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High-Resolution Electron Microscopy and Kinetic Studies of Precipitation Hardening Reactions in Cast Al-5.8Zn-2.2Mg-2.5Cu

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Abstract

Precipitation age hardening (T6) response of Al-5.8Zn-2.2Mg-2.5Cu alloy in near-net-shaped cast condition was investigated. Cast samples were manufactured using the controlled diffusion solidification (CDS) technology combined with a metallic mold tilt pour gravity casting process. This study was conducted using (1) non-isothermal aging during differential scanning calorimetry (DSC) experiments and (2) isothermal aging during which transient bulk and micro-hardness measurements taken at four aging temperatures of 343, 393, 433 and 473 K. A detailed microstructural investigation combined with a quantitative image analysis was carried out on both DSC and hardness specimens using high-resolution electron microscopy (HREM) and energy-dispersive spectroscopy (EDS) elemental mappings. Hardness measurements were analyzed using a kinetic modeling approach (single state variable combined with a kinetic equation that is valid for the Al-Zn-Mg-Cu alloys). The results confirm the formation of hardening phases of GP zone, intermediate hardening metastable phase η′ and equilibrium phase η. The results of calorimetric measurements, electron microscopy and selected area diffraction patterns are in good agreement and confirm transient precipitation/dissolution sequences. A mean value of activation energy \(Q_{\text{A}}\) of 75 kJ/mol was evaluated for combined bulk diffusion rate of alloying elements during precipitation of strengthening phases within Al matrix.

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Notes

  1. The nominal composition of the material was (at.%): 96.08 Al, 2.30 Zn, 1.38 Mg and 0.09 Si, with Cu, Fe and Mn in the remainder.

  2. Al-6.4Zn-2.2 Mg-2.3Cu (wt.%).

  3. Al-8.5Zn-1.9 Mg-2.2Cu (wt.%).

  4. ImageJ, Image processing and Analysis in Java, 1.42q Java 1.6.0 (32 bits) (https://imagej.net)

    .

  5. Al-5.5Zn-0.80 Mg-0.16Zr [0.016Cu-0.010Mn-0.008Ti-0.002Cr-0.17Fe-0.05Si] (composition is in wt.%, and minor elements appear in brackets)

    .

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Acknowledgments

The authors express their gratitude to the Natural Sciences and Engineering Research Council (NSERC) of Canada for their financial support through the Discovery Grant program. Further, the first author would like to express his gratitude to Professor Hugh Shercliff (Senior Lecturer, Department of Engineering, University of Cambridge, Cambridge, UK) and Dr. Marjan Rajabi (Assistant Professor, Department of Advanced Materials and Renewable Energy, Iranian Research Organization for Science and Technology, Tehran, Iran) for their invaluable comments on this work.

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  1. Light Metal Casting Research Centre (LMCRC), Department of Mechanical Engineering, McMaster University, Hamilton, ON, L8S 4L8, Canada

    S. Reza Ghiaasiaan & Sumanth Shankar

  2. CANMET MATERIALS, Natural Resources Canada, McMaster Inovation Park, 183 Longwood South St, Hamilton, ON, L8P 0A1, Canada

    Babak Shalchi Amirkhiz

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Ghiaasiaan, S.R., Shalchi Amirkhiz, B. & Shankar, S. High-Resolution Electron Microscopy and Kinetic Studies of Precipitation Hardening Reactions in Cast Al-5.8Zn-2.2Mg-2.5Cu. J. of Materi Eng and Perform 28, 4630–4646 (2019). https://doi.org/10.1007/s11665-019-04219-4

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