strength mech

strength mech - Precipitation Hardening Particles impede...

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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 1 Outline • Precipitation Hardening • Dispersion Strengthening • Martensite • Mechanical Properties of Steel • Effect of Pearlite Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 2 • Particles impede dislocations. • Things that slow down/hinder/impede dislocation movement will increase, σ y and σ TS • And also other phases - especially v. small, well dispersed particles. In some alloys can get small, uniform particles to precipitate out of (solid) solution. Hence name “ precipitation hardening ”, also known as " AGE " - hardening. Examples include : Al-Cu Cu-Be Cu-S Mg-Al Some alloy and stainless steels Precipitation Hardening • Many Al-alloys are precipitation hardenable Al - Cu is best known alloy, e.g. Al - 4%Cu Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 3 Precipitation Hardening Procedure: • Heat to T o ; Hold until only α -phase present. • Quench ( rapid cooling ) to T 1 ; because rapid, no diffusion occurs - SSSS - Super-saturated solid solution of α formed. B atoms “ trapped ”in α . Not thermodynamically stable. • Reheat to T 2 ; diffusion can occur, small precipitates of β -phase form. M = Max. solubility of metal B in metal A. Solid solubility decreases to N as T Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 4 •E x : A l -C u s y s t em • Procedure: - Pt A : solution heat treat (get a solid solution) - Pt B : quench to room temp. - Pt C : reheat to nucleate small θ crystals within α crystals. Pt A (sol’n heat treat) Pt B Pt C (precipitate θ) Temp. Time Precipitation Hardening 300 400 500 600 700 01 02 03 04 05 0 wt%Cu (Al) L α +L α α+θ θ θ +L T(°C) A B composition range needed for precipitation hardening CuAl 2 α + θ→ Heat (~550 o C) Quench (0 o C) →α
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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 5 Precipitation Hardening Initially have a supersaturated solid solution of Cu in Al Single phase - α (SSSS) α + θ ppt - form various zones or small Cu clusters - upon further aging form θ '' particles Lattices are trying to match up with each other ( stay …………. ) - large lattice strain - high resistance to dislocation movement α + θ ppt Can no longer match up with each other (………….) - less resistance to dislocation movement incoherent coherent Dr. M. Medraj Mech. Eng. Dept. - Concordia University
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This note was uploaded on 01/30/2011 for the course EMSE 103 taught by Professor Ggh during the Spring '10 term at Case Western.

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strength mech - Precipitation Hardening Particles impede...

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