【摘要】：變形稀土鎂合金具有較好的室溫、高溫力學(xué)性能和抗腐蝕性能,符合航空航天、武器裝備和汽車輕量化的要求,具有廣闊的應(yīng)用和發(fā)展前景。目前變形稀土鎂合金主要采用鍛造和擠壓的方式進(jìn)行塑性變形,其產(chǎn)品規(guī)格和形狀受到很大的限制。而軋制作為最經(jīng)濟(jì)高效的塑性加工方法,在金屬材料塑性加工領(lǐng)域得到廣泛的應(yīng)用,鎂合金板材軋制的研究也越來越受到重視。隨著鎂合金板材需求的增多,對板材的強度提出了更高的要求,致使開發(fā)高強度的寬幅鎂合金板材成為必然。根據(jù)目前稀土鎂合金的相關(guān)文獻(xiàn)可知,稀土元素質(zhì)量百分比在10%以上的合金,強度高,塑性低,變形熱加工窗口窄,只能通過特定的加工方式制備,不適合開發(fā)高強度的鎂合金板材。因此,在本課題組對EW75、WE93、WE91和WE83合金研究的基礎(chǔ)上,通過適當(dāng)?shù)慕档拖⊥梁?設(shè)計開發(fā)出一種塑性較好、強度較高和熱加工窗口較寬的稀土鎂合金,為WE71合金。其兼顧了強度和塑性,為開發(fā)高強度稀土鎂合金寬幅板材提供了可能。本文以Mg-7Y-1Nd-0.5Zr(WE71)合金為研究對象,對鑄態(tài)、均勻化態(tài)、變形態(tài)等不同狀態(tài)合金的組織、結(jié)構(gòu)和性能進(jìn)行了系統(tǒng)的研究。為研制高性能的稀土鎂合金板材提供實驗基礎(chǔ)和理論依據(jù)。確定鑄態(tài)WE71合金微觀組織是由a-Mg基體、“骨骼狀”的共晶相、富Y的方塊相和Zr顆粒組成。經(jīng)過均勻化熱處理后,共晶組織基本回溶到基體中,均勻化態(tài)合金的組織主要由a-Mg基體和富Y的方塊相組成,方塊相的結(jié)構(gòu)和成分與鑄態(tài)合金中的相同。均勻化后的合金進(jìn)行直接擠壓,擠壓過程中合金發(fā)生了完全動態(tài)再結(jié)晶,合金的微觀組織由平均晶粒尺寸為18μm的細(xì)小等軸晶粒組成。研究了均勻化態(tài)WE71合金的熱模擬變形過程中的組織演變規(guī)律。晶界是主要的動態(tài)再結(jié)晶的形核位置。變形量較小時,晶界發(fā)生遷移形成鋸齒狀,具有非連續(xù)動態(tài)再結(jié)晶的特征。變形量為60%時,亞晶界通過吸收位錯不斷增加其取向差,形成大角度晶界,屬于連續(xù)動態(tài)再結(jié)晶的特征。均勻化態(tài)合金的變形激活能Q為212.37kJ/mol,本構(gòu)方程可以表為：ε=3.337×1012[sinh(0.01108|σ|)]445 exp[-(212.37 ×103)/8.314T]。鑄態(tài)合金軋制后,共晶組織破碎,成點鏈狀或線狀分布,促進(jìn)動態(tài)再結(jié)晶的形核,細(xì)化晶粒：組織中產(chǎn)生大量的孿晶,道次間退火,發(fā)生了孿晶誘導(dǎo)再結(jié)晶的現(xiàn)象,細(xì)化組織,改善塑性,降低變形抗力,有利于下一道次的軋制變形。將動態(tài)再結(jié)晶與靜態(tài)再結(jié)晶有機結(jié)合,調(diào)控合金組織與性能。鑄態(tài)WE71合金軋制后共晶組織的比表面積增加、晶界面積增加、位錯密度增加,加快了溶質(zhì)原子的擴散,縮短了均勻化時間。經(jīng)過軋制+均勻化后合金的晶粒尺寸明顯小于鑄態(tài)直接進(jìn)行均勻化合金的晶粒尺寸。說明軋制+均勻化的工藝制度在消除共晶組織和細(xì)化晶粒方面具有良好的效果。擠壓態(tài)合金通過軋制變形后,獲得了較為均勻的顯微組織。隨著軋制溫度的升高,基面織構(gòu)的最大極密度先增加后減小,在450℃軋制時達(dá)到最大值,為7.175。隨著道次變形量的增大,發(fā)生動態(tài)再結(jié)晶區(qū)域越大,細(xì)化了晶粒,改善了組織的均勻性。隨著總變形量的增大,發(fā)生動態(tài)再結(jié)晶區(qū)域面積越來越大,顯微組織由細(xì)小的等軸再結(jié)晶晶粒組成,說明要獲得更加細(xì)小的再結(jié)晶晶粒,在熱軋過程中必須采用較大的總變形量進(jìn)行軋制。在保證軋制行為有效的前提下,通過對組織和性能的考察,得出WE71合金的最佳軋制工藝為：溫度為500℃,道次變形量為10%,總變形量約為50%。研究了WE71合金的時效沉淀析出序列及其強化機制。通過對WE71合金在175℃時效過程進(jìn)行研究,發(fā)現(xiàn)了含Y元素較高的稀土鎂合金在低溫時效過程中,析出相形核和長大過程緩慢。WE71合金在200℃等溫時效過程中的主要沉淀相為β'和β相。β'相是以(1010)為慣習(xí)面的底心正交結(jié)構(gòu),晶格常數(shù)為a=O.64nm,b=2.22nm,c=0.52mn。β'相與基體處于完全的共格關(guān)系,β'相與基體的共格關(guān)系及其不同變體間相互交錯成網(wǎng)狀是其在200℃時效具有較高的熱穩(wěn)定的主要原因。T5態(tài)合金的高溫拉伸試驗表明,隨著溫度的升高,合金的屈服強度迅速降低,而延伸率增加；隨著保溫時間的延長,強度值下降,而延伸率增加。本研究發(fā)展的理論和制訂的新工藝將很好的指導(dǎo)高強度稀土鎂合金板材的軋制,很大程度上提高了軋制成形性和生產(chǎn)效率,具有重大生產(chǎn)和學(xué)術(shù)的意義。
[Abstract]:Wrought rare earth magnesium alloys have good mechanical properties at room temperature, high temperature and corrosion resistance, which meet the requirements of aerospace, weaponry and automotive lightweight, and have broad application and development prospects. As the most economical and efficient plastic processing method, rolling has been widely used in the field of metal plastic processing, and the research of magnesium alloy sheet rolling has been paid more and more attention. According to the related literatures of rare earth magnesium alloys, the alloys with the mass percent of rare earth elements more than 10% have high strength, low plasticity and narrow deformation hot working window, which can only be prepared by specific processing methods, and are not suitable for developing high strength magnesium alloy sheets. On the basis of this, a kind of rare earth magnesium alloy with good plasticity, high strength and wide hot working window was designed and developed by reducing the content of rare earth properly, which is called WE71 alloy. The microstructure, structure and properties of as-cast and deformed WE71 alloys were systematically studied. The experimental and theoretical basis was provided for the development of high-performance RE-Mg alloy sheets. It was determined that the microstructure of as-cast WE71 alloy was composed of a-Mg matrix, skeletal eutectic phase, Y-rich cubic phase and Zr particles. After heat treatment, the eutectic structure is basically dissolved back into the matrix. The microstructure of homogenized alloy is mainly composed of a-Mg matrix and Y-rich cubic phase. The structure and composition of the cubic phase are the same as those of as-cast alloy. The homogenized alloy undergoes direct extrusion, and the alloy undergoes complete dynamic recrystallization during extrusion. Microstructure evolution of homogenized WE71 alloy during thermal simulation deformation was studied. Grain boundary is the main nucleation site of dynamic recrystallization. When deformation is small, grain boundary migrates to form serrated shape, which is characterized by discontinuous dynamic recrystallization. The deformation activation energy Q of the homogenized alloy is 212.37 kJ/mol, and the constitutive equation can be expressed as follows: e = 3.337 *1012 [sinh (0.01108 | _ |)] 445 exp [-(212.37 | 103) / 8.314T]. After rolling, the eutectic structure of the as-cast alloy is broken and the point chain is formed. Distribution in shape or linearity promotes nucleation of dynamic recrystallization and refines grains: a large number of twins are produced in the microstructure, annealing between passes, twin-induced recrystallization occurs, refines microstructure, improves plasticity, reduces deformation resistance, and is conducive to the next rolling deformation. The specific surface area of eutectic structure, intergranular area and dislocation density of as-rolled WE71 alloy increase, which accelerates the diffusion of solute atoms and shortens the homogenization time. The extruded alloys obtained uniform microstructure after rolling deformation. With the increase of rolling temperature, the maximum extreme density of basal texture first increases and then decreases, reaching a maximum value of 7.175 when rolling at 450 C. The larger the dynamic recrystallization region is, the finer the grain is, and the more homogeneous the microstructure is. With the increase of total deformation, the area of dynamic recrystallization region becomes larger and larger. The microstructure consists of fine equiaxed recrystallized grains, which indicates that to obtain finer recrystallized grains, a larger total transformation must be adopted during hot rolling. On the premise of guaranteeing the effective rolling behavior, the optimum rolling process of WE71 alloy is obtained by investigating the microstructure and properties of the alloy. The optimum rolling process is as follows: the temperature is 500 C, the pass deformation is 10%, and the total deformation is about 50%. The aging precipitation sequence and strengthening mechanism of WE71 alloy are studied. The main precipitation phases of WE71 alloy during isothermal aging at 200 C are beta and beta phases. The main reason for the high thermal stability of the alloy aged at 200 C is the complete coherence of the matrix. The tensile test at high temperature shows that the yield strength of the alloy decreases rapidly and the elongation increases with the increase of temperature. The theory developed in this study and the new technology formulated will guide the rolling of high strength RE-Mg alloy sheets, greatly improve the formability and production efficiency, and have great production and academic significance.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TG146.22;TG339

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本文編號：2229669