Yield Strength Σ0 Research Articles

Investigation of new steel with increased strength for low temperature applications

Abstract The work established the patterns of a structural and phase transformations which occurs in the new steel of the 0,21C-1,0Mn-1,8Ni-0,3Cu-0,3Mo-0,03Ti-0,004B (%, mass) alloying system at different cooling rate. A thermokinetic diagram has been constructed. Laboratory samples of rolled products were manufactured and the mode of hardening of rolled sheets from a temperature of 860 ° C in water was tested to develop the industrial production of new steel using specialized equipment of the laboratory complex of LLC “EC Termodeform-MSTU”. The results of mechanical tests of rolled sheet samples after heat treatment showed that the steel under study in a hardened state has high strength characteristics in complex with increased impact strength at a test temperature of minus 70 °C: tensile strength σUTS = 1500 MPa, yield strength σ0.2 = 1150 MPa, relative elongation A5 = 12.5 %, hardness 429 HBW, impact strength KCV−70 = 50.3 J/cm2. This confirms its high quality and the possibility of using it for the production of heavily loaded mechanical engineering structures operated at low temperatures in the conditions of the Far North and the Arctic zone of the Russian Federation.

Corrosion and mechanical properties of bioresorbable composite based on Fe-Cu-hydroxyapatite powders

In this work, bimetallic Fe-Cu nanopowders with the addition of hydroxyapatite (HAp) were used for the fabrication of composite biodegradable material suitable for extrusion additive manufacturing processes. Varying the metal and organic binder constituents in the feedstock has led to high performance composite material. Increasing the organic binder content in the initial feedstock from 50 to 60 wt% contributes to the reduction of porosity of the obtained composites from 20.6 % to 8.9 %. The 45Fe-Cu-HAp samples were characterized by the highest mechanical properties in the tensile test: yield strength σ0.2 being 110 MPa and tensile strength σB being 150 MPa. The Young's modulus of all composites is close to that of the cortical bone tissue modulus (≈ 15 GPa). The microhardness of the composite has been shown to exceed that of pure iron more than in 2 times. Corrosion tests have demonstrated that the composite material with minimal content of the organic binder (50Fe-Cu-HAp) showed the highest corrosion rate, which makes it more attractive for its application in the fabrication of a biodegradable implant. From the findings, the presented composite material showed significant antibacterial activity (more than 99 % reduction) against S. aureus. The composites showed excellent biocompatibility against sensitive fibroblast cell line 3T3. The more than 90 % cell viability was observed after 24 hours incubated with sample.

Effect of Pinless Friction Stir Processing on Microstructure and Properties of Surface Modification Layer of 2024 Aluminum Alloy

Abstract Friction stir processing (FSP) is an advanced material surface modification technology that is both green and energy-efficient. This technology plays a crucial role in regulating the surface microstructure of alloys and improving their material surface properties. It achieves this through the synergistic effect of non-equilibrium thermodynamics and surface mechanical deformation. In this work, the surface modification of an aluminum alloy was performed using pin-less FSP. And the modified surface was then analyzed using stress-strain curves, optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical tests to investigate the impact of spindle velocity on the modified layer's properties. Results of the study show after undergoing pin-free FSP modification treatment, the surface of the alloy appears bright and flat. The modified layer displays refined grains and numerous dispersed second-phase particles. Furthermore, the grains in the modified layer exhibit a gradient distribution from the surface to the matrix. Regarding the properties, Compared to the base material (BM), the yield strength (σ0.2) and tensile strength (σb) of the alloy-modified layer were increased by 34.8% and 29.4%, respectively. The maximum elongation (δ) of the modified coating reached 22.3%. The modified layer exhibits a tough-brittle mixed fracture pattern. Additionally, the modified layer's corrosion resistance significantly improves. The performance of the modified coating shows the most significant improvement when the spindle speed reaches 1000 rpm. The study aims to explore the effect of spindle speed on the properties of alloy-modified layers produced by FSP without a stirring pin. This research could be used to create superior metal matrix composites by adding reinforcement particles.

Achieving excellent uniform tensile ductility and strength in dislocation-cell-structured high-entropy alloys

Body-centered-cubic (BCC) high-entropy alloys (HEAs) encounter significant challenges in obtaining a high uniform tensile ductility (UTD). A dense dislocation-cell (DC) structure is produced in a heterogeneously grained HEA under tensile deformation, resulting from the anchored dislocation motion by grain interior elemental segregation. This fluctuation in elemental concentration is facilitated by thermomechanical processing. The activation of multiple-slip mechanisms, prompted by strain incompatibility among grains of varying sizes, significantly propels this process forward. This novel DC structure simultaneously increased the UTD (by 349.1 %) and yield strength (σ0.2, by 29.0 %) for a stable BCC HEA. Specifically, the single-phase alloy achieved a record-high UTD of 7.5 % and an σ0.2 of > 1,200 MPa, outperforming the counterparts of all the single-phase BCC HEAs. We employed a combination of transmission electron microscopy, in-situ scanning electron microscopy tensile testing coupled with an electron backscatter diffraction technology to investigate underlying strengthening mechanisms and identified that the serious stress concentration as a result of prevalent planar slip caused premature failure and localized strain of common BCC HEAs. At the initial stage of deformation, the DC structure promoted the activation of multiple slip systems and facilitated the extension of a plastic flow across the sample volume, effectively weakening stress concentration and premature failure. The extended plasticity zone and intensified dislocation interactions contributed to the increased UTD and σ0.2. These findings offer valuable inspiration for tailoring alloy properties via microstructure strategies and promoting their adoption in advanced manufacturing.

Effect of oxygen on the microstructure, tensile properties and deformation behaviours of a biocompatible Ti40Zr25Nb25Ta10 high entropy alloy

The effect of oxygen on the microstructure, mechanical properties and deformation behaviours of as-cast biocompatible Ti40Zr25Nb25Ta10Ox (x = 0.5, 1.0 and 2.0 at.%) high entropy alloys (HEAs) was investigated. All three oxygen-doped HEAs solidified as a single body-centred cubic (BCC) phase grain structure with predominantly high-angle grain boundaries following the Mackenzie prediction. Increasing oxygen content significantly increased tensile strength at a rate of about 180 MPa/1.0 at.%, but decreased tensile ductility. However, at the addition level of 0.5 at.% O, the as-cast Ti40Zr25Nb25Ta10O0.5 HEA can achieve a yield strength (σ0.2) of 947 ± 44 MPa and an elongation at break (εf) of 9.5 % ± 1.8 %. These properties make this HEA comparable to medical grade Ti-6Al-4V (wt.%) alloy (ASTM Grade 23 titanium) (σ0.2 ≥ 759 MPa; εf ≥ 10 %) in its ability to absorb energy in plastic deformation, while offering greater resistance to permanent shape changes. Due to the possible strong interaction between oxygen atoms and dislocations through pinning and de-pinning, all oxygen-doped HEAs exhibited discontinuous yielding, whereas the low oxygen base HEA underwent normal yielding. No oxygen clusters were detected through atom probe tomography (APT) analysis. The deformation mechanism depends on oxygen content. The plastic deformation of the Ti40Zr25Nb25Ta10O0.5 HEA occurred through the formation of primary and secondary shear bands. In contrast, planar slip bands and a limited number of primary shear bands (without secondary shear bands) were observed in the Ti40Zr25Nb25Ta10O2.0 HEA. To ensure sufficient ductility, the oxygen content should be limited to 0.5 at.%. Furthermore, at this oxygen content, the corrosion resistance of the Ti40Zr25Nb25Ta10O0.5 HEA in Hank's solution is comparable to that of Ti-6Al-4V.

A strong and ductile biocompatible Ti40Zr25Nb25Ta5Mo5 high entropy alloy

A non-equiatomic high entropy alloy (HEA) Ti40Nb25Zr25Ta5Mo5 (at.%) was designed by replacing 5% Ta with 5% Mo in a precursor Ti40Nb25Zr25Ta10 HEA for improved mechanical properties and reduced density. The Ti40Nb25Zr25Ta5Mo5 HEA fabricated by copper mould casting exhibited a nearly equiaxed grain structure (grain size: 96 ± 16 μm), but within each equiaxed grain, dendritic structures are discernible in the backscattered electron imaging mode. These dendrites are enriched in Ta, Nb and Mo, while the inter-dendritic regions contain higher Zr and Ti. The as-cast HEA achieved tensile yield strength (σ0.2) of 976 ± 8 MPa and strain to fracture (εf) of 16.80 ± 1.86%, well above the minimum requirements of σ0.2 and εf for medical-grade titanium alloy Ti–6Al–4V (759 MPa and 10%). The formation of microscale and nanoscale shear bands and the presence of significant dislocations in the shear band boundary region played a key role in allowing this as-cast HEA to achieve the excellent combination of tensile strength and ductility. Furthermore, the HEA exhibited a lower wear rate, lower coefficient of friction and much higher corrosion resistance in Hank's solution (electrochemical assessments) than Ti–6Al–4V, making it a promising alternative biomedical alloy.

Insight into the role of Sc on microstructure, mechanical properties and high temperature oxidation resistance of NiCoCrAlSiSc alloys

The microstructure and mechanical properties of Ni–20Co–20Cr–2Al–1Si-xSc (x = 0, 0.1, 0.3, 0.5, 1 wt %) alloys were investigated, and the oxidation behavior of the alloys with different Sc contents at 1000°C was studied. The results indicate that NiCoCrAlSi alloy without Sc solidified into coarse columnar grains, with the addition of Sc, the grain size significantly decreased. Sc enriched at the grain boundaries of the alloy and promotes the segregation of Si and Ni towards the grain boundaries, forming the compound NiScSi. Within the range of 0∼1% (wt.%) Sc content, as the Sc content increased, there is an increase in the yield strength (σ0.2) of the alloy, a decrease in elongation, a little variation in tensile strength, and an enhancement in Vickers hardness. After cyclic oxidation at 1000°C and 120 h for alloys with different Sc contents, composite oxide films were formed on the surfaces of the alloys, but the properties and microstructure of the oxide films were significantly different. As the Sc content rised, the integrity of the oxide films was significantly improved, and the oxidation resistance of the alloys gradually enhanced at a high temperature of 1000°C.

TRANSFORMATION OF RETAINED AUSTENITE AND PORTEVIN-LE CHATELIER EFFECT IN 44CRMN2SI2MO STEEL UNDER TENSION

The 44CrMn2Si2Mo steel heat treated by quenching and partitioning demonstrates a unique combination of strength characteristics: yield strength σ0.2 = 1140 MPa, ultimate strength σВ = 1690 MPa, and elongation δ = 20.7%. Quenching and partitioning leads to the formation of a multiphase structure consisting of primary martensite, retained austenite, bainite, and secondary martensite. Primary martensite and bainite contain Fe2C transition carbides. The high ductility of the steel is due to the transformation of retained austenite into strain-induced martensite during tension, which ensures high strain hardening. Stable plastic flow is observed at low strain, when a significant fraction of retained austenite is transformed into strain-induced martensite. The plastic flow instability, which is referred to as the Portevin-Le Chatelier effect on deformation curves and plastic flow localization in deformation bands, occurs at higher strains and is associated with the transformation of film-like retained austenite. The velocity of deformation bands decreases with a decrease in the volume fraction of retained austenite. Localization of plastic flow in the neck and fracture occur when the transformation of retained austenite into strain-induced martensite cannot provide strain hardening, and deformation bands lose their mobility.

Phase evolution and mechanical properties of Ti1.5NbVAlx(x=0.25, 0.5, 0.75, 1) lightweight refractory medium-entropy alloys

A series of Ti1.5NbVAlx(x = 0.25, 0.5, 0.75 and 1) lightweight refractory medium-entropy alloys was synthesized through vacuum arc-melting, and the effects of Al addition on the phase types, microstructure and properties were investigated. Al0.25 alloy showed a single-phase BCC structure, and the increase in Al content induced a transition from disordered-BCC to ordered-B2, which caused short-range ordered strengthening. The relationship between Al content and ordered B2 was revealed by TEM. The mechanical properties of this alloy system were measured by compression and tensile tests and results indicated that Al0.25 and Al0.5 show excellent tensile strength (800, 857 MPa) even without any microstructural homogenization or optimization, meanwhile, the tensile ductility is 20 and 19 %. TEM observation demonstrates that multiple slip systems are activated in Al0.5 alloy and highly concentrated dislocation bands cross each other to form substructures, which has an important effect on the strength and elongation. Furthermore, the four alloys showed excellent compression plasticity, and the deformation was more than 50 % without fracture. When the Al content increased to 0.75, the maximum compressive yield strength σ0.2 = 1.29 GPa. This work is of great significance for the design of high specific yield strength and good ductility RH/MEAs.

Microstructure and mechanical properties of extruded and rolled Mg–Y–Zn–Ni–Co alloy

The as-cast Mg–2Y-0.5Zn-0.5Ni-0.5Co alloy (MYZNC, at%) underwent composite hot working, extrusion and rolling to further improve the mechanical properties, and the corresponding microstructure and mechanical properties were also investigated. The extruded and rolled MYZNC alloy (MYZNC-R) consisted of α-Mg, a long period stacking order (LPSO) phase comprising lamellar and fine block 18R LPSO phase as well as a thin strip 14H LPSO phase, along small amounts of Mg2Y, MgY(Co,Zn,Ni)4 and Mg3(Co,Zn,Ni,Y) particle phases. After composite processing, the dispersion and amounts of LPSO phase in the alloy was significantly enhanced, accompanied by a substantial introduction of low angle grain boundaries (LAGBs) during rolling process. The MYZNC-R alloy exhibited the high mechanical properties, with an ultimate tensile strength (σb) of 350 MPa, a yield strength (σ0.2) of 305 MPa and an elongation to failure (εf) of 10.6 %. In comparison to the extruded MYZNC alloy, both σb and σ0.2 demonstrated significant improvements by 25 % and 35.6 %, respectively. The elongation to failure only decreased by mere 1 %, indicating that the alloy had a good strength-plasticity. The high tensile strengths of MYZNC-R alloy were mainly attributed to grain refinement, LPSO phase strengthening and texture strengthening. The good ductility of MYZNC-R alloy was associated with the coordinated deformed LPSO phase and the formation of a high-volume fraction (∼76.5 %) of LAGBs.

STUDY OF LOW-TEMPERATURE THERMOMECHANICAL BEHAVIOR OF THE Ti-18Zr-15Nb SUPERELASTIC ALLOY UNDER DIFFERENT TEMPERATURE-RATE CONDITIONS

The biomedical alloy Ti-18Zr-15Nb (at.%) with shape memory was subjected to a true deformation e = 0.7 in three different modes: in the temperature range from 20°C to 600°C at deformation rate ξ = 0. 1 s-1; at temperatures 250°C and 300°C with strain rates ξ = 0.1, 1 and 10 s-1; deformation at 300°C and strain rate ξ = 0.1 s-1 after annealing at 300°C (τ = 10, 60, 300, 600 and 1200 s). It is established that with increasing temperature the conditional yield strength σ0.2 continuously decreases, at that in the temperature range of deformation temperatures 250°С-300°С the increase of maximum stress σmax is observed. In the temperature range from 200°С to 400°С fluctuations are observed on the flow curves, the amplitude of which increases with increasing temperature. The change σ0.2 and σmax, as well as the presence of oscillations on the strain diagrams are connected with the course of dynamic strain aging accompanied by the release of excess ω-phase particles at temperatures 200°С - 400°С. Increasing the strain rate at temperatures of 250°С - 300°С has a strong effect on the deformation behavior of the alloy due to a significant increase in temperature during the deformation process. Thus, increasing the strain rate up to ξ = 10 s-1 leads to a stepwise decrease in stress, starting from e ≈ 0.3, after which the plastic flow curve acquires a wave-like form with a low frequency of stress fluctuations. The main phase after all modes of thermomechanical tests is the BCC β-phase. At annealing of 300°С and an exposure of more than 300 s weak ω-phase lines are observed, and at deformation after aging significantly broadened ω-phase lines are observed only after prolonged exposure (1200 s).

Effect of Nano‐TiC Ceramic Particles on Microstructures and Properties of High‐Cr Steel under Different Heat‐Treatment Conditions

Herein, to improve the service life of die steels, many methods are used to improve their properties. However, some properties may be weakened after different heat treatments. In this study, a novel method of nanoparticle treatment is used to improve the service performance of high‐Cr steel (HCS), and the effect of nano‐TiC ceramic particles on its microstructures and properties under different heat‐treatment conditions is investigated. After forging and isothermal spheroidization, the nano‐TiC ceramic particles treatment HCS has a mixed grain with equiaxed grain and striated grain. Under different heat‐treatment conditions, the nano‐TiC reinforced HCS has refined martensite laths, carbides, and a higher content of retained austenite. In contrast, the nano‐TiC ceramic‐particles‐reinforced HCS that have higher strength and strain, the yield strength (σ 0.2), tensile strength (σ b), fracture strain, product of strength and strain, and impact toughness of HCS‐51 with nano‐TiC (HCSN‐51) are 1349 MPa (1531 MPa), 1662 MPa (1770 MPa), 9.4% (12.1%), 12 405 MPa% (18 766 MPa%), and 266 J cm−2 (445 J cm−2), which is 13.5%, 6.5%, 28.7%, 51.3%, and 67.3% higher, respectively. The primary strengthening mechanisms included fine‐grain strengthening and precipitation strengthening. The main toughness mechanism was fine‐grain and transformation‐induced plasticity.

Hydrogen’s Effect on the Shape Memory Effect of TiNi Alloy Single Crystals

Hydrogen’s effect on the shape memory effect (SME) of [1¯17]-oriented Ti49.7-Ni50.3 (at.%) alloy single crystals, with a B2–B19′ martensitic transformation (MT), was studied after being electrolytically hydrogenated at a current density of 1500 A/m2 for 3 h at room temperature under isobaric tensile deformation. It was shown that, under the used hydrogenation regime, hydrogen was in a solid solution and lowered the elastic modulus of B19′ martensite. The hydrogen in a solid solution increased (i) the yield strength σ0.1 of the initial B2 phase by 100 MPa at Md temperature, (ii) the σ0.1 of the stress-induced B2–B19′ MT by 25 MPa at Ms temperature, and (iii) the plasticity of B19′ martensite relative to the hydrogen-free crystals. At the same level of external stresses, the SME in the hydrogenated crystals was greater than that in hydrogen-free crystals. At external tensile stresses σex = 200 MPa, the SME was 4.4 ± 0.2% in the hydrogenated crystals and 1.8 ± 0.2% without hydrogen. Hydrogen initiated a two-way SME of 0.5 ± 0.2% at σex = 0 MPa, which was absent in the hydrogen-free crystals. The physical reasons leading to an increase in the SME upon hydrogenation are discussed.

Precipitated phase characteristics and fracture behaviour of cast Al–Cu–Mn alloy

The cast Al4·4Cu0·6Mn alloy samples were subjected to quenching at 530 °C for 14 h, followed by aging treatment at 170 °C for 6, 8, 10, and 12 h. The effects of aging treatment time on the mechanical properties including the tensile strength (σb), yield strength (σ0.2), elongation (δ5), hardness (HB), and microstructure morphology of the alloy were investigated using metallographic microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, tensile-property, and hardness tests. The highest tensile strength of the alloy was 481.4 MPa when the aging time was 8 h. The size of precipitates increased gradually with the increase in aging time, and their morphology developed gradually from granular and bundle to reticulation. The main strengthening phases of the alloy were the θ- and T-phases as determined by X-ray diffraction and energy dispersive spectroscopy. Dimples and tear ridges were observed in the fracture morphology of four samples after different aging times, indicating that the fracture mode for the studied alloy is quasi-cleavage fracture. The T-phase has better thermal stability. Therefore, the mechanical properties of the alloy under the heat treatment conditions in this paper were stable and were not significantly affected by the aging time.

Realizing good combinations of strength-ductility and corrosion resistance in a Co-free Fe4Ni4Mn2CrTi high-entropy alloy via tailoring Ni/Ti-rich precipitates and heterogeneous structure

The development of alloys with good combinations of strength-ductility and corrosion resistance is a long-standing research theme for advanced materials engineering, which also holds true for the newly emerged high-entropy alloys (HEAs). Here, Ni/Ti-rich precipitates and heterogeneous structure were introduced for the primary purpose of improving strength-plasticity synergy of a Co-free Fe4Ni4Mn2CrTi HEA with good anti-corrosion performance. Specifically, four typical states of this HEA were tailored and compared, including i) as-cast, ii) homogenized, iii) post deformation annealing (PDA) sample with heterogeneous structure and iv) PDA sample with homogeneous structure. The structural features, mechanical properties, and underlying phase transformation mechanisms were systematically investigated. Phase transformation of η-D024 to γ′-L12 was realized during annealing treatments, based on the intrinsic stacking faults formed by repeating removal of the (0001) planes in the η structure. Compared with the homogenized counterpart, the strength-ductility synergy was achieved in the alloy iv, showing an evident increment of ∼43.4% in yield strength (σ0.2) and ∼57.8% in ultimate tensile strength (σUTS), respectively, resulting from solid-solution strengthening, grain-boundary strengthening and precipitation strengthening, yet without sacrificing ductility. Good combinations of strength and ductility were also achieved in the alloy iii with heterogeneous structure, possessing an even higher σ0.2 and σUTS yet maintaining a moderate elongation, whose strength contribution from hetero-deformation induced hardening is recognized as a significant strengthening mechanism. Additionally, in comparison with the conventional corrosion-resistant 304 stainless steel, compact TiO2 and Cr2O3 in the passive film give rise to more superior anti-corrosion properties of the homogenized HEA. This paper provides a new paradigm in the controllable design of novel low-cost high-performance HEAs for achieving their potential structural and functional engineering applications.

Enhanced strength and crack resistance in CoCrNi-based medium entropy alloy with nano-precipitates, 9R structures and nanotwins produced by hot isostatic pressing

In the present study, hot isostatic pressing (HIP) subjected to high intermediate cooling rate is exploited to seal keyholes and tune the microstructure of equiatomic CoCrNi and (CoCrNi)94Al3Ti3 medium entropy alloys (MEAs) prepared by spark plasma sintering (SPS). For equiatomic CoCrNi, fracture toughness (KIC) is significantly increased but the yield strength (σ0.2) is almost constant when the relative density is increased from 98.9% to 99.7% by HIP. By contrast, σ0.2 and KIC of (CoCrNi)94Al3Ti3 are considerably advanced by 27.3% and 38.6%, respectively. Meanwhile, the change of grain size introduced by HIP treatment is very inconspicuous in present study. Obvious plastic deformation traces are observed near the crack in the samples subjected to HIP treatment. Systematic microstructure observations indicate that the nano-precipitates and the concurrent of the long split 9R structure and nanotwins in (CoCrNi)94Al3Ti3 formed during HIP treatment are responsible for the enhancement of σ0.2. The formation mechanisms of 9R phase in terms of the hierarchical architecture twins and dislocations activity are explored. Our findings offer new insights on design and processing of MEAs parts by the powder metallurgy.

Understanding the yield strength difference in dual-phase eutectic high-entropy alloys

Face-centered cubic (FCC)/ordered body-centered cubic (B2)-structured Al–Co–Cr–Fe–Ni system eutectic high-entropy alloys (EHEAs) have a favorable combination of high strength and ductility in the as-cast state. However, the mechanical properties of this type of alloy vary significantly with composition. In this study, we attempted to determine the yield strength differences in FCC/B2 EHEAs. Based on reported Al0.8CrFeNi2.2, we successfully prepared three low-cost Co-free EHEAs: Al0.82Cr0.8Fe1.2Ni2.18, Al0.83Cr0.6Fe1.4Ni2.17, and Al0.85Cr0.4Fe1.6Ni2.15. All the EHEAs are shown to exhibit the same phase constituent (FCC/B2) and complete eutectic microstructure, with similar lamellar thickness but different tensile properties. We constructed a theoretical calculation model based on the Hall-Petch regime which fits the measured yield strength σ0.2. The yield differences for current FCC/B2 EHEAs results mainly from different intrinsic strengths of their soft FCC phases.

Low-Temperature Superplasticity and High Strength in the Al 2024 Alloy with Ultrafine Grains.

This study aims to achieve an ultrafine-grained (UFG) Al 2024 alloy superplasticity at temperatures lower than the traditional ones for commercial Al alloys (400-500 °C). The UFG structure with a mean grain size of 100 nm produced in the alloy by high-pressure torsion at room temperature provided a very high strength-microhardness (HV0.1) of 286 ± 4, offset yield strength (σ0.2) of 828 ± 9 MPa, and ultimate tensile strength (σUTS) of 871 ± 6 MPa at elongation to failure (δ) of 7 ± 0.2%. Complex tensile tests were performed at temperatures from 190 to 270 °C and strain rates from 10-2 to 5 × 10-5 s-1, and the values of flow stress, total elongation and strain rate-sensitivity coefficient were determined. The UFG alloy was shown to exhibit superplastic behavior at test temperatures of 240 and 270 °C. For the first time, 400% elongation was achieved in the alloy at an unusually low temperature of 270 °C (0.56 Tm) and strain rate of 10-3 s-1. The UFG 2024 alloy after superplastic deformation was found to have higher strength (150-160 HV) than that after the standard strengthening heat treatment T6.

Preparation and Characterization of In Situ (TiC-TiB2)/Al-Cu-Mg-Si Composites with High Strength and Wear Resistance.

This study involved the preparation and characterization of in situ (TiC-TiB2)/Al-4.7Cu-0.32Mg-0.44Si composites with excellent mechanical and abrasive wear properties. The composites were synthesized in an Al-Ti-B4C system by combining combustion reaction synthesis with hot-pressed sintering and hot extrusion. The in situ TiB2 and TiC particles were of multi-scaled sizes ranging from 20 nm to 1.3 μm. The TiB2 and TiC particles effectively increased the yield strength (σ0.2), ultimate tensile strength (σUTS), hardness (HV), and abrasive wear resistance of the composites. The 40 wt.% (TiC-TiB2)/Al-4.7Cu-0.32Mg-0.44Si composite exhibited the highest σ0.2 (569 MPa), σUTS (704 MPa) and hardness (286 HV), which were 74%, 51% and 110% higher than those of the matrix alloy, respectively. Compared with the matrix alloy, the abrasive wear resistance of the 40 wt.% (TiC-TiB2)/Al-4.7Cu-0.32Mg-0.44Si composite was increased by 4.17 times under an applied load of 5 N and Al2O3 abrasive particle size of 13 µm. Micro-ploughing and micro-cutting were the main abrasive wear mechanisms for the Al-Cu-Mg-Si alloy and the composites.

Fabrication of Al-YOF/TiB2 composites by molten-salt-assisted stirring casting

The poor wettability between oxides and Al melt and presence of alumina film on Al melt surface seriously limits the fabrication of Al matrix composites (AMCs) reinforced by oxides via casting method. The wettability of oxides can be improved by modification with fluorine, and the oxyfluoride is more feasible to incorporate into Al melt, but the strength of the oxides may decrease after modification. Therefore, it is necessary to assess the strengthening effect of oxyfluoride by comparison with classical reinforcement. In this work, AMCs reinforced with yttrium oxyfluoride (YOF) and TiB2 were successfully fabricated by molten-salt-assisted stirring casting. YOF particles exhibit a better distribution in Al matrix than TiB2 particles, and the YOF particles show better bonding strength with Al matrix than TiB2 due to reactive wetting. Al-YOF composite shows higher hardness, tensile strength, and ductility than Al-TiB2 composite. The values of microhardness, yield strength (σ0.2) and ultimate tensile strength (σb) of Al-YOF composite are 27.2 HV, 55 MPa and 81 MPa, which are 25.9 %, 62 % and 53 % higher than that of pure Al.