The large strain deformation of some aluminum alloys
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I.
INTRODUCTION
T H E stress-strain response to large strains of both single and polycrystalline material has been receiving increasing attention in the recent literature. 1-5 The importance of this behavior in influencing material formability and fracture has also been discussed.3 In the case of polycrystalline material, the published results show that regardless of composition, materials show a two-stage behavior, an initial stage where the work hardening rate decreases quite rapidly, followed by a region beyond a true strain of 1.0 of approximately constant work hardening rate. There is also some evidence for a third stage at very high strains where the work hardening rate goes to zero. 6 The constant work hardening regime has been discussed in some detail by Kuhlmann-Wilsdorf 7 showing that an inverse dependence of flow stress on subgrain size is consistent with such behavior. The development of a zero work hardening rate region or of a saturation in the flow stress led Kocks 8 to suggest that the stress-strain behavior to large strains is consistent with the constitutive equation suggested by Voce:
o',
-
o"
o's -- o'l
= exp
(-e
-
ec
el)
[1]
where o-i is the measured yield stress, el is an arbitrary strain usually taken as zero, ec is a characteristic strain for the material, o- is the value of the flow stress at a strain e, and % is the stress at which the strain hardening rate becomes zero. Some recent results on a series of particle containing A1 alloys 5 indicated that Eq. [1] needed to be modified to fit the stress-strain behavior of these alloys but the concept of a saturation stress was maintained. In the case of single crystal deformation Essman and Mug~abi 9 have considered the microscopic process of dislocation annihilation associated with a saturation in the flow stress. The present paper considers the stress-strain behavior of a series of A1 alloys deformed to large strains. Several different modes of deformation have been utilized, such as wire drawing, torsion, and compression. The latter two are D. J. LLOYD, Research Scientist, and D. KENNY, Research Staff, are with Alcan International Limited, Kingston Laboratories, Kingston, Ontario, Canada. Manuscript submitted April 13, 1981. METALLURGICAL TRANSACTIONS A
continuous tests in that the stress and strain were measured continuously during the deformation. In the case of wire drawing the stress-strain behavior was assessed after the deformation by means of an additional tensile test.
II.
EXPERIMENTAL REMARKS
Torsion tests were carried out on cylindrical specimens with a 2.5 cm gauge length of 0.5 cm diameter. The shear stress in the outer fiber was obtained from the torque-twist curve using the analysis due to Nadai. 17The shear stress (~') and shear strain (7) were converted to the effective tensile stress (-~) and strain (3) via -~ = V 3 r and -e = y / ~ / ' 3 . Compression tests were carried out on 0.5 cm diameter cylinder, 1.0 cm long using teflon as a lubricant. Wire drawing was carried out on a laboratory drawing bench at slo
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