A Study on the Microstructures and Mechanical Properties of the ZA85 Magnesium Alloy Fabricated by Equal-Channel Angular Extrusion

Autor: Lin, Che-Yi, 林哲毅
Rok vydání: 2014
Druh dokumentu: 學位論文 ; thesis
Popis: 102
In this study, the as–cast and solution–heat–treated Mg–8 wt.% Zn–5 wt.% Al (ZA85) alloys were subjected to the equal–channel angular extrusion (ECAE). The microstructural evolutions and tensile properties of the experimental alloys were investigated. In the as–cast ZA85 alloy, the initial grain size and precipitate size of 150 and 100 μm were greatly reduced to 4 and 1 μm, respectively, after the ECAE process. The grain–refinement mechanism of the experimental alloy fabricated by the ECAE process is dynamic recrystallization. At room temperature (RT), the ultimate tensile strength (UTS) and yield strength (YS) of the ECAE processed specimens were 402 and 281 MPa, respectively, compared with 175 (UTS) and 131 MPa (YS) for the as–cast specimens. At 200 °C, the UTS and YS of the ECAE processed specimens improved to 249 and 162 MPa, respectively, compared with 105 MPa (UTS) and 74 MPa (YS) for the as–cast specimens. This improvement in tensile properties of the ZA85 alloy was attributed to the refined grains and the well–distributed fine Mg32(Al,Zn)49 (τ–phase) precipitates. In order to further improve the mechanical properties of the ZA85 alloy, the as–cast ZA85 alloy was subjected to solution heat treatment (SHT). Dynamic precipitation was then induced using two–step ECAE process. After the SHT process, almost all the non–continuous τ–phase dissolved into the α–Mg matrix and the average grain size slightly increased to 170 μm. After six ECAE passes, the average grain size was greatly reduced to 4 μm, and fine τ–phase particles with ~100 nm in size were uniformly distributed in the α–Mg matrix by dynamic precipitation. The combination of SHT + ECAE process was demonstrated to greatly improve the tensile properties of the experimental alloy. By testing over a range of temperatures, the maximum ultimate tensile strength and the yield strength of 415 MPa/284 MPa and 261 MPa/173 MPa were obtained at RT and 200 °C, respectively. The strengthening factors for the SHT + ECAE alloy are the grain refinement, precipitation hardening, and presence of fine and well–distributed τ–phase particles. It was also demonstrated that ECAE processing produces superplasticity. By testing over a range of temperatures and strain rates, the ECAE processed ZA85 alloy exhibits both low temperature superplasticity (elongations of 147% and 400% at 300 °C with initial strain rates of 1.0 × 10-3 s-1 and 1.0 × 10-4 s-1, respectively; an elongation of 205% at 250 °C with the initial strain rate of 1.0 × 10-4 s-1) and high strain rate superplasticity (an elongation of 113% at 400 °C with the initial strain rate of 1.0 × 10-2 s-1). The dominant deformation mechanism for the specimens tested at 300 and 350 °C with the initial strain rates ranging from 1.0 × 10-4 s-1 to 1.0 × 10-3 s-1 is GBS controlled by grain boundary diffusion. At the higher testing temperature of 400 °C, the deformation mechanism for the experimental alloy is dislocation creep.
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