Improvement Of High-temperature Strength Research Articles

Strengthened high-temperature resistance in B4C/SiC/2024Al composite via SiC nanowires pinning grain boundary

Aiming at developing new B4C/Al neutron absorbing materials with both high thermal neutron absorption performance and high-temperature strength for neutron shielding applications, B4C/SiC/2024Al composites were fabricated by powder metallurgy process. TEM results revealed that SiC nanowires were distributed at Al grain boundaries and bonded tightly with Al matrix. Grain boundary sliding and grain rotation were efficiently restricted and prevented dislocation annihilation at GBs. The thermal stability of composite was improved by adding SiC nanowires and achieved a high-temperature strength of 163.4 ± 1.8 MPa at 573 K, which was increased by 92.2 % compared to the composite without addition of SiC nanowires. By analyzing the strengthening mechanism, dislocation strengthening was the dominant factor in the improvement of high-temperature strength. As for neutron absorption application, B4C/SiC/2024Al composite exhibited not only good high-temperature strength but also remarkable neutron absorption performance and matched thermal conductivity and coefficient of thermal expansion.

Constructing high-density dislocations by primary (Nb,Ti)(C,N) to induce massive secondary precipitations in austenitic heat-resistant cast steel

The microstructural evolution and the high-temperature tensile properties of a novel Nb-Ti-N alloyed austenitic heat-resistant cast steel were investigated during the isothermal aging at 900 °C for up to 1000 h. The mechanisms of enhancing strength-ductility synergy and the high-temperature tensile fracture behaviors after the aging process were analyzed in detail through tensile tests at 900 °C. Obtained results show that the high-density dislocation regions and the Cr segregation, formed around the intragranular primary (Nb,Ti)(C,N), provided numerous nucleation sites for submicron-sized secondary M23C6 precipitates. The density of secondary M23C6 significantly increased, while the coarsening rate was notably reduced during the aging process. Simultaneously, the uniform dispersed precipitation of nano-sized secondary (Nb,Ti)C was promoted by massive dislocations, which exhibited high resistance to coarsening during long-term aging. The improvement of high-temperature strength was attributed to the precipitation strengthening effects induced by the submicron-sized M23C6 and nano-sized (Nb,Ti)C particles. Additionally, the enhancement of ductility during aging was mainly due to the more effective stress transmission and the ameliorated interface relationship between primary (Nb,Ti)(C,N) and austenitic matrix, which was facilitated by the abundant precipitation of secondary phases. The current findings indicate that high-density dislocations constructed by the intragranular primary (Nb,Ti)(C,N) induce the extensive dispersion of secondary strengthening phases, which enhanced both ultimate tensile strength of 158 MPa and elongation of 45.3% at 900 °C after isothermal aging for 1000 h.

Improvement of Strength and Oxidation Resistance at High Temperature in AISI 4140 Steel by Micro-Alloying Chromium and Tungsten for Automotive Engine Applications

Increasing the operating temperature and pressure of an automotive engine and reducing its weight can improve fuel efficiency and lower carbon dioxide emissions. These can be achieved by changing the engine piston material from conventional aluminum alloy to high-strength heat- resistant steel. American Iron and Steel Institute 4140 modified steels (AISI 4140 Mod.s), which have improved strength, oxidation resistance, and wear resistance at high temperature were developed by adjusting the AISI 4140 alloy compositions and optimizing the heat treatment process for automotive engine applications. In this study, the effects of modifying alloy compositions on the microstructure, mechanical properties (both at room and high temperatures), and oxidation of AISI 4140 Mod.s were investigated. Effective grain refinement occurred due to the influence of high-temperature stable carbide forming elements such as Mo, and V. The bainite structure changed to martensite structure under the influence Cr and Ni. As the Cr and W contents increased, the oxidation resistance was improved, and the oxide layer thickness decreased after 10 hours exposure at 500°C. The AISI 4140 Mod. exhibited a 35% improvement in room temperature strength, 70% improvement in high-temperature strength, and 40% improvement in high-temperature oxidation resistance compared to conventional AISI 4140.

Improvement in tensile strength of GH3536-TiB2 composites fabricated by powder metallurgy

To further improve the tensile strength of the GH3536 alloy (Hastelloy X), the GH3536-1 wt%TiB2 composites were prepared successfully by powder metallurgy. The results show that numerous M3B2 and TiC particles precipitated in the GH3536-TiB2 composite system. Interestingly, M3B2 particles were distributed along the grain boundaries in the composites, while TiC particles were in-situ synthesized along the prior particle boundaries of the spherical GH3536 powders. In addition, the grain size of the composite matrix was decreased to 6.7 μm compared with that of the GH3536 alloy (9.5 μm) since the grain growth being hindered by numerous intergranular phases, which further caused the increase of twin boundary proportion (from 26.5% in the GH3536 alloy to 38.8% in the composites). Consequently, the yield strength of the prepared composites was improved to 361 MPa and 298 MPa, respectively, from 240 MPa and 200 MPa, representing a roughly 50% improvement over the GH3536 alloy at 700 °C and 815 °C. The improvement in high-temperature strength is mainly ascribed to grain refinement strengthening, twin strengthening, hetero-deformation-induced hardening, and the additional grain boundary strengthening effect induced by secondary phases.

Temperature Effect On Reactive Powder Concrete Using Sillimanite As Fine Aggregate

This study examines the use of sillimanite as a fine aggregate in the production of temperature-resistant reactive powder concrete. The impact of high temperatures on the mechanical strength of reactive powder concrete with High Alumina cement is investigated. The effectiveness of utilizing glass powder and sillimanite on the mechanical properties of RPC at high temperatures is investigated in this research. The samples were heated in the muffle furnace to the desired temperatures and then tested for their residual compressive strength, flexural strength and split tensile strength. The residual values of compressive strength, flexural strength and split tensile strength were measured at the temperature of 270C to 800°C. The weight loss of the specimens after exposure to the elevated temperatures was measured and the values showed enhanced resistance to the high temperature effects. The results demonstrate the greater contribution of glass powder and sillimanite towards the significant improvement of high temperature strength of reactive powder concrete than those made with normal quartz sand as fine aggregate.

Substantial Improvement of High Temperature Strength of New-Generation Nano-Oxide-Strengthened Alloys by Addition of Metallic Yttrium.

Oxide-dispersion-strengthened (ODS) Fe-Al-Y2O3-based alloys (denoted as FeAlOY) containing 5 vol. % of nano-oxides have a potential to become top oxidation and creep-resistant alloys for applications at temperatures of 1100–1300 °C. Oxide dispersoids cause nearly perfect strengthening of grains; thus, grain boundaries with limited cohesive strength become the weak link in FeAlOY in this temperature range. One of the possibilities for significantly improving the strength of FeAlOY is alloying with appropriate elements and increasing the cohesive strength of grain boundaries. Nearly 20 metallic elements have been tested with the aim to increase cohesive strength in the frame of preliminary tests. A positive influence is revealed for Al, Cr, and Y, whereby the influence of Y is enormous (addition of 1% of metallic Y increases strength by a factor of 2), as it is presented in this paper.

Origin of the O phase and its effect on the mechanical properties of rolled Ti-22Al-25Nb alloy sheets

ABSTRACT The microstructure and mechanical properties of a hot-rolled Ti-22Al-25Nb (at%) alloy sheet after aging and solution-aging treatment were studied. A typical core-shell structure formed in the as-received specimens upon aging at 650 °C to 950 °C for 6 h. O phases precipitated around the grain boundaries or in the B2 matrix for the solution-treated specimens upon aging at 650 °C to 950 °C. The combined effect of the precipitation and solution strengthening led to a decrease in microhardness while aging from 650 °C to 900 °C and an increase in microhardness while aging from 900 °C to 1000 °C for both the as-received and solution-treated specimens after the aging treatment. Finer O phases contributed to the increase in yield strength and elongation and were more beneficial to the improvement of high temperature strength than of room temperature strength.

Development of thermally stable powder metallurgy Al–Mn alloy extrusions and prediction of their terminal strength after prolonged service periods at high temperatures

The demand for thermally stable aluminum alloys is increasing, but mechanical strength of most of existing alloys inevitably decreases when exposed to high temperature environments. In this study, newly developed powder metallurgy (P/M) Al–Mn alloy extrusions were subjected to full annealing treatment and temperature-accelerated test, to simulate decrease in hardness after prolonged service periods at high temperatures, and were found to possess higher thermal stability than a conventional ingot metallurgy (I/M) 3000 series Al–Mn alloy extrusion. Such improvement of high temperature strength was attributed to the increased volume fractions of thermally stable Al6Mn phase, and quantitatively explained according to the underlying strengthening mechanisms, by which their terminal strength after prolonged and thus practically unmeasurable periods could be predicted. Prediction of the microstructures and the corresponding Vickers hardness was also achieved by combining phase-field simulation and classical micromechanics approach, confirming that the utilized simulation model could enable to predict the terminal strength of the developed P/M alloy extrusions.

Improvement of high temperature strength of 2219 alloy by Sc and Zr addition through a novel three-stage heat treatment route

Significant improvement of high temperature strength of 2219 alloy was achieved by addition of equal atom percentages (at%) of Sc and Zr and using a novel three-stage heat treatment route. Processing of the alloy involved casting in a water-cooled copper mould with a cooling rate in the range of 102 to 103 K/s. This was followed by direct ageing of the cast alloy at 375 ℃ to form thermally stable L12 type coherent Al3(Sc, Zr) precipitates in the matrix. Subsequently, the alloy was solutionized at 535 ℃ and further aged at 200 ℃ which caused the formation of highly dense θ′′ (Al3Cu) and θ′ (Al2Cu) precipitates. From microstructural characterization it was found that the high number density of these precipitates is due to heterogeneous nucleation on pre-existing Al3(Sc, Zr) precipitates. Presence of Al3(Sc, Zr) precipitates together with high number density of θ′′ and θ′ precipitates results in remarkably high 0.2% proof stress of 456 MPa at room temperature, 295 MPa at 200 °C and 227 MPa at 250 °C along with 5% ductility. It was found that Al3(Sc, Zr) precipitates also suppress the coarsening of θ′′ and θ′ precipitates thus making them thermally stable at high temperature.

Simultaneously increasing the high-temperature tensile strength and ductility of nano-sized TiCp reinforced Al-Cu matrix composites

Adding various amounts of nano-sized TiCp (0.1–0.7 wt%) made a simultaneous improvement in high-temperature strength and ductility of the Al-Cu matrix alloy. The influences of temperature and strain rate on the high-temperature tensile properties and θ′ precipitate sizes in the Al-Cu matrix alloy and the nano-sized TiCp/Al-Cu composites were investigated. Theoretical calculations suggest that the yield strength increment of the composites at 453 K was primarily attributed to the strengthening effect of both the refined θ′ precipitates and TiCp, while at 493 K only the strengthening effect of TiCp was predominant due to the significant coarsening of θ′ precipitates at this temperature.

Super high-temperature strength in hot rolled steels dispersing nanosized oxide particles

Further improvement of high-temperature strength is required for the martensitic oxide-dispersion-strengthened (ODS) steels that are the promising structural materials for the advanced fission and fusion reactors, since they have better formability and isotropic mechanical property emphasized as the engineering structural materials. We conducted severe hot rolling at the elevated temperature and subsequent air-cooling for the martensitic 9CrODS steels, and their tensile and creep strength recorded the highest worldwide for the engineering class of ferritic-martensitic steels. Significant strength improvement is originated by replacing the martensitic structure with a transformed coarse ferrite during hot rolling and subsequent air-cooling. The formation of the transformed coarse ferrite doesn’t follow the widely accepted ultrafine grain formation by the similar processing in the ferrite-martensite steels. New structural control and improving high-temperature strength for the ODS steels are greatly important and opened the window for realizing high-performance fission and fusion reactor materials.

Improvement of High Temperature Strength by Addition of Vanadium Content of Ni–Cr–Mo Steel for Brake Discs

Improvement of High Temperature Strength by Addition of Vanadium Content of Ni–Cr–Mo Steel for Brake Discs

Effects of Intermediate Layer in DLC Thin Film on Al2O3for Improvement of High Temperature Strength

DLC coating on ceramics is very useful for manufacturing the materials with hardness and low friction. Adhesion of DLC thin film on ceramics, on the other hand, is usually very weak. Adhesion of DLC film depends on many parameters such as contamination and chemical bonding between thin film and substrate. In this study, adhesion of DLC film on ceramics was improved by the intermediate layer when the plasma immersion ion deposition (PIID) technique was applied. It is found that the chemical composition and the thickness of intermediate layer have significantly an effect on the adhesion of DLC thin film on <TEX>$Al_2O_3$</TEX>.

Effect of Ti, W, Mn, Mo and Si on microstructure and mechanical properties of high carbon Fe–10·5 wt-%Al alloy

Effect of quaternary alloying elements Ti, W, Mn, Mo and Si on the microstructure and mechanical properties of Fe–10·5Al–0·7C (wt-%) alloy has been investigated. Six different alloys were prepared by a combination of air induction melting with flux cover and electroslag remelting (ESR). The composition of the quaternary alloying element was ∼2 wt-% and was substituted for iron. The ESR ingots were hot forged and hot rolled at 1375 K. The hot rolled alloys were characterised with respect to microstructure and mechanical properties. Alloys containing W, Mn and Si exhibited two phase microstructure of Fe3AlC0·5 precipitates in α Fe–Al matrix. Whereas alloys containing Mo and Ti exhibited three phase microstructures, the additional phase being the respective carbides. Both α Fe–Al matrix and Fe3AlC0·5 precipitates have considerable amount of solubility for W, Mn and Mo whereas Si has very high solubility in α Fe–Al matrix as compared with Fe3AlC0·5 precipitates and titanium has very low solubility in both α Fe–Al matrix and Fe3AlC0·5 precipitates. Greater improvement in room temperature tensile properties was observed by the addition of Mn as compared with the addition of W. Significant improvement in tensile and creep properties was observed by the addition of Mo. Though the addition of silicon has resulted in poor room temperature ductility, there has been remarkable improvement in high temperature strength and creep properties.

Mechanical Performances and Failure Modes of Direct Diffusion Bonding Joints of 316L Stainless Steel

Direct diffusion bonding of 316L stainless steel was performed at 850-1100°C for 1-3 h under a pressure of 10MPa in this study. The effect of bonding temperature and holding time on mechanical performances of the joints was investigated. Tensile tests were conducted to evaluate strength and elongation of the joints at room temperature and elevated temperature of 550°C. The microstructure and fracture surfaces of the joints were examined by optical microscope (OM) and scanning electronic microscope (SEM). The results indicated that the elongation of the joints increased with the increase of bonding temperature and holding time. However, overlong holding time had a side effect on the strength of the joint. Moreover, the change of the mechanical properties was closely related to the variation of the microstructure of the joints. The X-ray diffraction (XRD) analysis revealed that FeCr and Fe0.64Ni0.36 were formed at the DB6 joint during bonding process. It is suggested that FeCr should be detrimental to the improvement of high temperature strength of the joint.

Nano-structure control in ODS martensitic steels by means of selecting titanium and oxygen contents

The effects of mechanical alloying (MA) conditions on oxygen contamination were investigated to establish the manufacturing technology of oxygen-controlled oxide dispersion strengthened (ODS) martensitic steel. High-temperature mechanical tests and microstructure observations were performed in oxygen-and-titanium-controlled steels in order to demonstrate high-temperature strength improvement using oxygen control. Oxygen content was drastically reduced by the purification of MA atmosphere, Ar gas. It was shown that the lower speed agitation at the beginning of MA was effective for the reduction of oxygen contamination because oxygen was mixed mainly at the beginning of mechanical alloying due to the imperfect replacement of Ar gas. Dense distribution of oxide particle and significant improvement in high-temperature strength were achieved by applying 99.9999 wt% Ar in MA atmosphere and reducing oxygen contents. It was revealed that the merely increasing Y 2O 3 and titanium addition cannot improve the high-temperature strength. The excess oxygen is an important parameter for making oxide particles finely and densely distributed as well as improving high-temperature strength.

Fe3Al-Fe3AlC intermetallics for high temperature applications: An assessment

The advantage of carbon containing dual-phase iron aluminides is that they exhibit relative improvement in high temperature strength as well as reduction in probability of hydrogen embrittlement (HE) over the single-phase iron aluminides, which were the main impediment for the commercialisation of these materials. Owing to its practical and scientific importance a study on the dual-phase Fe3Al-Fe3AlC intermetallics is presented, which deals with the current status of research and developments, understanding of physical metallurgy, HE, oxidation resistance and thermal stability; and its limitation and future challenges.

Experimental description of thermomechanical properties of carbon fiber-reinforced TiC matrix composites

Titanium carbide ceramic is a good potential material used in high temperature environment for its good strength, erosion resistance and thermal stability. Unfortunately, the low thermal shock resistance and low fracture toughness are the well-known impediments to its application as high temperature structure components. In order to extend the application of TiC ceramics at high temperature, 20 vol.% short carbon fiber was added into TiC matrix to improve the thermomechanical properties. With the incorporation of carbon fiber, the thermal expansion coefficient of TiC composites was decreased and the thermal conductivity was increased slightly below 900 °C. The flexural strength was improved from 471 MPa for monolithic TiC to 593 MPa for TiC composites, and the strengthening effect of carbon fiber became more prominent at high temperatures. The addition of fiber decreased the elastic modulus of TiC composite. The elastic modulus of the composite decreased with increasing temperature. The improvement of high temperature strength and thermal conductivity and the decrease of thermal expansion will benefit the application of TiC composites in high temperature environment where the temperature usually varies.

Tensile Properties from Room Temperature to 673 K of Mg-0.9 mass%Ca Alloy Containing Lamella Mg&lt;SUB&gt;2&lt;/SUB&gt;Ca

Tensile properties were investigated from room temperature to 673 K of a binary Mg-0.9 mass%Ca alloy where Mg 2 Ca phase was dispersed as lamella. The 0.2% proof stress significantly increased by the Mg 2 Ca phase in the temperature range investigated. It was suggested that strengthening by stable lamella Mg 2 Ca in the grains and limitation of deformation related to grain boundaries by Mg 2 Ca phase at grain boundaries contribute to improvement of high temperature strength for the Mg-Ca alloy.

Improvement of High Temperature Strength and Creep of α-Sialon by Grain Boundary Crystallization

Improvement of High Temperature Strength and Creep of α-Sialon by Grain Boundary Crystallization