Invariant characterizing thermally activated plastic flow
Keywords:
thermal activation, strain rate sensitivity, plastic flow, Cottrell-Stokes law, Al alloysAbstract
Thermal activation leads to a reduction of the flow stress below the athermal value (the zero Kelvin flow stress). The ratio between the flow stress at a given temperature and the athermal stress (at given material structure) is known as the Cottrell-Stokes ratio – a fundamental parameter describing thermal activation. This ratio is independent of strain in pure metals. This observation represents the CottrellStokes law. Here it is shown that the ratio is also independent of the annealing state of the material, i.e. an invariant of the deformation and microstructure in Al alloys. A model of thermally activated dislocation motion across fields of obstacles is used to investigate the physics of the process. It is also used to discuss whether the validity of the Cottrell-Stokes law can be determined based on strain rate jump experiments and the Haasen plot, as usual in the current practice.
References
COTRELL, A.H., STOKES, R.J., Effects of temperature on the plastic properties of aluminum crystals, Proc. R. Soc. London, A233, pp. 17-34, 1955.
BASINSKI, Z.S., Thermally activated glide in FCC metals and its application to the theory of strain hardening, Phil. Mag., 4, pp. 393-432, 1959.
MECKING, H., LUCKE, K., Die Bestimmung des Aktivierungsvolumens durch Wechsel der Dehngeschwin insbesondere an Silbereinkristallen, Mater. Sci. Eng., 1, pp. 342-348, 1967.
BASINSKI, Z.S., Forest hardening in FCC metals, Scripta Metall., 8, pp. 1301-1308, 1974.
BASINSKI, Z.S., FOXALL, R.A., PASCUAL, R., Stress equivalence of solution hardening, Scripta Metall., 6, pp. 807-814, 1972.
ZEYFANG, R., BUCK, O., SEEGER, A., Thermally activated plastic deformation of high purity Cu single crystals, Phys. Stat. Sol., B61, pp. 551-561, 1974.
WIELKE, B., TIKVIC, W., SCHOECK, G., Cottrell-Stokes law in Cd and Zn, Phys. Stat. Sol., A40, pp. 271-277, 1977.
MECKING, H., KOCKS, U.F., Kinetics of flow and strain-hardening, Acta Metall., 29, pp. 1865- 1885, 1981.
SAIMOTO, S., SANG, H., A re-examination of the Cottrell-Stokes relation based on precision measurements of the activation volume, Acta Metall., 31, pp. 1873-1881, 1983.
BOCHNIAK, W., The Cottrell-Stokes law for FCC single crystals, Acta Metall. Mater., 41, pp. 3133-3140, 1993.
EZZ, S.S., SUN, Y.Q., HIRSCH, P.B., Strain rate dependence of the flow stress on work hardening of ?’, Mat. Sci. Eng., A192/193, pp. 45-52, 1995.
KRUML, T., CODDET, O., MARTIN, J.L., The investigation of internal stress fields by stress reduction experiments, Mat. Sci. Eng., A387-389, pp. 72-75, 2004.
HOLLANG, L., HIECKMANN, E., BRUNNER, D., HOLSTE, C., SKROTZKI, W., Scaling effects in plasticity of Ni, Mat. Sci. Eng., A424, pp. 138-153, 2006.
SEEGER, A., The generation of lattice defects by moving dislocations, and its application to the temperature dependence of the flow-stress of fcc crystals, Phil. Mag., 46, pp. 1194-1198, 1955.
ADAMS, M.A., COTTRELL, A.H., Effect of temperature on the flow stress of work-hardened copper crystals, Phil. Mag., 46, pp. 1187-1193, 1955.
BULLEN, F.P., COUSLAND, S.M., Energy levels of electron and hole traps in the band gap of doped anthracene crystals, Phys. Stat. Sol., 27, pp. 501-508, 1968.
HAASEN, P., Plastic deformation of nickel single crystals at low temperatures, Phil. Mag., 3, p. 384, 1958.
XU, Z., PICU, R.C., Thermally activated motion of dislocations in fields of obstacles: the effect of obstacle distribution, Phys. Rev., B76, pp. 094112(1)-(8), 2007.
PICU, R.C., LI, R., XU, Z., Strain rate sensitivity of thermally activated dislocation motion across fields of obstacles of different kind, Mat. Sci. Eng., A502, pp. 164-171, 2009.
FREIDEL, J., Dislocations, Addison-Wesley, Reading, MA, 1967.
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