NRC-M Workshop on Microstructural Engineering
25-30 May 2009
UGC Networking Resource Centre for Materials
Department of Materials Engineering
Indian Institute of Science
Bangalore 560 012.
Alloy Design : Some Vignettes
S. Ranganathan
Department of Materials Engineering
Indian Institute of Science
Bangalore 560012
The nature of materials research will be illustrated as a progressive evolution from discovery through development to design. In the beginning many discoveries of new materials and processes were accidental. Even now many accidental or serendipitous discoveries are made and include metallic glasses and quasicrystals. As the materials science paradigm evolved, it became evident that processing-structure-properties and performance are intimately intertwined. Examples from aluminium base alloys for aerospace and plutonium alloys for nuclear applications will be described. The contrast between sing;;e component Element Six and multicomponent alloys will be vivdly brought out by the use of Materials Tetrahedron. With growth in knowledge and computing power, it has become possible to design materials from first principles. “Quantum Steels” by Greg Olson is an impressive achievement. The advances in atomic resolution microscopy, 3D-microscopy and materials informatics will be highlighted.
Mathematical methods in applied microstructural engineering
Asim Tewari
Staff Researcher
GM India Science Lab
Bangalore
Microstructure plays a major role in today’s materials development. The first and foremost requirement to engineer microstructure is to be able to quantitatively describe it. All microscopic observations are inevitably biased and cannot be directly used to quantify microstructure. A matured field of applied mathematics exists which deals with this problem of microstructural quantification. A practical introduction to these mathematical methods and relations is presented. Specific topics covered include volume, interfacial surface area, mean curvature, mean size and size distribution measurements. Specific applications from the field of composites, single phase poly-crystalline & multiphase alloys are discussed. Finally, for the sake of completeness, an elementary introduction to the formal mathematical basis of these relations in provided. Apart from this present advances in multispectral microscopy and three-dimensional reconstruction are briefly described.
Solidification Microstructure: Issues and Development
Krishanu Biswas
Department of Materials and Metallurgical Engineering
IIT Kanpur, Kanpur -208016
Solidification is one of the important techniques of manufacturing. Almost every metallic component we see in real life today undergoes at least one solidification step during manufacturing. The important parameter controlling the properties of the components is the microstructural development during solidification processing. The present talk will highlight the microstructural evolution of materials during different types of solidification processing conditions.
A microstructure is generally defined by the morphology, size, distribution, crystal orientation, and correlation among different crystals of multiple phases. Phase and microstructure selection describes the variety of phases and microstructures that develop under given growth conditions and growth geometries. Microstructural development during solidification is either nucleation controlled or growth controlled or mixed controlled. Control experiments have been carried out to simulate each of the above situations, such as electromagnetic levitation for nucleation controlled and laser resolidification for growth controlled microstructural evolution. Sufficiently large undercooling attainable during electromagnetic levitation under ultra high vacuum ensures the attainment of condition close to the nucleation controlled solidification of small levitated droplet. In case of laser surface treatments, the molten liquid remains in contact with the underlying solid and thereby ensures the solidification controlled by growth kinetics of different phase.
Convection in the liquid phase also plays an important role in microstructural evolution of single and multiphase alloys. These convections include gravity driven as well as maragoni convection. This talk will also highlight the effect of convection on microstructural evolution so that the techniques of solidification on microgravity platform can be understood.
Issues pertaining to development of microstructure and texture during severe plastic deformation
Satyam Suwas
Department of Materials Engineering,
Indian Institute of Science
Bangalore - 560 012.
Equal Channel Angular Extrusion (ECAE) is a severe plastic deformation technique that produces grain sizes up to submicron level. Microstructures of ECAE processed materials are characterized by relatively lower dislocation density compared to the conventionally processed materials subjected to the same amount of strain. These two aspects taken together lead to many important attributes. Since these processes are associated with large amount of strain, depending on the strain path, characteristic crystallographic textures develop. The texture evolved after ECAE are typified as shear texture. In the presentation, the speaker will share his experiences with processing-microstructure-texture relationship in FCC, BCC and HCP materials. The key issues in texture development during ECAE will be focused, in particular. It will also be shown that there are inherent limitations in refining grain size below 200-300 nm. Thermal stability of microstructures evolved after ECAE will be discussed; in light of experience of the speaker vis-à-vis the literature. Finally, a state of the art survey will be presented with regard to the applications of ECAE.
Design of microstructure for improving mechanical properties and corrosion resistance in materials for nuclear reactor applications
S. Saroja, T. Karthikeyan and R. Mythili
Physical Metallurgy Divison
Indira Gandhi Centre for Atomic Research
Kalpakkam 603 102
The optimization of microstructure and its long term thermal and irradiation stability are important criteria for selection of a material for nuclear reactor applications. The choice of material for a specific application is dictated by the engineering properties, which in turn depends on the alloy composition and microstructure. The selection is complex due to stringent demands on the material to perform under the influence of severe environmental conditions like high temperature, stress, corrosion and radiation or a combination of these, all of which could bring about microstructural changes leading to performance degradation.
A variety of austenitic and ferritic steels have been developed over the years to meet the above requirements. The microstructures have been tailored to meet the required strength and creep properties, at the same time avoid effects like sensitization, substructural changes and formation of metastable embrittling phases, which are limiting factors for the life time of a component during service. This has been achieved by appropriate design of chemistry and processing conditions like thermal and thermo-mechanical treatments. Further, the experience gained by examination of ‘in service’ components w.r.t microstructural evolution has provided vital inputs for a systematic material development program to improve the high temperature mechanical properties and radiation resistance.
Apart from the basis of microstructural design for reactor applications, the lecture would also include specific case studies based on the work carried out in our laboratory. This includes the enhancement in fracture properties in a ferritic 9Cr-1Mo steel that has been achieved by grain refinement and grain boundary engineering. Another case study would illustrate the role of morphological features in the microstructure in improving the corrosion resistance of a Ti alloy.
Structure and stress in thin films
Srinivasan Raghavan
Materials Research Centre
Indian Institute of Science
Bangalore 560 012.
Crystals can be "defect free" and "stress free" in only one way, but defective and/or stressed in an infinite number of ways. As in bulk-polycrystalline materials, stresses and defects are the rule even in thin films. The so called "perfect single crystals" on which current computer chips are based and one which subsequently "defect-free" thin film devices are grown, have a dislocation density of about 100 per sq.cm. Stresses in thin films can routinely exceed 1 GPa. In comparison, the yield stress of mild steel is 250 MPa! While often construed as a disadvantage, if controlled, stresses and defects can be a boon to an engineer, as one can now design an infinite variety of materials. Indeed, the current computer chips that use Si-Ge technology use stressed films as a means of altering the band structure such that it increases the mobility of electrons and holes in the materials. This is turn can be used to make faster and smaller devices (remember resistance to electron flow increases as the wire gets thinner which means that for a smaller device to work at the same speed the conductivity or in turn mobility has to be increased). After a brief introduction to thin film growth, the talk will focus on strategies to obtain thin films with the desired levels of defects and stress so as to engineer films with the required properties.
Controlling Temper Embrittlement in Petroleum Reactor Pressure-vessel Steel Shells
N. Prabhu
Department of Metallurgical Engineering and Materials Science
Indian Institute of Technology - Bombay
The ferritic steel reactor vessels used in modern petroleum and petrochemical processing are operated under conditions as severe as metal temperatures of 565 C and pressures of 27 MPa. Component-failure scenarios consider not only the condition for steady operation but also those that apply during start-stop transients. In addition to the minimum Boiler and Pressure Vessel code requirements for the fabricated condition, steels for high-temperature, high-pressure hydrogenation service are required to withstand environmental-degradation processes such as temper-embrittlement, hydrogen-embrittlement, hydrogen attack and creep embrittlement. Temper embrittlement is a major cause of degradation of ferritic steels. Segregation of tramp elements to prior austenite grain-boundaries in steel is the principal cause of temper embrittlement. The history of temper embrittlement studies has revealed its extreme complexity. The uncertainties largely arise from the presence of various non-metallic impurities, alloying elements and the type of interactions between them. Experimentally, these issues are not easy to resolve because they are related to local chemistry and mechanical behaviour of grain boundaries on an atomic scale.
The paper will review the basic mechanism of segregation, the thermodynamics and kinetics of segregation for binary solid solutions, interactive segregation in multi-component systems and site competition. The literature data available on temper embrittlement will be analysed and various semi-empirical approaches used to estimate potential embrittlement susceptibilities of steels will be presented. The paper will conclude by discussing the various methods and practices adopted by the industry in controlling temper embrittlement in pressure-vessel steels.
Thermal and deformation induced microstructure evolution in superalloys
M. Sundararaman, J. B. Singh and S. A. Nalawade
Structural Metallurgy Section, Mechanical Metallurgy Section
Bhabha Atomic Research Centre
Mumbai 400085.
Superalloys are multi component alloys developed for use at service conditions at elevated temperatures, high mechanical stresses and aggressive environments. The service life of components made of these alloys is decided by the stability of microstructure which includes the stability of grains as well as second phase particles within them.
Superalloys are generally used either in solid solution or precipitation hardened condition depending upon the service conditions. In the solid solution strengthened alloys, the stability of microstructure is governed by the stacking fault energy of the material and also the ease or difficulty of grain growth during service.
The thermal stability precipitation hardened alloys is decided mainly by the misfit between particles and the matrix. The nature and the magnitude of misfit in different unit cell directions play an important role in governing the morphology, the habit plane and the growth of precipitates. The propagation of deformation across precipitates can either destroy order within them or can generate different type of defects which can act as nuclei for formation of new phases.
All these aspects which control the evolution of microstructure under thermal and deformation conditions will be discussed in the present talk with illustrations from the work carried out in our institute on nickel base superalloys and intermetallics. The importance of alloy chemistry on microstructure stability will also be presented.
Crystallographic Texture an Microtexture
Indradev Samajdar
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Mumbai – 400 076.
A microstructure consists of size, shape and orientation of grains, phases and defects. While size and shape can be estimated by conventional microscopy, the orientation often involves specialized measurements, representation and analysis – the broad subject of crystallographic texture. Like any other structural indices, the subject can be used to understand a process or a mechanism or for structure-property co-relation. The later may range from formability issues in low carbon steel to magnetic properties of electrical steel, from localized corrosion in austenitic stainless steel to resistance to hydriding in zirconium alloys, from earthquake predictions to fine-tuning advanced electroceramics – the list can indeed be exhaustive. Few selected examples can, however, justify the potential of the subject – and that will be attempted in this presentation.
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The Role of Interfaces in High Temperature Deformation and Failure
Atul H. Chokshi
Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012.
Interfaces are ubiquitous microstructural features in materials. It is well known that interfaces play an important, but frequently opposing, role in many deformation and failure processes. Thus, for example, a reduction in grain size enables superplasticity by facilitating grain boundary sliding; however, grain boundary sliding also leads to stress concentration and premature failure by concurrent cavitation.
This presentation will highlight the many roles played by interfaces from both the theoretical and experimental viewpoints, with examples from contemporary research on nanocrystalline materials and ceramics.
Development of textured coated high temperature superconducting (HTS) cables
V. Subramanya Sarma
Department of Metallurgical and Materials Engineering
Indian Institute of Technology - Madras
Chennai - 600 036.
Coated conductors (CC) project is underway in many countries with the aim to produce long lengths of (second generation (2G) YBCO based) high temperature superconducting (HTS) cables. The performance of these CCs is strongly dependent on the quality of the texture. There are 2 main routes used in the production of CCs, 1) Ion Beam Assisted Deposition (IBAD) approach, and 2) Rolling Assisted Bi-axially Textured Substrates (RABiTS) approach. First part of the talk will give an overview of the above two processes. The RABiTS process is thought to be cost effective in comparison to the IBAD route. For the RABiTS process, the development of thin, mechanically stronger, non-magnetic and highly cube-textured substrates is of great technological importance for increasing the engineering current density. Nickel is a suitable substrate in view of its ability to form strong cube texture after heavy cold rolling and annealing and its excellent oxidation resistance. However, nickel is very soft (yield strength ~ 40 MPa) and this limits the processing to thin tapes. The ferromagnetism of Ni is also undesirable for alternating current applications of coated conductors in magnetic fields. The second part of the talk will deal with the development of high strength, strongly cube textured Ni-based substrates with reduced magnetisation losses. In the last part, some results of transport measurements on YBaCuO7-d films deposited by pulsed laser deposition (PLD) on Ni, Ni-alloy substrates will be presented along with the current status worldwide in the production of long lengths of CCs.
Participation of microstructure in deformation, fatigue and fracture processes
Soumitra Tarafder
Fatigue & Fracture Group
National Metallurgical Laboratory (CSIR)
Jamshedpur 831 007.
Fracture of materials, whether through fatigue or by overload, is intimately connected to their deformation behaviour, which in turn is controlled by the microstructure. It is therefore not difficult to envisage that microstructure will have a profound influence on both deformation and fracture. This lecture will focus on the role of microstructural constituents in the process of deformation, fatigue and fracture.
Fracture may be considered to be the inevitable consequence of continued deformation. The interactions of the microstructure with strains evolved during deformation activate the mechanisms of fracture. With respect to ductile fracture, this concatenation of mechanisms is apparent from the fact that initiation, growth and coalescence of microvoids are the result of localization of dislocation activity that is primarily responsible for deformation. Such localization is triggered by interactions of intense dislocation configurations with particulate constituents of the microstructure – at precipitates, particles, inclusions and interfaces.
It is not very easy to demonstrate the relationships between microstructure, mechanical properties, and fracture properties explicitly, mainly because of the fact that appropriate parameters are not available to universally quantify the microstructure. Except for the grain size, which has been used effectively in the Hall-Petch relation to correlate well with yield strength, no other unambiguous parameter that is able to represent the various constituents of the microstructure and their complex distribution is unanimously acceptable. The understanding of the interplay is therefore employed in an intuitive or heuristic manner to model the fatigue and fracture resistance of materials. A rigorous quantitative framework for controlling and estimating fatigue and fracture properties, however, is still evolving.
In order to explore the role played by the microstructure in deformation and fracture, a material system has to be systematically engineered to produce a variety of microstructures with evolving fractions of certain constituents. At NML, controlled microstructures in a Cu-strengthened HSLA steel, capable of exhibiting various strengths and ductility combinations during deformation and fractures that are ductile to varying degrees, have been employed to reveal the role played by the microstructure. This will be discussed in the lecture. The microstructural modifications in a 304LN stainless steel during monotonic and cyclic deformation will also be highlighted to elucidate how stress induced transformation of the microstructure control the development of substructural features and fracture morphologies.
Characterization of Materials by Transmission Electron Microscopy
G. K. Dey
Materials Science Division
Bhabha Atomic Research Center
Mumbai 400 085.
The transmission electron microscope (TEM) is a very versatile tool for probing the different aspects of microstructure of a material. Starting from the basic information like morphology of the phases and the number of phases it can provide very specific and otherwise difficult to get data about the local composition and even the chemical state of the atoms present in the specimen.
Structural information can be obtained from the TEM indirectly by making use of the various diffraction techniques available in the TEM. Direct information about structure is obtainable by high-resolution electron microscopy (HREM). It can reveal the nature of crystallographic defects. TEM is capable of yielding composition analysis at nano level. It is very suitable for examining the initial stages of transformation. It is equally powerful in analyzing the structure of interfaces.
A recently developed technique known as fluctuation microscopy in the TEM has been found to be very useful in ascertaining the medium range order in amorphous materials. With its multifaceted capabilities such as nano-beam diffraction and composition analysis and imaging abilities at angstrom level, it has emerged as an instrument for complete characterization of microstructure of materials. The modern TEM has been taken to new heights of performance with the aid of the aberration correction and monochromator technologies.
This presentation describes the different abilities of a modern TEM. The improvements in these capabilities with aberration correction and monochromator technologies have been described. The level of advancement in the country vis-a-vis the rest of the world in this area is discussed. Examples have been cited from electron microscopy of a wide variety of materials.
Recrystallization Textures of FCC Materials-Revisited
Sandip Ghosh Chowdhury and Pinaki Prasad Bhattacharjee
Materials Science & Technology Division
National Metallurgical Laboratory
Jamshedpur 831007, India
Understanding the origin and development of prominent crystallographic textures in FCC materials following heavy deformation and annealing is often quite critical for engineering the end properties of the materials. In this work the origin and development of recrystallization textures have been critically discussed with special reference to Al alloys and austenitic steels with widely different stacking fault energy.
The most important recrystallization texture component in Al alloys for beverage can applications is the cube component ({001}<100>). The presence of second phase particles in these alloys can give rise to particle stimulated nucleation (PSN) and may significantly affect the quality of final recrystallization texture. The two prevailing theories, namely, the oriented nucleation (ON) and oriented growth (OG) have been discussed in the context of the formation of recrystallization texture in these alloys.
On the other hand, in austenitic steels, the formation of recrystallization texture is dominated by the formation of twin chains leading to gradual weakening of texture. The recrystallization texture of the two materials quite adequately represents the complexities in the development of the recrystallization textures and substantiates the necessity for further research in this area.
Microstructural effects on the properties of Al and Ti-alloys used for aerospace applications
M. Sujata
Materials Science division
National Aerospace Laboratories
Bangalore-560 017
Structural materials used for aerospace applications require a certain balance of physical and mechanical properties for safe and efficient use. The selection of a material/alloy in these cases is generally guided by the design requirements for specific applications. In aerospace industries, the choice of the material is based on a trade-off between physical properties such as density and melting point, and the mechanical properties. Often, a particular alloy is used in different microstructural conditions to maximize the performance depending on the application environment. In such cases, the chemistry and the morphology of the microstructural constituents of the alloy are controlled during the secondary processing stages such as thermo-mechanical processing and heat treatment. The morphology and distribution of constituent phases in the microstructure are tailored during the heat treatment to obtain the desired mechanical properties. This talk deals with high strength Al-alloys (2XXX and 7XXX series) and Ti-alloys wherein the effect of processing on the microstructural evolution and their effects on the mechanical properties such as toughness, fatigue and stress corrosion cracking will be highlighted. The use of basic phase diagram(s) in the selection of secondary processing parameters and heat treatment schedule will be brought out with a few practical examples.
Microstructure and Properties of Light Metals and Alloys
Subodh Kumar
Department of Materials Engineering
Indian Institute of Science
Bangalore 560 012.
The lecture will start with the definition and classification of light metals. The mechanical properties of selected light metals will be reviewed. Then, the ways to improve these properties by microstructural changes imparted by alloying additions, processing and heat treatment will be discussed.
Crystallographic texture -- An integral part of microstructural engineering
R.K.Ray
It has been known for a long time that the properties(mechanical, electrical and others) of a material are very much functions of its microstructure. Therefore, in order to achieve the right kind of property, one has to engineer the microstructure of a material by whatever processes one can. For a polycrystalline material, the term "microstructure" usually refers to the aggregate of grains or crystals the material is made up of, how the grains look like, their sizes, the nature of the grain boundaries and the various features in the grain interiors. The information which is missing from from this description is about the crystallographic orientations of the individual grains. The collective knowledge of the orientations of the aggregate of grains in a material is what is defined as the "Texture" of the material. If it so happens that the orientations of the different grains are present in a random manner, then the texture of the material is considered as random, that is the material does not possess any texture at all. If, on the other hand, the grain orientations are not random, but show distinct patterns, then the material is considered to have a texture or preferred orientation.
It is now known that any property of a material is dependent not only on its microstructure, but also on its texture. It has therefore become essential now a days to routinely evaluate the texture of a material, in addition to determining its microstructure, in order to come to any meaningful conclusion about the properties of the said material.
This presentation will deal with the basics of texture, such as the definition of texture and the various techniques of representation and measurement of texture. The utility of texture measurement along with the strong correlation that exists between texture, microstructure and property of a material will then be demonstrated with the help of a few case studies.
Beyond Kirkendall
Aloke Paul
Department of Materials Engineering
Indian Institute of Science, Bangalore
The innovative experiment that was conducted by Kirkendall and his student Smigelkas changed the mindset of the researchers working in the area of solid state diffusion. They first time showed that different species diffuses with different rates. Following the concept of direct exchange and ring mechanism as possible substitutional diffusion mechanism were discarded. Their experiment indicated that diffusion rather occurs by vacancy mechanism. In next two decades, many theories were developed and the relations to determine different kinds of diffusion parameters were established. Further impetus in this area is being noticed with the experimental finding that the inert markers used can be stable and sometimes unstable (no particular plane!). Moreover, on certain conditions more than one marker plane can be found. These findings helped us to develop a physico-chemical approach following which we can determine the diffusion parameters with the added advantage that one can explain/predict the morphogenesis of the interdiffusion zone. This will further help us to understand the effect of stress and electric field on diffusion of elements in thin films.
TRIP-aided steels for auto body applications
Shiv Brat Singh
Department of Metallurgical and Materials Engineering
Indian Institute of Technology, Kharagpur
Kharagpur-721302, India
A major portion of the car body (≈ 60 %) is made of steel. However, in recent years, the demand for lighter vehicles with reduced fuel consumption has led to increasing use of new and lighter materials (Al and Mg alloys, plastics). But the mechanical properties of these new materials are much inferior compared with steels. Besides, the weight of the automobile can be reduced appreciably without compromising on cost, passenger safety and comfort by substituting conventional low strength steels with higher strength steels. Evidently, the superior strength makes it possible to use steel products of thinner gauges, resulting in reduction of the weight of the vehicle while retaining or indeed improving its rigidity and crashworthiness.
Conventional high strength steels (HSS) have inferior ductility that decreases almost linearly with increasing strength. During the past few years, a lot of research has been carried out to develop new varieties of high strength steels designated as Advanced High Strength Steels (AHSS) that combine outstanding strength and formability properties. In contrast to conventional HSS such as CMn steels, high strength interstitial free (HS-IF) steels, bake hardenable (BH) steels and high strength low alloy (HSLA) steels, AHSS derive much of its properties from their dispersed multiphase microstructure (at least two different microstructural components). The AHSS family consists of dual phase (DP) steels, transformation induced plasticity (TRIP) aided steels, complex phase (CP) steels and martensitic (MART) steels.
Microstructure and concomitant mechanical properties of various AHSS grades will be described with particular emphasis on TRIP aided steel which is the focus of the present work. The microstructure of TRIP aided steels typically consists of a continuous ferrite matrix with 25-35% bainite and 5-20% metastable retained austenite as other phases. During deformation, the dispersion of hard second phase in soft ferrite creates a high initial strain hardening rate. Progressive strain-induced transformation of retained austenite to martensite helps in maintaining high strain hardening rate even at higher strains unlike in DP steel where strain hardening rate diminishes at higher strains. This high strain hardening rate delays the onset of necking and ultimately leads to higher uniform and total elongation. TRIP aided steels exhibit low YS to TS ratio like DP steels, but at equal TS their total elongation (TEL) is much higher. It is important to note that the transformation strain per se, resulting from austenite to martensite transformation, cannot account for high uniform elongation of TRIP aided steel.
Deformation induced transformation of retained austenite is the most important aspect of TRIP aided steels which determines its unique mechanical behaviour. An in-depth study was carried out to examine the combined effects of forming conditions like strain, strain rate and temperature on the deformation induced transformation behaviour of retained austenite and hence on the mechanical properties of the TRIP aided steels under investigation. Tensile tests were carried out at different strain rates at room temperature and at 150 oC. Intermittent tests were carried out at intermediate strain rates to estimate the extent of deformation induced transformation of austenite to martensite. The mechanical properties of the TRIP aided steels were found to be significantly better when the transformation of retained austenite to martensite was slow, gradual and spread over the entire straining regime. Rapid transformation at lower strains led to inferior mechanical properties.
It is important to develop a mathematical model for the strain-induced transformation of retained austenite. This would not only help in a better understanding of the transformation behaviour of retained austenite but also aid in a better design of the microstructure of TRIP aided steels. A large number of empirical as well as semi-empirical models have been developed over the years to describe and predict the variation of retained austenite with strain. A comparative study of the different available models will be presented in this paper. An attempt will be made to interpret the results of various models in terms of metallurgical theory of stress or strain induced transformation of austenite to martensite.
Microstructural evolution in steel welds
Md. Zaheer K. Yusufzai, R. Prasad and S. Pandey(*)
Department of Applied Mechanics and
(*) Department of Mechanical Engineering
I.I.T.-Delhi
New Delhi-110016
Basics of weld solidification and subsequent transformations that lead to the development of microstructure in steel welds will be discussed. Microstructure obtained in friction stir welding of mild steel from our work at Delhi will also be presented.
Modelling elastic stress driven morphological instabilities
M.P. Gururajan
Department of Applied Mechanics
Indian Institute of Technology - Delhi
New Delhi - 110 016
The problem of a non-hydrostatically stressed solid in contact with a melt is very old and was, in fact, studied using variational techniques by Gibbs during the second half of 1870s. At present, it is widely believed that the elastic stress driven morphological instabilities (also known as Asaro-Tiller-Grinfeld instabilities) play a key role in the microstructural evolution in materials as diverse as solid-liquid interface in quantum He-IV, dislocation-free Stranski-Krastanov growth patterns in semi-conductor quantum dots, and crack patterns in polymeric thin films and stressed minerals. In this presentation, after a review of some of the important theoretical concepts and their experimental verification, I will discuss the phase field modelling of ATG instabilities in thin film assemblies.
Surface Microstructure and texture evolution during phase transformation annealing in low carbon steels
J. Gautam (1,2,3) R. Petrov(2), Elke Leunis (4) and LA.I. Kestens (2,3)
(1) Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
2) Department of Materials science and Engineering, Ghent University, Technologiepark 903, Ghent, B-9052, Belgium
3) Department of Materials science and Engineering, Mekelweg 2, 2628CD, Delft, The Netherlands
4) OCAS N.V., Arcelor Mittal Research Industry, Zelzate,Ghent, Belgium
The austenite-to-ferrite phase transformation, which is an inherent feature of low-alloyed ultra low carbon steels, has scarcely been investigated to control surface texture and microstructure evolution. This paper investigates the systematic evolution of texture and microstructure at the metal-vapour interface during interrupted annealing in vacuum. Interrupted annealing experiments were carried out on three ultra low carbon steel sheets alloyed with Mn, Al and Si. The texture and microstructures have been investigated by X-ray diffraction and SEM-EBSD techniques. These results reveal a very clear variation in the surface texture components as well as in the surface microstructure after BCC recrystallisation and double a-g-a transformation annealing. The transformation texture at the surface exhibits a <100>// ND fibre in combination with components of the <110> //ND fibre. It has been revealed that the latter specific surface texture was present in a monolayer of outer surface grains which were in direct contact with the vapour atmosphere. This observed phenomenon could be explained by considering the role of surface energy anisotropy occurring during phase transformation annealing.
Mobilities of species and the growth of the superconductor Nb3Sn phase in Nb/Cu(Sn) diffusion couple
A. Kiran Kumar and A. Paul
Study on Nb3Sn intermetallic compound with A15 structure has drawn renewed interest because of its ability to function at low temperatures and can sustain higher current density with high critical temperature and high upper critical field. Since intermetallic compounds are brittle and cannot be drawn as wire, Nb3Sn is grown by interdiffusion process between the (Cu-Sn) bronze alloy and Nb. The position of the Kirkendall pores in the interdiffusion zone indicated that Sn is faster diffusing species though the product layer, which is rather unusual considering the atomic mechanism of diffusion. Further, we noticed that with very minor change in the Sn concentration in the bronze alloy, the growth rate of the product phase and the activation energy for diffusion changes dramatically. By utilizing combined thermodynamic and kinetic analysis, we have explained the unusual behavior, as it was felt, in this system.
Effect of strain rate and temperature on compression behaviour of a near eutectic Al-Si alloy
Sudha Joseph and Subodh Kumar
The deformation behaviour of a heat treated near eutectic as-cast Al-Si alloy has been studied over a range of strain rates (3x10-4s-1 to 100s-1 with an interval of one decade) and temperatures (RT, 100°C and 200°C) under uniaxial compression. It was found that the flow stress of the alloy is strain rate sensitive and the strain hardening rate is low for this alloy. The morphology of eutectic Al-Si is found to play an important role in the compressive deformation behaviour of the alloy. Fracture and debonding of silicon particles is an important aspect of damage evolution in this alloy. In addition, fracture of intermetallic particles is also observed. The overall fracture mechanism of the alloy was studied in this work.
Microstructural evolution and phase formation in laser ablation-deposited Nb/Si multilayers
Sanjay Kashyap and K. Chattopadhyay
Metal silicides films have been the subject of numerous investigations because of their potential use in structural applications as well in microelectronic industries. In the present investigation, we have done the detail microstructural study of Nb/Si multilayers using transmission electron microscope (TEM). Our main focus is on microstructure, phase evolution, atomic stacking (at interfaces) and coherency. At as deposited conditions Nb layer is polycrystalline in nature where as Si layer is amorphous. We have carried out the diffusion studies at preliminary stage of these films. For this we have annealed multilayers at different temperatures and observed under microscope to see the phase formation and interdiffusion layer. We have observed continuous formation of NbSi2 layer. There is depletion in the thickness of Si layers and increment in Nb layers, which shows the Nb is diffusion fast than Si. We have calculated the activation energy for these multilayers which is 56 kJ/mole; suggest that grain boundary diffusion plays a role in the growth of the silicide layers.
Laser Surface Modification of Creep Resistant Magnesium Alloy MRI 230D
George Rapheal and Subodh Kumar
Creep resistant Mg alloy MRI 230D was subjected to laser surface melting and laser cladding with Al and Al+Al2O3 using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. Laser surface melting was found to be beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarization resistance and AC impedance measurements confirmed that the polarization resistance values of laser melted alloy were twice as high as that for the untreated alloy. The improved corrosion resistance was attributed to the absence of the second phase Al2Ca at the grain boundary, microstructural refinement and increased solid solubility, particularly of Al, in α-Mg matrix owing to rapid solidification. Laser melting was found to be beneficial in increasing surface hardness and wear resistance considerably due to grain refinement and solid solution strengthening. Laser Cladding with Al exhibited poorer corrosion resistance than Substrate, while Cladding by a two-step process with Al and Al+Al2O3 exhibited slightly better corrosion resistance. This can be attributed to closure of cracks and pores created by the first cladding by the subsequent one. EDS measurements revealed dilution of clad layer with Mg from substrate and this may be the additional reason for the poor corrosion performance. Improvement in wear resistance after laser cladding is due to the presence of hard Al2O3 particles and increased solid solubility of Al in the clad layer due to rapid solidification.
Creep, wear and corrosion behaviour of AE42 magnesium alloy and its composites
A. K. Mondal and S. Kumar
Department of Materials Engineering
Indian institute of Science, Bangalore-560012.
A creep-resistant AE42 magnesium alloy and its composites reinforced with Saffil short fibres and SiC particles in various combinations have been investigated for their creep, wear and corrosion behaviour. All the composites exhibit lower creep rate as compared to the AE42 alloy. The creep resistance of the hybrid composites, in which Saffil short fibres are partially replaced by SiC particles, is observed to be comparable to that of the composite reinforced with Saffil short fibres alone. Wear rate of all the composites is found to be lower than the alloy. Wear rate progressively decreases with the partial replacement of Saffil short fibres by SiC particles. The AE42 alloy exhibits the best corrosion resistance and the addition of the Saffil short fibres and/or SiC particles in the AE42 alloy deteriorates its corrosion resistance. There is no systematic trend of corrosion resistance with SiC particles content. The creep resistance of the hybrid composites is comparable, wear resistance is better and corrosion resistance is slightly inferior to the composite reinforced with Saffil short fibres alone. Therefore, from the commercial point of view, the use of the hybrid composites, replacing a part of the expensive Saffil short fibres by cheap SiC particles, is beneficial.
Enhancement in lattice expansion of Bi & Ag and depression in melting point of embedded Bi due to size effect during mechanical milling
S. Chitra and K. Chattopadhyay
The behavior of nano Bi particles embedded in Ag matrix synthesized using the composition Ag 5.3 at% Bi from elemental powders by means of planetary ball mill was investigated. Particle size and lattice parameter estimations using X-Ray Diffraction and Transmission Electron Microscopy were carried out on powders collected at different milling hours. There observed a lattice expansion in Ag and Bi as a function of size reduction. It was found that supersaturated solution of Ag forms once Ag and Bi reaches a critical size of 10nm and 13nm respectively even though the heat of mixing between Ag and Bi is positive (2.1KJ/mol). Differential Scanning Calorimetric analysis with multiple heating and cooling cycles was performed to study the first order phase transformation of the embedded Bi particles in Ag matrix. The onset of the decomposition of the solid solution occurs at 383 K. There observed melting point depression of the embedded Bi as a function of size reduction can be attributed to Gibbs-Thompson effect i.e the melting temperature shows an approximate linear relationship with reciprocal of the crystallite size.
Microstructure and Texture Evolution by Severe Plastic Deformation of Mg and Mg alloy
Somjeet Biswas
Non-conventional processing by severe plastic deformation (SPD) such as equal channel angular extrusion (ECAE), multi-axial forging (MAF), accumulative roll bonding (ARB), high pressure torsion (HPT) etc. can be used to fabricate bulk nano-structured material with advanced properties. Conventional processing techniques like rolling, forging, extrusion etc. are used to fabricate metals and alloys at industrial scale, however, could not produce materials with properties comparable to SPD processes. Though not yet integrated in industries, these methods have tremendous potential to replace or to be applied prior or posterior to conventional processing. In this presentation, the concepts and principles of SPD processes such as ECAE and MAF will be discussed as they have been applied to produce bulk nano-structured metals and alloys particularly magnesium and magnesium alloys. Main emphasis is laid on the relationship of microstructure and texture with the property of these SPD processed materials.
Effect of grain size on deformation texture evolution in nickel
Nilesh Gurao and Satyam Suwas
Department of Materials Engineering
Indian Institute of Science
Bangalore 560 012.
Texture evolution during large strain deformation imparted by cold rolling of pulse electrodeposited nanocrystalline (grain size ~ 20 nm), macrocrystalline nickel (grain size ~ 500 μm) and oligocrystalline nickel was studied experimentally as well through crystal plasticity based visco-plastic self consistent simulations. It was found that the course of texture evolution in nanocrystalline nickel is different from that of normal grain material due to (i) the activation of additional grain boundary dominated mechanisms and
(ii) grain growth. Dislocation annihilation mechanism like mechanical recovery has been found to play an important role during the course of deformation in this grain size range. The characteristic texture evolution in this case is an indication of normal slip mediated plasticity in nanocrystalline nickel. A higher volume fraction of the characteristic Brass component was observed in nanocrystalline as well as macro and oligo crystalline nickel which is unlike that observed in normal grain high Stacking Fault Energy nickel. The exact causes for similar texture evolution in the extreme grain size ranges will be discussed.
An EBSD investigation of texture and microstructure development in titanium alloys
Shibayan Roy and Satyam Suwas
Department of Materials Engineering
Indian Institute of Science
Bangalore 560 012
The EBSD technique can effectively determine the underlying mechanisms responsible for microstructure and texture development during thermo-mechanical as well as thermal treatments of materials as this technique has the unique benefit of coupling the orientation information with the microstructural details. In order to emphasize the capabilities of EBSD technique, the microstructure and texture development in titanium alloys has been investigated in the present study and complex mechanisms responsible for the same has been explained.
Deformation – Thermal Coupling Effect on Grain Growth in Nano-Nickel
M.J.N.V. Prasad and Atul H. Chokshi
Department of Materials Engineering
Indian Institute of Science
Bangalore - 560 012.
The high strength property of nanocrystalline metals and alloys at room temperature decreases rapidly on thermal treatment as well as during deformation due to their poor microstructure stability. Microindentation studies were carried out on electrodeposited nano-Ni as a function of indentation load and test temperature. At room temperature the hardness of nano-Ni gradually decreased with indentation load and then saturated at higher loads. The critical indentation load for saturation decreased with increasing temperature. Microstructure evolution beneath the indenter by focused ion beam (FIB) technique reveals thermally activated deformation induced grain growth in nano-Ni.
Influence of weak electric field on grain growth in tetragonal zirconia
Santonu Ghosh(1), Atul H. Chokshi (1), Pilhwa Lee (2) and Rishi Raj (2)
(1) Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012.
(2) Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309.
Grain growth in nanocrystalline yttria stabilized tetragonal zirconia was significantly retarded under an influence of very weak electric field. Grain growth experiments were carried out at 1573K for 10 hrs. under an applied electric field in the range 4 V/cm to 100 V/cm. It was interesting to notice an existence of a threshold electric field above which an enhanced grain growth was observed. The data were compared with the grain growth experiments carried out without electric field. The retardation in grain growth was attributed to the reduction in the driving force for grain growth under an applied electric field. This study adds a new dimension to the rate processes in ceramics such as sintering and superplasticity and promises a potential energy saving step during high temperature processing of ceramics.