Synthesis And Properties Of Zr-based Bulk Metallic Glass Matrix Composites

In this dissertation, bulk metallic glass (BMG) and metallic glass matrix composites Zr(65-x)HfxCu17.5Ni10Al7.5 (at.%) with 3 mm diameter were successfully synthesized by using copper mold suction casting method. The effect of replacement of Zr with Hf on microstructure, glass formation, thermal stability, crystallization kinetics, compressive mechanical behavior and fracture feature of the as-quenched Zr(65-x)HfxCu17.5Ni10Al7.5 (x=1-65) alloys were investigated by means of X-ray diffraction (XRD), optical microscopy (OM), differential scanning calorimetry (DSC), differential thermal analysis (DTA), mechanical testing system (MTS) and scanning electron microscopy (SEM). The results of XRD and OM indicate that the substitutions of 1 and 2-10 at.% Hf for Zr lead to the formation of BMG and metallic glass matrix composites, respectively. The composites are characterized by a homogeneous distribution of micron-scale dendrites embedded in the amorphous matrix, and the size of the dendrites increases gradually with the increase of Hf content from 2 to 10 at.%. A comparison of DSC and DTA traces shows that no discernible endothermic peaks of glass transition exist for the alloys with x=15-65. The thermal stability enhanced while the glass forming ability decreased with the substitution of Hf for Zr in Zr(65-x)HfxCu17.5Ni10Al7.5 alloys when x=1-10. Among these alloys, composite Zr55Hf10Cu17.5Ni10Al7.5 has the best thermal stability with the supercooled liquid region of 130 K. Fully crystalline structure was obtained when Zr was completely replaced with Hf. In addition, the results of thermal analyses present that the volume fraction of quenched-in crystalline increases significantly with the substitution of Zr by Hf.The results of non-isothermal crystallization kinetics demonstrate that the metallic glass matrix composite has the same kinetic characterization with single-phase BMG in that both glass transition and crystallization have a significant kinetic effect. Apparent activation energy for crystallization of BMG matrix composite Zr60Hf5Cu17.5Ni10Al7.5 is somewhat larger than that of BMG Zr64Hf1Cu17.5Ni10Al7.5, further confirming that the thermal stability of BMG matrix composite is higher than that of fully amorphous alloy. Study of compressive mechanical behavior and fracture feature reveals that the grain size and volume fraction of quenched-in crystalline play an important role in mechanical properties of BMG matrix composite. Compared with single-phase BMG Zr64Hf1Cu17.5Ni10Al7.5, the dendrites in BMG matrix composites Zr63Hf2Cu17.5Ni10Al7.5 and Zr60Hf5Cu17.5Ni10Al7.5 can hinder the propagation of shear bands and initiate the formation of multiple shear bands, therefore, causing an improvement of plasticity. However, composite Zr55Hf10Cu17.5Ni10Al7.5 exhibits brittle fracture behavior due to its large grain size and high volume fraction of quenched-in crystalline.