Jae-Gil Jung

Quantitative analyses of dissolution and precipitation kinetics of θ-Al2Cu phase in an Al-6.2Si-2.9Cu alloy using electrical resistivity

The dissolution kinetics of the θ phase during solution treatment and isothermal precipitation kinetics during aging in an Al-6.2Si-2.9Cu alloy were quantitatively examined through electrical resistivity, hardness, and transmission electron microscopy (TEM). The electrical resistivity successfully predicted that pre-existing θ particles in the as-cast Al-Si-Cu alloy were fully dissolved after 4.5 h at 500 °C, which was in a good agreement with that directly observed by TEM. The electrical resistivity immediately decreased during aging at 250 °C due to the precipitation of θ′ phase and was saturated within 2 h. A maximum hardness peak appeared in 1 h at 250 °C due to the formation of a metastable θ′ phase, and then gradually decreased due to the coarsening of the θ′ phase as well as the annihilation of the θ″ phase. The θ′ phase was finally transformed to θ phase after 3.5 h at 250 °C.

Quantitative Evaluation of Dynamic Precipitation Kinetics in a Complex Nb-Ti-V Microalloyed Steel Using Electrical Resistivity Measurements

The kinetics of dynamic precipitation in austenite of a complex Nb-Ti-V microalloyed steel during hot compression at 900 °C with a strain rate of 6.7 s−1 was quantitatively investigated through electrical resistivity measurements. The dynamic precipitation in the Nb-Ti-V microalloyed steel started at a strain of 0.15. The amount of tiny Nb-rich (Nb,Ti,V)C carbides, which were precipitated at crystal defects gradually increased up to 0.02 wt% at a maximum strain of 0.67. The electrical resistivity was successfully applied to the quantitative evaluation of dynamic precipitation kinetics in microalloyed steel by excluding the effects of crystal defects and interstitial atoms on the electrical resistivity.

Effect of Heat-Treatment on Microstructure and Mechanical Properties of Sonicated Multicomponent AlMgSiCuZn Alloy

In this study, an AlMg8Si9Cu10Zn10 (in wt%) alloy is fabricated with a high volume fraction of coarse secondary phases, which is higher fraction than in the conventional piston alloys. Ingots are cast in a permanent mold after an ultrasonic melt treatment for 60 s at the temperature range of 750–700 °C. Microstructure of AlMgSiCuZn alloy consists of Si, Zn, Mg2Si, Q-Al5Cu2Mg8Si6, and θ-Al2Cu phases. By the heat-treatment at 440 °C, Q-phase at the vicinity of blocky Mg2Si phase grows and the roundness of the second phases increases with respect to the heat-treating time. Compared with the as-cast specimen, room-temperature ultimate compressive strength of the heat-treated specimens increases. However, maximum compressive stress at 350 °C is slightly decreased by heat treatment. The formation of fine clusters increases the ultimate compressive strength, while the spheroidization of bulky secondary phases during heat treatment deteriorates the ultimate compressive strengths.

Effect of Ultrasonic Melt-Treatment and Cooling Rate on Microstructure of Multi-phase Reinforced Al Alloy

Al–xMg–10Si–10Cu–10Zn (in wt%) alloy is fabricated with a high volume fraction of coarse multi-phase particles in the aluminum matrix. Permanent mold casting is carried out with a selective ultrasonic melt-treatment for 60 s at 1000 K. A step mold is used to achieve three different cooling rates. Blocky primary Mg2Si particles are homogeneously distributed through the Al matrix and eutectic phases such as Si, Q–Al5Cu2Mg8Si6, and θ-Al2Cu are found in the dendritic boundaries. Ultrasonication significantly decreases the porosity and the size of microstructural constituents such as primary Mg2Si phases. Refinement of primary Mg2Si by ultrasonication is more effective in the specimens with higher cooling rates. The compressive properties at room temperature and 623 K are enhanced in the ultrasonicated alloys. By heat treatment at 713 K, very fine θ′ precipitates with size less than 10 nm are formed through the Al matrix and blocky Mg2Si and Q phases were spheroidized.

Microstructures and Mechanical Properties of Multiphase-Reinforced In Situ Aluminum Matrix Composites

We investigated the microstructures and mechanical properties of multiphase-reinforced in situ aluminum matrix composites (AMCs) prepared with various combinations and contents of Li, Mg, Si, Cu, Zn, Sn, and Ni. The area fractions of the secondary phases in the as-cast AMCs ranged from 26% to 58%, and the types of secondary phases depended on the alloy chemical compositions. The type and amount of secondary phases were more important than matrix strengthening in determining the alloy mechanical properties. Composite hardness and compressive stress increased while fracture strain decreased with increasing total area fraction of the secondary phases up to 40%. The formation of coarse primary and soft/heavy Sn-containing phases significantly deteriorated the alloy mechanical properties. Annealing also influenced the mechanical properties of the AMCs by changing the microstructures of the secondary phases and Al matrices.