Quantitative analysis on boundary sliding and its accommodation mode during superplastic deformation of two-phase Ti-6Al-4V alloy

A study has been made to investigate boundary sliding and its accommodation mode with respect to the variation of grain size and α/β volume fraction during superplastic deformation of a two-phase Ti-6Al-4V alloy. A load relaxation test has been performed at 600 °C and 800 °C to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that grain matrix deformation (GMD) is found to be dominant at 600 °C and is well described by the plastic state equation. Whereas, at 800 °C, phase/grain boundary sliding (P/GBS) becomes dominant and is fitted well with the viscous flow equation. The accommodation mode for fine-grained microstructures (3 μm) well agrees with the isostress model, while that for large-grained structures (11 μm) is a mixed mode of the isostress and isostrain-rate models. The sliding resistance analyzed for the different boundaries is lowest in the α/β boundary, and increases on the order of α/β≪α/α≈β/β, which plays an important role in controlling the superplasticity of the alloys with various α/β phase ratios.     

Effects of Recovery Behavior and Strain-Rate Dependence of Stress–Strain Curve on Prediction Accuracy of Thermal Stress Analysis During Casting

Recovery behavior (recovery) and strain-rate dependence of the stress–strain curve (strain-rate dependence) are incorporated into constitutive equations of alloys to predict residual stress and thermal stress during casting. Nevertheless, few studies have systematically investigated the effects of these metallurgical phenomena on the prediction accuracy of thermal stress in a casting. This study compares the thermal stress analysis results with in situ thermal stress measurement results of an Al-Si-Cu specimen during casting. The results underscore the importance for the alloy constitutive equation of incorporating strain-rate dependence to predict thermal stress that develops at high temperatures where the alloy shows strong strain-rate dependence of the stress–strain curve. However, the prediction accuracy of the thermal stress developed at low temperatures did not improve by considering the strain-rate dependence. Incorporating recovery into the constitutive equation improved the accuracy of the simulated thermal stress at low temperatures. Results of comparison implied that the constitutive equation should include strain-rate dependence to simulate defects that develop from thermal stress at high temperatures, such as hot tearing and hot cracking. Recovery should be incorporated into the alloy constitutive equation to predict the casting residual stress and deformation caused by the thermal stress developed mainly in the low temperature range.

     

On the microstructural aspects of the nonhomogeneity of superplastic deformation at the level of grain groups

The surface relief of a superplasticity deformed magensium alloy (Mg-1.5% Mn-0.3%Ce) was studied. Zones/bands of localized deformation were detected by means of vacuum etching. Between the localized deformation bands were less-deformed regions. After 20% elongation in vacuum (1.33 × 10⁻³ Pa), zones of intensive vacuum etching were observed as two intersecting bands of localized deformation oriented at 35–60° to the tensile axis. Spacing between the localized deformation bands is 6–8 grain diameters after a tensile elongation of 20% and 3–4 grain diameters after an elongation of 160%. The observed bands of strain localization are explained from the viewpoint of cooperative grain boundary sliding, i.e. shifting of grain groups as a whole unit along grain boundary surfaces oriented close to the maximum shear stress direction. It is suggested that the cooperative nature of GBS be taken into account when evaluating the real local strain rate and the real strain-rate sensitivity of grain boundary sliding process.     

An analysis of the flow stress of a two-phase alloy system,Ti-6Al-4V

An analysis of the tensile deformation behavior of a two-phase body-centered cubic (bcc)-hexagonal close-packed (hcp) alloy, Ti-6Al-4V, has been made. This has shown that the temperature dependence of the flow stress, the logarithm of the effective stress, and the strain-rate sensitivities can be described by simple analytical equations if the thermally activated strain-rate equation contains the Yokobori activation enthalpyH=H ^o ln (σ*₀/σ*), whereH ^o is a constant,σ* the effective stress, andσ*₀ its 0 K value. The flow stress-temperature plateau region (500 to 600 K) also can be rationalized analytically in terms of oxygen dynamic strain aging in the alpha phase     

New aspects on the superplasticity of fine-grained 7475 aluminum alloys

Thermomechanical processes were developed which give fine grain sizes of 6 and 8 μm in the 7475 Al alloy. Superplastic properties of this material were evaluated in the temperature range of 400 °C to 545 °C over the strain-rate range of 2.8 x 10^-4 to 2.8 X 10^-2 s^-1. The maximum ductility exhibited by the alloy was approximately 2000 pct, and optimum superplasticity was achieved at a strain rate of 2.8 X 10^-3 s^-1 which is higher by an order of magnitude than other 7475 Al alloys. This result is attributed to the presence of fine dispersoids which maintain the fine grain size at high homologous temperatures. The flow stress and strain-rate sensitivity strongly depend on the grain size. The superplastic 7475 Al alloy has strain-rate sensitivities of 0.67 (6 μm) and 0.5 (13 μm) and an activation energy which is similar to the one for grain boundary diffusion of aluminum. Microstructural investigation after superplastic tests revealed zones free of dispersoid particles at grain boundaries primarily normal to the tensile direction. These dispersoidfree zones (DFZs) appear even after 100 pct elongation and are occasionally as large as 5 μm across. This result demonstrates the importance of diffusional flow in superplastic deformation of the fine-grained 7475 Al alloy especially at low elongations.     

Mechanical behavior of a cryomilled near-nanostructured Al-Mg-Sc alloy

In the present study, the mechanical behavior of a cryomilled Al-7.5 pct Mg-0.3 pct Sc alloy was investigated at temperatures in the range of 298 to 648 K. The grain size of the as-extruded alloy was determined to be approximately 200 nm by transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. The data indicate that as a result of cryomilling, a supersaturated solid solution with high thermal stability was formed in the Al-Mg-Sc alloy. The high strength at room temperature was primarily attributed to three types of strengthening: grain size effect, solid solution hardening, and Orowan strengthening. The elevated temperature mechanical behavior of the Al-Mg-Sc alloy exhibits the following: (a) a strain-rate sensitivity, m, of less than 0.2; and (b) an activation energy, Q, that increases from 139 to 193 kJ/mol with increasing applied stress. An analysis of the experimental data at elevated temperatures shows that despite the fine-grained structure of the alloy, the deformation characteristics are not consistent with those arising from a superplastic deformation process that incorporates a threshold stress. On the other hand, the analysis suggests that the deformation characteristics agree with those associated with the transition in the creep behavior of Al-based solid solution alloys from that for the intermediate-stress region, where m=0.33 and Q=Q _D (Q _D is the activation energy for self-diffusion in Al), to that of the high-stress region, where m<0.2 and Q>Q _D .     

Effects of microstructural evolution on superplastic deformation characteristics of a rapidly solidified Al-Li alloy

This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10⁻²/s to 10⁻¹/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.     

Strain-Rate Sensitivity and Failure Peculiarities in Compression of the Nanocrystalline Ni-20 Pct Fe Alloy at Low Temperatures

The strain-rate sensitivity of the applied stress was measured, and the values of the activation volume of the process of the plastic deformation V along the deformation curve in compression of the nanocrystalline (NC) Ni-20 pct Fe alloy (with average grain size ~22 nm) in the temperature range of 300 to 77 K (27 to −196 °C) were calculated. It was found that the decrease of the temperature from 300 to 77 K leads to a decrease of the value of V from 20 to 8 b ³, and values of V do not depend on the strain. Fractographic features of the failure surfaces were studied in the temperature range 300 to 4.2 K (27 to −269 °C). Observed traces of melting on these surfaces at temperatures below 300 K indicate the intense heating in the catastrophic shear band of the alloy in the moment of failure. Causes of low-temperature decohesion along grain boundaries are discussed in terms of the sulfur segregation influence.     

Mechanical Behavior and Microstructural Change of a High Nitrogen CrMn Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200 °C) and strain rate range 0.01 to 10 s⁻¹ was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s⁻¹) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.     

High-temperature deformation mechanisms and constitutive equations for the oxide dispersion-strengthened superalloy MA 956

An experimental study of the constitutive response of the oxide dispersion-strengthened (ODS) superalloy MA 956, which consists of an Fe-Cr-Al matrix dispersion strengthened with yttria, has been performed.Single-crystal specimens of MA 956 having remarkably simple initial microstructures have been tested in compression in the temperature range of 900 °C to 1200 °C and in the axial strain-rate range of 1.8 x 10^-4 s^-1 to 10^-2 s^-1. The deformation response of the material has been examined by performing constant true strain-rate tests, strain-rate jump tests, and stress relaxation tests. The orientation dependence of the stress-strain response of the single crystals has been compensated for by determining the operative slip systems and resolving the stresses and strains accordingly. These experiments, together with electron-microscopic observations of deformed and quenched specimens, allow a number of conclusions to be drawn about the physics of particle strengthening in this simple ODS alloy at high temperatures. Further, drawing on this physical understanding, a set of phenomenological internal variable constitutive equations which model the high-temperature deformation behavior of this alloy is also developed. These equations reasonably well model not only the temperature and strain-rate sensitivity of the flow stress but also the strain-hardening behavior of the material.     

An intergranular creep crack growth model based on grain boundary sliding

An intergranular crack growth model is developed to describe the effect of microstructural features such as grain size, grain boundary precipitates, and serrated grain boundaries on creep crack growth under grain boundary sliding (GBS) conditions. The model considers quantitatively that several deformation mechanisms contribute to the stress redistribution ahead of the crack tip through a stress relaxation process. The crack tip region is divided into three zones: (a) the intragranular-deformation-controlled stress relaxation zone, (b) the GBS-controlled stress relaxation zone, and (c) the elastic region. Intergranular creep crack growth is considered to occur as a result of the GBS-controlled process in all cases. The derived creep crack growth model shows a complex dependence of the creep crack growth rate (CCGR) on fracture mechanics quantities, such as C(t) (the path-independent energy integral with its steady-state value as C*) and K (the stress intensity factor). For creep-brittle materials, the model predicts that the CCGR depends on K to the power of 2 and this is verified experimentally; however, when environmental effects contribute to the crack growth process, the power exponent will increase. A semiempirical factor is introduced to account for the effects of oxidation on CCGR.     

Flow Behaviour of Ti-6Al-4V Subjected to Multi Directional Isothermal Compression

Flow behavior of Ti-6Al-4V was investigated under multi directional isothermal compressions. Flow curves of multi directional isothermal compressions were analyzed to determine softening parameter, which is the ratio of maximum flow stress (σ_i) to steady state flow stress (σ_s). Multi directional isothermal compressions were carried out at a optimum strain-rate of 10^?2 s^?1. Softening parameter increased gradually from first to third stage of compression. The huge strain imposed on the specimen reduced grain size from 16 µm to approximately 7–8 µm, which is suitable for superplastic deformation and also improved strength.     

A Deformation Mechanism Map for the 1.23Cr-1.2Mo-0.26V Rotor Steel and Its Verification Using Neural Networks

A deformation mechanism map is constructed for the 1.23Cr-1.2Mo-0.26V rotor steel as a function of temperature, stress, and strain rate using published creep test results and the current understanding of time dependent deformation mechanisms operative in complex engineering alloys. Instead of diffusional creep, grain boundary sliding (GBS) accommodated by different deformation processes is considered dominant at lower strain rates. The GBS dominated region is further sub-divided into two parts, where GBS is accommodated by wedge type cracking at temperatures below 0.5T/T _m and the accommodation process changes to creep cavitation at temperatures above 0.5T/T _m. The map is verified using experimental data and artificial neural network modeling. The proposed artificial neural network model is capable of predicting the dominance of different deformation mechanisms in 1.23Cr-1.2Mo-0.26V steel over a wide range of stress and temperature. This modeling procedure can potentially be used to construct or expand deformation mechanism maps for other engineering alloys.     

Effect of the lamellar grain size on plastic flow behavior and microstructure evolution during hot working of a gamma titanium aluminide alloy

The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal, constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between 80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s⁻¹ on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset test specimens revealed the presence of fine, equiaxed grains of γ + α ₂ + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling of the lamellae, and the formation of equiaxed particles of γ + β ₂ + α ₂ on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions.     

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。     

A study of superplasticity in a modified 5083 Al-Mg-Mn alloy

The superplastic (SP) properties of a modified 5083 alloy (Al-4.7Mg-1.6Mn) were evaluated by tensile tests and microstructural characterization over a range of strain rates from 0.0005 to 0.1 s⁻¹, temperatures from 500 °C to 550 °C, and initial grain sizes from 8.7 to 17 μm. The fine-grained material was found to exhibit strain-rate sensitivity values of greater than 0.5 over the strain-rate range of 0.002 to 0.1 s⁻¹, while the coarser-grained material appeared to deform as a Class I solid solution by glide-controlled dislocation creep. It was found that the mechanical properties could be adequately represented by a semiempirical constitutive equation which reflected the flow hardening due to dynamic grain growth, the change in m with strain and strain rate, and the transition between SP deformation and dislocation creep with strain rate. Microstructural examination revealed the presence of several pre-existing cavities associated with intermetallic particles. Tensile elongations of up to 525 pct were obtained at a strain rate of 10⁻³s⁻¹.     

Effect of <Emphasis Type="Italic">β</Emphasis> Grain Size on Stress-Induced Martensitic Transformation in <Emphasis Type="Italic">β</Emphasis> Solution-Treated 51.1Zr-40.2Ti-4.5Al-4.2V Alloy

The effect of β grain size on stress-induced martensitic transformation in β solution-treated 51.1Zr-40.2Ti-4.5Al-4.2V alloy was investigated by using XRD and TEM techniques. The results show that initial β grain size has a profound effect on the triggering stress of the stress-induced martensitic (SIM) transformation. The triggering stress increases with increasing initial β grain size. The SIM transformation significantly affects the deformation behavior of the alloy. A typical double yielding is observed in the stress-strain curves due to the occurrence of the SIM transformation. The curve of work hardening rate vs. true strain is divided into three stages for the samples with small β grain size. The work hardening rate at stage ΙΙ or ΙΙΙ decreases with increasing initial β grain size, which is attributed to the effect of the SIM transformation during a tensile test.