Peter Wong, Yung Joon Jung
Date of Award
Doctor of Philosophy
Department or Academic Unit
College of Engineering. Department of Mechanical and Industrial Engineering.
I-Phase, Mg-Zn-Y, UDS, W-Phase, X-Phase
Engineering | Mechanical Engineering
The magnesium consumption worldwide has been rapidly increasing in the recent years as magnesium alloys replace other materials in a variety of structural applications in industrial machinery, materials handling, commercial and aerospace equipment, and more recently in audio-video-computer-communication equipment and automobiles. Development of new high-performance alloys, however, is hampered in part by magnesium's pyrophoricity, which poses a serious barrier to applying conventional droplet-based processing to magnesium alloys. This research addresses the processing of high-performance magnesium alloys with mono-size droplets produced by the capillary breakup-based uniform-droplet spray (UDS) process which permits stringent control of the thermal state of droplets while assuring safe spraying. Strengthening magnesium alloys for elevated temperature service is another key to increasing the magnesium utilization in manufacturing. The Mg-Zn-Y system, having various intermetallic phases such as the I-phase (Mg3Zn6Y1), X- phase (Mg12ZnY) and W-phase (Mg3Zn3Y2), offers unique opportunities for the development of high-performance Mg alloys.
Three Mg-Zn-Y alloys, Mg97Zn1Y2, Mg88Zn10Y2 and Mg76.5Zn20Y3.5, were chosen for this research to investigate the potentials for the droplet-based processing of high-performance alloys by the UDS process. Spray deposits produced with uniform (mono-size) droplets 700-1000 µm in diameter were characterized by full density and fine equiaxed grains, in contrast to the coarse dendritic structure of the original cast ingot. While the primary equiaxed grains were the HCP Mg-rich solid solution in the deposits of all three compositions, the Mg97Zn1Y2 deposits also had X-phase, and the Mg88Zn10Y2 and Mg76.5Zn20Y3.5 deposits had the I- and W-phases in the intergranular regions. The primary grains in the Mg97Zn1Y2 deposits exhibited striations indicative of X-phase precipitation on basal planes. The occurrence of I-phase in the Mg88Zn10Y2 and Mg76.5Zn20Y3.5 alloys is in line with previous reports on the existence of a pseudo-binary system between Mg and I-phase.
Subsequent thermomechanical processing of the spray deposits by rolling resulted in further microstructural refinement and improved room- and elevated- temperature strength, especially in the case of Mg97Zn1Y2 which had a room-temperature tensile strength of 340 MPa and an elevated-temperature (473K) tensile strength of 271 MPa. The other two alloys, Mg88Zn10Y2 and Mg76.5Zn20Y3.5, respectively, had 305 MPa and 271 MPa at room temperature, and 237 MPa and 216 MPa at 473 K. These strength values significantly exceed those of commercial wrought Mg alloys, which typically range 200 MPa to 300 MPa at room temperature and 150 MPa to 200 MPa at 473 K. Although the process parameters still need to be further optimized, UDS processing, combined with thermomechanical processing, enables safe and effective processing of high-performance magnesium alloys.
Fukuda, Hiroki, "Droplet-based processing of magnesium alloys for the production of high-performance bulk materials" (2009). Mechanical Engineering Dissertations. Paper 10. http://hdl.handle.net/2047/d20000640
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