M. Freels

Direct Comparisons of the Fatigue Behavior of Bulk-Metallic Glasses and Crystalline Alloys

Recent research works on bulk-metallic glasses (BMGs) have opened a window to create a new generation of structural materials for applications. Although the mechanical behavior of BMGs is being studied widely, the fatigue characteristics are poorly understood. The uniaxial tension-tension high-cycle fatigue (HCF) studies were performed on zirconium (Zr)-based bulk-metallic glasses (BMGs): Zr50Cu40Al10, Zr50Cu30Al10Ni10, Zr50Cu37Al10Pd3, and Zr41.2Cu12.5Ni10Ti13.8Be22.5, in atomic percent. The HCF experiments were conducted using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1, where R = σmin./σmax., where σmin. and σmax. are the applied minimum and maximum stresses, respectively. The fatigue-endurance limit of Zr50Cu37Al10Pd3 was significantly greater than those of Zr50Cu40Al10, Zr50Cu30Al10Ni10, and Zr41.2Ti13.8Cu12.5Ni10Be22.5. In order to compare the fatigue property with the crystalline alloys, the same HCF experiments were also performed on Ti-6-4, drill tool steel, and Al 7075. The fatigue lifetime of Zr-based BMGs is generally comparable to those of Ti-6-4 and drill-tool-steel crystalline alloys and is greater than that of Al 7075 alloy. The fracture morphology of BMGs indicates that fatigue-crack-propagation region included the distinct rough striations and the fine striations. The possible mechanism for the striation formation was proposed.

Fatigue Study of a Zr-Ti-Ni-Cu-Be Bulk Metallic Glass

High-cycle fatigue (HCF) studies were performed on zirconium (Zr)-based bulk metallic glasses (BMGs): Zr 41.2 Ti 13.8 Ni 10 Cu 12.5 Be 22.5 , in atomic percent. The HCF experiments were conducted using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 and under tension-tension loading, where R = σmin./σmax., where σ min. and σmax. are the applied minimum and maximum stresses, respectively. The test environment was air. A high-speed and high-sensitivity thermographic-infrared (IR) imaging system has been used for nondestructive evaluation of temperature evolution during fatigue testing of BMGs. Limited temperature evolution was observed during fatigue. However, no sparking phenomenon was observed at the final moment of fracture of this BMG. At high stress levels (σmax. > 864 MPa), the fatigue lives of Batch 59 are longer than those of Batch 94 due to the presence of oxides in Batch 94. Moreover, the fatigue-endurance limit of Batch 59 (703 MPa) is somewhat greater than that of Bath 94 (615 MPa) in air. The fatigue-endurance limit of Ti-6–4 is greater than this BMG, but Al 7075 has the lowest fatigue life. The vein pattern with a melted appearance were observed in the apparent melting region. The fracture morphology indicates that fatigue cracks initiate from some defects.