The development of plastic failure modes in crystalline materials: Shear bands in Fcc polycrystals
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PREFACE
THIS paper
is based on our presentation at the AIME (Cincinnati, OH, 1987) fall symposium on nondestructive evaluation. It represents a report on our new work conducted on the formation of shear bands in polyerystalline metals. Shear bands are a common deformation mechanism in crystalline solids and play an important role in determining material constitutive behavior, as well as being fail~re modes that limit ductility and toughness. The discussion of this work at the symposium was aimed at defining the need to develop quantitative nondestructive methods to observe shear band formation and to measure the density of shear bands in heavily deformed solids. This aim was achieved in the symposium; the paper itself is concerned with a presentation of new experimental results on the mechanisms of shear band initiation and propagation. I. I N T R O D U C T I O N It is well understood now that localized plastic deformation is a natural outcome of large strain plasticity, tLzj The localization of plastic flow affects a wide range of material properties; in metals, localized deformation influences constitutive behavior, texture development, and, in general, serves as a direct precursor to failure, thus limiting ductility and toughness. Shear localization, where plastic flow is concentrated in thin bands, is one of the most common types of nonuniform deformation modes that affect material properties and restrict ductility and toughness. As such, the phenomenon of shear band formation has received a great deal of attention for several H.E. DI~VE, Postdoctoral Research Associate, and R.J. ASARO, Professor, are with Brown University, Box D, Division of Engineering, Providence, RI 02912. This paper is based on a presentation made in the symposium "Nondestructive Evaluation--Predictive and Descriptive Probes of Mechanical Behavior of Metals" presented at the TMS-AIME fall meeting, Cincinnati, OH, October 13, 1987, under the auspices of the TMS Mechanical Metallurgy Committee. METALLURGICAL TRANSACTIONS A
decades, and recent reviews are available, t3,4,5] In forming processes which require large deformations like forging, cold rolling, or machining, localization of plastic flow into shear bands can become the predominant deformation mechanism. [4,61 The review of experimental observations given by Hatherly t4] showed that in cold rolling, shear band formation is almost an inevitable occurrence. Shear localization is often associated with strain softening mechanisms or adiabatic heating and thermal softening. It is commonly recognized now that shear bands also develop in damage free, strain hardening materials and, therefore, should also be viewed as an instability in the material constitutive behavior, t7-12~ In heavily deformed metals, shear localization progressively replaces the current deformation mode, viz., slip a n d / o r twinning. This change in deformation mode contributes to the development of the material anisotropy caused by texture formation and, as such, modifies material properties.t3,4,s~ The review of H
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