TB17钛合金热压缩流变应力分析及本构方程

使用Gleeble-3500热模拟试验机在变形温度为800~1000℃、应变速率0.001~10 s~(-1)以及真应变为1.2的条件下对TB17钛合金进行热变形行为研究。根据热压缩数据,分析真应力-真应变曲线,计算TB17钛合金变形激活能,并建立了TB17钛合金应力-应变本构模型,对金相组织进行分析,并进行了本构模型的验证。结果表明,TB17钛合金在热压缩变形过程中,出现动态回复和动态再结晶现象,在低应变速率0.001和0.01 s~(-1)下,以动态再结晶为主要软化机制,在高应变速率1和10 s~(-1)下主要以动态回复为软化机制;流变应力随应变速率的下降和变形温度的升高而降低;峰值应力计算值和实验值的平均误差为6.5%,表明该模型有很高的精确度。研究为TB17钛合金塑性加工过程的模拟和控制提供了参考。     

6061铝合金热变形动态再结晶行为研究

利用Gleeble-3500试验机对6061铝合金进行单道次等温恒应变速率压缩试验,研究合金在应变速率为0.001～1s~(-1),温度为350～500℃热变形条件下的动态再结晶行为。统计试验所得流变应力曲线峰值应力数据,确定合金热变形激活能Q为307.528kJ·mol~(-1),建立合金在不同热变形条件下的流变应力方程,动态再结晶峰值和临界应变模型;依据流变应力曲线特征,计算合金在不同变形条件下的动态再结晶体积分数,据此建立动态再结晶动力学模型。分析流变应力曲线可知铸态6061铝合金在350～500℃下变形,应变速率较低时(0.01s~(-1)),合金组织更容易发生动态再结晶,应力软化现象更明显。     

粗晶GH4720Li合金热变形行为与动态再结晶特点

为了模拟难变形镍基高温合金GH4720Li开坯锻造过程,采用Gleeble-3800热模拟试验机研究经均匀化处理的GH4720Li铸锭高温压缩变形时的力学流动行为,分析高温变形过程中微观组织演化规律。结果表明,GH4720Li合金在1100℃, 0.1 s~(-1)条件下应力水平达到250 MPa,且应力对热变形温度和应变速率敏感,动态再结晶是主要的软化机制。粗晶组织提高了合金动态再结晶临界变形温度和应变速率,如在变形量为60%,变形条件为1140℃, 0.001 s~(-1)和1180℃,0.001s~(-1)才能发生完全动态再结晶。计算的粗晶GH4720Li合金热变形激活能Q=1171kJ/mol,较高的热变形激活能表明粗晶组织不利于热塑性变形和动态再结晶的发生。基于本研究,铸态GH4720Li合金开坯温度应高于1140℃,同时保证较低的应变速率,以确保动态再结晶的充分发生,实现枝晶组织破碎。     

TB18钛合金热变形行为及动态再结晶机制

本文利用Gleeble 3800热模拟试验机和电子背散射衍射（EBSD）技术研究了TB18钛合金在700℃~ 900℃、应变速率0.01~10 s-1时的热变形行为和动态再结晶机制。研究表明该合金的流动应力大小对应变速率和变形温度敏感。变形初期流动应力皆在达到峰值应力后快速软化,随后有不同程度的上升。通过数据回归得到了该合金在两相区和单相区的高温变形Arrhenius型本构方程,其变形激活能分别为340 kJ/mol和185 kJ/mol。其单相区的变形软化机制主要为β相的动态回复,两相区主要为β相的动态再结晶。结合了EBSD技术,金相观察和流变曲线特点的分析表明,在高变形温度,低应变速率时（900℃,0.01s-1）主要以几何动态再结晶（GDRX）为主。在温度较低,或变形速率较高下,变形初期发生不连续动态再结晶（DDRX）,应变增大后发生连续动态再结晶（CDRX）。     

6061铝合金热变形行为及动态再结晶

通过Gleeble-3500热压缩模拟试验机对6061铝合金进行热压缩实验,借助金相显微镜和透射电子显微镜研究合金在变形温度为340～490℃,应变速率为0.001～1 s~(-1)条件下热变形和动态再结晶行为。结果表明:6061铝合金的动态再结晶行为对变形温度和应变速率十分敏感,温度的升高和应变速率的减小都会促进动态再结晶的发生。基于峰值应力建立了合金热变形本构方程,计算得出热变形激活能为235.155 kJ·mol~(-1)。采用加工硬化率-流变应力曲线确定了合金热变形过程中的临界应力(应变)和峰值应力(应变)与Z参数的关系模型。随着温度的升高和应变速率的减小,DRX临界应力(应变)和峰值应力(应变)减小。依据Avrami方程建立了合金动态再结晶体积分数模型,动态再结晶体积分数随应变的增加,呈现先缓慢增加后迅速增加再缓慢增加的特征,所建模型能够较为准确地预测该合金的动态再结晶行为。     

Ti60合金热变形过程中的软化机制

采用Thermecmaster-Z型热加工模拟试验机对Ti60合金进行等温恒应变速率压缩实验,通过分析流动应力-应变曲线的流动特征,计算加工硬化率,观察变形微观组织,并结合变形激活能的计算,研究该合金在变形温度为850～950℃、应变速率为0.001～10 s~(-1)、真应变为0.51热变形条件下的软化机制。结果表明:Ti60合金在低应变速率(0.001～0.1s~(-1))和高应变速率(1～10s~(-1))区间流动应力-应变曲线分别呈现流动稳态型和流动软化型两种;加工硬化率曲线呈现无拐点特征;变形微观组织为动态回复组织,未出现动态再结晶现象;变形激活能在低应变速率区间和高应变速率区间分别为484.35 kJ/mol和500.76 kJ/mol,两者相差不大。综合这些结果可以判定,Ti60合金的软化机制以动态回复为主。     

挤压态阻燃钛合金高温变形行为及机理

通过等温恒应变速率热模拟压缩试验,研究一种挤压态组织的阻燃钛合金在变形温度900~1150℃,应变速率0.001~1 s~(-1)条件下的高温流变应力和组织演变,基于热压试验数据计算变形激活能。结果表明:此种阻燃钛合金流变应力-应变曲线具有应力峰值和流动软化特征,高温变形是扩散控制的过程,软化机制以动态回复为主,但在应变速率较高时会发生局部连续再结晶,而在应变速率较低时晶界运动比较明显。     

低成本Ti-Al-V-Fe-O合金热变形行为及热加工图

采用真空非自耗熔炼炉制备了低成本Ti-6Al-2.5V-1.5Fe-0.15O合金。利用Gleeble-1500D热模拟机,研究了其热加工参数为:变形温度875～1100℃、应变速率0.001～1 s~(-1),变形量为70%时的热变形行为。建立了Ti-6Al-2.5V-1.5Fe-0.15O合金考虑应变量的Arrhenius本构方程,基于动态材料模型建立热加工图。结果表明:变形温度升高,应变速率降低,流变应力降低。通过本构方程计算可得两相区平均热激活能为398.824 kJ/mol,远大于纯钛自激活能,表明热变形软化机制与动态再结晶有关。单相区热激活能为210.93 kJ/mol,略大于纯钛自激活能,以动态回复为主。通过热加工图确定2个失稳区,中等变形温度(950～1070℃)、高应变速率(0.31～0.1 s~(-1))易发生绝热剪切。结合热加工图确定适合的加工区间:应变速率为0.001～0.01 s~(-1),变形温度为875～925℃。     

基于软化机制的TC18钛合金本构关系研究

通过TC18钛合金热模拟压缩实验,得到不同变形条件下的高温变形真应力-真应变曲线.通过加工硬化和动态软化效应,分析变形参数变化对TC18钛合金应力-应变曲线形态和峰值应力的影响.不同变形条件下,TC18钛合金流变曲线呈现出相似的特征,而峰值应力对变形参数的变化却十分敏感.通过Poliak-Jonas准则,分析了不同条件下TC18钛合金在高温变形过程中的软化机制.相同温度下,动态再结晶机制主要发生在低应变速率下的高温变形过程中,并且软化机制的选择对温度不敏感.基于传统的Arrhenius型方程,针对TC18钛合金热变形过程中不同的软化机制,分别建立动态再结晶和动态回复机制下的本构方程.针对识别出的TC18合金在不同变形条件下的软化机制,通过适用的本构模型来描述TC18合金在应变为0.7时真实应力对变形温度、应变速率的响应过程.以动态再结晶为主要软化机制的变形过程,其变形激活能和应变速率敏感系数远远大于以动态回复为主的过程.     

7150铝合金高温热压缩变形流变应力行为

在Gleeble-1500热模拟机上对7150铝合金进行高温热压缩实验,研究该合金在变形温度为300~450 ℃和应变速率为0.01～10 s~(-1) 条件下的流变应力行为.结果表明:流变应力在变形初期随着应变的增加而增大,出现峰值后逐渐趋于平稳;峰值应力随着温度的升高而减小,随着应变速率的增大而增大;可用包含Zener-Hollomon参数的Arrhenius双曲正弦关系来描述合金的热流变行为,其变形激活能为226.698 8 kJ/mol;随着温度的升高和应变速率的降低,合金中拉长的晶粒发生粗化,亚晶尺寸增大,再结晶晶粒在晶界交叉处出现并且晶粒数量逐渐增加;合金热压缩变形的主要软化机制由动态回复逐步转变为动态再结晶.     

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).     

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.     

Theoretical Analysis for the Motion and Temperature of Melt Droplets in Spray Degassing Process

The motion of melt droplets in spray degassing process was analyzed theoretically. The height of the treatment tank in spray degassing process could be determined by the results of theoretical calculation of motion of melt droplets. To know whether the melt droplets would solidify during spraying process, the balance temperature of melt droplets was also theoretically analyzed. Then proof experiments for theoretical results about temperature of melt droplets were carried. In comparison, the experimental results were nearly similar to the calculation results.     

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.