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- 1. This lecture unit is part of a set created by Mike Ashby to help introduce students to materials, processes and rational selection. The Teaching Resources website aims to support teaching of materials-related courses in Design, Engineering and Science. Resources come in various formats and are aimed primarily at undergraduate education. Some of the resources are open access and students can access them. Others are only available to educators using CES EduPack. Mike Ashby Hugh Shercliff Department of Engineering University of Cambridge www.grantadesign.com/education/resources © M. F. Ashby, H.R. Shercliff, 2011 For reproduction guidance see back page Unidad 4. Manipulando las propiedades vía: química, microestructura, configuración
- 2. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Reseña Fuentes: • Libro: “Materials: engineering, science, processing and design” 2a Edición por M.F. Ashby, H.R. Shercliff y D. Cebon, Butterworth Heinemann, Oxford 2010, Capítulo 19. • Libro: Callister, Budinski, Askeland and others – lectura recomendada • Software: CES EduPack 2011 (Grantadesign.com) • Transparencias Powerpoint: Unidad 3 Base de Datos CES EduPack Elementos Fuentes: • Libro: “Materials: engineering, science, processing and design” 2a Edición por M.F. Ashby, H.R. Shercliff y D. Cebon, Butterworth Heinemann, Oxford 2010, Capítulo 19. • Libro: Callister, Budinski, Askeland and others – lectura recomendada • Software: CES EduPack 2011 (Grantadesign.com) • Transparencias Powerpoint: Unidad 3 Base de Datos CES EduPack Elementos Motivación: Necesidad de entender: el origen de las propiedades de los materiales y cómo modificarlas. Para explicar el origen y las modificaciones de las propiedades se utilizan: Módulo de Young y densidad Resistencia y tenacidad Coeficiente de expansión térmica y conductividad
- 3. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Elementos-densidad • Hay 96 elementos puros disponibles. ¿Cómo combinarlos para crear materiales para ingeniería? Densidad 20.000 Rango 1.000 Gráfico realizado con CES EduPack'10 Elementos . Versión no disponible en castellano
- 4. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Elementos- Módulo de Young Gráfico realizado con CES EduPack'10 Elementos . Versión no disponible en castellano Módulo de Young 1 1.000 Rango
- 5. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Origen: densidad y módulo de Young Densidad: peso atómico y factor de empaquetamiento Módulo de Young: fuerzas intermoleculares y factor de empaquetamiento
- 6. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Materiales: módulo de Young- densidad 20.0001.000 1.00 0 1
- 7. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Materiales compuestos: Configuración – Componentes, Capas Manipulando las propiedades: módulo de Young-densidad Química, microestructura y configuración Materiales cristalinos: Química – enlace metálico frente a enlace iónico/covalente. Pequeñas burbujas- Comportamiento similar al de los elementos Polímeros y elastómeros: Química – Cadenas poliméricas unidas por puentes de hidrógeno. Microestructura – Entrecruzamiento y cristalinidad Espumas: Configuración – estructura celular
- 8. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control del módulo de Young vía química y microestructura Enlaces y empaquetamiento Compuestos inorgánicos óxidos, nitruros, carburos, compuestos intermetálicos Compuestos orgánicos Termoplásticos, termoestables y elastómeros estructuras poliméricas
- 9. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Materiales cristalinos: Química – enlace metálico frente a enlace iónico/covalente. Pequeñas burbujas- Comportamiento similar al de los elementos Manipulando las propiedades: módulo de Young-densidad Química y microestructura Polímeros y elastómeros: Química – Cadenas poliméricas unidas por puentes de hidrógeno. Microestructura – Entrecruzamiento y cristalinidad
- 10. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control del módulo de Young vía configuración Materiales compuestos: “híbridos formados por dos sólidos” Límite superior : Eu Límite inferior : El CFRP GFRP
- 11. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control del módulo de Young vía configuración Espumas: “híbridos formados por un sólido y aire” Espuma cerámica Espuma metálica 2 sólido espuma sólido espuma ρ ρ E E = ~~
- 12. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Materiales compuestos: Configuración– Componentes, Capas Manipulando las propiedades: módulo de Young-densidad Configuración Espumas: Configuración – estructura celular
- 13. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Sumario • El módulo de Young y la densidad se pueden predecir a partir de su estructura cristalina – Modificación vía Química Configuración • Vía química - componentes con alta o baja energía libre de formación • Vía configuración – Escala desde mm a micras para mejorar las propiedades físicas óxidos, nitruros, carburos, compuestos intermetálicos Puentes de hidrógeno y fuerzas de Van der Waals en polímeros Estructuras flexibles - Espumas Estructuras reforzadas – materiales compuestos
- 14. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Manipulando las propiedades: resistencia Metales y cerámicas: Microestructura: tratamientos térmicos y mecánicos Polímeros y elastómeros: Química: entrecruzamient o y ramificación de cadenas Espumas: Configuración: Estructura celular Materiales compuestos Configuración: Componentes y Capas Química, microestructura y configuración
- 15. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Profundizando en la resistencia Metales: mecanismos de endurecimiento-bloqueo del movimiento de las dislocaciones Endurecimiento por solución sólida (Química) Endurecimiento por precipitación (Microestructura) Endurecimiento por deformación (Microestructura)
- 16. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control de las propiedades vía química y microestructura Ejemplos: Aleaciones de aluminio (endurecimiento por envejecimiento) Acero al carbono y aceros aleados (Temple y revenido) Metales: Endurecimiento por precipitación
- 17. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 control vía química y microestructura Al -Cu 2024 Al -Si S413 Aleaciones de aluminio: endurecimiento por solución sólida, precipitación y deformación
- 18. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control vía química: aceros Ferrita + perlita Aceros: resistencia, tenacidad y contenido en carbono
- 19. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control vía microestructura: aceros Normalizado (ferrita + perlita) Aceros aleados- Temple (martensita) Aceros: Cambios en la microestructura para una composición constante
- 20. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Química y microestructura: aceros Aceros: Sumario de composición + procesos
- 21. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Química, microestructura y configuración: polímeros Polímeros: (PP) tenacidad a fractura-módulo de Young: Evolución con la composición
- 22. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Química y microestructura : aleaciones de cobre Aleaciones de cobre: Evolución de las propiedades con la composición y el procesado Soluto: Mayor resistividad eléctrica 70-30 Latón (C26000) Cobre - berilio (C17510) Endurecimiento por precipitación: Mayor resistencia
- 23. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Sumario • La resistencia y la tenacidad dependen de la estructura del material - Modificación vía Química Microestructura Configuración • Vía química – endurecimiento por solución sólida • Vía configuración– Escala desde mm a micras para mejorar las propiedades físicas • Vía microestructura– -tratamientos térmicos y mecánicos Latones, bronces, acero inoxidable Temple, revenido (aceros) Temple, envejecimiento (Aleaciones de aluminio) Deformación (forja, laminado,embutición) Estructuras reforzadas- materiales compuestos Estructuras celulares- espumas
- 24. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Propiedades térmicas: coeficiente de expansión térmica y conductividad Metales y cerámicas: Alta conductividad, baja expansión Polímeros y elastómeros: Baja conductividad, alta expansión
- 25. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Origen del coeficiente de expansión térmica y conductividad Fuerzas intermoleculares no lineares causan la expansión Coeficiente de expansión térmica Dispersión de electrones, fonones limitan la conductividad Conductividad térmica
- 26. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Control vía química, microestructura y configuración Espumas: Configuración – el gas contenido reduce la conductividad Metales altamente aleados : Química : Los solutos reducen la conductividad Invar Química: transformación magnética Polímeros y elastómeros: Las estructuras amorfas dispersan los fonones
- 27. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Sumario • El coeficiente de expansión térmica y la conductividad térmica se pueden modificar vía química y configuración • Vía química – introduciendo puntos de dispersión – introduciendo transformaciones de fase • Vía configuración – conductividad, introduciendo barreras térmicas – expansión, aumento o disminución con termopares solución sólida – aceros inoxidables Contracción por pérdida de magnetismo Espumas –gas Actuadores bimetálicos Termoelementos compensados
- 28. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Resumen Modelo de enseñanza de materiales basado en el diseño: - Explota los mapas de propiedades para la selección de materiales - Estudia el origen microestructural de las propiedades Estudio de las propiedades de los materiales: - Profundizar en las propiedades que se pueden manipular - Concepto fundamental: “Composición + Procesado → Microestructura + Propiedades” Mapas de propiedades: - Representación gráfica de la evolución de las propiedades con la composición y el procesado - Aplicable a todo tipo de materiales: metales, polímeros, espumas, materiales compuestos...
- 29. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Fin de la Unidad 4
- 30. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 Serie completa de unidades Las presentaciones de cada unidad están disponibles en la web de Recursos docentes Cada transparencia contiene notas explicativas. Se pueden consultar abriendo la vista de Notas o bien pulsando en el botón de la barra de herramientas inferior de PowerPoint
- 31. www.grantadesign.com/education/resourcesM. F. Ashby & H.R. Shercliff, 2011 www.grantadesign.com/education/resources © M. F. Ashby, 2011 Los recursos docentes de la página web de Granta Design son una herramienta de apoyo al aprendizaje de materiales en ingenierías, ciencias, procesos y diseño. Los recursos están disponibles en varios formatos y distintos niveles de información para adecuarlos a todos los estudiantes. Este recurso es parte de una serie de lecciones creadas por Mike Ashby para introducir a los estudiantes en el diseño y selección de materiales. La página web también contiene otros recursos didácticos y han contribuido académicos de las más de 800 universidades que usan CES EduPack en el mundo. La página web contiene tanto recursos que requieren el uso de CES EduPack como una selección de recursos que no lo necesitan. Autor Mike Ashby Universidad de Cambridge, Granta Design Ltd. www.grantadesign.com www.eng.cam.ac.uk Reproducción Los derechos de autor están protegidos por copyright. Los recursos pueden ser reproducidos para su uso con fines docentes y pueden ser consultados en la página web de Recursos docentes, una vez adquirido el producto .Asegúrese de que tanto Mike Ashby como Granta Design están al corriente de la reproducción. No se pueden utilizar estos recursos con fines comerciales. Mayor calidad Desde Granta Design se trabaja para mejorar la calidad de estos recursos. Si tiene cualquier sugerencia o mejora, no dude en contactar vía e-mail a la siguiente dirección: teachingresources@grantadesign.com Hay más de 200 recursos disponibles, que incluyen: 77 clases magistrales en power point Ejercicios con soluciones desarrolladas Seminarios Pósters Publicaciones Manuales con soluciones Casos prácticos interactivos *Recursos traducidos por Mar Estellés Gil (UNIVERSIDAD POLITÉCNICA DE VALENCIA)
Notas del editor
- There are notes here for all of the slides The 23 Lecture Units For 2011 TopicNumberName Finding and Displaying Information Unit 1The materials and processes universe: families, classes, members, attributes Unit 2Materials charts: mapping the materials universe Material Properties Unit 3The Elements: Property origins, trends and relationships Unit 4Manipulating Properties: Chemistry, Microstructure, Architecture Unit 5Designing New Materials: Filling the boundaries of materials property space Selection Unit 6Translation, Screening, Documentation: the first step in optimized selection Unit 7Ranking: refining the choice Unit 8Objectives in conflict: trade-off methods and penalty functions Unit 9Material and shape Unit 10Selecting processes: shaping, joining and surface treatment Unit 11The economics: cost modelling for selection Sustainability Unit 12Eco Selection: the eco audit tool Unit 13Advanced Eco design: systematic material selection Unit 14 Low Carbon Power: Resource Intensities and Materials Use Special Topics Unit 15Architecture and the Built Environment: materials for construction Unit 16Structural sections: shape in action Unit 17CES EduPack Bio Edition: Natural and man-made implantable materials Unit 18Materials in Industrial design: Why do consumers buy products? Advanced Teaching and Research Unit 19Advanced Databases: Level 3 Standard, Aerospace and Polymer Unit 20Hybrid Synthesizer: Modelling Composites, Cellular structures and Sandwich panels Unit 21Database creation: Using CES Constructor in Research Unit 22Research: CES Selector and Constructor Unit 23Campus: Overview of this commercial polymers database
- Property charts are central to the CES EduPack methodology for selection of materials in design. But engineers and material scientists will also explore the underpinning physics of the properties – in particular to appreciate how manufacturing processes enable us to manipulate properties. Since changes in composition and process history lead to new property profiles, we can readily track how pairs of properties evolve on a chart – a so-called process trajectory. So charts not only show the trade-offs between properties in design – they also conveniently illustrate the property compromises that are made when materials are manufactured. The concept is illustrated here with some standard combinations of properties that crop up in design and material selection – two mechanical, and one thermal. Examples of the approach are scattered throughout Ashby, Shercliff and Cebon – but particularly in Chapter 19 “Follow the recipe: processing and properties”.
- We start by looking at density and modulus for the pure elements (using the CES EduPack Elements database). The figure shows the density of all 96 usable elements. Excluding the gases (at the bottom), we see that all solid elements fall within a range spanning a factor of about 20. There is a general trend of density increasing with atomic number, though with bands of atomic number showing their own trend of rising then falling density.
- The corresponding figure for Young’s modulus of the pure elements (with the gases now excluded) shows a much bigger spread (a factor of 1000) – but there are similar trends within the bands of atomic number with peaks in the modulus. But we don’t use pure elements much: how do the ranges of modulus and density compare with those for engineering materials?
- The conventional starting point for interpretation of modulus and density is the “hard sphere” model, with atoms of one type packed into a crystal (or a the dense random packing of an amorphous structures). Density simply reflects weight of the atoms and how many are packed into a given volume. Modulus is interpreted by treating the interatomic bonds as approximately linear springs, with equilibrium spacing determined by an energy well. This is more or less sufficient to account for the behaviour of metals (including their alloys) – metals fall in the same domain as the elements on which they are based, governed by efficient crystalline packing and the metallic bond: the “bubbles” on the property chart are small. All other material classes involve some degree of manipulation; these can be classified into some combination of chemistry, microstructure and architecture – with processing being key to achieving the target manipulation.
- The shaded rectangle indicates where the pure elements would all fall on a modulus-density chart. Crystalline materials fall into the same space – but we see that ceramics are stiffer than the metals. Composites are also in the same territory – but at densities below all the metals, and comparable (or better) modulus. Polymers, elastomers and foams extend the range of modulus and density to much lower values. The charts can be used as a lead-in to ideas of materials science. This chart shows two properties: Young’s modulus and density, as before. The positions occupied by the families on the charts are explained through an understanding of inter-atomic bonding and crystal packing; other concepts of materials science explain their positions on the other charts. The Science Notes attached to each field name introduce the property and explain its origins in more detail. The references at the end of each Science Note direct the you to the relevant chapters of standard Materials Science and Engineering Materials texts Hardcopy versions of the Charts can be copied for teaching purposes without restriction of Copyright from the Texts, or downloaded from www.Grantadesign.com. The CES EduPack software allows any pair of properties to be plotted as a chart (even functions of properties can be plotted), giving enormous freedom to explore the world of materials visually.
- So what controls these differences? Designers might just take all this as “given data”. But by “drilling down” into the origin of the properties, we not only understand the physical world around us, but can turn the problem around and ask how we engineers can manipulate properties. The Charts allow a discussion of the ways in which properties can be manipulated. Thus combining two materials to make a composite gives a new material (the composite) with properties that lie, on the chart, between those of its components. Combining a material with a gas gives a foam with properties that lie below, and to the left, of the solid starting material.
- First, examples of the role of chemistry and microstructure: Ceramics are inorganic compounds between metal (or silicon) with oxygen, carbon or nitrogen. The mixtures of elements still pack efficiently in crystals, but the nature of the bonding changes to ionic or covalent – stiffer than the metallic bond (hence giving the higher modulus). The extreme example of covalent bonding is 100% carbon as diamond (and the closely related SiC) – the highest modulus of all. Intermetallic compounds too have enhanced modulus due to stiffer bonding. Polymers and elastomers are organic compounds in a molecular architecture. This provides the interesting contrast of a material containing stiff covalent carbon bonds – but with much weaker hydrogen bonds between the chains principally governing the modulus, opening up a whole regime of moduli well below those of the pure elements. Densities are also low – the elements are light (C, H, Cl etc) and the tangled molecular structure gives lower packing with more free volume. The different polymers are determined by the monomer chemistry forming the molecule – then on top we manipulate the internal microstructure in various ways:- thermoplastics may be semi-crystalline, enabling more efficient packing and bonding of the chain molecules- elastomers and thermosets exploit cross-linking , i.e. direct covalent bonding between the chains. Bulk polymers are therefore low modulus, low density materials. Not illustrated, but worth noting in passing, is the processing of polymers into fibres, aligning the molecules such that the modulus does reflect the covalent bonds along the chain. This enables production of ropes and fabrics, with much greater stiffness-to-weight characteristics than bulk polymers.
- Summarising these observations on the chart, we now understand the impacts of chemistry and microstructure on these properties: Both metals and ceramics show little sensitivity to microstructure for these properties – the composition alone closely defines the modulus and density (100s of steels in a tiny bubble). In particular note the many orders of magnitude covered by polymers and elastomers.
- The second example is composites – hybrids of two solids. Density is again straightforward – mixing two fractions of different density materials leads to a composite density given by a linear rule of mixtures. Simple stress-strain analysis produces two familiar bounds for the modulus – upper and lower limits on the property, between which the actual value must fall. These bounds can also be interpreted graphically as a “lozenge” of property space between the bulk values for the two constituents.
- Modulus and density are also manipulated by architecture – by making hybrids of more than one bulk material. The first example is foams – porous solids, or a hybrid of solid and air. The density is easily explained – the air is effectively weightless, so the relative density (the proportion of the foam that is solid) determines the density of the foam, given the density of the slid in the foam. Interpreting the modulus requires some simple solid mechanics. In many foams, the overall displacement imposed is accommodated internally by bending the solid edges of the cells. This leads to simple scaling laws, suggesting for example that a foam which is 10% solid will have a modulus 100th that of the solid in the cell walls. Here is an example of a “trajectory” on a property chart – foaming a solid moves the resulting properties downwards to the left on a characteristic slope (of 2, for the model illustrated).
- Summarising these observations on the chart, we now understand the impacts of architecture on modulus and density: Foams fall where we expect them to, from the model – down to the left of the polymers from which they are made (down to very low combinations of modulus and density). Composites lie between the constituents from which they are made, as expected from the upper and lower bound models (e.g. CFRP lies between thermosets and ceramics – C fibre bonding being the same as in a ceramic). This methodology may be exploited in designing new hybrids that fill blanks on the map (or compete with existing materials) – this is explored further in Unit 5.
- To sum up so far, the property chart for modulus and density acted as a prompt to explore and understand the underlying physics of the different material classes. A convenient classification of the ways in which the properties are manipulated is to consider chemistry, microstructure and architecture. Examples of each have been highlighted:- chemistry starts at the atomic and molecular scale, and opens up different bonding energies between atoms (and hence stiffness) - microstructure captures the re-arrangement of molecules (here effective in polymers)- architecture moves to the micron to mm length scale, mixing porosity into a solid (foams) or combining two solid constituents in a way that exploits the good characteristics of both (composites) Next we illustrate that these three classes of material manipulation can also be applied to other properties.
- Strength vs. density is another standard property combination for lightweight design. The behaviour of crystalline materials differs significantly compared to modulus. First the strength of ceramics (in tension) is dominated by fracture, whereas metal strength is governed by yielding (though the actual strength values cover the same range). Metals in particular now show wide ranges in a given alloy system (long thin bubbles), reflecting sensitivity to the microstructure (which itself depends on the alloy composition and process history). For polymers and elastomers, the same factors govern the strength as modulus: monomer chemistry, crystallinity and cross-linking. he strength of foams reflects the strength of the cell edges (bending leading to yield or fracture of the cell walls). And the architecture of composites leads to strengths which are some combination of the strengths of the components. The greatest diversity in behaviour (and also commercial significance) is seen in the strength characteristics of metallic alloys – hence “drilling down” will highlight examples in this area.
- Understanding alloy strengthening requires an appreciation of dislocation behaviour. The key generic concepts are dislocation pinning, and the spacing and effectiveness of the obstacles introduced as pinning points: solute atoms, precipitates and other dislocations. Solute level reflects composition directly. Precipitation is the most effective, exploiting microstructures that depend in complex ways on the composition and the process history (examples below). Dislocation microstructures evolve in complex ways with deformation. .
- Precipitation hardening is most effective when fine-scale second phase particles are dispersed throughout every grain. – this microstructure is achieved by heat treatment. The science behind the evolution of precipitates requires a basic understanding of phase diagrams and phase transformations – taking alloying elements into solution, quenching to avoid the formation of coarse two-phase distributions, with controlled low temperature, solid-state transformation of fine-scale precipitates. Two prominent examples are: age hardening of Al alloys, quenching/tempering of steels. Property charts again offer a neat graphical illustration of the consequences of these heat treatments.
- A common trade-off in processing for higher yield strength is the consequential evolution of fracture toughness, the resistance to crack propagation – hence charts showing this pairing are of interest. The figure shows the composition and process trajectories for commercial Al alloys. For wrought alloys the chart illustrates that the hardening mechanisms all increase strength with minimal impact on fracture toughness. The relative effectiveness of the different mechanisms is apparent – the strongest are the age hardened aerospace alloys. We can then annotate the figures with typical micrographs – here a TEM image of the needle shaped precipitates in 2024 alloy (noting the nm length scale). The figure also shows data for casting alloys – alloying + heat treatment again raises strength, but with a corresponding decrease in toughness. This can be correlated with the coarser microstructure formed on casting (2nd micrograph), and the presence of brittle second phases such as Si in casting alloys. (By zooming in, we could easily illustrate the effectiveness of chemical additions to modify the Si for greater toughness, as in Na-modified Al-Si casting alloys).
- Here we follow the same thought process for steels, on the same property chart. First, we examine the effect of composition alone. First consider plain carbon steels in the normalised (slow cooled) condition. Starting with pure iron (100% ferrite), raising the carbon content steadily inroduces hard iron carbide in the form of two-phase pearlite, reaching 100% pearlite at the eutectoid composition (0.8%C). Strength increase, toughness falls.; the parallel micrographs illustrate why this is the case. Progressing to cast irons (at 3-4% C), little further strengthing is achieved but toughness falls further (due to the presence of brittle graphite in the microstructure). (Cast irons too are modified by chemical additions and heat treatments – e.g. to spherodise the graphite. Zooming in would again allow this to be explored graphically).
- Next, consider a given composition (medium carbon steel), to illustrate the quench and temper heat treatment, and consequent microstructure and properties. Quenching leads to the brittle phase martensite (with C trapped in solid solution in a fine-scale needle-grained microstructure, as in the micrograph). This is very hard, but dangerously brittle. Tempering restores toughness while bringing the strength back to a level that is significantly above the initial normalised strength (from ferrite and pearlite, illustrated). The figure also shows that the exact combination of properties can be tuned by varying process conditions – here the temperature used to temper the steel.
- Steels show remarkable diversity, with many more combinations of properties achieved through composition, processing and resulting microstructure. Several classes of alloy steels exploit the different hardening mechanisms. Low and high alloy steels enhance the response to heat treatment and introduce harder alloy carbides, giving very high strength in combination with fracture resistance. Stainless steels are not usually precipitation hardened – those shown in the chart use high levels of solute (for corrosion resistance), producing very tough materials of good strength.
- Now consider polymers again. Here we examine fracture toughness and modulus – since polymer design is often limited by stiffness rather than strength. The figure shows trajectories for polypropylene (PP) – exploiting chemistry, microstructure and architecture. First, we can toughen PP (with some loss of stiffness) by making a copolymer with another monomer, or introducing fine-scale rubbery particles. Then a range of composites can be produced at a more macroscopic scale, all increasing the stiffness. Adding ceramic powders causes some loss of toughness; adding glass or carbon fibres is a win-win scenario – both stiffer and tougher.
- The concept has so far been established for properties relevant to purely mechanical design – here we extend it to the combination of yield strength and electrical resistivity (e.g. alloys for strong electrical conductors). Copper alloys can be strengthened by the same three mechanisms: solute, precipitates and dislocations. But resistivity depends on how conduction electrons interact and are scattered by these obstacles. All the trajectories show increases in both properties relative to pure copper – but solute is most detrimental to resistivity. Hence conductor alloys are strengthened by work hardening or precipitation hardening. (The same is true for aluminium conductor alloys).
- To sum up again, the diversity of strengthening mechanisms (and consequences for other properties) are captured clearly using property charts and trajectories. The behaviour can always be attributed to some combination of chemistry, microstructure and architecture. leading on to discussion of process histories and micrographs as appropriate. It is well-known that chemistry and microstructure are particularly prominent in discussing metallic alloys – the novelty here is a single framework for capturing widely diverse examples. And commercial polymers are frequently hybrids, in the form of powder and fibre composites to enhance their properties.
- Finally, what can we learn about thermal properties from charts? The figure shows thermal expansion vs. thermal conductivity – central to design in which temperature changes lead to distortion or induced thermal stress. The familiar classification of polymers as thermal insulators and metals as thermal conductors is apparent (with the less familiar evidence that ceramics too are reasonable conductors – not to be confused with their electrical insulation properties). Polymers also show much greater thermal expansion than crystalline materials – one source of the poorer dimensional control in hot moulding of polymers. Drilling down into the underlying physics seeks to explain this big picture, and to understand why some materials are much better conductors / insulators than others.
- First to understand thermal expansion we return to the force-displacement characteristic of neighbouring atoms. The energy-well responsible for an equilibrium spacing is somewhat asymmetrical, leading to non-linear spring behaviour. As temperature rises, atoms vibrate at an enhanced energy level across the well, with the average spacing creeping upwards due to the asymmetry in the curve. So at the simplest level, thermal expansion is tied to the interatomic stiffness, with greater expansion in lower stiffness materials. Thermal conductivity is complex – heat is transferred by phonons, elastic waves in which atoms vibrate in a manner that is correlated with their neighbours. The more regular the structure, the easier it is for phonons to propagate and transfer thermal energy without scattering – the simplest explanation for the poor conductivity of amorphous molecular polymers, compared to the crystalline materials. But metals also convey heat via the free electrons. The greater the mean free path between scattering centres, the higher the conductivity. So alloys with high levels of solute (such as stainless steels) have lower thermal conductivity than the pure metal (iron). But phonons play a role here too – metals become more resistive with increasing temperature, since the greater phonon energy scatters the conduction electrons more effectively. (In passing, the thermal and electrical conductivities of metals both depend on electrons interacting with scattering centres such as solute – a property chart showing the two properties confirms the close correlation between the two).
- Returning to the property chart:, we can now appreciate the patterns in the data: Metals are good conductors, polymers are insulators. And within the metals, solute plays a major role in scattering. The sequence of decreasing stiffness – ceramics, metals, polymers – translates into the sequence of increasing expansion. Note also the unusual outlier of Invar, a Ni-Fe alloy , exploiting a rather unique physical trick, whereby a progressive loss of magnetism with temperature counteracts the conventional thermal expansion. Architectured hybrids enable new combinations of thermal properties:- foams are even better insulators than polymers, due to the trapped air- composites again bridge the gap between their constituents, leading to comparable expansion to metals at lower thermal conductivity.
- To conclude this preliminary look at thermal properties, we see chemistry, microstructure and architecture all play a part. Thermal design also exploits other tricks at a component scale – perhaps the extrapolation of architecture. An example is a bimetallic strip, a two-part composite designed to curve with temperature in a controlled way.
- To conclude: Property charts can act as the common ground for both design-led and science-led approaches to teaching materials. Once a design context has been established, and properties defined, the charts offer great freedom to explore the physics of properties and processing, or to go on to performance analysis and material selection, or both. Perhaps the key shift in emphasis for more science-led teaching is to provide the design context first – then “drilling down” into the underlying physics (the heart of conventional materials science teaching) means that students know something about why these properties matter. Engineers automatically expect the design context, and a “light touch” approach to the science is often an appropriate level to cover in parallel with material selection and manufacturing processes. The examples show that the core concepts of “Composition + Processing Microstructure + Properties” illustrated via “trajectories” on charts, can lead in several directions. Here the emphasis has been on understanding which properties can be manipulated, and how. This understanding is essential for control of properties in manufacturing, e.g. appreciating the sensitivities of the outcome to the composition and process history. This methodology also guides the considered development of new materials, recognising the physical limitations of what can and can’t be manipulated, and suggesting where on the charts we might be trying to head (more on this in Unit 5).
- Granta is always interested in hearing about good teaching resources in the materials area. If you use something successfully with your students that you think we should link to from our web site, please get in touch. [email_address] We continue to coordinate annual Materials Education Symposia. You can read about that here: http://www.grantadesign.com/symposium/index.htm