| With the development of science and technology and the variety of spacecraft,the demand for lightweight magnesium alloy materials is becoming more and more urgent,and the requirements for strength and plasticity and performance under extreme conditions are also increasing.Conventional magnesium alloys have low strength and poor plasticity.At present,they cannot be used on the main bearing structural parts,and it is urgent to develop ultra-high strength and high-toughness magnesium alloys.Moreover,the space conditions are harsh,and the temperature difference between day and night is very large.It is required that aerospace materials not only have good cold resistance but also good heat resistance.Therefore,in order to safely use the high-strength and tough magnesium alloys studied in structural members,it is necessary to first understand the fracture failure behavior under different service conditions.In summary,this thesis tests the mechanical behavior and failure mechanism of ultra-high strength Mg-9Gd-4Y-1Zn-0.8Mn alloy with LPSO phase under simulated aerospace service conditions(high temperature,low temperature,etc.).On this basis,through the addition of alloying elements,its properties are improved,and ultra-high strength and high-toughness magnesium alloys satisfying aerospace service conditions are developed.The research of this subject not only has guiding significance for the design and preparation of ultra-high strength and high toughness magnesium alloy,but also provides reference for the development of aerospace magnesium alloy.The research results have both academic significance and application value.First,we investigated the mechanical properties of Mg-9Gd-4Y-1Zn-0.8Mn alloy under extreme service temperature conditions.The liquid nitrogen of Mg-9Gd-4Y-1Zn-0.8Mn alloy was studied for a long time(10 h,20 h,50 h,100 h).After immersion,and the microstructure change and mechanical properties of the alloy at a high temperature of 250~400 °C.After low temperature treatment,the grain of the alloy was remarkably refined,and the mechanical properties were high after 50 h treatment: tensile strength 529 MPa and elongation 7.34 %.The high temperature tensile test shows that the alloy has better heat resistance below 300 °C.With the increase of temperature,the fracture mode of the alloy changes from cleavage brittle fracture to dimple-type ductile fracture.Secondly,we studied the regulation of microalloying element Sr(0.4,0.8,1.2(wt.%))on the microstructure of Mg-9Gd-4Y-1Zn-0.8Mn alloy alloy.The trace amount of Sr can effectively refine the grain size of the alloy and inhibit the precipitation of layered LPSO phase in the alloy.To make the second phase in the alloy more diffuse and fine.The addition of the component is 0.4Sr,the mechanical properties of the alloy are most obvious.The tensile strength of the extruded alloy reaches 403 MPa and the elongation is 10 %.After the aging treatment,the strength of the alloy is further improved to 530 MPa.5.2 %.With the further increase of the Sr content,the second phase has a tendency to form a network structure at the grain boundary,resulting in deterioration of performance.Then,we conducted experiments on fracture toughness and impact toughness of the Mg-9Gd-4Y-1Zn-0.8Mn alloy and the alloy with the best performance in the previous chapter to understand the fracture failure behavior of the high-strength magnesium alloy studied and analyze the fracture mechanism.Due to the refinement effect of the element Sr on the grain and the second phase of the alloy,the fracture toughness and impact toughness of the alloy are effectively improved after the addition of the element,and the crack propagation path is more tortuous and consumes more energy.After alloying elements,the alloy impact toughness is significantly improved by more than 50 %.The fracture mechanism of the alloy is the cleavage fracture mode of the intergranular and transgranular mixed type. |
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