Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 1991"

Plastic deformation of alloys with submicron-grained structure

Abstract: The peculiarities of a submicron-grained structure formation in a series of aluminium- and magnesium-based alloys by means of a special strain-heat treatment are examined in this paper. The unusual mechanical properties of alloys displayed in this state at room temperature as well as at enhanced temperatures are found. The nature of the plastic deformation of these materials is discussed.

Magnetization process in nanocrystalline ferromagnets

Abstract: Annealing of initially amorphous FeCuNbSiB alloys at temperatures above their crystallization temperature leads to an ultrafine grain structure of α-FeSi with average grain sizes down to D ≈ 10 nm and random texture. Owing to the nanocrystalline structure the magneto-crystalline anisotropy is randomly averaged out by exchange interaction. Moreover magnetostriction decreases on crystallization owing to the formation of α-FeSi. As a consequence the material reveals excellent soft magnetic properties.

Operational texture analysis

Abstract: A comprehensive overview is presented of the elements that enter quantitative techniques of texture measurement, analysis and representations. There are many potential errors to be corrected and many choices to be made in all these stages, and we present those that we consider most appropriate. They have been implemented in a software package that is available publicly. A number of novel techniques are used including, for example, in the representation of measured textures, both for quantitative visual inspection and for use in the prediction of anisotropic properties. The symmetry of the test sample is allowed to be general, and that of the crystal structure may be as low as trigonal.

Plasma surface engineering of low alloy steel

Abstract: The surface of low alloy steel (En40B) has been engineered in the plasma of a glow discharge via plasma nitriding and ion plating of titanium nitride (TiN) coatings on the nitrided substrates with the purpose of enhancing the surface properties and fatigue strength. The nitriding response of the steel has been accessed by the evaluation of phase composition, layer thickness, hardness profile, residual stresses and nitrogen and carbon distributions. The wear and fatigue characteristics of the plasma-nitrided steel have been investigated and simple models have been developed to describe the influence of such properties as depth and strength of the nitrided case on the fatigue limit and load-bearing capacity of the nitrided steel. In order to further improve the tribological properties and load-bearing capacity of the low alloy steel, a duplex plasma surface-engineering technique has been developed. This is achieved by plasma nitriding the steel first so as to produce a thick, strong subsurface and then depositing a thin, hard and wear-resistant TiN coating on the nitrided substrate by ion plating. Dry-sliding wear tests demonstrated that the duplex-treated steel, i.e. the TiN coating-nitrided steel composite, not only exhibited enhanced wear resistance over the as-nitrided steel (by a factor of 2–8) but also had much higher load-bearing capacity than the TiN coating on unnitrided steel. Optimization of the coating-substrate combination can be achieved by correct control of the plasma-nitriding, surface preparation and ion-plating processes.

Universal high performance ball-milling device and its application for mechanical alloying

Abstract: We describe the design and modes of operation of a novel ball mill device in which the ball movement pattern is controlled by an external magnetic field. Examples of applications of this ball mill to the synthesis of NiZr alloys, high melting point intermetallics, MgZn alloys and aluminium-based alloys are given. In particular, various modes of operation may result in a different path for the solid state reaction occurring during milling.

A review of the methods for the evaluation of coating-substrate adhesion

Abstract: The adhesion between coating and substrate is often the predominant factor in determining the performance and reliability of coated engineering components. Methods which have been developed for evaluating coating-substrate adhesion often have a number of advantages and disadvantages depending on the specific coating-substrate system. The performances of a number of these techniques, namely the pressure-sensitive tape test, indentation testing, the scractch test, laser spallation, etc., are reviewed.

Cracks on bimaterial interfaces: elasticity and plasticity aspects

Abstract: Substantial progress has been made on the mechanics of interface fracture. An engineering program has emerged which allows the fracture resistance of interfaces to be measured and utilized. This recent development, assessed in an Acta-Scripta Metallurgica Proceedings, is discussed. Several results have been obtained on the plasticity aspects of interface cracks in which one (or both) of the constituent materials can deform plastically. The crack tip fields are members of a family parametrized by plastic mode mixity parameter ξ. The J integral scales each member field. Analyses of finite width crack geometries loaded by remote tension show that the effects of load, ligament plasticity and geometry on the near-tip fields are adequately accounted for by the J integral. The progress is summarized.

Magnetic properties of FeCuMSiB (M = Cr, V, Mo, Nb, Ta, W) alloys

Abstract: Magnetic properties, microstructures and formed phases of nanocrystalline alloys prepared by annealing iron-based amorphous alloys added Cu and M (Cr, V, Mo, Nb, Ta, W) were investigated. Nb, No, Ta, W addition is very effective to improve soft magnetic properties. Especially, Nb is the most effective element to improve the soft magnetic properties. The order of the effect of decreasing the grain size is: Nb = Ta > Mo = W > V > Cr and is similar to that of the sofr magnetic properties.

Transition wear behavior of SiC-particulate- and SiC-whisker-reinforced 7091 Al metal matrix composites

Abstract: The wear behavior of unreinforced and reinforced 7091 Al, the latter containing either 20 vol.% SiC particulates (SiC p ) or 20 vol.% SiC whiskers (SiC w ), was studied as a function of sliding distance and sliding velocity under unlubricated conditions. At sliding velocities below 1.2 m s −1 , SiC reinforcement does not affect wear resistance. Wear debris produced from both the unreinforced and reinforced materials was predominantly metallic and was small in dimension and dark in color. The mechanism of wear under these conditions was surface-fatigue-related surface cracking. At sliding velocities greater than 1.2 m s −1 , the wear rates of the reinforced materials were lower than for the unreinforced matrix. Both the unreinforced alloy and the SiC-reinforced composites exhibited elevated wear rates during the initial period of sliding, the mechanism of wear under these conditions, i.e high velocity and short sliding distance, being controlled by subsurface-cracking-assisted adhesive transfer and by abrasion. During steady state sliding, these elevated wear rates were maintained by the unreinforced alloy, reduced wear rates being observed in the reinforced composites. The initial wear rates of the composites depend strongly upon reinforcement orientation, the highest wear rates being observed the perpendicularly oriented SiC w composite. However, the steady state wear rates of the composites were generally independent of reinforcement geometry (particulate vs. whiskers) and orientation (perpendicular vs. parallel) with the exception of wear at 3.6 m s −1 where the parallel-oriented SiC w composite was superior.

Wettability of SiO2 and oxidized SiC by aluminium

Abstract: The silica layer grown naturally or artificially on the surface of SiC fibres or particles used in alumina-based matrix composites is supposed to have two functions: protection of the SiC from aluminium attack and improvement of the wettability of SiC by aluminium which would result from the reaction between aluminium and SiO 2 . The effective role of silica in the wetting of aluminium on SiC was studied using the sessile drop method and the immersion-emersion tensiometric technique. Aluminium contact angles were measured first on amorphous SiO 2 and then on thermally oxidized SiC monocrystals (silica layers of 10–50 nm), between 933 K and 1173 K, and under a dynamic vacuum of 10 −4 −10 −5 Pa. In the two systems it appeared that silica acts as an oxygen source which causes oxidation of liquid aluminium. As a result the wetting kinetics was slowed down and even blocked: the apparent contact angle at 973 K is very high (above 150°). At higher temperatures (above 1073 K) deoxidation of aluminium by evaporation of the alumina layer allowed a real interface to be established between the solid and the liquid. However, as the silica reduction reaction occurred before the wetting, the stationary contact angle of aluminium on SiO 2 was found to be that of aluminium on alumina, and the steady contact angle of aluminium on oxidized SiC was that on alumina (at temperatures less than 1073 K) or on SiC (at temperatures higher than 1173 K). The strong reactivity between aluminium and SiO 2 cannot be used to improve the wetting of this metal on SiC. Consequently, silica layers on SiC cannot help the incorporation of particles or the infiltration of fibres by aluminium.

Electrochemistry of electroless plating

Abstract: The present review describes the electrochemical aspects of electroless plating in four sections: (1) electrochemical polarization behaviours; (2) catalytic aspects of electroless plating; (3) mechanism of anodic oxidation of reductants; (4) methods for determination of the rate of electroless plating. It is stressed that electrochemical polarization studies in an electroless plating bath is a powerful method for assessing the adaptability of the bath and for obtaining mechanistic information, although the anodic and cathodic reactions are more or less interdependent when they occur simultaneously. Catalytic aspects of electroless plating are discussed on the basis of the polarization curves for the anodic oxidation of reductants. The mechanism of anodic oxidation of reductants is discussed on the basis of a computer simulation of the current density-potential curve assuming several reaction schemes. Attention is focused especially on the occurrence of the volcano-shaped polarization curve. It is established that the polarization resistance (the slope of the current-potential curve at the electroless plating potential) is inversely proportional to the rate of electroless plating.

Microstructure and mechanical properties of Laves-phase alloys based on Cr2Nb

Abstract: Mechanical properties of Laves-phase alloys based on Cr2Nb at temperatures up to 1000°C were examined and correlated with microstructures and phase relationships. Single-phase Cr2Nb alloys are very hard and brittle at ambient temperatures, indicating the difficulty in generation and glide of dislocations due to the complicated crystal structure (C-15). Examination of the CrCr2Nb two-phase region revealed the following: (a) the hardness decreases with increasing amounts of the soft chromium-rich phase; (b) the eutectic composition has a niobium concentration of 17 at.%, instead of 12 at.% as reported in the currently existing phase diagram; (c) heat treatments produce uniform dispersion of fine Laves-phase precipitates in primary chromium-rich patches for the hypoeutectic alloys, and these particles are very stable even at temperatures above 1000°C; (d) the soft particles are very effective in preventing crack propagation originating in the brittle Laves-phase matrix, which results in a high yield strength with moderate ductility up to 1000°C. These results demonstrate that the introduction of a soft chromium phase has promising effects in improving the mechanical properties of brittle Cr2Nb Laves-phase alloys.

Reaction-formed silicon carbide

Abstract: The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

Metastable phase transformations induced by ball-milling in the CuW system

Abstract: For the first time, based on X-ray diffraction, scanning electron microscopy/energy dispersion X-ray analyses, differential thermal analysis and differential scanning calorimetry experiments, the crystal to non-equilibrium phase transition induced by ball-milling has been evidenced in the CuW system. This system exhibits a total immiscibility in both solid and liquid states. Starting from elemental copper and tungsten powders, a partial solubility of copper into the cubic tungsten lattice as well as of tungsten into the f.c.c. copper lattice is induced by ball-milling. Such an enhancement of the solubility is revealed by an effect on the lattice parameter of both Cu(W) and W(Cu) crystalline solid solutions over the whole investigated composition range, i.e. Cu 5 W 95 to Cu 95 W 5 (wt.%). Such an experimental investigation of the crystal to amorphous phase transition induced by ball-milling in a system which exhibits a positive heat of mixing either in the solid or in the liquid states supports our previous results leading to the conclusion that the mechanisms of the phase transitions induced by ball-milling are different from those of the so-called classical solid-state amorphization.

Nanocrystalline materials by crystallization of metal-metalloid glasses

Abstract: The formation of ultrafine microstructures by crystallization of metal-metalloid glasses was investigated by means of electron microscopy as well as in situ time-resolved X-ray diffraction. The results can be understood on the basis of nucleation and growth theories, taking into account the effect of recalescence during massive crystallization and the differences in the mode of crystallization and the diffusivity. In a polymorphic crystallizing Fe66Ni10B24 glass the finest microstructure can be achieved by annealing at temperatures significantly below the “nose” of the TTT diagram; the finest grain size can be calculated and observed to be in the range of about 0.1 μm. In glassy Fe73.4Cu1Nb3.1Si13.4B9.1 (FINEMENT) the combination of a reduced growth rate due to the niobium content as well as with increasing size of the primary crystals and an accelerated nucleation rate due to the copper additions allows the formation of extremely fine-grained microstructures in primary crystallizing metal-metalloid glasses at temperatures above the glass transition.

The physics of superplastic deformation

Abstract: Superplasticity is an important mode of deformation in metallic alloys with very small grain sizes (usually less than 10 μm). In general, high elongations are observed over a rather limited range of intermediate strain rates and there is a decrease in the superplastic effect at both high and low strain rates. The major experimental observations in superplastic metals are summarized and the physical mechanisms of flow are discussed with reference to the behavior at high, intermediate and low strain rates, respectively. Superplastic-like behavior has been reported recently in some ceramics but the experimental evidence suggests that the mechanism of flow in these materials in not the as in metals.

Formation of a super-saturated solid solution in the AgCu system by mechanical alloying

Abstract: Mechanical alloying in the AgCu system by the ball-milling and repeated rolling methods was performed. A super-saturated f.c.c. solid solution was formed in the entire composition range by ball milling. The lattice parameter change due to the super-saturated solid solution formation matched quite well with the reported result obtained by rapid quenching. Upon heating, the super-saturated solid solution decomposed into base silver and copper solutions, resulting in a large reduction of electrical resistivity.

Mean-field micromechanics model and its application to the analysis of thermomechanical behaviour of composite materials

Abstract: This overview article is concerned with the mean-field micromechanics model that has been developed by several investigators as a powerful modification of Eshelby's theory for the study of the macro-mechanical behaviour of composite materials. First, a general framework of the model is reformulated with emphasis on clarification of the key approximation involved. The model is then applied to the prediction of overall thermoelastic properties of some specific composites of practical interest. For the system with randomly oriented spheroidal particles in particular, the overall bulk and shear moduli, computed as functions of the particle aspect ratio, are shown to vary systematically within the margins which coincide with those bounded by Hashin and Shtrikman. The remainder of this article is devoted to the analysis of anisotropic elastic-plastic responses of metal matrix composites to multiaxial thermomechanical loads. The energy approach that has been proposed is generalized and its applications to specific problems are presented and discussed in comparison with experimental data.

Properties of sputtered stainless steel-nitrogen coatings and structural analogy with low temperature plasma nitrided layers of austenitic steels

Abstract: Structural analogies are revealed between 310 stainless steel-nitrogen deposits prepared by reactive sputtering and diffusion layers resulting from low temperature plasma nitriding of the same grade of stainless steel Both the coatings and the diffusion layers show high nitrogen supersaturations in the fcc lattice of austenite and present a rather favourable compressive stress and a good level of microhardness An interesting corrosion resistance is also expected With these properties, future applications can be considered

Spray casting: an integral model for process understanding and control

Abstract: This paper discusses the scientific and technological aspects of spray casting An integral model is presented which encompasses and addresses each unit stage of the overall process, namely atomization, spray, consolidation, shape, pre-form solidification and microstructure The atomization model uses an empirical relationship to correlate the droplet size distribution with spray-forming process parameters The spray model quantities the condition of the spray upon impact with the substrate; the condition of the spray is represented in terms of the fraction of liquid in the spray and the proportion of solid, mushy and liquid droplets The sticking efficiency is obtained from the consolidation model and was found to be a critical parameter which governs the yield, shape and microstructure of sprayed deposits The shape model dynamically predicts the evolution of pre-form shape for different combinations of substrate and spray motion The model for pre-form solidification uses a two-dimensional heat transfer analysis to compute temperature-liquid fraction profiles in the perform The microstructure model predicts the final grain or cell size in sprayed deposits as a function of the local solidification time Individual models are interlinked to identify and asses the effect of critical process parameters on the integrity of spray-cast product

Pressure infiltration casting of metal matrix composites

Abstract: Pressure infiltration casting ∗ is a unique form of liquid infiltration which utilizes pressurized inert gas to force liquid metal into a preform of reinforcement material. The methods and equipment used for pressure infiltration casting allow for inexpensive development of composite materials, prototypes, and net-shape component production. Pressure infiltration casting's use of an enclosed die chamber with controlled pressurization makes it possible to cast in low strength molds with high infiltration pressures. The development of a number of solidification systems has enabled parts to be infiltrated and directionally solidified, producing high quality composites. The basic principles behind pressure infiltration casting and its history will be discussed in this paper. Results of research in different methods for casting composites will also be presented. These methods include: 1. (1) top fill casting-liquid metal is forced downward by a pressurized gas into a preform; 2. (2) bottom fill casting-liquid metal is forced up a fill tube into a preform by pressurized gas acting on the surface of a melt; 3. (3) top pour casting-a method developed for infiltration of high temperature alloys where the reinforcement and melt must be prevented from reacting. The methods and apparatus illustrated include sample components with aluminum and copper matrices, their microstructures, and tolerancing for net shape component production.

Orientation splitting of cube-oriented face-centred cubic crystals in plane strain compression

Abstract: Orientation splitting of cube-oriented f.c.c. crystals during plane strain compression is analysed by assuming the development of deformation bands in the direction of extension. The bands are considered to undergo the macroscopic normal strain components but are allowed to shear in opposite senses along the elongation direction (local relaxed constraints model). It is shown that this local deformation banding leads, for both the (001)[010] and (001)[110] orientations, to large lattice rotations of opposite sign about the transverse axis and hence the formation of transition bands. Analytical expressions are derived for the lattice rotations up to true strains of about unity. Experimental plane strain compression tests on (001)[010] and (001)[110] aluminium single crystals confirm the predicted behaviour and indicate the spatial distribution of the bands and their slip systems. It is shown that the bands in the (001)[110] orientation are characterized by double coplanar slip while those in the cube crystals behave according to a single-slip model.

Mechanical approach to the residual stress field induced by shot peening

Abstract: A simplified analytical model for calculating the compressive residual stress field due to shot peening is given. For eight shot-peening conditions. experimental verifications were carried out on 40Cr steel with four different heat treatment conditions. The results show good agreement with predictions of the model.

Plasma Immersion Ion-Implantation of Steels

Abstract: Plasma immersoin ion implantation (PI3) is a new technique with certain advantages over conventional ion implantation. We have developed an implanter based on an inductively coupled r.f. glow discharge plasma. Ion densities of 1010 cm−3 are obtained with filling pressures of about 10−3 mbar. Ions are accelerated from the plasma by high voltage pulses (typically – 45 kV) applied directly to the work-piece. In this paper we report on the application of PI3 to nitrogen implantation in steels. Dramatic increases in microhardness and wear resistance have been observed for a number of steels, ranging from 0.3 wt.% C mild steel to austenitic stainless steels. Despite the relatively implantation energy, the modified layer can be greater than 1 μm in thickness. The nitrogen concentration profile can be controlled by the implantation temperature and dose. Glancing-angle X-ray diffraction has been used to determine the structural changes that occur in the surface layer.

Amorphization of CuTa alloys by mechanical alloying

Abstract: The amorphization in the system CuTa is investigated in the concentration range Cu20Ta80 to Cu70Ta30. The milled powders are examined by X-ray diffraction. The crystallization temperatures, the activation energy of crystallization, the crystallization enthalpies and the stored energy of the elements are determined by DSC and DTA measurements. The amorphization of alloys in the immiscible system CuTa is discussed in the context of stored deformation energy and interface energy.

About the AlCuFe icosahedral phase formation

Abstract: In the present work, results obtained by differential thermal analysis (DTA) and X-ray diffraction (XRD) confirm [5, 8] that the icosahedral phase (i-phase) is formed by a peritectic reaction. A single i-phase can be obtained after annealing around the compositions Al64Cu24Fe12Al61.75Cu25.5Fe12.75. For the composition Al62Cu25.5Fe12.5, the XRD lines of the i-phase are narrow whatever the annealing temperature (800 °C or 600 °C). Thus, this result establishes that there exists a single phase domain where the i-phase is perfect (without phasons) and remains stable at 600 °C. A provisional vertical section along the line Al70Cu20Fe10Al64Cu24Fe12Al58CuPin28Fe14 is proposed and shows the phases that exist equilibrium at various temperatures in this range of compositions.

Interfacial phenomena in the solidification processing of metal matrix composites

Abstract: Capillary phenomena pertinent to solidification processing of metal matrix composite materials are reviewed. Recent research on the design of wettability measurement methods that are pertinent to metal matrix composite fabrication and overcome limitations of the sessile drop technique is presented. The discussion also addresses non-chemical aspects of wetting of a reinforcement by liquid metal, with emphasis on the infiltration process, and reviews engineering approaches for improved wettability and control of interface microstructure.

The influence of microstructure on the fracture behaviour of particulate metal matrix composites

Abstract: Fracture processes have been studied in a range of model particle-reinforced metal matrix composite systems: commercially pure aluminium matrices reinforced with silicon carbide particles of two volume fractions and three particle sizes. The fracture micromechanisms have been identified by a combination of fractography and in situ experiments. A transition in fracture mechanisms from decohesion at the particle-matrix interface to particle cracking has been observed on increasing both the particle size and the volume fraction. Fracture nucleation criteria have been developed to explain these phenomena as a function of the microstructural parameters.

Microstructure-property relationships of in situ reacted TiC/AlCu metal matrix composites

Abstract: A novel technique was utilized for the fabrication of in situ titanium carbide reinforced aluminum alloy metal matrix composites. The reacted, cast, extruded and heat-treated samples exhibited a homogeneous distribution of fine (0.1–3 μm) TiC platelets in a fine-grained recrystallized Al4.5wt.%Cu matrix. Elevated temperature tensile testing indicated that the composite retains its room temperature strengths up to 250 °C and compared favorably with composites fabricated by more complex and costly processes. When compared with Al4.5wt.%Cu alloy processed similarly but without the TiC reinforcement, the additions of TiC resulted in a yield strength and tensile strength increase of 130% and 65% respectively. Fractographic analysis indicated ductile failure, although the ductility and strength were limited by the presence of coarse titanium aluminides (Al3Ti).

Novel reinforced ceramics and metals: a review of Lanxide's composite technologies

Abstract: Lanxide Corporation is developing and commercializing novel technologies for ceramic matrix and metal matrix composites. The ceramic composite technology is based on the use of a unique directed-metal-oxidation process to grow ceramic matrices around pre-placed composite fillers or reinforcements. The metal matrix composites are made by a pressureless molten metal infiltration process involving excellent wetting of the reinforcement by the matrix alloy. This paper provides an overview of the processing methods, a description of some of the composites that have been made and their key properties, and an indication of some of the applications being addressed with these materials.