Characterization of hot deformation and microstructure evolution of a new metastable β titanium alloy

The hot deformation characteristics of as-forged Ti?3.5Al?5Mo?6V?3Cr?2Sn?0.5Fe?0.1B?0.1C alloy within a temperature range from 750 to 910 °C and a strain rate range from 0.001 to 1 s^?1 were investigated by hot compression tests. The stress?strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate. The microstructure is sensitive to deformation parameters. The dynamic recrystallization (DRX) grains appear while the temperature reaches 790 °C at a constant strain rate of 0.001 s^?1 and strain rate is not higher than 0.1 s^?1 at a constant temperature of 910 °C. The work-hardening rate θ is calculated and it is found that DRX prefers to happen at high temperature and low strain rate. The constitutive equation and processing map were obtained. The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map, which indicates excellent hot workability. At the strain rate of 0.1 s^?1, the stress?strain curves show an abnormal shape where there are two stress peaks simultaneously. This can be attributed to the alternation of hardening effect, which results from the continuous dynamic recrystallization (CDRX) and the rotation of DRX grains, and dynamic softening mechanism.     

A Modified Constitutive Equation for Aluminum Alloy Reinforced by Silicon Carbide Particles at Elevated Temperature

In this paper, the constitutive relationship of an aluminum alloy reinforced by silicon carbide particles is investigated using a new method of double multivariate nonlinear regression (DMNR) in which the strain, strain rate, deformation temperature, and the interaction effect among the strain, strain rate, and deformation temperature are considered. The experimental true stress-strain data were obtained by isothermal hot compression tests on a Gleeble-3500 thermo-mechanical simulator in the temperature range of 623-773 K and the strain rate range of 0.001-10 s^?1. The experiments showed that the material-softening behavior changed with the strain rate, and it changed from dynamic recovery to dynamic recrystallization with an increase in the strain rate. A new constitutive equation has been established by the DMNR; the correlation coefficient (R) and average absolute relative error (AARE) of this model are 0.98 and 7.8%, respectively. To improve the accuracy of the model, separate constitutive relationships were obtained according to the softening behavior. At strain rates of 0.001, 0.01, 0.1, and 1 s^?1, the R and AARE are 0.9865 and 6.0%, respectively; at strain rates of 5 and 10 s^?1, the R and AARE are 0.9860 and 3.0%, respectively. The DMNR gives an accurate and precise evaluation of the flow stress for the aluminum alloy reinforced by silicon carbide particles.     

Mg-6Zn-1Al-0.3Mn 镁合金的热压缩变形行为及变形组织

采用Gleeble热模拟方法研究Mg?6Zn?1Al?0.3Mn 变形镁合金在温度为200~400°C,应变速率为0.01~7 s?1条件下的热压缩变形行为。结果表明,变形温度和应变速率显著影响其热变形行为。通过计算获得了热变形激活能及应力指数分别为Q=166 kJ/mol,n=5.99,且其本构方程为ε&=3.16×1013[sinh(0.010σ)]5.99exp [?1.66×105/(RT)]。热压缩显微组织观察表明：在应变速率为0.01~1 s?1的条件下,在250°C热压缩变形时初始晶粒晶界及孪晶处发生了部分动态再结晶,而在高温(350~400°C)条件下,发生了完全动态再结晶且再结晶晶粒尺寸随着应变速率的增加而减小。获得的较优的变形条件为温度330~400°C、应变速率为0.01~0.03 s?1以及350°C、应变速率为1 s?1。     

Effect of strain rate on microstructure of polycrystalline oxygen-free high conductivity copper severely deformed at liquid nitrogen temperature

The microstructure of polycrystalline oxygen-free high conductivity copper subjected to severe uniaxial single compression at liquid nitrogen temperature and strain rates ranging from 10^?2 to 10⁵ s^?1 is characterized using transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. A difference in strain rate leads to a change in the density, character and arrangement of dislocations, as well as the size and configuration of dislocations cells/(sub)grains in the deformed sample. A threshold strain rate of 10³ s^?1 is identified for the formation of localized deformation bands, which characterizes heterogeneity of deformation at high strain rates. These bands are composed of grains that are significantly smaller than those outside them, as well as those obtained at strain rates lower than 10³ s^?1. Under particular conditions, grains as small as several nanometers can be generated in the vicinity of these bands, through the activation of rotational dynamic recrystallization. Amorphization is identified as a deformation mechanism in structures consisting of grains smaller than ～13 nm, and this offers an explanation for the “inverse Hall–Petch effect”. A model that illustrates the initiation and propagation of an amorphous phase during deformation is proposed. Deformed samples exhibit the tendency of an increase in strength with the value of the Zener–Hollomon parameter, which captures strain rate and temperature rise during deformation. This study suggests that a strain rate in the order of 10² s^?1 should be adopted in severe plastic deformation techniques to produce nanometer-sized grains.     

Distribution characterization of boundary misorientation angle of 7050 aluminum alloy after high-temperature compression

Compression tests of 7050 aluminum alloy have been conducted at different temperatures (340, 380, 420, and 460 °C) and different strain rates of 0.1, 1, 10, and 100 s^?1. The microstructure characteristics of the alloy after deformation are investigated using OM, electron backscatter diffraction (EBSD) technique and TEM. Results show that the volume fraction of recrystallized grains and the average misorientation angle increase with the increase of deformation temperature with the strain rate of 0.1 s^?1. When the 7050 aluminum alloys were deformed at 460 °C, the volume fraction of recrystallized grains and average misorientation angle decrease with increasing strain rate. The primary softening mechanism of the 7050 aluminum alloy deformed at 340, 380, and 420 °C with the strain rate of 0.1 s^?1 is dynamic recovery. Dynamic recrystallization is the main softening mechanism of the alloy deformed at 460 °C and different strain rates. The softening mechanism of the alloy is not sensitive to strain rate.     

Research on the Hot Deformation Behavior of Al-Zn-Mg-Sc-Zr Alloy During Compression at Elevated Temperature

The flow behavior of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation was studied by isothermal compression test using Gleeble-1500 thermo-mechanical equipment. Compression tests were performed in the temperature range of 340-500 °C and in the strain rate range of 0.001-10 s^?1.The results indicate that the flow stress of the alloy increases with increasing strain rate at a given temperature, and decreases with increasing temperature at a given imposed strain rate. The relationship between flow stress and strain rate and temperature was derived by analyzing the experimental data. The constitutive equation of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation can be described by the Arrhenius relationship of the hyperbolic sine form. The values of A, n, and α in the analytical expression of strain rate are fitted to be 1.49 × 10¹⁰ s^?1, 7.504, and 0.0114 MPa^?1, respectively. The hot deformation activation energy of the alloy during compression is 150.25 kJ/mol. The temperature and strain rate have great influences on microstructure evolution of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation. According to microstructure evolution, the dynamic flow softening is mainly caused by dynamic recovery and dynamic recrystallization in this present experiment.     

Hot Deformation Mechanisms of an As-Extruded TiAl Alloy with Large Amount of Remnant Lamellae

The hot deformation mechanisms of an as-extruded Ti-44Al-5V-1Cr alloy with a large amount of remnant lamellae were investigated by hot compression tests at temperatures of 900-1250 °C and strain rates of 0.001-1 s^?1. The hot processing map of the as-extruded Ti-44Al-5V-1Cr alloy was developed on the basis of dynamic materials modeling and the Prasad criteria. There were four different domains in the hot processing map, according to the efficiency of power dissipation, η. The flow soft and hot deformation mechanisms for different domains were illustrated in the context of microstructural evolution during the process of deformation. As a result, the dynamic recrystallization and superplastic deformation occurred at 1125-1150 °C near 0.001 s^?1, and this region is suitable for superplastic forming. The α phase dynamic recrystallization and dynamic recovery occurred at 1250 °C and 0.1 s^?1. The existence of small amount of the γ and β phases effectively inhibited the growth of α grains.     

Isothermal compression behavior and constitutive modeling of Ti-5Al-5Mo-5V-1Cr-1Fe alloy

Hot compression behavior of Ti-5Al-5Mo-5V-1Cr-1Fe alloy with an equiaxed (α+β) starting microstructure was investigated by isothermal compression test and optical microscopy. Based on the true strain–stress data with temperature correction, constitutive models with a high accuracy were developed and processing maps were established. Strain inhomogeneity at different locations in the compressed sample is reduced by raising temperature, leading to a uniform distribution of α phases. For the temperature range of 800–840 °C with a strain rate of 10 s^?1, the transformed volume fraction of α phase increases and the average grain size of α phase decreases slightly with increasing the temperature, indicating co-existence of dynamic recovery and dynamic recrystallization. Flow localization and faint β grain boundaries are observed at the strain rate of 10 s^?1 in the temperature range of 860–900 °C. The processing map analysis shows that hot working of Ti-5Al-5Mo-5V-1Cr-1Fe alloy should be conducted with the strain rate lower than 0.01 s^?1 to extend its workability.     

Dynamic recrystallization kinetics of as-cast AZ91D alloy

The flow behavior and dynamic recrystallization (DRX) behavior of an as-cast AZ91D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220–380 °C and strain rate range of 0.001–1 s^?1. The effect of temperature and strain rate on the DRX behavior was discussed. The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates. To evaluate the evolution of dynamic recrystallization, the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves. It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves. A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91D alloy.     

Recrystallization in hot vs cold deformed commercial aluminum: a microstructure path comparison

Static, isothermal recrystallization at a temperature of 400 °C was studied by means of quantitative microscopy in a well-characterized, commercial purity aluminum-alloy AA1050 that had undergone plane strain deformation at 400 °C at a strain rate of 2.5 s⁻¹ to an equivalent strain of 2. The microstructural properties, V_v, the volume fraction recrystallized, S_v, the interfacial area density separating recrystallizing grains from deformed volumes and <λ>, the mean recrystallized grain free length, were all measured stereologically as a function of time and the reaction kinetics, microstructural path, grain boundary migration rates and nucleation characteristics of the recrystallization were quantified experimentally. The results are compared to a recently published study of recrystallization in the identical pre-deformation starting material but after room temperature deformation by rolling to a comparable strain. Recrystallization kinetics differences between the two materials include: the hot deformed material had a higher, by at least 120 °C, recrystallization temperature; had many fewer recrystallization nuclei leading to a factor of about three larger as-recrystallized grain size; lacked a Cahn-Hagel growth rate transient like the cold deformed exhibited; and required a slightly different impingement model for the microstructural path analysis. In both cases particle stimulated nucleation (PSN) was thought to be operative but it seemed to be much more potent after cold deformation.     

Constitutive modeling and dynamic softening mechanism during hot deformation of an ultra-pure 17%Cr ferritic stainless steel stabilized with Nb

The hot deformation behavior of an ultra-pure 17%Cr ferritic stainless steel was studied in the temperature range of 750–1000 °C and strain rates of 0.5 to 10 s^?1 using isothermal hot compression tests in a thermomechanical simulator. The microstructural evolution was investigated using electron backscattered diffraction and transmission electron microscopy. A modified constitutive equation considering the effect of strain on material constant was developed, which predicted the flow stress for the deformation conditions studied, except at 950 °C in 1 s^?1 and 900 °C in 10 s^?1. Decreasing deformation temperature and increasing strain was beneficial in refining the microstructure. Decreasing deformation temperature, the in-grain shear bands appeared in the microstructure. It is suggested that the dynamic softening mechanism is closely related to deformation temperature. At low deformation temperature, dynamic recovery was major softening mechanism and no dynamic recrystallization occurred. At high deformation temperature, dynamic softening was explained in terms of efficient dynamic recovery and limited continuous dynamic recrystallization. A drop in the flow stress was not found due to very small fraction of new grains nucleated during dynamic recrystallization.     

Evaluation of the ductile-to-brittle transition temperature of a silicon steel under various strain rate conditions with a servo-hydraulic high speed testing machine

This paper is concerned with the construction of an empirical model of the Ductile-to-Brittle Transition Temperature (DBTT) for 3.4% silicon steel based on tensile test results at strain rates ranging from 0.001 s^?1 to 100 s^?1. Dynamic tensile tests are conducted using an in-house servo hydraulic tensile test machine at strain rates of 1 s^?1, 10 s^?1, and 100 s^?1 and quasi-static tensile tests are conducted using Instron 4206 at strain rates of 0.001 s^?1 and 0.01 s^?1 with an environmental chamber. Fracture elongations are measured by a DIC method during all tests using the high-speed camera for accurate measurement. The DBTT of 3.4% silicon steel is presented in terms of fracture strain with the variation of the temperature and the strain rate. It is demonstrated from the test results that the DBTT increases as the strain rate increases. An empirical model of the DBTT is constructed in terms of strain rate, temperature and fracture elongation. The parameters of the empirical model are calculated from experimental results obtained at various temperatures and strain rates.     

Constitutive modeling and processing map for elevated temperature flow behaviors of a powder metallurgy titanium aluminide alloy

The flow behaviors of PM titanium aluminide alloy were studied by isothermal compression simulation test. The apparent activation energy of deformation was calculated to be 313.53 kJ mol^?1 and a constitutive equation had been established to describe the flow behavior. Processing map was developed at a strain of 0.7. With an increase of strain, two domains can be found: dynamic recrystallization and superplastic deformation, which are further confirmed by microstructural observations. The dynamic recrystallization occurs extensively at 1000 °C and 10^?3 s^?1, with a peak efficiency of 50%, and the superplastic deformation occurs at 1100 °C and 10^?3 s^?1, with a peak efficiency of 60%. At a strain rate higher than 10^?1 s^?1, the alloy exhibits flow instability.     

Activation Energy in Hot Forming and Recrystallization Models for Magnesium Alloy AZ31

Kinetics of the dynamic, as well as postdynamic recrystallization of the wrought magnesium alloy AZ31, was ascertained. Continuous compression tests associated with the study of dynamic recrystallization were realized at temperatures from 523 to 723 K and at the strain rates from 0.001 to 10 s^?1. The activation energy in hot forming was determined as Q = 158 kJ/mol for stress-strain curves of conventional shape, or Q = 146 kJ/mol for stress-strain curves with the concave initial phase affected by twinning. If the Zener-Hollomon parameter Z > 9.1 × 10¹² s^?1 the deformation necessary for the initiation of dynamic recrystallization is almost independent on the forming parameters. Using the results of the stress relaxation tests, equations describing the kinetics of metadynamic recrystallization and the grain size originated in such a way were developed and the effect of individual variables was evaluated.     

A phenomenological study of the incomplete grain evolution in a deformed vanadium alloy

The dynamic recrystallization (DRX) behavior of a V-5Cr-5Ti (wt%) alloy was studied with a view to optimizing its hot working behavior. Uniaxial compression tests were performed over the temperature range 1373 to 1673 K and strain rate range 0.001 to 1.0 s^− 1 and the microstructural changes were examined by EBSD. Discontinuous dynamic recrystallization (d-DRX) was observed to take place in addition to continuous dynamic recrystallization (c-DRX) despite the bcc nature of this alloy. The new grains nucleated at triple junctions and along grain boundaries to form a necklace structure. Some DRX grains formed within shear bands and deformation bands as well as in the matrix when the Zener-Hollomon (Z) parameter and the strain were increased. The critical stresses and strains increased with Z while the DRX grain size decreased with Z.     

Hot deformation behavior of Al–9.0Mg–0.5Mn–0.1Ti alloy based on processing maps

Hot deformation behavior of extrusion preform of the spray-formed Al–9.0Mg–0.5Mn–0.1Ti alloy was studied using hot compression tests over deformation temperature range of 300–450 °C and strain rate range of 0.01–10 s^?1. On the basis of experiments and dynamic material model, 2D processing maps and 3D power dissipation maps were developed for identification of exact instability regions and optimization of hot processing parameters. The experimental results indicated that the efficiency factor of energy dissipate (η) lowered to the minimum value when the deformation conditions located at the strain of 0.4, temperature of 300 °C and strain rate of 1 s^?1. The softening mechanism was dynamic recovery, the grain shape was mainly flat, and the portion of high angle grain boundary (>15°) was 34%. While increasing the deformation temperature to 400 °C and decreasing the strain rate to 0.1 s^?1, a maximum value of η was obtained. It can be found that the main softening mechanism was dynamic recrystallization, the structures were completely recrystallized, and the portion of high angle grain boundary accounted for 86.5%. According to 2D processing maps and 3D power dissipation maps, the optimum processing conditions for the extrusion preform of the spray-formed Al–9.0Mg–0.5Mn–0.1Ti alloy were in the deformation temperature range of 340–450 °C and the strain rate range of 0.01–0.1 s^?1 with the power dissipation efficiency range of 38%–43%.     

The Microstructural Evolution and Special Flow Behavior of Ti-5Al-2Sn-2Zr-4Mo-4Cr During Isothermal Compression at a Low Strain Rate

The microstructural evolution and special flow behavior of Ti-5Al-2Sn-2Zr-4Mo-4Cr during isothermal compression at a strain rate of 0.0001 s^?1 were investigated. The dislocation climbs in elongated α grains resulted in the formation of low-angle boundaries that transform into high-angle boundaries with greater deformation, and the elongated α grains subsequently separated into homogenous globular α grains with the penetration of the β phase. The simultaneous occurrence of discontinuous dynamic recrystallization and continuous dynamic recrystallization in the primary β grains resulted in a trimode grain distribution. The β grains surrounded by dislocations presented an equilateral-hexagonal morphology, which suggests that grain boundary sliding through dislocation climbs was the main deformation mechanism. The true stress–strain curves for 1073 and 1113 K abnormally intersect at a strain of ~0.35, related to the α → β phase transformation and distinct growth of the β grain size.     

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

Hot compression tests on samples of the TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) titanium alloy have been done within the temperatures of 750–950 °C and strain rate ranges of 0.1–10 s^?1 to 40–60% height reduction. The experimental results show that the flow stress behavior can be described by an exponential law for the deformation conditions. The hot deformation activation energy (Q) derived from the experimental data is 538 kJ mol^?1 with a strain rate sensitivity exponent (m) of 0.107. Optical microstructure evidence shows that dynamic recrystallization (DRX) takes place during the deformation process. Moreover, only α DRX grains are founded in the titanium alloys. The influences of hot working parameters on the flow stress behavior and microstructural features of TC11 alloy, especially on the type of phase present, the morphologies of the α phase, grain size and DRX are analyzed. The optimum parameters for hot working of TC11 alloy are developed.     

Evolution of microstructure and microtexture during the hot deformation of Mg–3% Al

The orientation relationships associated with different mechanisms of new grain formation during the hot deformation of a Mg–3.4% Al–0.33% Mn alloy were investigated using electron back-scattered diffraction (EBSD) techniques. Compression tests were carried out at 350 °C with a strain rate of 0.001 s^?1 on samples machined from extruded tubes. Three types of microstructural features were produced at this temperature: (i) microbands (MBs); (ii) bulged regions; (iii) new grains formed by continuous dynamic recrystallization (cDRX). The formation of the MBs is attributed to the collection of basal dislocations in the MB boundaries. Both the bulges as well as the new cDRX grains are formed as a result of dislocation-based processes that produce c-axis rotations toward the loading axis (i.e. away from the radial direction–transverse direction (RD–TD) plane). Once nuclei have formed, however, the new grains have their c-axes located fairly close to the RD–TD plane. In this way dynamic recrystallization leads to the retention of the main characteristics of the initial RD–TD texture.     

The Microstructure Evolution of Dual-Phase Pipeline Steel with Plastic Deformation at Different Strain Rates

Tensile properties of the high-deformability dual-phase ferrite-bainite X70 pipeline steel have been investigated at room temperature under the strain rates of 2.5 × 10^?5, 1.25 × 10^?4, 2.5 × 10^?3, and 1.25 × 10^?2 s^?1. The microstructures at different amount of plastic deformation were examined by using scanning and transmission electron microscopy. Generally, the ductility of typical body-centered cubic steels is reduced when its stain rate increases. However, we observed a different ductility dependence on strain rates in the dual-phase X70 pipeline steel. The uniform elongation (UEL%) and elongation to fracture (EL%) at the strain rate of 2.5 × 10^?3 s^?1 increase about 54 and 74%, respectively, compared to those at 2.5 × 10^?5 s^?1. The UEL% and EL% reach to their maximum at the strain rate of 2.5 × 10^?3 s^?1. This phenomenon was explained by the observed grain structures and dislocation configurations. Whether or not the ductility can be enhanced with increasing strain rates depends on the competition between the homogenization of plastic deformation among the microconstituents (ultra-fine ferrite grains, relatively coarse ferrite grains as well as bainite) and the progress of cracks formed as a consequence of localized inconsistent plastic deformation.