Elsevier

Materials Science and Engineering: A

Volume 655, 8 February 2016, Pages 321-330
Materials Science and Engineering: A

Self-organization of adiabatic shear bands in ZK60 Magnesium alloy

https://doi.org/10.1016/j.msea.2016.01.008Get rights and content

Abstract

Self-organization of shear bands in the ZK60 magnesium alloy was investigated by means of the radial collapse of the thick-walled cylinder technique. The distribution, the width, and the spacing of the multiple shear bands were examined under different effective strains. Optical microscopic examination showed the evolution of multiple shear bands was a competitive process, and influenced each other. The perturbation source such as precipitated phase and grain boundary can affect shear-band propagation, and leading to merging and bifurcation. The shear bands propagated along clockwise or counter clockwise direction at 45° or 135° with the radius from the internal boundary of the ZK60 cylindrical specimens, and exhibited approximately a symmetrical distribution pattern without obvious optimum selecting in direction. Results from an experimental investigation, the shear-band averaged width increased with the increase of the effective strain, varied between 3.83 µm and 9.87 µm with the effective strain varying from 0.57 to 0.88. The shear-band spacing was measured and compared with theoretical predictions in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models. The results are closer to the theoretical predictions of the perturbation models, which indicated the shear-band spacing was determined by small perturbation of initiation, and interaction effect of multiple shear bands also cannot be ignored during the propagation process.

Introduction

Magnesium (Mg) alloys have received considerable attention in recent years due to their low density and high specific strength. The applications in the aerospace and automotive become wider and wider. And Mg alloys are also potential candidate materials in armor applications. They would suffer inevitably dynamic loading in applications. When subjected to high strain rates, most ductile materials develop highly localized shear deformation in narrow bands with the width of 1–100 µm. These bands are called adiabatic shear bands (ASBs). The great majority of studies [1], [2], [3], [4], [5] addressed an individual shear band to investigate the mechanistic and microstructural features of ASB. However, in engineering conditions multiple shear bands always are observed during dynamic impact. Micro-cracks preferentially nucleated from the shear bands, and subsequently gathered, grew and extended along shear band in certain condition [6]. The distribution of cracks depends on self-organization of shear bands.

Self-organization of shear bands ultimately exhibited steady shear-band spacing and trajectory. The thick-walled cylinder (TWC) method was introduced by Nesterenko [7], [8], and was successfully produced multiple shear bands to study shear-band spacing. Grady [9], Grady and Kipp [10], Wright and Ockendon [11], and Molinari [12] proposed momentum diffusion and perturbation theoretical model, which used to describe the shear-band spacing. Meyers [13] carried out many experiments using the TWC methods in several materials, including metals (Ti, Ti–6Al–4V, 304SS), ceramics (Al2O3 and SiC), polymers (teflon), and metallic glass(Co58Ni10Fe5Si11B16), revealed that the shear bands do not occur in a random manner, but with a well-established and self-organized pattern. Xue investigated self-organization of shear bands in commercially pure titanium, Ti–6Al–4V alloy [14] and stainless [15], [16]. Liu [17] proposed a numerical model for adiabatic shear bands with application to a thick-walled cylinder in 304 stainless steel and found a close agreement on shear band spacing and propagating velocity of the shear band between the simulation and experimental results. The author previously conducted some work on self-organization of shear bands in Ti-1300 alloy [18] and 7075 aluminum alloy [19]. To date, only very limited information is available on the dynamic properties of any Mg alloys. Thus, it is significant to investigate self-organization behavior of Mg alloy for understanding the dynamic damage laws of material.

In this work, ZK60 alloy was used to investigate high strain rate behavior by means of TWC technique. ZK60 belongs to series of Mg–Zn–Zr and is representative high strength magnesium alloy. ZK60 offers good comprehensive performances and it is often applied for car body panels, space frame and aircraft envelope [20], [21]. Self-organization of shear bands was analyzed on ZK60 specimens for understanding the dynamic damage laws of Mg alloys.

Section snippets

Materials and experiment

Table 1 shows the chemical composition of the ZK60 alloy. The materials used in this work are extruded rod of ZK60 magnesium alloy (T5).

The experiment was conducted through the radial collapse of a thick-walled cylinder under high strain rate. The TWC method was introduced by Nesterenko et al. The technique is described in detail elsewhere [22]and will be only briefly presented here. The experimental configuration is showed in Fig. 1. The specimen is sandwiched between the internal and external

The shear-band character during propagation

Fig. 3 shows the shear-band characteristic pattern during propagation of multiple shear bands. The evolution of multiple shear bands was competitive process, and coupled with some important features of shear bands, such as merging and bifurcation. Some shear bands disappeared, and some shear bands continued to propagate along spiral direction. The merging of major shear band 1 and shear band 2 was observed in Fig. 3(a), shear bands propagated along the same direction after encountering of the

Conclusion

Multiple shear bands were produced by means of the TWC technique in ZK60 Mg alloy. The shear band nucleated from the internal boundary of the ZK60 cylindrical specimens and developed outwards. The evolution of multiple shear bands with different effective strains, namely self-organization behavior, including shear-band distribution, shear-band width characteristics, and shear-band spacing, was investigated in the work.

The evolution of multiple shear bands was a competitive process. Shear bands

Acknowledgments

This work is supported by the National Natural Science Foundation of China (No. 51274245 and 51574290), NSAF (No. U1330126), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20120162130006), the Hunan Provincial Natural Science Foundation of China (No. 14JJ2011), and the key project of State Key Laboratory of Explosion Science and Technology (No. KFJJ11-1).

References (32)

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