Strengthening effects of deformation twins and dislocations introduced by short duration shock pulses in cu-8.7ge
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S H O C K pulses provide a unique means by which large amounts of hardening can be effected in many materials with little or no permanent macroscopic deformation. In addition, because of the time dependent nature of plastic deformation processes, very short pulse durations can be used as a tool to generate fundamental information about the processes, as well as to evaluate the strengthening contributions of various defect configurations? Although the effects of shock pulses on the properties and substructures of materials have been the subject of numerous studies, much of this early work is of questionable value because only the pulse amplitude was controlled? -6 It is now well established that changes in pulse duration can also have profound effects. 2-9 In the current work a fcc solid solution alloy Cu-8.7 Ge was subjected to a wide range of pulse amplitudes and pulse durations. The substructures formed have been characterized in detail and the results have been related to both the degree of hardening and the generation of dislocations and deformation twins. It will be shown that finely-spaced twin boundaries are effective in strengthening and that deformation twins can form in much shorter times than previously indicated. In addition, the formation of the dislocation substructure is strongly time dependent, however, the strengthening contribution at any time is in agreement with the empirical relation A~ ~ Or2.
S. LAROUCHE and E. T. MARSH, formerly Graduate Students at Michigan TechnologicalUniversity,are now with Ford Motor Co., Dearborn, M1 48121 and Joslyn Manufacturing Co., Woodstock, IL 60098, respectively.D. E. MIKKOLA is Professor of Metallurgical Engineering, Michigan TechnologicalUniversity,Houghton, MI 49931. Manuscript submitted April 16, 1979. METALLURGICALTRANSACTIONSA
EXPERIMENTAL PROCEDURE The Cu-8.7 at pet Ge used in the study was initially in the form of 10 mm thick bar stock. Disk-shaped specimens, 18 mm in diam, were cut from the material after roiling, annealing, and then rolling 45 pct reduction in thickness to a thickness of 1.5 ram. In order to minimize radial release effects during shocking, the specimens were press fit into 39 mm diam brass guard rings, s.~0Brass was chosen because the shock impedance matches that for Cu-8.7 Ge. The specimenguard ring assemblies were metallographically polished through 0.3/~ alumina abrasive to give a final thickness o f I mm +_ 0.025 mm with the surfaces kept flat and parallel well within 0.025 mm. The polished assemblies were then given a recrystallization anneal at 380 ~ for 3 h. in vacuum. This gave a final grain size of ~ 3/xm with a hardness of 160 VHN. Close-fitting brass spall plates 1 cm thick were used to eliminate back surface reflections and to ensure that each specimen was subjected to only a single shock pulse. The shock loading of the specimens was done by Effects Technology, Inc. with the exploding foil-flyer plate technique. A schematic of the shock loading set-up along with the specimen assemblies is shown in Fig. 1. Shocking is accom
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