【作者】 周伟；

【导师】 彭晓东；

【作者基本信息】 重庆大学 ， 材料加工工程， 2011， 硕士

【摘要】 镁锂合金是目前最轻的合金,具有低密度、高比强度、高比刚度、阻尼性能好等一系列优点,因而在航空航天、电子通讯等领域得到日益广泛的应用。然而镁锂合金由于高温下强度较低、抗蠕变性能差大大限制了其应用。本论文以Mg-Li-Al系合金为基础,采用“对掺法”制备Mg-Sr中间合金和Mg-9Li-3Al-xSr合金,利用光学显微镜、SEM扫描电镜、X射线衍射分析仪和高温力学性能测试等检测手段对铸态、热处理态和挤压态实验合金进行表征,研究了不同Sr含量对不同状态合金微观组织和高温力学性能的影响规律,分析了合金的高温强化机理及断裂机制。研究结果表明,碱土Sr在合金中生成了高温硬质相Al₄Sr,主要沿晶界分布,具有细化晶粒的作用。当Sr的含量为2.5wt%时,合金晶粒明显细化。合金在挤压过程中发生了动态再结晶,晶粒大小变得更加细小均匀,Al₄Sr化合物相从连续网状分布转变为块状和颗粒状均匀分布于合金基体,进而提高了合金的力学性能。对合金进行100℃和150℃高温拉伸测试,结果得出,锶的加入提高了合金的耐热性能,含Sr的合金高温抗拉强度相对于Mg-9Li-3Al合金明显提高,挤压态合金高温力学性能明显优于铸态。合金高温的断裂面形态特征表明,随着Sr元素的含量增大,Mg-9Li-3Al合金沿晶断裂的趋势有所增加。挤压态合金在100℃时,延伸率均在10%以上,当Sr的含量在2.5wt%左右时,合金抗拉强度达到最大值225.3Mpa,比Mg-9Li-3Al合金提高15.7%,此时实验合金的综合力学性能最佳。Sr提高Mg-9Li-3Al合金高温性能的机理主要表现为:Sr优先与偏聚在晶界的Al结合而形成高熔点（1040℃）的Al₄Sr相,Al₄Sr的数量随Sr加入量的增加而增多,使Mg₁₇Al₁₂、AlLi数量减少,而Al₄Sr热稳定性好,有效地阻止位错滑移和晶界滑动,增强了合金的耐热性能。Sr含量过高则导致Al₄Sr呈聚集分布,降低合金的力学性能,相反,适量的Sr会提升合金的性能。更多还原

【Abstract】 Mg-Li alloy is known as the lightweight alloy.Due to its low density,high strength-to-weigh, rigidity-to-weigh ration and good damping capacity properties, Mg-Li alloys are applicable to aerospace,electronics and communication industries ,which would be likely to expanding at a high rate.Research works of this thesis aim at Mg-Li-Al alloy. Mg-Sr intermediate alloy and Mg-9Li-3Al alloy were prepared using“Incorporation method”. The influence of Sr on the microstructure of as-cast,heat-treated, and hot-extruded test alloys were investigated by optical microscopy （OM）, scanning electron microscopy （SEM）, X-ray diffraction （XRD） and high-temprature mechanical property test. Microstructure and high-tempreture mechanical properties of Mg-9Li-3Al alloy with Sr addition were studied, the fracture mechanism of the Mg-9Li-3Al-xSr alloys was investigated as well.The results showed that Sr has refining effect on test alloys and the new stable phase Al₄Sr is formed in the alloys, which distributes along grain boundary.When the as-cast alloy with 2.5wt% Sr content, the grains are refined efficiently. The dynamic recrystallization occurs during the hot-extruding ,so the grains are refined more smaller than as-cast alloys which makes the tensil strength of test alloy increase greatly. Besides,Al₄Sr phase distributes dispersively in the form of block structure or particle structure after extrusion.Mg-9Li-3Al-xSr alloys as-cast and extuded were subjected to the high-tempreture mechanical properties test at 100℃and 150℃.The result of mechanical properties test indicates that mechanical properties of hot-extruded test alloys are much better than the as-cast alloys. Mg-9Li-3Al alloy of high-tempreture tensile strength declined clear compared with the test alloy with Sr addition which indicated that amount of Sr performed good mechanical properties. Elevated temperature fractural morphology showed that the trends of intergranular fracture extended of Mg-9Li-3Al alloy with addition of Sr.Not only the elongation is all above 10% for hot-extruded test alloys at 100℃,the tensile strength of the hot-extruded alloy can reaches to its peak value 235.2MPa with 2.5wt%Sr content. The tensil strength, yield strength respectively increases by 19.8% compared with the Mg-9Li-3Al alloy. So the Mg-9Li-3Al-2.5Sr extruded alloy shows the best high-tempreture mechanical properties with the highest strength and a relative good elongation. Moreover,the improved high-tempreture properties with Sr addition are analysed. Sr tends to separately form stable Al₄Sr（1040℃） phase with Al in the alloys which leads to the reduction in amount of Mg₁₇Al₁₂、AlLi phases. The highly thermal stablility of Al₄Sr phase could hinder dislocation climbing and slipping along the grain boundaries. The addition of Sr might cause the phase Al₄Sr to be concentrated to the boundaries, which reduce the high-temperature properties of Mg-9Li-3Al alloy. On the contrary, minor content of Sr can improve the mechanical properties.更多还原