Aspects of dynamic recrystallization in shaped charge and explosively formed projectile devices
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I.
INTRODUCTION
U N U S U A L conditions apply for witnessing jet and slug formation from a shaped charge (SC) or an explosively fbrmed projectile (EFP) device under a high explosive shock. The total time lapse is 100 to 200 ms. In many fbrming operations such as extrusion, deep drawing, and shear forming, the metal flow is conformed to closely follow the contour of a die. But no dies are used to guide the metal flow in jet and slug formation. The phenomena have been observed on numerous occasions for metals and alloys which are prepared by different methods. While copper is commonly used for SC, tantalum is used for both SC and EFP applications. Both metals, when properly prepared, are well known for their excellent plastic flow properties which, as will be discussed later, are essential for successful jet and slug formation. Characteristically, differences exist between copper and tantalum in their respective jet and slug formations. An important factor lies in the intense heat that is generated by the high strain rate plastic flow at >104/s. It gives rise to a number of metallurgical reactions. Among them are recovery and recrystallization of the grain structure which are closely related to a successful continuation of plastic flow and are believed to play a key role in jet and slug formation. In a series of recent studies,t1 71 special attention was given to dynamic recrystallization and its effect on the SC
C. FENG is retired from the Materials Technology Branch, Aeroballistics and Materials Technology Division, AED, ARDEC, Picatinny, NJ 07806-5000. L.E. MURR, Murchison Professor and Chairman, and C.-S. NIOU, Senior Staff Scientist, are with the Department of Metallurgical and Materials Engineering, The University of Texas at E1 Paso, El Paso, TX 79968. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICALAND MATERIALSTRANSACTIONS A
and EFP formation process. It was established that varying degrees of dynamic recrystallization occurred in the respective copper and tantalum jet and slug formations.t4] The present article is largely an extension of the previous studies. It intends to examine other material parameters which may also exert some influence on the process. Grain rotation which was found to accompany slug formation will be analyzed. An attempt will be made to trace the origins of the rotation and to examine the implications arising from the difference in grain rotation between the tantalum and copper slugs.
II.
EXPERIMENTAL ANALYSES
A. Phenomenological Description of Slug Formation At detonation, the high shock pressure induces a stress acting obliquely to the surface of a SC or an EFP liner, less oblique for the EFP. The stress causes the liner to collapse and transform into a jet and a slug. Extensive plastic flow in the grain structure is fou
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