Initial Agehardening and Thermally Activated Process in Al–Zn Alloys

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  • Tanoue Koji
    Department of Metal Processing Engineering, Kyushu Institute of Technology
  • Umemoto Yasushi
    Graduate Student, Kyushu Institute of Technology
  • Matsuda Hidehiko
    Department of Metal Processing Engineering, Kyushu Institute of Technology

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  • Initial Agehardening and Thermally Activated Process in Al–Zn Alloys

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Measurements of the electrical resistivity, Simple tensile tests and stress relaxation tests are made to investigate the hardening mechanism at the early stage of ageing in Al–Zn alloys. There exists a good linear relationship between the flow stress and the square root of the plastic strain, ε1⁄2, at any stage of ageing for pure Al and Al–Zn alloys deformed at \dotε=10−4s−1 and 77 K. The strain independent flow stress σ0 obtained by extrapolating ε1⁄2 to zero increases drastically in the concentrated alloys as ageing proceeds. This initial agehardening can be explained by the increment in internal stress due to the interaction between dislocations and the coherent strain fields as proposed by Gerold et al.<BR>The strain exponent n of the activation volume for dislocation motion is obtained to be n1=−0.3 and n2=−0.6 on both sides of a certain strain εc for the alloys on the assumption that V*∝εn. The dislocation intersection mechanism can give a satisfactory explanation for n2\simeq−1⁄2. The reason why n1 does not coincide with n2 may be that parallel processes of the dislocation intersection and the shearing of GP zones by dislocations rate-controll the deformation in the strain region less than εc.

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