Microstructural study of adiabatic shear band
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INTRODUCTION
I N materials deformed at high strain rates, plastic flow localizes in narrow regions which are known as adiabatic shear bands, tl-41 These shear band regions often act as precursor sites for eventual failure of the structural component through a loss in the load carrying capacity of the highly deformed material. Failure of materials by adiabatic shearing occurs in ballistic impact, machining, and high strain rate shaping and forming. [5-27J Materials susceptible to forming shear bands include steels, [5-~51Ti alloys, tl6-19] Cu alloys, [2~ A1 alloys, [24-26] A1-SiCw composites,t27] e t c . Since adiabatic shear banding is a very important phenomenon in dynamic deformation, a number of mechanical and metallurgical investigations have been performed on ballistically impacted structures. [5-23.28-30] It is generally believed that strain localization is attributed to a thermomechanical instability. I1,31 Temperature rise within the locally deformed region leads to softening of materials. If the influence of this thermal softening outweighs the effect of strain-hardening rate in the rapidly deforming region, the strain localization process tends to become more accentuated. 15,61Temperature rise within the shear band of an HY-100 steel has been measured by Marchand and Duffy161and Chi and Duffy. t5j Their results have shown that the higher the local strain, the higher the temperature rise within the shear band, and that the temperature of the shear band was measured to be about 600 ~ Because of the temperature rise and the high plastic strains, the microstructure within the shear band might be modified. There is, however, only limited information available on the formation mechanism of the SUNGHAK LEE, Associate Professor, is with the Department of Materials Science and Engineering, Pohang Institute of Science and Technology, Pohang, 790-600 Korea. KYUNG-MOX CHO, Assistant Professor, is with the Department of Metallurgical Engineering, Pusan National University, Pusan, 609-735 Korea. CHANG SUN LEE, Research Engineer, and WUNG YONG CHOO, General Manager, are with the Department of Steel Products, Research Institute of Industrial Science and Technology, Pohang, 790-600 Korea. Manuscript submitted July 17, 1992. METALLURGICAL TRANSACTIONS A
adiabatic shear band in high strength steels, despite the fact that it is a critical issue in analyzing ballistic performance. The shear band microstructure has been explained differently among investigators, e.g., untempered martensite containing carbides, t9,1~ highly dislocated cell s t r u c t u r e , [8,13-15,19,32-33] and 6-ferrite, u21 thereby leading to confusion in microstructural analyses of the adiabatic shear band. The objectives of the present study are to observe the detailed microstructures of adiabatic shear bands formed in an HY-100 steel upon ballistic impact and to elucidate fundamental understanding of the microstructural development within the adiabatic shear bands. Possible mechanisms for the shear banding processes are also suggested to explain
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