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Unformatted text preview: ME 382 Lecture 05 14/ix/07 1 P ROPERTIES OF E NGINEERING M ATERIALS Material properties depend on: (a) Type of atomic bonding (b) Three dimensional arrangements of atoms and molecules (crystal structure) (c) Atomic-scale defects in the crystal structure (d) Interactions of these defects with micron-scale features (microstructure). Structureinsensitive properties depend primarily on bonding and crystal structure. Not affected by processing E.g ., elastic modulus, density, melting temperature for metals & ceramics Structuresensitive properties affected by defects and microstructure. Strongly influenced by details of composition and processing. Must add information on processing in material specifications E.g. , yield strength, ultimate tensile strength, elongation for metals. F ORCES ON ATOMS AND MOLECULES Work = Force x Distance Can be used to give a general expression for the force acting on a body: F x = " dU dx where x is distance and U is energy of a body Example: Gravitational potential energy: U = mgx Force due to gravity = - mg (positive force acts in - x direction) Atoms & molecules try to minimize energy and maximize entropy (disorder) Gibbs free energy is a thermodynamic quantity that incorporates both terms G = H- TS H = enthalpy (energy); S = entropy ; T is absolute temperatur e Force acting on an atom or molecule can be expressed as F x = " dG dx Energy and entropy terms for the interaction between atoms or molecules ME 382 Lecture 05 14/ix/07 2...
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- Spring '08