Dislocations interacting with precipitate - interpretation bhadeshia123 2024-10-23 | Dislocations interacting with precipitate - interpretation
Phase field modelling of microstructure in multicomponent alloys bhadeshia123 2023-03-13 | Professor Nils Warnken’s research currently focuses on the study and modelling of phase transformations in metallic alloys, particularly solidification-related phenomena. This includes thermodynamic modelling and other techniques, but also the modelling of mould filling and convection in castings, microstructure formation in solidification during additive manufacturing and conventional castings. This talk covers the background to the phase field modelling method, explained in a way that is understandable to a general audience in higher education. He gives examples of the innovative use of the method, together with the development of non-equilibrium phase field formulations.The seminar is organised and chaired by Associate Professor Prakash Srirangam Venkata, as a part of a series of seminars at the Warwick University Manufacturing Group.
Steeling the show - creating exceptional alloys of iron : the inaugural lecture of Harry Bhadeshia bhadeshia123 2023-03-09 | Harry Bhadeshia's inaugural lecture, Queen Mary University of LondonSir Harry discusses the often haphazard yet remarkably graceful motion of atoms within solid iron, and how his work seeks to exploit these properties to build inspiring new physical forms, such as the world’s first bulk-produced nanostructured alloy.A highly respected figure in the field of metallurgy, Sir Harry has authored and co-authored over 650 papers and several books on the subject. As a fellow of the Royal Society and the Royal Academy of Engineering, Sir Harry has also been awarded the Bessemer Gold Medal by the Institute of Materials, Minerals and Mining and was appointed the first Tata Steel Professor of Metallurgy in 2008. Sir Harry’s numerous collaborations include partnering with British Steel to develop alloys for the Channel Tunnel rails and working on high-performance armour steel with the Ministry of Defence.Sir Harry’s lecture is part of our Night of Science and Engineering event, an evening hosted by Queen Mary’s Faculty of Science and Engineering in celebration of our latest cutting-edge research. Guests will have the opportunity to take part in demonstrations and immersive experiences showcasing our work, whilst networking with our academics and distinguished partners.phase-trans.msm.cam.ac.uk
Phase transformations from a modelling perspective bhadeshia123 2023-02-17 | A talk by Professor D. Ernst Gamsjäger, Montan Universität Leoben (Austria), delivered as a part of the ``Metallic Microstructures European Lectures Online" series.Information about stability ranges for various phases is crucial for materials design and process engineering. These stability ranges are represented by phase diagrams, which are constructed by means of appropriate modelling approaches using thermodynamic databases.In the first part of the lecture the Debye-Einstein fitting approach for pure components and end members is compared with the frequently used temperature polynomials. Modelling of non-equilibrium processes also benefit from equilibrium thermodynamics. A thermodynamic system not far from equilibrium can be divided into a sufficiently large number of sub-systems, where local equilibrium is assumed to hold in each sub-system. As long as the neighbouring sub-systems are in a different local equilibrium state a thermodynamic force (e.g. a difference in chemical potentials) drives a thermodynamic flux (e.g. the diffusive flux of a component in the system).Irreversible state changes will be considered in the second part of the lecture where the kinetics of growth and shrinkage of oxide crystals is investigated. The interfacial reaction and diffusion in the liquid bulk are considered as possible rate-controlling processes. The modelling approach combined with data analysis from key experiments reveals the underlying dissipative processes of solidification and melting phenomena, here, diffusion in the liquid bulk material and/or the interfacial reactions.Solid/solid phase transformations are the topic of the third part of the lecture. In particular the consequences of various heat treatments on the microstructure of steels is explored. Whereas dilatometry, metallographic investigations and hardness measurements help to distinguish between martensite, ferrite, bainite and pearlite, evaluating the X-ray diffractograms allows to quantify the phase fraction of austenite. The experimental investigations are evaluated by machine learning algorithms with the potential to relate microstructures to material properties. The classification of martensite, ferrite, bainite and pearlite is supported by an unsupervised machine learning algorithm (here hierarchical clustering) on the basis of measured X-ray diffractograms.
Virtual simulation of metal processes, Dr Zhanli Guo bhadeshia123 2023-02-16 | This talk is about the quantitative simulation of a variety of methods used in the processing of metals, for example, casting, forging, rolling, heat treatment, welding and additive manufacturing. Dr Guo explains that there remain problems in that the entire phenomena cannot as yet be truly vitrual and that there is a need for phenomena coupling. Dr Zhanli Guo received his BEng and MEng in Materials Science and Engineering from Tsinghua University, China in 1994 & 1997 respectively. His PhD degree was obtained from Queens University, Belfast in 2000 where he continued working as a post-doctoral researcher. He joined Sente Software Ltd. in 2003 focused on developing JMatPro®.He has written more than 100 technical papers, including over 40 journal publications and a book on Maraging Steels. Dr Guo currently runs the team responsible for the development of new models in JMatPro®.
Spinodal decomposition - a simulation bhadeshia123 2023-02-05 | A simulation of spinodal decomposition showing phase separation in an Fe-40Cr at.% alloy at 850 K. Movie courtesy of M. Honjo and Y. Saito of Waseda University in Tokyo, Japan. ISIJ International 40 (2000) 916--821.http://www.phase-trans.msm.cam.ac.uk/mphil/spinodal.movie.html
Mathematical modelling of cold rolling and beyond bhadeshia123 2023-01-11 | Associate Professor of Applied Mathematics at the University of Warwick; a joint appointment between the Warwick Mathematics Institute and WMG (formerly the Warwick Manufacturing Group). He is a UKRI "future leader" as well.Here he talks about the quantitative representation of the cold rolling of metals, including asymmetric rolling and some suggestions for progress in creating a better model of the process. The work should permit the processing of complex products, the use of recycled materials and less waste. There may be outcomes that lead to cheaper tooling and greater customisation.The seminar is organised and chaired by Associate Professor Prakash Srirangam Venkata, as a part of a series of seminars at the Warwick University Manufacturing Group.
Taipei 101 - new year celebrations, 2023, Taiwan bhadeshia123 2023-01-02 | Taipei 101 is the tallest building in Taiwan, and probably the most earthquake resistant tall building in the whole world. You can see more about its construction and metallurgy atphase-trans.msm.cam.ac.uk/2005/t101/t101.htmland also in the booklink.springer.com/book/10.1007/978-3-030-57612-7The video has kindly been provided by Professor Jer Ren Yang of the National Taiwan University.
Dance performance, Chowmahalla Palace, Hydrerabad, India bhadeshia123 2022-12-28 | Videos courtesy of Professor Jer Ren Yang of the National Taiwan University. Taken at the 12th Asia-Pacific Microscopy Conference.
Phase transitions - 9 bhadeshia123 2022-12-09 | Alloys of iron are by far the most successful structural material; there are simply no challengers for the vast majority of applications. Just one interesting example -- Elon Musk's largest rocket, the Starship, is made from steel, not aluminium nor carbon fibre composites.The vast majority of steels are in the austenitic condition attemperatures in excess of 900°C. Austenite has a cubic-close packedcrystal structure and tends to decompose into ferrite (body-centeredcubic) and cementite.This lecture deals also with precipitation hardened aluminium alloys of the kind used in aerospace engineering.Haixue YanMore information onphase-trans.msm.cam.ac.uk/teaching.html
Phase transitions - 8 bhadeshia123 2022-12-02 | We have seen that homogeneous nucleation is difficult. For example, in container-less experiments where a pure liquid is isolated from its environment by levitation or by allowing it to free fall over a long distance, it is possible to cool the liquid by an enormous amount (~200°C). In general, however, solidification proceeds from the surfaces of the container (the mould) into which the liquid is poured, by heterogeneous nucleation at the container surface. The way in which it proceeds, i.e. by the stable advance of a planar interface, or an interface which when perturbed develops instabilities, depends on the local conditions at the solidification front.Haixue YanMore information onphase-trans.msm.cam.ac.uk/teaching.html
High-entropy alloys for nuclear applications bhadeshia123 2022-11-21 | Dr Ed Pickering from the University of Manchester talks about the special properties of high-entropy alloys that make them potentially suitable for the construction of nuclear power plants.High-entropy alloys are those that comprise five or more principal alloying elements, often in near-equiatomic ratios. Their compositions are, therefore, quite different from conventional alloys, which tend to be based around a single component. For example, popular they may include CrMnFeCoNi, Al0.5CrCoFeNiCu, and VNbMoTaW. The expanded compositional freedom afforded by these alloys may represent a unique opportunity for nuclear applications, especially where current engineering alloys fall short. This talk will provide a critical review of the work done to date in the field of high-entropy alloys for nuclear applications, and discuss related challenges and opportunities. Specific topics for discussion will include: (i) whether there is evidence that the entropy-rich alloys possess ‘special’ irradiation resistance in comparison to conventional alloys, (ii) opportunities to ‘tune’ the composition for particular irradiation responses, and (iii) challenges associated with alloy design and manufacture for nuclear.This seminar is a part of a series of talks given at the Warwick Manufacturing Group of Warwick University, organised by Dr Prakash Srirangam.
Phase transitions - 7 bhadeshia123 2022-11-19 | Phase fluctuations occur as random events due to the thermal vibration of atoms. An individual fluctuation may or may not be associated with a reduction in free energy, but it can only survive and grow if there is a reduction. There is a cost associated with the creation of a new phase, the interface energy, a penalty which becomes smaller as the particle surface to volume ratio decreases. In a metastable system this leads to a critical size of fluctuation that must be achieved, beyond which growth is favoured.Haixue YanMore information onphase-trans.msm.cam.ac.uk/teaching.html
The Curie temperature bhadeshia123 2022-11-14 | On heating beyond the Curie temperature, a ferromagnetic material becomes paramagnetic. It therefore will not be attracted by a permanent magnet.(In a ferromagnet, the magnetic dipoles are aligned. At high temperatures, the entropy term dominates so they point randomly to create the paramagnetic state).H. K. D. H. Bhadeshiaphase-trans.msm.cam.ac.uk
Phase transitions - 6 bhadeshia123 2022-11-03 | Diffusion is at first sightly difficult to appreciate for the solid state. A number of mechanisms have been proposed historically. This includes a variety of ring mechanisms where atoms simply swap positions, but controversy remained because the strain energies associated with such swaps made the theories uncertain. One possibility is that diffusion occurs by atoms jumping into vacancies. But the equilibrium concentration of vacancies is typically 10^{-6}, which is small. The theory was therefore not generally accepted until an elegant experiment by Smigelskas and Kirkendall.This talk deals with the mechanism of diffusion, for both interstitial and substitutional solutes. It ends with an understanding of uphill diffusion where the chemical potential gradient opposes that of the concentration.More information onphase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Phase transitions - 5 bhadeshia123 2022-10-27 | Diffusion is often described as "the spreading of things" but it can in fact involve "un-spreading" if the dependence of chemical potential on the composition is appropriate. We begin with a consideration of Brownian motion and the root mean square distance achieved during a random walk. This distance is found to change with the square root of time, rather than a linear dependence. The difference between Brownian motion and diffusion is that the latter can be directed (oriented), i.e, non-random motion. This leads to Fick's first law of diffusion, which is used widely, for example in considering the passage of medical drugs through cell membranes. Fick's first law relies on a flux that is proportional to a constant concentration gradient, with the proportionality constant being the diffusion coefficient. When the gradient is not constant, the concentration at any point should change with time. This leads to Fick's second law of diffusion, with the classical exponential solution (e.g., the penetration of lithium into copper) and to the error function solution (e.g. the penetration of chloride ions into concrete.We note finally that an effective diffusion coefficient suffices when dealing with heterogeneous substances, such as the cell membrane or concrete, both of which contain structure.Download lecture notes from:phase-trans.msm.cam.ac.uk/2022/EMS523U_book.pdf
Phase transitions - 4 bhadeshia123 2022-10-20 | The idea of equilibrium has many connotations - for a pure substance, phases in equilibrium have identical free energies. In multicomponent systems, the chemical potential of each component must be uniform everywhere, i.e., equal in every phase present, to achieve equilibrium even when the chemical compositions of the phases are different. These concepts can be used to construct equilibrium phase diagrams that delineate domains (of temperature, pressure, composition) where a particular phase is the most stable. Phase boundaries between two phases then identify the locus of all points where the two phases are in equilibrium, and triple points similarly define the conditions where three phases are simultaneously in equilibrium. In some cases, solid and liquid phases are able to a accommodate any amount of solute, leading to a simple phase diagram. This is not always the case, for example when a eutectic reaction occurs.However, phase diagrams become difficult to visualise, and therefore to get quantitative information from, when dealing with higher than ternary alloys. But all the information that we have in phase diagrams can be expressed by simple numbers for any number of components and phases. To be able to use elementary thermodynamics for creating concepts and materials is the epitome of imagination. Think about the entropy of mixing as not dealing with atom, but with particles that each contain billions of atoms. How might we calculate the microstructure of a non-equilibrium phase? All this and more is addressed in this lecture, ending with the physical meaning of chemical potential and its implications on the meaning of equilibrium.Download lecture notes from:phase-trans.msm.cam.ac.uk/2022/EMS523U_book.pdf
Phase transitions - 3 bhadeshia123 2022-10-13 | To be able to use elementary thermodynamics for creating concepts and materials is the epitome of imagination. Think about the entropy of mixing as not dealing with atom, but with particles that each contain billions of atoms. How might we calculate the microstructure of a non-equilibrium phase? All this and more is addressed in this lecture, ending with the physical meaning of chemical potential and its implications on the meaning of equilibrium.Download lecture notes from:phase-trans.msm.cam.ac.uk/2022/EMS523U_book.pdf
Phase transitions - 2 bhadeshia123 2022-10-07 | Pippard, in his wonderful book "Elements of Classical Thermodynamics", states that "the function of thermodynamics is to link together the many observable variables so that they can all be seen to be a consequence of a few". In this lecture, I begin with this quote and explain with an example calculation, how a lot of variables can be expressed in terms of a few thermodynamic quantities to enable the minimisation of experiments.We cover the meaning of basic thermodynamic quantities such as internal energy, enthalpy and entropy, with a derivation of configurational entropy of mixing. Download lecture notes from:phase-trans.msm.cam.ac.uk/2022/EMS523U_book.pdf
Geiger counter output from X-ray source, at constant diffraction angle bhadeshia123 2022-10-05 | Shows that the diffracted X-ray intensity is not constant because the emissions are stochastic. The Geiger counter was set to measure diffracted intensity at a constant two theta angle, over a period of time.phase-trans.msm.cam.ac.uk
Phase transitions - 1 bhadeshia123 2022-10-01 | To paraphrase Cottrell, "there are three main frontiers of science today. First, the science of the very large, i.e., cosmology. Second, the science of the very small, the elementary particles of matter. Third, and by far the largest is the science of the very complex, which includes materials science." In fact, the complexity of structures in a materials is impressive; its control by manipulating phase transformations helps achieve a huge range of useful properties.A "phase transformation" involves a change in atomic arrangement but there may also be a change in chemical composition. An understanding of these changes is essential in the design of new materials, and the optimisation of existing materials for challenging conditions.This is the first of a set of lectures designed to introduce the concepts to undergraduates. The course is given by Harry Bhadeshia and Haixue Yan at Queen Mary University of London.Download lecture notes from:phase-trans.msm.cam.ac.uk/2022/EMS523U_book.pdfp.s. the image of Nikola Tesla is in fact of Dimitri Mendeleev. Tesla's image can be found on phase-trans.msm.cam.ac.uk/2015/classic/index.html
Atomic mechanism of acicular ferrite transformation in steels bhadeshia123 2022-09-28 | Non-metallic inclusions are anathema when it comes to the design of strong steels because they become the initiation sites for fracture. Huge efforts have been made devoted to making clean steels - the oxygen concentration of a hard bearing steel is routinely less than 10 ppm. However, there are other structural steels that have to be welded where the localised heat input generates microstructures in the heat-affected zone that are undesirable. The alloys that are used to deposit the weld must have good properties in the as-cast state. In both of these circumstances, specific non-metallic inclusions are a positive boon in that they provide substrates for the intragranular nucleation of bainite. As a consequence, highly organised sheaves of bainite are altered into a more chaotic arrangement that frequently deflects propagating cracks and hence enhances the toughness. This is the so-called acicular ferrite that is the subject of this lecture.
Validation of heat-treatment simulation bhadeshia123 2022-09-07 | Kyozo Arimoto presented this talk at the HT2021 meeting held in USA. The work has been recognised as an `ASM Heat Treating Society Prime Contributor'. Kyozo Arimoto has been a consultant at Arimotech Ltd since 2002; he is the founder of that industry. He was a mechanical engineer for Hitachi Shipbuilding Co. Ltd. after graduating from Osaka Prefectural Technical College in 1972. He then moved to Century Research Center Corporation in 1986 and created some specialised computer codes until 1997. After that he was involved in developing a heat treatment module of a code at SFTC, USA, for 4 years. He has been developing a system for determination of cooling characteristics of quenchants.Heat treatment simulation has progressed to the point where commercial software is widely available, and validations of simulation functions using experimental results has played a big role in getting there. Kyozo has a number of illustrative validation cases and can explain how simple experiments can provide insight into heat treatment and help validate calculations. This includes a treatment of distortion.
Multi-metal additive manufacturing: in-situ alloying, composition control and species mixing bhadeshia123 2022-08-08 | Additive manufacturing holds a great deal of promise in techniques for the manufacture of complex components, particularly when it comes metallic components. Chinnapat Panwisawa, Queen Mary University of London, is an expert on developing the quantitative understanding of the process, with a deep metallurgical basis.
Microstructural modelling of steels - the need to consider upstream requirements bhadeshia123 2022-07-14 | Martin Strangwood is affiliated with both the University of Birmingham and the University of Warwick (Warwick Manufacturing Group). This talk is to emphasise how academic research can be engineered to think about the needs of large-scale processing. The talk is nicely explained with the use of specific examples.The talk is a part of a series organised by Prakash Srirangam of Warwick University.
Rapid industrial adoption of new steels bhadeshia123 2022-06-23 | This talk, by Professor Claire Davis of the Warwick University's manufacturing group, deals with methods of designing new steels in a way that makes the idea consistent with industrial constraints, and hence enable rapid adoption. In a complex industrial process, it is necessary to account for mass production, factors such as chemical segregation, a whole plethora of other metallurgical phenomena that must be accounted for.The talk is a part of the series of lectures under the generic title "Metallic Microstructures: European Lectures Online", organised by KTH Stockholm, Gent University and Delft University of Technology.
Chemistry and processing of structural steels bhadeshia123 2022-06-15 | Professor Andrew Kostryzhev of the University of Queensland's Centre for Microscopy and Microanalysis explains his laboratories and facilities, in the context of metal processing, characterisation, mechanical and non-destructive testing and the development of new and improved technologies and materials.The talk is a part of a series of lectures held at Warwick University (Warwich Manufacturing Group), organised by Professor Prakash Siriangam.
22 scraps of paper bhadeshia123 2022-06-12 | An exhibit in Boston, by Arthur Granson, entitled 22 scraps of paper.
Electrochemical pathways towards deep decarbonization and profitable sustainability, Donald Sadoway bhadeshia123 2022-05-24 | Professor Donald R. Sadoway of the Massachusetts Institute of Technology, talks about the electrolytic production of liquid iron from iron ore. The technology is important in reducing the carbon dioxide burden of steel production, assuming that the electricity needed can be provided from "green" sources. The technology established has a strong potential of being upscaled into a commercial scenario, and the quality of the iron produced is said to be superb. The process has led to the establishment of "Boston Metal", a startup company with a wide spectrum of investors.The lecture was kindly organised by Professor Fabio Miani of the University of Udine in Italy.
Materials research and development at Rolls-Royce plc, by Neil DSouza bhadeshia123 2022-05-19 | Talk by Dr Neil D'Souza of Rolls-Royce plc, giving an overview of the variety of materials research in progress, in the aerospace, digital, nucelar, power systems, turbines, rotatives joining, coating, high-entropy alloys, etc. He also covers aspects such as fatigue, cyclic flexure and principles of alloy design.
Trolley lift pulled by pearlitic steel ropes, Zurich, Switzerland bhadeshia123 2022-04-14 | This is a system in which trolleys are pulled by steel wires bundled into ropes, up a steep incline that leads to a forest in Zurich, Switzerland.The ropes are made of very strong steel, by cold-drawing pearlite.http://www.phase-trans.msm.cam.ac.uk/phase-trans
Casting and solidification research at TATA Steel Europe bhadeshia123 2022-04-13 | Dr Begoña Santillana, Principal Scientist at TATA Steel Europe, presents an overview of the methods used in assessing whether new alloys will be susceptible to solidification cracking when cast.A combination of equilibrium thermodynamics and non-equilibrium solidification methods such as those embedded in the Scheil technique can be used to define parameters such as the expected solidification range. With this knowledge, and when temperature-depended mechanical properties are available, they can be used to either modify the solidification process or to change the steel composition in order to avoid difficulties as the material solidifies. The seminar is organised and chaired by Associate Professor Prakash Srirangam Venkata, as a part of a series of seminars at the Warwick University Manufacturing Group.
Electro-slag remelting (ESR) bhadeshia123 2022-03-28 | Professors Ganna Stovpchenko and Lev Medovar of the E. O. Paton Welding Institute and Elmet-Roll, Ukraine, describe the electro-slag remelting process. This is an important process for special steels and other metals used in critical applications. It uses an alloy as a consumable electrode to create a new ingot via a special slag. The consumable droplets travel through the slag to the bottom of a mold, with the slag having the function of cleaning the molten metal.
Steels: nanostructured alloys bhadeshia123 2022-03-16 | A nanostructured material is here defined as one containing an exceptionally large density of strong interfaces, rather than one which simply contains a minor fraction of features such as precipitates, which are small in size. The desire for such materials in the engineering context comes from the expectation of novel mechanical properties, particularly the stress that can safely be tolerated in service. It is difficult to invent such materials because any design must address three basic issues: (i) it should ideally be possible to make samples which are large in all dimensions, not simply wires or thin sheets;(ii) there are commercially available steels in which the distance between interfaces is of the order of 250--100~nm. The novelty is in approaching a structural scale in polycrystalline metals which is an order of magnitude smaller.(iii) The material concerned must be cheap to produce if it is not to be limited to niche applications. A good standard for an affordable material is that its cost must be similar to that of bottled water when considering weight or volume.An alloy system based on iron is described in which it has been possible to create a high density of interfaces by heat--treatment alone. The resulting structure consists of a mixture of slender platelets of bainitic ferrite, just 20--40\,nm in thickness, embedded in a matrix of carbon--enriched austenite. The rate at which this structure evolves is slow by conventional standards, but this permits components to be made which are large in all three dimensions, with uniform properties throughout. The fundamental mechanisms behind this novel nanostructured steel are reviewed, along with the factors determining its strength, ductility and fracture toughness. It is argued that although reasonable toughness can be achieved in the context of strength levels exceeding 2000\,MPa, the impact toughness remains poor and that it may not be possible to improve this particular parameter.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Phase transformations in heat-affected zones of ferritic heat-resistant steels bhadeshia123 2022-03-14 | A talk by Professor Cong Wang of the oxide metallurgy group at Northeastern University in China. After introducing power plant issues he describes the need for new creep-resistant materials that would be needed to increase steam temperatures in the context of thermodynamic efficiency. This in turn leads to welding, and problems associated with the microstructures within the heat-affected zones of the welds, leading to cracking. The focus then shifts to acicular ferrite that nucleates intragranularly in austenite grains containing non-metallic inclusions. Confocal laser microscopy is used to monitor the growth of plates. The properties in the heat-affected zones are also explained.
The Titanic disaster and the continuing effort to improve the impact toughness of ferritic steels bhadeshia123 2022-03-12 | Professor Debalay Chakrabarti of the Indian Institute of Technology Kharagpur, India, provides a historical context to the brittle failure of ferritic steels, and then goes on to explain the fundamental science of the ductile-brittle transition in steels. He contrasts, using dislocation theory, the difficulty in moving dislocations in ferrite against the relative ease in austenite. This explains why the ductile-brittle transition is generally absent in austenitic steels. Detailed fracture mechanics arguments are presented to estimate the fracture stress as a function of variables such as the grain size, hard-particle cleavage, stress triaxiality etc.Specific comments added after the lecture, on the role of manganese sulphides:Being softer than the matrix, MnS inclusions elongate along the direction of metal-flow during hot-deformation and that creates anisotropy in ductility in toughness, particularly affecting the properties along transverse direction of rolled / forged plates. It was mentioned that MnS decreases toughness at room temperature, but the accurate statement is that it significantly reduces the upper shelf energy level and thus brings down the entire transition curve towards lower energy side.
Steels: twinning-induced plasticity steels bhadeshia123 2022-03-11 | There are three essential modes by which steels can be permanentlydeformed at ambient temperature, without recourse to diffusion.Individual dislocations whose Burgers vectors correspond to latticevectors can glide, leading to a change in shape without altering thecrystal structure or volume. In contrast, a displacive transformation(e.g. martensite or bainite) results not only in a plastic strain, butalso a change of crystal structure and density; this is the phenomenonexploited in the TRIP steels. The third mode of deformation is mechanical twinning, in which thecrystal structure of the steel is preserved but the twinned region is{reoriented} in the process. Mechanical twinning results in a much largershear strain (0.7071...), compared with displacive transformationswhere the shear is typically 0.26. There is a particular class ofextraordinarily ductile alloys of iron, known as the TWIP steels, whichexploit mechanical twinning to achieve their properties.TWIP stands for twinning-induced plasticity. The alloys are austeniticand remain so during mechanical deformation, but the material is able toaccommodate strain via both the glide of individual dislocations andthrough mechanical twinning. The alloys typically contain a large amount of manganese, some aluminium and silicon (e.g. Fe-25{Mn}-3Si-3Al wt%) with carbon and nitrogen in some versions presentessentially as impurities. Larger concentrations of carbon may be added to enhance strength. At high manganese concentrations, there is atendency for the austenite to transform into epsilon-{martensite}(hexagonal close packed) during deformation.The alloys have a rather low yield strength at 200-300 MPa but the ultimate tensile strength can be much higher, in excessof 1100 MPa. This is because the strain-hardeningcoefficient is large, resulting in a great deal of uniform elongation,and a total elongation of some 60-95\%. The effect of mechanicaltwinning is two-fold. The twins add to plasticity, but they also have apowerful effect in increasing the work-hardening rate by subdividing the{untwinned} austenite into finer regions.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Discharge crucible method, Professor Hani Henein, University of Alberta bhadeshia123 2022-03-08 | There are many processes where the properties of the melt are important. It is necessary in computations and design to have reliable data on variables such as density, surface tension, viscosity etc. For example, viscosity is required to model convection and segregation during solidification; surface tension can control the mould filling ability of a melt.The discharge crucible method allows the simultaneous measurement of viscosity, surface tension and density in a simple manner.
Steels: TRIP-assisted steels bhadeshia123 2022-03-06 | Solid-state phase transformation during the course of deformation in tension, can retard the onset of plastic instability, i.e. the necking that leads to ultimate failure. The mechanism by which this occurs is explored, somewhat quantitatively, to illustrate that the transformation strains themselves make only a minor contribution. Instead, it is the production of high-carbon, hard martensite, that is responsible for enhancing the work-hardening rate that in turn delays the onset of necking. A variety of other issues are discussed, for example the contrast between TRIP steels and those that are TRIP-assisted.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Steels: overall transformation kinetics bhadeshia123 2022-02-26 | In calculating a volume fraction of transformation as a function of time, temperature and chemical composition, it is necessary to possess models for nucleation and growth of each of the product phases, based on their respective mechanisms of transformation. However, this is not sufficient to estimate the volume fraction, because growing particles will eventually collide and the nucleation of new particles cannot occur in already transformed regions. This "hard impingment" is taken into account in Avrami theory by calculating first, an extended volume where particles can grow into each other, and then correcting that by multiplying by the fraction of untransformed austenite. This theory is then generalised to multiple, simultaneous transformations and applied to continuous cooling transformation. A case study is presented at the end of how to use such knowledge to design a hydrogen-resistant steel.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Steels: Pearlite bhadeshia123 2022-02-24 | The two-phase mixture of ferrite and cementite that constitutes "pearlite" is a wonderful example of cooperative growth. The ferrite and cementite share a common transformation front with the austenite, and in Fe-C, the average composition of the pearlite is precisely the same as that of the austenite, meaning that growth occurs at a constant rate. The theory for the growth of pearlite is deriver, and its strength and toughness rationalised, both in terms of interlamellar spacing and nodule size. Some quite spectacular applications of pearlite are illustrated.The process and consequences of wire drawing and rope manufacture are also covered, including the classic Embury and Fisher equation.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Steels: ferrite bhadeshia123 2022-02-21 | The kinetics and mechanism of the allotriomorphic ferrite transformation is described for binary Fe-C alloys, with a brief discussion of idiomorphic ferrite. The thickness of ferrite is shown to increase with the square root of time, i.e., growth slows down as the ferrite thickens. This is because of the accumulation of carbon ahead of the transformation front. Thermomechanical processing, the limits to grain refinement and applications are also described.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Recent progress in micromechanics-based approaches to ductile fracture bhadeshia123 2022-02-20 | Lecture by Professor T. Pardoen of the Université catholique de Louvain, Belgium, discussing progress on the characterisation and modelling of void nucleation, and new non-local version of an extended Gurson model with shear and tensile coalescence yield surfaces.There are many interesting observations, including the fact that void nucleation on particles becomes more difficult at small particle sizes. The initial porosity has a large effect on the final fracture strain.
Widmanstätten ferrite bhadeshia123 2022-02-17 | The nature of Widmanstätten ferrite in steels is described, beginning with the definition of a "crystallographic grain size" that controls the path of la propagating cleavage crack. This is followed by a description of the atomic mechanism of transformation, in essence a carbon diffusion-controlled displacive mechanism in which the composition of the ferrite is given by paraequilibrium with the austenite. The lengthening rate of the plate of Widmanstätten ferrite is derived and shown to be consistent with experimental measurements. The idea that the plates are a consequence of interfacial instability in a carbon diffusion-field, as modelled using phase field theory is dismissed, one reason being that Widmanstätten ferrite is found in many interstitial-free alloys.Finally, there is a rationalisation of the shear transformations in steels (martensite, bainite and Widmanstätten ferrite), revealing that not all three occur in all steels.This series of lectures is organised by Professor Fabio Miani of the University of Udine in Italy.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Inclusion processing in steelmaking, implemented using electrical fields bhadeshia123 2022-02-15 | Professor Rongshan Qin of The Open University explains how the fact that the electrical properties of non-metallic inclusions contained in liquid metal can be manipulated by the application of electrical stimulation. Both experimental data and theory are described in some detail.This seminar is a part of a series of talks given at the Warwick Manufacturing Group of Warwick University
Bainite - the connection between theory and manufacture bhadeshia123 2022-02-12 | Theory on its own can be bland, but if used to create new steels, can be inspiring. It is demonstrated using specific examples, how the theory can enable the manufacture of these novel steels, on a large scale, in enlightened industry. In particular, the T-zero concept when combined with the physical basis of toughness in carbide-free bainitic steels has led to remarkable developments.This series of lectures is organised by Professor Fabio Miani of the University of Udine in Italy.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Computational thermodynamics and OpenCalphad, Bo Sundman bhadeshia123 2022-02-11 | Emeritus Professor Sundman describes the OpenCalphad project in which he creates the software that can interpret thermodynamic data and find equilibria , constrained equilibria and well-defined deviations from equilibrium, such as the T-zero concept. This is as a function of temperature, pressure and composition. The software is open access. The lecture is a part of a series organised by Professor Fabio Miani of the University of Udine in Italy
Steels 2022: bainite, Lecture 3 of 11 bhadeshia123 2022-02-11 | There is so much understood today about the bainite transformation in steels that it is now routinely possible to use the theory in order to design bainitic steels without doing many experiments. In this lecture, which is the first of two on bainite, I describe the choreography of atoms as austenite changes into bainite by a displacive mechanism in which the product phase then becomes tempered.This series of lectures is organised by Professor Fabio Miani of the University of Udine in Italy.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
Some steels research at the Universidad Federal Fluminense, Brazil bhadeshia123 2022-02-10 | After introducing activities related to steels at his University, Professor André Costa e Silva talks about improving the efficiency of steel production, the consequences of steel production and how to reduce the carbon burden due to steel production.The lecture is a part of a series organised by Professor Fabio Miani of the University of Udine in Italy
Real-time observations of transformations in steels bhadeshia123 2022-02-09 | Real-time observations can help determine problems that are otherwise difficult to prove. For example, the hgh-resolution determination of the growth rate of plates of bainite that are too fine to resolve using hot-stage optical or laser-confocal microscopy; the development of surface relief as a sample of austenite transforms by a displacive mechanism. Proof that austenite does not undergo a separation into carbon-rich and carbon-poor domains prior to phase transformation. Acoustic emissions that place a lower limit to the rate of transformation. The simultaneous real-time monitoring of the development of stress, dimensional change and phase change to design novel welding alloys that leave little in the way of residual stress on reaching ambient temperature.A lecture given by Harry Bhadeshia, for the Mettle'22 series organised by the National Institute of Technology, Tiruchirappalli, India.More detail onphase-trans.msm.cam.ac.uk
Steels 2022: martensite, Lecture 2 of 11 bhadeshia123 2022-02-05 | The crystallographic description and its mathematical expression is dealt with first, bringing a closure between the crystal structure change, the irrational habit plane, the irrational orientation relationship and the shape deformation. The is followed by a thermodynamic assessment that permits the calculation of transformation temperatures, and example applications that relate phase transformation theory to practical problems such as the design of strong and tough steels that can be mass produced. This series of lectures is organised by Professor Fabio Miani of the University of Udine in Italy.Associated teaching materials can be found on:http://www.phase-trans.msm.cam.ac.uk/teaching.htmlH. K. D. H. Bhadeshia
