Enhancement of ductility and improvement of abnorm

International Journal of Minerals, Metallurgy and Materials Volume 25, Number 4, April 2018, Page 444
https:///10.1007/s12613-018-1590-y
Corresponding author: Ji-heng Li E-mail: lijh@
© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2018
Enhancement of ductility and improvement of abnormal Goss grain growth of magnetostrictive Fe–Ga rolled alloys
Ji-heng Li, Chao Yuan, Xing Mu, Xiao-qian Bao, and Xue-xu Gao
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
(Received: 13 October 2017; revised: 30 November 2017; accepted: 8 December 2017)
Abstract: The influences of initial microstructures on the mechanical properties and the recrystallization texture of magnetostrictive 0.1at% NbC-doped Fe83Ga17 alloys were investigated. The directionally solidified columnar-grained structure substantially enhanced the tensile elongation at intermediate temperatures by suppressing fracture along the transverse boundaries. Compared with tensile elongations of 1.0% at 300°C and 12.0% at 500°C of the hot-forged equiaxed-grained alloys, the columnar-grained alloys exhibited substantially increased tensile elongations of 21.6% at 300°C and 46.6% at 500°C. In the slabs for rolling, the introduction of <001>-oriented columnar grains also pro-motes the secondary recrystallization of Goss grains in the finally annealed sheets, resulting in an improvement of the saturation magneto-striction. For the columnar-grained specimens, the inhomogeneous microstructure and disadvantage in number and size of Goss grains are improved in the primarily annealed sheets, which is beneficial to the abnormal growth of Goss grains during the final annealing process. Keywords: magnetostriction; iron-gallium alloys; columnar grain; ductility; abnormal grain growth
1. Introduction
Magnetostrictive Fe–Ga alloys are of great interest be-cause of their potential application in sensors, actuators, and transducers [1–4]. Single-crystalline Fe–Ga alloy exhibits a maximum magnetostriction as high as approximately 400 × 10−6 along the <001> crystal direction [2] and has a tensile strength greater than 500 MPa [3]. Compared with the cor-responding single-crystalline alloy, the highly <001>- oriented polycrystalline Fe–Ga alloy with large magneto-striction represents an effective lower-cost alternative. For application in a high-frequency alternating field, rolled thin sheets are preferred to avoid the eddy-current losses due to the high conductivity of Fe–Ga alloys. Although Fe–Ga al-loys are robust, they are not sufficiently ductile to undergo rolling deformation and tend to fracture along grain bounda-ries. It is thus desirable to enhance the rollability and to im-prove the <001> orientation in rolled Fe–Ga polycrystalline sheets.
Within the past decade, several investigations focused on element addition to improve the magnetostriction and rolla-bility of Fe–Ga alloys have been reported [5–9]. In sin-gle-crystalline alloys, trace amounts of interstitial atoms can improve the alloys’ magnetostriction [5]. By contrast, most substitutional elements will reduce magnetostriction [6–7]. Therefore, most substitutional elements are not suitable for maintaining a large magnetostriction. However, the addition of alloying elements is an effective approach to improving the mechanical properties of Fe–Ga alloys by suppressing the grain-boundary brittleness [8–9]. In previous studies, the additions of ternary elements such as B and NbC improved the ductility and rollability of Fe–Ga alloys [8–10]. NbC particles have also been reported to promote the develop-ment of the <001> orientation via the abnormal grain growth (AGG) of Goss grains [11], and single-crystal-like Goss-oriented Fe–Ga sheets have been achieved via the combined effect of sulfur annealing [12]. Thus, maximizing the magnetostrictive performance of Fe–Ga alloys requires sharply <001>-oriented Fe–Ga sheets with smaller amounts of added ternary components. However, reducing the amounts of added ternary components is expected to de-crease the ductility of the resultant Fe–Ga alloys. Because
J.H. Li et al., Enhancement of ductility and improvement of abnormal Goss grain growth of (445)
Fe–Ga alloys always fracture along grain boundaries, the ductility may be improved by a reduction in the number of transverse grain boundaries. The literature contains several reports that the columnar grains induced by directional soli-dification remarkably increased the tensile elongation of al-loys such as Ni3Al [13] and Fe–6.5wt%Si [14]. In most of the related prior works [8–12], the Fe–Ga sheets were rolled from hot-forged slabs with the equiaxed-grained micro-structure. Few studies about the influences of initial micro-structure on the mechanical properties and recrystallization textures in rolled Fe–Ga alloys have been reported.
In our previous works, instead of investigating the ab-normal grain growth and formation of strong Goss texture in the binary Fe–Ga alloy or the Fe–Ga alloy with NbC added in concentrations as high as 1.0mol%, as investigated by Na et al. [11–12], we investigated the abnormal grain growth and strong Goss texture formation in Fe83Ga17 sheets with only 0.1at% NbC added. We noted that, when the <001>-oriented columnar-grained slabs were used for roll-ing, the abnormal growth of Goss grains and sharp Goss texture could be achieved even without the combined effect of sulfur annealing [15]. We subsequently investigated the effect of different annealing atmospheres on the secondary recrystallization behavior in the rolled columnar-grained 0.1at% NbC-doped Fe83Ga17 alloys by interrupting the sec-ondary recrystallization process [16].
In the present work, the influences of equiaxed-grained and columnar-grained microstructures on the mechanical properties of 0.1at% NbC-doped Fe83Ga17 alloys are com-pared and studied. The effects of initial microstructures on the secondary recrystallization of Goss grains in rolled 0.1at% NbC-doped Fe83Ga17 alloy sheets are also discussed.
2. Experimental
Alloy ingots with nominal composition 0.1at% NbC-doped Fe83Ga17 were prepared from Fe (99.9wt% purity), Ga (99.99wt% purity), and master alloys of Nb–Fe and Fe–C. The as-cast ingot was then hot-forged to both reduce the number of casting defects and improve homogenization. Directionally solidified alloys with the same composition were prepared at a growth rate of 720 mm·h−1, and the <100> orientation was obtained along the solidification di-rection. Detailed descriptions of the directional solidification process and microstructure of the directionally solidified al-loy are reported in our previous works [17–18]. The hot-forged and directionally solidified alloys obtained herein are referred to as EG (equiaxed-grained) and CG (colum-nar-grained) specimens, respectively. To detect the me-chanical properties of the EG and CG specimens, we pre-pared rod samples from the hot-forged and directionally so-lidified alloys. The rod samples were annealed at 1100°C for 1 h and then cooled in the furnace to 730°C and held for 3 h, followed by air-cooling to room temperature. Subse-quently, the rod samples were mechanically machined into tensile specimens with a strain area of 3 mm × 15 mm. In addition, some slabs for rolling with a thickness of 18–20 mm were cut by electrical discharge machining. The slabs were hot rolled at 1150°C to 2.1 mm, followed by warm rolling at 600°C to 1.1 mm. After an intermediate an-nealing at 850°C for 5 min, the specimens were further cold rolled to a final thickness of 0.3 mm. Both the EG and CG specimens were subjected to the same rolling process. The long axes of columnar grains were arranged along the roll-ing direction when the CG specimen was rolled, as shown in Fig. 1. The as-rolled sheets of 12 mm × 16 mm with addi-tional elemental S (approximately 1 mg·cm−2) were en-closed in quartz ampoules using 0.03 MPa Ar as the pro-tecting gas. Primary recrystallization annealing was used at 850°C for 6 min, and then a continuous heating process was used from 900 to 1080°C at a rate of 0.25°C·min−1 to pro-mote secondary recrystallization. The final annealing process was carried out at 1200°C for 6 h under a flowing Ar/H2 (25vol% H2) mixed atmosphere to remove precipi-
tates and enable full grain growth.
Fig. 1. Schematic of the rolling method of CG specimens (RD means the rolling direction).
We conducted static tensile tests at a constant velocity of 1 × 10−3s−1 to investigate the influence of the specimens’ initial microstructure on their ductility. The tensile tests were carried out at room temperature, 300°C, and 500°C. Fracture morphologies of specimens were observed by scanning electron microscopy (SEM). Electron backscatter diffraction (EBSD) patterns were collected and analyzed to obtain the texture component, orientation imaging micro-scopy (OIM), inverse pole figure (IPF) images, and orienta-tion distribution function (ODF) plots. Magnetostriction was measured by strain gauges positioned along the rolling di-rection. The saturation magnetostriction was calculated as
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(3/2)λs = λ// − λ⊥, where λ// and λ⊥ are the magnetostriction with the magnetic field parallel and perpendicular to the rolling direction, respectively.
3. Results and discussion
3.1. Influence of initial microstructure on tensile ductility of 0.1at% NbC-doped Fe 83Ga 17 alloy
Fig. 2 shows the microstructures of the as-cast and the directionally solidified 0.1at% NbC-doped Fe 83Ga 17 alloys. In Fig. 2(a), numerous equiaxed grains are observed in the as-cast alloy. By contrast, some large columnar grains with a width greater than 1000 µm are distributed in the alloy fa-bricated by directional solidification, as shown in Fig. 2(b). The grain boundary is parallel to the direction of grain growth, which is related to the direction of heat dissipation. The stress–strain curves are shown in Fig. 3, and the speci-mens after the tensile tests are shown in the inset figures. In the tensile tests at room temperature, the EG and CG speci-mens both exhibit very low ductility and no yield occurs
before fracturing. However, whereas the EG alloys exhibit 0.3% elongation, the CG specimen elongation as large as 0.7%, which is close to the 0.8% elongation before the dis-continuous slip in the single-crystalline Fe 83Ga 17 alloy [3]. The increase of tensile temperature dramatically improves the ductility of both the EG and CG specimens but espe-cially improves that of the CG specimen. The tensile elon-gation of the CG specimen greatly increases to 21.6% at 300°C and further increases to 46.6% when the test temper-ature approaches 500°C. Compared with the CG specimen, the EG specimen exhibits much lower tensile elongation, with values of 1.0% and 12.0% under temperatures of 300°C and 500°C, respectively. In addition, in the CG spe-cimen, drastic serration fluctuations occur in the strain–stress curve at 300°C; these serration fluctuations weaken at 500°C. The results indicate that the colum-nar-grained microstructure can obviously improve the ten-sile ductility of 0.1at% NbC-doped Fe 83Ga 17 alloy at inter-mediate temperatures, which is important with respect to the
rolling of Fe–Ga alloys.
Fig. 2. Optical microstructures of as-cast (a) and directionally solidified (b) 0.1at% NbC-doped Fe 83Ga 17
alloys.
Fig. 3. Tensile stress–strain curves at different temperatures: (a) EG specimens; (b) CG specimens. The rod specimens after tensile tests are shown in the inset photos (RT denotes room temperature).
The fracture surfaces are shown in Fig. 4, and the macro fractographs are also given as inset figures in the top-right corners of each subfigure. In the EG specimen, intergra-nular fracture features with several slip bands appear at
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room temperature and at 300°C, as shown in Figs. 4(a) and 4(c). At 500°C, ductile fracture is observed in most areas of the macro fractographs; however, the intergranular fracture feature remains, as shown in the inset of Fig. 4(e). Compared with the EG specimen, the CG specimen with-out transverse grain boundaries exhibits no intergranular fracture feature. Figs. 4(b) and 4(d) show that the fracture morphology appears as cleavage fracture at room temper-ature and gradually changes into plastic fracture with a few dimples at 300°C. At 500°C, the fracture morphology en-tirely changes into ductile fracture with numerous dimples; the corresponding macro fractograph displays an irregular shape due to the anisotropy of the Fe–Ga alloys, as shown in Fig. 4(f).
Fig. 4. SEM fractographs of rod specimens after tensile tests: (a) EG specimens at room temperature; (b) CG specimens at room temperature; (c) EG, 300°C; (d) CG, 300°C; (e) EG, 500°C; (f) CG, 500°C. The corresponding macro fractographs are shown as in-sets in the top-right corners.
In Fe–Ga alloys, grain boundaries are considered poten-tial sources of cracks because of the low strength of grain boundaries [8–9]. Compared with the addition of 1.0at% B [8], the addition of 0.1at% NbC to the EG specimen is insuffi-cient to suppress grain-boundary brittleness, as shown in Figs. 4(a) and 4(c), which results in low tensile elongation at room temperature and at 300°C. As the mobility of disloca-tions increases at 500°C, the ductile fracture feature emerges, accompanied by an increase of tensile elongation. Com-pared with the EG specimen, most of the transverse grain boundaries, which are the potential sources of cracks, are removed from the CG specimen; thus, the tensile elongation remarkably increases in the CG specimen. The dislocations have low mobility at room temperature, which leads to cleavage fracture and to low tensile elongation. The mobili-ty of dislocations greatly increases with increasing tempera-ture, and the tensile elongation substantially increases at 300°C and 500°C, without crack initiation at the boundaries. In addition, some large-angle boundaries along the tensile axis exist in the CG specimen. These boundaries stop the
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slips from moving into the adjacent grains, and cracks oc-cur at the boundaries, as shown in Fig. 4(d). The crack propagation is accompanied by a sudden decrease in stress, which is likely responsible for the large serrations in the stress–strain curves of the CG specimen [13]. By taking into account transversal grain boundaries, we considered that the intergranular fracture is substantially suppressed in the CG specimen, which results in the observed large im-provement of tensile elongation of the 0.1at% NbC-doped Fe 83Ga 17 alloy.
3.2. Influences of initial microstructure on recrystallization texture and magnetostriction in rolled 0.1at% NbC-doped Fe 83Ga 17 alloy sheets
The evolution of the recrystallization texture is closely related to the magnetostrictive performance of Fe–Ga rolled sheets. Among all of the recrystallization textures, the {111}<110> and {111}<112> textures often appear as the primary recrystallization texture, whereas Goss {110}<001> and cube {100}<001> textures are the most likely second-ary recrystallization texture. For the rolled 0.1at%NbC- doped Fe 83Ga 17 alloy sheets, the OIM images of grains within 15° deviation and ODF plots (φ2 = 45°) for the pri-marily recrystallized EG and CG specimens are shown in Fig. 5. In the ODF plots, the intensity of preferred Goss tex-tures is very weak in both specimens; however, strong {100}<001> and γ-fibered textures appear in the CG speci-men, as shown in Figs. 5(b) and 5(d). The area fractions of {110}<001>, {100}<001>, {111}<112>, and {111}<110> textures in the CG specimen are greater than those in the EG specimen, as shown in the insets of Fig. 5. In addition, for the likely nucleus for AGG, the Goss grains have no ob-vious advantages in size compared with other grains, as shown in Figs. 5(a) and 5(c). Meanwhile, Goss grains within 15° deviation cover less than 4% area in both the EG and CG primarily recrystallized specimens. In the CG specimen, although the Goss grains have no advantage in number, the area fraction of cube grains is approximately 1.5 times greater than that in the EG specimen. Figs. 6(a) and 6(b) show the grain size distributions in the primary recrystal-lized EG and CG specimens. The grain size distribution in the EG specimen shows a longer tail than that in the CG specimen, indicating a greater D max /D avg (D max : maximum diameter; D avg : average diameter) ratio. An evident disad-vantage in size is observed for Goss grains in the EG speci-men, as shown in Fig. 6(c). The maximum grain size of Goss grains is only 43 μm, which is much smaller than that of grains with other random orientations. By contrast, a more uniform grain structure is observed in the CG speci-men. In the case of the CG specimen, the maximum grain size is 57 μm and the average grain size is 9.5 μm (1253 grains). Furthermore, compared with the γ-fibered and ran-dom grains, although the disadvantage in size is also ob-served for Goss grains in the CG specimen, the maximum grain size of Goss grains is larger than that of the γ-fibered
grains, as shown in Fig. 6(d).
Fig. 5. OIM images of grains within 15° deviation from ideal crystal orientation on the whole thickness in an RD–ND section of the primarily recrystallized sheets: (a) EG specimen; (c) CG specimen. ODF plots (φ2 = 45°): (b) EG specimen; (d) CG specimen. ND means the normal direction and RD means the rolling direction.
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Fig. 6. Grain size distributions of all grains and the main texture components in the primarily recrystallized sheets: (a) and (c) EG specimen; (b) and (d) CG specimen.
The secondarily recrystallized Goss grains are important for maximizing magnetostrictive performance [12,15]. The optical metallography photos of the finally annealed 0.1at% NbC-doped Fe 83Ga 17 alloy sheets are shown in Fig. 7. Some abnormally grown grains with sizes of several millimeters are observed in both the secondarily recrystallized EG and the secondarily recrystallized CG specimens because of the combined effects of Nb-rich precipitates and sulfur anneal-ing. However, numerous normally grown grains remain in the EG specimen, whereas the CG specimen becomes sin-gle-crystal-like after the final annealing. In addition to the abnormal grain growth, the orientation of abnormally grown grains also determines the magnetostriction of the seconda-rily recrystallized sheets. To characterize the grain orienta-tion, we captured and analyzed EBSD patterns correspond-ing to the area within the white rectangle in Fig. 7. The IPF maps in Fig. 8 show that, in the scanning area, the abnor-mally grown (110) grains cover 100% area in the CG spe-cimen, whereas many normally grown non-(110) grains re-main in the EG specimen. This result is consistent with the results in Fig. 7. Magnetostrictive performance strongly de-pends on the extent of deviation from the preferred orienta-tion. Fig. 9 shows the deviation angle profiles from ideal Goss orientation. Goss grains within 20° deviation cover only 53.1% area in the secondarily recrystallized EG speci-
men, and two deviation peaks appear at 10° and 21°. Com-pared with the EG specimen, Goss grains within 20° devia-tion cover approximately 100% area in the CG specimen; the deviation angle of abnormally grown grains is approx-imately 9° and 15° in the CG specimens, respectively. The results demonstrate that the initial columnar-grained micro-structure of the feedstock substantially promotes the sec-ondary recrystallization of Goss grains during the final an-
nealing process.
Fig. 7. Optical metallography photos of the secondarily re-crystallized sheets: (a) EG specimen; (b) CG specimen. EBSD patterns of grains within the white rectangles were captured for analysis of the orientation.
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Fig. 8. IPF images corresponding to the region within the white rectangles of the sample in Fig. 7: (a) EG specimens; (b) CG specimens. The red, green, and blue colors indicate the (001), (011), and (111) grains, respectively. TD means the
transverse direction.
Fig. 9. Deviation angle profile from ideal {110}<001> orienta-tion in the secondarily recrystallized sheets.
The average values of saturation magnetostriction, (3/2)λs = λ// − λ⊥, for the primarily and secondarily recrystallized 0.1at% NbC-doped Fe 83Ga 17 alloy sheets are shown in Fig. 10. Compared with the (3/2)λs values for the EG specimen, those for the CG specimen are larger. The average value of (3/2)λs from the test samples is 67.5 × 10−6 in the primarily recrystallized EG specimen. An increase of approximately 23 × 10−6 is obtained in the primarily recrystallized CG spe-cimen, which is mainly attributed to the more cube-textured grains in these specimens. After the final annealing, the av-erage values of (3/2)λs both greatly increase to values great-er than 200 × 10−6 because of the secondary recrystallization. In the secondarily recrystallized EG specimen, the value of (3/2)λs approaches 201 × 10−6. However, a large deviation of ±20 × 10−6 is also observed, which is attributed to the sec-ondary recrystallization being incomplete in the EG speci-men, which contains numerous normally grown grains with other orientations. The Fe–Ga alloy exhibits a substantial magnetostrictive anisotropy, where the largest magnetostric-tion coefficient is approximately 400 × 10−6 along the <100> direction but is only approximately 40 × 10−6 along the <111>direction [2,19]. Thus, the magnetostriction of the secondarily recrystallized EG specimen is weak. By contrast, in the secondarily recrystallized CG specimen, the average value of (3/2)λs is as high as 246 × 10−6, which represents an increase of approximately 45 × 10−6 compared with the av-erage value for the EG specimen. Additionally, a small dev-iation in magnetostriction is observed in the case of the CG specimen. A single-crystal-like microstructure and strong Goss texture with accurate orientation are responsible for the higher value of (3/2)λs and lower deviation in magneto-
striction in the secondarily recrystallized CG specimen.
Fig. 10. Average values of saturation magnetostriction, (3/2) λs = λ// − λ⊥, for the primarily and secondarily recrystallized sheets.
Abnormal grain growth in grain-oriented silicon steel has been widely investigated, and numerous factors can affect the abnormal growth of Goss grains. The most common trigger for abnormal grain growth is the inhibition effect by fine particles of secondary phases such as AlN and MnS [20–21]. For the rolled Fe–Ga sheets, the dispersive NbC precipitates are the proper inhibitor. In addition, sulfur annealing can also promote abnormal grain growth in the Fe–Ga sheets [12,16]. Therefore, the combined effects of NbC precipitates and sulfur annealing result in the secondary recrystallization both in the EG and CG rolled sheets. Notably, after the same rolling and annealing protocols, the secondary recrystalliza-tion in the CG specimen is much more complete than that in the EG specimen. The improvement of abnormal growth of Goss grains can be attributed to the initial columnar-grained microstructure. Although the Goss-oriented nucleus has a close area fraction, there is an obvious increase of the γ-fibered grains ({111}<112> and {111}<110>) in the pri-marily recrystallized CG specimen. The primary recrystalli-zation texture of {111}<112> is considered to be beneficial for the abnormal growth of Goss grains in the grain-oriented silicon steel [22]. The {111}<112>-oriented grains are ro-
J.H. Li et al., Enhancement of ductility and improvement of abnormal Goss grain growth of (451)
tated by approximately 35° with respect to the {110}<001> orientation, which produces boundaries close to the Σ9 coincident orientation. Thus, greater grain-boundary mobil-ity can be obtained at the boundaries between {111}<112> and Goss grains. Therefore, the primary recrystallization matrix with the preferred {111}<112> texture provides a favorable surrounding for the abnormal growth of Goss grains. In addition, the heredity of the initial orientation leads to a strong cube texture in the primarily recrystallized CG specimen. Zhang et al. [23] have reported that, in the late stage of secondary recrystallization, the growth advan-tage of Goss grains is more obvious than that of cube grains in the columnar-grained silicon steel. As a result, the intro-duction of <001>-oriented columnar grains into the slabs before rolling improves the secondary recrystallization of Goss grains and greater magnetostriction is attained in the CG specimens.
4. Conclusion
The columnar-grained microstructure greatly enhances the tensile elongation of 0.1at% NbC-doped Fe83Ga17 alloy at in-termediate temperatures by removing transverse boundaries perpendicular to the tensile direction. The tensile elongation in the hot-forged equiaxed-grained specimen is 1.0% at 300°C and 12.0% at 500°C, respectively. The introduction of a columnar-grained structure greatly increases the tensile elongation to 21.6% at 300°C and 46.6% at 500°C. The ini-tial columnar-grained microstructure is also beneficial for the secondary recrystallization of Goss grains in 0.1at% NbC-doped Fe83Ga17 alloy rolled sheets. Compared with the equiaxed-grained specimen, the columnar-grained specimen exhibits a remarkable increase of γ-fiber and cube textures in its primary recrystallization textures. The inhomogeneous primarily recrystallized microstructure and disadvantage in number and size of Goss grains are improved in the colum-nar-grained specimen, which is beneficial for the secondary recrystallization of Goss grains during the final annealing process. Higher saturation magnetostriction is attained in the secondarily recrystallized columnar-grained specimens be-cause of the single-crystal-like microstructure and the im-provement of Goss orientation. Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (No. 51501006), State Key Laboratory for Advanced Metals and Materials (No. 2017Z-11), the Fundamental Research Funds for the Central Universities (No. FRF-GF-17-B2) and partly supported by a scholarship from the China Scholarship Council. References
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