19~chapter 19 - Materials engineering science processing...

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Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Processing for Properties Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Intrinsic properties such as strength and resistivity depend on microstructure and microstructure depends on processing The ability to tune microstructure and properties is central to materials processing and design, so it brings with it the need for good process understanding and control
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Process History Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 19.1 Figure 19.1 depicts the process history of an aluminum bike frame Materials processing involves more than one step Each process step has a characteristic thermal history Designers should watch out for unintended side-effects in the joining stage Design focuses on the properties of finished products, but some of these properties are also critical during processing
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Microstructure Overview Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 19.2 Metals Figure 19.3 Glasses and Ceramics
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Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 19.4 Polymers and Elastomers The role of shaping processes is to produce the right shape with the right final properties – the first is achieved by controlling viscous flow or plasticity – the second requires control of the nature and rate of microstructural evolution
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Phases Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Phase : region of a material with a specified atomic arrangement Phase diagram : maps showing the phases expected as a function of composition and temperature, if the material is in its lowest free energy state Phase transformation : occur when the phases present change – requires a driving force and a mechanism
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Thermodynamics of Phases Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Microstructures can only evolve from one state to another if it is energetically favorable to do so The Gibbs free energy G helps determine what phase an alloy will be in at a given composition held at a fixed temperature U – intrinsic energy of the material p – pressure V – volume U + pV – enthalpy T – temperature S – entropy The state of lowest free energy is the state of thermodynamic equilibrium – at equilibrium no phase change is physically possible
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Phase Diagram: Lead-Tin System The diagram divides up into single- and two-phase regions, separated by grain boundaries At any point in the two-phase region, the present phases are those found at the phase boundaries at either end of the of a horizontal tie-line through the point
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