The influence of hydrostatic pressure on fracture of single-crystal and polycrystalline NiAl
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I.
INTRODUCTION
THE use of hydrostatic pressure in fracture experiments is one means for carefully controlling the stress state in order to determine the failure criteria of a variety of materials. The materials typically studied in such investigations have predominantly focused on ductile metalstL2,3~or brittle ceramics, t4-81 with recent investigations into the pressure response of a variety of polymerslg:~ and metal-matrix composites, t~2-~s~where changes in the microstructure and alloy type are shown to exhibit significant effects on the pressure response. Significantly fewer results have been reported for intermetallic compounds. In addition to the experimental data on pressure effects on materials, a variety of models exist with reference to failure criteria under different stress states. The purpose of this work is to investigate the effects of hydrostatic pressure on the fracture behavior of singlecrystal and polycrystalline NiA1, in order to examine the possible failure criteria for such a material. A brief review of the various experimental works and failure criteria is given in Section II. H.
REVIEW OF PREVIOUS STUDIES
A . Effects o f Pressure on Ductility
The pressure-induced increase in tensile ductility in polycrystalline materials has been the subject of many studies. BridgmantL21 first recorded the effects of pressure on the ductility of a variety of nominally ductile metals (e.g., A1, Cu, brass, and bronze), where ductile rupture (er = 1.1 to 2.3) was exhibited by specimens tested under high pressures. Similar pressure-induced increases in ductility were recorded for nominally brittle R.W. MARGEVICIUS, formerly Graduate Student, Case Western Reserve University, is Max-Planck Society Visiting Postdoctoral Researcher, Max-Planck Institut, Stuttgart, Germany 70174. J.J. LEWANDOWSKI, Professor, is with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted July 8, 1993. METALLURGICALAND MATERIALSTRANSACTIONS A
materials, such as Be and phosphor bronze, although pressures of up to 2.8 GPa were required in these cases in order to produce fracture strains of 0.65 and 1.62, respectively. Subsequent work by Davidson et al. ttg] on Zn, Mg, Co, W, and 1045 martensitic steel revealed that the magnitude of pressure-induced ductility increases varied with both the crystal structure [e.g., hexagonal close-packed (hcp) vs body-centered cubic (bcc)], as well as with changes in the microstructure for a given material. For example, the hcp metals Zn and Mg exhibited an abrupt increase in ductility over a narrow pressure range, fracturing at a true strain of 0.1 at 0.1 MPa (i.e., 1 atm) and increasing to a fracture strain in excess of 3.0 at a pressure above this transition pressure. In contrast, the ductility of the bcc metals Co and W increased to an asymptotic level of approximately 0.5 and did not increase with further increases in pressure, while the ductility of the martensitic 1045 steel increased linearly with pressure up to 2.2 GPa
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