Microstructural Evolution and Mechanical Properties of Zr-Cu-Ni-Al Bulk Metallic Glasses by the Bridgman Solidification

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I.

INTRODUCTION

DUE to their high glass-forming ability (GFA), easy fabrication, outstanding mechanical properties, and lack of both noble elements and toxic elements, Zr-Cu-Ni-Al bulk metallic glasses (BMGs) have been examined extensively,[1–10] and several BMGs have been developed that can be produced with centimeter size dimensions.[1–6] However, due to the complexity and sensitivity in the composition governing the GFA, some results have proved difficult to repeat by different research groups; for example, the critical cast diameter sizes of Zr55Cu30Ni5Al10 and Zr65Cu17.5Ni10Al7.5 were estimated to be 30 and 16 mm, respectively, using the conventional metallic mold cast technique in References 4 and 5, but are less than 10 and 6 mm in Reference 2. Thus, it is important to understand the foundation of the GFA in Zr-Cu-NiAl BMGs. In this study, we designed a novel alloy Zr51.7Cu30Ni8.3Al10, which is close to the famous Inoue alloy Zr55Cu30Ni5Al10[4] and to Zr50.7Cu28Ni9Al12.3, which has a critical diameter of 14 mm,[2] and used a Bridgman growth technique to relate the microstructural evolution to the withdrawal velocity. In so doing, we demonstrated that the competition between glass formation and crystal nucleation and growth controls the GFA. Further, we also investigated the effects of the withdrawal velocity and, hence, the microstructural evolution on the mechanical properties. J.L. CHENG and H.W. XU, Graduate Students, G. CHEN, Professor, and F. XU and Y.L. DU, Associate Professors, are with the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China, and the Jiangsu Institute of Advanced Materials, Danyang 212300, Jiangsu, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted January 8, 2011. Article published online June 17, 2011 2620—VOLUME 43A, AUGUST 2012

II.

EXPERIMENTAL PROCEDURE

Alloy button ingots of Zr51.7Cu30Ni8.3Al10 (at. pct) were prepared by arc melting the mixtures of Zr, Cu, Ni, and Al metal chips with purities higher than 99.9 wt pct under a Ti-gettered argon atmosphere. The rod-shaped samples with 3-mm diameters were fabricated in an Ar atmosphere through induction melting the master alloy ingot in a quartz tube and ejecting it into a copper mold. Then, the rod-shaped samples were placed into a Bridgman furnace (details are described in Reference 11). The Bridgman technique grew using withdrawal velocities of 0.1 to 6 mm/s (V). A temperature gradient (G) of about 17 K/mm (17 C/mm) was used. To avoid oxidation, the remelting process of the alloys was carried out used a vacuum of 5 9 10 3 Pa. The structure of the samples was analyzed by using a RIGAKU* D/max 2038 X-ray diffractometer (XRD) *RIGAKU is a trademark of Rigaku, Tokyo, Japan.

with Cu Ka radiation operated at 40 KV and 40 mA. Measurements were performed by step scanning 2h from 20 to 90 with a 0.02 deg step size, and a count time of 1 second per step was used. Thermal analysis was

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