Microstructural characteristics and mechanical behavior of B4Cp/6061Al composites synthesized at different hot-pressing temperatures

B₄Cp/6061Al composites have become important structural and functional materials and can be fabricated by powder metallurgy and subsequent hot rolling. In this work, the effects of the hot-pressing temperature on microstructures and mechanical behaviors of the B₄Cp/6061Al composites were investigated. The results showed that compared with the T4 heat treated B₄Cp/6061Al composite hot pressed at 560 °C, the yield strength and failure strain of the composites hot pressed at 580 °C were increased to 235 MPa and 18.4%, respectively. This was associated with the interface bonding strength between the B₄C particles and the matrix. However, the reaction products, identified to be MgAl₂O₄ phases, were detected in the composites hot pressed at 600 °C. The formation of the MgAl₂O₄ phases resulted in the Mg depletion, thus reducing the yield strength to 203.5 MPa after the T4 heat treatment due to the effect of the solid solution strengthening being weakened. In addition, the variation of hardness and electrical conductivity was mainly related to the Mg content in the matrix. Based on the as-rolled microstructures observed by SEM, SR-μCT and fracture surfaces, the deformation schematic diagram was depicted to reflect the tensile deformation process of the composites.     

Fatigue Crack Growth and Fracture of 30 wt% B4C/6061Al Composites

This paper focuses on studying the fatigue crack growth (FCG) characteristics and fracture behaviours of 30 wt% B₄C/6061Al composites fabricated by using powder metallurgy and hot extrusion method. Compact tension (CT) specimens having incisions parallel to the extrusion direction (T‐D) and perpendicular to the extrusion direction (E‐D) were investigated through FCG tests. Results show that, at low/medium stress‐intensity factor range levels (ΔK ≤ 9), crack propagation rate in E‐D specimens is lower than that in T‐D specimens because the elongated B₄C particles parallel to the extrusion direction in E‐D specimens can deflect the crack. The scanning electron microscope micrographs of the fractured surface illustrate that crack mainly propagates in the matrix alloy at the initial stage of its propagation and propagates more remarkably near the particle‐matrix interface with the increase of ΔK value. B₄C particles are also found to be easy to fracture during the rapid crack propagation. Based on fracture analyses, considering the impacts of factors like crack deviation, plastic zone size at the crack tip, and crack driving force, a 2‐D crack propagation model was developed to study the fatigue crack propagation mechanism in the 30 wt% B₄C/6061Al composite.     

Strategical parametric investigation on manufacturing of Al–Mg–Zn–Cu alloy surface composites using FSP

Friction stir processing (FSP) is a unique approach being presently researched for composite fabrication. In the present investigation, Al-B₄C surface composite was fabricated through FSP by incorporating B₄C powder particles into Al–Mg–Zn–Cu alloy (AA 7075) matrix. The influence of varying powder particle reinforcement strategies on the microstructure, powder distribution, microhardness, and wear resistance of the surface composite is reported. In addition, AA 6061/B₄C composites were prepared using the same parameter set and the powder distribution in the composite was compared to that in the AA 7075/B₄C composite. More homogeneous dispersion of B₄C powder was observed in AA 6061 as compared to AA 7075 substrate. Among the prepared AA 7075/B₄C composites, the best B₄C powder distribution was detected in samples processed using fine powder and incorporating the change in stirring direction between passes. The hardness and wear resistance of the prepared composites were almost doubled attributing to several strengthening mechanisms and B₄C powder distribution in the AA 7075 matrix.     

高能球磨工艺对B4C/Al复合粉体结构演变及分布均匀性的影响

制备B4C增强Al基复合材料存在的难点主要是B4C颗粒在Al基体中的均匀分布及界面结合。本研究采用卧式搅拌高能球磨法制备了B4C/Al复合粉体,研究了搅拌轴转速和球磨时间对B4C/Al复合粉体结构演变及分布均匀性的影响。结果表明,随搅拌轴转速的提高,复合粉体受磨球碰撞时所获能量增大,增强体颗粒瞬间被破碎同时使Al粉发生较大的塑性变形,随球磨时间的延长,破碎的B4C颗粒逐渐在Al基体中分散均匀并与基体焊合,利于粉体实现均匀分布和良好的界面结合。球磨过程中B4C沿颗粒棱边脆性断裂,在Al粉的冷焊变形过程中被嵌入,形成一种片状化的Al粉基体包裹B4C增强相的复合粉体。在搅拌轴转速为600/800r/min(交变转速,交变频率为1min),球磨时间为2h时,B4C/Al复合粉体的粒度得到细化,B4C颗粒在Al基体中分布均匀、界面结合紧密。     

Thermally stable microstructures and mechanical properties of B4C-Al composite with in-situ formed Mg(Al)B2

B₄C particulate-reinforced 6061Al composite was fabricated by powder metallurgy method. The as-rolled composite possesses high tensile strength which is comparable to that of the peak-aged 6061Al alloy. More importantly, the microstructures and mechanical properties are thermally stable during long-term holding at elevated temperature (400 °C). The microstructual contributions to the strength of the composite were discussed. Transmission electron microscopy (TEM) analysis indicates that the in-situ formed reinforcement Mg(Al)B₂, as products of the interfacial reactions between B₄C and the aluminum matrix, show not only good resistance to thermal coarsening but also strong pinning effect to the grain boundaries in the alloy matrix.     

The dynamic properties of B4C/6061Al neutron absorber composites fabricated by power metallurgy

ABSTRACT

The dynamic compression properties of B₄C/6061Al neutron absorber composites (NACs) with three B₄C volume fractions (20–40%), fabricated by power metallurgy, were studied. Compression tests were conducted at strain rates ranging from 760 to 1150?s^?1, using a split Hopkinson pressure bar. The damage mechanism was studied through microstructural analysis. Results show that the B₄C particles exhibited a uniform distribution in the 6061Al matrix. The NACs dynamic strength was found to improve with increasing quantities of B₄C particles, and with strain rate. The damage mechanisms include particle fracture and interface debonding. Dislocation pile-up was observed at grain boundaries and at the interface between particles and the matrix. A constitutive model under dynamic compression was developed based on the Johnson–Cook model.

This paper is part of a thematic issue on Nuclear Materials.     

Different reinforcement strategies of hybrid surface composite AA6061/(B4C+MoS2) produced by friction stir processing

Aluminum surface composites have gained huge importance in material processing due to their noble tribological characteristics. The reinforcement of solid lubricant particles with hard ceramics further enriches the tribological characteristics of surface composites. In the current study, friction stir processing was chosen to synthesize hybrid surface composites of aluminum containing B₄C and MoS₂ particles with anticipated improved tribological behavior. B₄C and MoS₂ powder particles in 87.5: 12.5 ratio were reinforced into the AA6061 by hole and groove method. Microstructural observations indicated that reinforcement particles are well distributed in the matrix. The hardness and wear resistance of hybrid surface composites improved as compared to the base material, due to well distributed abrasive B₄C and solid lubricant MoS₂ particles in AA6061. The hybrid surface composites achieved ∼32 % increased average hardness as compared to the base material. Hole method revealed ∼13 % better wear resistance compared to the groove method for friction stir processed hybrid surface composite, attributing to an improved homogeneity of particle distribution shown by zigzag hole pattern. Moreover, friction stir processed AA6061 without reinforcement particles exhibited reduced hardness and wear resistance due to loss of strengthening precipitates during multi-pass friction stir processing.     

The microstructure and mechanical properties of the B4C/Cu matrix composite fabricated by SPS-HR

Copper matrix composites containing different volume fractions of B₄C particles (0–15%) were first fabricated by spark plasma sintering followed by hot rolling in atmospheric environments, then their microstructures, phase compositions, mechanical properties and sintering mechanism were investigated. It was found that B₄C particles distributed relatively homogeneously in the copper matrix. Reaction products of CuC₈ and B were observed and identified in the composite. Under increasing B₄C particle content, the ultimate tensile, yield strength and elongation to fracture of the composites decreased. Failure mode of composites included: (1) the interfacial debonding and (2) the cleavage fracture of copper. Moreover, micro-discharge between the adjacent particles occurred, and its led to local high temperature at the interface.     

Production and characterization of AA6061–B4C stir cast composite

This work focuses on the fabrication of aluminum (6061-T6) matrix composites (AMCs) reinforced with various weight percentage of B₄C particulates by modified stir casting route. The wettability of B₄C particles in the matrix has been improved by adding K₂TiF₆ flux into the melt. The microstructure and mechanical properties of the fabricated AMCs are analyzed. The optical microstructure and scanning electron microscope (SEM) images reveal the homogeneous dispersion of B₄C particles in the matrix. The reinforcement dispersion has also been identified with X-ray diffraction (XRD). The mechanical properties like hardness and tensile strength have improved with the increase in weight percentage of B₄C particulates in the aluminum matrix.     

Development of light composite materials with low coefficients of thermal expansion

Abstract

Composite materials based on aluminium are used in different fields where weight, thermal expansion, and thermal stability are key requirements. The aim of the present study was to develop a universal method and scientific approach for evaluating the design of lightweight, Al matrix composites with low coefficients of thermal expansion (CTE) and high dimensional stability, and to produce such composites using the vacuum plasma spray (VPS)process. The methodology is general and could be applied to other composite systems. The VPS-produced Al and Al alloy 6061 based composites were reinforced with a variety of ceramic particles including Si₃N₄, B₄C, TiB₂, and 3Al₂O₃.2SiO₂. These composites have low CTE values ((12–13)×10^-6 K^-1), similar to that of steel, and high dimensional stability (capable of keeping dimensions stable with changes in temperature). They have low porosity (98–99%dense) and a uniform distribution of the strengthening particles. Hot rolling of the VPS-formed composites, followed by heat treatment, resulted in a significant improvement in the mechanical properties. Deformed and heat treated 6061 based composites, containing 20 wt-%TiB₂ and 40 wt-%3Al₂O₃?2SiO₂, showed excellent mechanical properties (ultimate tensile strength 210–250 MPa, elongation >4%).     

Microstructure and property characterization of Al-based composites reinforced with CuZrAl particles fabricated by mechanical alloying and spark plasma sintering

In the present work, CuZrAl metallic glass particles were synthesized by mechanical alloying method. High relative density Al-based composites (ABCs) reinforced with different volume fraction of CuZrAl particles have been fabricated by spark plasma sintering (SPS) technique. The microstructures, mechanical properties and corrosion resistance in seawater solution of the ABCs were investigated. The sintered products are all composed of fcc-Al, Al₃Zr and CuAl₂ phases. For CuZrAl addition, bright and network precipitates are clearly observed in the Al matrix. On account of the interdiffusion of Al and Cu atoms between matrix and reinforcement, the ABCs present the good interfacial bonding. Compared with SPS-ed pure Al bulk, ABCs possess the excellent mechanical properties. It is mainly ascribed to the second phase strengthening, continuously distributed precipitates, high relative density or bonding interface, and grain refinement strengthening. Thereinto, combined with a degree of plastic strain, the composite with 20?vol% CuZrAl reinforcement reveals the best micro-hardness (290?HV), and the highest yield strength and fracture strength of 408 and 459?MPa, respectively. Moreover, the ABCs bear the better pitting resistance with wide passive region in seawater solution.     

Microstructures and properties of high-fraction Sip–6061Al composites fabricated by pressureless sintering

Pre-treated Si powder (Sip) and 6061Al powder were used to fabricate high-fraction Sip/6061Al composites via pressureless sintering, and the effects of the Sip content and the sintering temperature on the microstructures and properties of the composites were studied. The results show that in the composites, there exist MgAl₂O₄ nanocrystalline particles, and the Si phase varied from a discontinuous particulate state to a semi-continuous skeleton state as the Si content increased from 30 to 50?wt-%. Densities, bending strengths, hardness, and thermal conductivities of the composites all increased initially and then decreased with the sintering temperature. The 680°C sintered 30?wt-% Sip/6061Al composites and the 700°C sintered 50?wt-% Sip/6061Al composites have the optimal mechanical and thermophysical properties.     

双屏蔽(B4C-W)/6061Al层状复合板设计与性能

基于B₄C和W良好的屏蔽中子和γ射线性能,采用6061铝合金作为基体,设计了一种新型双屏蔽(B₄C-W)/6061Al层状复合材料,通过放电等离子烧结后加热轧制成板材,对制备的复合材料微观组织和力学性能进行了研究。结果表明,屏蔽组元B₄C和W颗粒均匀地分布在6061Al基体中,层界面、B₄C/Al、W/Al异质界面之间结合良好,无空隙和裂纹。在颗粒与基体界面处形成扩散层,扩散层的厚度约为6 μm (W/Al)和4 μm (W/Al)。轧制态的(B₄C-W)/6061Al层状复合板的屈服强度(109 MPa)和极限抗拉强度(245 MPa)明显优于烧结态的复合材料,但断裂韧性降低。强度提高的原因主要是轧制后颗粒的二次分布、均匀性及界面结合强度提高,基体合金的晶粒尺寸减小,位错密度增加。层状复合板的断裂方式为基体合金的韧性断裂和颗粒的脆性断裂。      

基于SPS法B4C/6061Al中子吸收复合材料组织及性能

B₄C中B的同位素¹⁰B具有较大的热中子吸收截面,是良好的中子吸收体。采用放电等离子烧结法（SPS）制备了B₄C体积分数为10%~40%的B₄C/6061Al中子吸收复合材料,对B₄C/6061Al中子吸收复合材料的微观组织形貌及物相组成进行了观察分析,并测试了其拉伸性能。结果表明：B₄C颗粒均匀地分布在6061Al基体中,颗粒尖端放电产生的等离子体能够促进B₄C颗粒/6061Al基体界面结合,材料内部的物相主要有Al、B₄C、AlB₂和Al₃BC。随着B₄C体积分数的增加,B₄C/6061Al中子吸收复合材料的致密度降低,抗拉强度先增加后降低,断裂机制主要为6061Al基体及B₄C颗粒/6061Al基体界面的撕裂。     

Processing of heat-treated silicon carbide-reinforced aluminum alloy composites

The present study investigates the processing of heat-treated silicon carbide (SiC) particle-reinforced 6061 aluminum alloy (AA) composites. As-received SiC powders were heat treated at 1300ºC, 1400ºC, and 1500ºC in nitrogen atmosphere for 2 h, and the 6061 AA–SiC composites were developed by spark plasma sintering at 560ºC and 60 MPa for 5 min in argon atmosphere. The amorphous silicon nitride is found to form in SiC particles as a result of heating at 1400ºC. The microstructure of the composites exhibited uniform distribution of SiC or SiC/Si₃N₄ particles in 6061 AA matrix. Further, the heat-treated SiC-reinforced 6061AA composites exhibited improved mechanical properties. A typical combination of UTS of 240 MPa and elongation of 21% is obtained for the 6061 AA composites prepared using SiC powders heated at 1400ºC.     

Influence of boron carbide content on the microstructure,tensile strength and fracture behavior of boron carbide reinforced aluminum metal matrix composites

In the present work, an indigenously developed low cost modified stir casting technique is developed for the processing of 6061 Al‐B₄C composites containing high‐volume fraction of boron carbide particles (up to 20 vol. %). The influence of varying reinforcement content on the spatial distribution of boron carbide in the aluminum matrix is qualitatively characterized using scanning electron microscope. At a lower volume fraction of reinforcement, wide particle free zone and large interparticle spacing were observed in the matrix while the composite with high reinforcement content displayed relatively homogeneous and discrete particle distribution. X‐ray diffraction analysis confirms the presence of only aluminum and boron carbide diffraction peaks, indicating that no significant reaction occurs during composite processing. The tensile behavior of composites revealed that strength and ductility are influenced by varying particulate content. The quantitative analysis of strengthening mechanism in the casted composites showed that higher volume fraction of boron carbide lead to larger values of thermal dislocation strengthening, grain size and strain gradient strengthening. The morphology of fracture surfaces reveals the presence of dimple network and the average size of dimples gradually decreases with the increase in particulate content, which indicates the co‐existence of ductile and brittle fracture.     

Microstructural features influencing the strength of Trimodal Aluminum Metal-Matrix-Composites

Trimodal Al Metal-Matrix-Composites (MMCs), consisting of a nanocrystalline Al phase (NC-Al), B₄C reinforcement particles, and a coarse-grain Al phase (CG-Al), were successfully fabricated on both lab and commercial scales. Multi-scale microstructural features contributing to the exceptional high strength of Trimodal Al MMCs were examined via comprehensive microstructural and spectroscopic analysis. Size and distribution of nanocrystalline Al grains, B₄C particles, coarse-grain Al, and uniformity in distribution were examined and quantified. Other features such as dispersoids with and without nitrogen (e.g., Al₂O₃, Al₄C₃), dislocation density, and interfacial characteristics were also examined with due respect for their contributions to the strength of Trimodal Al MMCs.     

B4C/Al Composites Processed by Metal-assisted Pressureless Infiltration Technique and its Characterization

In order to improve the wettability between Al melt and B₄C ceramic preform during fabricating B₄C/Al composites by pressureless infiltration technique, trace amount of Ti particulates with high melting point was added into the starting materials as infiltration inducer. A simple and cost-effective method, metal-assisted pressureless infiltration technique, was developed to fabricate light-weight B₄C/Al composites. The microstructure, phases, and mechanical behavior of B₄C/Al composites were characterized by SEM, XRD, and mechanical property test. The density of the as-fabricated B₄C/Al composites was about 2.75 g/cm³ and the relative density of this kind of composites was over 97%. The as-fabricated B₄C/Al composites exhibited rather well wear resistance. The flexural and compressive strengths of the as-fabricated B₄C/Al composites were about 200 MPa and 670 MPa, respectively.     

Phosphorus‐Rich Colloidal Cobalt Diphosphide (CoP2) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution

Developing earth‐abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realizing a high‐performance solar‐to‐hydrogen energy conversion process. Herein, phosphorus‐rich colloidal cobalt diphosphide nanocrystals (CoP₂ NCs) are synthesized via hot injection. The CoP₂ NCs show a Pt‐like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve ?10 mA cm^?2 and a very low Tafel slope of 32 mV dec^?1. Density functional theory (DFT) calculations reveal that the high P content both physically separates Co atoms to prevent H from over binding to multiple Co atoms, while simultaneously stabilizing H adsorbed to single Co atoms. The catalytic performance of the CoP₂ NCs is further demonstrated in a metal–insulator–semiconductor photoelectrochemical device consisting of bottom p‐Si light absorber, atomic layer deposition Al–ZnO passivation layers, and the CoP₂ cocatalyst. The p‐Si/AZO/TiO₂/CoP₂ photocathode shows a photocurrent density of ?16.7 mA cm^?2 at 0 V versus reversible hydrogen electrode (RHE) and an output photovoltage of 0.54 V. The high performance and stability are attributed to the junction between p‐Si and AZO, the corrosion‐resistance of the pinhole‐free TiO₂ protective layer, and the fast HER kinetics of the CoP₂ NCs.     

Effect of heat treatment on strength and abrasive wear behaviour of Al6061-SiC<Subscript>p</Subscript> composites

In recent years, aluminum alloy based metal matrix composites (MMC) are gaining importance in several aerospace and automobile applications. Aluminum 6061 has been used as matrix material owing to its excellent mechanical properties coupled with good formability and its wide applications in industrial sector. Addition of SiC_p as reinforcement in Al6061 alloy system improves its hardness, tensile strength and wear resistance. In the present investigation Al6061-SiC_p composites was fabricated by liquid metallurgy route with percentages of SiC_p varying from 4 wt% to 10 wt% in steps of 2 wt%. The cast matrix alloy and its composites have been subjected to solutionizing treatment at a temperature of 530°C for 1 h followed by quenching in different media such as air, water and ice. The quenched samples are then subjected to both natural and artificial ageing. Microstructural studies have been carried out to understand the nature of structure. Mechanical properties such as microhardness, tensile strength, and abrasive wear tests have been conducted both on matrix Al6061 and Al6061-SiC_p composites before and after heat treatment. However, under identical heat treatment conditions, adopted Al6061-SiC_p composites exhibited better microhardness and tensile strength reduced wear loss when compared with Al matrix alloy.