Showing papers in "Materials Science and Technology in 1988"

Cast metal matrix composites

Abstract: Metal matrix composites have been available in certain forms for at least two decades, e.g. boron fibre reinforced aluminium and various dispersed phase alloys and cermets. Recently, a range of alumina and silicon carbide fibres, whiskers, and particles with diameters <20 μm have become available. The possibilities of incorporating these materials into metals to improve stiffness, wear resistance, and elevated temperature strength without incurring weight penalties have attracted the attention of design engineers in the aerospace and automobile industries. The aim of the present paper is to outline the manufacturing processes for such composites, in particular those based upon liquid metal technology, e.g. squeeze casting and spray forming. Some of the mechanical and physical properties which have been determined for these materials are described. An analysis of how matrix alloy selection may influence tensile and fracture behaviour of short fibre and particle reinforced composites is attempted.MS...

Application of fracture mechanics to failure of surface oxide scales

Abstract: A new simple method is described for determining the dimensions of a ‘composite defect’ in surface scales by summing the actual defects present in the scale using well established techniques of linear elastic analysis. From such estimates, fracture toughness can be assessed and the stress intensity factor for growing surface oxide scales can be calculated. The technique also enables an estimate to be made of the residual internal stress of growing surface oxide scales. Data are presented for the stress intensity of the oxide scales on Nimonic 75 (which is a Ni–Cr alloy), Armco iron, and dilute titanium alloys, at temperatures in the range 20–850°C. Such data can be used to predict oxide stability on components at high temperature when subjected to applied stress or thermal cycling.MST/854

Structure maps for. Pseudobinary and ternary phases

Abstract: The empirical database of Villars and Calvert on the structures of pseudobinary and ternary phases is ordered within a limited number of two dimensional plots by using the recently proposed Mendeleev number M, which takes a unique value for each element in the periodic table The pseudobinaries are characterised by average Mendeleev numbers MA and MB The structure maps (MA, MB) for the AB and AB3 stoichiometries respectively are found to display similar domains of structural stability as those of the pure binaries Most of the non H, C, N, or O containing phases of ternary structure type having more than one representative compound each can be displayed and separated within only seven two dimensional plots Two each are required for the boride and chalcogenide structure types and a further three for the remaining ternary phases The importance of these plots for the design of new alloys is stressedMST/814

Mechanism of σ-phase precipitation in Cr–Ni austenitic steels

Abstract: The mechanism of σ-phase precipitation in industrial Cr–Ni austenitic steels has been found to depend on the chemical composition of the austenite after precipitation of the interstitial phases whose formation always precedes nucleation of the σ-phase. Depending on the chemical composition of the austenite, the σ-phase may precipitate directly from the austenite and partially through the M23C6 carbide or through the ferrite, or only through the M23C6 carbide. In this last case the quantity of σ-phase precipitated does not exceed 0·5%.MST/529

Mechanical properties of polycrystalline diamonds

Abstract: The room temperature mechanical properties of polycrystalline diamonds, i.e. tensile strength, transverse rupture strength, compressive strength, impact strength, fracture toughness, and elastic constants, have been determined. The applied test techniques are described and the results compared with those obtained by other authors. The fracture mode under the present experimental conditions was primarily transgranular. A grain size dependence, where strength increases with decreasing grain size, has been found. Fracture toughness was found to go through a maximum for grain sizes between 10 to 30 μm. The modulus of elasticity increases with increasing grain size. An influence of cobalt content on strength and modulus of elasticity has been found, while no significant influence on toughness could be determined. Increasing the cobalt content increases strength, but has the inverse effect on the modulus of elasticity. The results of strength, toughness, and elastic constants measurements are discussed ...

Wagner theory and short circuit diffusion

Abstract: The growth of an oxide film on a metal by thermal oxidation is usually discussed in terms of Wagner's theory in which the oxidation rate is controlled by the transport of ions across the oxide film under the combined effects of concentration gradients and electric fields. This leads to parabolic growth kinetics with a rate constant which in most cases is controlled by the diffusive properties of the oxide. However, the expected relationship between growth rate and diffusion constants is not generally confirmed experimentally and usually the oxidation rate is many orders of magnitude greater than expected. For Ni oxidation, the fast rates are a result of rapid diffusion along grain boundaries in the oxide and when this is taken into account the oxidation rates and diffusion data can be reconciled. However, in the case of Cr oxidation even fast diffusion along grain boundaries is still apparently too slow to account for the measured oxide growth rates. Short circuit diffusion of oxygen appears to oc...

Deformation characteristics of Ti–6Al–4V

Abstract: The deformation characteristics of Ti–6Al–4V have been established by torsion testing in the temperature range 800–1150°C. Constitutive equations are proposed for both the β-region and the α+β-region which, it is suggested, may have some practical applications. Extensive optical and electron microscopy have established that dynamic recovery is the operative deformation mode in the β-region, while dynamic recrystallisation predominates in the α+β-region.MST/806

Criteria for formation of single layer, duplex, and breakaway scales on steels

Abstract: It is generally agreed that differences in oxidant transport behaviour explain the development of single, duplex, and breakaway oxide scales. In view of the low lattice mobility of oxygen, it is necessary to postulate the transport of oxidant in molecular form through micropores in the scale. It is proposed that the oxidant has some degree of access to the oxide/metal interface in all types of scale and that volume constraints on the gaseous reactions at this site determine which type of scale forms. Single layers form when dislocation climb eliminates space created at this interface by outward metal diffusion. Factors which inhibit this volume collapse, such as alloying or sample corners, lead to duplex scales. However, the rate of reaction is still limited by the rate at which cation effusion creates the volume. Breakaway occurs when factors such as non-sealing interfaces caused by oxide carburisation overcome this constraint. The breakaway rate is then controlled by the rate the oxide can grow ...

Effects of coolant on temperature distribution in metal machining

Abstract: The effects of coolant on tool temperature in metal turning operations were studied both experimentally and theoretically. Values for the coefficient of heat transfer from the tool to the coolant were obtained. Experimentally, a medium carbon steel was turned at a feed rate of 0·254 mm rev −1, at cutting speeds of 33–61 m min−1, with high speed steel tools and water based coolant. Tool temperatures estimated from structural changes in the tool were compared with values calculated by finite element analysis. Heat transfer coefficients from tool to coolant between 103 and 5 × 103 W m2K−1, depending on coolant flow rate, best match theory and experiment. Further, it is shown by the calculations that tool temperatures are particularly sensitive to changes in heat transfer coefficient between 103 and 104 W m−2K−1. This is the range in which coefficients are to be found in practice, explaining the critical nature of cutting fluid formulation, supply rate, and direction of application in those operations...

Mechanism of scale growth on liquid aluminium

Abstract: Metal loss is an unavoidable consequence of the high production melting of aluminium and its alloys. Losses must be minimised, for reasons of economy and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting result primarily from the formation of dross, a mixture of oxide and melt. Results are presented from an initial study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the initial stages in oxide scale growth, a study of growth kinetics, and observations of the subsequent breakdown of these initial scales to form dross. It was found using TEM that a thin surface oxide of γ-alumina forms rapidly on melts of commercially pure aluminium at 750°C providing a highly effective barrier confining the molten aluminium. This thin surface oxide reduces scale growth to a low level before the onset of breakaway oxidation. Localised failure of this protective oxide film results in ...

Fabrication and properties of metal matrix composites based on SiC fibre reinforced aluminium alloys

Abstract: Composites made with Al–7 wt-%Si alloys and discontinuous Nicalon SiC fibres were prepared by compocasting followed by squeeze casting. This technique gives rise to macroscopically homogeneous materials with, however, some segregation of the fibres in the eutectic phase, especially for the lowest volume fractions. The SiC fibres appreciably improve wear resistance, hardness, and compressive strength of the matrix, both in the as cast and heat treated conditions. An analysis of the compressive test results is given based on the mixture rule modified to apply for discontinuous randomly oriented fibre composites. However, it is shown that agreement between predictions and experimental results can only be obtained by considering the increase of the strength of the matrix as a result of the presence of the fibres. This increase is discussed in terms of structural changes and residual stresses created during cooling resulting from the difference in the coefficients of thermal expansion of the components...

Diffusion wear in milling titanium alloys

Abstract: In this paper, diffusion wear during milling of titanium alloys is reported. In high speed milling, tool wear is mainly caused by diffusion. The wear pattern is characterised by the combined extension of crater wear on the rake face and glacier wear on the flank. Evidence of diffusion of cobalt and carbon at the interface between the milling cutter and the workpiece has been obtained. It was demonstrated for the first time that, as diffusion wear occurred, a carbon rich layer was formed at the tool/workpiece interface while the tool material below the flank wearland was depleted in carbon. Wear occurred as a result of embrittlement and weakening of the tool surface resulting from the diffusion process.MST/674

Cavity formation and metallurgical changes induced by growth of oxide scale

Abstract: Even under conditions of protective oxidation, significant structural changes can develop in the metal substrate. One observation frequently reported is that of subsurface cavitation, but it is not likely that the same physical processes always apply. In some cases, vacancy injection by the removal of cations and subsequent Kirkendall void formation does seem to occur, but it is shown that such injection requires vacancies to have a low binding energy to an oxide/metal interface that has widely spaced sink sites. Other examples of subsurface cavities can be associated with particle dissociation, creep damage, and gas bubble formation. Some metallurgical changes are considered, which can be induced either by the selective removal of cations or by the injection of large amounts of oxygen. The basis of theoretical development is also discussed. Finally, consideration is given to the role of secondary reaction products, specifically carbon and hydrogen, which can cause precipitation when they enter th...

Evaluation of glass forming ability in binary and ternary metallic alloy systems – an application of thermodynamic phase diagram calculations

Abstract: The glass forming ability (GFA) of a wide range of binary and ternary alloy systems (Au–Si, Pd–Si, Ti–Be, Zr–Be, Hf–Be, Cu–Ti, Co–Zr, Ni–Zr, Cu–Zr, Ni–P, Pd–P, Ni–Pd–P, Cu–Pd–P, Co–Ti–Zr, Zr–Be–Hf, Ti–Be–Hf, Ti–Be–Zr) was calculated using a combined thermodynamic and kinetic approach. There is good agreement between the predicted glass forming ranges and those experimentally observed. By using this combined approach it has also proved possible to estimate critical cooling rates for phases not observed in the equilibrium phase diagram. A significant advantage of the approach is that, for multicomponent alloys, the melting temperatures and thermodynamic input parameters for the kinetic equations are derived using the constituent binary thermodynamic phase diagram calculations and, therefore, it has the potential to predict GFA in multicomponent systems using information from mainly binary systems.MST/788

Hydrogen and temper embrittlement in 9Cr–1Mo steel

Abstract: The synergism between hydrogen embrittlement and temper embrittlement has been investigated in a 9Cr–1Mo martensitic steel. Measurements of tensile ductility were used to monitor the development of embrittlement with increasing hydrogen content in material as tempered and aged for up to 5000 h at 500 or 550°C. A detailed examination was made of associated changes in fracture mechanism, precipitate microstructure, and interfacial and precipitate chemistry. A strong interaction between hydrogen and temper embrittlement was observed. Both types of embrittlement in isolation reduced tensile ductility by promoting a ductile interlath fracture mechanism: ‘chisel fracture’. Hydrogen and temper embrittlement acted synergistically to reduce ductility further by the promotion of brittle intergranular fracture and transgranular cleavage. The dominant factor controlling the interaction was the precipitation of a brittle intermetallic Laves phase containing phosphorus in solution. Phosphorus segregated to inte...

Spallation of oxide from stainless steel AGR nuclear fuel cladding: mechanisms and consequences

Abstract: 20Cr–25Ni–Nb stabilised stainless steel is used in the advanced gas cooled reactors (AGR) as cladding for the UO2 fuel. Extensive studies have been made and are continuing in an attempt to understand the spallation of surface oxide from this steel and in this paper recent progress is surveyed. In particular, it is shown how the decohesion of oxide can be thought to occur when the strain energy in the oxide layer, resulting from differential thermal contraction during cooling, attains a critical value. This value can be equated to the fracture energy of the oxide/metal interface. With continued cooling, the quantity of spall increases as an increasing fraction of the surface oxide reaches the fracture condition. The intrinsic distribution in the oxide strain energy required to cause the observed spallation kinetics is derived. The paper is concluded with a consideration of the consequences of such spallation. First, the release of radioactive particles of oxide into the primary circuit can cause co...

Plane section and spatial characteristics of equiaxed β-brass grains

Abstract: A study was undertaken to define better the concept of the statistical grain. A cube having edges of 52·1 mm was cut from a slowly cooled β-brass ingot which had equiaxed grains. The distributions of sizes and shapes of grain sections on the faces of the cube were determined. The grains were then separated in a solution of mercurous nitrate and nitric acid. Observations were made of the relationship between grain diameter and the number and shapes of faces. For the 941 separated whole grains, the frequency of distribution of grain diameters obeyed the log–normal law. A plane distribution curve was derived for the statistical grain which enables spatial distributions to be calculated from measurements made on a plane section.MST/825

Solubility of hydrogen in liquid aluminium

Abstract: The solubility of hydrogen in liquid pure aluminium has been redetermined for temperatures from 943 to 1123 K and pressures from 67 to 113 kPa (0·66 to 1·12 atm), using an appropriate version of Sieverts' method. The results fit the Van't Hoff isobar and Sieverts' isotherm. Recommended values for the solubility (S) are given, based on critical assessment of both the present results and corresponding results from recent previous determinations. The recommended values are given by the equation log(S/S°)−1/2log(p/p°)=(−2700/T)+2·72, where S° is a standard value of solubility equal to 1 cm3 of diatomic hydrogen measured at 273 K and 101·325 kPa (1 atm) per 100 g of metal, p° is a standard pressure equal to 101·325 kPa (1 atm), and T is absolute temperature.MST/502

Stresses and decohesion of oxide scales

Abstract: Stresses in oxide scale–metal systems can lead to decohesion of the scales when critical values are exceeded, which manifests itself in stable growth of interfacial cracks and separation along the scale/metal interface or in sudden spalling of the scale or of parts of it as a result of unstable cracking processes. The discussions in this paper are focused on decohesion of scales resulting from stable interfacial crack growth. It is attempted to summarise the critical conditions for separation at the scale/metal interface, the effect of stresses on the scale/metal interface behaviour, and the effect of decohesion on the protective effect. With the help of experimental results, the location of the plane of decohesion in multilayer scales and the decohesion of oxide scales on heat exchanger tubes as components used in practice are discussed. Finally, some considerations on the measurement of adhesion strength of scales are carried out leading to the recommendation of a fracture mechanics approach to ...

High resolution microanalysis of binder phase in as sintered WC–Co cemented carbides

Abstract: A direct determination is presented of the as sintered composition of the cobalt rich binder phase in a commercial WC–Co cemented carbide grade (80WC–20Co, wt-%) using atom probe time-of-flight spectrometry and analytical electron microscopy. The carbon content of the binder phase was found to be less than 0·1 at.-% throughout the material and it is shown that this result can be generalised to apply to most commercial cemented carbide materials. A tungsten depleted zone, of the order of 50 nm wide, is present in the binder at the phase boundaries. The observed variation of the tungsten content as a function of distance from the phase boundary could be reproduced by numerically solving the diffusion equation. It can be inferred from these calculations that the tungsten concentration retained in the interior of the submicrometre sized binder phase domains has been frozen-in at approximately 1000°C during cooling after sintering.MST/353

Expression for solubility product of niobium carbonitride in austenite

Abstract: An expression for the solubility product of niobium carbonitride in the austenite is obtained that agrees reasonably well with published data. A generalisation of such an expression is also given which provides a simple thermodynamical estimate for the solubility product of the multicomponent fcc carbonitride of the form (Nb, Ta, V, Ti, Zr) (C, N).MST/576

Methods of improving adherence

Abstract: The present paper considers some of the factors which determine the susceptibility of an oxide–metal system to scale spallation and methods by which this can be modified in practice. Although much of the discussion relates to such systems in general, particular emphasis is placed on high temperature alloys which develop Cr2O3, Al2O3, and/or SiO2 scales, together with their base metals such as iron and nickel. Considerable reference is made to the reactive element effect because this has been studied extensively in terms of scale adhesion processes and can be used to illustrate methods of improving scale spallation resistance. Other systems, such as oxide scales on tantalum or niobium and spinel oxides on Fe–Cr alloys, are not discussed specifically, but similar criteria are likely to be relevant.MST/852

Static recrystallisation and recrystallisation during hot deformation of Al–1Mg–1Mn alloy

Abstract: Plane strain compression tests at 5 s−1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 103 times faster in the material free from small particles.MST/751

Use of adhesives in repair of cracks in ship structures

Abstract: The structures of modern warships frequently contain significant quantities of aluminium alloy: a material especially prone to fatigue induced cracking. The satisfactory repair of such cracks is often vital if the fighting efficiency of a warship is to be maintained properly and would usually be carried out by welding. The welding of aluminium and its alloys is often difficult and onboard is further complicated by the necessity to remove inflammable linings, cables, and items of equipment to reduce the fire risk and provide access for the repair. Consequently, repair welds are often carried out from the most accessible side and are seldom full penetration. The finished weld is invariably little more than a sealant with a fatigue life inferior to that of the original plate. Initiation of further cracking in the repair weld can be prevented by adhesively bonding a reinforcement across the weld to provide a strength at least as good as that of the original plate and a stiffness sufficient to prevent ...

Effect of pearlite morphology on impact toughness of eutectoid steel containing vanadium

Abstract: The effect of a change in the morphology of the pearlite colonies on the Charpy impact energy of a fully pearlitic steel containing 0·76%C, 1·20%Mn, and 0·085% V was examined over the range of testing temperatures from −50 to 200°C. The change from a multicolony nodular pearlite structure produced from austenite of grain size 185 μm to a structure composed of individually formed colonies produced from austenite of grain size 25 μm caused a decrease in the transition temperature of 75 K and an almost 100% increase in the Charpy impact energy measured at room temperature. It is proposed that the impact toughness of pearlitic steel can be affected by pearlite morphology, at constant interlamellar spacing, only at temperatures above the ductile–brittle transition temperature of the ferrite, when local plastic deformation in the pearlitic ferrite at high angle boundaries can arrest propagating brittle cracks.MST/730

Effect of antimony additions on hardness and tensile properties of directionally solidified Al–Si eutectic alloy

Abstract: The effect of antimony additions on the hardness and tensile properties of Al–Si eutectic alloy has been examined during directional solidification. Antimony, provided that its concentration in the melt was maintained at ~0·1 wt-%, improved the mechanical properties of the eutectics by almost 20–25%, all effects being additive to those that can be gained through increasing the growth velocity of the samples. Mechanical properties were found to correlate directly with the spacing between the silicon particles of the eutectic phase of the solidification structure, a Hall–Petch type relationship existing between the yield strength and silicon particle spacing.MST/653

Fracture behaviour of C–Mn steel multipass MMA weld metals at −60°C in Charpy V testing

Abstract: By observation of the fracture surfaces and of appropriate metallographic sections of C–Mn steel multipass MMA (manual metal arc) weld metals and simulated weld specimens which were fractured at −60°C in Charpy V tests, it was found that the impact toughness of the specimen could be correlated with the length of the fibrous crack which was limited by unstable propagation of the cleavage crack; the latter could be initiated at a type of second phase particle transformed from carbon rich regions or non-metallic inclusions. The weakest zone in which the cleavage crack initiated was characterised by coarse grains of ferrite and the critical event which gave rise to unstable propagation of a cleavage crack was a crack in the ferrite grain larger than 30 μm cutting through the boundary and extending over the specimen. On the basis of these results, a model of the fracture mechanism is proposed and the effect of Mn content on increasing toughness is explained using the model.MST/647

Production and characterisation of fibres for metal matrix composites

Abstract: A wide range of ceramic fibres is available for incorporation in metal matrix composites (MMCs). The driving force for development of MMCs is savings in weight in high temperature applications. The properties of the main families of fibres are reviewed and the relevance of their use in MMCs is discussed.MST/740

Laser surface modification of ductile iron: Part 1 Microstructure

Abstract: Laser induced microstructural modification of ductile iron has been studied as a function of processing parameters such as power density and beam–substrate interaction time. The high energy density CO2 laser source changes and refines the microstructure of the near surface layer, leading to enhanced hardness and wear resistance. Two basic types of microstructure are produced by laser processing, depending on the solidification/cooling rate of the melted zone. High solidification/cooling rates produce a mechanically metastable austenitic matrix, and a dendritic microstructure with interdendritic cementite films. Low solidification/cooling rates produce a very hard, lamellar ferrite+cementite microstructure. A mixed microstructure of intermediate hardness is produced at intermediate cooling rates. Detailed optical and electron microscopy studies of these microstructures as a function of laser processing parameters are reported.MST/701

Effect of austenite grain boundary mobility on hardenability of steels containing vanadium

Abstract: A correlation has been established between the rate of grain boundary migration during austenitisation and the hardenability of steels containing 0·2–0·3%C, 1·5–1·7%Mn, up to 0·35% V, and small additions of Al or Ti. Interaction between the austenite grain boundaries and pinning particles was investigated using transmission electron microscopy and segregation to the austenite grain boundaries was examined using Auger electron spectroscopy. It has been concluded that the velocity of grain boundary migration during austenitisation influences the extent of equilibrium segregation to the austenite grain boundaries which, in turn, affects the hardenability. Pinning of the austenite grain boundaries enabled the potential hardenability effect of the alloying elements to be increased. Mechanisms have been discussed for the ways in which segregation, particularly of V, occurs to pinned or immobilised austenite grain boundaries, and the conditions by which most effective grain boundary pinning can be achiev...