Fracture toughness of discontinuously reinforced Al-4Cu-1.5Mg/TiB2 composites

The effect of particle size, particle volume fraction, and matrix microstructure on the fracture initiation toughness of a discontinuously reinforced aluminum composite was examined. The composites were Al-4 wt pct Cu-1.5 wt pct Mg reinforced with 0 to 15 vol pct of TiB₂ having an average particle diameter of 1.3 or 0.3μm producedin situ by the XD process. The room-temperature plane-strain toughness measured using compact tension specimens ranged from 19 to 25 MPa . Toughness was adversely affected by increases in TiB₂ volume fraction. The fracture toughness of all composites was affected by changes in the matrix microstructure produced by aging. The response of the composites to artificial aging deviates from that of the matrix. Fractography revealed that these composites failed in a ductile manner, with voids initiating at the reinforcing TiB₂ particles. The experimentally measured plane-strain toughness properties of Al-4Cu-l .5Mg composites with well-dispersed, 1.3-μm TiB² reinforcements agree with the Rice and Johnson model.     

Effects of SiCp reinforcement by electroless copper plating on properties of Cu/SiCp composites

Abstract

Silicon carbide reinforced copper matrix composites containing 50–80 vol.-%SiC_p were fabricated by hot pressing copper coated SiC_p powder. The results show that the densification, thermal expansion coefficients, flexural strength, and thermal conductivity of Cu/SiC_p composites reinforced by electroless copper plating and their corrosion resistance in 5%NaCl solution are better than those without electroless plating. Physical properties and flexural strength of the composites decrease with an increase in SiC_p content, whereas the corrosion resistance increases with an increase in SiC_p volume fraction. By observing the fracture surface after a flexural test, it can be seen there are two types of fracture model: the cracking of Cu/SiC_p interface and the pulling out of SiC_p particles. The experiment also proved that the bonding strength of the Cu/SiC_p interface and the pressure of the hot pressing operation are the two main factors which influence the fracture of these composites.     

MgO/HA复合材料的强韧化与冷处理

以多种不同粒径的MgO颗粒为第二相,以HA为基体,采用无压烧结法制备MgO/HA复合材料;研究MgO粒径与MgO/HA复合材料的抗弯强度和断裂韧性之间的关系,探讨冷处理对复合材料性能的影响。结果表明:添加适宜粒径的MgO颗粒能够提高HA复合材料的抗弯强度和断裂韧性,其断裂韧性可达基体断裂韧性的1.5倍,抗弯强度可达基体抗弯强度的1.29倍,MgO颗粒增韧的粒径范围为15～35μm,增强的粒径范围为<25μm。冷处理可以进一步提高复合材料的强度和韧性,而且可以改变增韧和增强的MgO粒径范围,使增强与增韧粒径的重叠范围变宽。     

Fabrication and characterisation of bulk Al2O3/Mo nanocomposite by mechanical milling and sintering

Abstract

In this study, fabrication and mechanical properties of alumina based ceramic matrix nanocomposite reinforced with 15 and 26·6 vol.-%Mo particles were investigated. Alumina–molybdenum nanocomposite powders were prepared by ball milling of Al and MoO₃ in an SPEX8000 type ball mill. The powder particles were consolidated by cold uniaxial pressing followed by sintering in vacuum atmosphere at 1300 and 1400°C. The structural evaluation of as milled and sintered samples was studied by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. Sintered samples were examined by hardness measurements and three-point flexural strength. Results show a significant improvement in flexural strength of Al₂O₃–Mo nanocomposites in comparison to monolithic alumina and increases by Mo content. During sintering, grain growth and α-Al₂O₃ to γ-Al₂O₃ transformation occurred. In addition, an increase in temperature of sintering resulted in higher density and hardness of consolidated nanocomposites.     

Microstructure and mechanical properties of SiC particles reinforced Mg–8Al–1Sn magnesium matrix composites fabricated by powder metallurgy

The new type of Mg–8Al–1Sn (AT81) magnesium matrix composites reinforced with different volume fractions (5, 10, 15, 20, 25 and 30 vol.-%) of SiC particles (average size of 10 μm) was fabricated by powder metallurgy. With the increasing volume fraction of SiC particles (SiC_p), the particles gradually show more homogeneous distribution. Compared with the AT81 alloy, the yield strength (YS) and ultimate compressive strength of the SiC_p/AT81 composites are improved simultaneously. With the increasing SiC_p from 0 to 30 vol.-%, the YS and ultimate compressive strength increase from 69 to 239 MPa and 286 to 385 MPa respectively, while the corresponding fracture strain (ε) decreases from 19·3 to 4·8%. The improvement of the YS and ultimate compressive strength of the SiC_p/AT81 composites benefits from the more homogeneous microstructure due to the increase in the SiC particles.     

预扩散处理对原位生成TiC_p/Fe基粉末冶金复合材料组织与性能的影响

将水雾化Fe粉与Ni粉、Mo粉、Ti粉混合均匀,然后在800℃、50%H2+50%Ar(体积分数)气氛保护下进行预扩散处理,将预扩散粉与电解Cu粉和石墨粉混合,通过压制与烧结,制备Ti C颗粒增强Fe基复合材料。通过X射线衍射分析、扫描电镜及能谱分析和力学性能测试等手段,研究预扩散处理对原位生成Ti Cp/Fe基粉末冶金材料组织与性能的影响。结果表明:Fe-Ni-Mo-Ti混合粉在800℃下预扩散处理后形成表面粗糙的团球状预扩散粉末颗粒,但合金元素在Fe粉颗粒内分布不均匀。与用混合粉制成的Ti Cp/Fe基复合材料相比,用预扩散粉制备的材料孔隙率略有增加。随预扩散时间延长,材料中富Ti区的尺寸减小,组织明显细化,珠光体分布更均匀,同时形成大量弥散分布的粒径在0.1~0.5μm的Ti C颗粒。材料的硬度和抗弯强度都随原料粉预扩散时间延长而提高,用60 min预扩散粉制成的Ti Cp/Fe基复合材料的硬度HRB和抗弯强度分别达到63.6和613.7 MPa,比用混合粉制成的Ti Cp/Fe基复合材料分别提高11.8%和38.3%。用预扩散粉末制备的Fe基复合材料的断裂形式为具有一定韧性断裂特征的脆性断裂。     

Effect of overaging and particle size on tensile deformation and fracture of particle-reinforced aluminum matrix composites

The effect of reinforcement particle size and overaging treatment on the tensile behavior and fracture morphology of a 2080/SiC/20 _p composite was investigated. Tensile behavior was profoundly influenced by particle size and matrix strength. The composite strength increased with a decrease in particle size, while overaging greatly reduced the strength of the composite, independent of particle size. Almost all particles on the fracture plane were fractured, and the amount of particle fracture in the composites was insensitive to overaging and particle size, due to the excellent bonding between SiC particles and the Al matrix. Fractography showed that void nucleation in the matrix of peak-aged composites took place primarily at very fine SiC particles, which were much smaller than the average SiC particle size. Subsequent failure took place by the tearing topography surface (TTS) mechanism. In the overaged composite, composites failed by a more conventional void nucleation and growth process, where void nucleation took place at coarsened S precipitate particles, resulting in smaller and more elongated voids.     

颗粒形状及基体热处理对SiCp/LD2 断裂韧性的影响

对普通SiC颗粒和钝化处理过的SiC颗粒增强LD2铝复合材料的研究表明,颗粒经钝化处理后,几乎去除了很尖锐的部分,使颗粒呈近等轴状,但颗粒的形状对两种热处理态的复合材料(热挤出态和T6态)的断裂韧性K     

颗粒形状及基体热处理对SiCp/LD2断裂韧性的影响

对普通ＳｉＣ颗粒和钝化处理过的ＳｉＣ颗粒增强ＬＤ２铝复合材料的研究表明,颗粒经钝化处理后,几乎去除了很尖锐的部分,使颗粒呈近等轴状,但颗粒的形状对两种热处理态的复合材料的断裂性ＫＱ均无影响,而热挤出态的复合材料ＫＱ低于Ｔ６态。     

Mechanical behaviour of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> based alloys and composites

The demand for materials to be used in the components operating above 1100°C in advanced aero-engines drives the development of the silicide-based intermetallic alloys and composites, including the titanium silicides. The mechanical behaviour of Ti₅Si₃ and its composites has been reviewed with emphasis on the microstructure-property relationships. It is found that the grain size is a critical parameter, and smaller grain sizes are desirable for reducing the magnitude of internal residual stress caused by the crystallographic anisotropy in coefficients of thermal expansion. The reduction in grain size leads to significant improvement in hardness, room temperature flexural strength and fracture toughness. On the other hand, the high temperature strength observed at slow strain rates and creep resistance are higher in the samples with the coarser grain sizes. Further improvements in the strength, fracture toughness and high temperature creep resistance are possible, either through the development of multiphase alloys, or by the use of ceramic reinforcements in composites.     

Fracture toughness and fatigue crack growth in rapidly quenched Nb-Cr-Ti In situ composites

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr₂Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr₂Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr₂Nb particles.     

Correlation of Microstructure,Hardness, and Fracture Toughness of Fe-Based Surface Composites Fabricated by High-Energy Electron Beam Irradiation with Fe-Based Metamorphic Alloy Powders and VC Powders

Iron-based surface composites were fabricated with Fe-based metamorphic alloy powders and VC powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and fracture toughness was investigated. Mixtures of metamorphic powders and VC powders were deposited on a plain carbon steel substrate, and then the electron beam was irradiated on these powders without flux, to fabricate surface composites. The composite layers 1.3 to 1.8 mm in thickness contained a large amount (up to 47 vol pct) of hard Cr₂B and V₈C₇ particles formed in eutectic colony regions and inside colonies, respectively. The hardness of the surface composites was approximately 2 to 4 times greater than that of the substrate because of Cr₂B and V₈C₇ particles. According to the microfracture observation of the composite fabricated with mixing 30 wt pct VC powders, microcracks initiated at coarse V₈C₇ particles ins inside colonies as well as at Cr₂B particles in colony regions, and were connected with other microcracks in a zigzag shape. Thus, it showed a higher fracture toughness and hardness twice as high as the composite fabricated without mixing VC powders.     

Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites

The effects of microstructure (namely, particulate volume fraction and particulate size) and the counterface materials on the dry-sliding wear resistance of the aluminum matrix composites 2014A1-SiC and 6061Al-Al₂O₃ were studied. Experiments were performed within a load range of 0.9 to 350 N at a constant sliding velocity of 0.2 ms^-1. Two types of counterface materials, SAE 52100 bearing steel and mullite, were used. At low loads, where particles act as loadbearing constituents, the wear resistance of the 2014A1 reinforced with 15.8 μm diameter SiC was superior to that of the alloy with the same volume fraction of SiC but with 2.4 μm diameter. The wear rates of the composites worn against a steel slider were lower compared with those worn against a mullite slider because of the formation of iron-rich layers that act asin situ solid lubricants in the former case. With increasing the applied load, SiC and A1₂O₃ particles fractured and the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. The transition to this regime was delayed to higher loads in the composites with a higher volume percentage of particles. Concurrent with particle fracture, large strains and strain gradients were generated within the aluminum layers adjacent to contact surfaces. This led to the subsurface crack growth and delamination. Because the particles and interfaces provided preferential sites for subsurface crack initiation and growth and because of the propensity of the broken particles to act as third-body abrasive elements at the contact surfaces, no improvement of the wear resistance was observed in the composites in this regime relative to unreinforced aluminum alloys. A second transition, to severe wear, occurred at higher loads when the contact surface temperature exceeded a critical value. The transition loads (and temperatures) were higher in the composites. The alloys with higher volume fraction of reinforcement provided better resistance to severe wear. Wearing the materials against a mullite counterface, which has a smaller thermal conductivity than a counterface made of steel, led to the occurrence of severe wear at lower loads.     

A Comparative Study of Mechanical Properties,Thermal Conductivity,Residual Stresses,and Wear Resistance of Aluminum-Alumina Composites Obtained by Squeeze Casting and Powder Metallurgy

Squeeze casting and powder metallurgy techniques were employed to fabricate AlSi12/Al₂O₃ composites, which are lightweight structural materials with potential applications in the automotive industry. The impact of the processing route on the material properties was studied. Comparative analyses were conducted for the Vickers hardness, flexural strength, fracture toughness, thermal conductivity, thermal residual stresses, and frictional wear. Our results show that the squeeze cast composite exhibits superior properties to those obtained using powder metallurgy.

     

Effect of Dispersoid Size and Volume Fraction on Aging Behavior and Mechanical Properties of TiO2-Dispersed AA7075 Alloy Composites

TiO₂-dispersed AA7075 alloy composites were produced by mechanical milling followed by hot uniaxial compaction and sintering. The effects of volume fraction and dispersoid size on precipitation kinetics, densification, and hardness of the composites were studied in detail. While the sinterability of the composites decreases with increasing volume fraction of the particulate reinforcement (dispersoid), the same increases with decreasing particle size of the reinforcement. Microstructural analysis using X-ray diffraction and scanning electron microscopy shows an improvement in the distribution of reinforcement with decreasing particle size. The hardness of the composites increases with increasing volume fraction and decreasing TiO₂ particle size. Further, the reinforced composites do not show age hardenability unlike unreinforced AA7075 alloy. Microstructural analysis reveals the formation of MgTiO₃ and ZnO near the TiO₂-AA7075 interface, which suppresses the formation of Guinier-Preston (GP) zone resulting in no age hardenability of the composites.     

Plasma densification of agglomerated Cr2O3 powder for thermal spray coating

Abstract

Double plasma flame treatments were carried out on spray dried Cr₂O₃ agglomerated powders to increase their apparent density. The powders that were subjected to the first densification treatment didn't show the entirely melted state, and were fully melted only after the second plasma treatment. Plasma densification resulted in powder size decreasing as well as apparent density of particles and also resulted in the fluidity increasing due to the powder melting and surface smoothing effects. However, some parts of the particles after the second treatment showed a hollowed structure, especially for a particle size above 30 µm. The influence of the thermal conductivity of powder and the gas pressure within aggregates exposed to the plasma flame in the particle densification process was discussed in detail. The powder density strongly influenced the structure of plasma sprayed coatings. The dense coatings with high hardness and high bond strength was achieved in the coatings produced from Cr₂O₃ powders after plasma densification.     

Observations on the effect of cementite particles on the fracture toughness of spheroidized carbon steels

The results of experimental studies of the influence of cementite particles on the fracture toughness of a number of spheroidized carbon steels at low temperatures were analyzed in terms of current theories of crack-tip behavior. The fracture toughness parameterK _IC was evaluated by using circumferentially notched and fatigue-cracked cylindrical specimens. The conclusions are summarized as follows: 1) In general,K _IC decreases with increasing volume fraction and increasing size of the carbide particles. 2) Crack initiation occurs at the carbide particles. 3) Crack propagation occurs by cleavage if the stress conditions satisfy the Ritchie, Knott and Rice criterion that a critical cleavage stress is achieved over a minimum microstructural size scale. The critical stress is that required to propagate a crack from a particle and the minimum size scale is of the order of 1 to 2 grain sizes. 4) Crack propagation occurs initially by fibrous rupture if the stress intensification is insufficient to attain the critical cleavage stress. P. Rawal was formerly affiliated.     

Effect of Nanocrystalline Powders on the Structure and Properties of Dispersion-Hardened Alloy TiC – 40% KhN70Yu

This article examines aspects of the effect of nanocrystalline powders of ZrO₂, Al₂O₃, W, WC, WC–Co, NbC, and Si₃N₄ on the combustion, structure, and physical and physico-mechanical properties of new dispersion-hardened electrode alloy TiC – 40%KhN70Yu. This heat-resistant hard alloy, based on titanium carbide and a nickel alloy, was obtained by self-propagating high-temperature synthesis (SHS). It is shown that the addition of a nanocrystalline powder decreases combustion rate, with the magnitude of the reduction depending on the specific surface of the addition. It was determined that the structure of the synthesis products is modified appreciably by introducing a mixture of nanocrystalline powders into the initial charge. Here, additions of ZrO₂ and NbC have a positive effect on the main physico-mechanical characteristics of the alloy (strength, hardness, fracture toughness). Nano-powders of Al₂O₃ and Si₃N₄ have a negative effect on the alloy's physico-mechanical properties. The addition of WC–Co increases the flexural strength of the material, while the addition of W and WC increases its fracture toughness.     

Processing,Microstructure, and Mechanical Properties of B4C ― TiB2 Particulate Sintered Composites. Part II. Fracture and Mechanical Properties

The fracture response of pressureless sintered boron carbide ceramics containing 5-25 vol.% TiB₂ phase produced via the in-situ chemical reaction between B₄C, TiO₂ and elemental carbon was studied. Both strength and fracture toughness depend on TiB₂ volume fraction, reaching their maximum values of 500 MPa and 4.6 MPa·m^1/2, respectively, at 15 vol.% TiB₂. The observed increase in strength and fracture toughness was ascribed to the interaction between the propagating crack front and local thermal mismatch stress associated with TiB₂ particles. Induced circumferencial microcracking and crack impedance are discussed as the major toughening mechanisms. Spontaneous circumferencial microcracking due to thermal mismatch stress in TiB₂ particles was found to occur when the particle size exceeds its critical value. The theoretical interpretation of spontaneous circumferencial microcracking, toughening via induced microcracking, and crack impedance was justified experimentally.     

Mechanical properties of β-SiC pressureless sintered with Al2O3 additions

Mechanical properties of pressureless sintered SiC with Al₂O₃ addition were measured. The increase in fracture toughness and strength is attributed to the presence of a liquid phase which results in the formation of platelets of an α-SiC. The highest values for flexural strength and toughness were measured on samples with ∼ 10 wt% Al₂O₃ addition. Pull out, crack bridging and crack deflection are identified as the major strengthening and toughening mechanisms.