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Unformatted text preview: ME 382 Lecture 32 7/iv/06 1 D EFORMATION MECHANISM MAPS • For steady-state creep (ignores microstructural changes, cavitation and elasticity) • Different mechanisms can operate simultaneously • Fastest mechanism will dominate ⇒ Depends on temperature, stress and grain size • Deformation-mechanism map ⇒ Dominant mechanisms for stress & temperature Includes lines of constant strain rate Note on this map: “High-temperature” power-law creep is “lattice-diffusion” power-law creep “Low-temperature” power-law creep is “core-diffusion” power-law creep • Boundary-diffusion creep ∝ d-3 ; lattice-diffusion creep ∝ d-2 ME 382 Lecture 32 7/iv/06 2 • With small grain sizes, boundary diffusion may dominate for all temperatures • Lattice diffusion may dominate at higher temperatures with large grains (because Q l > Q b ) • Increase in grain size reduces diffusional creep rate, but not power-law creep ∴ Regime of power-law creep dominance increases Example: Cylindrical pressure vessel of pure nickel with grain size of 0.01 mm. Average radius of cylinder = 200 mm; wall thickness = 1 mm; internal pressure of 0.1 MPa. What is life time at 860 ºC if failure occurs at an effective strain of 0.01 ! "" = PR / t ; !...
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