Plastic constraint and ductility in tensile notched specimens of amorphous pd 78 cu 6 si 16
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INTRODUCTION
MOSTamorphous alloys can be bent completely back onto themselves without fracturing, but tensile plastic strain is very small because the specimen undergoes highly-localized shear deformation. 1,2,3The measure of ductility in this case, i.e., the bend-back angle of 180 deg, indicates that an amorphous alloy is very ductile. However, the latter quantity, the small fracture-energy given by the stress-strain curve, shows that this alloy is a low-toughness material, in the engineering sense of the term. The ductility of an amorphous alloy depends on the measure of variable which a testing method provides for ductility. The extensive ductility is demonstrated by: (1) a total area reduction of 93 pct in drawing after multiple passes for a wire specimen of amorphous Pd77.sCu6Si16.5; 4 (2) thickness-reduction in rolling of 40 pct for amorphous Pd80Si20(in these processes, the materials flow without fracture); 5 and (3) complete sliding off in a tensile specimen of amorphous Pd80Si20 (i.e., area reduction of 100 pct) in an extremely "hard" testing machine. 6 These measures of ductility, however, are too empirical to allow a quantitative understanding of the origin of ductility in amorphous alloys. They do not provide a test condition of the level of hydrostatic tension that is conducive to cracking. On the other hand, the fracture toughness (K~c, Kmc) of some Fe-based amorphous alloys is comparable to that of high-strength materials such as a maraging steel.7'8'9 Linear fracture mechanics is, needless to say, useful in a quantitative description of the resistance of materials to crack propagation. However, most as-quenched amorphous alloys (i.e., amorphous Pd78Cu6Si16 in V-notch bending tests) do not exhibit crack formation below general yielding, 10.. and so these fracture modes do not represent valid conditions of fracture toughness. Furthermore, the values of K1c for Febased alloys and their plate-thickness dependence vary strongly on quenching conditions. 12 This dependence indicates that, at present, it appears to be difficult to offer the fracture toughness value (K~c) as an accepted parameter
H. KIMURA, Research Associate, and T. MASUMOTO, Professor, are both with The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan. Manuscript submitted September 2, 1982. METALLURGICALTRANSACTIONS A
of toughness for a variety of amorphous compositions, nor can we easily formulate a rule for the mechanical deformation process which is based on any concept of fracture toughness. In order to evaluate the ductility of amorphous alloys both beyond general yielding and in a triaxial stress state, we are going to present as fracture parameters the plastic constraint factors of notched specimens. At the same time, we attempt to set up a mechanics of notched tensile deformation that will be applicable to a variety of amorphous alloys. Notched-bar fracture mechanics successfully account for annealing-induced embrittlement for amorphous Pd78Cu6Si16, since a well-known slip-line-field
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