2B resulted in increased fracture toughness values, from ∼6 to 12 MPa m to ∼17–23 MPa m, in the temperature range 200 °C–800 °C. Interrupted fracture toughness testing revealed that markedly higher loads were required to propagate the crack tip as compared to those required for the comparable alloy without quartz. The improvement in fracture toughness behavior is also seen when quartz is added to two-phase alloys containing MoSS and the T2 phase. Tensile tests above 900 °C show that the addition of quartz results in softening and decreased ductility as compared to the base alloy. It is proposed that compressive elastic stresses induced in the composite due to differential thermal expansion of quartz and the Mo–Si–B matrix contribute to the observed increase in fracture toughness of the composites.

Improving the High-Temperature Creep Resistance of a Cast Co-Cr-W-Ni Superalloy with Additions of B and Zr

Improving the High-Temperature Creep Resistance of a Cast Co-Cr-W-Ni Superalloy with Additions of B and Zr

Microstructural and mechanical properties of precipitation-strengthened Al-Mg-Zr-Sc-Er-Y-Si alloys

We study the precipitation behavior of two L12-strengthened alloys with Mg and Y additions: ultralow-Sc, Si-free, Al-1Mg-0.09Zr-0.007Sc-0.006Er-0.02Y-0.01Si, and low-Sc, Si-added, Al-1Mg-0.09Zr-0.013Sc-0.006Er-0.02Y-0.08Si (at.%), and their ambient temperature strength and high-temperature creep resistance. Scanning transmission electron microscopy analyses reveal that β-Mg2Si precipitates or their precursors (β’, β’’) form in the Si-added alloy at ∼ 200°C, which act as preferential nucleation sites for the L12-nanoprecipitates and cause partial depletion of Si solute atoms, which decelerates L12 precipitation-kinetics, resulting in a microhardness peak at the same isochronal temperature, 475°C, in both alloys. Atom-probe tomography analyses reveal that the L12-nanoprecipitates in both alloys exhibit Sc/Er/Y-rich cores and Zr-rich shells, as well as Mg segregation at the interfacial regions. The L12-nanoprecipitates in the Si-added alloy has a smaller Sc/Er/Y and higher Zr and Si concentrations. The Si-free alloy exhibits superior creep properties at 300°C, due to a larger lattice parameter mismatch of the L12-nanoprecipitates with the Al(f.c.c.) matrix provided by higher Sc/Er/Y concentrations. Both alloys exhibit slower L12 nanoprecipitation-kinetics, extremely high coarsening resistance, and similar high-temperature creep resistance compared to a recently developed Mg/Y-free low-Sc, Al-0.08Zr-0.014Sc-0.008Er-0.09Si (at.%) alloy. The extremely high coarsening resistance of the Si-added alloy is attributed to the smaller Si concentration in the Al(f.c.c.) matrix. Silicon is scavenged by the β-Mg2Si precipitates, thermodynamically stable at < 475°C, which is then not available in the matrix to accelerate the coarsening of the L12-nanoprecipitates.

Infiltration of silica slurry into silica-based ceramic cores prepared by selective laser sintering based on pre-sintering

The silica-based ceramic core is an important mold for manufacturing hollow blades. The preparation of silica-based ceramic cores by combining selective laser sintering (SLS) and vacuum infiltration (VI) technologies is a promising approach. To enhance infiltration effect, pre-sintering was introduced to obtain the bodies with high porosity and hydrophilicity. The infiltrated silica content increased from 29.58 wt% for samples infiltrated once to 68.91 wt% for samples infiltrated three times. The increase of silica content in the matrix reduced the porosity of final ceramics, thus improving the room-temperature flexural strength. The infiltrated silica gradually transformed into cristobalite during the high-temperature test, which improved the high-temperature flexural strength and creep resistance. When the final-sintering temperature was 1225 °C, room-temperature and high-temperature flexural strength reached 21.17 MPa and 21.21 MPa, respectively, due to the enhanced sintering necks and low porosity. These results indicate that our work provides a promising route to fabricate silica-based ceramic cores by SLS technology.

Improved creep resistance of 20Cr25NiNb heat resistant steels through grain boundary intermetallic precipitation strengthening

The creep performance of 20Cr25NiNb austenitic steel through the addition of W, Mo, and B elements was investigated. W and Mo enhanced high temperature creep resistance primarily through solid solution strengthening σss and precipitation strengthening σp by introducing Laves phase. Boron enrichment was predominantly observed in Laves phase at grain boundaries (GBs) rather than in σ and G phases. The significant improvement in creep life of B-bearing steel was primarily attributed to a remarkable increase in σp, as trace amounts of B greatly accelerated Laves precipitation at GBs and inhibited σ phase coarsening. The creep resistance decreased significantly after 5000 h thermal aging due to reduced dislocation density, grain coarsening, and decreased solid solution elements in austenite. Creep cracks mainly propagated along GBs, where coarse σ phase provided sites for creep cavity nucleation. The applied stress accelerated σ precipitation and coarsening, resulting in the absence of tertiary creep stage during 4700 h creep of B containing 2025WMoB steel.