C.L. Wu

Complex Precipitation Sequences of Al-Cu-Li-(Mg) Alloys Characterized in Relation to Thermal Ageing Processes

The Al-Cu-Li-(Mg) alloy is a high-performance lightweight material strengthened by complex coexisting precipitates that form in the alloy upon thermal ageing. Using high-resolution (scanning) transmission electron microscopy in association with first-principles energy calculations, we systematically studied the complex coexisting precipitates in the alloys and correlated their precipitation sequences with thermal ageing processes applied. The principal results are the following: (1) eight types of precipitates can be observed in the alloy; (2) of these precipitates, the T1-phase is most stable. The S-phase precipitates with segregated Li atoms at their interfacial edges are unexpectedly more stable than the σ-phase; (3) the T1-phase has a characteristic precursor that plays the key role in its nucleation and growth.

Relationship Between the Strengthening Effect and the Morphology of Precipitates in Al–7.4Zn–1.7Mg–2.0Cu Alloy

The morphological evolution of the precipitates in Al–7.4Zn–1.7Mg–2.0Cu (wt%) alloy was studied by high-resolution transmission electron microscopy (HRTEM). Statistics reveal that the hardness of the alloy changes accordingly with the change of the average thickness–diameter ratio of precipitates. The GPII zones are mainly responsible for the first and also the highest hardness peak. They grow in diameter and keep 7-atomic-layer in thickness. Once the thickness changes, the phase transformation from GPII zone to η′ or η-precursor would occur. The resultant metastable η′ and η-precursor precipitates grow in both diameter and thickness, but much faster in the former. After the first hardness peak, the metastable η′ precipitates and η-precursor, coexisting with part of GPII zones, are counted as the main hardening precipitates.

Application of the Peierls–Nabarro Model to Symmetric Tilt Low-Angle Grain Boundary with Full Dislocation in Pure Magnesium

Three types of symmetric (11

The crystallographic and morphological evolution of the strengthening precipitates in Cu–Ni–Si alloys

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Microstructure and mechanical properties of Mg–6Zn–xCu–0.6Zr (wt.%) alloys

2¯0) tilt low-angle grain boundaries (LAGBs) with array of basal, prismatic, and pyramidal edge full dislocations in pure Mg have been studied by using the improved Peierls–Nabarro model in combination with the generalized stacking fault energy curve. The results show that with decreasing distance between the dislocations in all the three types of tilt LAGBs, the stress and strain fields are gradually suppressed. The reduction extent of the stress and strain fields decreases from the prismatic to basal to pyramidal dislocations. The variation of dislocation line energy (DLE) for all tilt LAGBs is divided into three stages: DLE changes slightly and linearly when the distance is larger than 300 Å, ~10%; DLE declines exponentially and quickly when the distance goes from 300 to 100 Å, ~70%; and finally, the descent speed lowers when the distance is smaller than 100 Å and the dislocation core energy is nearly half of the DLE. The grain boundary energy (GBE) decreases when the tilt angle of LAGB increases from 1° to 2° for all cases. The tilt LAGB consists of pyramidal dislocations always has the largest GBE, while that with array of prismatic dislocations has the smallest one in the whole range. The Peierls stress of dislocation in tilt LAGB is nearly unchanged, the same as that of single dislocation. This work is useful for further study of dissociated dislocation, solute segregation, precipitate nucleation in tilt LAGB and its interaction with single dislocations.