A Preliminary Study on The Behavior of Notched Ni 3 Al(B): Fracture Mode Transition
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A PRELIMINARY STUDY ON THE BEHAVIOR OF NOTCHED Ni3AI(B): FRACTURE MODE TRANSITION
YIXIN XU AND ERLAND M. SCHULSON Thayer School of Engineering, Dartmouth College, Hanover, NH 03755
ABSTRACT Tensile tests have been carried out on notched Ni3 AI(B) specimens with different thickness/width ratios. The result indicate a transition in the fracture mode upon increasing the specimen thickness, from ductile transgranular to intergranular fracture. Experiments at high strain rates show that the transition cannot be explained in terms of the strain rate at the root of the notch. Rather, it is explained in terms of an increasing degree of triaxiality of the stress state. I. INTRODUCTION As a potential structural material, Ni 3AI(B) may suffer from the tendency to fracture intergranularly in the presence of a notch [1,2]. Notch-induced intergranular fracture has also been seen in another potentially useful, strongly ordered L12 alloy, Zr3Al, where a notch strength ratio (the ratio of maximum stress of notched specimen to the ultimate tensile stress of unnotched specimen), NSR, of 0.72 was reported [3]. For Ni3 AI(B), NSR=0.56 [1]. In comparison austenitic stainless steel does not show this effect. The question concerns the origin of the phenomenon. One factor may be strain rate at the notch tip. Since the radius of the notch is small, plastic deformation will be concentrated in a narrow region, resulting in a high and possibly embrittling strain rate locally. Another factor is the stress state behind a loaded notch [4]. Under uniaxial loading, a triaxial stress state will develop if the specimen is thick enough. Correspondingly, the material will deform plastically at higher applied stresses. As a result, brittle fracture could occur before extensive plastic strain is imparted, particularly if the fracture criterion involves a critical tensile stress. A third factor is the environment. Many intermetallic compounds fracture prematurely even in non-aggressive environments like air and water [5]. A notch could exacerbate the situation. Another factor is the work hardening rate. Strongly ordered L12 intermetallic compounds generally have high work hardening rates [6-10], which, when normalized with respect to the shear modulus, are about 3 to 4 times higher than those of elemental metals and random f.c.c. solid solutions. A high work hardening rate will probably reduce the plastic strain which can be imparted locally, should brittle fracture be triggered by a critical tensile stress. This paper focuses on Ni3 AI(B) and on the first two possible factors; namely, strain rate and stress state. The strain rate is varied in the usual manner. The stress state is varied through the use of notches. II. EXPERIMENTAL The material used in this study was the same as that used earlier [11]; namely, Ni 24at.% Al - 0.3at.% B, hot-extruded from powder. All notched specimens were machined and/or ground from the same as-extruded rod and were subsequently annealed at 700 'C for 0.5 hour. Their grain size was 9.5 gm. Smooth-bar specimens were teste
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