Fatigue behavior and fracture morphology of Zr 50 Al 10 Cu 40 and Zr 50 Al 10 Cu 30 Ni 10 bulk-metallic glasses G.Y. Wang a, * , P.K. Liaw a , W.H. Peter a , B. Yang a , M. Freels a , Y. Yokoyama b , M.L. Benson a , B.A. Green a , T.A. Saleh a , R.L. McDaniels a , R.V. Steward a , R.A. Buchanan a , C.T. Liu c , C.R. Brooks a a Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA b Materials Science and Engineering, Himeji Institute of Technology, Shosha 2167, Himeji, Japan c Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Available online 7 June 2004 Abstract In the present work, high-cycle fatigue (HCF) experiments were conducted on zirconium (Zr)-based bulk-metallic glasses (BMGs): Zr 50 Al 10 Cu 40 and Zr 50 Al 10 Cu 30 Ni 10 , in atomic percent. The HCF tests were performed, using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 and under tension – tension loading. Note that R ¼ s min =s max ; where s min and s max are the applied minimum and maximum stresses, respectively. The test environments were air and vacuum. A high-speed and high-sensitivity thermographic-infrared (IR) imaging system was used for the nondestructive evaluation of temperature evolutions during fatigue testing of the BMGs. A sparking phenomenon was observed at the final fracture moment of Zr 50 Al 10 Cu 30 Ni 10 in air, while a bright notch section was observed near the final fracture moment of these two BMGs in vacuum. The effect of the chemical composition on the fatigue behavior of the Zr-based BMGs was studied. The fatigue-endurance limit of Zr 50 Al 10 Cu 30 Ni 10 (865 MPa) was somewhat greater than that of Zr 50 Al 10 Cu 40 (752 MPa) in air. The fatigue lives in vacuum and air were generally found to be comparable. The fatigue-fracture surface consisted of four main regions: the fatigue crack-initiation, crack-propagation, final-fast-fracture, and apparent-melting areas. Apparent fracture toughness was determined from the measurement of the crack-propagation region of the fatigue-fractured surface. The fracture-toughness values of Zr 50 Al 10 Cu 40 were greater than those of Zr 50 Al 10 Cu 30 Ni 10 . The vein pattern and droplets with a melted appearance were observed in the apparent melting region. There were microcracks on the outer surface of the specimen, which could be associated with multiple fatigue-crack-initiation sites. These microcracks might result from shear bands and shear-off steps. q 2004 Elsevier Ltd. All rights reserved. Keywords: B. Glasses, metallic; B. Mechanical properties at ambient temperature; F. Electron microscopy, scanning; F. Non-destructive evaluation 1. Introduction Since the synthesis of glasses in metallic systems in 1960 [1] and the development of bulk-metallic glasses (BMGs) during the early 1990s [2,3], several novel multi- component BMGs, such as Zr–Al–Ni [3], Zr–Al–Cu–Ni [4], and Zr–Ti–Cu–Ni–Be [2], have been discovered, which exhibit exceptional glass-forming abilities. Thus, BMGs broke through the limitation of the specimen geometry, because it is possible that the critical-cooling rate could be less than 10 3 K/s. The lack of any long-range order and the subsequent absence of microstructures have led to a range of interesting properties, which are desirable for the applications as potential structural materials. These properties include high strengths (, 2 GPa), ductilities in compression, low coefficients of friction, high wear resist- ances, high corrosion resistances, low shrinkages during cooling, and almost perfect as-cast surfaces [5–8]. The mechanical behavior of metallic glasses is being widely studied, but the real nature of the deformation mechanisms in these amorphous alloys still remain unclear because traditional notions of crystal defects (e.g., dislo- cations and grain boundaries) do not apply [9]. The flow in BMGs, which appears to be related to a local change in the viscosity in shear bands near planes of the maximum shear, is extremely inhomogeneous at high stresses and low temperatures. Two hypotheses emerged in the 1970s to explain why this trend may be the case: the free-volume 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.04.038 Intermetallics 12 (2004) 1219–1227 www.elsevier.com/locate/intermet * Corresponding author. Tel.: þ 1-865-974-1000. E-mail address: gwang@utk.edu (G.Y. Wang).