Shear Localization in Dynamic Deformation: Microstructural Evolution YONGBO XU, JINGHUA ZHANG, YILONG BAI, and MARC ANDRE ´ MEYERS Investigations made by the authors and collaborators into the microstructural aspects of adi- abatic shear localization are critically reviewed. The materials analyzed are low-carbon steels, 304 stainless steel, monocrystalline Fe-Ni-Cr, Ti and its alloys, Al-Li alloys, Zircaloy, copper, and Al/SiC p composites. The principal findings are the following: (a) there is a strain-rate- dependent critical strain for the development of shear bands; (b) deformed bands and white- etching bands correspond to different stages of deformation; (c) different slip activities occur in different stages of band development; (d) grain refinement and amorphization occur in shear bands; (e) loss of stress-carrying capability is more closely associated with microdefects rather than with localization of strain; (f) both crystalline rotation and slip play important roles; and (g) band development and band structures are material dependent. Additionally, avenues for new research directions are suggested. DOI: 10.1007/s11661-007-9431-z Ó The Minerals, Metals & Materials Society and ASM International 2008 I. INTRODUCTION LOCALIZED shear deformation in the form of intensive deformation in a narrow band generated during dynamic deformation under high strain rates has been a topic of great interest for decades and, thus, a great deal of investigation has been conducted experi- mentally and theoretically since Zener and HollomonÕs classic article of 1944. [1] It is interesting to note that Tresca [2] had already observed this phenomenon in the nineteenth century. Localized shear is an important mode of deformation; it leads to catastrophic failure with low ductility and it occurs frequently during high- strain-rate deformation, such as is found in ballistic impact, explosive fragmentation, high-speed shaping and forming, dynamic compaction and welding, machining, and grinding. This deformation mode may also occur during quasi-static loading, such as uniaxial extension and cyclic fatigue. The failure of metallic glasses and, in particular, bulk metallic glasses is a classic example of shear localization, and it has been shown by Lewandowski and Greer [3] that temperature plays a role: significant temperature rises were mea- sured. Similarly, nanocrystalline metals are prone to shear localization when deformed at low strain rates and, although the imposed displacement velocities are small, the evolution of shear bands is dynamic and leads to early failure by virtue of a near absence of work hardening (e.g., Jia et al. [4] and Wei et al. [5] ). The phenomenon is clearly recognizable in most steels and in other metals, including aluminum alloys, copper, titanium, zirconium, and uranium and their alloys; aluminum composites reinforced with SiC particles and whiskers; and engineering plastics. Mechanical engineers have focused their efforts on the macrodescription of the constitutive model, developing the criteria required for the plastic deformation insta- bility (Recht in 1964, [6] Culver in 1973, [7] Clifton in 1980, [8] Bai in 1981, [9] Burns and Trucano in 1982, [10] Pan in 1983, [11] Semiatin et al. in 1984, [12] Wu and Freund in 1984, [13] Wright and Batra in 1985, [14] Fressengeas and Molinari in 1987, [15] Johnson in 1981, [16] Drew and Flaherty in 1984, [17] Lemonds and Needleman in 1986, [18] Tvergaard in 1987, [19] and Anderson et al. in 1990 [20] ). Most of the approaches consist of a combination of mechanical and thermal instability analysis. On the other hand, materials scien- tists have focused on the material and structural aspects of localized shear deformation, emphasizing the effect of the microstructures on the formation of the shear bands. Regarding the microstructural aspects of shear local- ization, there are a number of reviews: Rogers, [21,22] Stelly and Dormeval, [23] Timothy, [24] Murr, [25] Dormeval, [26] and Meyers. [27,28] Among the numerous articles on the topic, the articles indicated in Table I are noteworthy. In this article, we will review results of the micro- structural aspects of the adiabatic shear localization generated under an imposed strain rate range of 10 3 to 10 4 s -1 at ambient temperature, resulting from research carried out by the authors over the past 20 years. YONGBO XU and JINGHUA ZHANG, Professors, are with the Shenyang National Laboratory of Materials Sciences, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: ybxu@imr.ac.cn YILONG BAI, Professor, is with the State Key Laboratory for Non-Linear Mechanics of Continuous Media, Institute of Mechanics, Chinese Academy of Sciences, Beijing, P.R. China. MARC ANDRE¢ MEYERS, Professor, is with the Materials Science and Engineering Program and Department of Mechanical and Aerospace Engineering, University of California at San Diego, La Jolla, CA 92093-0411. This article is based on a presentation made in the symposium entitled ‘‘Dynamic Behavior of Materials,’’ which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ ASM Mechanical Behavior of Materials Committee. Article published onlined February 13, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A, APRIL 2008—811