Selective Abnormal Growth Behavior of Goss Grains in Magnetostrictive Fe-Ga Alloy Sheets
2016 Volume 57 Issue 12 Pages 2083-2088
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2016 Volume 57 Issue 12 Pages 2083-2088
Selective abnormal growth behavior of Goss grains was investigated by interrupting secondary recrystallization process in magnetostrictive (Fe83Ga17)99.9(NbC)0.1 rolled alloy sheets. The evolution of microstructure and texture was analyzed by electron back-scattered diffraction while ramping the temperature from 1123 to 1353 K. The results indicate that before temperature increasing up to 1203 K, Goss grains had no advantages in size and quantity, and the abnormal grain growth did not occur. Goss grains grew abnormally from about 1218 K due to the inhibitory action of Nb-rich precipitates on normal growth of other orientated grains. The γ-fiber texture was the predominant texture before the onset of secondary recrystallization. When the secondary recrystallization got fully development, a sharp Goss texture and an average magnetostriction up to 210 ppm were obtained in the sample annealed at 1353 K.
Magnetostrictive Fe-Ga alloys, also named “Galfenol”, have received increasing attention as a new magnetostrictive smart material for actuator, sensor, and energy harvesting applications1,2). This interest stems from facts that unlike existing smart material systems, Galfenol is the first to offer the combination of good magnetostrictive properties and mechanical properties. Magnetically3), the addition of Ga increases the magnetostrictive capability of Fe over tenfold up to 400 ppm ($\frac{3}{2} \lambda_{100} $) along <100> direction in single crystal material. Mechanically4), Fe-Ga alloys are robust not exhibited by materials such as PZT, PMN, or Terfenol-D. In addition, Fe-Ga alloys have high permeability (μr > 100)5), high Curie temperature (Tc > 923 K)6)and low temperature dependence (～0.5 ppm/K)7). This combination of magnetic and mechanical properties makes Fe-Ga alloy a unique material.
Due to the high conductivity, Fe-Ga alloys need to be formed into thin sheets to avoid eddy current losses, especially in the ultrasonic application. Thermal mechanical processes involving deformation and recrystallization have been employed to produce rolled Fe-Ga sheets8–12), and <001> orientation along the rolling direction (RD) is preferred to maximize the magnetostrictive performance. Up to now, the secondary recrystallization, also named by abnormal grain growth (AGG), of Goss oriented ({110}<001>) grains has been the most effective way to achieve the sharp <001> orientation and large magnetostriction along the RD in the rolled Fe-Ga sheets13–18). Previous studies have reported the achievement of sharp Goss orientation, by the combined effects of NbC particles as inhibitors and sulfur-induced surface energy effects15–17). However, most of studies focused on the influence of final annealing time and temperature, and annealing processes at temperatures above 1373 K for a special time were employed to produce the AGG. During the temperature rising process, few work reported the nucleation and selective growth of Goss grains before abnormal growth. The selective growth behavior of Goss grains is important for the better understanding of the secondary recrystallization of Goss oriented Fe-Ga sheets.
Our recent work18) has prepared sharply Goss oriented Fe-Ga sheets with small amount of NbC (0.1 at.%) as inhibitors, by using rolling processes on <001> oriented column-grained alloys. In this work, the selective growth behavior of Goss grains during the heating process from 1123 to 1353 K was investigated. The abnormal growth of Goss grains started at about 1218 K. The average deviation degree to ideal {110}<001> of Goss grains without abnormal growth is above 13°, and the angle decreases to about 6° with the abnormally growing of Goss grains. The orientation becomes accurate gradually and the secondary recrystallization gets a full development with rising of temperature, resulting in an obvious improvement of magnetostriction.
The alloy with nominal composition (Fe83Ga17)99.9(NbC)0.1 was prepared from Fe (99.9%), Ga (99.99%), and master alloys of Nb-Fe and Fe-C, and the <100> oriented columnar-grains were produced by directional solidification (DS) processes at a growth rate of 720 mm·h−1. The slabs with a thickness of ～18 mm were obtained from the DS alloys by electrical discharge machining. The slabs were hot rolled along the growth direction of columnar grains at 1423 K to ～2.1 mm, followed by warm rolling at ～873 K to ～1.1 mm. After an intermediate annealing at 1123 K for 5 min, further rolling was undertaken to the final thickness of ～0.3 mm. The Fe-Ga sheets, 12 mm × 16 mm and 22 mm × 37 mm cut by electrical discharge machining, were enclosed in quartz ampoules using 0.3 atm. Ar as protecting gas. The sheets enclosed in the ampoules were primarily annealed at 1123 K for 6 min. After the primary annealing, samples were rapidly heated from 1123 to 1173 K with a rate of 5 K/min in the furnace, and then they were slowly heated from 1173 to 1353 K at a controlled rate of 0.25 K/min. Different samples heated to 1173, 1188, 1203, 1218, 1233 and 1353 K were cooled in the air and taken out to investigate the secondary recrystallization behavior at the corresponding temperature, respectively.
Precipitates were examined by scanning electron microscopy (SEM), and energy dispersive X-rays spectroscopy (EDS) was used to identify the composition of precipitates. Electron backscatter diffraction (EBSD) patterns were captured and analyzed to obtain microstructure image, the texture component and orientation distribution function (ODF) plots. The magnetostriction at room temperature was measured along the RD using strain gauges with the gage area of 2.8 mm × 2.0 mm (base area of 6.4 mm × 3.5 mm). Saturation magnetostriction was calculated by (3/2)λs = λ//-λ⊥, and the λ// and λ⊥ were the maximum magnetostriction when the magnetic field parallel and perpendicular to the RD are applied, respectively.
The macrostructures of samples annealed at various interrupt temperatures were observed. The abnormal grain growth was not observed before 1203 K. Beyond 1203 K, grains grew abnormally. A photo of low magnification microstructure of the sample heated to 1353 K is shown in Fig. 1. The size of Goss grain is up to several centimeters.
Low magnification microstructure of abnormal growth of Goss grains.
Figure 2 shows the microstructure of samples annealed at different interrupt temperatures. Goss grains, using a maximum angular deviation of 20° from ideal {110}<001>, are identified as dark grains. The sizes of average grain and different oriented grains were collected and analyzed by using the TSL OIM Analysis 5 software of electron backscatter diffraction, and the dependence on temperatures is shown in Fig. 3. It can be seen that grains grow slowly with the temperature rising, and Goss grains have no advantages in size and quantity before 1203 K. The slowly growth of average grain is attributed to the inhibitory action of precipitates. The pinning effect of Nb-rich precipitates with size of 60～110 nm (see Fig. 4) on grain boundary migration hinders the grain growth. Fig. 3(a) shows the average grain size increases from 18.6 µm to 24.2 µm, while that of Goss grains increases from 15.3 to 21.9 µm. Despite the fact that Goss grains have no advantage in size before abnormal growth, the growth rate of Goss grain is a little higher than that of the average grain. The temperature dependence of average size of different oriented grains is given in Fig. 3(b). After primary recrystallization at 1123 K for 6 min, the grain size of different oriented grains is not uneven. With a further raising of temperature from 1173 to 1203 K, in addition to the γ-textured ({111}<112> and {111}<110>) grains, the average size of grains with other orientation all increases slightly. It is noteworthy that a decrease of the average size of γ-textured grains is observed after 1188 K, meanwhile the average size of Goss grains is bigger than that of other oriented grains, suggesting the abnormal growth of Goss grains is about to begin. Figure 5 shows the growth rate of different oriented grains during the temperature increasing from 1123 to 1203 K. For Goss grains, the growth rate of 43.1% is significantly greater than that of other oriented grains, which indicates that although Goss grains have no advantage on size, but their growth is faster than other oriented grains.
EBSD images of samples at different temperature: (a) 1123 K; (b) 1173 K; (c) 1188 K; (d) 1203 K; Goss grains are shown as dark color.
Average grain size (a) and average size of different oriented grains at various temperatures (b).
SEM image (a) and EDS profile (b) of precipitates in the sample at 1173 K. The inset is the enlarge image of an Nb-rich precipitate.
Growth rates of different oriented grains.
Figure 6 displays the EBSD images of abnormal grown Goss grains. As the temperature increasing to 1218 K, although some small grains are not yet annexed, most of the grains are very large due to abnormal grain growth, as shown in Fig. 6(a). In addition, the grain boundary morphology of Goss grains is uneven. For the oriented silicon steels, this kind grain boundary is called as a solid-state wetting grain boundary by some researchers19,20). Their results indicate that AGG by wetting is likely to occur if a particular grain has a relatively high frequency of low energy grain boundaries with its neighbors. But, the formation of this kind grain boundary morphology is also interpreted by the coincidence site lattice (CSL) model and the high-energy grain boundary (HEGB) model21,22). On the other hand, we consider an additional reason may be the little quantity of secondary nuclei of Goss grains. Although the secondary nuclei of Goss grains can grow fully, grain boundary migration is not synchronous. Several Goss grains merge into a big grain, and produce some inner concave grain boundaries. A large Goss grain with some embedded island-like grains is shown in Fig. 6(b). The formation of island-like grains could be attributed to that Goss grain growth is hindered by some adjacent large grains during abnormal grain growth process.
EBSD images of abnormal growth of Goss grains at 1218 K.
EBSD patterns were captured and analyzed to obtain information of crystal orientation. ODF plots (φ2 = 45°) of samples annealed at various interrupt temperatures are shown in Fig. 7. For the primarily recrystallized sheet annealed at 1123 K for 6 min, Fig. 7(a) shows the γ-fiber texture and cubic texture are predominant textures, while the intensity of Goss texture is very weak. Upon heating to 1203 K, the γ-fiber texture is still the predominant texture, suggesting the texture has been relatively stable, as shown in Fig. 7 (b) to (d). For the oriented silicon steels, it is very important for the eventually formation of a sharp Goss texture that there are a large number of γ-textured grains before the abnormal grain growth23). Combined Fig. 7 and Fig. 8, it shows that as compared to the matrix texture, the {100}<001> texture component decreases immensely, while the {110}<001> texture gradually becomes stronger. With further increase of temperature to 1218 K, a sharp Goss texture appears in the sample, as shown in Fig. 7(e). Before the onset of secondary recrystallization, the grains grow slowly due to the inhibition of precipitates, resulting in a relatively stable microstructure and texture. During the process of abnormal grain growth, Goss grains annex other oriented grains, and then several adjacent Goss grains meet and merge into a large Goss grain, resulting in a sharp Goss texture. This growth pattern is perhaps the reason that the secondary recrystallization could be completed quickly from the beginning to the end.
ODFs of samples at different temperature: (a) 1123 K; (b) 1173 K; (c) 1188 K; (d) 1203 K; (f) 1218 K.
Volume fractions of different oriented grains at various temperatures.
As for Fe-Ga alloy, theoretical explanations for the development of secondary recrystallization in the oriented silicon steels could be also applicable for Fe-Ga alloy, because they have the same bcc structure. For the oriented silicon steels, various mechanisms of the development of secondary recrystallization have been already proposed. The CSL model and the HEGB model are frequently used to quantify the grain boundary characteristics in Goss textured silicon steel during AGG24–27). CSL boundaries, especially ∑9 boundaries, are believed to have somewhat lower energies than general high angle boundaries and so are less strongly inhibited by Zener drag. These ∑9 boundaries occur relatively frequently for Goss grains existing in certain types of primary texture and are considered to confer the necessary mobility advantage24,25). HEGB model demonstrated that the role of the high-energy boundary is important in the late stage of secondary recrystallization when large Goss grains consume matrix grains26,27). Evidence from Y. Hayakawa28) shows that high-energy boundaries move quickly and disappear during the grain growth process, suggesting the high-energy boundary has a high mobility. Na et al.29) have pointed out that Fe-Ga alloys and silicon steels show relatively high frequencies of occurrence of these high energy grain boundaries for both alloys, corresponding to 49.5% and 54.3%, respectively. The finding of these grain boundary character distribution (GBCD) analyses regarding both the CSL and HEGB models suggests the mechanisms that lead to abnormal growth of Goss grains in Fe-Ga alloy is similar to the mechanisms that promote abnormal growth of Goss grains in silicon steels.
Figure 9 shows the deviation of Goss grains with and without abnormal growth from ideal {110}<001> texture. Clearly, before the onset of secondary recrystallization, the average deviation angle of primary Goss grains without abnormal growth is larger than 13°. As the temperature increases to above 1218 K, Goss grains grow abnormally, resulting in an obviously decrease of the deviation angle. It shows the deviation is less than 8°. In other words, the orientation of Goss grains becomes accurate gradually during the process of abnormal grain growth. It is likely that only those Goss grains with precise orientation could grow abnormally, and form a sharp Goss texture finally.
Average angle deviation of Goss orientation and magnetostriction of samples at various temperatures.
The measured average magnetostriction values of samples annealed at various temperatures are shown as a function of temperature in Fig. 9, and the error bars show the standard deviation in the average magnetostriction. The observed magnetostriction values are averaged on three samples. As evident from Fig. 9, the magnetostriction of the sheets annealed at temperatures below 1203 K, are less than 100 ppm, and with a small deviation. As the temperature increases to above 1218 K, a sharp increase in the magnetostriction with the average value above 190 ppm is obtained due to the abnormal growth of Goss grains. But, a high deviation value above ±30 ppm is also apparent. These results indicate that the grain orientation has changed greatly at around 1218 K, but the microstructure is not uniform, because the secondary recrystallization has not been developed fully. Subsequently, the deviation is slightly dropped with the further growth of Goss grains.
From the above analysis, it could be concluded that the temperature range of secondary recrystallization of magnetostrictive Fe-Ga sheets is very narrow, which is about between 1203 and 1233 K. The change of average size and quantity of Goss grains is small before abnormal grain growth. This period may be referred to as the incubation period of the secondary recrystallization. In order to promote the development of secondary recrystallization, the incubation period should to be met the following conditions: I. Before the onset of secondary recrystallization, the size of primary grain should be essentially unchanged, ensuring the stability of the primary recrystallization microstructure. Furthermore, It is necessary to ensure that the γ-fiber texture (especially {111}<112>) is the predominant texture and is relatively stable. II.There must be a certain number of Goss oriented grains, and with a further rising of temperature, these grains will become the nuclei of secondary recrystallization. III. Goss grains maybe do not have advantages in size and quantity, but the growth rate should be obviously higher than that of other grains at a certain temperature before the coarsening of the inhibitor.
(1) For the (Fe83Ga17)99.9(NbC)0.1 alloy sheet, during process of temperature rising, abnormal grain growth does not occur before 1203 K, and the average grain size is unchanged essentially. In this period, although Goss grains do not have advantages in size and quantity, Goss grains grow faster than other orientated grains. Abnormal growth of Goss grains start at around 1218 K, and secondary recrystallization gets fully development with the temperature increasing to 1353 K.
(2) In incubation period of the secondary recrystallization, γ-fiber texture is the predominant texture, and has been relatively stable. Meanwhile, {100}<001> component is immensely decreased as compared to the matrix texture, while the {110}<001> is slightly increased. Along with start of secondary recrystallization, the texture has changed greatly, and a sharp Goss texture is obtained finally.
(3) The magnetostriction of (Fe83Ga17)99.9(NbC)0.1 alloy sheet is improved sharply due to the abnormal growth of Goss grains. When the secondary recrystallization gets fully development, a high magnetostriction up to 210 ppm with small deviation is achieved.
This study was financially supported by the National Natural Science Foundation of China (No.51271019, 51501006).
