本文選題：鎂合金　切入點：顯微組織　出處：《沈陽工業(yè)大學》2015年博士論文　論文類型：學位論文

【摘要】：鎂合金以高比強度、比模量,良好的散熱性、消震性以及資源豐富等優(yōu)勢,在航空航天、現(xiàn)代汽車及通訊電子等領域應用前景廣闊。提高服役性能包括提高強度以及改善室溫可成型性是促進其作為結構材料工業(yè)化發(fā)展的先決條件。而優(yōu)化合金成分,改善加工及熱處理工藝是改善鎂合金綜合性能的重要手段。本文以形成準晶I相性能優(yōu)異的三元Mg-Zn-Y合金系作為研究對象,在研究合金元素對準晶I相形成影響規(guī)律的基礎上,通過調(diào)控合金化元素的含量以及結合熔鑄熱加工等工藝優(yōu)化合金組織,以期進一步提高該合金系的力學性能。具體研究了Zn、Y元素含量及Zn/Y質(zhì)量比對合金中第二相形態(tài)及其形成過程的影響;對鑄態(tài)性能優(yōu)異的合金進行了熱擠壓及熱軋變形,探討了熱變形過程對合金組織、織構及力學行為的影響;分析了變形合金在靜載及動載作用下的變形機制。研究結果表明:在普通鑄造條件下,添加稀土Y元素促使Mg-Zn合金形成準晶I_Mg3Zn6Y及立方W_Mg3Zn3Y2為主要析出相的微觀組織。Zn、Y元素含量不僅影響合金中析出相的數(shù)量,而且影響析出相的形態(tài)。當Zn/Y質(zhì)量比接近5時,合金中析出相呈顆粒狀;Zn/Y質(zhì)量比為其他值時,第二相以“三叉”共晶及條片狀存在。鑄態(tài)合金性能隨第二相數(shù)量的增加而提高。當析出相為顆粒狀存在時,鑄態(tài)合金的綜合力學性能達到最佳,抗拉強度達到191.8MPa,斷后伸長率為9.8%。優(yōu)異的性能主要源于準晶I相自身高硬度、低表面能及其與基體共格等優(yōu)良特性。經(jīng)過350℃擠壓比為12.75的熱擠壓后,合金中大部分α-Mg晶粒發(fā)生了再結晶,形成極其細小的再結晶晶粒。擠壓態(tài)合金再結晶α-Mg晶粒平均尺寸為2μm左右,析出相以細小顆粒分布于基體中。經(jīng)過擠壓后合金獲得優(yōu)異的力學性能,其中擠壓態(tài)Mg-5Zn-1Y合金的抗拉強度和屈服強度高達353.6MPa及325.8MPa,斷后伸長率為13.4%。合金性能的改善來自于細晶強化,顆粒狀準晶I相強化以及細小晶粒引發(fā)的非基面位錯的滑移。另外,擠壓變形使得合金中形成了較強的(0002)纖維織構,從而產(chǎn)生織構強化。經(jīng)過425℃/15min退火,擠壓比為12.75的Mg-5Zn-1Y及Mg-6Zn-1Y合金獲得了完全的再結晶等軸晶。晶粒平均尺寸增大到10μm,析出相顆粒變化不大。退火處理使合金的強度降低,斷后伸長率大幅度提高。退火態(tài)Mg-5Zn-1Y合金的抗拉強度降為279.2MPa,斷后伸長率高達29.8%。EBSD結果分析表明,拉伸塑性的提高來自于退火過程中織構弱化引起({1012}-{1011})二次孿生交互發(fā)生對變形的協(xié)調(diào)。通過熱分析及時效硬度變化確定了Mg-5Zn-1Y合金合理的時效工藝為225℃/8h。合金在處理過程中析出相以極其細小顆粒分布于基體中,同時析出長周期堆垛有序(LPOS)相,有利于合金強化。然而時效過程中合金晶粒明顯粗化。晶粒的粗化引起的強度下降占主體地位導致合金強度最終表現(xiàn)為下降。擠壓態(tài)Mg-5Zn-1Y的應變疲勞測試表明,應變幅較小(0.7%)時,疲勞滯回曲線基本對稱,疲勞的拉伸階段表現(xiàn)為明顯的應變率強化,壓縮階段均表現(xiàn)為循環(huán)軟化;應變幅較大(0.7%)時,疲勞滯回曲線明顯不對稱,所包含的面積隨應變幅的增大而明顯增大。主要是由于低應變幅下,合金的變形機制以位錯滑移及滯彈性為主,位錯滑移受第二相顆粒阻礙形成高密度位錯引起循環(huán)硬化;而高應變幅下變形機制以位錯滑移和孿生-去孿生為主,去孿生的過程引起循環(huán)軟化。擠壓態(tài)Mg-5Zn-1Y的動態(tài)(SHPB)壓縮測試表明,合金抗壓強度表現(xiàn)出明顯的應變率強化,但屈服強度受影響較小。在相同加載速率下,沿擠壓方向(ED)與垂直擠壓方向(TD)性能差異較大,主要由于極高的應變速率以及合金中(0002)織構引起變形機制不同。高速應變條件下,與基體結合良好的析出相對性能提高極為有利。
[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.

【學位授予單位】：沈陽工業(yè)大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG146.22

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