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Unformatted text preview: ME 382 Lecture 05 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”). Structure–insensitive properties depend primarily on bonding and crystal structure. • Not affected by processing • E.g ., elastic modulus, density, melting temperature for metals & ceramics Structure–sensitive 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 • Attractive forces dominate when atoms are remote ME 382 Lecture 05...
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This note was uploaded on 09/08/2011 for the course MECHENG 382 taught by Professor Thouless during the Fall '08 term at University of Michigan.
- Fall '08