An investigation of contact deformation, fracture, and fatigue behavior in bulk metallic glasses

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8/8/03

3:28 PM

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An Investigation of Contact Deformation, Fracture, and Fatigue Behavior in Bulk Metallic Glasses C. MERCER, P.T. ANGLIN, R. CIRINCIONE, C. FENTON, M. HUANG, and W.O. SOBOYEJO This article presents the results of a study of the contact-induced deformation, fracture/resistancecurve behavior, and fatigue-crack-growth behavior of two bulk metallic glasses (BMGs), one in the fully amorphous condition and the other containing a dispersion of microscale crystallites. Hertzian contact indentation experiments were performed on both materials under monotonic loading conditions. The contact-induced deformation characteristics observed in the Hertzian experiments are then compared with the stress distribution determined from finite-element analyses. In addition, the cracking patterns associated with resistance-curve behavior in single-edge notched bend (SENB) specimens are incorporated into a fracture-mechanics framework for the estimation of toughening due to microcrack shielding. The predicted steady-state toughness values are shown to be comparable to the measured values obtained from resistance-curve experiments. Subsequently, fatigue-crack-growth rare data obtained at stress ratios of 0.1 and 0.5 are presented and analyzed using a crack-tip opening displacement (CTOD) model. The implications of the results are then discussed.

I. INTRODUCTION

OVER the past two decades, the development of bulk metallic glasses (BMGs)[1] has stimulated significant research efforts to understand their deformation and fracture behavior.[2–10] The pioneering research on deformation mechanisms was done by Spaepen[3] and Argon.[4] These studies demonstrated the importance of free volume in the deformation behavior of metallic glasses. Subsequent studies also revealed the importance of shear banding and the movement of free volume in the deformation behavior of bulk metallic glasses.[1–6] Studies of deformation have been carried out by Donoran, [7] Wright et al., [8] and Lowhaphandu et al.[9,10] The studies by Wright et al. show clearly that compressive deformation is associated with shear bands. The work of Donovan and Lowhaphandu et al. has explored the effects of stress state on flow and fracture processes in amorphous alloys. These investigations show that flow processes in bulk amorphous alloys follow the normal stress acting on the shear plane. Such observations by Donovan resulted in the support of a Mohr–Coulomb criterion, that was later verified in subsequent work by Lowhaphandu et al. However, the flow and fracture stress were not significantly affected by the superposition of hydrostatic pressure. More recently, a number of research groups have studied the fatigue and fracture behavior of BMGs.[11–19] The initial measurements of fracture toughness in BMGs revealed that some Zr-Be–based systems could have precracked fracture-toughness values of 60 to 130 MPa 1m.[11,12,15] C. MERCER, Staff Scientist, P.T. ANGLIN, R. CIRINCIONE, and C. FENTON, Undergraduate Research Assistants, M. HUANG, Postdoctoral Research

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