302.L8.age.harden.18Nov02

302.L8.age.harden.18Nov02 - 1 Objective Age- hardening...

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Unformatted text preview: 1 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Microstructure-Properties: II Age Hardening 27-302 Lecture 9 Fall, 2002 Prof. A. D. Rollett 2 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Materials Tetrahedron Microstructure Properties Processing Performance 3 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Objective The objective of this lecture is to describe the relationship between precipitation and hardness as an example of a key microstructure-property relationship. 4 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms References Phase transformations in metals and alloys, D.A. Porter, & K.E. Easterling, Chapman & Hall. Materials Principles & Practice , Butterworth Heinemann, Edited by C. Newey & G. Weaver. Mechanical Metallurgy , McGrawHill, G.E. Dieter, 3rd Ed. Hull, D. and D. J. Bacon (1984). Introduction to Dislocations . Oxford, UK, Pergamon. Courtney, T. H. (2000). Mechanical Behavior of Materials . Boston, McGraw-Hill. 5 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Notation a := lattice parameter := strain, misfit (or similar quantity to describe a hardening mechanism) G := shear modulus b := Burgers vector r := Particle size (radius) f V V ( ) := volume fraction (of precipitates) := stress (macroscopic) := shear stress ( critical value, in some cases ) := boundary energy, e.g. anti-phase boundary <L 3 > := mean intercept length (of precipitates) 6 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Age Hardening Curves The most quoted age hardening curve is that for Al-Cu alloys performed in the late 40s. Keep in mind that age hardening was known empirically (Alfred Wilm) as a technologically useful treatment from the early days of aluminum alloys. Higher Cu contents result in higher maximum hardnesses because larger volume fractions of precipitate are possible. 7 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Al-Cu precipitation sequence The sequence is: 1 + GP-zones 2 + 3 + 4 + The phase are: n- fcc aluminum; n th subscript denotes each equilibrium GP zones - mono-atomic layers of Cu on (001) Al - thin discs, fully coherent with matrix - disc-shaped, semi-coherent on (001) bct. - incoherent interface, ~spherical, complex body-centered tetragonal (bct). 8 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Al-Cu ppt structures GP zone structure 9 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Al-Cu microstructures This tableau shows which of the different ppt types are associated with which part of the hardening curve. GP zones 10 Objective Age- hardening Al-Cu Al-Ag steels mecha- nisms Al-Cu driving forces Each precipitate has a different free energy curve w.r.t composition. Exception is the GP zone, which may be regarded as continuous with the alloy (leading to the possibility of spinodal decomposition, discussed later).possibility of spinodal decomposition, discussed later)....
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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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302.L8.age.harden.18Nov02 - 1 Objective Age- hardening...

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