Yong’an Zhang’s research while affiliated with General Research Institute for Nonferrous Metals (GRINM) and other places

This study investigated the microstructure and mechanical properties of AlNbTiVZr series high-entropy alloys (HEAs) through both experimental studies and density functional theory calculations. Significant improvements in the microstructures and mechanical properties were achieved for the AlNbTiVZr series HEAs by meticulously adjusting the alloy composition and employing homogenization heat treatment. Notably, the specimen designated as Al0.5NbTiVZr0.5 demonstrated excellent mechanical properties including a compressive yield strength of 1162 MPa and a compressive strength of 1783 MPa. After homogenization heat treatment at 1000 °C for 24 h, the Al0.5NbTiVZr0.5 alloy exhibits brittle-to-ductile transition. Further atomic-scale theoretical simulations reveal that the decrease of Al content intrinsically enhances the ductility of the alloys, thereby indicating that the mechanical properties of the AlNbTiVZr series HEAs were significantly influenced by the chemical composition. Additionally, specific atomic pair formations were observed to adversely affect the microstructure of the AlNbTiVZr series HEAs, particularly in terms of ductility. These findings provide valuable insights for the design and optimization of light weight HEAs, emphasizing the synergistic adjustment of alloy composition and heat treatment processes to achieve a balance between the strength and ductility.

To investigate the comprehensive effects of the Al and Zr element contents on the microstructure evolution of the AlNbTiVZr series light-weight refractory high entropy alloys (HEAs), five samples were studied. Samples with different compositions were designated Al1.5NbTiVZr, Al1.5NbTiVZr0.5, AlNbTiVZr, AlNbTiVZr0.5, and Al0.5NbTiVZr0.5. The results demonstrated that the actual density of the studied HEA samples ranged from 5.291 to 5.826 g·cm−3. The microstructure of these HEAs contains a solid solution phase with a BCC structure and a Laves phase. The Laves phase was further identified as the ZrAlV intermetallic compound by TEM observations. The microstructure of the AlNbTiVZr series HEAs was affected by both the Al and Zr element contents, whereas the Zr element showed a more dominant effect due to Zr atoms occupying the core position of the ZrAlV Laves phase (C14 structure). Therefore, the as-cast Al0.5NbTiVZr0.5 sample exhibits the best room temperature compression property with a compression strength (σp) of 1783 MPa and an engineering strain of 28.8% due to having the lowest ZrAlV intermetallic compound area fraction (0.7%), as characterized by the EBSD technique.

Aviation-grade aluminum alloy is one of the most important structural materials for aircraft body structure. It is the material basis for developing national defense industry, constructing a modern economic system, and strengthening the manufacturing sector of China. Aluminum alloys with high strength/toughness, high specific strength/modulus, and Sc addition have been developed in the recent decades. At present, research on next-generation alloy design, processing methods, and new alloy research and development paradigm have been drawing attention. The aviation-grade aluminum alloy industry in China has been developing rapidly. Especially in the increasingly complex international and domestic macro situation, development opportunities and challenges coexist. However, deficiencies are becoming increasingly apparent, such as lacking independent innovation capability to support the leadership of aviation-grade aluminum alloy materials, prominent risks of specific materials and equipment, lacking competitiveness in the international market, and the accumulation of test and application data. It is necessary to improve the competitiveness of aviation-grade aluminum alloy materials, develop new materials, cooperate with upstream and downstream research and development, promote the application of new materials, and establish material and application evaluation systems to improve the innovation and development of China’s aviation-grade aluminum alloy industry.

A quantitative relationship between microstructure and tensile properties of Al-Zn-Mg-Cu alloys with various alloying degrees was investigated in this study. Three kinds of high strength Al-Zn-Mg-Cu alloys with little strength difference were used for the microstructural characteristic and statistical analysis. A strength model with high accuracy was established through the systematical calculation of strengthening theories, which was validated by the microstructure and ultimate tensile strength of a testing sample. The model gives the precise ratio of the contribution of precipitation hardening and even the component achieved from the bypass mechanism. Moreover, guidance on the regulation of precipitation phases was proposed from the strength model. It can be disclosed that the alloys will show the best strength when the precipitates of 7xxx series Al alloy are distributed near the right of critical radius, which is identified as 2.5 nm in this model.

To investigate the dynamic recrystallization behavior of 7xxx aluminum alloys, the isothermal compression tests were carried on the 7056 aluminum alloy in the temperatures range of 320–440 °C and in the strain rates range of 0.001–1 s−1. In addition, the microstructure of samples were observed via electron back scanning diffraction microscope. According to the results, true stress and true strain curves were established and an Arrhenius-type equation was established, showing the flow stress increases with the temperature decreasing and the strain rate increasing. The critical strain (εc) and the critical stress (σc) of the onset of dynamic recrystallization were identified via the strain hardening rate and constructed relationship between deformation parameters as follows: εc=6.71×10−4Z0.137 3 and σp=1.202σc+12.691. The DRX is incomplete in this alloy, whose volume fraction is only 20% even if the strain reaches 0.9. Through this study, the flow stress behavior and DRX behavior of 7056 aluminum alloys are deeply understood, which gives benefit to control the hot working process.

Precipitation behavior of Al3(Sc,Zr) particles in a new high-alloyed Al–Zn–Mg–Cu–Zr–Sc aluminum alloy during homogenization was investigated by use of three-dimensional atom probe, transition electron microscope and high-resolution transition electron microscope. The results indicate that Al3Sc was the precursor of Al3(Sc,Zr) precipitate. The supersaturated solid solution containing Sc decomposed at 250 °C, forming Sc-rich clusters and gradually developing into Al3Sc nuclei. In multicomponent system containing Zr and Sc, the process of Al3Sc nucleus transforming to mature Al3(Sc,Zr) particle was controlled by diffusion. In the early stage of nuclei growth, Zr atoms did not have long-range diffusion capacity, but only Sc atoms diffused freely. With the increase in temperature, Zr atoms began to diffuse over long distance and approach to Al3Sc nuclei driven by chemical potential gradient, and then enrich around them to form Zr-rich thin layer, thus forming Al3(Sc,Zr) structure of Al3Sc core + Zr-rich shell. During isothermal annealing at 440 °C for different holding hours, the mean diameters of the Sc-containing particles after 1 h, 10 h, 36 h and 100 h were 10.9 nm, 15.5 nm, 14.3 nm and 16.7 nm, respectively. The coarsening coefficient for Al3(Sc,Zr) precipitates is about three orders of magnitude smaller than that of Al3Sc particles, showing much better thermal stability.

The plasticity, fracture toughness and stress corrosion cracking resistance of a high-alloying Al-Zn-Mg-Cu alloy with various recrystallization degrees introduced by solution treatment were investigated. The results exhibited that the elongation increased from 14.73% to 16.38% with the deepening of recrystallization degree from 8.80% to 20.20%, as well as the enhancement of fracture toughness (KIC) from 25.7 MPa·m1/2 to 28.9 MPa·m1/2. On the contrast, the critical stress intensity factor for stress corrosion cracking (KISCC) declined from 29.9 MPa·m1/2 to 25.0 MPa·m1/2. The recrystallization benefited plasticity and fracture toughness by not only breaking the tight connection between two deformed grains and eliminating the dislocations density but also increasing the “lubricating” thin grain boundary phases. Meanwhile, it degraded stress corrosion cracking resistance due to the increase of coarsening precipitated particles distributed on recrystallized grain boundaries. This work revealed the opposition trend among properties by modifying recrystallization in the high-alloying Al-Zn-Mg-Cu alloy.

The 6005A-T5 aluminum alloy welded joints were prepared by use of friction stir welding (FSW) and metal inert gas welding (MIG). The difference in microstructure and mechanical properties of the two types of welded joints were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscope (TEM), Vickers hardness, and tensile tests. The results showed that both two methods could be used to weld this alloy successfully. The nugget zone (NZ) of FSW joint experienced a mass of heat input, hence the fine equiaxed grains appeared and the β″ phases dissolved completely. The grown and elongated grains have been preserved in the thermo-mechanically affected zone (TMAZ). The grains in heat-affected zone (HAZ) grew significantly. The microstructure in weld metal of MIG joint shows an evident feature of dendrites. The fusion zone (FZ) is composed of large columnar crystals formed along the direction of heat dissipation. The upgrowth of grains in the HAZ region was more significant than that of FSW. Both the HAZ of the FSW and MIG joints consist of β′ phase and Q′ phase. The minimum hardness of FSW joints is located in the HAZ region, while that of MIG joints is located in the weld zone. The tensile strengths of the FSW and MIG joints reach 80.3 and 72.8% of the BM, respectively. Both of FSW and MIG joints show the ductile fracture.

The microstructural evolution of an Al–Zn–Mg–Cu aluminum alloy during an optimized two-step homogenization treatment was investigated by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy-dispersive spectrometer (EDS) and differential scanning calorimetry (DSC). It mainly focused on secondary phase transformation and dissolution. Methods such as and differential scanning calorimetry (DSC) was used to investigate the heat change of each specimen as an indication of phase transformation involved in the homogenization. The results showed that the lamellar eutectic phase had the trend of spheroidizing during the treatment at 400 °C, and the T-Al2Mg2Zn3 phase was transformed into S-Al2CuMg phase. Further dissolution of T phase was not obvious at second step homogenization even after the holding time was extended to more than 24 h.

The microstructural evolution and phase transformation of cast Al–1.5Mg–0.6Si–3Li (mass %) alloy during homogenization were investigated. The results show that severe dendritic segregation exists in the as-cast ingot. Mg and Si elements segregate at grain boundaries to form intermetallic Mg2Si phase. In addition, there also exists Li-containing phases, including T-Al2LiMg and δ-AlLi phase in the α-Al matrix. These Li-containing phases completely dissolve into the matrix and the segregation of dendrite is eliminated after homogenization at 570 °C for 24 h. During the homogenization, most of the Mg2Si phase at grain boundaries disappear, but AlLiSi ternary compounds precipitate and disperse at interior and boundary of grains because of the strong binding capacity between Li and Si element. The AlLiSi phase is detrimental to the properties of the alloy, therefore homogenization treatment may be not profitable for microstructural refinement of Al–1.5Mg–0.6Si–3Li alloy.