等通道角挤压制备细晶ZK60镁合金的组织与力学性能

利用两种等通道角挤压(ECAP)方法(普通单步ECAP和两步ECAP)制备细晶ZK60合金。采用金相显微镜、扫描电镜、透射电镜和X射线衍射仪对合金的组织和织构进行观察,通过拉伸试验研究不同ECAP方法对合金力学性能的影响。结果表明:与单步ECAP变形相比,两步ECAP变形,由于降低了变形温度,晶粒细化效果更好;经过(240℃,4道次)+(180℃,4道次)两步ECAP变形后,合金晶粒细化至约0.8μm;合金的力学性能与材料的织构密切相关,由于存在织构软化效应,与挤压态相比,经单步ECAP变形后合金的强度有所降低,而伸长率明显提高;但经两步ECAP变形后,由于细晶强化和亚结构强化的作用,合金的强度得到提高。     

Effect of Forming Process on Microstructure and

用OM, SEM, TEM和电子万能试验机对不同方法制备的ZK60镁合金薄带的组织和力学性能进行了研究。常规铸造ZK60镁合金轧制后仍为等轴晶组织,晶粒尺寸明显细化,双辊铸轧ZK60镁合金条带温轧变形后,显微组织由树枝晶转变为纤维状变形组织,且有高密度剪切带产生,温轧过程中没有明显的动态再结晶发生。轧制后两种合金均具有良好的力学性能,轧制态铸轧合金的强度明显高于传统铸造合金,伸长率略低于传统铸造合金。退火热处理后两种合金均发生了再结晶,得到等轴晶组织,且铸轧合金的组织比传统铸造合金的组织更加均匀细小。退火热处理使薄带的强度略有下降,而伸长率大幅度提高,退火后双辊铸轧合金和传统铸造合金的抗拉强度、屈服强度和延伸率分别为：388 MPa,301 MPa,22.9%和311 MPa,219 MPa,19.3%。镁合金薄带制备过程的晶粒细化归因于剪切带、位错和挛晶的产生及后续退火过程中再结晶。     

成形过程对ZK60镁合金薄带组织和力学性能的影响

用OM,SEM,TEM和电子万能试验机对不同方法制备的ZK60镁合金薄带的组织和力学性能进行了研究.常规铸造ZK60镁合金轧制后仍为等轴晶组织,晶粒尺寸明显细化,双辊铸轧ZK60镁合金条带温轧变形后,显微组织由树枝晶转变为纤维状变形组织,且有高密度剪切带产生,温轧过程中没有明显的动态再结晶发生.轧制后两种合金均具有良好的力学性能,轧制态铸轧合金的强度明显高于传统铸造合金,伸长率略低于传统铸造合金.退火热处理后两种合金均发生了再结晶,得到等轴晶组织,且铸轧合金的组织比传统铸造合金的组织更加均匀细小.退火热处理使薄带的强度略有下降,而伸长率大幅度提高,退火后双辊铸轧合金和传统铸造合金的抗拉强度、屈服强度和延伸率分别为:388 MPa,301 MPa,22.9％和311MPa,219 MPa,19.3％.镁合金薄带制备过程的晶粒细化归因于剪切带、位错和挛晶的产生及后续退火过程中再结晶.     

等通道挤压镁合金的微观组织与硬度

对ZK60镁合金进行了C方式(相邻道次间旋转180°)等通道角挤压.采用光学显微镜、扫描电子显微镜和透射电子显微镜分析了试样ECAP变形前后的显微组织变化;采用显维维氏硬度计进行了硬度测试,并分析了ECAP变形对材料硬度的影响规律.结果表明:在ECAP挤压后的组织中有高密度的位错,它促成再结晶并晶粒细化到1～3μm;在经过6道次挤压后,合金的硬度达到76.3 HV.     

ECAP对粗晶AZ31镁合金的晶粒细化机制的影响

采用等通道转角挤压(ECAP)方法对AZ31镁合金进行挤压加工,并研究了变形过程中试样各部位的显微组织和硬度的变化.显微组织分析表明,孪晶在剪切变形阶段可以协调晶粒变形,而且在变形后成为动态再结晶晶粒形成的中心.ECAP过程中AZ31镁合金的组织是以一种非均匀的方式细化的,再结晶晶粒主要在孪晶界上形核,这形成了双模态的晶粒分布.研究还发现,经过ECAP后的试样纵截面上部残留的大晶粒明显多于下部的,这是由于在静态再结晶阶段试样各部位受力状态的不同造成的,这很好地解释了ECAP不同路径对镁合金均匀性的影响.     

ECAP及二次挤压对ZK60镁合金变形过程的模拟与验证

采用Deform-3D软件模拟了ZK60镁合金关于等通道角挤压(ECAP)及二次挤压的变形过程并进行试验验证。研究了ECAP和二次挤压对晶粒细化效果的影响,讨论了ECAP和二次挤压过程中材料流动和变形均匀性以及挤压载荷和有效应变的变化规律。模拟结果表明,在275～300℃进行ECAP,再在150℃下进行二次挤压,试样可以获得较大的分布均匀的应变,从而获得良好性能的超细ZK60镁合金。试验所得晶粒细化的结果与模拟结果符合。     

二次挤压对挤压和ECAP变形后ZK60镁合金显微组织和力学性能的影响

测试四种状态下ZK60合金的显微组织和力学性能,四种状态分别为：挤压;挤压＋4道次ECAP;挤压＋4道次ECAP＋二次挤压;挤压＋4道次ECAP＋退火＋二次挤压。在室温下成功地进行ZK60的二次挤压,得到超细晶组织。结果表明：ECAP和二次挤压可以显著细化晶粒。挤压＋4道次ECAP＋二次挤压后的ZK60合金的屈服强度为342MPa,但是其伸长率只有0.8%。在二次挤压之前进行退火,ZK60合金的伸长率可以提高到4.5%,而屈服强度基本不变,抗拉强度达到 388 MPa。     

超细晶Mg-2.5Zn-1Ca合金制备及其组织性能研究

通过挤压+等通道转角挤压(ECAP)复合加工工艺制备了超细晶Mg-2.5Zn-1Ca合金,采用OM、SEM、XRD、EBSD等手段分析变形过程中微观结构演变特征,结合力学性能变化,研究变形过程中合金强化机制。结果表明,经挤压+ECAP变形后,晶粒与第二相颗粒明显细化,其中挤压+2道次ECAP后获得了均匀的细晶组织,平均晶粒尺寸约1.1μm;同时,细小的Ca₂Mg₆Zn₃颗粒弥散分布于基体中。晶粒细化是剧烈塑性变形、动态再结晶和细小弥散的Ca₂Mg₆Zn₃相共同作用的结果。ECAP变形使合金的力学性能显著提高,2道次有最高的抗拉强度和延伸率,分别为275 MPa和17%。随着ECAP变形道次的增加,织构强度逐渐减弱,基面织构逐渐转变为一种新的织构,并且ECAP变形合金有较高的非基面施密特因子,组织均匀细化,使得材料有更好的延伸率。     

累积叠轧焊AZ31镁合金微观组织和织构演变的EBSD研究

对AZ31镁合金热轧板在350℃进行了累积叠轧焊(ARB)变形,采用EBSD技术研究了AZ31镁合金的微观组织和织构演变.结果表明,ARB可以显著细化AZ31镁合金的晶粒组织,经过3道次变形后平均晶粒尺寸为2.18μm,后续的ARB变形使AZ31镁合金的微观组织更均匀,但晶粒不会再显著细化,说明存在临界ARB变形道次,使晶粒细化和晶粒长大之间达到动态平衡.AZ31镁合金在ARB变形过程中的晶粒细化机制为连续动态再结晶,尤其还观察到了旋转动态再结晶.动态再结晶的形变储存能来源于多道次累积的剧烈应变和沿厚度方向分布复杂的剪切变形.ARB变形过程中旋转动态再结晶和剪切变形使新晶粒c轴发生旋转,导致基面织构弱化.     

ZK60镁合金新型变形工艺数值模拟与实验研究

为了解决等通道转角挤压(ECAP)和正挤压(FE)工艺缺陷,结合两者特点,提出了ECAP-FE新型大塑性变形挤压工艺。利用DEFORM软件进行ZK60镁合金新型变形工艺数值模拟,分别获取新型工艺成形特征、三种工艺等效应变场及ECAP-FE工艺组织模拟。模拟结果显示:新型工艺成形过程可分为4个阶段,并且存在两大变形区的ECAP-FE挤压工艺比ECAP或FE变形更能够积累大变形量。实验结果表明:ZK60镁合金经过ECAP-FE变形后平均晶粒尺寸约为2. 8μm,比铸态晶粒尺寸减小32倍,晶粒细化效果显著。     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.     

Directional Growth of Tin Crystals Controlled by Combined Solute Concentration Gradient Field and Static Magnetic Field

A new method was introduced to achieve directional growth of Sn crystals. Microstructures in liquid(Pb)/liquid(Sn) diffusion couples were investigated under various static magnetic fields. Results show that the β-Sn crystals mainly reveal an irregular dendritic morphology without or with a relatively low static magnetic field(B0.3 T). When the magnetic field is increased to 0.5 T, the β-Sn dendrites close to the final stage of growth begin to show some directional character. With a further increase in the magnetic field to a higher level(0.8–5 T), the β-Sn dendrites have an enhanced directional growth character, but the dendrites show a certain deflection. As the magnetic field is increased to 12 T, the directional growth of the β-Sn dendrites in the center of the couple is severely destroyed. The mechanism of the directional growth of the β-Sn crystals and the deflection of the β-Sn crystals with the application of static magnetic field was tentatively discussed.     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.