lecture7-materials-pdf - 6 Chapter Phase Diagrams Dr....

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Unformatted text preview: 6 Chapter Phase Diagrams Dr. Mubarak ١ Outline Definitions and basic concepts Phases and microstructure Binary isomorphous systems (complete solid solubility) Binary eutectic systems (limited solid Dr. Mubarak ٢ solubility) Binary systems with intermediate phases/compounds The iron-carbon system (steel and cast iron) Definitions: Components and Phases Component : chemically recognizable species (Fe and C in carbon steel, H 2 O and NaCl in salted water). A binary alloy contains two components, a ternary alloy – three, etc. Dr. Mubarak ٣ Phase : a portion of a system that has uniform physical and chemical characteristics. Two distinct phases in a system have distinct physical or chemical characteristics (e.g. water and ice) and are separated from each other by definite phase boundaries. Definitions: Components and Phases A phase may contain one or more components. A single-phase system is called homogeneous, Dr. Mubarak ٤ systems with two or more phases are mixtures or heterogeneous systems. Definitions: Solubility Limit Solvent : host or major component in solution. Solute : minor component. Solubility Limit of a component in a phase is the maximum amount of the component that Dr. Mubarak ٥ can be dissolved in it (e.g. alcohol has unlimited solubility in water, sugar has a limited solubility, oil is insoluble). The same concepts apply to solid phases: Cu and Ni are mutually soluble in any amount (unlimited solid solubility), while C has a limited solubility in Fe. Microstructure The properties of an alloy depend not only on proportions of the phases but also on how they are arranged structurally at the microscopic level. Dr. Mubarak ٦ Thus, the microstructure is specified by: • The number of phases, • Their proportions, • And their arrangement in space. This is an alloy of Fe with 4 wt.% C. There are several phases. The long gray regions are flakes of graphite. The matrix is a fine ٧ Dr. Mubarak Phase diagrams will help us to understand and predict the microstructures like the one shown in this photo. mixture of BCC Fe and Fe 3 C compound. A system is at equilibrium if at constant temperature, pressure and composition the system is stable, not changing with time. Equilibrium is the state that is achieved given States Metastable Equilibrium and ٨ Dr. Mubarak sufficient time. But the time to achieve equilibrium may be very long (the kinetics can be slow) that a state along the path to the equilibrium may appear to be stable. This is called a metastable state. Free energy is a function States Metastable Equilibrium and In thermodynamics, equilibrium is described as the state of system that corresponds to the minimum of the thermodynamic function called the free energy of the system....
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lecture7-materials-pdf - 6 Chapter Phase Diagrams Dr....

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