The Duality of Fracture Behavior in a Ca-based Bulk-Metallic Glass
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I.
INTRODUCTION
CA-BASED bulk-metallic glasses (BMGs) have a low density (~2.0 g/cm3), low Young’s modulus (~20 to 30 GPa), low shear modulus (~8 to 15 GPa), low glasstransition temperature (Tg ~373 K [100 C] to 463 K [190 C]), low crystallization temperature (Tx ~403 K to 513 K [130 C to 240 C]), and a wide super-cooled liquid temperature range (DTxg = Tx – Tg 30 K [30 C] to 80 K [80 C]).[1–3] In addition, Ca-based BMGs have a good glass-forming ability (GFA) and are based on two simple metals—Ca and Mg.[3] Because Ca-based BMGs were synthesized successfully,[4,5] numerous Ca-based BMG systems have been discovered and studied.[6–12] The elastic modulus of Ca-based BMGs is comparable with the modulus of human bones and Ca, Mg, and Zn are biocompatible. These features make Ca-Mg-Zn-based alloys attractive for biomedical applications.[2,3,13] Brittleness at temperatures well below Tg is a common feature of many metallic glasses, such as Zr-, Cu-, Mg-, and Fe-based BMGs.[14] Their brittleness generally is explained by limited deformation carriers that can accommodate the loading conditions, such as linear and planar defects, as well as the absence of strain hardening in amorphous structures.[15] Ca-based BMGs are extremely brittle at room temperature.[1,15,16] During compression testing of a Ca65Mg15Zn20 (atomic percent) BMG, many thin pieces were observed to shed progressively from free surfaces of samples as a result of splitting fracture, which eventually exploded into numerous small pieces in the final catastrophic failure.[15,16] GONGYAO WANG, Research Assistant, and PETER K. LIAW, Professor, are with the Department of Material Science and Engineering, The University of Tennessee, Knoxville, TN 37996. Contact e-mail: [email protected] OLEG N. SENKOV and DANIEL B. MIRACLE, Senior Scientists, are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, Dayton, OH 45433. Manuscript submitted April 30, 2010. Article published online November 12, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
A good understanding of the fracture behavior is critically important for the application of Ca-based BMGs. The unusual, catastrophic failure of Ca BMGs makes it difficult to study the fracture mechanisms that operate during the early stages of failure. In the current study, the fracture modes are characterized on fracture surfaces produced by interrupted compressive loading of a Ca65Mg15Zn20 BMG, and the associated failure mechanisms are discussed.
II.
EXPERIMENT
The Ca65Mg15Zn20 (at. pct) BMG alloy was fabricated by induction melting pure elements (99.9 wt pct) in a water-cooled copper susceptor in an argon atmosphere. The prepared alloy was placed subsequently in a quartz crucible with a 2-mm diameter hole at the bottom, induction melted in an argon atmosphere, and injected into a water-cooled copper mold with a 15 mm 9 15 mm 9 4 mm cavity.[1,2] X-ray diffraction and differential scanning calorimetry were used to assess the structure of the 4-mm-thick plates produc
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