Effects of Microstructure on the Mechanical Properties and Stress Corrosion Cracking of an Al-Zn-Mg-Sc-Zr Alloy by Various Temper Treatments

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  • Wu Ling-Mei
    Department of Mechanical Engineering, National Taiwan University
  • Wang Wen-Hsiung
    Department of Mechanical Engineering, National Taiwan University
  • Hsu Yung-Fu
    Department of Materials and Mineral Resources Engineering, National Taipei University of Technology
  • Trong Shan
    Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology

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Abstract

The high strength Al-Zn-Mg alloy used in the aerospace industry is strengthened by coherent G.P. zones and semicoherent η′ phase. However, this series of aluminum alloys are susceptible to stress corrosion cracking (SCC), particularly when aged to the peak-aged state of T6 temper. In this study, the effect of microstructure on mechanical properties and stress corrosion cracking of the alloy was investigated for alloys tempered to T6, RRA, Two-Step and T7 conditions by tensile test in air, SCC test and polarization test in a 3.5%NaCl solution. It is shown that the improvement in SCC resistance correlates very well with the size of matrix precipitates and grain boundary precipitates. T7 temper can produce larger sizes of both the matrix precipitates and grain boundary precipitates than that of T6, RRA and Two-step tempers, causing a decrease in the length and density of dislocation lines, which results in the decrease of stress concentration at grain boundary and greatly improving the SCC resistance. The addition of Sc and Zr to high strength Al-Zn-Mg alloys has been found to simultaneously improve the tensile strength and SCC resistance.

Journal

  • MATERIALS TRANSACTIONS

    MATERIALS TRANSACTIONS 48 (3), 600-609, 2007

    The Japan Institute of Metals and Materials

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