Effects of substrate temperature on the magnetic and electrical properties of cobalt ferrite/metal composite thin films

The magnetic and electrical properties of the cobalt ferrite/metal composite thin films, prepared by reactive sputtering, were studied as a function of substrate temperature. With increasing substrate temperature, the saturation magnetization of the thin films increased owing to precipitation of the Co_0.67Fe_0.33 phase. Also, the electrical resistivity of the thin films decreased. From Hall experiments, the decrease of electrical resistivity of the composite thin films was mainly attributed to the increase of electron concentration. The Seebeck coefficient measurement shows that the electrical conduction mechanism of the thin films containing 37.8 and 33.7 at % Co changes from p-type to n-type and that of the thin films containing 28.5 at % Co remains n-type with increasing substrate temperature. This might be attributed to the change in composition of the cobalt ferrite matrix to Fe-excess with precipitation of Co-rich Fe alloy. ©1999 Kluwer Academic Publishers     

Influence of autofrettage on metal matrix composite reinforced gun barrels

Advanced materials are considered as candidates for the replacement of traditional gun barrel steel with the hope that weapons as durable as steel but at a fraction of the weight will be developed. Through an analytical model that simulates the effects of autofrettage on a cylindrical gun barrel, the resultant compressive residual stresses are quantified, and different materials examined as to their possible resistance to fracture under repeated internal pressure loads. This study investigates a traditional low-alloy gun steel, a high temperature SiC/titanium-alloy metal matrix composite, as well as various hybrid combinations of these materials, for their ability to develop the necessary residual stress and inelastic strain states necessary for durability. It is discovered that a hybrid composite comprised of low-alloy gun steel on the inner region of the gun barrel and circumferentially wound SiC/Ti–24Al–11Nb on the outer region can still exhibit the same compressive residual stress (and corresponding inelastic strains) seen in homogeneous steel barrels, but with a weight savings of up to 37%, while maintaining the original barrel dimensions.     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

Recrystallized grain size prediction in a particulate reinforced metal matrix composite

The recrystallized grain size following cold rolling and annealing of an Al alloy (AA2014) and a particulate reinforced AA2014 composite was investigated. The composite contained 20 vol% alumina particulates of average diameter 15 m. The recrystallized grain size in the composite was finer than in the alloy, for a given set of conditions; this was most notable for material strained less than 50% cold-work. This behaviour was attributable to a higher nucleation efficiency in the matrix adjacent to coarse alumina particles in the composite. A model was presented for the composite to predict the recrystallized grain size as a function of strain, with respect to the size distribution and number density of alumina particles. This model predicted the strain dependence of the recrystallized grain size and, in particular, the grain size insensitivity to strain at moderate-to-high levels of cold-work.     

Opportunities for new graphitic aluminium metal matrix composite

Abstract

Nickel coated graphite particles have been incorporated into aluminium with a second particulate phase to produce graphitic aluminium metal matrix composites (Gr A-Ni) with improved processing, wear, and scuffing resistance. Excellent wear behaviour is provided by a combination of solid lubrication by graphite as well as high temperature strengthening of the matrix alloy by nickel present as Al₃ Ni intermetallics. Applications being developed include cylinder liners, pistons, connecting rods, various types of brakes, air diffusers and bushings. Neutral buoyancy of two particles, one of which is lighter and the other heavier than the aluminium matrix alloy, makes this a readily sand and die castable material. The presence of graphite and Al₃Ni intermetallics reduces the amount of ceramic particulate required to achieve the desired wear properties, with resulting improved machinability. The composition of the material can be tailored to the application. All these factors influence the finished part cost.     

Tensile fracture behavior of notched fiber reinforced titanium metal matrix composite

The static fracture behavior of a titanium based metal matrix composite (MMC) with a central hole or a straight notch was investigated. The MMC used was SCS-6/Ti-β21-S with a quasi-isotropic lay-up. Different sizes of hole or notch were used which provided cut-out size to specimen width ratios from 0·1 to 0·4. Two test temperatures were used: ambient and 650°C. At both temperatures, the tested MMC showed a mild hole size effect or notch sensitivity. The failure mechanisms involved the debonding of fibers followed by failure of fibers, and then by failure of the matrix.     

Phase identification of cobalt ferrite/metal composite thin films using convergent beam electron diffraction

Cobalt ferrite/metal composite thin films with a saturation magnetization (Ms) of 0.729 Weber m-2 were prepared by a reactive sputtering method. The Ms of the thin films increased with increasing substrate temperature. The microstructures of the thin films were identified by a convergent beam electron diffraction method. For the thin films deposited at high substrate temperatures (>300 °C), CoxFe1-x (x0.62) metal alloys were separated from the cobalt ferrite matrix. A cobalt ferrite phase was determined as CoFe2O4 with a cubic structure (a0=0.839 nm) and a space group of Fd3m, while a metal phase CoxFe1-x (x0.62) with a b.c.c. structure (a0=0.289 nm) and a space group of Im3m. © 1998 Kluwer Academic Publishers     

界面对颗粒增强金属基复合材料强化性能的影响

为提高颗粒增强金属基复合材料的力学性能,采用基于微观组织的胞元模型建模方法,并利用有限元软件ABAQUS着重分析了界面层厚度以及界面层强度对复合材料性能的影响,通过对复合材料中各组成部分的应力、应变云图的获取,形象地说明了各部分的变形规律.研究结果表明,在弱界面层下,随着界面层厚度的增加,复合材料的强化效果并不显著,而在强界面层下,随着界面层厚度的增加,强化效果非常明显;就界面层强度来说,界面越强,所表现出的强化效果就越明显,但当界面层强度比基体大得多时,随着界面层强度的增加,虽然复合材料的强化呈递增趋势,但是递增的幅度已逐渐降低.     

钴铁氧体纳米粒子制备及其镧掺杂的磁性能研究

为了制备性能良好的钴铁氧体及改善其磁性能,通过改进的溶胶凝胶自蔓延燃烧法成功地制备了钴铁氧体(CoFe2O4)及掺镧(La)钴铁氧体纳米粒子.采用X射线衍射(XRD),透射电镜(TEM)、能谱分析(EDS)、振动样品磁场计(VSM)对所得粒子进行了结构、形貌、成分及磁性能表征.测试结果表明,利用改进的溶胶凝胶法制得钴铁氧体粒度均匀,且成相温度较低,500℃煅烧1h时平均粒径12nm左右;通过掺杂稀土镧元素对所得铁氧体的相结构有较强的影响,所得掺镧钴铁氧体与目标产物一致;所得钴铁氧体具有较高的矫顽力(737.33Oe),并且通过稀土元素镧的掺杂提高了钴铁氧体的矫顽力.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

Hot Working behaviour of AI-AI2O3 particulate reinforced metal matrix composite

Abstract

The hot deformation behaviour of a particulate reinforced metal matrix composite, manufactured via a casting route and consisting of a 2000 series matrix reinforced with 20 vol.-%Al₂O₃ particles, was investigated over a range of temperatures and strain rates. The behaviour was compared with the unreinforced alloy deformed under the same conditions. Both materials exhibited similar hot working behaviour. However, under all deformation conditions the composite exhibited flow stresses higher than that of the alloy, but as the deformation temperature increased and the strain rate decreased, this difference became negligible. The activation energy for deformation was determined using constitutive equations. The value determined for the composite was slightly higher than that for the alloy. This suggested that the ceramic particles in the composite force the matrix to undergo additional strain hardening during deformation. Dynamic recovery was the sole restoration process in both materials. No evidence of dynamic recrystallisation was found.     

Laser beam processing of a SiC particulate reinforced 6061 aluminium metal matrix composite

SiC particulate reinforced 6061 Al metal matrix composites were laser beam cut using a 3kW continuous wave CO2 laser. The influence of laser processing parameters such as cutting speed, laser power, and shielding gas on the quality of the cuts were investigated. Optical microscopy, scanning electron microscopy and X-ray diffraction were used to analyse the laser treated zone. Experimental results show that 6061 Al metal matrix composites can cut be successfully using laser. A number of Al4C3/Al4SiC4 plates were formed in the heat affected zones due to a chemical reaction between Si and Al that occurred during the laser processing. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructural characterization of a silicon carbide whisker reinforced 2124 aluminium metal matrix composite

The microstructure of a silicon carbide whisker (SiC_w) reinforced 2124 aluminium metal matrix composite was characterized using scanning transmission electron microscopy (STEM). The SiC whiskers ranged in length from approximately 2 to 10 µm, and demonstrated good bonding to the aluminium matrix. In a few cases, the interface between SiC whiskers and the aluminium matrix exhibited wavy characteristics. The size of subgrains in the aluminium matrix was found to be dependent upon that of SiC whiskers. In addition, two types of intermetallic compounds were observed in the composite.     

Development of novel specimens for mechanical testing of fibre reinforced titanium metal matrix composite

Abstract

Specimens used to date for testing titanium metal matrix composites (Ti MMCs) have severe limitations in the data obtained. In the present work, novel specimens have been developed to ensure that data obtained are indicative of the material as it appears in engineering components. For the longitudinal orientation, a modified dogbone with large shoulder radii is successful in improving the integrity of data. In the transverse orientation, selective reinforcement of monolithic Ti with MMC has allowed the production of a cruciform specimen in which the‘uprights’ are made of monolithic Ti and are gripped in the testing machine and the ‘cross’ is made of MMC and acts as a gauge length. The cruciform geometry ensures that surface defects, which blight conventional specimens, do not have such a deleterious effect. A unique specimen has been developed to enable the MMCs to be loaded in the through thickness direction, thus allowing a comparison of mechanical properties for the three geometric axes.     

Fatigue and fracture of a 316 stainless steel metal matrix composite reinforced with 25% titanium diboride

Fatigue and fracture mechanisms have been studied in a steel-based metal matrix composite (MMC), comprising a 316L austenitic matrix reinforced with 25 wt.% particulate titanium diboride (TiB₂). The fracture toughness was determined in the as-HIPped condition as being slightly below 30 MPa√m. Fatigue crack growth rates have been determined, and corrected for the effects of crack closure. The fracture surfaces have been studied to determine the mechanisms of damage during crack advance, which are determined as matrix fatigue, reinforcement particle fracture, and ductile rupture of the matrix. We show that the occurrence of damage mechanisms during fatigue of the material is linked to K_max, rather than to ΔK. This is rationalised in terms of a semi-cohesive process zone within the monotonic plastic zone ahead of the crack tip.     

The role of cobalt ferrite magnetic nanoparticles in medical science

The nanotechnology industry is rapidly growing and promises that the substantial changes that will have significant economic and scientific impacts be applicable to a wide range of areas, such as aerospace engineering, nano-electronics, environmental remediation and medical healthcare. In this area, cobalt ferrite nanoparticles have been regarded as one of the competitive candidates because of their suitable physical, chemical and magnetic properties like the high anisotropy constant, high coercivity and high Curie temperature, moderate saturation magnetization and ease of synthesis. This paper introduces the magnetic properties, synthesis methods and some medical applications, including the hyperthermia, magnetic resonance imaging (MRI), magnetic separation and drug delivery of cobalt ferrite nanoparticles.     

Hot isostatic pressing of metal reinforced metal matrix composites

Metal reinforced Metal Matrix Composites (MMMCs) made by combining an aluminium alloy matrix with stainless steel reinforcing wires are potentially cheaper and tougher than continuous fibre ceramic reinforced Metal Matrix Composites (MMCs). Although they do not give as great enhancements in stiffness and strength, worthwhile gains are achieved. Such MMMCs can be produced by Hot Isostatic Pressing (HIPping), which reduces interfacial reactions in comparison with liquid metal routes. Here, stainless steel (316L) and commercial purity aluminium wires were used to make bundles which were inserted into mild steel cans for HIPping at 525 °C/120 min/100 MPa. Some stainless steel wires were pre-coated with A17Si, to examine the effect of coatings on mechanical properties. Specimens were evaluated in terms of their tensile and fatigue properties. During HIPping, cans collapsed anisotropically to give different cross-section shapes, and for larger diameter cans, there was also some longitudinal twisting. Wires tended to be better aligned after HIPping in the smaller diameter cans, which produced material having higher modulus and UTS. Higher volume fractions of reinforcement tend to give better fatigue properties. Composites with coated stainless steel wires gave higher composite elongation to failure than uncoated wires. Both uncoated and coated wires failed by fatigue during fatigue testing of the composite. This contrasts with ceramic reinforced MMCs where the fibres fracture at weak points and then pull out of the matrix.