• Volume 12, Issue 3-4

      September 1989,   pages  171-447

    • Foreword

      P Rama Rao P M Prasad P Ramachandra Rao S Lele

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    • Some comparisons in the behaviour of materials at elevated temperatures under uniaxial and under multiaxial stress

      G W Greenwood

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      Most data on the mechanical behaviour of materials at elevated temperature concerns the influence of a uniaxial stress. For many purposes such information may suffice but there is increasing awareness of its inadequacy when used in the design or service life estimation of engineering components operating under more complex stress systems. Such stress systems do not only arise from the external stresses that are applied but may also result from thermal effects, particularly at interfaces and joints, or from the special geometrical features of a component.

      Some experimental techniques to provide information on creep behaviour under multiaxial stresses are described together with a discussion and evaluation of the results obtained. It is noted that data from uniaxial stress tests can be used to predict such behaviour when the material is isotropic and is not subject to volume changes, microstructural instability or creep damage. Frequently, materials do not fulfill these conditions and information is presented on the influence of some of these complicating features both on creep rate and on fracture.

    • Theory of propagating deformation bands

      J Schlipf

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      A model for propagating deformation bands is developed, based on a mechanical equation of state and on evolution equations for the structure parameters. It is shown that, contrary to normal deformation, it is necessary to discriminate between global parameters which depend solely on time elapsed and local parameters which are functions of the strain history. Correspondingly, two sets of equations are presented, one describing the development of the global parameters in the course of time, the other giving the evolution of the local state parameters in the course of deformation, i.e. as a function of strain. Measurable quantities derived from the first set are the load serrations and the band velocity, while the second set yields the flow stress and the strain profile of the propagating band.

    • Defect structures produced by cyclic deformation

      B Ramaswami

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      The application of dislocation theory and electron microscopy to problems of low-cycle fatigue has attracted considerable interest in the last three decades. The evolution of defect structure and microstructure during cyclic deformation and its effect on cyclic hardening and cyclic softening have been studied intensively. Studies of strain localization in persistent slip bands have led to a sound understanding of the cyclic stress-strain curve. The sliding and the migration of boundaries during high temperature fatigue have been investigated. These developments are illustrated with selected examples from the vast literature on the subject.

    • Application of finite element method for determination of stress intensity factor and plastic zone geometry in an aluminium alloy sheet under uniaxial tension

      A K Panda S Misra S C Misra

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      High strength materials have gained prominence in the fields of aero-structures, space missiles, ship-building, pressure vessels etc. However, high strength materials are often characterised by low values of crack resistance or fracture toughness. Knowledge of stress intensity factor (SIF) is essential to predict their fracture toughness. SIF values can be obtained both theoretically and experimentally. Theoretical methods include analytical techniques as well as the finite element method (FEM). The former is used for simpler geometries and the latter for complicated geometries of engineering structures. The SIF as a function of crack size in an aluminium alloy 2024-T3 (Al-4·5% Cu, 1·5% Mg, 0·6% Mn) sheet was determined by a computer method. These values were obtained directly from the stresses as well as indirectly from strain energy release rateG andJ integral. The results agree well with the normalised values obtained from an ASTM formula. The size and shape of the plastic zone at the crack tip have been determined as a function of nominal stress for a fixed crack length. The plastic zone has the form of two ellipsoids with their maximum spreads oriented around 69° to the crack axis.

    • Metal matrix composites. A bird’s eye view

      A Deruyttere L Froyen S De Bondt

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      Composite materials, in general, have so far been used mainly for structural applications. However, with regard to metal matrix composites, interest is growing on account of their physical properties. Indeed, customer requirements in this field cannot always be met by traditional materials.

      This paper first presents a brief overview of the interaction between fabrication, microstructure and properties of metal matrix composites. Further, some changes in the strategy for modelling and designing these materials are discussed. Finally, future prospects are outlined.

    • Current trends in the development and applications of superconducting materials

      C V Sundaram T S Radhakrishnan

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      The discovery of the phenomenon of superconductivity by Kamerlingh Onnes in 1911 was the first indication of the possibility of electrical conduction without any associated Joule loss. The technological application of the property (which was essentially manifested at liquid helium temperatures) had to await the development of stable superconducting materials capable of withstanding high currents and large magnetic fields. Although many materials — elements, alloys, ternary chalcogenides, and recently oxides — have been found to be superconducting, only a few of them have received attention for significant applications. This is based on three important parameters namelyTc, the transition temperature,Hc2, the upper critical field andJc, the critical current density.Tc andHc2 are considered intrinsic to the material, whileJc is influenced by the microstructure, and has to be optimised during fabrication of the material in the useful form. On these considerations, Nb-Ti, Nb3Sn and V3Ga have emerged as proven materials for significant applications while PbMo6S8 is still under development. Despite the fact that all these materials have to be used only at liquid helium temperatures on account of their lowTc, major developments have taken place in harnessing particularly the niobium alloys to produce superconducting magnets.

      Towards the end of 1986, a break-through has been achieved in the direction of raising theTc. Many ceramic oxides, notably Y1Ba2Cu3O7, have exhibitedTc in the vicinity of 100 K. These materials have also been shown to have highHc2, about 180 Tesla. Attempts are now being made to realise a highJc. It is too early to say whether such materials can be fabricated in suitable forms capable of carrying high currents.

      Among the major areas in which superconducting materials have so far been used, mention should be made of superconducting magnets for high energy particle accelerators, magnetohydrodynamic power generation, magnetic resonance imaging, and fusion research programmes. In other potential applications such as motors and magnetically levitated transportation, economic break-even has not been achieved, mostly on account of the need to use liquid helium. The discovery of the high temperature superconductors capable of operating at liquid nitrogen temperatures thus promises a revolution in electrical technology.

      The paper reviews the development and applications of superconducting materials, with reference to work being done in India.

    • Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates

      D Pandey V S Tiwari A K Singh Sangeeta Chaudhry

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      The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 − y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 − y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics.

    • Ferritic stainless steel — A coinage material

      V P Sardana D A Pikle

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      General economic and metallurgical requirements, and processing methods for coinage materials are reviewed. The Indian effort at the development of ferritic stainless steels for coinage is discussed. The results of several trials at the Salem Steel Plant towards establishing the appropriate composition and processing route are presented and evaluated.

    • Quasicrystalline materials

      K M Knowles

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      The report in late 1984 of phases in rapidly cooled aluminium alloys whose electron diffraction patterns exhibit icosahedral symmetry has stimulated intensive research into the structure and properties of quasicrystalline materials. Here I review our current understanding of quasicrystalline materials and quasiperiodic tilings, and in particular I discuss in detail the interpretation of transmission electron microscopy observations of quasicrystalline phases.

    • Control of microstructure and component shape in rapidly solidified/powder metallurgy titanium alloys

      F H Froes D Eylon R G Rowe C F Yolton

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      Extensive use of titanium rapidly solidified/powder metallurgy (RS/PM) components requires not only careful control of the microstructure for optimum mechanical properties but also cost-effective processing. A new direct reduction process for production of titanium alloy powder will be presented. Control of the microstructure in conventional alloys such as Ti-6Al-4V and in non-conventional dispersion strengthened terminal and intermetallic alloys will be discussed. It will be shown that RS/PM processing allows production of a fine grain size and useful dispersions of rare earth and metalloid phases; phases which normally form as gross undesirable particles. The use of hydrogen as a temporary alloying element, thermochemical processing, will be discussed and it will be demonstrated how this treatment can lead to refined microstructures with enhanced mechanical behaviour. Cost-effective processing using near-net shape techniques such as the ceramic mold process, rapid omnidirectional compaction (ROC), and the use of RS/PM preforms for subsequent isothermal forging will be presented. Microstructural control and shape-making used in unison should lead to increased use of titanium components in advanced aerospace systems.

    • Studies on age-hardenable aluminium alloys by atom-probe field-ion microscope

      Ken-Ichi Hirano

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      Field-ion microscopy connected with the successive field-evaporation technique and the atom-probe analysis has been applied to studying precipitation processes, especially the early stage of precipitation, in age-hardenable aluminium alloys, such as Al-Cu, Al-Ag and Al-Sc alloys. In Al-Cu alloys, coexistence of the single-layer G.P.(1) zones and the multilayer G.P.(1) zones has been confirmed and the difference between G.P.(1) zones and G.P.(2) zones has been clearly recognized. In Al-Ag alloys, the octahedral shape of η-G.P. zones has been confirmed. In Al-Sc alloys, it has been found that the equilibrium Al3Sc phase particles precipitate homogeneously from the beginning without any preprecipitation stage and coherency between the precipitate and the matrix is maintained even in an alloy aged at relatively higher temperature for prolonged periods of time.

    • Physical metallurgy of aluminum-lithium alloys

      G J Kulkarni D Banerjee T R Ramachandran

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      The addition of lithium to aluminium reduces the density and increases the elastic modulus; precipitation of the metastableδ′(Al3Li) phase from supersaturated Al-Li solid solution leads to appreciable increase in strength. The enhanced values for specific modulus and specific strength favour the use of the Al-Li alloys as structural materials for aerospace applications. However the binary alloys suffer from problems of poor ductility and toughness associated with strain localisation (resulting from the ease with whichδ′ particles are sheared during deformation), the presence ofδ′-free zones near grain boundaries and the heterogeneous nucleation of the equilibriumδ phase on the grain boundaries. These problems have been overcome by the development of ternary and quaternary alloys containing copper and magnesium. A small amount (∼0·1%) of zirconium is added to these alloys to improve the recrystallisation characteristics. The properties of alloys developed for commercial exploitation are briefly discussed. An overview of the physical metallurgy of the Al-Li alloys is presented with emphasis on the following features: (i) phase equilibria and precipitation reactions in Al-Li, Al-Cu-Mg, Al-Cu-Li and Al-Mg-Li systems and extension of these results to Al-Li-Cu-Mg alloys, (ii) insoluble particles and their effect on precipitation in the alloys, (iii) microstructural studies on Al-2·3%Li-1·2%Cu-0·7%Mg-0·12%Zr alloy, (iv) lithium depletion during solution treatment, (v) coarsening ofδ′ particles and development of precipitate-free zones near grain boundaries and (vi) microanalysis of the lithium containing phases.

    • Zero-flux planes and flux reversals in multicomponent diffusion

      M A Dayananda

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      The development of zero-flux planes (ZFP) and flux reversals of individual components is a phenomenon commonly encountered during isothermal diffusion in multicomponent systems. At a zero-flux plane, the interdiffusion flux of a component goes to zero and exhibits a flux reversal from one side of the plane to the other. The identification of ZFP’s is based on the determination of interdiffusion fluxes directly from the concentration profiles of diffusion couples without the knowledge or use of interdiffusion coefficients. The development of ZFP and flux reversals is reviewed and presented for both single-phase and multiphase isothermal diffusion couples in selected Cu-base multicomponent systems. The significance of ZFP is discussed with the aid of diffusion paths and thermodynamic data.

    • Role of mathematical modelling in metallurgy

      S P Mehrotra

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      Mathematical modelling is identified as one of the several techniques which may be used for process analysis. The scope of modelling in metallurgy and the steps involved in model formulation are described. Limitations of mathematical modelling in process analysis are pointed out. The need for a sound experimental programme/plant trials that must go on in parallel with the model exercise is emphasized. Three examples of mathematical modelling of metallurgical processes are presented to illustrate some of the points made above. The examples include mathematical modelling of (i) a copper convertor, (ii) interfacial phenomena in metal-ceramic systems, and (iii) mixing in a gas-stirred liquid bath.

    • A cold model study of mass transfer in Q-BOP

      N Prasad S Singh S L Malhotra

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      At steel-making temperature, chemical kinetics can rarely be the rate-limiting step. Thus most of the reactions are limited by the rate of mass transfer to and from the reaction interface. The overall rate of mass transfer may be controlled by gas phase mass transfer or liquid phase mass transfer. Since in Q-BOP, the rate of reaction may be controlled by the rate of mass transfer in gas phase or in liquid phase, both were studied in a cold model. The different variables studied were tuyere diameter, jet direction, flow rate of gas and tuyere depth. The results of gas phase mass transfer indicate that the effect of tuyere diameter and jet direction is very small. For Reynolds number less than 9000 the effect of flow rate and tuyere depth is given by the equation,KgA/L0Q = 0·02d0 + 0·043, whereas for Reynolds number greater than 9000 the effect of flow rate and tuyere depth is given by the equation,KgA/L0Q=0·061d0+0·046. Similarly the liquid phase mass transfer coefficient is independent of the tuyere diameter and the shrouding gas, and is not much affected by the jet direction. The effect of gas flow rate and tuyere depth is given by the equation,KLA=0·077 (Q)0·75(L0)0·61.

    • Process technology — rare and refractory metals

      C K Gupta D K Bose

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      India has fairly rich reserves of rare and refractory metals. Abundant sources of ilmenite, rutile, zircon and rare earths are found in the placer deposits of the southern and eastern coasts of the country. Columbite-tantalite occur in mica and the mining belts of Bihar and cassiterite deposits are found in Bastar (Madhya Pradesh). Vanadium as a minor associate occurs in bauxites and in the vast deposits of titaniferrous magnetites.

      Over the years, research and development and pilot plant works in many research organisations in India have built up a sound technological base in the country for process metallurgy of many refractory and rare earth metals starting from their indigenous sources. The present paper provides a comprehensive view of the developments that have taken place till now on the processing of various refractory and rare earth metals with particular reference to the extensive work carried out at the Department of Atomic Energy. The coverage includes mineral beneficiation, separation of individual elements, preparation of pure intermediates, techniques of reduction to metal and final purification. The paper also reviews some of the recent developments that have been taken place in these fields and the potential application of these metals in the foreseeable future.

    • Pyrometallurgical processes for the separation of hafnium from zirconium

      R Mallikarjunan J C Sehra

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      The paper reviews the various pyrometallurgical processes developed for the separation of hafnium from zirconium and also discusses the criteria for the selection of the process to be adopted for the preparation of nuclear grade zirconium in the country.

    • Recent trends in iron and steelmaking technology

      B N Singh

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      Iron and steelmaking technology, including the preparation of raw materials and utilisation of wastes, has undergone marked changes over the last five decades. The hand mining of ores/minerals has almost completely been replaced by mechanised mining. The role of mineral beneficiation and sizing has therefore increased. The current emphasis is on maximising the use of mined materials through multiple beneficiation stages and agglomeration of fines.

      Whilst emerging technologies for producing liquid iron are making their advent, blast furnaces continue to be the unchallenged source of hot metal production. However, the efficiency of ironmaking, both in terms of productivity and quality of hot metal has improved markedly.

      Gone are the days of open hearth furnaces and Bessemer converters. LD steelmaking has incorporated in it a number of improvements, namely combined blowing, dynamic control along with the use of the sublance, improved refractories for lining and ladle treatment of liquid steel.

      Trends emerging in the area of steel-casting include, horizontal casting, thin strip casting and hot charging of con-cast products without soaking/re-heating.

      The paper deals with some of these areas including the advances in rolling mill operation.

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