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I. INTRODUCTION

THERE are many practical applications involving material deformation under dynamic conditions, from metal forming to projectile impact and penetration. Understanding material behavior under such extreme conditions is fundamental to improving performance and minimizing failure in many engineering systems. This has driven the considerable progress that has been achieved modeling macroscopic dynamic deformation of materials.[1–5] The agreement between experiments and models has been generally good; however, most models cannot account for local deformation modes that lead to failure, including strain localization at and around grain boundaries (GBs) and the propagation of slip bands through the microstructure. Hence, predictions of failure in polycrystalline materials require the consideration of operative failure mechanisms at GBs and inside the grains. In particular, GBs may act as barriers to the propagation of transgranular failure modes or as sites of damage initiation. These modes can be affected by strain heterogeneity at GBs due to material anisotropy. Experimental studies have shown that initiation and propagation of damage in fcc and bcc bicrystals and polycrystals under quasi-static loading are related to boundary misorientations.[6] The failure mechanisms under dynamic conditions have been examined by Mescheryakov et al.,[7] who studied the dynamic response of steel. They found that the dispersion of the velocity at the grain level is of the same order of magnitude as the mean flow and that local deformation can exert a strong influence over the dynamic response. The study of these local modes P. PERALTA, Associate Professor, and E. LOOMIS and C.H. LIM, Graduate Students, are with the Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287-6106. Contact e-mail: [email protected] D. SWIFT and K.J. McCLELLAN, Technical Staff Members, are with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted June 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

in polycrystals is complicated due to the microstructure variations. Hence, bicrystals are appropriate models of individual boundaries for use in understanding the dynamic deformation mechanisms associated with interfaces. In this article, initial results are presented of the dynamic behavior of the NiAl single crystals and bicrystals tested using laserinduced shock waves.[8,9,10] Details of the experimental techniques used are given next. II. EXPERIMENTAL Dynamic behavior was studied with small-scale samples (3 to 5 mm in diameter, 100- to 400-
m thick) using a laserdriven testing setup at the TRIDENT facility at Los Alamos National Laboratory (LANL) in New Mexico. Direct laser irradiation was used, whereby a laser beam is shined directly on the back of the sample, forming hot plasma that supports the shock,[11] as shown in Figure 1. Laser-driven techniques offer a number of advantages over traditional methods; e.g., the smaller sample size makes it more practical to study valuable sin