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The composition of precipitates in peak-aged Al-Zn-Mg-Cu alloys with various Zn/Mg ratios has been investigated by atom probe tomography (APT). The results show that the η′ and the T′ phases have different compositions with a similar Zn/Mg ratio in the same alloy. The Zn/Mg ratio within precipitates was shown to be positively correlated with the Zn/Mg ratio in the alloy. Moreover, the size dependency of composition for ageing precipitates in Al-Zn-Mg-Cu alloys has been determined. This study enriches the understanding of precipitate composition relative to that of the bulk in Al-Zn-Mg-Cu alloys. [ABSTRACT FROM AUTHOR]

The addition of Ag significantly increases the early ageing response, overall ageing response and peak age-hardening of an Al-Zn-Mg-Cu alloy. Ag is the most segregated of the solutes during ageing and a positive correlation between Ag and Cu exists in clusters and larger aggregates throughout the precipitation sequence (1–120 min). [ABSTRACT FROM AUTHOR]

The natural aging (NA) and artificial aging (AA) behavior of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were systematically investigated by means of hardness test, atom probe tomography, transmission electron microscopy, and Monte Carlo simulation. The Si-rich low-Cu alloys displayed higher hardness compared to the Mg-rich equivalents because Si atoms play a dominant role in clustering of solute atoms during both natural and artificial aging. In the high-Cu alloys, Cu did not obviously change the cluster distribution during NA, but significantly refines the clusters and precipitates due to the strong interaction of Cu atoms with Mg atoms during AA. In contrast to the low-Cu alloys, the Mg-rich high-Cu alloys exhibit higher hardness in the early and over-aged stages of artificial aging, with similar or slightly higher hardness in the peak aging condition compared to their Si-rich counterparts. Three types of precipitates ( β″, Q′, and L) are favored in the high-Cu alloys. The Mg-rich high-Cu alloy has more L phase, while the Si-rich variant is abundant in Q′ phase. The negative effect of NA on subsequent AA behavior is less dependent on Mg/Si ratio in the high-Cu alloys due to a synergistic action of the residual Si and Cu atoms, but is closely related to Mg/Si ratio in low-Cu alloys. [ABSTRACT FROM AUTHOR]

The clustering behaviour of a pre-aged Al-Mg-Si alloy during natural ageing (NA) and paint baking (PB) was systematically investigated by hardness measurements and atom probe tomography (APT). The results show that pre-ageing (PA) can effectively retard the negative effect of NA and enhance the paint-baking response even after long-term NA (up to one year). The pre-aged alloy shows a good resistance against clustering within a short NA period of one week, and the capability is enhanced with increasing pre-ageing time. However, the clustering becomes significant after prolonged NA, especially for one year. The increasing number density of clusters containing 10–30 solute atoms contributes most to the hardness increase after prolonged NA. The mechanism about two synergistic processes of 'vacancy trap' and 'vacancy escape' is introduced to explain the phenomenon. The alloy with a prior PA above 2 h exhibits a good PB response even after one-year NA. Such performance is attributed to large-size precipitates (GP zones or pre-β″ precipitates) formed during PA, which can serve as nuclei for β″ due to a similar composition. Moreover, a strength model has been established to predict the different strengthening effect and evaluate contributions from clusters, GP zones and pre-β″/β″ precipitates during multi-stage ageing in the Al-Mg-Si alloy. • Pre-ageing (PA) can retard clustering and facilitate paint baking (PB) response even after one-year natural ageing (NA). • The enhanced NA stability and PB response is due to the generation of GP zone or pre-β″ during PA. • The deteriorated stability after prolonged NA can be ascribed to the vacancy behaviour. • The final hardness after PB largely depends on priorPA time rather than NA period. [ABSTRACT FROM AUTHOR]

Effects of Ag addition on the age hardening and precipitation behavior of aluminum alloy 7075 were investigated. The transmission electron microscopy (TEM) and atom probe tomography (APT) were used to study the aggregate distribution and identify the role of Ag in precipitation hardening of the alloy. The hardness and strength of the Ag–containing alloy were increased by more than 10% due to a higher number density of fine aggregates, which was 2.5 times that of the Ag–free alloy 7075. Both T–type and η–type precipitates existed in the alloys, and two precipitation sequences occurred simultaneously as: SSS → η–type cluster → GP zone → η′ → η → T, and SSS → T–type cluster → GP zone→ T′ → T. Particularly, Ag promoted the formation of T–type clusters due to the interaction among Ag, Mg and Zn atoms as well as vacancies at the early stage of aging. The Mg–Ag, Zn–Ag, and vacancy–Ag co–clusters acted as nucleation sites for the subsequent precipitates (η′, T′, etc.). This also caused the increase of Mg concentration in T–type clusters at the beginning of aging and the narrowing of the precipitation free zone (PFZ) near grain boundaries at the peak aging. • Ag accelerates the aging precipitation process of the alloy 7075. • The peak-aged alloy 7075 + Ag exhibits higher mechanical properties. • Ag increases the number density of aggregates in aged 7075 alloy. • Both T/η-types precipitates exist in the alloys 7075 and 7075 + Ag during aging. • Strong interacts of Mg–Ag/vacancy promote precipitation of T in alloy 7075 + Ag. [ABSTRACT FROM AUTHOR]

The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloys were systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. In under-aged alloys, most of precipitates have a disordered structure, with a substructure of β″ (LDC) and Cu sub-unit cluster or C unit cell. After aging to peak strength, disordered precipitates including β″, QP1 and QP2 phases are formed. The disordered QP1 and QP2 phases, which contain the unit cells of Q′ and C phases, respectively, are the precursor phases of Q′ phase in these alloys. The β″ phase can transform into the disordered QP1 phase by incorporating Cu atoms, forming Cu sub-unit clusters and QP lattice. When the alloy is over-aged, the ordering and transformation of QP1 to Q′ occurs by the formation of Cu sub-unit clusters, the ordering of QP lattice, and the ordering of QC lattice. In contrast, the transformation of the disordered QP2 phase are rather sluggish. After sufficient aging, Q′, C and disordered QP2 transform into the Q phase. During the evolution of the precipitates in these alloys, a continuous incorporation of Mg, Si and Cu atoms and release of Al atoms occur. These findings provide new insights in understanding precipitation in Al-Mg-Si-Cu alloys. [ABSTRACT FROM AUTHOR]

The formation mechanism of T 1 (Al 2 CuLi) precipitate in an Al-Cu-Li alloy was investigated by using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, atom probe tomography (APT) and first-principles energy calculations. The HAADF-STEM images reveal the existence of the T 1 precursor phase, which is termed as T 1p in this study. The T 1p phase has a lattice structure identical to the Al matrix, and consists of a stacking of Li-rich and Cu-rich layers along the {111} Al planes. APT analysis further confirms the existence of T 1p phase, in which the concentrations of Li and Cu are significantly lower than those in T 1 plate. Based on the first-principles energy calculations, the structural evolution from T 1p phase to T 1 phase is proved to be energetically favorable. Correspondingly, one possible evolution process from T 1p phase to T 1 phase involving the change of stacking sequence is proposed and discussed. [ABSTRACT FROM AUTHOR]

The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3rd generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ∼ 240 Hv was achieved on ageing at 110 oC/60h after HPT. A further improvement in the hardness to ∼ 260 Hv was accomplished by a pre-ageing 110 oC/24h before HPT combined with a post-HPT ageing process at 110 oC for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (∼ 120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions. [ABSTRACT FROM AUTHOR]

The ageing precipitates and strengthening effects of Al-Zn-Mg-Cu alloys with different Zn contents were investigated. The results showed that increasing Zn contents can significantly improve the age-hardening response and influence the type of precipitates as well. The low Zn-containing alloy (1.70 at.%) is strengthened by nano-scale T′ phases in the peak aged condition. However, the microstructure of the high Zn content (2.34 at.%) alloy in the peak aged condition consists of a higher density of globular T′ phases and a small proportion of plate-like η′ phases. The results of atom probe tomography revealed that clusters formed at the early stage of ageing with a Zn/Mg atomic ratio higher than 1.3 transform into η′ phases and these clusters with a lower Zn/Mg ratio transform into T′ phases. The composition of a T′ phase varies with its size, where the concentrations of Zn and Mg increase with its increasing radius, while the concentration of Al presents an opposite trend. • Co-precipitation of T´ and η´ phase in Al-Zn-Mg-Cu alloys was investigated. • The precipitation of η´ phase was promoted by the increase of Zn content. • Clusters with different Zn/Mg ratios showed different phase transformation path. [ABSTRACT FROM AUTHOR]