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Douglas C. Hofmann Keck Laboratory for Engineering Materials, California Institute of Technology, Pasadena, California 91125; and Liquidmetal Technologies, 30452 Esperanza, Rancho Santa Margarita California 92688

William L. Johnson Keck Laboratory for Engineering Materials, California Institute of Technology, Pasadena, California 91125

Upadrasta Ramamurtya) Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India (Received 14 August 2009; accepted 22 September 2009)

A major drawback in using bulk metallic glasses (BMGs) as structural materials is their extremely poor fatigue performance. One way to alleviate this problem is through the composite route, in which second phases are introduced into the glass to arrest crack growth. In this paper, the fatigue crack growth behavior of in situ reinforced BMGs with crystalline dendrites, which are tailored to impart significant ductility and toughness to the BMG, was investigated. Three composites, all with equal volume fraction of dendrite phases, were examined to assess the influence of chemical composition on the near-threshold fatigue crack growth characteristics. While the ductility is enhanced at the cost of yield strength vis-a`-vis that of the fully amorphous BMG, the threshold stress intensity factor range for fatigue crack initiation in composites was found to be enhanced by more than 100%. Crack blunting and trapping by the dendritic phases and constraining of the shear bands within the interdendritic regions are the micromechanisms responsible for this enhanced fatigue crack growth resistance. I. INTRODUCTION

Bulk metallic gasses (BMGs) have many attractive mechanical properties that include high values of stiffness, strength, hardness, and crack initiation toughness. However, lack of ductility and crack growth resistance and extremely poor fatigue properties are some of the major impediments for widespread structural applications of these materials.1 Plastic deformation in BMGs at low temperatures (i.e., at temperature below 70% of the glass transition temperature, Tg) occurs through localization of shear into narrow bands, referred to as shear bands.1 In the absence of a microstructure, shear bands propagate unhindered, ultimately leading to failure. This is the reason for poor ductility in monotonic loading of unconstrained BMG specimens. Under cyclic loading, shear bands form at relatively low stresses (sometimes as low as 10% of the tensile strength of the BMG) and become crack initiation sites that grow under fatigue loading, leading to relatively low endurance limits vis-a`-vis crystalline metals and alloys.2 Consequently, improving fatigue performance of BMGs has been a major area of recent research. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0439 J. Mater. Res., Vol. 24, No. 12, Dec 2009

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