Microstructure, Mechanical Properties And Deformation Mechanisms Of Mg-Dy-Zn Alloys

In this dissertation, the effect of different Zn contents on the microstructure, age hardening behavior and tensile properties of the Mg-2Dy(at.%) alloy is investigated. Firstly, a new style heat resistance Mg-2Dy-0.5Zn alloy containing a high volume fraction of the long period stacking ordered (LPSO) phases is optimized through appropriate solid solution and ageing treatments. Furthermore, the microstructure, age hardening behavior and tensile properties at room and elevated temperatures for the extruded Mg-2Dy-0.5Zn alloy are investigated. Also, the deformation behavior of the extruded Mg-2Dy-0.5Zn alloy in the peak-aged state at different temperatures and strain rates is also discussed. It is noted that the high volume fraction of LPSO phases significantly enhance the age hardening behavior, effectively improve the elevated temperature tenisle properties of the extruded Mg-2Dy-0.5Zn alloy and are helpful to forming superplastic behavior of the alloy due to their good thermal stability.The investigated results in this dissertation are summarized as following:1. The addition of 0.5 at.%Zn into Mg-2Dy alloy leads to the precipitation of the lamellar 18R LPSO phase at the grain boundaries. This phase has a good thermal stability. After solid solution treatment at 525℃for 10h, 18R LPSO phase transforms into 14H LPSO phase in the grain interior. As the Zn content is 1.0at.%, the Mg₃Zn₃Dy₂ particle phases precipitat in theα-Mg matrix and still exist in the solid solution and ageing treatments. Also, the grain size of the alloy is significantly refined due to Zn addition. The addition of 0.5at.% Zn enhances age hardening behavior and improves the tensile properties of the Mg-2Dy alloy at room temperature and 200℃. In contrast, as Zn content is 1.0at.%Zn, which decreases the age hardening behavior of the alloy. The high tensile strengths are attributed to grain refinement, LPSO phase strengthening and precipitation strengthening of Mg₃Zn₃Dy₂ particle phases. 2. During ageing process, 14H LPSO phase forms and grows in theα-Mg supersaturated solid solution. The volume fraction of the phase increases with increasing ageing time. Also, the size, distribution and amount of the (Mg, Zn)xDy particle phases occur to change with ageing time. The as-cast Mg-2Dy-0.5Zn alloy has two maximun hardeness values being with 81Hv ang 79Hv at 36h and 80h, respectively. The values of the two ageing peaks for the Mg-2Dy-0.5Zn alloy increase by 11%, as compared with that for the Mg-2Dy alloy. The as-cast Mg-2Dy-0.5Zn alloy exhibits a double-peak age behavior. The corresponding investigation reveals that the double-peak age behavior of the alloy is attributed to the LPSO strengthening and the precipitation strengthening of a large number of the fine (Mg, Zn)xDy particle phases.3. Hot extrusion refines the grain, disperses (Mg, Zn)xDy particles of the Mg-2Dy-0.5Zn alloy and makes the LPSO phase bend and form kink bands and kink boundaries. During the deformation, the dislocation density increases in theα-Mg matrix located in those kink bands and kink boundaries of LPSO phases, which results in an increasement of the tensile strength. The tensile testing results indicate that the extruded Mg-2Dy-0.5Zn alloy in the peak-aged state exhibits an outstanding tensile strength. The yield strength, ultimate tensile strength and elongation to failure of the alloy at 300℃are 245Mpa, 260Mpa and 36%, respectively. These high tensile strengths mainly arise from 14H LPSO phase strengthening, grain refinement and precipitation strengthening of (Mg, Zn)xDy particle phases.4. At low temperatures (RT²00℃) and different strain rates, the flow stress of the extruded Mg-2Dy-0.5Zn alloy increases with increasing strain rates, while elongation to failure appears a little chang. m is estimated to be 0.01949. At the same time, some shear bands appear on the surface of the fractured tensile samples. At 300℃and different strain rates, the alloy still exhibits high flow stress, the elongation to failure decreases with increasing strain rates. As (ε|.)=3×10^-5s^-1, the elongation to failure is 100.5%. The alloy exhibits a superplastic behavior. It is noted that the alloy has two m values at 300℃and different strain rates. At high strain rates (8×10^-4s^-1﹤(ε|.)﹤3×10-1s-1), m=0.01445. At low strain rates (3×10^-5s^-1﹤(ε|.)﹤8×10^-4s^-1), m =0.37944. The deformation activation energy is estimated to be 88.79kJ/mol. The investigation indicates the deformation mechanism of the alloy at low temperatures and different strain rates is dislocation sliding accommodated by the shear band. While the deformation mechanism of the alloy at 300℃and high strain rates is similar to that at temperatures below 200℃. The superplastic deformation mechanism of the alloy at 300℃and low strain rates is grain boundary sliding accommodated by the grain boundary diffusion.