Mechanical Behaviors Of A ZrCuNiAl Bulk Metallic Glass

In this thesis, a Zr50.7Cu28Ni9A1l2.3 bulk metallic glass(BMG) was selected as a model materials. The effect of heat treatment on the microstructural evolution of this BMG was studied. The relationship between the microstructure and the related mechanical properties of this BMG was studied. A mechanical model regarding the correlation between the annealing time and mechanical performances was established.The microstructures of the BMG samples with different heat treatment states were investigated using differential scanning calorimetry(DSC), X-ray diffraction(XRD), transmission electron microscope(TEM), and synchrotron radiation. The XRD results demonstrated that annealed samples remained amorphous microstructure. The results of the autocorrelation function(ACF) calculation of high resolution transmission electron microscopy(HRTEM) images indicated that the order degree of the BMG samples gradually increase with increasing the annealing time. The high energy X-ray diffraction results suggested that the atomic configuration of the BMG samples become denser after annealing. The samples with different crystalline fraction volume can be obtained after heat treatments between the glass transition temperature and crystallization temperature. The images of TEM showed that the shape of precipitated phase transformed block and mass into acicular and diffusion,when the crystalline fraction volume was up to 40%. With the increasing of crystalline fraction volume, the component of precipitated phases changed. The results of synchrotron radiation revealed the significant changes in microstructure of the crystallized samples.Three-points bending tests at room temperature showed that for the annealed samples or crystallized samples, the free volume content dramatically decrease, and the crystalline phases precipitate from the amorphous matrix, respectively. The extension in the heat treatment time will result in the decrease in the plastic deformation capacity of the samples, for which, the deformation mode will change from ductile to brittle. The results of the compressive tests indicated that the nucleation and propagation of the shear bands was inhibited due to the decrease of free volume content, which will lead to the enhanced yield stress. The dispersion strengthening induced by a trace of precipitated phase elevated the fracture strength of 6% crystallized samples. With the increasing of the crystallized phase fraction, the fracture strength dramatically decreased, and the mode of deformation converted to brittle fracture. Due to the difficulty of shear banding, Vickers hardness of samples after 8 hours annealing was the highest, and the dispersion strengthening resulted in the maximum hardness in crystallized samples at 6% crystalline fraction volume. On the analysis of nano-indentation tests, the hardness grew with compacting the atomic arrangement gradually, when the time of annealing rose, and the capablity of plastic deformation fell with the decreasing of the free volume. Due to the precipitated acicular phase, the microhardness of 40% and 100% crystallized samples significantly reduced.A mechanical model regarding the bending behaviors of Zr50.7Cu28Ni9A1l2.3 BMG with different heat treatment states was established. The relationship between the annealing time, the offset of neutral plane and the shear band spacings was built, which was verified by the experimental results.