几种Mg-Zn-Y合金的组织性能及变形机制研究

发布时间：2018-03-07 13:41

本文选题：镁合金　切入点：显微组织　出处：《沈阳工业大学》2015年博士论文　论文类型：学位论文

【摘要】：镁合金以高比强度、比模量,良好的散热性、消震性以及资源丰富等优势,在航空航天、现代汽车及通讯电子等领域应用前景广阔。提高服役性能包括提高强度以及改善室温可成型性是促进其作为结构材料工业化发展的先决条件。而优化合金成分,改善加工及热处理工艺是改善镁合金综合性能的重要手段。本文以形成准晶I相性能优异的三元Mg-Zn-Y合金系作为研究对象,在研究合金元素对准晶I相形成影响规律的基础上,通过调控合金化元素的含量以及结合熔铸热加工等工艺优化合金组织,以期进一步提高该合金系的力学性能。具体研究了Zn、Y元素含量及Zn/Y质量比对合金中第二相形态及其形成过程的影响;对铸态性能优异的合金进行了热挤压及热轧变形,探讨了热变形过程对合金组织、织构及力学行为的影响;分析了变形合金在静载及动载作用下的变形机制。研究结果表明:在普通铸造条件下,添加稀土Y元素促使Mg-Zn合金形成准晶I_Mg3Zn6Y及立方W_Mg3Zn3Y2为主要析出相的微观组织。Zn、Y元素含量不仅影响合金中析出相的数量,而且影响析出相的形态。当Zn/Y质量比接近5时,合金中析出相呈颗粒状;Zn/Y质量比为其他值时,第二相以“三叉”共晶及条片状存在。铸态合金性能随第二相数量的增加而提高。当析出相为颗粒状存在时,铸态合金的综合力学性能达到最佳,抗拉强度达到191.8MPa,断后伸长率为9.8%。优异的性能主要源于准晶I相自身高硬度、低表面能及其与基体共格等优良特性。经过350℃挤压比为12.75的热挤压后,合金中大部分α-Mg晶粒发生了再结晶,形成极其细小的再结晶晶粒。挤压态合金再结晶α-Mg晶粒平均尺寸为2μm左右,析出相以细小颗粒分布于基体中。经过挤压后合金获得优异的力学性能,其中挤压态Mg-5Zn-1Y合金的抗拉强度和屈服强度高达353.6MPa及325.8MPa,断后伸长率为13.4%。合金性能的改善来自于细晶强化,颗粒状准晶I相强化以及细小晶粒引发的非基面位错的滑移。另外,挤压变形使得合金中形成了较强的(0002)纤维织构,从而产生织构强化。经过425℃/15min退火,挤压比为12.75的Mg-5Zn-1Y及Mg-6Zn-1Y合金获得了完全的再结晶等轴晶。晶粒平均尺寸增大到10μm,析出相颗粒变化不大。退火处理使合金的强度降低,断后伸长率大幅度提高。退火态Mg-5Zn-1Y合金的抗拉强度降为279.2MPa,断后伸长率高达29.8%。EBSD结果分析表明,拉伸塑性的提高来自于退火过程中织构弱化引起({1012}-{1011})二次孪生交互发生对变形的协调。通过热分析及时效硬度变化确定了Mg-5Zn-1Y合金合理的时效工艺为225℃/8h。合金在处理过程中析出相以极其细小颗粒分布于基体中,同时析出长周期堆垛有序(LPOS)相,有利于合金强化。然而时效过程中合金晶粒明显粗化。晶粒的粗化引起的强度下降占主体地位导致合金强度最终表现为下降。挤压态Mg-5Zn-1Y的应变疲劳测试表明,应变幅较小(0.7%)时,疲劳滞回曲线基本对称,疲劳的拉伸阶段表现为明显的应变率强化,压缩阶段均表现为循环软化;应变幅较大(0.7%)时,疲劳滞回曲线明显不对称,所包含的面积随应变幅的增大而明显增大。主要是由于低应变幅下,合金的变形机制以位错滑移及滞弹性为主,位错滑移受第二相颗粒阻碍形成高密度位错引起循环硬化;而高应变幅下变形机制以位错滑移和孪生-去孪生为主,去孪生的过程引起循环软化。挤压态Mg-5Zn-1Y的动态(SHPB)压缩测试表明,合金抗压强度表现出明显的应变率强化,但屈服强度受影响较小。在相同加载速率下,沿挤压方向(ED)与垂直挤压方向(TD)性能差异较大,主要由于极高的应变速率以及合金中(0002)织构引起变形机制不同。高速应变条件下,与基体结合良好的析出相对性能提高极为有利。
[Abstract]:Magnesium alloy with high specific strength and modulus, good heat dissipation, shock and abundant resources and other advantages, in the aerospace, automobile and electronic communications and other fields has broad application prospect. To improve the service performance including improving the strength and improving the room temperature formability is a prerequisite for promoting industrial development as its structural material while optimizing the composition of alloy, improve the processing and heat treatment technology is an important means to improve the comprehensive performance of magnesium alloy. In this paper, the formation of quasicrystalline I phase with excellent performance of three yuan Mg-Zn-Y alloy as the research object, on the basis of quasicrystal alloy elements on the formation of I phase influence law, by regulating the contents of alloying elements and combination casting hot working process optimization of microstructure, in order to further improve the mechanical properties of the alloy. The research of Zn, Y element content and the mass ratio of Zn/Y alloy in the second phase morphology The influence of its forming process; cast alloy with excellent properties of hot extrusion and hot rolling deformation, discusses the hot deformation process of the alloy, the effect of texture and mechanical behavior; analysis of the deformation mechanism of the deformation in the alloy the effect of static and dynamic loads. The results show that: in the normal casting conditions, add rare earth element Y to Mg-Zn alloy I_Mg3Zn6Y and W_Mg3Zn3Y2 cubic quasicrystal formation as the main precipitates in the microstructure of.Zn, the amount of precipitates in the alloy not only affects the content of Y, and the influence of the precipitate shape. When the mass ratio of Zn/Y is close to 5, the alloy granular precipitates; the mass ratio of Zn/Y to other values, the phase to trigeminal eutectic and strip ". The alloy performance increases. When the number of second phase precipitates as granular, mechanical properties of as cast alloy has the best tensile strength. Reach 191.8MPa, elongation is 9.8%. excellent performance is mainly due to the quasi crystal I phase high hardness, low surface energy and coherent matrix and other excellent properties. After 350 DEG C extrusion ratio of 12.75 after hot extrusion, the majority of alpha -Mg grain recrystallization occurred in the alloy, recrystallization grain forming extremely fine the extruded alloy recrystallization. The average size of a -Mg grain size is about 2 m, with small particle size distribution of precipitates in the matrix. After extrusion alloys have excellent mechanical properties, the tensile strength of the extruded Mg-5Zn-1Y alloy and the yield strength of up to 353.6MPa and 325.8MPa, the elongation is the performance improvement from 13.4%. alloy grain strengthening, slip and I particle quasicrystal phase strengthening fine grains caused by nonbasal dislocation. In addition, the extruded alloy formed a strong (0002) fiber texture, resulting in strong texture At 425 DEG /15min after annealing, the extrusion ratio is Mg-5Zn-1Y and Mg-6Zn-1Y alloy 12.75 was completely recrystallized equiaxed grains. The grain size increases to 10 m, the precipitation changed little. The annealing treatment of the alloy strength decreased and the elongation increased. The tensile strength of annealed Mg-5Zn-1Y alloy drop for 279.2MPa, the elongation up to 29.8%.EBSD results show that the improved tensile ductility from texture during annealing ({1012}-{1011}) caused by the weakening of the two twin interactions of deformation coordination. By changing the stiffness of thermal analysis and aging to determine the aging process of Mg-5Zn-1Y alloy is reasonable 225 DEG /8h. alloy precipitated in the process the extremely small particles distributed in the matrix, while the precipitation of Long-period Stacking Order (LPOS) is conducive to strengthening. However, alloy alloy during aging process of crystal grain coarsening. Coarse grain decreased strength due to the dominant position in the strength of the alloy is decreased. The final strain fatigue test of extruded Mg-5Zn-1Y showed that the strain amplitude is smaller (0.7%), fatigue hysteresis curve symmetry, tensile fatigue stage obvious strain rate strengthening, the compression phase showed cyclic softening; strain a large (0.7%), fatigue hysteretic curves were asymmetric and contained in the area with increasing strain amplitude significantly increased. Mainly due to the low strain amplitude, deformation mechanism of the alloy by dislocation slip and anelastic, dislocation slip by second phase particles hinders the formation of high density dislocations caused by cyclic hardening and high stress; the dislocation slip and twinning twinning deformation mechanism to amplitude, to twinning process caused by cyclic softening. Dynamic extruded Mg-5Zn-1Y (SHPB) compression test showed that the alloy compressive strength showed that Strain hardening rate, but the yield strength is less affected. At the same loading rate, along the extrusion direction (ED) and perpendicular to the extrusion direction (TD) with different properties, mainly due to high strain rate and alloy (0002) texture caused by different deformation mechanisms. High strain conditions, combined with the matrix good performance of precipitates is extremely beneficial.

【学位授予单位】：沈阳工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.22

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