热轧工艺对20%B4C/Al复合材料显微组织及缺陷的影响

采用热等静压烧结与热轧相结合的方法制备了20%B_4C/Al(质量分数,下同)复合材料,采用排水法及SEM、EDS等手段研究了热轧工艺(道次变形量、总变形量)对复合材料缺陷及显微组织的影响。研究结果表明,热等静压制备的B_4C/Al复合材料坯体密度可达2.66g/cm3(相对密度100%),B_4C颗粒分布均匀且与Al界面处结合紧密;B_4C/Al复合材料轧制道次变形量应控制在10%以内,进一步增加道次变形量复合材料内出现宏观裂纹。复合材料经热轧后,B_4C颗粒仍分布较为均匀,且与Al基体结合紧密,复合材料内部未观察到明显的显微缺陷。     

Robust design and manufacturing of ceramic laminates with controlled thermal residual stresses for enhanced toughness

Boron carbide-silicon carbide ceramic composites are very promising armor materials because they are intrinsically very hard. However, their fracture toughness is not very high. Their ballistic performance could be significantly increased if the brittleness of these materials could be decreased. Here we report development of boron carbide-silicon carbide layered ceramics with controlled compressive and tensile stresses in separate layers. Such B₄C-SiC laminates with strong interfaces can provide high apparent fracture toughness and damage tolerance along with high protection capabilities. The theory of heterogeneous layered systems was used to develop optimal design parameters allowing the evaluation and maximization of apparent fracture toughness. The layered composites were designed in a way to achieve high compressive residual stresses in thin B₄C-SiC based layers and low tensile residuals stresses in thick B₄C layers. The residual stresses were controlled by the phase composition of layers and the layers thickness. The estimated apparent fracture toughness was calculated for both three layered and nine layered composites. B₄C-30 wt%SiC/B₄C laminates were made based on the optimized design for high apparent fracture toughness. Processing of laminates involved preprocessing of powders, forming green tapes and hot pressing. Work is in progress to measure fracture toughness of laminates, as well as their strength, hardness and the ballistic performance.     

Mechanical properties Of Si3N4 cerarnics reinforced with SiC whiskers and SiC platelets

Silicon nitride ceramics reinforced with SiC whiskers and SiC platelets were fabricated by hot pressing and their mechanical properties were studied. They showed higher fracture energy than conventional composites, particularly when they were consolidated by gas-pressure hot pressing at high temperature. A high fracture toughness (10.7 MPa m^1/2) which was measured by the single-edge pre-cracked beam method was achieved. Furthermore, a unique method to observe the crack propagation behaviours directly in a scanning electron microscope with loading devices was developed. As a result, much bridging and pull-out of the whiskers and the elongated Si₃N₄ grains, and crack deflection along the platelets, were observed behind the crack tip. This means that these grains are effective in enhancing the fracture resistance during crack propagation.     

Properties of B4C/Al–B4C composite with a two-layer structure

In order to obtain a material with a promising bulletproof performance, a two-layer structure composite consisting of B₄C/Al-B₄C was obtained using a two-step method for both hot pressing and infiltration aluminum in vacuum. Before aluminum infiltration the B₄C porous layer of the two-layer preform looked like a three-dimensional network of interconnected capillaries. For the B₄C ceramics layer the microstructure showed no apparent change before and/or after aluminum infiltration. The two-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C-Al material.     

Effect of cold rolling and subsequent annealing on hot pressed Ni/Al laminates

Nickel/Aluminum laminates have been prepared by hot pressing of stacked alternate layers of very thin foils of nickel and aluminium. It is found that ductile nickel layers exist in hot pressed Ni/Al laminates which are annealed up to 600 °C. However, at these temperatures Al-rich intermetallic compounds are also formed with a very thin irregular layer of pure aluminium and few voids. As the annealing temperature is increased, nickel layer is consumed and Ni-rich intermetallic compounds are formed. Cold rolling up to 8% reduction in area resulted in the cracking of the regions having intermetallic compounds with cracks perpendicular to the rolling direction. The tensile strength values are high for the laminates which contain ductile Ni layers and reach a value of 320 MPa for the specimens which are hot pressed at 420 °C. However, the laminates which are annealed at 800 °C show brittle fracture and lower tensile strength of 174 MPa. The fractographic examination indicated that Ni/Al laminates which are hot pressed at 420 °C undergo plastic deformation before breaking. On the other hand, Ni/Al laminates which are annealed at 800 °C with Ni present in the form of intermetallic compounds only, exhibit brittle intergranular fracture.     

Reciprocal sliding wear behaviour of B4C particulate reinforced aluminum alloy composites

Aluminum based composites reinforced with B₄C particles were prepared by cryomilling and subsequent hot pressing steps. The cryomilled powders dispersed with 5 wt.% or 10 wt.% B₄C particles were hot pressed under a pressure of 600 MPa at 350 °C. Microstructural studies conducted on the composites indicated that homogeneous distribution of the B₄C particles in the Al matrix and a good interface between them had been achieved. According to the results of reciprocating wear tests carried out by utilizing alumina and steel balls, wear resistance increased with increasing B₄C particle content.     

Microstructure and mechanical properties of a three-layer B4C/Al–B4C/TiB2–B4C composite

A three-layer structure material, consisting of B₄C/Al, B₄C/TiB₂ and B₄C composites, was obtained using a two-step method for both hot pressing and aluminum infiltration in vacuum. The three-layer B₄C/Al–B₄C/TiB₂–B₄C composite showed good interfacial bonding. Before aluminum infiltration the B₄C porous layer in the three-layer preform looked like a three-dimensional network of interconnected capillaries. The microstructures of both B₄C/TiB₂ and B₄C layers showed no apparent changes before and/or after aluminum infiltration. The three-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C/Al material.     

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.     

Microstructures and fracture toughness of Ti–(43–48)Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

In the present paper, the microstructures of directionally solidified Ti–(43–48)Al–2Cr–2Nb alloys prepared by electromagnetic cold crucible directional solidification technique were studied in detail. The results show that with the decrease of Al content, the interlamellar spacing of α₂/γ decreases, but the volume fraction of B₂ phase increases. In addition, the fracture toughness of these alloys with different lamellar orientation was investigated by single edge notched beam three-point bending test (SENB). It is revealed that B₂ phase has greatly impacted on the fracture toughness. When the laminates are perpendicular to the loading stress, the increasing of B₂ phase leads to a lower fracture toughness value. On the contrary, B₂ phase becomes barriers for crack propagations and increases fracture toughness value when the laminates are parallel to the loading stress.     

Silicon nitride: the engineering material of the future

The purpose of this review is to present the recent developments in silicon nitride (Si₃N₄) ceramics and to examine the achievements regarding our understanding of the relationship between processing conditions, chemical composition, microstructure and mechanical properties of Si₃N₄. Si₃N₄ is one of the most important structural ceramics because it possesses a combination of advanced properties such as good wear and corrosive resistance, high flexural strength, good fracture resistance, good creep resistance and relatively high hardness. These properties are obtained through the processing method involving liquid phase sintering in which a tailored microstructure, with high aspect ratio grains and chemistry of intergranular phase, triggers the toughening and strengthening mechanisms leading to the development of high fracture toughness and fracture strength. However, despite high fracture toughness and strength, Si₃N₄ ceramic materials still break catastrophically, and the fracture behaviour of this ceramic is considered to be the major obstacle for its wider use as a structural material. In addition to the macrostructure–mechanical properties relationship, this paper also reviews new designs involving laminates possessing no plane of weakness and some theoretical developments involving crack opening displacement. Proposals of how to improve the fracture resistance were also discussed.     

Mechanical property and R-curve behavior of the B4C/BN ceramics composites

The B₄C/BN ceramics composites were fabricated by the hot-pressing process. In this paper, the mechanical property and R-curves behavior of the B₄C/BN composites were investigated. The fracture strength and fracture toughness of the B₄C/BN microcomposites and the B₄C/BN nanocomposites decreased gradually with the increasing content of h-BN. The fracture strength and fracture toughness of the B₄C/BN nanocomposites were significantly improved in comparison with the B₄C/BN microcomposites. The damage resistance and R-curves behavior of the B₄C monolith and the B₄C/BN composites were evaluated by the indentation-strength in bending technique (ISB). The fracture strength of the B₄C monolith, the B₄C/BN microcomposites and the B₄C/BN nanocomposites decreased gradually with the increase of the indentation load. The B₄C/BN nanocomposites retained relative higher fracture strength in comparison with the B₄C monolith and the B₄C/BN microcomposites under the equivalent indentation load. The B₄C monolith, the B₄C/BN microcomposites and the B₄C/BN nanocomposites all exhibited the rising R-curves behavior. The B₄C/BN nanocomposites exhibited the higher rising R-curve behavior than that of the B₄C monolith and the B₄C/BN microcomposites. The toughness mechanisms of the composites were investigated. The B₄C/BN composites with the h-BN content more than 20 wt.% exhibited excellent machinability. The slowly rising R-curves behavior remarkably improved the machinability of the composites.     

Fracture behaviour of chemical vapour deposited and hot isostatically pressed zinc sulphide ceramics

Fracture behaviour of zinc sulphide ceramics prepared by chemical vapour deposition (CVD) followed by hot isostatic pressing (CVD + HIP) was investigated in terms of flexural strength (σ_f), plane-strain fracture toughness (K_Ic), even conditional fracture toughness (K_IQ), R-curve behaviour (variation of total fracture energy release rate, J_c with crack extension, δ/δ_c) and fracture mode. The corresponding Knoop Hardness number (KHN) and its correlations to flexural strength (σ_f) are also evaluated and reported. The present study showed that the zinc sulphide (ZnS) ceramics processed by CVD exhibited higher fracture resistance compared to ZnS processed by CVD + HIP condition. This observation is principally attributed to higher grain size associated with post-CVD HIPing process. In both conditions, the ZnS materials exhibited conditional fracture toughness (K_IQ) that decreased moderately with increased crack length due to the change in fracture mode form grossly tensile to predominant shear. A constantly rising R-curve behaviour was indicated in both the materials with significant increase in total fracture energy release rate (J_c with the normalised displacement (δ/δ_c), a parameter representing crack extension.     

Analysis of Si3N4+β-Si3N4 whisker ceramics

Dense Si₃N₄+-Si₃N₄ whisker composite ceramics were fabricated by hot pressing powder-whisker mixtures. Addition of -Si₃N₄ whiskers had no significant influence on the densification behaviour for up to 20 wt% addition. Light microscopy and scanning and transmission electron microscopy were used to study their microstructure and fracture behaviour. An increase in fracture toughness was observed for -Si₃N₄ whisker additions of up to 10 %. The main toughening mechanisms observed were crack deflection, crack branching, whisker-matrix debonding and whisker pull-out.     

In situ experimental study on fracture toughness and damage mechanism of TiB2-reinforced steel matrix composites

It is crucial to obtain better fracture property of particle-reinforced metal matrix composites (PRMMCs) for application in structural parts. In this study, three-point bending tests are conducted on a TiB₂-reinforced steel matrix composites (SMCs) with 9% and 13% TiB₂ volume fractions to understand the effect of hot rolling and particle content on fracture toughness. Results show that increasing particle content has a negative effect on the fracture toughness of SCMs as a whole. Many microcracks induced by large-size particle fracture initiate in the front of crack tip and coalesce with one another are observed, thus accelerating the main crack propagation. However, hot rolling can effectively improve the fracture toughness and hardness of SMCs with two particle contents. Particle characteristics and matrix plasticity of the SMCs are optimized by hot rolling, which finally enhances the crack propagation resistance. The present work provides guiding suggestions for effectively improving fracture properties of PRMMCs.     

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.     

Design of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-based ceramic laminates by the residual stresses

Ceramic laminates with strong interfaces between layers are considered a very promising material for different engineering applications because of the potential for increasing fracture toughness by designing high residual compressive and low residual tensile stresses in separate layers. In this work, Si₃N₄/Si₃N₄-TiN ceramic laminates with strong interfaces were manufactured by rolling and hot pressing techniques. The investigation of their mechanical properties has shown that the increase in apparent fracture toughness can be achieved for the Si₃N₄/Si₃N₄-20 wt.%TiN composite, while further increase of TiN content in the layers with residual tensile stresses lead to a formation of multiple cracks, and as a result, a significant decrease in the mechanical performance of the composites. Micro-Raman spectroscopy was used to measure the frequency shift across the Si₃N₄/Si₃N₄-20 wt.%TiN laminate. These preliminary Raman results can be useful for further analysis of residual stress distribution in the laminate.     

Effect of Si additions on the microstructure and mechanical properties of hot-pressed B<Subscript>4</Subscript>C

Dense B₄C-based materials containing up to 4.9 wt % Si have been produced by hot pressing in the temperature range 1600–1700°C. In this process, the silicon melts to form a liquid phase, which improves the sinterability of the material. The boron carbide partially dissolves in the liquid Si to form silicon carbide between the B₄C grains. The relative density, bending strength, Vickers hardness, and fracture toughness of the materials obtained in this study are 99.0 ± 0.1%, 584 ± 12 MPa, 39.4 ± 0.1 GPa, and 5.3 ± 0.2 MPa m^1/2, respectively. The materials experience predominantly transcrystalline fracture.     

Microstructure and properties of silicon nitride ceramics with calcium aluminate additions

Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength, and air oxidation resistance has been studied. The results demonstrate that the Si₃N₄ ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and is stable to air oxidation up to 1300°C.     

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.     

Hot pressed ZrB2–SiC–C ultra high temperature ceramics with polycarbosilane as a precursor

ZrB₂–SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB₂ and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also improved.