【作者】 周浩；

【导师】 王渠东； Yuntian T. Zhu；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2015， 博士

【摘要】 镁合金是实际应用中最轻的金属结构材料,在国防军事、航空航天、汽车、电子通信等工业领域具有非常广阔的应用前景。相比于其他金属结构材料,镁合金在室温下的强度和塑性都比较低,开展镁合金的强韧化研究对推广其应用具有重要意义。由于镁没有同素异构变化,无法利用相变强化来提高它的性能。目前,镁合金最为有效的强化方式为固溶强化、细晶强化和析出强化等。本文以Mg-GdY系合金为研究对象,以反复镦压和轧制为变形手段,细化和均匀化合金的组织结构,研究的重点着眼于合金在变形和热处理中的组织、结构和织构的演变,以及相应的力学性能变化规律。本研究系统地研究了细晶强化、固溶强化、时效强化、位错强化和织构控制等多种强化手段对提高合金力学性能的作用;深入地分析了位错运动,孪晶、亚晶和晶界形成,第二相析出和转变,界面偏析等变形和析出机理;揭示了如何有效利用多种强韧化机制,充分发挥复合强化作用制备高性能稀土镁合金关键科学问题。具体研究内容如下:利用Gleeble热模拟试验机,研究了挤压态Mg-Gd-Y（GW103K）合金的热压缩变形过程中的流变行为。GW103K合金的流变应力对加工温度和应变速率敏感,在任意应变速率下,峰值应力随温度的降低而增高;而当变形温度一定时,峰值应力随着应变速率的增加而提高。基于流变应力结果,对材料进行了本构分析,获得挤压态GW103K合金的变形激活能计算值为229.5k J/mol,及其本构模型:。热加工图表明变形温度420-450℃,变形速率0.01-0.1s-1是优化的热变形工艺参数。热加工图预测了应变量较少时,主要变形失稳区是低温低应变速率和高温高应变速率;而应变量较大的主要变形失稳区集中在高应变速率。用有限元模拟软件DEFORM对不同温度、道次反复镦压变形进行了模拟。研究结果表明,样品在变形过程中存在小范围搅拌紊流;应力场显示,较高的变形应力发生在镦压初期,尤其是样品侧棱位置有明显的应力集中;应变场显示,单道次变形后,材料应变在挤压方向分布不均,呈波浪形分布特征,多道次变形应变累积上升,未发现变形死区,反复镦压4道次变形后,材料的最小应变值达到³.7。研究了不同工艺参数下反复镦压样品的微观组织。反复镦压加工道次对组织细化效果明显,晶粒尺寸随着加工道次的增加,从139μm细化到1μm以下的超细晶;晶粒尺寸的均匀性也随加工道次增加而逐渐提高;织构强度随加工道次增加逐渐降低。反复镦压变形温度对组织结构演化有显著影响,随着变形温度增加,晶粒尺寸也从¹μm增加到4⁵μm和10²0μm;织构强度也随变形温度升高而提高。反复镦压不同加工路径的晶粒细化能力有区别,路径A细化效果显著,并发现有剪切带形成,路径B细化效果较差,4道次变形后依然存在尺寸为5¹0μm的大晶粒,未见明显的剪切带。研究了不同工艺参数下反复镦压样品的力学性能。力学性能随加工道次的增加而提高,变形前后屈服强度、抗拉强度和断裂延伸率分别比初始状态提高了150%、52%和110%;随着变形道次增加,力学性能均匀性也显著提高。随着变形温度增加,屈服强度呈逐渐降低的规律,抗拉强度变化规律与之相似,断裂延伸率的变化规律是随着加工温度提高而提高。反复镦压加工路径B与路径A相比,得到的样品具有更高的延伸率,但是屈服强度和抗拉强度都相对较低,路径B适合制备延伸率要求较高的样品,而路径A更适合制备高强度GW103K合金。研究了反复镦压后样品的时效析出行为。时效析出可进一步提高反复镦压样品的屈服强度和断裂强度,时效峰值样品的抗拉强度可在时效前350MPa的基础上再提高100MPa左右,但延伸率大幅降低。峰值时效晶内析出相形貌为“乌龟形”,经细致的微观结构表征确认,该形貌是由β″相、βT相和β′相共同组成。新发现的βT相位于β″相和β′相之间,其晶体结构为b.c.o结构,晶格常数为a=2·a Mg≈0.64nm,b=8·d（1010）Mg≈3.33 nm,c=cMg≈0.52 nm,与基体的取向关系是:（100）Tb//（1120）a、[001]Tb//[0001]a。βT相的化学成分为Mg₅RE,其中RE由Gd和Y组成,因此也可写成Mg₅Gdx Y1-x（0<x<1）。提出了新的Mg-Gd-Y三元合金的时效析出序列:S.S.S.S→β″→βT→β′→β1→β。Ag元素对Mg-Gd-Y系合金的界面偏析结构有影响。未添加Ag的GW103K合金,孪晶和晶界的原子偏析呈直线型,而添加了Ag元素后,Mg-Gd-Y-Ag合金（GWQ1032K）,在{1012}孪晶（TBII）和片层晶界（LGB）上会形成“脊椎骨”形貌的周期性偏析新结构,该新结构比直线型偏析有更高的溶质原子密度,因而具有更高的钉扎界面的能力。出现这种新偏析结构的主要原因是,Ag原子的fcc结构以及较小的原子半径。此外,孪晶、层错、晶界等界面结构和能量也会对偏析形式产生影响:“脊椎骨”形周期性偏析结构容易在晶界和TBII等高界面能的界面上形成;而低界面能的TBI上形成的偏析结构为直线型;能量最低的层错上甚至无法形成任何周期性偏析结构。更多还原

【Abstract】 As a lightweight structural metal material, magnesium alloys have widely potential applications for use in national defense, aerospace, automotive and electronic industries. However, the application of Mg alloys has been substantially hindered by their relatively low strength and limited ductility at room temperature. It is very important to investigate the toughening of magnesium alloys for promoting their applications. Owing to absent of allotropic transformation, mechanical properties of magnesium alloys cannot be enhanced by phase transformation. Recently, the most effective ways for strength enhancement are solid solution strengthening, grain refinement and precipitation hardening. Combining these mechanisms, high performance rare earth magnesium alloys are prepared by a novel super plastic deformation technique, repeated upsetting(RU), and followed by aging treatment in this study. Systematic studies of microstructure evolution, mechanical properties changing and strengthen mechanism interaction during the deformation and annealing are carried out to develop the toughening technology of magnesium alloys. In this way,some key issues in fabrication of high performance rare earth magnesium alloys are solved. The detailed researches are as followed:The deformation behavior of as-extruded Mg-Gd-Y(GW103K) alloy is investigated by compression test with Gleeble-3500 thermal simulator at temperature of 648-723 K and strain rate of 0.01-5 s-1. It is found that the flow stress is sensitive to deformation temperature and strain rate for GW103 K alloy. At any strain rate, the peak stress increases along with the decrease of temperature suggesting that the working hardening effect is significant at low temperatures. On the other hand, the peak stress increases along with the increase of strain rate at a given temperature indicating that the release of work hardening factors like dislocation accumulation is more effective during slower deformation processes. Base on the results of flow stress, we proposed a hyperbolic sine constitutive equation in which the determined average activation energy is 229.5 k J/mol. Processing maps describing the variation of power dissipation efficiency is constructed as a function of temperature and strain rate, which exhibit a domain of dynamic recrystallization(DRX) occurring at temperature of 420-450 °Cand strain rate of 0.01-0.1s-1corresponding to the optimum hot working window. The instability zones of flow behavior are also recognized from the maps.Finite element simulation was carried out to investigate deformation temperature and passes effect on material flow, stress and strain evolution. Flow field results show that the deformation of RU can be divided into two components of pure shear and uniform extend, which leads to a turbulent flow during deformation. Stress results show that stress concentration occurs at edges of the samples during the beginning stage of deformation. The sample processed by a single pass of RU exhibits a wave-type distribution along extrusion direction. As the number of passes increase, the flow strain shows normal distribution. The minimum strain of the sample reaches ~3.7, indicating no dead zone existed in RU deformation.After pre-extrusion and 4 passes of RU at 350 °C, initial grain size reduces from139μm to less than 1μm, forming a homogenous and ultra-fined grain structure. As the deformation passes increasing from 1 to 4, the maximum texture intensity gradually decreases from 7.6 to 2.0. The grain size of the samples deformed at 350 °C, 400 °C and 450 °C for 4 passes are ~ 1μm, 4 ~ 5μm and 10 ~ 20μm, respectively. As temperature decreases from 450 °C to 400 °C and 350 °C, the maximum pole density slightly increases from 2.0 to 2.2 and 2.5. Effective grain refinement is achieved by producing with route A, which also induces many shear bands into the microstructure.However, the samples deformed after 4 pass of Route B did not show equivalent refinement. Many coarse grains of 5~10μm retained, while no obvious shear bands are found in the microstructure.Mechanical properties of the samples with different parameters were investigated.After 4 passes of RU processing, the yield strength, ultimate strength and uniform elongation improved 150%, 52% and 110%, respectively, which reached 250 MPa,350MPa and 12%. The homogeneity of mechanical properties are also improved with the increase of RU passes. The range of micro hardness changes from 650~850MPa to850~950MPa. After 4 passes of RU processing, the yield strength is improved to 271 MPa in tension and 262 MPa in compression without showing the yield strength asymmetry. As the temperature increase from 350 °C to 400 °C and 450 °C, the yield strength decrease from 250 MPa to 210 MPa and 190 MPa, which is similar to the trend of ultimate strength. Elongation improve from 12% to 18.5% and 24% with the temperature increase from 350 °C to 400 °C and 450 °C. The elongation of the sample produced by route B is larger than route A, while the strength of route B is lower.Therefore, route B is suitable for fabricating the sample with better ductility, while routeA is better to produce high performance GW103 K alloy.Mechanical properties can be further improved by annealing after RU. The ultimate strength improves from 350 MPa to 450 MPa after peak aged, while the elongation decreases to ~3.3 %. A previously unobserved metastable phase(βT) is discovered to coexist with reported β″ and β′ metastable phases under peak aging conditions. The βT phase has an orthorhombic crystal structure with lattice parameters of a = 2· a Mg ≈ 0.64 nm, b = 8· d(1010)Mg ≈ 3.33 nm, c = c Mg ≈ 0.52 nm, with a composition of Mg5RE(RE= Gd and Y). It can be seen that the orientation relationship between aβT precipitate and the matrix is:(100)Tb//(1120)aand [001]Tb//[0001]a. According to our observations, a precipitation sequence of S.S.S.S → β″ → βT → β′ → β1 → β is proposed, where some phases may coexist under some aging conditions.Ag addition is found to significantly affect segregations a high-energy interfaces.Ag-assisted segregation at{10 12}twin boundaries(TBII)and lamellar grain boundaries(LGB)exhibits a new periodic spinal-shaped structure that is different from the single-lined segregation in the alloy without Ag.The segregation consists of Gdand Ag-rich columns.It appears that high Ag content in the spinal-shaped segregation induces fcc-like cells structures.更多还原