An inverted die casting technique has been developed for the rapid and reproducible production of high quality lightweight bulk metallic glass (BMG) castings. Comprehensive processing maps for producing lightweight BMG samples of cross section 3.15 mm x 7 mm and a length of 125 mm were developed as a means of identifying the optimum casting conditions for producing casting of high structural integrity, maximum length and enhanced surface quality. Utilising these maps, Mg65CU25Y10 and Ca65Mg15Zn20 BMGs were consistently produced using the inverted injection die casting method and a naturally cooled copper mould, by choosing injection parameters that stabilise the molten metal flow front within the mould cavity. Highest quality Mg65CU25Y10 BMG bars were produced in the casting temperature range of 560 C to 580 C and gate velocities in the range of 12.5 to 15 m/s. Highest quality Ca65Mg15Zn20 BMG bars were produced in the casting temperature range of 480 C to 520 C and gate velocities in the range of 13.8 to 14.7 m/s. The casting parameter range for achieving the highest quality castings for the lightweight BMGs examined was found to be practically identical and related to the casting system geometry. The use of higher holding pressures when casting was also found to increase the sample surface quality due to a post-casting consolidation process during sample cooling. As part of the experimental program, critical cooling rate experiments were carried out, whereby the change in casting temperature over time was measured between Tl and Tg. The resulting castings were analysed using x-ray diffraction (XRD). The Mg65CU25Y10 BMG was found to have a critical cooling rate between 49 and 61 C/sec, and may be gravity cast in a copper mould to achieve a completely amorphous structure between 3 and 3.75 mm, or readily cast using the inverted injection method successfully to obtain a thickness of 3.15 mm. The Ca65Mg15Zn20 BMG was found to have a critical cooling rate between 150 and 170 C/sec, and may be cast using the inverted injection method to achieve a completely amorphous structure of a diameter 8 to 9 mm. From the as-cast samples, differential scanning calorimetry (DSC) experiments were carried out as to determine the thermal properties of both materials where it was found that the Mg65CU25Y10 BMG had glass transition and crystallisation temperatures that varied with heating rate. Tg varied from 138 C for a heating rate of 2 C/min to 148 C for a heating rate of 20C/min. Tx varied from 195 C for a heating rate of 2C/min to 213 C for a heating rate of 20C/min. This indicates a supercooled liquid (SCL) interval of 57 to 65 C. The Ca65Mg15Zn20 BMG was found to have glass transition and crystallisation temperatures that were almost independent of heating rate. Tg varied from 102 C for a heating rate of 5 C/min to 105 C for a heating rate of 20 C/min. Tx remained relatively unchanged with heating rate at 137 C, indicating a SCL interval of 32C. Isothermal DSC results show that the onset of crystallisation occurs much more quickly in the Ca65Mg15Zn20 BMG and follows a non-Arrhenius type relationship as opposed to the slower, Arrhenius crystallisation kinetics displayed by the Mg65CU25Y10 BMG. In conjunction with this work, the elevated temperature mechanical properties of these BMGs was studied. When deformed in tension at an elevated temperature under constant strain rate conditions, it was found that an increase in test temperature resulted in a decrease in both peak stress and flow stress. It was also found that an increase in strain rate resulted in an increase in both peak stress and flow stress. It was established that Newtonian flow occurred at high temperatures in the SCL region and at lower strain rates. The Ca65Mg15Zn20 BMG was found to be far more strain rate sensitive with respect to brittle fracture, exhibiting a maximum achievable strain rate for homogeneous flow of 10 -3/S compared to 10 -1/S for the Mg65CU25Y10 BMG. Elongations achieved for the Mg65CU25Y10 BMG exceeded 1300% compared to a maximum elongation of 598% for the Ca65Mg15Zn20 BMG under constant temperature/ constant strain rate conditions, with elongation usually limited due to the onset of crystallisation. Both BMGs were found to crystallise under certain deformation conditions. For these conditions, the Mg-based BMG was found to display a stress increase due to crystallisation prior to the times determined by static crystallisation experiments due to dynamic segregation of the amorphous phase into Cu rich and Y rich regions, as determined by atom probe tomography (APT). Where crystallisation occurred in the Ca-based BMG under dynamic conditions a delayed stress increase due to crystallisation was observed in comparison to static crystallisation experiments. The dynamic stabilisation (time delay to crystallisation) of the amorphous phase in the Ca65Mg15Zn20 alloy was found to decrease with increasing test temperature and decreasing strain rate. Constant load defomation experiments were carried out at a constant heating rate of 5 C/sec for the Ca65Mg15Zn20 BMG. It was found that stress overshoot behaviour was avoided and a strain of 850% was achieved prior to crystallisation hardening and subsequent failure which is larger than that observed in constant strain rate testing.
| Identifer | oai:union.ndltd.org:ADTP/258289 |
| Date | January 2007 |
| Creators | Laws, Kevin J., Materials Science & Engineering, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. Materials Science & Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Laws Kevin J.., http://unsworks.unsw.edu.au/copyright |
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