Hard Phase Research Articles

Integrated manufacturing of powder metallurgy preparation-thixoforming for Al0.8Co0.5Cr1.5CuFeNi HEA: Excellent formability and enhancement by in-situ generated Al2O3 in liquid phase

Thixoforming displays wild application prospects in manufacturing Cu containing high entropy alloy (HEA) product. In current work, we proposed a novel manufacturing strategy on powder metallurgy preparation-thixoforming for Cu containing HEAs, integrating material preparation, shaping and strengthening objectives. During thixoforming, oxide in-situ generation in liquid phase was utilized to reinforce soft phase, achieving the strengthening of component. Based on this strategy, manufacturing of disc-shaped component for a model Al0.8Co0.5Cr1.5CuFeNi HEA was studied. Compared to 1175 °C thixoforming, room-temperature strength experiences a breakthrough increase at 1200 °C and 1225 °C, accompanied by an increase in yield strength of 323.8 MPa and 314.8 MPa, an increase in ultimate strength of 374.2 MPa and 470.0 MPa, meanwhile, fracture strain improves, which is caused by a considerable amount of in-suit generated Al2O3. Homogeneous and almost defect free microstructure obtained under higher temperature conditions ensures uniformity of mechanical properties for the whole component. As thixoforming temperature increases, preferred orientation of grain fiber shape becomes random, plastic deformation of solid grain (PDS) turns to flow of liquid incorporating solid particles (FLS), sliding between solid particles (SS) during material flow filling, which effectively reduces material damage risk caused by strain localization. In-situ generated Al2O3 in liquid phase helps to hinder crack propagation and blunt crack tip in Cu rich soft phase. Al2O3 only strengthens soft phase, relieving mismatch of strength-plasticity properties between soft and hard phase, accordingly, failure of soft phase is delayed and simultaneous improvement of strength and plasticity is achieved.

High strain rate tensile deformation of similar and dissimilar AA6082 and AA7075 friction-stir-welded blanks

Friction-stir-welding process has become an established solid-state technique for joining of dissimilar lightweight materials over the past decade by overcoming fundamental welding challenges such as solidification cracking, phase segregation and surface oxidation. Despite these unique capabilities, the resulting microstructure feature in the weld zone consisting of fine grains with a high dislocation density, challenges further heat treatment and forming processes. Hence, a high solution annealing temperature results in degradation of the adjusted microstructure and in a lower to reduced mechanical properties in case of dissimilar joints of precipitation-hardenable aluminum alloys. Subsequent hot forming therefore involves a great effort and requires further heat treatment steps. Thus, the present study investigates the effect of a recently introduced novel thermo-mechanical forming process on local deformation behavior of similar as well as dissimilar joints processed by friction-stir-welding technique of thin-walled AA6082 and AA7075 blanks. To avoid the complete elimination of the adjusted microstructure, the welding process is performed after a thermo-mechanical process consisting of solution annealing, die cooling and peak aging. Uniaxial tensile tests are then carried out at high strain rates ranging from ε˙ = 40 s-1 to ε˙ = 400 s-1. The results show an increase in yield and ultimate tensile strength as well as in total elongation after failure with increasing strain rates. As the strain rate increases, the flow stress of similar weld of AA7075 is higher than those of AA6082 and the dissimilar weld of AA6082 and AA7075. Local deformation measurements reveal higher strain localization in the welded zone for similar welds, which leads to micro-crack initiation and failure due to strain accumulation. Microstructure analysis shows very fine equiaxed grain structure in the nugget zone and homogenous distribution of precipitates after friction stir welding process, which explain the plastic deformation behavior.

Multimodal assessment of mechanically induced transformation and damage in TRIP steels using X-ray nanotomography and pencil-beam diffraction tomography

A comprehensive multimodal measurement environment integrating X-ray computed tomography (CT) and special X-ray diffraction (XRD) using a collimated beam has been developed and was applied for the first time to investigate martensitic transformation and damage behaviour in transformation-induced plasticity (TRIP) steels. The transformation and damage behaviour of numerous γ grains were analysed individually to understand the behaviours and to identify the factors controlling these processes.A transition was observed from a stress-assisted transformation in the early loading phase, controlled by mechanical driving forces, to a strain-induced transformation controlled by dislocation density. A characteristic reversal in the transformation rate was observed whereby γ grains with a crystallographic orientation that initially undergo rapid transformation due to high mechanical driving force underwent a sudden deceleration of transformation. Conversely, γ grains with an orientation that initially underwent gradual transformation rapidly completed transformation. The preceding hard phase inhibited the transformation of neighbouring γ grains through the triple effects of reducing stress triaxiality, relaxing stresses and constraining plastic deformation.The growth of pre-existing voids was modest, and material damage was dominated by voids that were newly formed during deformation from α′ grains. The initiation and progress of damage were influenced by the combined effects of the γ grain morphology and their interaction with surrounding α′ grains. The complexity of the local morphology governed the martensitic transformation and damage behaviour, and geometrical parameters were identified as highly sensitive indicators for describing this phenomenon. Finally, industrial measures to design damage-resistant TRIP steels based on these research findings are discussed.

Improvement of High Temperature Wear Resistance of Laser-Cladding High-Entropy Alloy Coatings: A Review

As a novel type of metal material emerging in recent years, high-entropy alloy boasts properties such as a simplified microstructure, high strength, high hardness and wear resistance. High-entropy alloys can use laser cladding to produce coatings that exhibit excellent metallurgical bonding with the substrate, thereby significantly improvement of the wear resistance of the material surface. In this paper, the research progress on improving the high-temperature wear resistance of high entropy alloy coatings (LC-HEACs) was mainly analyzed based on the effect of some added alloying elements and the presence of hard ceramic phases. Building on this foundation, the study primarily examines the impact of adding elements such as aluminum, titanium, copper, silicon, and molybdenum, along with hard ceramic particles like TiC, WC, and NbC, on the phase structure of coatings, high-temperature mechanisms, and the synergistic interactions between these elements. Additionally, it explores the potential of promising lubricating particles and introduces an innovative, highly efficient additive manufacturing technology known as extreme high-speed laser metal deposition (EHLMD). Finally, this paper summarizes the main difficulties involved in increasing the high-temperature wear resistance of LC-HEACs and some problems worthy of attention in the future development.

Microstructural evolution and mechanical properties of FeNiVAlx medium-entropy alloy

FeNiV alloys exhibit potential applications in magnetic, radiation-resistant, mechanical, and superconducting fields, however, the formation of the hard yet brittle sigma phase enriched with Fe and V hindered their application in extreme environments. Therefore, improving the mechanical properties of FeNiV alloys has been a crucial problem. In this study, a series of as-cast FeNiVAlx (values of x is 0, 0.1, 0.2, 0.3) medium-entropy alloys (MEAs) were fabricated by vacuum arc melting, and the evolution of microstructure and mechanical properties with different Al microalloying were investigated systematically. As the Al content increased, FeNiVAlx MEAs underwent a structural transformation from a dual-phase structure consisting of a FCC phase (46.4 %; phase content) and a sigma phase (53.6 %) at x=0 to a single BCC phase in FeNiVAl0.3, while the mechanical properties of the alloy show a decrease in strength and a significant increase in elongation. Notably, the FeNiV alloy exhibits the highest yield strength of 1851 MPa with 7.8 % ductility. In contrast, the FeNiVAl0.2 alloy demonstrated superior comprehensive compressive properties, with a high yield strength of 1255 MPa and an exceptional ductility exceeding 50 % without fracturing. The elimination of the harmful sigma phase, coupled with the formation of BCC phase, contributes to the observed decrement in strength and increment in ductility. This study elucidates the intrinsic relationship between microstructure and mechanical properties with Al contents in FeNiVAlx alloys, providing insights for the design of MEAs with excellent strength-ductility balance through microalloying.

Effect of laser remelting treatment on the tribological properties of plasma sprayed WC–Cr3C2–Ni reinforced NiCrBSi coatings

The incorporation of single hard phases, such as tungsten carbide (WC), and chromium carbide (Cr3C2), into NiCrBSi coatings effectively enhances their performance, thereby fulfilling the tough requirements of high-temperature friction condition. To investigate the collaborative reinforcing effect of multiple hard phases and further enhance the high-temperature wear resistance of the coating, a novel WC–Cr3C2–NiCrBSi binary hard phase reinforced coating was developed and subjected to laser remelting treatment. The as-sprayed coatings demonstrate a distinct lamellar structure that diminishes upon remelting, resulting in significant densification of the coatings and a reduction in porosity to 0.3 %. The microhardness of the remelting coatings is reduced by about 10 % due to dilution of the coating by the lower hardness of the base material. However, during the laser remelting process, WC and Cr3C2 decompose and dissolve into the alloying binder phase of the coating, serving as solid solution agents. In cases where the concentration of W elements is elevated, a considerable precipitation of W-rich carbide is observed within the coating. Notably, N30 coatings demonstrate remarkable were resistance at both ambient and 700 °C, resulting in a significant reduction in wear volume by 86.5 % and 55.2 % respectively in comparison to original NiCrBSi coatings. The remelting treatment enhances the densification of the coating, improves the homogeneity of the microstructure, and reduces the coefficient of friction of the coating. The laser-remelted coating exhibits a decreased wear volume, attributed to a transition in its wear mechanism from fatigue-dominated to abrasive wear at room temperature. Conversely, under frictional conditions at 700 °C, the remelted coating experiences an augmented wear volume, attributed to severe abrasive wear.

Exploring the effect of exchange coupling in (SrFe9.8Al2La0.2O19)1-x/(Co0.7Zn0.3Fe2O4)x nanocomposites synthesised by sol–gel auto combustion method

Extensive research has been conducted over the last decade to develop permanent magnets utilizing less rare earth materials. Exchange-coupled nanocomposite magnets are a fascinating development in the field of permanent magnets. They involve the combination of hard and soft ferrites, resulting in a unique and evolving magnet. Here we present an in-depth structural, morphological and magnetic characterization of ferrite-based nanostructures obtained through a bottom-up sol−gel approach. The combination of high coercivity of a hard phase SrFe9.8Al2La0.2O19 (SFALO) and high saturation magnetization of a soft phase, Co0.7Zn0.3Fe2O4 (CZFO), allowed us to develop exchange-coupled bi-magnetic nanocomposites by varying the mass percentage (x = 0, 0.1, 0.2, 0.3, 0.4, 1). Thermogravimetric analysis (TGA) up to 1200 °C was used to investigate the material’s thermal properties with the phase formation temperature. Detailed atomic/nano-scale structural characterizations of these nanocomposites were performed by X-ray diffraction and Transmission Electron Microscopy. The average particle size was calculated from the SEM analysis and it was observed that most of the composites with ratio x = 0.1, 0.3 & 0.4 was 145 nm and x = 0.2 was 165 nm. The Switching Field Distribution (SFD) curve revealed the exchange coupling between soft and hard magnetic particles. The findings indicate that the inclusion of a soft magnetic phase has a considerable impact on the exchange coupling between the hard and soft ferrite phases. As the spinel concentration increased, in the composite the saturation magnetization increased from 18 emu/g to 48 emu/g. From the stroner-wholfrathmodel, the decrease in the remanence ratio below 0.5 concludes the crystallites are randomly oriented. The nanocomposite with the ratio x = 0.1 was observed with the high magneto crystalline anisotropy of 2183.23 × 102 (J/m3) and a high energy product (BH)max of 5.5 kJ/m3. The aforementioned features of nanocomposites demonstrate their potential for use in permanent magnet applications.

Multimodal deep learning framework to predict strain localization of Mg/LPSO two-phase alloys

This study proposes a method for predicting three-dimensional (3D) local strain distribution under compressive deformation of as-cast Mg/LPSO two-phase alloys from 3D microstructure images. The 3D local strain distribution was obtained by applying the digital volume correlation method to X-ray CT images before and after compression tests. Three microstructure descriptors were extracted from the 3D microstructure images around each strain measurement point: volume fractions of the phases, persistent diagrams that can express the connectivity of the phases, and two-phase spatial correlation that can express the spatial distribution of the phases. A deep learning model was then constructed to predict local strain from the three microstructure descriptors. Since two types of descriptors were used in this study, numerical data and image data, multimodal deep learning was employed to make predictions. Thus, the use of multiple microstructure descriptors enabled predictions to be made with higher accuracy than when predictions were made from a single descriptor. Feature importance of the descriptors was assessed through correlation analysis and occlusion sensitivity analysis. The results revealed that high strain tended to occur in the region where the hard phase, LPSO phase, had a large elongated phase oriented at a 45° direction to the loading direction. This result is consistent with other previous studies and indicates that the proposed method is effective in elucidating the relationship between the microstructure and the deformation behavior of the material.

Using pulse current to tailor hydride networks while improving the strength and plasticity of pure titanium

Interstitial hydrogen atoms in titanium usually deteriorate the mechanical properties of titanium by brittle hydride phase or hydrogen-enhanced localized plasticity. In this study, hydride was used as the second phase to improve the tensile strength and elongation of TA1 pure titanium. The continuous coarse hydride network is precipitated in TA1 pure titanium after the hydrogen-charged. Then pulse current treatment is used to decompose the original coarse hydride network and reduce the size and number of hydride strips. Finally, a small uniform hydride network is formed in TA1 pure titanium. The results of tensile experiments indicate that the tensile strength of hydrogen-charged TA1 pure titanium treated by pulse current increases from the 286.4 MPa to 316.1 MPa and the elongation increases from the 47.6 % to 56.8 %. The improvement of mechanical properties demonstrate that the small and uniform hydrides can significantly improve the mechanical properties of TA1 pure titanium. In hydrogen-charged TA1 pure titanium treated by pulse current, the increment of strength is mainly caused by hard phase hydrides, and the increase in plasticity is attributed to the role of twins in coordinating deformation during plastic deformation. This study manifests that in addition to being used as a temporary alloying element to optimize the microstructure of titanium, hydrogen can also be directly act as a second phase in titanium to improve the mechanical properties.

Achieving ultra-low coefficient of friction of FeCoNiW multi-bonded composite coatings by Co-coated WC

It is of paramount importance to enhance the surface wear resistance and reduce the impact of corrosion caused by the marine environment in order to guarantee the propeller transmission components’ exterior protection. FeCoNiW multi-bonded Co-WC composite coatings were prepared with an ultralow friction coefficient (0.17) and wear rate (7.22 ×10−7 mm3⋅N−1⋅m−1) in this research. The uniform and fine hard phase skeleton, which is cobalt-rich, ensures that the coating exhibits reliable tribological characteristics. Furthermore, this coating exhibits high hardness (1000HV0.3 at 80 wt.%Co-WC) and sufficient low current density (7.485 ×10−6 A/cm2). The primary reason for these enhanced attributes is a reduction in the density of flaws in the coatings. Consequently, Co-WC-added composite coatings have considerable potential for utilization in drive shaft components and propeller applications.

Eutectic Solidification Morphologies in Rapidly Solidified Hypereutectic Sn–Ag Solder Alloy

The effect of rapid solidification upon hypereutectic Sn–Ag solder alloy has been investigated using a 6.5 m drop tube. Powder sizes ranging from > 850 to < 38 μm were produced, with equivalent cooling rates of 250 to 14,800 K s−1 for 850 and 38 μm droplets, respectively. At all cooling rates investigated, dendritic β-Sn was observed as the primary solidification phase, not proeutectic Ag3Sn as predicted by the phase diagram. The volume fraction of interdendritic eutectic was observed to decrease with increasing cooling rate, with the Ag concentration in the residual interdendritic liquid estimated at 12.5–15 wt pct Ag, far in excess of the eutectic concentration of 3.8 wt pct. Much of the Ag3Sn observed within the eutectic had a blocky, divorced eutectic appearance. A model is proposed which can explain these observations in terms of sluggish nucleation of the Ag3Sn intermetallic, coupled with a metastable phase diagram that permits significant supersaturation of Ag at modest undercooling.

Education and vocational training in Kosovo, a challenge and objective of the 21st century

Classes in Kosovo are held in private and public educational institutions. Education in Kosovo went through very hard phases and challenges especially in the years 1990/1999 since differences and the need for change made us not entirely prepared. After the establishment of Kosovo Institutions, the Department of Education and Science (DES) was established within the Ministry of Education. The primary aim was to establish the legal and professional basis as a frontline of the reform of our education system, especially the acceptance of the Bologna processes that facilitate the radical reform of general and vocational education. This paper will address the aspects of reforming and progression of vocational secondary education, vocational schools, and correspondence to the needs of the EU market and the possibility of implementing in post-secondary and university higher education.

Magnetic exchange coupled composite behavior in the doped manganite nanoparticles: A proposed phenomenological model

In this paper, we comprehensively investigate the isothermal magnetization behavior of doped perovskite manganite nanoparticles. The focus is on understanding the impact of variation of particle sizes on the soft and hard magnetic phases with respect to the changes in the coercive field and remanent magnetization, both theoretically and experimentally. The study seeks to correlate experimental findings with the proposed phenomenological model to gain deeper insights into the underlying mechanisms governing exchange coupling and anisotropy effects in the nanocrystalline composites. The proposed phenomenological model beautifully demonstrates how the values of saturation magnetization and coercive field changes with changing the particle size in the nanocrystalline La0.48Ca0.52MnO3 (LCMO48) and La0.46Ca0.54MnO3 (LCMO46) compounds. In addition, the model provide an insights into the limitations of critical radius, size and shape of the nanocrystalline particle. This investigation looks into how the size of particles affects their magnetic properties, specifically coercive field and remanent magnetization.

Epitaxial growth of quasi-2D van der Waals ferromagnets on crystalline substrates

Intrinsic magnetism in van der Waals materials has instigated interest in exploring magnetism in the 2D limit for potential applications in spintronics and also in understanding novel control of 2D magnetism via variation of layer thickness, gate tunability and magnetoelectric effects. The chromium telluride (CrxTey) family is an interesting subsection of ferromagnetic materials with highTCvalues, also presenting diverse stoichiometry arising from self-intercalation of Cr. Apart from the layered CrTe2system, the other non-layered CrxTeycompounds also offer exceptional magnetic properties, and a novel growth technique to grow thin films of these non-layered compounds offers exciting possibilities for ultra-thin spin-based electronics and magnetic sensors. In this work, we discuss the role of crystalline substrates in chemical vapor deposition growth of non-layered 2D ferromagnets, where the crystal symmetry of the substrate as well as the misfit and strain are the key players governing the growth mechanism of ultra-thin Cr5Te8, a non-layered ferromagnet. The magnetic studies of the as-grown Cr5Te8reveal the signatures of co-existing soft and hard ferromagnetic phases, which makes this system an intriguing system to search for emergent topological phases, such as magnetic skyrmions.

Sea Cucumber-Inspired Polyurethane Demonstrating Record-Breaking Mechanical Properties in Room-Temperature Self-Healing Ionogels.

Practical applications of existing self-healing ionogels are often hindered by the trade-off between their mechanical robustness, ionic conductivity, and temperature requirements for their self-healing ability. Herein, this challenge is addressed by drawing inspiration from sea cucumber. A polyurethane containing multiple hydrogen-bond donors and acceptors is synthesized and used to fabricate room-temperature self-healing ionogels with excellent mechanical properties, high ionic conductivity, puncture resistance, and impact resistance. The hard segments of polyurethane, driven by multiple hydrogen bonds, coalesce into hard phase regions, which can efficiently dissipate energy through the reversible disruption and reformation of multiple hydrogen bonds. Consequently, the resulting ionogels exhibit record-high tensile strength and toughness compared to other room-temperature self-healing ionogels. Furthermore, the inherent reversibility of multiple hydrogen bonds within the hard phase regions allows the ionogels to spontaneously and efficiently self-heal damaged mechanical properties and ionic conductivity multiple times at room temperature. To underscore their application potential, these ionogels are employed as electrolytes in the fabrication of electrochromic devices, which exhibit excellent and stable electrochromic performance, repeatable healing ability, and satisfactory impact resistance. This study presents a novel strategy for the fabrication of ionogels with exceptional mechanical properties and room-temperature self-healing capability.

Molten Aluminum-Induced Corrosion and Wear-Resistance Properties of ZrB2-Based Cermets Improved by Sintering-Temperature Manipulation.

During the hot dip aluminum plating process, components such as sinking rollers, pulling rollers, and guide plates will come into long-term contact with high-temperature liquid aluminum and be corroded by the aluminum liquid, greatly reducing their service life. Therefore, the development of a material with excellent corrosion resistance to molten aluminum is used to prepare parts for the dipping and plating equipment and protect the equipment from erosion, which can effectively improve the production efficiency of the factory and strengthen the quality of aluminum-plated materials, which is of great significance for the growth of corporate profits. With AlFeNiCoCr as the binder phase and ZrB2 as the hard phase, ZrB2-based ceramic composites were prepared by spark plasma sintering (SPS). SEM, EDS and XRD were used to characterize the microstructure and properties of the sintered, corroded, and abraded material samples. The density, fracture toughness, corrosion rate and wear amount of the composite material were measured. The results show that ZrB2-AlFeNiCoCr ceramics have compact structure and excellent mechanical properties, and the density, hardness and fracture toughness of ZrB2-AlFeNiCoCr increase with the increase in sintering temperature. However, when the composite material is at 1600 °C, the relative density of the sintering at 1600 °C decreases due to the overflow of the bonding phase. Therefore, when the sintering temperature is 1500 °C, the high entropy alloy has the best performance. The average corrosion rate of ZrB2-1500 at 700 °C liquid aluminum is 1.225 × 10-3 mm/h, and the wear amount in the friction and wear test is 0.104 mm3.

Attaining exceptional wear resistance in an in-situ ceramic phase reinforced NbMoWTa refractory high entropy alloy composite by Spark plasma sintering

The refractory high-entropy alloys (RHEAs) exhibit great potential as structural components for aerospace equipment. However, their lack of wear resistance and increased coefficient of friction at room temperature (RT) impose limitations on their practical applications. Therefore, further enhancements are required to improve their friction and wear properties under RT. In this context, the development of NbMoWTa(h-BN)x RHEA ceramic composites in this work offers a viable solution to address this issue. Experimental results demonstrate that the addition of h-BN leads to the in-situ generation of (Nb,Ta)N/(Nb,Ta)2N and (Nb,Ta)B2 ceramic phases, significantly enhancing the hardness and wear resistance of the composites. The wear rate of NbMoWTa(h-BN)0.5 reaching as low as 1.32 × 10−8 mm3/Nm, which is four orders of magnitude lower than that of the RHEA. The NbMoWTa RHEA exhibits significant adhesive wear, which can be effectively mitigated in composites through the uniform dispersion of ceramic phase particles with lower mean free path. The abrasive particles primarily interact with the hard strengthening phase, effectively inhibiting plastic deformation in their vicinity. Consequently, the reduced mean free path between the ceramic phases limits the likelihood of metal matrix removal. Subsequently, aided by the presence of ceramic phases, the spontaneous formation of protective third bodies further inhibit surface material removal and ultimately ensures exceptional wear resistance.

Influence of Ni content on electrochemical corrosion and tribological behavior of Cu-10Sn coatings by laser cladding

In this work, Cu-10Sn-xNi (x = 0, 3, 6, 9, and 12 wt%) alloy coatings were prepared using laser cladding technology. The influence of Ni content on the phase composition, microstructure, crystallographic characteristics, microhardness, electrochemical corrosion, dry sliding wear and corrosive wear behavior of the coatings were studied. The results indicated that the coatings formed good metallurgical bond with the substrate. The coatings were composed primarily of the α-Cu matrix and small γ precipitates. The Ni element effectively reduced component segregation, and with the increased of Ni content, the grain size in the coating was significantly refined, resulting in fine equiaxed grains. Under the effect of fine grain strengthening, the Cu-10Sn-12Ni (C12NS) coating achieved a higher microhardness of approximately 222.9 ± 5.3 HV0.2. In the electrochemical corrosion test, the increased in the number of grain boundaries significantly reduced the corrosion resistance of the Cu-10Sn-xNi coatings. The Cu-10Sn-6Ni (C6NS) coating exhibited excellent corrosion resistance, with an Icorr of only 1.37 μA/cm2. The results of the dry sliding wear test showed that under the influence of the hardness gradient between the hard precipitate phase and the soft matrix of the coating, the C6NS coating achieved satisfactory wear resistance with a specific wear rate of 3.30 × 104 μm3/Nm. Meanwhile, due to the good corrosive resistance, the C6NS coating showed the best tribological performance in 3.5 wt% NaCl environment, and the specific wear rate was 1.58 × 104 μm3/Nm.

Microstructure evolution and tribological properties over a wide temperature range of Ni-based composite coatings with Ti3AlC2 addition

Ni60A alloy powders doped with varying concentrations (0, 10 wt% and 20 wt%) of Ti3AlC2 MAX phase were coated on S31008 NiCr stainless steel substrate using atmosphere plasma spraying technique in this work. Comparative investigation was conducted to examine the effects of Ti3AlC2 additions on the microstructure evolution, microhardness, and wide temperature range (room temperature to 800 oC) tribological performance of the composite coatings. The findings demonstrated that the composite coatings were composed of γ-Ni, intermetallic hard metal borides, Ti3AlC2 and TiC phases, without any noticeable oxide phases. The 20 wt% Ti3AlC2 additions to the Ni60A alloy coating exhibited minimum imperfections in microstructure with a porosity drop of 65.8 %, and the highest microhardness which was enhanced by 64 % to 924 HV0.515. After introducing of Ti3AlC2, the coefficient of friction decreased and appeared to distinguish itself by remaining minimal and stable at low-medium temperatures compared with an increase of more than 70 % at high temperatures. Ni60A-10 wt% Ti3AlC2 coating resulted in the lowest friction coefficient of approximately 0.45 from room temperature to 400 oC and it peaked to 0.78 at 800 oC, reducing by 19.6 % and 6 % compared to non-composite coating, respectively. The wear resistance of the composite coating was considerably improved with the increment of Ti3AlC2, and the coating with 20 wt% Ti3AlC2 showed the most significant reduction in wear rate: 18.5 % at room temperature, 45.2 % at 200 oC, 37.2 % at 400 oC, 60 % at 600 oC, 90.8 % at 800 oC. The lubricate layered Ti3AlC2 phases and the tribo-oxide film predominate composed of derivative NiO, Cr2O3 Al2O3 and TiO2 played a critical role in antifriction and wear resistance. The wear mechanisms were accordingly conducted as abrasion and adhesion wear at low-medium temperature and transferred into tribo-oxidation wear at elevated temperature.

A novel method for reducing the brazing temperature of C/C composite with TiZrHfTa/Ni composite interlayers

C/C composite was successfully brazed with TiZrHfTa/Ni composite interlayers at lower temperature far below the melting point of TiZrHfTa refractory high entropy alloy. The influence of brazing parameters on the joint morphology, indentation fraction toughness of the reaction layer, shear strength at room temperature and at 1000 °C, and fracture behavior was investigated. The results show that all of the C/C composite joints contained two main phases: an equimolar (Ti-Zr-Hf-Ta)C hard phase and a near-pure Ni binder phase. The maximum indentation fracture toughness of the obtained joint reaction layer material was 15.52 ± 0.64 MPa·m1/2. This was proved to be beneficial to improve the strength and toughness of the joint. Meanwhile, the maximum shear strengths of C/C-TiZrHfTa/Ni-C/C joint at room temperature and at 1000 °C reached 33.24 ± 1.85 MPa and 21.70 ± 2.12 MPa, respectively. Abundant slip lines, in-situ dimples, and tearing ridges resulting from Ni plastic deformation suggest a predominantly ductile fracture mode in the joint. This work introduces an innovative method, which can not only ensure the service of C/C composite in high temperature environment, but also effectively reduce the joining temperature.