Deformation Induced Dilatations and New Observations on Fracture in Compression in Metallic Glasses at Low Temperatures
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B.H. Kear,
B.C.
283
Giessen, and M. Cohen, editors
DEFORMATION INDUCED DILATATIONS AND NEW OBSERVATIONS ON FRACTURE IN COMPRESSION IN METALLIC GLASSES AT LOW TEMPERATURES
JANEZ MEGUSAR, ALI S. ARGON AND NICHOLAS J. GRANT Massachusetts Institute of Technology, Cambridge, Mass.
02139,
USA
ABSTRACT Deformation induced dilatations were measured on a Pd-Cu-Si glass by a differential dilatometric method during crystallization. At large pre-strains, these dilatations are large and of the order of 20%. Fracture surfaces of specimens in compression show either the characteristic features of tensile separation or evidence for repeated nucleation of fracture. Rubbing was observed only in a small fraction of a fracture surface.
INTRODUCTION It has been proposed that metallic glasses, viewed as dense, randomly packed, hard sphere solids, should undergo large shear induced dilatations during plastic flow at low temperatures, and that this effect should be responsible for the strong shear localization found in these materials [1,2]. Such dilatancy has been observed directly by Argon and Kuo [3] in sheared soap bubble rafts which, as they have discussed, are ideal analogs to metallic glasses. Based on these observations and making certain assumptions on the thermal dispersal of free volume, Argon [2] calculated the rate of shear localization in metallic glasses at low temperatures and suggested that the rate of accumulation of excess free volume should be substantial inside the bands, and that the rate of localization would be very rapid. The geometrical aspects of such localization in tension have been studied by several workers - most recently by us [4]. On the whole, the subject of shear localization resulting from dilatancy in solids is, of course, much broader. It has been studied by geophysicists and workers in applied mechanics who have calculated the angles that shear localization zones make with the principal axes of deformation and how these angles depend on the level of dilatancy [5,6]. Here we report experimental results on dilatancy in glassy Pd77.5Cu6Si 16.5 We also report new observations on fracture in compression. EXPERIMENTAL DETAILS Since shear localization in glassy alloys in tension normally initiates separation by shearing-off a fracture by the meniscus convolution process [7], it is essential to perform the experiments in compression where the sample can be mechanically stabilized. Since the critical cooling rates for retention of a glassy structure are relatively low (~103 K/s) in Pd 7 7 5 Cu6 Si 1 6 . 5 alloy, specimens of several millimeters diameter are obtainable with this material on which useful compression experiments can be performed [8]. Rods of approximately 2 mm diameter of this alloy were obtained in glassy form by the technique of Pampillo and Chen [8], were centerless ground, electropolished, and contacted with polishing cloth bearing 1 Um diamond polish while spinning on a lathe to inscribe circumferential reference marks on the lateral surfaces to serve as convenient shear displacem
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