Resistometric Determination of the Upper Temperature Limit for G.P. Zones in Al&ndash;Mg<SUB>2</SUB>Si Alloys

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  • Resistometric Determination of the Upper Temperature Limit for G.P. Zones in Al–Mg<SUB>2</SUB>Si Alloys

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As it is difficult for Al–Mg2Si alloys to apply the X-ray small angle scattering technique to the study of the G.P. zones, an attempt was made to estimate the upper temperature limit of G.P. zones from the relation between the maximum resistivity increase Δρm observed during isothermal ageing and the ageing temperature Ta. This relation obtained from the Thomson-Freundlich equation and Panseri’s assumption is<BR>Δρm=Δρz[Calloy−exp(ΔS⁄k)exp{(2σV⁄r−ΔH)⁄kT}],<BR>where Δρz is the contribution to the resistivity of a unit concentration of the solute atom inside the G.P. zones with the radius corresponding to the resistivity maximum, Calloy is the alloy concentration, σ is the interfacial energy between the G.P. zone and the matrix, V is the atomic volume of the solute atom, r is the zone radius corresponding to the resistivity maximum, and ΔS and ΔH are the entropy and the enthalpy of solution, respectively.<BR>The values of Δρm at several ageing temperatures were experimentally determined for three different Calloy. Δρm vs. Ta curves suggested that at least two kinds of zones exist. The change of Δρz with Ta and Calloy may be also explained by the presence of the two kinds of zones.<BR>The temperature limit for the zones stable up to a higher temperature and having the radius corresponding to the resistivity maximum was represented as<BR>C=106exp(−0.19⁄kT).<BR>The application of this proposition to the Al–Zn alloy gave the values of the interfacial energy and the upper temperature limit of G.P. zones which were in agreement with the previous results.

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