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Mechanisms of metastable phase transformations in Al-Cu alloys with additions of Si, Ti and B

Different metastable phases formed during thermal treatment of Al-Cu alloys were investigated by a combination of HRTEM, TEM+EDS, and HRSEM techniques. The based Al - 4.97 wt. % Cu - 0.56 wt. % Ag alloy (A201) was modified by different additions of Si, Ti and B. Microstructure and mechanical properties were studied in the as-cast, solution treated (at 550Ë?C for ~20 hours) and aged (at 170Ë?C up to 32 days) conditions. The precipitation sequence during aging was the following: supersaturated solid solution (SSSS) ï?  GP zones ï?  θ'' ï?   θ' + Ω ï?  θ. During the early stages of aging GP, zones are nucleated as single layers of Cu parallel to {100} planes of the -Al matrix. Then these GP zones are united and generate the metastable θ''- CuAl3 phase consisting of several single atomic layers of Cu, each of them separated by three atomic layers of Al. The Ag, Ti, and B additions resulted in the nucleation of metastable semi-coherent ï? phase formed at {111} -Al planes. The Si addition increased the nucleation of GP zones and inhibited the Ω phase. The following aging resulted in θ" transformation to semi-coherent metastable θ'- CuAl2. The mechanism of this transformation is discussed The next step of microstructure evolution is the diffusional dissolution of θ'' precipitates in the presence of more stable θ' and ï? phases.  The maximum microhardness corresponded to the simultaneous formation of semi-coherent θ' and Ω precipitates. After extended aging, the θ' transforms to stable incoherent BCT θ-phase


S. Abd El Majid

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