Dynamic Globularization Research Articles

Evolution of equiaxed and lamellar α during hot compression in a near alpha titanium alloy with bimodal microstructure

Microstructural evolution of equiaxed α and lamellar α during hot compression in a bimodal-structure near alpha Ti-6Al-3Nb-2Zr-1Mo alloy were investigated. Focus was concentrated on their own dynamic restoration mechanism via electron backscatter diffraction (EBSD) technique. Based on analysis of grain boundary misorientation distribution and misorientation gradient within α grain, it was revealed that limited continuous dynamic recrystallization (CDRX) as well as adequate dynamic recovery (DRV) took place within equiaxed α, whereas CDRX occurred as the main restoration mechanism in lamellar α. Further, the deformation heterogeneous among equiaxed α grains was found to be relevant to the difference in slip systems. Under the same nominal strain of 75% height reduction, process of weak DRV, strong DRV and CDRX coexisted in distinct equiaxed α grains owning to small, medium and high Schmid Factor of prismatic slip. Besides, globularization heterogeneity induced by CDRX in lamellar α was considered to be closely related with its initial orientation to compression axis and the imposed strain. For flat colony α, the microstructure was relatively stable during straining showing a thinning process and a weak dynamic globularization, while the kinked colony α with large strain was globularized sufficiently.

Hot tensile deformation behavior and globularization mechanism of bimodal microstructured Ti−6Al−2Zr−1Mo−1V alloy

Abstract The hot deformation behavior, microstructure evolution and fracture characteristics of bimodal microstructured Ti−6Al−2Zr−1Mo−1V alloy were investigated by isothermal tensile tests. Results reveal that flow softening is caused by dynamic globularization of the bimodal microstructure, which also results in a relatively high stress exponent and thermal activation energy. The corresponding SEM, EBSD and TEM observations indicate that the dynamic globularization at 750 and 800 °C is accomplished by the formation of α/α sub-grain boundary and penetration of the β phase. However, dynamic recrystallization (DRX) is the main globularization mechanism at 850 °C, which was proved by the generation of fine grains with a necklace-like character due to the transformation of low-angle boundaries (LABs) into high-angle boundaries (HABs). With an increase in the deformation temperature or a decrease in the strain rate, the fracture mechanism changes from microvoid coalescence to intergranular fracture.

Modelling for the flow behavior of a new metastable beta titanium alloy by GA-based Arrhenius equation

The hot compression tests of a new metastable beta titanium alloy (Ti-5Al-5Mo-5V-3Cr-1Zr alloy) were conducted at the temperature range of 1033–1123 K with an interval of 30 K and the strain rates of 0.001–1 s−1 on a Gleeble-3500 isothermal simulator for the acquisition of true stress strain curves. The true stress strain curves at the experimental conditions show different curve-types which reveal different microstructure evolution mechanisms including dynamic recrystallization (DRX), dynamic globularization (DG) and dynamic recovery (DRV). The Arrhenius constitutive model of this alloy was developed based on the experimental results and used to predict the true stress. Based on the developed Arrhenius model, a genetic-algorithm-based (GA-based) Arrhenius model was constructed and applied in true stress prediction. To validate the superiority of the GA-based Arrhenius model in true stress prediction, statistical indicators including mean value (μ), standard deviation (ω), correlation coefficient (R) and average absolute relative error (AARE) were introduced to assess and compare the experimental values and the prediction values. As a result, the μ-value, ω-value, R-value and AARE-value of Arrhenius equation method are −3.8998%, 5.505 62%, 0.9960 and 5.1817% respectively and of the GA-based Arrhenius equation method are 0.0505%, 0.8083%, 0.9999 and 0.6099% respectively, which means the GA-based Arrhenius equation method is promoted in true stress prediction and high accuracy finite element simulation can be achieved by this method.

Hot deformation behavior and microstructure evolution of Ti-6Cr-5Mo-5V-4Al alloy during hot compression

The true stress strain curves of Ti-6Cr-5Mo-5V-4Al alloy at the deformation condition range of 963–1113 K & 0.001–1 s−1 were acquired by hot compression tests carried out on a Gleeble-3500 isothermal simulator. The higher softening values in the α+β phase field reveal the occurrences of dynamic recrystallization (DRX) and dynamic globularization (DG), and the lower softening values in the β phase field reveal that dynamic recovery (DRV) predominates. The constitutive relationships of this alloy in α+β phase field and β phase field were developed based on an improved Arrhenius model. The strain rate sensitivities (m) were determined and the temperature sensitivities (θ) were calculated by a modified function. The stacking contour map consisted of temperature sensitivity, strain rate sensitivity and microstructures at the strain of 0.9 was developed. By comparing the relationship between the three factors, the deformation defects are risky to occur at the deformation condition with high θ-value (>0.5) and low m-value (<0.17) and reasonable microstructure could be obtained at the condition with low θ-value (<0.4) and reasonable m-value (>0.2). Based on the obtained criterions, an optimal working area map was constructed, which is beneficial for intrinsic hot workability identification for this alloy.

Hot working of Ti-6Al-4V with a complex initial microstructure

The hot deformation behaviour of wrought Ti-6Al-4V, with an initial microstructure of equiaxed and elongated α phase and intergranular β, was investigated. Isothermal hot compression testing was performed using a Gleeble 3500 thermomechanical simulator at strain rates of 0.01–10 s−1, up to a total strain of 0.6, and at temperatures of 750–950 °C, i.e. in the α + β phase region. The stress-strain data were used to develop constitutive equations and processing maps so that the stress exponent, activation energy and the most advantageous processing conditions for deforming the alloy could be determined. Microstructural examination for validation of the processing maps was carried out by optical microscopy and scanning electron microscopy. The average activation energy (Q) and stress exponent (n) at all strains were typical of dynamic recrystallisation values reported for α + β titanium alloys. The processing maps showed different features at different strains. There was no domain of instability when samples were deformed to a total strain of 0.2 but regions of instability were observed at strains of 0.5 and 0.6. The optimum processing conditions were identified at ~900 °C/0.05 s−1 and 940 °C/1.7 s−1(0.2 strain); 900 °C/0.02 s−1 and 945 °C/1.5 s−1 (0.5 strain); and 800 °C/0.01 s−1 and 940 °C/1.2 s−1 (0.6 strain). Power dissipation efficiency values and microstructural features confirmed that the main deformation mechanism corresponded to dynamic globularisation of the α phase. Increased transformation of α-Ti to β-Ti also enhanced flow softening at higher deformation temperatures.

High-Temperature Deformation Behavior of Ti-6Al-2Sn-4Zr-2Mo Alloy with Lamellar Microstructure Under Plane-Strain Compression

High-temperature plane-strain compression of a Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy with a lamellar structure was applied by a thermal simulation machine Gleeble 3800 at a temperature of 1223-1370 K and a strain rate of 0.01-10 s−1. Constitutive relations between the flow stress and strain at different temperatures and strain rates were constructed based on the Arrhenius equation. Processing maps based on the dynamic material model at strains of 0.5, 1.0, and 1.25 were also constructed to analyze the mechanism and instability of high-temperature deformation. Dynamic recrystallization was found to occur at 1223-1313 K and 0.01-0.1 s−1, with a peak efficiency of power dissipation of 70%. Observations of the microstructure demonstrated that α platelets and dynamic globularization were responsible for dynamic softening in the α + β phase field. Flow instability behaviors occurred at 1263-1343 K/1.78-10 s−1, 1233-1283 K/0.1-3.16 s−1, and 1353-1373 K/0.56-1.78 s−1, and adiabatic shear bands and microcracks were observed in these domains. Thus, for the lamellar Ti-6242 alloy, the recommended hot rolling parameters were 1243-1333 K and 0.01-0.1 s−1. Electron backscattered diffraction results revealed that the dynamic restoration mechanism in the β phase field was dynamic recovery when the strain rate was as high as 10 s−1.

On the microstructure evolution during isothermal low cycle fatigue of β-annealed Ti-6242S titanium alloy: Internal damage mechanism, substructure development and early globularization

The present work deals with the room and high temperature microstructure evolutions of a near alpha titanium alloy (Ti-6242S) during isothermal low cycle fatigue. The initial fully lamellar microstructure has been purposefully annealed to form α/β interface phase layers. The shearing and displacement of the β layers is identified as the main governing internal damage mechanism. This is phenomenal owning to the very low imposed strain; and is justified considering: (i) the large colony size and thin thickness of the β layers within the initial lamellar microstructure, (ii) the presence of the interface phase, and (iii) the intensified substructure development within the α and β phases. The shearing and displacement of the β layers lead to the segmentation and this is followed by initial stage of dynamic globularization even at room temperature condition. The globularization kinetic is intensified at higher temperature cyclic loading. In this regard, the sub-boundary induced boundary splitting/grooving is characterized as an involved mechanism. Interestingly, the early dynamic globularization of the α lamellae is also traced through the high temperature fatigued microstructures. The β phase penetration along the α phase sub-boundary is realized as the main globularization mechanisms. The interface phase layer stimulates the possibility of globularization via providing the high-energy non-coherent interfaces in structure. In addition, the deformation twinning within the interface phase, so called as “twinning induced micro-plasticity effect”, appears to have a significant effect on the fatigue damage behavior via compensation of incompatibility between α and β phases and deformation twin formation.

Dynamic Globularization Behavior of O-phase Lamellae in Ti-22Al-25Nb (at%) Alloy during Deformation at Elevated Temperatures

Abstract To clarify the globularization mechanism and model of the O-phase lamellae of Ti2AlNb-based alloys during deformation at elevated temperatures, the hot deformation behavior and microstructure evolution of Ti-22Al-25Nb (at%) alloy during hot compression deformation in (α2+O+B2) region were investigated. The results show that the flow softening is relevant to the dynamic globularization of O-phase lamellae and the flow instability including 45° shear deformation, 45° shear cracking and micro-cracks. The deformation constitutive model based on the exponential law was established and the deformation activation energy was calculated to be 831 kJ·mol−1. The globularization of O-phase lamellae is hard to occur at low temperatures or at high strain rates due to slow diffusion rate or insufficient diffusion time. The main globularization mechanism is the kink and shear of the O-phase lamellae, which is confirmed to be a dynamic recrystallization behavior. The dynamic globularization kinetics was also studied, which is sensitive to deformation conditions including deformation temperatures and strain rates and follows an Avrami type sigmoid equation.

High-temperature deformation behavior of the extruded Ti-22Al-25Nb alloy fabricated by powder metallurgy

The hot-deformation behavior of the extruded Ti-22Al-25Nb alloy was investigated by compression testing in the 1213–1333 K temperature range, under a 0.001–1.0 s−1 strain-rate range. The activation energy, Q, at a deformation strain of 0.3, was calculated to be 574.80 kJ mol−1. A strain-compensated Arrhenius-type constitutive model was established for this alloy, with an average absolute relative error (AARE) and correlation coefficient (R) of 7.64% and 0.994, respectively. Based on the dynamic materials model (DMM), processing maps were developed by combining the power dissipation and instability maps. An instability region was identified in the 1213–1278 K temperature range and 0.022–1.0 s−1 strain-rate range. The microstructure of specimens deformed under different conditions were analyzed to characterize the corresponding deformation mechanisms. The main softening mechanisms in the stability regions were dynamic recrystallization (DRX) and dynamic globularization. The grain size of the specimen deformed at 1313 K/0.001 s−1 was approximately 15.2 μm.

High Temperature Deformation Behavior of TC17 Alloy

The high temperature deformation behavior and microstructures evolution of TC17 alloy in the temperature range of 820°C - 930°C, strain rate range of 0.01 s-1 - 10 s-1 and height direction reduction of 20% - 80% have been studied by hot compressing testing. The microstructures of TC17 alloy were observed and analyzed using Olympus/PMG3 optical microscope. The flow stresses were correlated with strain rate and the temperature by the constitutive equation. The results show that the flow stress of TC17 alloy increase quickly with the strain, then decrease with a steady value. The deformation activation energy obtained in the α + β region for TC17 was 407 kJ/mol, and in the β region was 155 kJ/mol. It was also found that the degree of dynamic globularization of α phase increases with increasing strains, increasing temperature and decreasing strain rate in α + β region, the dynamic re-crystallization is obvious at low strain rate and dynamic recovery is obvious at high strain rate in β region.

Evolution characterization of α lamellae during isothermal compression of TC17 alloy with colony-α microstructure

The α lamellae evolution of TC17 alloy with colony-α microstructure during isothermal compression in α/β phase field was investigated via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques. Special attention was paid to orientation evolution and α/α interfaces of the α lamellae. The results showed that the strain rate played a more significant role in the α lamellae evolution than the deformation temperature. The dynamic globularization via boundary grooving mainly characterized the α lamellae evolution at the strain rates of 0.01s−1 and 0.1s−1, and the high-angle boundaries (HABs) induced by the continuous dynamic recrystallization (CDRX) provided the main driving force for boundary grooving. The globularization fraction of α lamellae increased with the increasing of deformation temperature and decreasing of strain rate, in which the diffusion-controlled boundary grooving was enhanced. The α lamellae evolution at the strain rates of 1.0s−1 and 5.0s−1 was dominated by fragmentation, in which the localized shear flow driven by the synergistic effect of strain localization and inadequate dynamic recovery governed the fragmentation of α laths.

Forging property, processing map, and mesoscale microstructural evolution modeling of a Ti-17 alloy with a lamellar (α+β) starting microstructure

This work identifies microstructural conversion mechanisms during hot deformation (at temperatures ranging from 750 °C to 1050 °C and strain rates ranging from 10−3 s−1 to 1 s−1) of a Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy with a lamellar starting microstructure and establishes constitutive formulae for predicting the microstructural evolution using finite-element analysis. In the α phase, lamellae kinking is the dominant mode in the higher strain rate region and dynamic globularization frequently occurs at higher temperatures. In the β phase, continuous dynamic recrystallization is the dominant mode below the transition temperature, Tβ (880~890 °C). Dynamic recovery tends to be more active at conditions of lower strain rates and higher temperatures. At temperatures above Tβ, continuous dynamic recrystallization of the β phase frequently occurs, especially in the lower strain rate region. A set of constitutive equations modeling the microstructural evolution and processing map characteristic are established by optimizing the experimental data and were later implemented in the DEFORM-3D software package. There is a satisfactory agreement between the experimental and simulated results, indicating that the established series of constitutive models can be used to reliably predict the properties of a Ti-17 alloy after forging in the (α+β) region.

Compressive response and microstructural evolution of bimodal sized particulates reinforced (TiB+La2O3)/Ti composites

The hot deformation behavior and microstructural evolution of bimodal sized particulates reinforced (TiB+La2O3)/Ti composites are investigated in the temperature range of 850–1100 °C and strain rate range of 0.001–1 s−1 using isothermal compression tests. The constitutive equations in different phase regions are established in terms of the flow stress data, and the apparent activation energies are calculated to be 753 kJ/mol in α+β phase region and 185 kJ/mol in β phase region, which are much higher than those of matrix alloy due to the incorporation of bimodal sized reinforcements. The optimal hot processing window for (TiB+La2O3)/Ti composites is determined at 900–950 °C/0.01–0.1 s−1, which is associated with the continuous dynamic recrystallization (CDRX) of primary α grains and dynamic globularization of lamellar α. The necklace recrystallization of β grains and dynamic recovery (DRV) of lamellar α are observed in β phase region, and the instability mechanisms include inhomogeneous deformation and breaking or debonding of TiB whiskers. Additionally, TEM observations are used to study the effects of reinforcements on microstructural evolution. The results indicate that both micro-sized TiB and submicro-sized La2O3 can impede dislocation movement and TiB can facilitate the dynamic recrystallization (DRX) of α phase and grain refinement.

Influence of High-Energy Pulse Current on the Mechanical Properties and Microstructures of Ti-6Al-4V Alloy

This study reported the effect of the high-energy pulse current on the mechanical properties and microstructure evolution of Ti-6Al-4V alloy in the compression deformation process. The electroplastic compression (EC) of Ti-6Al-4V alloy was performed with discharge voltages of 50 and 70 V and strain rates of 0.01 and 0.03 s−1, respectively. The results show that the mechanical properties of Ti-6Al-4V alloy greatly change under the influence of the high-energy pulse current. Metallographic examination and x-ray diffraction are performed to observe the microstructural evolution of the alloy. An obvious dynamic globularization occurs at the later stage of the EC process even though the temperature of the samples is relatively low. An α–β phase change also occurs during the EC process. The microstructural evolution of Ti-6Al-4V alloy during the EC process is mainly attributed to the thermal and athermal effects of electropulsing, which promotes the dislocation motion and nucleation rate of dynamic globularization.

Mechanical and Microstructural Response of Near Beta Ti Alloys to Hot Tensile Testing

AbstractHot tensile tests were carried out on Timetal-125 and Timetal-LCB near beta Ti alloys at temperatures in range of 600-1000°C and constant strain rate of 0.1 s−1. At temperatures below 700-800°C, the homogenuous and total strains for Timetal-LCB were greater than those for Timetal-125. In contrast, at temperatures over 800°C, Timetal-125 showed better hot ductility. The yield point phenomena was observed in Timetal-LCB at all temperatures. Unlikely, for Timetal-125, it was observed only at temperatures over 800°C. The weaker yield point phenomena in Timetal-125 could be attributed to the negative effect of Al on the diffusion of V. At all temperatures Timetal-LCB exhibited higher strength than Timetal-125. It was found that there should be a direct relationship between the extent of yield point phenomena and strength and dynamic softening through hot tensile testing. It was observed that at temperatures beyond 800°C (beta phase field in both alloys) dynamic recrystallization can progress more in Timetal-125 than in Timetal-LCB. These results were in good agreement with the better hot ductility of Timetal-125 at high temperatures. At low temperatures, i.e. below 700-800°C, partial dynamic recrystallization occurs in beta and dynamic globularization in alpha phase. These processes progress more in Timetal-LCB than in Timetal-125.

Effect of initial alpha lamellar thickness on deformation behavior of a near-α high-temperature alloy during thermomechanical processing

The hot deformation behavior of a near-α high-temperature Ti-5.4Al-3.7Sn-3.3Zr-0.5Mo-0.4Si alloy with initial alpha lamellar thicknesses from 1µm to 10µm was investigated by conducting a series of isothermal hot compression tests. The effect of initial alpha lamellar thickness on the flow behavior, apparent activation energy, strain-rate sensitivity and microstructure evolution under different hot deformation conditions were analyzed based on the experimental data. The results show that the flow stress of the alloy display an overall decreasing trend with the increasing of initial alpha lamellar thickness for a given temperature and strain rate. The peak stress and the apparent activity energy during thermomechanical processing were quantitatively described as a function of alpha platelet thickness based on the Hall-Petch behavior. According to various deformation mechanisms, the variations of apparent activity energy and strain rate sensitivity associated with different initial alpha lamellar thicknesses were calculated and analyzed for the alloy, which indicate that the thin initial alpha lamellar microstructure with higher apparent activation energy and strain rate sensitivity shows easier occurrence of dynamic globularization than thick initial alpha lamellar microstructure. Furthermore, the characterization of microstructure evolution for the alpha lamellae with different thicknesses were observed and the thin lamellae was more easily fragmented into fine α phase while most of the thick lamellae were elongated or buckled at same processing conditions.

Constitutive modeling for high-temperature flow behavior of Ti-6242S alloy

The high-temperature deformation behavior of near-alpha Ti-6242S alloy was investigated at temperatures in the range of 850–1000℃ and at strain rates of 0.001-1s−1. The results indicated a difference in the flow stress behavior of the lower (T≤950℃) and upper (T=1000℃) parts of the two-phase alpha/beta region. Dynamic recovery and recrystallization of beta phase occur at 1000℃, while distortion and dynamic globularization of alpha phase are the dominant events, at temperatures lower than 950℃. Flow behavior of the alloy was predicted through developing two phenomenological constitutive models, considering the coupled effects of deformation temperature, strain rate and strain. Furthermore, a comparative study was performed on the capability of the aforementioned models to predict the high-temperature flow behavior of the Ti-6242S alloy. Comparing the predicted average absolute relative error and correlation coefficient showed a more accurate flow behavior through Arrhenius-type equation against to the dynamic softening (Cingara) model. However, a novel approach was used to enhance the Cingara model and was verified with experimental result showing an increase in accuracy.

Role of processing parameters in the development of tri-modal microstructure during isothermal local loading forming of TA15 titanium alloy

It is critical to precisely control the tri-modal microstructure during the isothermal local loading forming of titanium alloy to obtain high-performance components. To this end, the effect of local loading processing parameters on the development of tri-modal microstructure were experimentally investigated. The key influence factors and laws are revealed as follows. In the first loading step, the deformation temperature plays a decisive role in the volume fraction of equiaxed α, which decreases with the increase of temperature. While, the deformation amount and cooling mode present little effects on the microstructure evolution. In the second loading step, the deformation temperature and amount mainly influence the volume fraction, spatial orientation distribution and globularization of lamellar α. The volume fraction of lamellar α increases with the temperature decreasing. The spatial orientation distribution of lamellar α gradually changes from homogeneous distribution to concentrated distribution with the increase of deformation amount. Besides, the dynamic globularization fraction of lamellar α producing in the second loading step increases with the deformation amount, and their relationship can be well fitted by Avrami type equation. Moreover, higher temperature in the second loading step is beneficial to decrease the critical strain for the initiation of dynamic globularization and promote the kinetic rate of dynamic globularization. On the other hand, the deformation amount of the second loading step has after-effects on the static globularization of lamellar α in the annealing treatment. If the deformation amount exceeded the critical strain for the initiation of dynamic globularization, the static globularization of lamellar α would produce in the annealing, and the static globularization fraction increases with the deformation amount of the second loading step.

The flow behavior and microstructure evolution during (α + β) deformation of β wrought TA15 titanium alloy

The hot compressions in (α+β) region were conducted on β wrought TA15 titanium alloy, so as to investigate the effect of β working on the flow behavior and dynamic globularization of lamellar microstructure. The results indicated that initial β grain size had no significant influence on flow stress. The dynamic globularization rate of α laths in smaller β grains was higher due to the difference in lamellar length. β predeformation would decrease the flow stress of (α+β) deformation. Most of α laths in the microstructure after β predeformation were short with a geometric orientation vertical to β grain boundary and rotated together during compression, which would promote the formation of α/α substructures, thus highly enhanced the dynamic globularization rate of α laths. While β heat treated microstructure had long α lamellae with a random orientation, which tended to become kinked first and then break up into short ones, resulting in the delay of dynamic globularization. In addition, prior β deformation changed the initial α phase texture and dynamic globularization rate would be affected by strain path. When the compressive direction in (α+β) region was vertical to that of β predeformation, the globularized fraction of α lamellae was apparently high.

Characterization of the hot deformation behavior and microstructural evolution of Ti–6Al–4V sintered preforms using materials modeling techniques

The hot deformation behavior of Ti–6Al–4V sintered preforms was investigated by hot compression tests over temperatures from 800 to 1100 °C and strain rates from 0.001 to 10.0 s−1. The flow curves for the α + β regimes exhibited continuous flow softening and broad oscillation after the peak stress under all of the processing conditions. Otherwise, for the β field, steady-state flow stress was obtained or stress oscillation appeared after the peak stress. In the α + β field, the dynamic globularization and platelet buckling/kinking were observed at lower and higher strain rates, respectively. Unlike in the α + β regions, dynamic recrystallization was found at higher strain rates in the β ones. The apparent activation energies for hot deformation in the α + β and β regimes from kinetic modeling technique were estimated to 147.35 and 233.95 kJ mol−1, respectively. The efficiency of power dissipation over all of the deformation conditions was also calculated by using dynamic material models and schematized into a processing map. The highest value was predicted to 46% at a temperature of 945 °C and at a strain rate of 0.001 s−1, and the processing condition seemed to be optimal for hot deformation of the Ti–6Al–4V sintered preforms.