Constitutive behavior and processing maps of low-expansion GH909 superalloy

The hot deformation behavior of GH909 superalloy was studied systematically using isothermal hot compression tests in a temperature range of 960 to 1040℃ and at strain rates from 0.02 to 10 s-1 with a height reduction as large as 70%. The relations considering flow stress, temperature, and strain rate were evaluated via power-law, hyperbolic sine, and exponential constitutive equations under different strain conditions. An exponential equation was found to be the most appropriate for process modeling. The processing maps for the superalloy were constructed for strains of 0.2, 0.4, 0.6, and 0.8 on the basis of the dynamic material model, and a total processing map that includes all the investigated strains was proposed. Metallurgical instabilities in the instability domain mainly located at higher strain rates manifested as adiabatic shear bands and cracking. The stability domain occurred at 960-1040℃ and at strain rates less than 0.2 s-1; these conditions are recommended for optimum hot working of GH909 superalloy.     

Hot deformation map and its application of GH4706 alloy

The hot deformation behaviors of GH4706 alloy were investigated using compression tests in a deformation temperature range from 900℃ to 1200℃ with a strain rate range of 0.001–1 s?1. Hot processing maps were developed on the basis of the dynamic material model and compression data. A three-dimensional distribution of power dissipation parameter (η) with strain rate and temperature reveals that η decreases in sensitivity with an increase in strain rate and a decrease in temperature. Microstructure studies show that the grain size of GH4706 alloy increases when η is larger than 0.32, and the microstructure exhibits local deformation when η is smaller than 0.23. The hot processing map at the strain of 0.7 exposes a domain peak at η=0.32 for the temperature between 940℃ and 970℃ with the strain rate from 0.015 s?1 to 0.003 s?1, and these are the optimum parameters for hot working.     

Hot compression deformation behavior of AISI 321 austenitic stainless steel

The hot compression behavior of AISI 321 austenitic stainless steel was studied at the temperatures of 950–1100℃ and the strain rates of 0.01–1 s?1 using a Baehr DIL-805 deformation dilatometer. The hot deformation equations and the relationship between hot deformation parameters were obtained. It is found that strain rate and deformation temperature significantly influence the flow stress behavior of the steel. The work hardening rate and the peak value of flow stress increase with the decrease of deformation temperature and the increase of strain rate. In addition, the activation energy of deformation (Q) is calculated as 433.343 kJ/mol. The microstructural evolution during deformation indicates that, at the temperature of 950℃ and the strain rate of 0.01 s?1, small circle-like precipitates form along grain boundaries; but at the temperatures above 950℃, the dissolution of such precipitates occurs. Energy-dispersive X-ray analyses indicate that the precipitates are complex carbides of Cr, Fe, Mn, Ni, and Ti.     

Dynamic recrystallization behavior of the Ti–48Al–2Cr–2Nb alloy during isothermal hot deformation

The hot deformation behavior of the as-cast Ti–48Al–2Cr–2Nb alloy was investigated by isothermal compression tests at deformation temperatures ranging from 1000℃ to 1200℃,and strain rates from 0.001 s~(-1)to 0.1 s~(-1).The single peak stress features common to all flow curves indicate that DRX is the dominating softening mechanism.The calculated values of the hot deformation activation energy Q and stress index n are 296.5 kJ mol~(-1)and 3.97,respectively.Based on this,the Arrhenius type constitutive equation was successfully established.The DRX critical condition model and relationship among DRX volume fractions,deformation temperatures and strain rates were obtained to optimize the process.Combined with microstructure analysis,it's concluded that 1200℃/0.01s~(-1)is the optimization parameter.Besides,both DDRX and CDRX were observed in theγphase evolution.The deformation mechanism from the inter-grain dislocation motion to the grain boundary migration and grain rotation was discussed.     

Hot deformation characteristics of as-cast high-Cr ultra-super-critical rotor steel with columnar grains

Isothermal hot compression tests of as-cast high-Cr ultra-super-critical (USC) rotor steel with columnar grains perpendicular to the compression direction were carried out in the temperature range from 950 to 1250°C at strain rates ranging from 0.001 to 1 s-1. The softening mechanism was dynamic recovery (DRV) at 950°C and the strain rate of 1 s-1, whereas it was dynamic recrystallization (DRX) under the other conditions. A modified constitutive equation based on the Arrhenius model with strain compensation reasonably predicted the flow stress under various deformation conditions, and the activation energy was calculated to be 643.92 kJ·mol-1. The critical stresses of dynamic recrystallization under different conditions were determined from the work-hardening rate (θ)–flow stress (σ) and -?θ/?σ–σ curves. The optimum processing parameters via analysis of the processing map and the softening mechanism were determined to be a deformation temperature range from 1100 to 1200°C and a strain-rate range from 0.001 to 0.08 s-1, with a power dissipation efficiency η greater than 31%.     

Hot deformation behaviors of WE71 alloy under plain strain compression at elevated temperature

Hot deformation behaviors of WE71 (Mg–7Y-1Nd-0.5Zr) alloy was investigated by plain strain compression tests conducted at temperatures ranging from 350 °C to 500 °C and strain rates varying from 0.01 s^-1 to 10 s^-1. Results show that the hot deformation of WE71 was accompanied by the precipitation of rich Zr phase with granular shape and block-shaped phase rich in element Y. When deformed at low temperature and high strain rate, the softening behavior of the alloy was synergically determined by shear bands propagation, adiabatic heating, twinning formation and dynamic recrystallization (DRX). For the conditions of high temperature and high strain rate, DRX was the major softening mechanism while the formation and annihilation of extension twinning resulted in a special flow curve characteristic at the strain of around 0.3. According to the microstructural observations, it can be concluded that the irregular flow curves of WE71 alloy during plain strain compression process are mainly ascribed to shear bands propagation, adiabatic heating, twinning formation and DRX.     

Mechanical behavior of a near α titanium alloy under dynamic compression: Characterization and modeling

Dynamic compression tests under strain rates from 870 s^?1 to 2100 s^?1 were conducted for a near α Ti–8Al–1Mo–1V titanium alloy with equiaxed microstructure. Compression behavior, adiabatic shearing and band microstructure were investigated via characterization and calculation. The results demonstrate that all dynamic constitutive curves exhibited obvious stress fluctuation phenomenon with double increase-decrease changing stages at the primary stage of compression. The dislocation multiplication theory can be used to explain this phenomenon. After the stress fluctuation period, work hardening coexisted with the thermal softening, resulting in the slow hardening tendency in constitutive curves. J-C model was utilized to quantify the dynamic constitutive curves. The deviations between the predicted and experimental curves under high strain rates may be attributed to the over-consideration of thermal softening effect in J-C model. Adiabatic shearing band (ASB) began to form under the strain rate of 2100 s^?1. A total shearing strain of 8.1 within ASB achieved in 8.9 μs, corresponding to a local strain rate of about 9.1 × 10⁵ s^?1 and is over 430 times of the macro strain rate. Post annealing was conducted on ASB before EBSD characterization. Due to the static recrystallization during annealing, the α phase within ASB generally presented as ultra-fine grains less than 1 μm in diameter.     

Hot deformation behavior andfl ow stress modeling of annealed AZ61 Mg alloys

In this study, the hyperbolic-sine type constitutive equation was used to model the flow stress of annealed AZ61 magnesium(Mg) alloys. Hot compression tests were conducted at the temperatures ranging from 250 1C to 450 1C and at the strain rates ranging from 1 10–3s 1to 1 s 1on a Gleeble-3500 thermo-simulation machine. Constitutive equations as a function of strain were established through a simple extension of the hyperbolic sine constitutive relation. The effects of annealing heat treatments on the variations in constitutive parameters with strain were discussed. The hot compressive flow curves exhibited typical features of dynamic recrystallization. Multiple peak flow curves were observed in the annealed specimens upon testing at a strain rate of 1 10 1s–1and at various temperatures. Variations in constitutive parameters with strain were related to flow behavior and dependent on the initial conditions of the test specimens. The flow stresses of annealed AZ61 Mg alloys were predicted well by the strain-dependent constitutive equations of the hyperbolic sine function under the deformation conditions employed in this study.     

Hot deformation behavior of microstructural constituents in a duplex stainless steel during high-temperature straining

The hot deformation characteristics of 1.4462 duplex stainless steel (DSS) were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800–1100℃ and strain rates of 0.001–1 s?1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-Hollomon parameter (Z) increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6.×1015 (lnZ=35.5), a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction (EBSD) confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.     

铜/石墨复合材料热变形行为研究

通过Gleeble-3500热模拟试验机对铜/石墨复合材料进行热压缩试验,研究变形温度为700～850 ℃、应变速率为0.001～1.000 s^-1时该复合材料的热变行为。通过光学显微镜研究复合材料显微组织的演变,根据实验数据构建该复合材料的本构方程和热加工图。使用Zener-Hollomon参数模型对该复合材料的流变应力进行研究。研究发现,铜/石墨复合材料的流变应力随着应变温度的升高而降低,随应变速度的增大而增大。计算得出该复合材料的热变形激活能为463.02 kJ/mol,表明材料具有良好的成形能力。通过构建的本构方程验证了最大应力的吻合性,发现计算值和试验值的误差在9.5%以内,说明该方程对复合材料的流变行为具有指导作用。热加工图表明了该复合材料的适宜加工温度为780～820 ℃,变形速率为0.050～0.100 s^-1;变形温度为830～850 ℃时,变形速率约为0.001 s^-1。     

Hot deformation behavior of Super304H austenitic heat resistant steel

The hot compression tests of Super304H austenitic heat resistant steel were carried out at 800–1200℃ and 0.005–5 s-1 using a Gleeble 3500 thermal-mechanical simulator, and its deformation behavior was analyzed. The results show that the flow stress of Super304H steel decreases with the decrease of strain rate and the increase of deformation temperature; the hot deformation activation energy of the steel is 485 kJ/mol. The hot deformation equation and the relationship between the peak stress and the deformation temperature and strain rate is obtained. The softening caused by deformation heating cannot be neglected when both the deformation temperature and strain rate are higher.     

Dynamic recrystallization and phase transformation behavior of a wrought β-γ TiAl alloy during hot compression

The dynamic recrystallization (DRX) and phase transformation (PT) behavior of a wrought β-γ TiAl alloy during hot compression under various deformation temperatures were investigated. The typical work hardening and flow softening features indicated that DRX was the dominating softening mechanism. Both γ-DRX and β-DRX took place during the hot compression. γ-DRX was triggered at all compression temperatures, while the β-DRX was induced when the compression temperature was above 1000 °C. The hot deformation kinetics was calculated, which showed that DRX behavior existed in the whole hot compression process, and the DRX volume fraction increased with the increase of the compression temperature. Combined with the microstructure observation, it concluded that the β/B2+α₂→γ PT occurred at 850 °C and 1000 °C, while the γ→β/B2 PT happened at 1050 °C during hot compression, which is important to optimize microstructure. Moreover, the hot compression mechanism changed from dislocation gliding to grain-boundary sliding was discussed.     

Deformation behavior and microstructure of an Al-Zn-Mg-Cu-Zr alloy during hot deformation

The hot deformation behavior and microstructures of Al-7055 commercial alloy were investigated by axisymmetric hot compression at temperatures ranging from 300℃ to 450℃ and strain rates from 10-2 to 10 s-1, respectively. Microstructures of deformed 7055 alloy were investigated by transmission electron microscopy (TEM). The dependence of peak stress on deformation temperature and strain rate can be expressed by the hyperbolic-sine type equation. The hot deformation activation energy of the alloy is 146 kJ/mol. Moreover, the flow stress curves predicted by the modified constitutive equations are reasonably consistent with the experimental results, which confirms that the proposed deformation constitutive equations can provide evidence for the selection of hot forming parameters. TEM results indicate that dynamic recovery is the main softening mechanism during hot deformation.     

Simulation of dynamic recrystallization for an Al-Zn-Mg-Cu alloy using cellular automaton

A cellular automata (CA) model has been developed to predict and control the microstructure evolution during hot deformation on 7085 aluminum alloy. The initial microstructure and thermal-mechanical parameters were used as the input data of the CA model. To link microstructure evolution with macroscopic flow stress, dislocation density was set as an important internal state variable. The hot deformation behavior of 7085 aluminum alloy was studied by isothermal compression tests under a deformation temperature range of 623–723 K and a strain rate range of 0.001-1s^?1 up to true strains of 0.53–1.20. Electron backscattered diffraction technique and the CA model were utilized to systematically investigate the effects of strain, strain rate and deformation temperature on the microstructure evolution, and further to predict the average grain diameter and the recrystallization fraction after deformation. The simulated results were validated by the experimental data to demonstrate the feasibility and predictability of the CA model.     

超纯Cr17铁素体不锈钢热变形过程的动态回复行为

采用热力模拟试验机进行单道次压缩试验,旨在揭示超纯Cr17铁素体不锈钢在热变形过程中的动态回复行为.在变形速率为1 s-1,最大真应变为0.8的条件下研究了900~1 150℃范围内的热变形行为及组织演化规律.结果表明,单道次压缩得到的应力-应变曲线均呈动态回复型.变形温度越高,动态回复越快.当变形温度较低时,微观组织演化以晶界拱出和变形晶粒的形成为主要特征;当变形温度较高时,微观组织演化以大量亚晶界和亚晶的形成为主要特征.     

Microstructure and texture evolution of TRC A8006 alloy by homogenization

The microstructure and texture evolution of twin-roll cast A8006 alloy by homogenization were characterized using scanning and transmission electron microscopy, and the microhardness was tested as well. According to the relationship between dendritic arm spacing and cooling rate the cooling rate of the as-cast twin-roll cast A8006 sheet of 6 mm in thickness was estimated as 1.48×103 K·s?1. It is found that the grains and the nanostructural precipitates of the twin-roll cast sheet become coarser after homogenization at 580℃ for 4 h in comparison with those after homogenization at 500℃ for 8 h. The textures formed after cold rolling and became weaker during homogenization. The increase in hardness of the as-cast twin-roll cast sheets is related to the supersaturated α-Al solid solution and fine microstructure, but the decrease in hardness after homogenization can be attributed to the coarsening of grains and Al₆Fe(Mn) precipitates.     

高锰TRIP钢热变形行为研究

通过单轴压缩实验,研究了高锰TRIP钢(Fe15Mn3Si3Al)在800～1050℃温度范围内、应变速率ε.=0.01～5.0s-1条件下的热变形行为和组织变化,讨论了热变形参数对流变应力和显微组织的影响.结果表明:动态再结晶只在较高变形温度和低应变速率下发生.实验钢对温度和应变速率都很敏感,而应变速率对实验钢的热变形行为影响较大.高锰TRIP钢的表观应力指数n=3.909,变形激活能Q=353.167kJ/mol.根据实验数据,建立了高锰TRIP钢高温变形的热加工方程.     

Synthesis of glass ceramics from kaolin and dolomite mixture

Cordierite-and anorthite-based binary glass ceramics of the CaO-MgO-Al₂O₃-SiO₂ (CMAS) system were synthesized by mixing local and abundant raw minerals (kaolin and doloma by mass ratio of 82/18). A kinetics study reveals that the activation energy of crystallization (Ea) calculated by the methods of Kissinger and Marotta are 438 kJ·mol-1 and 459 kJ·mol-1, respectively. The Avrami parameter (n) is estimated to be approximately equal to 1, corresponding to the surface crystallization mechanism. X-ray diffraction (XRD) analysis shows that the anorthite and cordierite crystals are precipitated from the parent glass as major phases. Anorthite crystals first form at 850℃, whereas the μ-cordierite phase appears after heat treatment at 950℃. Thereafter, the cordierite allotropically transforms to α-cordierite at 1000℃. Complete densification is achieved at 950℃; however, the density slightly decreases at higher temperatures, reaching a stable value of 2.63 kg·m-3 between 1000℃ and 1100℃. The highest Vickers hardness of 6 GPa is also obtained at 950℃. However, a substantial decrease in hardness is recorded at 1000℃; at higher sintering temperatures, it slightly increases with increasing temperature as the α-cordierite crystallizes.     

High-temperature mechanical properties and deformation behavior of high Nb containing TiAl alloys fabricated by spark plasma sintering

A high Nb containing TiAl alloy was prepared from the pre-alloyed powder of Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y (at%) by spark plasma sintering (SPS). Its high-temperature mechanical properties and compressive deformation behavior were investigated in a temperature range of 700 to 1050℃ and a strain rate range of 0.002 to 0.2 s-1. The results show that the high-temperature mechanical properties of the high Nb containing TiAl alloy are sensitive to deformation temperature and strain rate, and the sensitivity to strain rate tends to rise with the deformation temperature increasing. The hot workability of the alloy is good at temperatures higher than 900℃, while fracture occurs at lower temperatures. The flow curves of the samples compressed at or above 900℃ exhibit obvious flow softening after the peak stress. Under the deformation condition of 900-1050℃ and 0.002-0.2 s-1, the interrelations of peak flow stress, strain rate, and deformation temperature follow the Arrhenius' equation modified by a hyperbolic sine function with a stress exponent of 5.99 and an apparent activation energy of 441.2 kJ·mol-1.     

Peculiarities of the rheological behavior and structure formation of aluminum under deformation at near-solidus temperatures

This paper deals with a peculiar rheological behavior of aluminum at near-solidus temperatures. It has been experimentally established that there is an inverse strain rate dependence of strain resistance at temperatures ranging between 560 and 640°С and strain rates ranging from 0.06 to 1.2 s-1. Electron backscatter diffraction analysis has shown that at temperatures ranging between 540 and 640°С and strain rates ranging from 0.06 to 0.1 s-1, the main process of softening is dynamic polygonization, resulting in in situ recrystallization. At higher strain rates, ranging between 0.8 and 1.2 s-1, and temperatures ranging between 560 and 640°С, the recovery is dynamic. This unusual behavior of the mechanism of softening and the presence of the inverse strain rate dependence of strain resistance can be explained by blocking the motion of free dislocations by foreign atoms, which occurs at strain rates ranging between 0.06 and 0.1 s-1. This process results in dislocation pile-up, thereby causing in situ recrystallization. At strain rates exceeding 0.16 s-1, there is no intensive blocking of dislocations, leading to a direct strain rate dependence of strain resistance.