Lecture32.pdf

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Department
Mechanical Engineering
Course
MECHENG 382
Professor
All Professors
Semester
Winter

Description
Lecture 32 - Deformation mechanisms maps & creep-resistant materials 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 -3 -2 • Boundary-diffusion creep ∝ d ; lattice-diffusion creep ∝ d ME382 - 11/iv/14 1 Lecture 32 - Deformation mechanisms maps & creep-resistant materials • With small grain sizes, boundary diffusion may dominate for all temperatures • Lattice diffusion may dominate at higher temperatures with large grains (because Ql> 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 σθθ = PR/t; σ =zzR/2t; σ = 0 rr ⇒ σ H 3PR/2t = 17.3 MPa But, from Mohr’s circle of stress: σ H 3τ ; ∴ τ = 17.3/ 3 = 10 MPa ˙ −5 −1 ∴ At 860 ºC γ = 10 s But, from Mohr’s circle of strain: εH= γ / 3 ∴ At 860 ºC ε = 5.7 × 10 −6 s−1 H ∴ Life time = 0.01/(5.7 x 10 ) = 1700 secs = 28.9 minutes ME382 - 11/iv/14 2 Lecture 32 - Deformation mechanisms maps & creep-resistant materials • Transient creep maps can be plotted by including elastic contribution ∴ Give total strain at fixed time ∴ Should, in principle, include effects of microstructure change, but .... • Relative contribution of creep to total strain increases with time Case study • Turbine blade ω = 1000 rad/s, radius = 0. 3 m; design calls fγ <10 s8 −; T = 450 °C → 700 °C σ = rω ρl where l is distance from tip of blade; ρ (for Ni) = 8900 kg/m ∴ At root, the normal stress = 120 MPa (shear stress is about 60 MP
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