High-Temperature Deformation Behavior and Formability of a Zr-Cu-Al-Ni Bulk Metallic Glass
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ZIRCONIA-BASED bulk metallic glasses (BMGs) are now well known to have an excellent glass-forming ability due to their sufficiently low critical cooling rates as low as 1 to 100 K/s together with a very wide supercooled liquid region (SLR), which is commonly characterized by DT = (Tx – Tg) with Tx and Tg denoting the crystallization onset and the glass transition temperature, respectively.[1–3] These Zr-based BMGs have also attracted great attention because of their outstanding mechanical properties in terms of high strength up to 2 GPa, low YoungÕs modulus, and high toughness at room temperature.[3–5] Furthermore, the application fields of these alloys as structural materials can be widely extended with the ability to provide bulk specimens, which enables determination of the reliability and reproducibility through sufficient tests for mechanical properties and deformation behavior. However, these BMGs are generally known to have very low ductility and toughness at room temperature due to nucleation and propagation of shear bands,[5,6] so that the subsequent forming process is very difficult at room temperature. Different from this, by using the unique characteristics of viscous flow of BMGs in the SLR around the glass transition, the near-net-shape fabrication process is particularly considered as the most H.-J. JUN, Graduate Research Assistant, and Y.W. CHANG, Professor, are with the Department of Materials Science and Engineering, POSTECH, Pohang 790-784, South Korea. Contact e-mail: [email protected] K.S. LEE, Postdoctoral Research Fellow, and J. ECKERT, Director and Professor, are with the Institute for Complex Materials, IFW Dresden, Dresden D-01171, Germany. This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online September 13, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
promising one for forming applications.[7] On the other hand, the BMGs are prone to structural relaxation or crystallization to change the unique viscous properties of the amorphous matrix when deformed in the SLR for a considerable time, even at temperatures well below the crystallization temperature.[8,9] Excluding the aforementioned structural evolutions, monolithic BMGs were observed to exhibit two distinctly different deformation modes depending on the test conditions. At a high-stress and low-temperature regime, the BMGs exhibited an inhomogeneous plastic flow leading into a catastrophic failure without any macroscopic plasticity regardless of the strain rate applied, possibly due to the localized shear strain in a few shear bands.[10,11] Homogeneous deformation behavior has been observed, on the other hand, at a low-stress and high-temperature regime, in which every volume element of an entire specimen is strained or compressed homogeneously. The stress-strain curves obtained in this homogeneous
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