Precipitation Hardening Is there a change in the equilibrium

Precipitation Hardening Is there a change in the equilibrium composition of Theta phase in Al-Cu alloys when overaging occurs? Why or why not? Consider the following aging for 6022-T4 aluminium. Why does overegging occur earlier at higher temparatures? (Source: JJ Gracio. EF Barlat, EF.Rauch. PT lones. VF Nefo. AB Lopes Intl J plasticity Vol 20 (2004). pp. 427-145) Vickers Hardness (HV) vs aging time for 6022 aluminum.

Solution

a) There is a clear correlation between the phases that form and the change in hardness; typical increases in hardness are associated with the formation of GP (Guinier-Preston) zones and \'\' precipitates; in most cases, the formation of \' leads to a decrease in hardness.Immediately after quenching to room temperature the only contribution to strengthening (that is, resistance to the movement of dislocations) comes from the solid solution: copper atoms at the aluminium sites which resist the movement of dislocations. However, as the GP zones form, the elastic stresses associated with the coherent GP zones resists the movement of dislocations contributing to hardness. As the aging time increases, the coherent \'\' phases that form, due to the mis- fit strains that they produce, manage to resist the movement of dislocations and hence lead to further hardening. Finally the formation of semicoherent \' can also increase the strength; however, in the case of both \'\' and \' if the particles are coarser or the volume fractions of these phases are smaller (and hence are further apart) it leads to a decrease in hardness since the dislocations can bow between the preciptiates and hence move in the matrix contributing the plastic deformation.

b) At the higher temperature the peak hardness (the highest hardness that is achieved before overaging) is lower; this is because the lower driving force at the higher temperature for the formation of the \'\' phase leads to a coarsely dispersed phase with lower volume fractions.