The Detriment of Coherency Strains to the Electrical Conductivity of Naturally-Aged B319 Al Alloy
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UCTION
NATURAL aging is an effective process that can enhance the strength of various industrial cast Al alloys. For many 3xx series Al-Si-Cu alloys, this treatment involves three main processes that aim to transform and redistribute the Cu phases in the microstructure.[1,2] First, the coarse, as-cast Al2Cu phases are dissolved into the Al matrix during a high-temperature solution heat treatment. This process is typically accompanied by homogenization of the microstructure as well as fragmentation, spheroidization, and coarsening of the eutectic Si particles.[3–5] Second, the alloy is quenched to preserve the supersaturated Al solid solution of Cu and vacancies at ambient-temperature. Third, the controlled diffusion and precipitation of elements from the supersaturated solid solution is enabled during subsequent aging.
ELI VANDERSLUIS and COMONDORE RAVINDRAN are with the Centre for Near-net-shape Processing of Materials, Ryerson University, 101 Gerrard Street East, Toronto, ON, M5B1G8, Canada. Contact e-mail: [email protected] MENACHEM BAMBERGER is with the Department of Materials Science and Engineering, Technion Israel Institute of Technology, Technion City, 3200003, Haifa, Israel. Manuscript submitted June 10, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Although artificial aging at elevated temperatures is common for Al alloys, many alloys experience appreciable natural aging at ambient-temperature.[6] Despite the relatively low amounts of energy available for diffusion in the latter, the extensive lattice supersaturation of solute and vacancies can promote the segregation of fine, randomly-distributed, Cu-rich clusters called Guinier-Preston (GP) zones. These zones are typically 1 or 2 atoms thick and approximately 25 atoms in diameter, and they form along the {100} crystallographic planes of the Al lattice.[7] Given their coherency in the matrix crystal structure, these clusters impose local strains on the lattice. The ensuing distortions interact with dislocations to impede their motion through the microstructure, thereby increasing the strength and hardness of the alloy.[6] With enough energy, the GP zones can grow and the precipitation sequence can progress through the formation of several metastable Al-Cu phases and finally the stable Al2Cu phase. Yet, considerable precipitation of these phases usually necessitates the application of an elevated-temperature artificial aging treatment.[2] For many Al alloys, the formation of GP zones and the associated increases in strength attain maximum stable values within a few days, after which minimal further aging is observed at ambient-temperature. Accordingly, the naturally-aged temper T4 specifies a nominal aging time of 4 days. The naturally-aged
condition is useful for many applications, given its characteristic high fracture toughness, fatigue resistance, and ratio of tensile to yield strength.[7] A T4 treatment may not be appropriate for elevated-temperature applications, such as for automotive powertrain components, where further artificial aging wil
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