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Deformation introduces a large density of dislocations which can alter the precipitation sequence in a number of ways, either by heterogeneous precipitation on dislocations or by modification of kinetics of formation of precipitates in locations away from the dislocations (i.e. kinetics of bulk precipitation) [84]. First, dislocations are favourable nucleation sites for precipitates and short-circuit diffusion paths for solutes, which results in faster and coarser precipitation on dislocations [84]. Moreover, precipitation on structural defects naturally promotes stable rather than metastable phases [84, 85]. Second, the interaction between dislocations and solutes results in a solute flux to dislocations. Hence a lower solute fraction is available for bulk precipitation; therefore, the kinetics of bulk precipitation will be altered.

Given the above possible interactions, the effect of the presence of dislocations on precipitation hardening strongly depends on alloy family [42]. Three different responses can be identified [42]: (1) Enhanced hardening in Al-Cu, Al-Cu-Mg systems. (2) Little change in hardening, e.g. Al-Zn-Mg system. (3) Reduced hardening occurring in special circumstances, e.g. Al-Cu alloys with trace additions of cadmium, indium or tin. Currently the study and understanding of the precipitation in the pre-deformed samples are very limited compared with that of solution-treated samples. In the following, some studies on the effect of cold work on precipitation in Al-Mg-Si(-Cu) alloys will be reviewed.

The microstructure and mechanical properties developed by thermomechanical treatment in an Al-0.78wt%Mg-0.98wt%Si alloy (0.15wt%Fe and 0.3wt%Mn) have been studied by Ismail [86]. The results showed that no modification in the precipitate structure due to deformation prior to ageing at temperatures up to 150°C occurs. However, when aged at

200°C, needle-shaped β″ precipitates were observed in the deformed samples instead of the very fine GP zones present for the lower ageing temperature range up to 150°C. Moreover, the needle-shaped precipitates were noticed to become larger in size and more heterogeneously distributed with increasing deformation from 15% to 25%.

Yassar et al. [85, 87] have studied the effect of cold deformation on the precipitation of metastable phases in an AA6022 alloy (0.55wt%Mg-1.10wt%Si-0.06wt%Cu) by means of DSC and TEM. It was found that the introduction of deformation changes the precipitate

type from β″ phase to β′ and Q′ phases during the dynamic heating in a DSC scan at

20°C/min. That is, instead of the sequence: Clusters/GP zones → β″ → β′ + Q′ for the

undeformed condition, the following precipitation sequence for metastable phases in a

deformed condition was proposed: Clusters/GP zones → β′ + Q′ → Q′ [85]. The

comparison of the measured activation energy for precipitate formation showed that cold deformation facilitates precipitation by decreasing the activation energy [87].

The effect of cold work on the precipitation kinetics of an AA6111 alloy (Al-(0.5-1.0)Mg- (0.6-1.1)Si-(0.5-0.9)Cu) has been evaluated by means of tensile testing, DSC and TEM [88]. In this study, the DSC results showed acceleration in the precipitation kinetics and

the reduction of the activation energy for formation of β″ phase with increasing level of

cold work. Meanwhile, TEM results revealed the increased amount of strengthening precipitates with increasing level of cold work.

Microhardness, electrical conductivity and DSC measurements were employed to investigate the possible use of pre-strain to improve the bake hardening response of a twin-roll cast Al–Mg–Si alloy (0.47%Mg-1.15%Si-0.03%Cu) by Birol [89]. In this study, several changes were noted in the DSC curves upon pre-straining shortly after solution treatment. After one week natural ageing, in contrast to the sample processed without pre-

straining, the pre-strained samples reveal an exothermic peak between 100°C and 160°C,

which is linked to the formation of GP-1 zones. This effect was found to shift to lower temperatures with increasing pre-strain from 0 to 5%. On the other hand, the dissolution trough for the reversion of the clusters and smaller zones formed during natural ageing is

largely missing in the pre-strained samples. The β″ and β′ precipitation peaks have both

shifted to lower temperatures with increasing level of cold work, which means that the

kinetics of both β″ and β′ precipitation is accelerated with increasing pre-strain. Thus, in

precipitation is promoted when deformation is introduced shortly after solution treatment. The dislocations introduced by prestrain not only suppress clustering during natural ageing but also provide heterogeneous nucleation sites for GP-1 zones which readily grow to

stable nuclei for the β″ particles.

Zhen et al. have studied the effect of predeformation on microstructure and tensile properties of Al-Mg-Si alloys with various Si contents [90]. Their results show that the strength of Al–Mg–Si alloys in the underaged condition is greatly increased by pre-

stretching immediately after quench. Meanwhile, the β″ precipitates are observed to form

directly on dislocations with a larger size than those forming in the dislocation free areas. When pre-stretching is less than 5%, the increase in pre-stretching results in increased

density of β″ precipitates, whereas pre-stretching by 10% leads to larger sizes but lower

density of β″ precipitates.

The effect of plastic deformation on structure and properties of two types of 6XXX aluminium alloys (Al-(1-1.5)Mg-(0.9-1.1)Si-(1.1-1.3)Cu alloys) has been studied by Dutkiewicz et al. [91] through hardness tests and TEM studies. In this study, three different cold-rolling reductions were employed: 30%, 60% and 90%. The results confirm that the kinetics of the precipitation is accelerated with increasing pre-deformation. It also reveals that the precipitates form mainly on dislocations at peak hardness and their density increases with increasing deformation. TEM observations in this study suggest that

recovery process occurs after ageing at 165°C but no recrystallisation can be seen at this

temperature.

Ratchev et al. [64] have studied the effect of pre-deformation on the precipitation

hardening of an Al-4.2wt%Mg-0.6wt%Cu alloy, which is in the ternary α+S+T region in

Figure 2.8. It was found that pre-deformation before artificial ageing has a complex influence on precipitation hardening: deformation hampers the GPB zone precipitation but it introduces more heterogeneous nucleation sites. It was suggested that the main precipitation hardening occurring after pre-deformation is due to heterogeneous nucleation

and growth of S″ phase. The S″ nucleation might also prevent dislocations annihilation

and preserve to some extend the work hardening introduced during pre-deformation. Ringer et al. have investigated the effect of cold work on precipitation in an Al-4.0Cu- 0.3Mg alloy by means of hardness testing and TEM [42]. This study revealed that the responses of the alloy to hardening during natural ageing are reduced by 6% cold working

after quenching due to the retarded formation of GP zones whilst some amount of cold work prior to elevated temperature ageing promotes increased hardening in the alloy.