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I.

INTRODUCTION

A L T H O U G H adiabatic shear bands have been the object of numerous studies for the past 40 years, considerable controversy exists over their structure in steels. White-etching bands have been labeled as "transformed" without irrefutable evidence for such a transformation, while dark-etching bands have been named "deformed." This paper represents an effort at identifying the structure of adiabatic shear bands in steels. White-etching bands and layers are not restricted to adiabatic (high swain rate) deformation conditions. There are many reported cases of similar white-etching zones occurring in contact loading and frictional systems. In 1941, Trent ~11studied the formation of "martensitic" bands formed on rope wire by friction. Zener and Hollomon t2~ were the first to describe that adiabatic heating effects are responsible for the formation of narrow white-etching shear bands. This landmark study was followed by numerous investigations of both a mechanical and metallurgical nature. In a number of these studies, the bands have been classified into "transformed" and "deformed," whether they etch white or dark, respectively. I3'4] This controversy over the nature of the white-etching shear bands is due, in part, to the lack of careful observations and analyses by transmission electron microscopy (TEM). The thickness of these bands being typically of the order of a few microns, TEM is virtually the only

C.L. WlTTMAN, M.A. MEYERS, and H.-r. PAK, all formerly with the Department of Metallurgical and Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, are with Honeywell Defense Systems, Hopkins, MN 55343, the Center of Excellence for Advanced Materials, University of California at San Diego, La Jolla, CA 92093, and the Department of Metallurgy and Mineral Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, respectively. Manuscript submitted July 13, 1987. METALLURGICAL TRANSACTIONS A

technique available for the identification of their structure.* There are only two reports in the literature, and *A recent review on adiabatic shear bands is recommended, t2sj

they both fail to identify incontrovertibly the structure of white-etching shear bands. In 1971, Glenn and Leslie tS~ studied shear bands in 0.6 pct carbon steel produced by ballistic impact. The structure of the white-etching bands was difficult to resolve by TEM, because the grain size was thought to be less than 0.1/zm. The transition from the white-etching region to the matrix was gradual, with the diffraction pattern changing from a diffuse ring to a solid bct ring pattern. Glenn and Leslie tsl postulated that the white-etching region was very rapidly quenched martensite. W i n g r o v e , [6'7] alSO in 1971, observed in the whiteetching regions a high density of dislocations, with some cell boundaries. The microstructure was not typical of normal martensite observed in steel, yet the diffraction pattern indexed to martensite. Upon tempering of the white-etching zone, extra spots