医用镍钛合金的制备与热压缩变形行为

采用真空感应熔炼法制备了医用Ti-50. 7%Ni合金(原子数分数), 测试了铸态合金的成分、相变点、微观组织和硬度, 并采用Gleeble-3800热模拟实验机在变形温度750~950℃、应变速率0. 001~1 s-1, 应变量为0. 5的条件下对Ni-Ti合金进行高温压缩变形, 分析其流动应力变化规律, 建立了高温塑性变形本构关系和热加工图.结果表明: 当变形温度减小或应变速率增大时, Ni-Ti合金的流动应力会随之增大.应变速率为1 s-1时, 合金的真应力-真应变曲线呈现出锯齿状特征.根据热加工图, 获得了Ni-Ti合金的加工安全区和流变失稳区, 进而确定其合理的热变形温度范围为820~880℃, 真应变速率低于0. 1 s-1.从而为制定镍钛合金的锻造工艺参数提供理论和数据基础.      

Dislocation-particle interactions during high temperature deformation of two-phase aluminium alloys

Aluminium-silicon, and aluminium-nickel alloys containing various dispersions of secondphase particles have been deformed in compression at elevated temperatures. It is found that there is a sharp transition in the mechanical behaviour, microstructure, and microtexture, at a critical temperature or strain rate, which is dependent on particle size. The results are compared with various models of stress relaxation at particles, and it is shown that both interface and bulk diffusion controlled mechanisms are operative. The relevance of the transition to the hot working of two-phase alloys is discussed.     

Ti-25Al-14Nb-2Mo-1Fe合金热压缩变形流变应力行为与微观组织演变的研究

在Gleeble-3500热模拟试验机上对Ti-25Al-14Nb-2Mo-1Fe合金进行了等温恒应变速率压缩试验,研究了在变形温度为950~1 100℃,应变速率为0.001~1 s-1,最大变形程度为50%的条件下合金的热压缩变形流变应力行为与微观组织演变。结果表明:Ti-25Al-14Nb-2Mo-1Fe合金的流变应力对变形温度和应变速率均较为敏感,其流变应力曲线具有应力峰值、流变软化和稳态流变的特征。在变形温度为950℃,应变速率为0.001~0.1 s-1的条件下,Ti-25Al-14Nb-2Mo-1Fe合金的热变形特性为片层组织球化,其热变形机制可用晶界分离球化模型进行解释说明;在变形温度为1 000~1 100℃,应变速率为1 s-1的条件下,材料只发生了动态回复现象;在变形温度为1 050~1 100℃,应变速率为0.001~0.1 s-1的条件下,材料发生了动态再结晶现象。     

6005A与6082铝合金热变形流变行为

利用Gleeble-1500热模拟试验机对6005A和6082铝合金进行高温等温压缩试验,研究了在变形温度为450~550℃和应变速率为0.005~10 s-1条件下两种铝合金的热变形流变行为.6005A铝合金在低应变速率条件下,不同变形温度时的流变曲线均呈现波浪形特征,随着应变速率的增加,硬化和软化接近平衡,表现为稳态流变特征;在高应变速率条件下,硬化过程占据主导地位,回复和硬化过程的竞争使流变曲线呈现波浪形上升的趋势.6082铝合金在低应变速率情况下,不同变形温度时的流变曲线未出现周期性波动;在中等应变速率条件下也表现为稳态流变特征;在高应变速率条件下出现波浪形特征.两种铝合金均为正应变速率敏感材料,其热变形是受热激活控制.最后给出了铝合金热变形条件下流变应力、应变速率和变形温度三者之间的关系式.      

Improvement on Hot Workability of γ-TiAl Base Alloy

γ- Ti Al base alloys are potential aerospaceengine materials because of their low density andhigh performance at elevated temperature[1] .Thebrittleness at room temperature is still chiefroadblock to their application. During the lastdecade,the efforts to improve the ductility ofγ-Ti Al base alloy have been made[2 ] .A fine fullylamellar (FFL ) microstructure was founded to havegood ambient ductility and fracture toughness.Itcan be obtained through thermal mechanicalprocessing (TMP) and …     

Breakdown Behavior of Eutectic Carbide in High speed Steel during Hot Compression

 The evolution of eutectic carbide in as-cast M2 high-speed steel on different deforming conditions was investigated. Initially, specimens were deformed by hot compression in the range of 1223 to 1398K at strain rates of 0.01 to 1.0s-1. In a series of continuous deformation tests the flow stress curves were determined for the downstream process simulation. Subsequently, metallographic examination was carried out before and after deformed specimens for carbides analysis. As-cast microstructure is characterized as a spherical matrix of austenite and a continuous network of rodlike or irregular eutectic carbide, whereas deformed microstructure has broken carbide network and smaller granular products on all experimental conditions. It suggests that eutectic carbides fracture to particles during hot deformation by thermomechanical disintegration, while diffusion-controlled phase transformation was not remarkable. Combination with numerical simulation, the relationship between breakdown ratio of carbide network and deforming parameters were concluded. Strain was the most important one to shatter eutectic carbides and disperse products. Furthermore, critical strain values were obtained, beyond which carbide network disappeared, and fractured carbides kept a stable profile and deformed with matrix coordinately. A higher temperature or lower strain rate resulted in a lower critical strain.     

Micro‐scale Strain Distribution in Hot‐worked Duplex Stainless Steel

A modified microgrid technique has been applied to a laboratory‐made duplex stainless steel, to experimentally simulate the local state of deformation of the austenite‐ferrite microstructure of low‐alloy steels subject to intercritical deformation. A sample containing such a microgrid was deformed by plane strain compression at high temperature under conditions representative of hot rolling processes. The distortion of the microgrid after hot deformation revealed, in a quantifiable manner, the plastic flow of both phases and different deformation features. The micro‐strain distributions measured can be used to validate the models predicting the hot deformation of low alloyed C‐Mn steels during intercritical rolling.     

Hot Deformation Behavior of a Novel Cr–Mo–V Hot Work Die Steel

Herein, high-temperature hot compression tests are carried out to investigate the hot deformation behavior of a novel Cr–Mo–V hot work die steel. Meanwhile, a strain compensation Arrhenius constitutive equation is created, and the fitting correlation coefficient R between the experimental and predicted values is 0.98921, with a mean average absolute relative error (AARE) of 3.82%. According to the results, the hot deformation constitutive equation of the novel Cr–Mo–V steel can predict the flow stress of hot deformation with high accuracy. Furthermore, the equations for critical stress (strain), peak stress (strain), and Zener–Holomon parameters are constructed, and they demonstrate a satisfactory linear fit. Additionally, a hot processing map of the novel Cr–Mo–V steel is developed at strains of 0.4, 0.5, 0.6, and 0.7. By analyzing the hot processing map and viewing the microstructure, this study concludes that the optimal hot processing parameters of the novel Cr–Mo–V steel are 1050–1150 °C and 0.8–1 s⁻¹ at a strain of 0.7, with a peak power dissipation efficiency of nearly 0.68.     

Plastic relaxation of internal stresses in a dislocation microstructure

Internal stresses are present in plastically deformed metals because plastic strain gradients develop at obstacles to flow. The internal stress builds up until it causes local plastic flow. This is called plastic relaxation. Here dislocation-rich obstacles to flow in pure f.c.c. and b.c.c. metals are modelled as ellipsoidal inclusions. A method due to Eshelby is used to calculate the stress concentrations in the obstacles when the metal shears uniformly, on a single glide plane. It is found that planar obstacles which lie either close to the glide plane or roughly perpendicular to the glide direction concentrate the stress on secondary systems. It is suggested that such obstacles are reinforced and stabilised by plastic relaxation and that they are stable components of the microstructure.     

An analysis of the isothermal hot compression test

The nominally isothermal, uniaxial hot compression test has been analyzed with special reference to the effects of temperature nonuniformities and friction on sample deformation and flow stress estimates. A simple one-dimensional analysis was used to establish the influence of initial temperature nonuniformities, strain rate, and the temperature dependence of the flow stress on flow localization tendencies. Noticeable strain concentrations were predicted to occur only at high strain rates (∼10 s⁻¹) in materials such as titanium alloys, but not in steels, for typical values of the initial temperature nonuniformity. More extensive numerical (finite element method) simulations of the compression test with various values of the friction shear factor corroborated the conclusions of the flow localization analysis. In addition, it was established that initial temperature nonuniformities, as well as friction, have an almost negligible effect on flow stress data deduced from measurements of average pressurevs true height strain, at least for reductions of the order of 50 pct. The analysis results were supported by observations of the deformation behavior of a near-gamma titanium aluminide and a low-alloy steel. S.I. OH, formerly Research Leader, Net Shape Manufacturing Group, Battelle Memorial Institute, Columbus, OH, 43201. S.L. SEMIATIN, formerly Senior Research Scientist, Metalworking Group, Battelle Memorial Institute, Columbus, OH 43201.     

Processing-microstructure relationships for Ti-6Al-2Sn-4Zr-2Mo-0.1Si

The detailed relationships between thermal-mechanical processing parameters and resulting microstructures for Ti-6Al-2Sn-4Zr-2Mo-0.1 Si (Ti-6242) have been established through compression testing and heat treatment. Beginning with either an equiaxed alpha or Widmanstätten alpha preform microstructure, isothermal compression tests were run at strain rates typical of isothermal forging (10^?3 to 10^?1 s^?1) and conventional forging (1 to 100 s^?1). Metallographic investigation of these test specimens in as-deformed and heat treated conditions was used to characterize deformation-induced microstructures and transformations. For the equiaxed alpha microstructure, it was shown that deformation, as well as post-deformation heat treatment, were more effective in promoting microstructures close to the expected equilibrium ones than heat treatment alone, a finding similar to that for other alloy systems. For the metastable Widmanstätten alpha microstructure, the deformation and heat treatment parameters that promote the development of an equilibrium, equiaxed alpha microstructure have been determined. For this microstructure, two separate temperature-strain rate regimes have been identified, and the resulting microstructures correlated with the measured flow stress behavior. For the low temperature regime, deformation is highly nonuniform, and the microstructural features are shown to be similar to those in pearlitic steels and other lamellar alloys. In the higher temperature regime, on the other hand, deformation is much more uniform. The results presented can be applied to select hot forging parameters for the control of final microstructure and properties in Ti-6242 and similarα/β titanium alloys.     

超纯21%Cr铁素体不锈钢的热变形行为研究

采用热力模拟试验机对超纯21%Cr铁素体不锈钢进行了单道次压缩试验。研究了在650~1 000℃范围内铁素体不锈钢的热变形行为,确定了流变应力、变形温度和变形速率之间的关系及组织演变规律,并建立了变形抗力数学模型。结果表明,超纯铁素体不锈钢在试验温度范围内的热变形软化机制以动态回复为主。在800℃以下变形时,铁素体晶粒内部出现剪切变形带。随着温度的降低和压下量的增加试样中亚晶粒得到细化,位错密度增加。     

二次硬化超高强度钢的热变形行为

利用Gleeble-3500热力模拟试验机,在温度为1123～1423K,应变速率为0.5～10s-1的条件下,对航空用高强韧性的二次硬化超高强度钢(AF1410钢)进行了高温轴向压缩试验,测得了AF1410钢的高温流变曲线,并观察了变形后的显微组织。试验结果表明,AF1410钢的流变应力和峰值应变随着变形温度的升高和应变速率的降低而减小;AF1410钢在真应变为0.8,应变速率为0.5～10s-1的条件下,随着变形速率的提高,其发生完全动态再结晶的温度也逐渐升高。当变形速率为10s-1时,其变形温度高于1373K,才会发生完全动态再结晶;AF1410钢的热变形激活能Q值为430.39kJ/mol,并确立了其热变形方程。     

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

SAE9310钢热变形行为的研究

利用Gleeble-3500热力模拟试验机,在温度为1123~1423 K,应变速率为0.1~10 s-1,真应变为0.8的条件下,对一种传动部件用高强度渗碳钢(SAE9310钢)进行了高温轴向压缩试验,测得了SAE9310钢的高温流变曲线,并观察其变形后的显微组织。试验结果表明,SAE9310钢的流变应力和峰值应变随着变形温度的升高和应变速率的降低而减小;SAE9310钢在真应变为0.8的条件下,随着变形速率的提高,其发生完全动态再结晶的温度也逐渐升高,当热变形温度高于1323 K时,应变速率在0.1~10 s-1范围内,试验钢均会发生动态完全再结晶;测得9310钢的热变形激活能Q值为416.78 kJ/mol,并确立了其热变形方程。     

铸态超级双相不锈钢S32750热加工性能

 为研究热变形参数对铸态超级双相不锈钢S32750热变形行为和显微组织的影响,运用Gleeble-3800热模拟试验机对S32750进行不同温度和应变速率下的高温拉伸和压缩试验。结果表明,S32750在1000~1200℃范围内具有较好的热塑性。在变形温度较低、应变速率较低时,流变曲线表现出不同于单相不锈钢的“类屈服平台”特征;当应变速率较高或变形温度较高、应变速率较低时,流变曲线为典型的动态再结晶特征。微观组织演变显示,铁素体和奥氏体两相都发生动态再结晶,且铁素体的再结晶先于奥氏体。降低应变速率,提高变形温度,可促进动态再结晶发生。基于热变形动力学模型建立了本构方程,表观应力指数为3.99,热变形激活能为393.75kJ/mol。S32750的高温软化机制与Zener-Hollomon(Z)参数有关,随Z参数增加,热变形峰值应力增加。     

Dynamic Recrystallization of Hot Deformed 3Cr2NiMnMo Steel: Modeling and Numerical Simulation

Hot compression tests of 3Cr2NiMnMo steel were performed at temperatures in the range of 850 to 1100 °C and with strain rates of 10^?2s^?1 to 1s^?1. Both the constitutive equations and the hot deformation activation energy were derived from the correlativity of flow stress, strain rate and temperature. The mathematical models of the dynamic recrystallization of 3Cr2NiMnMo steel, which include the dynamic recrystallization kinetics model and the crystallization grain size model, are based on Avrami's law and the results of thermosimulation experiments. By integrating derived dynamic recrystallization models with the thermal-mechanical coupled finite element method, the microstructure evolution in hot compressive deformation was simulated. The distribution of dynamic recrystallization grains and grain sizes were determined through a comparison of the simulation results with the experimental results. The distribution of strain and dynamic recrystallization grain is also discussed. The similarity between the experimental results and the simulated results indicates that the derived dynamic recrystallization models can be applied effectively to predict and analyze the microstructure evolution in hot deformed 3Cr2NiMnMo steel.     

Texture evolution during plane strain deformation of aluminum

Plane strain forming in commercial processes commonly includes elevated temperature multi-step reductions. An understanding of microstructural development during this process is critical because it dictates the properties of the material subsequent to hot forming. Earing behavior, formability, and mechanical response of the final product are all dependent upon the processing parameters which dictate microstructural evolution. The present work focuses upon the development of hot deformation textures in aluminum during this type of processing. Commercial purity aluminum specimens exhibiting two different starting textures were deformed in channel die compression experiments to simulate the plane strain deformation conditions imposed by the rolling mill. Initial structures consisted of a randomly textured material and a preferentiallycube oriented texture to investigate the effects of starting texture on hot deformation processing and the resulting microstructures. Rate and temperature dependence of texture evolution was experimentally and theoretically investigated in conjunction with this. The relative stability of cube orientations within polycrystals deformed in plane strain is demonstrated for certain deformation conditions. Finally, experimental observations of the evolution of orientation flow from the alpha to the beta fiber are discussed.     

初始晶粒尺寸对2024铝合金热变形行为和组织的影响

利用永磁搅拌近液相线铸造和普通铸造方法制备不同晶粒尺寸的2024铝合金铸锭,利用Gleeble-1500热模拟试验机研究初始晶粒尺寸对不同压缩变形条件下2024铝合金的热变形行为和变形后显微组织的影响。研究表明:2024铝合金的热变形行为依赖于变形条件和初始组织。初始晶粒尺寸对流变应力的影响是:当应变速率小于0.1 s~(-1)时,流变应力随晶粒尺寸减小而减少;当应变速率为10 s~(-1)时,流变应力随晶粒尺寸减小而增大。降低变形温度会弱化晶粒尺寸对流变应力的影响。热压缩流变应力随应变速率增大而增大,随变形温度升高而减小。应变速率为10 s~(-1)时,热压缩应力应变曲线呈现周期性波动;只在粗晶2024铝合金中发现变形剪切带。