Weibull analysis of fracture strength for Zr55Ti2Co28Al15 bulk metallic glass: Tension-compression asymmetry and porosity effect

In this work, two-and three-parameter Weibull statistics were used for analyzing the variability of fracture strength for Zr55Ti2Co28Al15 bulk metallic glass (BMG), both in compression and in tension testing. In contrast to the compression in which the specimens fail via the massive shear-off, however, failure mode in tension for the as-cast BMG is flaw-controlled crack opening (mode I or mixed mode) due to the presence of cast defects such as porosity. As a result, dispersion of compressive fracture strength is rather uniform. For the BMG rods of 6 mm in diameter, the three-parameter Weibull modulus m(3p) and threshold stress sigma(mu) (below which no failure occurs) are 3.4 and 1780 MPa, respectively. However, tensile fracture strength of the BMGs manifests a large variability, in a range of 310-1690 MPa. In terms of fracture surface morphology, the specimen failure at different stress is associated with two types of defects: large pores on/near the surface of specimens and small internal pores. Using bimodal and three-parameter Weibull analysis, the Weibull modulus m(1) and threshold sigma(mu 1) at lower strength level are 1.8 and 250 MPa, respectively, suggesting a modest reliability. One should exercise caution, therefore, in interpreting the reliability of as-cast BMG materials only simply in terms of the compression tests, small-sized samples, and tow-parameter Weibull analysis. Like the conventional metal castings, controlling the processing conditions to minimize the cast defects is critical issue to ensure the reliability of BMG materials.