X.-Grant Chen’s research while affiliated with Université du Québec à Chicoutimi and other places

To ensure reliable power transmission, maintaining high thermal stability in aluminum conductor cables is essential, as strength degradation may occur due to the increased temperature of overhead cables during service. This study examined the strength degradation of an ultrafine-grained hypoeutectic Al-Si alloy by investigating its microstructural evolution during post-annealing. The primary strengthening mechanisms in the Al-Si wire include strain hardening and grain boundary strengthening. This study revealed that microstructural changes were influenced by both the temperature and duration of post-annealing. At 90 °C, a slight strength reduction occurred owing to defect recovery, such as vacancies and dislocations. At 250 °C, grain growth and dislocation density reduction contributed to strength degradation. These findings indicate that dislocation recovery, recrystallization, and grain growth are the main factors driving strength degradation in Al-Si wires.

Hypoeutectic Al-Si alloys are promising candidates for novel Al conductor cables; however, their limited electrical conductivity (EC) and mechanical strength hinder their widespread industrial applications. This study investigates the influence of two thermomechanical processing routes-conventional (C-TMP) and modified (M-TMP)-on the microstructural evolution and the resulting enhancements in mechanical and electrical properties of hypoeutectic AA4043 Al alloy. The C-TMP method improved the ultimate tensile strength from 180.7 MPa to 289.8 MPa and slightly increased the EC from 50.1 to 51.4 % IACS, however, it still remained below the industrial requirement threshold of 52.5 % IACS. In contrast, the M-TMP method successfully overcame the strength-EC trade-off by achieving simultaneous improvements in both properties: the UTS reached 231.4 MPa, while the EC increased to 59.2 % IACS, which represent enhancements of 28.1 % and 18.2 %, respectively, over the as-rolled (AsR) rod condition. The substantial improvement in the EC was attributed to the depletion of solute Si from the Al matrix through the formation of Si nanoprecipitates during pre-annealing. Microstructural analysis of the M-TMP sample revealed the development of an ultrafine-grained (UFG) structure containing embedded Si nanoprecipitates, with a lower dislocation density compared to the C-TMP sample. The underlying mechanisms contributing to the strength-EC synergy are discussed using constitutive models, focusing on Si nanoprecipitates, dislocation density, and grain refinement. These results demonstrate that M-TMP effectively resolved the strength-EC trade-off and yielded a high-strength, high-EC Al-Si conductor that is suitable for advanced electrical wiring applications.

The impact of annealing on the recrystallized grain structure and superplastic behavior of two Al-Mg 5xxx alloys used for high-speed blow forming (HSBF) was studied. The results revealed that both alloys demonstrated rapid static recrystallization after only a few minutes of annealing at 520 °C, forming fine and equiaxed grain structures. After four min of annealing, Alloy 2 (Al-4.0Mg-1.18Mn) exhibited a higher fraction of small grains (<10 µm) compared to Alloy 1 (Al-4.5Mg-0.74Mn). Moreover, Alloy 2 displayed enhanced resistance to grain coarsening with increasing annealing times, which was attributed to its higher amount of Al6(Mn,Fe) intermetallic particles and a higher number density of Mn dispersoids. Optimizing the annealing time can effectively develop a fine and stable grain structure in Al-Mg 5xxx alloys. During tensile deformation, Alloy 2 consistently showed higher ductility compared to Alloy 1 at low strain rates (170% vs. 138% at 0.001 s−1 and 163% vs. 134% at 0.01 s−1), whereas at a high strain rate of 1 s−1, both alloys displayed comparable tensile elongation. The high superplastic response of Alloy 2 at low strain rates renders it a superior superplastic alloy for HSBF applications.

The effects of minor additions of the transition elements Zr, Ti, and V on the microstructure, mechanical properties, and out-of-phase thermomechanical fatigue behavior of 224 Al-Cu alloys were investigated. The results revealed that the introduction of the transition elements led to a refined grain size and a finer and much denser distribution of θ″/θ′ precipitates compared to that of the base alloy, which enhanced the tensile strength but reduced the elongation at both room temperature and 300 °C. Constitutive analyses based on theoretical strength calculations indicated that precipitation strengthening was the primary mechanism contributing to the strength of both tested alloys at room temperature and 300 °C. The out-of-phase thermomechanical fatigue test results showed that the addition of transition elements caused a slight decrease in the fatigue lifetime, which was mainly attributed to the reduced ductility and higher peak tensile stress at low temperatures. During the fatigue process, the transition element-added alloy exhibited a lower coarsening ratio, indicating higher thermal stability, which mitigated the negative impact of the reduced ductility on the fatigue performance to some extent. Considering their various properties, the addition of Zr, Ti, and V is recommended to improve the overall performance of Al-Cu 224 cast alloys.

The precipitation microstructures in both primary α-Al and eutectic Al and mechanical properties of high-pressure vacuum die-cast AlSi10MgMn alloy under T5 aging treatments were systematically investigated. The results revealed that the three T5 treatments (peak aging, pre-aging, and prolonged aging) achieved similar levels of yield strength (YS) but different elongations (El). The El values of the T5-treated samples were effectively improved by pre-aging and prolonged aging. The best strength-ductility trade-off was achieved by prolonged aging at 185 °C for 24 h, providing a YS of 225 MPa and an El of 6.8%. Compared to the T6-treated samples, the El of the T5 prolonged-aged samples was 20% higher, but the YS was 15% lower. The microstructures after T5 primarily comprised nano-sized Si particles, Si clusters, and β″ precipitates in primary α-Al, but only Si clusters and β″ precipitates in eutectic Al. Si particles were the main and most stable strengthening phases. The T6-treated samples predominantly contained β″ precipitates in both primary and eutectic Al. The different precipitates in the primary and eutectic Al were quantified, and their strengthening contributions were analyzed by applying classical shearing and bypassing mechanisms. The predicted overall YS values are in good agreement with the experimental data.