| High strength and toughness magnesium alloy with lightweight has become one of the most important materials in the area of aerospace, national defence, modern industry and so on. It plays an important role on solving the problems of the shortage of resources, the energy dilemma and environmental pollution. Among Mg alloys, Mg-RE-TM series alloys with LPSO phases and precipitation phases show the best mechanical properties, with tensile properties over 500 MPa, which has basically reached the commercial strength of high strength aluminum alloy. However, the elongation is generally low(typically less than 5%), which is unable to meet as a key member of the project bearing applications. Therefore, it is an urgent scientific problems to use an effective strengthening and toughening method to improve the elongtaion in high-strength magnesium alloy and solve the contradiction between the strength and plasticity. If we want to design a high strength and high toughness Mg-RE-TM alloy, it is necessary to clarify the mechanism of strengthening and toughening of the alloy and to grasp the regulation of strength and plasticity. Then, some guidance for research and development of the high strength and high ductility magneiusm alloys would be supplied.In response to this situation, the main secondary phases in Mg-RE-TM alloys have been divided, and a single long period stacking ordered phase and a single precipitation hardening phase will be prepared respectively. According to these results, we considered the interactive effects of the two different mechanisms and design a multi-phase composite alloy containing LPSO phases and age-hardening precipitates. Then, the strengthening-toughening mechanism in Mg-RE-TM alloy would be tried to clear.In current study, Mg-RE alloys with single precipitation phases were studied first. The results showed that Gd and Y elements shows best strengthening effects among different RE elements which could form precipitation phase. Y could have excellent effect of aging with the content over 2.4 at%, while Gd shows fine age-hardening results over 2.0 at% content. Based on this, the designed Mg-1.2at.%Gd-1.0at.%Y alloy could reach mechanical properties with tensile strength of 400 MPa and elongation of 9%.Then, Mg-RE-TM alloys with single LPSO phases were also designed. By controlling Zn/Y ratio, the Mg-Zn-Y alloys with the value of 3/4 shows most outstanding mechanical properties. Based on the ratio, two types of LPSO phase alloys, the one with the LPSO phase could form during solidification and the other with the LPSO phase precipitates from the Mg-supersaturated solid solution at high temperatures, were prepared. According to the experiment, Mg-Ni-Y alloy has the highest tensile strength, while Mg-Zn-Y alloy is more excellent ductility and better overall performance. And Mg-Gd-Zn alloy does not include a LPSO phase in the as-cast ingots, as large lamer 14 H LPSO phase precipitates from alloy during annealing processing. Mg-Gd-Y-Zn alloy with mixture LPSO phases could get higher mechanical properties with tensile properties of 370 MPa and plasticity of 7%.Based on the comprehensive of strengthening effect LPSO phases and precipitates, a Mg-Gd-Y-Zn alloy with mixture strengthening phases was designed. By controlling Zn content and micro-alloying element of Mn and adopting solution annealing, extrusion and aging, the peak-aged alloy could achieve excellent mechanical properties with tensile strength of 493 MPa and plasticity of 5.5%.In addition, LPSO phases in Mg-RE-TM alloy with mixture strengthening phases were controlled by changing the shape, structure and distribution, for improving the mechanical properties further. The methods of subsitution of Ni for Zn, controlling RE elements, using different heat-treatment and adding micro-alloying element like Sn and Al were adopted. The results shows that the block-shaped LPSO phases, which are distributed along the grain boundaries, can induce the formation of isometric crystal grains and improve the ductility in as-extruded alloys. On the other hand, the lamellar LPSO phases precipitating in the grain interior can induce the formation of strip-shaped grains and enhance the strength of the as-extruded alloys. And the finer the lamellar LPSO phases are, the higher the tensile properties are. On this basis, GWZM3 alloy using annealing process at 540? for 4h and extrusion could get tensile strength over 400 MPa with elongation of 12.9%.What's more, the pecipitation behavior and the effects on mechanical properties in Mg-RE-TM alloy with mixture strengthening phases was studied. The results show that the tensile strength increased greatly while the plasticity declined in a large extent after aging treatment. However, no matter which morphology the LPSO phase is, the change of the tensile strength lift approximately the same. The plasticity of different alloys changes significantly differently. Some alloys shows aging platform during aging phenomenon, which makes the plasticity of Mg-RE-TM alloys decrease slower in a certain extent, implying enhancing the ductility of the alloys.Finally, based on these studies above, combined with the aging platform, we found that both the strength and ductility could improve during aging process with increasing aging time in GWZM3 alloy. Then, we gain a Mg-RE-TM alloy with tensile strength of 538 MPa and elongation of 10 %. On this basis, from the perspective of the composite strengthening and toughening of LPSO phase and precipitation, we discussion the mechanism of the mechanical properties of Mg-RE-TM alloys. The excellent properties were attributed to the the numerous plate-shaped ?' phase precipitates in the a-Mg matrix, the LPSO phases and the stacking faults. The author summarized the basic methods to design high strength and high toughness magnesium alloy, and put forward his own options. |
PDF Full Text Request