J. Cent. South Univ. (2019) 26: 550-559
DOI: https://doi.org/10.1007/s11771-019-4026-6
Temperature distribution and effect of low-density electric current on B2+O lamellar microstructure of Ti2AlNb alloy sheet during resistance heating
WANG Guo-feng(王国峰), LI Xiao(李骁), LI Dan-feng(李丹峰), GU Yi-bin(顾义斌), FANG Hui(方慧)
National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology,Harbin 150001, China
Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
Abstract: The resistance heating method has been one of the prospective techniques for hot processing and welding techniques. The thermal behavior under different densities of electric current and the effect of electric current at temperature of 780 oC using low density of electric current of 6.70 A/mm2 on the B2+O lamellar microstructure were investigated for Ti2AlNb alloy sheet. The stable temperature denoted a balanced state between the Joule heat and the dissipation of heat including heat conduction, convection and radiation while the distribution of temperature was nonuniform. The highest temperatures of electric current heating samples increased as the density of electric current was elevated. In order to understand the specific effect of electric current on B2+O microstructure, heat treatment for microstructural homogeneity was introduced to this study. After that, according to the microstructural observations by common characterization techniques in the resistance-heating sample and the isothermal furnace-heating sample after homogenizing treatment, few significant differences in content and orientation of phases can be directly and explicitly found except the thermal effect from the applied electric current. The results will provide reference to this prospective forming and welding techniques and the application for Ti2AlNb alloys using resistance heating in the near future.
Key words: Ti2AlNb; resistance heating; thermal behavior; Joule heat
Cite this article as: WANG Guo-feng, LI Xiao, LI Dan-feng, GU Yi-bin, FANG Hui. Temperature distribution and effect of low-density electric current on B2+O lamellar microstructure of Ti2AlNb alloy sheet during resistance heating [J]. Journal of Central South University, 2019, 26(3): 550–559. DOI: https://doi.org/10.1007/s11771-019-4026-6.
1 Introduction
Advanced intermetallic alloy has a good performance on the specific strength and specific stiffness. Furthermore, the Ti–Al series intermetallics present excellent properties such as resistance to creep, oxidation and their low density. However, brittleness at room temperature is a critical problem which limits the development and applications of Ti–Al series intermetallics until the orthorhombic Ti–Al–Nb structure (O-phase) was discovered by Indian scholars BANERJEE et al [1] in Ti–Al alloy enriched with Nb element. Because O-phase can markedly improve the comprehensive properties of Ti–Al series intermetallics, KUMPFERT [2] summarized the characteristics of the structure and properties of Ti2AlNb alloys and concluded that Ti2AlNb alloy has been one of promising materials in the astronautic and aeronautic fields. The operating temperature of Ti2AlNb is about 600–750 °C, and the forming temperature will exceed 900 °C for elevated temperature forming [3] and superplastic forming [4]. Therefore, the cost consumption is high for the conventional heating method. In recent years, there is an increasing interest of interaction between an external field and the internal structure and mechanical properties during processing of metals and alloys from scientific community and engineers. The ultrasonic treatment and electric current- assisted techniques are developing rapidly [5, 6]. Resistance heating and electrically-assisted heating have attached much attention from researchers owing to their high heating and cooling rate. Japanese scholar MORI et al [7] and MAENO et al [8] developed the resistance heating technique based on the outstanding progress and devoted to the processes and equipment. YANG et al [9] introduced the resistance heating technique to hot-stamping process of advanced high-strength steel and the thickness distribution and absorbed energy were calculated. The results of microstructural characterization and tensile test were given for the assessment of formability. As reported, the practicability of resistance heating has been hence validated for the hot forming of difficulty-deformation metal sheets.
Because of the considerable drift electrons in metals, the effect of electric current or electric field on microstructure evolution such as recrystallization, texture evolution and dislocation migration have been investigated by many researchers and a positive effect termed electroplasticity based on the substantive works in the processing of materials has been proven. TROITSKII’s [10] pioneering research has presented a direct effect simulated by electric current itself rather than the resistive heat from current-carry conductors. KUANG et al [11, 12] and ZHU et al [13] deeply studied the electroplastic rolling processing and the mechanism, and provided sufficient evidences about the special effect of current on the formability and microstructure for light-weight alloy sheets. Moreover, XIE et al [14] conducted experimental investigation of electroplastic deformation during uniaxial tension and validated the athermal effect in AZ31 magnesium alloy sheet. Therefore, the phenomena and the internal reason about resistance heating is significant to study.
In the present study, the thermal behavior of resistance heating for Ti2AlNb and the impact of a low-density electric current (6.70 A/mm2) on the bimodal lamellar microstructure (B2+O) were investigated by experiments including furnace heating and resistance heating. The purpose of this study is to provide a reference for the following electrically-assisted forming of Ti2AlNb alloy sheet.
2 Experimental details
The as-received material is a hot-rolled alloy sheet with a nominal thickness of 2 mm and a nominal composition of Ti-23Al-26Nb (at%), O-phase has the highest content with uneven distribution and varied shapes as shown in Figures 4(a) and 5(a).
The schematic diagram of electric resistance heating is shown in Figure 1, which mainly comprises a direct current power supply, copper electrodes and the Ti2AlNb sample. The sizes of samples for the resistance heating are 120 mm×60 mm×2 mm (S1) and 60 mm×40 mm×2 mm (S2), respectively. The temperature was measured by an infrared radiation thermometer and an infrared thermal camera (FLIR Systems, Inc). The cooling way of all samples was air cooling. Meanwhile, to avoid the complicated phase transformation during resistance heating, the current density was selected as 6.70 A/mm2 so that the microstructure can keep at an equilibrium state of B2+O lamellar microstructure.
Figure 1 Schematic diagram of resistance heating
As for microstructural analysis, samples were cut for the dimensions of 4 mm×5 mm×2 mm. The back scatter electron (BSE) microstructure was observed by a Quanta 200 FEG scanning electron microscope (SEM) and the electron back scatter diffraction (EBSD) images were gotten by a ZEISS Supra 55 SAPPHIRE machine. SEM and EBSD samples were prepared by electrochemical polishing with an applied current of 70 mA and a voltage of 30 V.
3 Results and discussion
3.1 Temperature distribution of resistance heating
The distribution of temperature during resistance heating was uneven and the highest temperature was always in the middle of the rectangular sample. Figure 2(a) shows the relationship between the temperature and heating time in the central point of S1, and Figure 2(b) shows the equilibrium temperatures between Joule heat and dissipated heat of S2. The heating time is 330 s for all samples.
Figure 2 Resistance heating behavior of Ti2AlNb sheet:
Figure 2(a) reveals that each equilibrium temperature is corresponding to a current density. The Joule heat can be considered as a constant value while the heat dissipated in the method of heat conduction, convection and radiation, and the dissipated heat increases when temperature rises. Therefore, the initial temperature increases sharply. Meanwhile, the temperatures perpendicular to the direction of current in Figure 2(b) show a similar characteristic, and the highest temperature is also in the central point of S2.
In order to visually understand the thermal behavior of resistance heating for Ti2AlNb, an infrared thermal camera is used to image the temperature distribution of S1 with the current density of 9.20 A/mm2, as shown in Figure 3.
Figure 3 Infrared thermography of equilibrium temperature of S1 heated by current of 9.20 A/mm2 at different temperature scale intervals:
The temperature of middle zone in Figure 2 is the highest. The electrodes gripped sample on the area of four corners where the current density is high. Therefore, the infrared map in Figure 3 presents a shape of a dumbbell. Due to the effect of heat dissipation, the temperature on the edge of sample is also markedly low.
3.2 Effect of low-density current on B2+O lamellar microstructure
The microstructure of as-received sheet is not homogeneous (Figure 4(a)). Therefore, a homogeneous treatment schedule is introduced to obtain B2+O lamellar microstructure. According to the phase diagram of Ti–22Al–xNb-based alloy [15], firstly, the samples were heated with furnace from room temperature to 1100 °C and then held for 2 h followed by air cooling; secondly, the samples were heated at 900 °C for 3 h and then water quenched. The microstructural result of heat treatment is given in Figure 4(c). The B2+O lamellar microstructure shown in Figure 4(c) can be also obtained through solution treatment at 900 °C for 1 h and then aging treatment at 650 °C for 24 h for Ti2AlNb–based alloy [16].
Because the highest temperature illustrated in Figure 2(a) is 780 °C for the Ti2AlNb sample with the current density of 6.70 A/mm2, the homogeneous microstructure was treated by the current density of 6.70 A/mm2 and the holding time was 20 min. Correspondingly, the furnace-heating sample was also kept at a temperature of 780 °C for the same time of 20 min. The resistance-heating sample and the furnace-heating sample were both cooled in the air.
In Figure 4, the difference between microstructures after resistance heating and furnace heating is not apparent, and the lamellar O-phase seems to be denser in comparison with the homogeneous microstructure. This result suggests that the drift electrons or low-density current (6.70 A/mm2) has no evident effect on the shape of lamellar O-phase and O-phase transformation. The precipitation of acicular O-phase is attributed to the Joule heat.
Figure 4 SEM images:
Figure 5 Phase distribution (a, c, e, g) and corresponding IPFs (b, d, f, h):
According to the characteristics of crystallography, EBSD technique can explicitly recognize the crystalline structure in Ti2AlNb alloy and show phases in a phase distribution map and the orientation relationship of grains in an inverse pole figure (IPF). Figure 5 shows the phases distribution figures comprised of B2-phase, O-phase and α2-phase as well as the corresponding IPFs with a step of 0.30 μm for collecting the data. The area fractions of three phases in Figure 5 are listed in Table 1.
Table 1 Area fraction of phases in Ti2AlNb alloy samples shown in Figure 5 (Lstep=0.30 μm, 1500×)
Because of the high fraction of no-solution area shown in Figures 5(e) and (h), a higher magnification and a smaller step size are selected to scan the phases and orientation relationship in Figure 6. The area fraction of phases is accordingly listed in Table 2.
3.2.1 Contents of phases
In comparison with the homogeneous microstructure (Figure 5(c)), the differences in phase distribution after resistance heating and furnace heating featured an increase in no-solution area and a decrease in the content of O-phase. Generally, EBSD technique cannot detect the defect or impurity such as the grain boundary or the grain with a size less than the step length, which will be determined as “no solution”. Additionally, compared to the resistance heating sample(Figure 5(e)), obvious difference can be given, and the no-solution area is increased by 17.80% and the area fraction of B2-phase is reduced by 17.61% in the furnace-treated sample (Figure 5(g)). Meanwhile, the grain boundaries of prior B2-phase mainly are comprised of α2-phase and O-phase as shown in heat-treatment microstructure (Figure 5(c)) where numerous α2-phase precipitates on the grain boundary of prior B2-phase.
The microstructural evolution of Ti2AlNb alloy is always elusive. WANG et al [17] studied the evolution of O-phase in Ti–22Al–25Nb alloy by solution and age treatment at various temperatures, and found that a great many secondary acicular O-phases (the average width of the secondary fine acicular was (0.09±0.003) μm and the length was (0.91±0.04) μm) precipitated from B2-phase after the solution treatment at 940 °C and age treatment at the 760 °C for 12 h. XUE et al [18] also conducted the similar investigation on the transformation of lamellar O-phase after solution and age treatment. XUE et al [18] concluded that the thickness of lamellar O-phase in B2+O bimodal microstructure would increase as the aged-treated time increased or the temperature rose (the thickness of O-phase solution-treated at 980 °C and then aged-treated at 760 °C for 1 h was 0.16 μm (6.7%)). Therefore, fine and acicular O-phases with large area of grain boundary precipitated after resistance heating and furnace heating so that the step of 0.30 μm was not small enough for EBSD to recognize, and the area fraction of no-solution area increased in Figures 5(e) and (g). The dispersed B2-phase among fine O-phase likewise enhanced the no-solution area. Normally, the precipitation of fine O-phase on the boundary and in the internal grain of prior B2-phase respectively derived from B2/β→B2/β+O and α2→α2+O, respectively. Meanwhile, SHAO et al [19] characterized the microstructure of Ti2AlNb alloy and concluded that the lamellar O-phase would transformed to B2-phase and α2-phase on the grain boundary of prior large B2 grain. Because of crystalline defect and high diffused rate, the α2-phase may favor to precipitate on the B2/B2 boundary or B2/O interphase as seen in Figures 5(c) and (d).
Figure 6 Phase distributions (a, c) and corresponding IPFs (b, d):
Table 2 Area fractions of phases in Ti2AlNb alloy samples shown in Figure 6 (Lstep=0.15 μm, 3000×)
Compared with Figure 5, Figure 6 shows a decrease in the area fraction of no-solution both in resistance-heating and furnace-heating microstructure and an increase in area fraction of O-phase. However, the no-solution area in sample of furnace heating is still high (34.25%) due to more fine O-phase and B2-phase, and this also can be reflected by the BSE images shown in Figure 4. Hence, the area EBSD cannot detect is mainly fine O-phase and B2-phase. O-phase transformation can be sustaining because of a uniform and stable temperature of 780 °C in the furnace. Without the temperature gradient and the skin effect [20], the diffused procedure is much faster for the O-phase transformation which is controlled by long-range diffusion. The microstructural evolution was dominated by the Joule heat rather than the athermal effect, because the difference was not outstanding and cannot directly demonstrate the effect of a low current-density of 6.70 A/mm2 on the content and the shape of lamellar O-phase based on the observations.
Moreover, the α2-phase distributed in B2-phase grains is not uniform in Figure 6(a). The content of α2-phase in one B2-phase grain is markedly higher than that in the neighbouring one. It can be explained by the difference in the content of Al in B2-phase grain before age treatment resulting in a high percentage of α2-phase (16.44%).
3.2.2 Orientations of phases
IPF is always an effective method used to show the crystalline orientations in metals and it seems to be a projection of sample coordinate on the crystalline coordinate. The more similar the color shows in IPF, the more marked the orientation displays. A colorful IPF reveals randomness for the oriented grains. Because of the solution and age treatment or homogenized treatment, there is no texture from the hot rolling processing.
The sizes of prior B2-phase grains after homogenized treatment in Figure 4 are more than 100 μm while the lamellar and acicular O-phase is fine. Therefore, the orientation of a single B2-phase grain was consistent in resistance-heating and furnace-heating microstructure. BOEHLERT et al [15] and BANERJEE [21] summarized orientation relationships in Ti2AlNb: (0001)α2//(001)O and 
(011)B2//(0001)α2 and 
(001)O//(110)B2 and 
It suggested a great deal of similar oriented O-phase precipitated from the B2-phase matrix. However, the lamellar O-phase can not only precipitate from B2-phase, but form in α2-phase [22] or transform from the peritectoid reaction between α2-phase and B2-phase, together with various variants distributed in B2-phase grains [23]. Although the lamellar O-phase precipitates from one single B2-phase grain with one basic color, the orientation would be various as shown in IPFs due to different formation methods. Meanwhile, the neighboring B2-phase grains markedly differed in the colors or orientations at the intersection of grain boundary.
DOLINSKY et al [24] and QIN et al [25] reported that electric current pulse can promote the formation of phases with a high electric conductivity while restrain the formation of phases with a low electric conductivity in a current-carry conductor. GUO et al [26] further suggested a difference in conductivity among crystalline planes or directions and a positive impact on the formation of oriented microstructure when high-density electric current passed through metals. WANG et al [27] applied a high-density current (J=18.6 kA/mm2, t=800 μs) into Cu–Zn alloy, lots of the α-phase nanotwins with a orientation of (111)
in parallel with the current direction were found in the current pulse treated sample. WANG et al [27] further compared the microstructure treated by a lower density current pulse (J=12.5 kA/mm2, t=800 μs) and only few α-phase nanotwins with random orientation relationship were observed in Cu–Zn alloy. As seen in Figures 5 and 6, the precipitated phases including granular α2-phase and lamellar O-phase do not show a directional or a consistent orientation relationship with the electric current because the IPFs of resistance-heating microstructure are far from a single color. Hence, an appropriate and high density electric current maybe a necessary prerequisite for the formation of directional structure.
The electric current accelerates or restrains the phase transformation by means of changing the Gibbs free energy or the driving force. Many researchers demonstrated the electron wind force effects on the multiplication and motion of dislocations. QIN et al [25] pointed out that the driving force for phase transformation is proportional to the square of the current density (ΔG∝J2). In order to study and explain the athermal effect on the microstructural change and the pure electroplastic effect, the Joule heat is expected to keep a minimum value [28]. Hence, the selected frequency of applied current is always below 200 Hz while the density is always high (102–103 A/mm2) in considerable experiments. However, the frequency of current used in this experiment is 40 kHz for the rapid heating though the current density is quite low (6.70 A/mm2). Therefore, the change of lamellar O-phase cannot be observed for the B2+O lamellar microstructure where a great many dislocations are recovered due to the heat treatment before.
4 Conclusions
The temperature of resistance heating depends on the electric current density and the dissipation of heat as a result of an equilibrium temperature after several minutes. Compared with the furnace- heating microstructure at a similar temperature, the content and orientation of phases in resistance- heating microstructure have few changes even though the fine O-phase precipitated from B2-phase matrix at 780 °C with a low-density electric current of 6.70 A/mm2. In short, because of the lack of driving force and free energy, the effect of a low-density electric current on the phase transformation is unconfirmed and obscure for Ti2AlNb alloy sheet other than the thermal effect.
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(Edited by FANG Jing-hua)
中文导读
Ti2AlNb合金板材自阻加热中温度分布及低电流密度对B2+O层片组织的作用
摘要：自阻加热技术是在热加工和焊接领域中具有潜力的加热方式。本文研究了不同电流密度下的Ti2AlNb合金板材的自阻加热行为和在6.70 A/mm2低电流密度，最高温度为780 °C时，电流密度对Ti2AlNb合金板材B2+O双相组织的作用。自阻加热时稳定的温度场意味着焦耳热和由热传导、热对流和热辐射引起的耗散热量的一种平衡，而此时的板材温度分布不均匀。板材自阻加热时的最高温度随着电流密度的提高而逐渐增加。为确切地研究电流对Ti2AlNb合金板材中B2+O相的作用，采用了均匀化热处理。通过表征均匀化热处理后自阻加热试样和等温度下的炉温加热试样的组织发现，除了电流直接带来的热效应，没有明确地发现O相、α2相和B2基体相的含量和取向上的差异。本研究可以为Ti2AlNb合金板材在电流辅助热成形及焊接方面提供参考，为未来的Ti2AlNb合金的自阻加热应用提供实验基础。
关键词：Ti2AlNb合金；自阻加热；热行为；焦耳热
Foundation item: Project(51875122) supported by the National Natural Science Foundation of China
Received date: 2018-01-18; Accepted date: 2018-08-07
Corresponding author: WANG Guo-feng, PhD, Professor; Tel: +86-13904516689; E-mail: gfwang@hit.edu.cn; ORCID: 0000-0002- 6001-2930
