Bauschinger effect and residual phase stresses in two ductile-phase steels: Part I. The influence of phase stresses on t
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I.
INTRODUCTION
THE
Bauschinger effect is an important mechanical phenomenon in metals and alloys. As put forth initially, the BE is inferred in the decrease in material yield stress following prestrain in the reverse direction. It was subsequently recognized that the BE can be more comprehensively thought of as a manifestation of anisotropic plasticity induced in an isotropic material by plastic deformation. Two main kinds of microscopic mechanisms have been advanced to explain the Bauschinger effect. H'21 First, long-range internal stresses may be present in single-phase polycrystalline as well as multiphase materials. Dislocation pileups at grain boundaries, Orowan loops, and the plastic incompatibility among grains and between particles and matrices are the main sources of such long-range internal stresses. Second, short-range stress fields can also induce the BE, even in superpure metal single crystals. For example, Orowan t31 suggested that anisotropy in the resistance to dislocation motion is introduced by prestrain; e.g., it is easier to move dislocations in the reverse than in the forward direction following prestrain. The Bauschinger effect in two ductile-phase materials has been widely investigated. It has been recognized that the BE in two-phase materials is much more pronounced than in single-phase materials, c4-1~ This was ascribed to the fact that long range internal stresses in each constituent phase produced by the misfit strain between them, which are often called "phase stresses" in two-phase materials, are much higher compared to the long-range internal stress acting over the scale of grain size in singlephase materials. However, until now, the relationship between the parameters of the BE and phase stresses is very sparsely identified. Finite-element methods (FEM's) have recently been used to calculate the tensile stress-strain curve of twophase structures from the stress-strain curves of the comLI ZHONGHUA, Associate Professor, and GU HAICHENG, Professor, are with the Department of Engineering Mechanics and Research Institute for Strength of Metals, respectively, Xi'an Jiaotong University, Xi'an, People's Republic of China. Manuscript submitted November 16, 1988. METALLURGICAL TRANSACTIONS A
ponent p h a s e s . [11-14] However, little attention has been paid to calculating the compressive flow curves subsequent to tensile deformation of two-phase materials. In the present study, an FEM was employed to simulate the BE in dual-phase steels, using both isotropic- and anisotropic-hardening models. The chief aim of our research is to assess the role of phase stress on the BE and to establish a clear physical picture and appropriate mechanical models for the BE phenomenon in two-phase materials.
II. EXPERIMENTAL P R O C E D U R E AND R E S U L T S Pure iron and three steels, having different carbon contents, were studied. The chemical analyses, heat treatments, and microstructures of these materials are given in Table I. Alloys 2 and 3 are dual-phase steels with 16 and 40 vol pct martensite, r
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