On the influence of the mode of deformation on the hardening of iron at low temperature and large strains
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On the Influence of the Mode of Deformation on the Hardening of Iron at Low Temperature and Large Strains J. GIL -SEVILLANO AND E. AERNOUDT If drawn into wire at room temperature, bcc metals show a linear hardening rate up to very large strains (Refs. 1 to 3). A recent paper on the flow-stress of iron by Young, Anderson and Sherby shows that, if the strain is given by torsion, an equivalent stress-strain behavior is found up to a strain of e = 2, but that a decreasing hardening rate follows afterwards, leading to a steady-stage flow stress.' It was suggested by these authors that continuous linear hardening was not a fundamental behavior of bcc materials but a consequence of the special structure developed by "the special complex form of deformation" imposed by drawing. The actual behavior of iron, the "normal" behavior of bcc metals, would be that encountered in torsion, formally similar to that of fcc metals. The present communication offers an explanation for the influence of the mode of deformation on the work hardening of iron, allowing for a fundamental accord between the strain hardening in torsion and in axisymmetric deformation, in spite of the fact that the two stress-strain curves differ so strongly. The macroscopic stress strain behavior of a polycrystal at large strains is mainly determined by two factors: the "microscopic" law of hardening * govern*The resolved shear stress as a function of the amount of crystallographic slip.
ing the strain hardening of each of the grains of the polycrystal, and the crystallographic texture developed by the specific mode of deformation imposed. Normally, those two factors are taken into account separately and
J. GIL -SEVILLANO is Associate Professor, Escuela Tecnica Superior de Engeneros Industriales, San Sebastian, Spain. E. AERNOUDT is Professor, Katholieke Universiteit Leuven, Leuven, Belgium. Manuscript submitted March 3, 1975. METALLURGICAL TRANSACTIONS A
the influence of texture represents merely an "orientation factor". This way of treating the strain hardening of polycrystals implies in fact the tacit assumption of independency between the two factors mentioned. If the origin of the (microscopic) strain hardening would be an isotropic distribution of obstacles to slip, such as a random dislocations distribution or an equiaxed cellular structure, the independency between the microscopic law of hardening and the crystallographic orientation would be a sound hypothesis. This condition is met, e.g. in the steady state of hot working, where an equiaxed structure is created by dynamic recovery or dynamic recrystallization, 5 or during cold deformation of copper (fcc) where the dynamic recovery is so important that a steady state with an almost equiaxed structure is reached at large strains. 6 In those cases, the stress strain curves obtained by different modes of deformation (e.g. wire dr. and torsion) are formally similar to the ratio of the two "orientation factors". 5 On the contrary, if the structural barriers to slip are not randomly distributed, say, if th
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