Effects of changes in chemistry and test temperature on deformation behavior and fatigue properties of aluminum-based amorphous alloys.

摘要

The extraordinary mechanical properties such as high strength, hardness, elasticity, and strength to density ratio, as well as good formability and shaping abilities make Al-based amorphous alloys good candidates for structural applications. In this dissertation, the effects of changes in chemical composition of Al-Gd-Ni-X (X= transition metals) were investigated. Three aspects of properties were examined: thermal stability, deformation behavior, and fatigue properties. Systematic additions of Fe and Co significantly improved Tg and Tx1 of the base Al87Gd6Ni7 amorphous alloy since Fe and Co exhibit a negative heat of mixing with Al. However, the value of DeltaTx (i.e. Tx - Tg) was not changed significantly over the range of alloying additions investigated. The deformation behavior of Al-Gd-Ni-X ribbons was determined by high temperature micro-hardness tests and dynamic mechanical analysis (DMA). Although these ribbons exhibited high hardness/viscosity at room temperature, a distinct reduction near their Tg was observed, with subsequent increases when the test temperature passed Tx. Such behavior and separate TEM analysis indicated that microstructural evolution occurred at this temperature. The deformation behavior also was studied by tensile tests at various temperatures. A transition from inhomogeneous flow to more homogeneous flow was obtained at low strain rates upon approaching Tg, while the deformation behavior at different strain rates and temperatures was captured in a deformation map. In addition, the fatigue performance was examined at both R= -1 and R= 0 conditions. It was demonstrated that the substitution for Al by Fe and Co additions increased the stress amplitude at the fatigue limit from 240 MPa for Al87Gd6Ni7 to 397 MPa for Al86Gd6Ni7Co1 when tested at R= -1. Conducting fatigue tests at R= 0 revealed that tensile mean stresses produced a detrimental effect on the fatigue performance, but not as great as that exhibited in commercial aluminum alloys. It also was demonstrated that the improvement of high cycle fatigue performance of higher alloyed glasses may be a result of their higher Tg, providing a lower value of the ratio of the test temperature to Tg. The importance of this was checked by conducting fatigue tests at lower test temperatures (i.e. different values of T/Tg). The increases in the Fe/Co level that produced improvements in the HCF behavior are partly attributed to the lower T/T g present during fatigue testing of the highly alloyed ribbons.