Effects of hydrostatic pressure on the flow and fracture of a bulk amorphous metal

The flow and fracture behaviour of a Zr-Ti-Ni-Cu-Be bulk amorphous metal have been determined in tension and compression at room temperature with levels of superimposed hydrostatic pressure ranging from 0.1 to 700 MPa. Metallographically polished cylindrical specimens tested in uniaxial tension and compression were utilized in the high pressure tests, while polished cylindrical torsion specimens were tested at 0.1 MPa (i.e. atmospheric pressure) in order to approach conditions of pure shear. All the tension and torsion tests, regardless of the level of superimposed pressure, exhibited linear elastic failure, as did the compression tests conducted with low levels (e.g. less than 450 MPa) of pressure. At the highest pressures (i.e. 450 MPa or higher), the compression tests exhibited elastic-perfectly plastic behaviour and an increase in the compressive elongation to fracture. The flow stress and fracture stress were not significantly affected by the superposition of pressure as failure occurred in shear, indicative of pressure-independent behaviour over the range tested. However, a change in fracture plane angle was detected. Tensile fracture surfaces were oriented at 50-59degrees; compression fracture surfaces were oriented at 40degrees. The flow and fracture behaviours were analysed in terms of a Mohr-Coulomb criterion of the form tau(c) = 950 MPa - 0.038sigma(n) over the range of stress states examined. The results are discussed in the light of the various yield criteria and the flow and fracture theories provided for amorphous metallic systems.