稀有金属(英文版) 2017,36(11),886-893

Inhibition force of precipitates for promoting abnormal grain growth in magnetostrictive Fe₈₃Ga₁₇-(B,NbC) alloy sheets

Ji-Heng Li Wen-Lan Zhang Chao Yuan Xiao-Qian Bao Xue-Xu Gao

State Key Laboratory of Advanced Metals and Materials,University of Science and Technology,Beijing

收稿日期：21 June 2017

基金：financially supported by the National Natural Science Foundation of China(Nos.51501006 and 51271019);

Inhibition force of precipitates for promoting abnormal grain growth in magnetostrictive Fe₈₃Ga₁₇-(B,NbC) alloy sheets

Ji-Heng Li Wen-Lan Zhang Chao Yuan Xiao-Qian Bao Xue-Xu Gao

State Key Laboratory of Advanced Metals and Materials,University of Science and Technology

Abstract：

Inhibition force of precipitate particles for promoting abnormal grain growth in magnetostrictive Fe₈₃Ga₁₇-(B,NbC) alloy sheets was investigated in this study. After a continuous heating and a high-temperature annealing, the Fe₈₃Ga₁₇ + 0.5 at% B alloy sheets do not occur significant abnormal grain growth. Correspondingly,textures of {111}<112> and {100}<001 > in addition to the Goss texture are obtained in the final annealed alloy sheets. By contrast, after the same annealing processes, the size of {110} textured grains is very large in the final annealed Fe₈₃Ga₁₇ + 0.5 at% NbC alloy sheets due to the abnormal grain growth, which results in a sharp Goss texture. BN precipitates were introduced into Fe₈₃Ga₁₇+0.2 at% B alloy sheets by nitriding annealing at 800 ℃ for 2 min under NH₃ atmosphere. The abnormal grain growth of Goss grains is achieved in 0.2 at% B-doped Fe₈₃Ga₁₇alloy sheets after a high-temperature annealing, which is attributed to the enhanced inhibition force by introducing BN precipitates. During the recrystallization annealing process, Fe₂B precipitates is easy to coarsen and decompose at high temperature due to the low thermal stability,resulting in a decrease or even disappearance of the inhibition force. For NbC and BN precipitates, the thermal stability and hardness of particles are both better than those of Fe₂B precipitates, leading to strong inhibition force.Because of the preferred Goss texture, the magnetostriction of 2.05 × 10^-4 and 1.81 × 10^-4 is obtained in the secondary recrystallized Fe₈₃Ga₁₇ + 0.5 at% NbC and Fe₈₃Ga₁₇ + 0.2 at% B alloy sheets, respectively.

Keyword：

Fe-Ga alloy; Magnetostriction; Grain growth; Inhibition force; Precipitates;

Author： Xue-Xu Gao e-mail:gaox@skl.ustb.edu.cn;

Received： 21 June 2017

1 Introduction

Since 2000,Fe-Ga alloys have received increasing attention due to the high magnetostriction under low saturation magnetic field [ 1, 2, 3] .Single-crystal alloys of Fe-Ga exhibit a maximum saturation magnetostriction of～4×10^-4 under a magnetic flied smaller than15.87 kA·m^-1.In addition,Fe-Ga alloys are robust,which is not exhibited by other smart materials,such as PZT,Ni-Mn-Ga or Tb-Dy-Fe alloys [ 4, 5] .Fe-Ga alloys exhibit a significant magnetostrictive anisotropy with the largest magnetostriction coefficient of above 4.0×10^-4along<100>direction and only about 0.4×10^-4 along<111>direction [ 6] .As eddy current losses are taken into account,it is desirable to obtain textured alloy sheets with preferred[100]orientation.In order to produce the thin and<001>textured sheets,efforts have been made to produce textured sheets by rolling and secondary crystallization processes [ 7, 8, 9, 10, 11, 12] .

As well known,it is an effective way to develop abnormal grain growth (AGG) that the dispersed secondphase particles are obtained in matrix alloy,because of the inhibiting effect on normal grain growth (NGG) during the recrystallization annealing process.Precipitated particles impede the normal grain growth by interacting with grain boundaries,which is referred to as“Smith-Zener pinning.”Zener pinning force (F) could be written as the following equation.

whereσis grain boundary surface tension,f and r are content in volume fraction and radius of the precipitated particles,respectively.Zener pinning force is opposite to the driving force for grain boundary movement.The smaller size and the larger content of the precipitated particles make a stronger ability of pinning the grain boundary or grain growth inhibition.

Up to now,the secondary recrystallization,also named abnormal grain growth,has been considered as the most effective way to achieve the sharp<001>orientation and large magnetostriction along the rolling direction in the rolled Fe-Ga alloy sheets [ 13, 14, 15, 16, 17, 18] .Goss texture{110}<001>and cube texture{100}<001>,which are usually obtained by the secondary recrystallization,are the ideal textures to achieve large magnetostriction in Fe-Ga alloy sheets.However,γ-textures ({111}<112>and{111}<110>) deteriorate the magnetostriction of Fe-Ga alloy sheets.In previous studies,ternary addition elements,such as B and NbC,were introduced to improve the ductility and rollability [ 10, 19] .Meanwhile,NbC particles could also promote the development of<001>orientation by the abnormal grain growth of Goss{110}<001>grains,and single-crystal-like Goss-oriented Fe-Ga sheets were achieved with the combined effect of sulfur annealing [ 14] .After annealing at high temperature (1100-1200℃),near cube texture ({100}<001>) was obtained in B-alloyed FeGa sheets,in which Fe₂B precipitates may play a role as inhibitors [ 20, 21] .Obviously,the influences of B and NbC precipitates on abnormal grain growth and texture evolution are different.However,few works focus on the inhibitor force of different precipitates and their effect on recrystallization texture evolution in magnetostrictive FeGa alloy sheets.

In this work,inhibition force of different precipitates for promoting abnormal grain growth in magnetostrictive Fe₈₃Ga_17-(B,NbC) alloy sheets was investigated.BN precipitates were introduced into the boron-added Fe₈₃Ga₁₇alloy sheets by nitriding annealing.The effects of Fe₂B,NbC and BN precipitates on abnormal grain growth were compared and studied,which could provide theoretical basis for the choice of inhibitors in magnetostrictive Fe-Ga alloy sheets.

2 Experimental

Fe₈₃Ga₁₇+0.2 at%B,Fe₈₃Ga₁₇+0.5 at%B and Fe₈₃Ga₁₇+0.5 at%NbC alloys were prepared from pure Fe (99.9 wt%),pure Ga (99.99 wt%) and master alloys of Nb-Fe,B-Fe and Fe-C.The columnar-grained ingot was produced by directional solidification method.The slabs with～18 mm in thickness were obtained from the directional solidified ingot by electrical discharge machining and then were rolled to the final thickness of～0.3 mm.A detail description of rolling process could be found in Ref [ 16] .The as-rolled sheets,with size of 12 mm×16 mm,were annealed at 850℃for 6 min using 0.03 MPa Ar as protecting gas,named as“primary recrystallization.”After the primary recrystallization,samples underwent a continuous heating process from 900 to 1080℃with a rate of0.2℃·min^-1.The final annealing process was undertaken at 1200℃for 6 h under flow Ar/H₂ (H₂ content of25vol%) mixed atmosphere.In order to study the effect of BN precipitates on promoting abnormal grain growth,the as-rolled sheets of Fe₈₃Ga₁₇+0.2 at%B alloy were annealed at 800℃for 2 min under NH₃ atmosphere,named as“nitriding annealing.”Then,the nitrided sheets were heated from 900 to 1080℃at rate of 0.25℃·min^-1.The final high-temperature annealing was undertaken at1250℃for 4 h under flow Ar/H₂ (H₂ content of 25vol%)mixed atmosphere.

Microstructures of the directional solidified alloys and the primary crystallized sheets were observed by optical microscope (OM,Axio Imager M2M,Carl Zeiss) and scanning electron microscope (SEM,Zeiss Supra 55,Carl Zeiss).Energy-dispersive spectroscopy (EDS,Zeiss Supra55,Carl Zeiss) was employed to identify the composition of precipitates.The orientation distributions and grain configurations of samples were determined by electron backscattered diffraction (EBSD,Zeiss Supra 55,Carl Zeiss) system.Magnetostriction measurements were taken along the rolling direction (RD) by using strain gauges.Saturation magnetostriction was calculated by (3/2)λ_s=λ_//-λ_⊥,whereλ_//andλ_⊥were the maximum magnetostriction when magnetic field parallel and perpendicular to RD was applied,respectively.

3 Results and discussion

The longitudinal optical microstructures of directional solidified Fe-Ga alloys doped with 0.5 at%B and NbC are shown in Fig.la,b.From OM images,it can be found that the grain growth direction is approximately parallel to the solidification direction,and the size of a single columnar crystal is very big.The curved grain boundaries could be attributed to the dragging effect of precipitates on the boundaries.In addition,it can be seen that a large amount of precipitates preferentially distributes along the grain boundaries in the 0.5 at%B-doped alloys.For Fe₈₃Ga₁₇+0.5 at%NbC alloy,the precipitates are located both in the grains and at grain boundaries.Backscattered electron (BSE) images and EDS were employed to further identification of precipitates.The results shown in Fig.1c,e demonstrate that the B-rich precipitates with composition near (FeGa)₂B and diameter of 2-5μm distribute along the gain boundaries,and the matrix alloy is homogeneous,which indicates that B is preferentially segregated at grain boundaries in the B-doped alloy.For comparison,a lot of Nb-rich precipitates,composition near NbC and a few with bar-shaped,can be observed inside grains in Fe₈₃Ga_17-0.5 at%NbC alloy,as shown in Fig.1d,f.Nb atom has a low solubility inα-Fe and tends to precipitate with C,N in metallic materials.The precipitate of NbC is widely used in steels,and its effective precipitation temperature in ferrite is thought to be 600-750℃ [ 22, 23] .

Fig.1 OM images of directionally solidified alloys:a Fe₈₃Ga₁₇+0.5 at%B and b Fe₈₃Ga₁₇+0.5 at%NbC (arrows showing direction of directional solidification);BSE images and EDS profiles of precipitates:c,e Fe₈₃Ga₁₇+0.5 at%B and d,f:Fe₈₃Ga₁₇+0.5 at%NbC

EBSD patterns were captured and analyzed to obtain the inverse pole figure (IPF) and the orientation distribution function (ODF).Figure 2 displays the microstructures of the primary recrystallized sheets.IPF images show the through-thickness structures on lateral face.After the primary recrystallization annealing at 850℃for6 min,the Fe₈₃Ga₁₇+0.5 at%NbC alloy sheet is completely recrystallized and composes of the equiaxed grains;meanwhile,the microstructure is homogeneous.In contrast,the microstructure of Fe₈₃Ga₁₇+0.5 at%B sample is remarkably inhomogeneous.One is the fine recrystallized grains located at surface layer,while the other one is the elongated non-recrystallized grains located at center layer.During the rolling processes,the stresses applied at surface layer and center layer are different.Because of the coarse initial grains,grains at surface layer are deformed heavily by the shearing stress due to the friction between roll and sheet,while grains at center layer are relatively slightly deformed under the compressive stress.After the primary annealing,the deformed grains with high stored energy at surface layer can easily recrystallize,while those at center layer recrystallize partially or remain recovered due to the lower stored energy.On the one hand,the size of recrystallized grains is closely related to the deformed grains where they are nucleated.The higher stored energy of the deformed grains can provide the higher nucleation rate and lead to smaller recrystallized grains.On the other hand,second-phase precipitates could promote the recrystallization,because there are more inhomogeneous deformation zones distributed around these precipitate particles,where the substructure is unstable due to the uneven distribution of dislocations.However,secondphase precipitates can also inhibit the grain growth through pinning the movement of grain boundaries.According to the results shown in Fig.2,it could be considered that the pinning force of NbC particles is larger than that of (FeGa)2B particles.

Fig.2 OM images and IPF images of primarily annealed sheets:a,c Fe₈₃Ga₁₇+0.5 at%B alloy and b,d Fe₈₃Ga₁₇+0.5 at%NbC alloy(grain boundaries:white lines being 0°-15°and black lines being≥15°;ND,normal direction;RD,rolling direction)

As well known,the primary recrystallized microstructure and texture are important for the final formation of Goss texture by abnormal grain growth.The primary annealed sheets went through a continuous heating process from 900 to 1080℃and holding at 1200℃for 6 h to induce the abnormal grain growth.IPF maps and ODF plots for the final annealed sheets are shown in Fig.3.Most of the grains in Fe₈₃Ga₁₇+0.5 at%B alloy sample are not very large,as shown in Fig.3a,which indicates that the abnormal grain growth does not occur.Correspondingly,Fig.3b displays the textures of{111}<112>and{100}<001>in addition to Goss texture.By contrast,Fig.3c shows that the size of{110}textured grains is very large in the final annealed Fe₈₃Ga₁₇+0.5 at%NbC alloy sheets due to the abnormal grain growth,which results in a sharp Goss texture,as shown in Fig.3d.SEM images for the cross section of the final annealed sheets are shown in Fig.4.As shown in Fig.4a,B-rich precipitates could hardly be found after annealing at 1200℃for 6 h under H₂/Ar atmosphere due to the dissolution or decomposition of B-rich precipitates at high temperature.Comparatively,some precipitates are still retained in the final annealed Fe₈₃Ga₁₇+0.5 at%NbC alloy sheets,as shown in Fig.4b.The obvious abnormal grain growth of Goss grains and the sharp Goss texture are ascribed to the effect of the stable NbC precipitated particles on abnormal grain growth.In previous studies with Fe-Ga-B alloy rolled sheets,Sun et al. [ 21] reported that significant abnormal grain growth of Goss grains was not observed in the(Fe₈₁Ga₁₉)₉₈B₂ rolled sheets after annealing at 1200℃,which may be attributed to the weak inhibition force of Fe₂B precipitate particles.Na and Flatau [ 24] found that a dominate Goss texture was obtained through surface energy-induced selective grain growth controlled by segregated sulfur on the surface in (Fe_81.3Ga_18.7)+0.5 at%B+0.005 at%S alloy sheets,and they observed the formation of FeS phase on the surface.It is considered that in addition to the surface energy effect,a probable enhanced inhibition force was achieved by introducing segregated sulfur element [ 25] .

In silicon steel,in order to increase the inhibition force,more AlN precipitates were obtained through nitriding treatment [ 26, 27] .The combination of nitrogen and boron elements easily leads to the formation of BN precipitates in metallic materials.BN particle has high hardness and good thermal stability.As mentioned about the enhanced inhibition force by introducing other phase precipitate particles,the as-rolled sheets of Fe₈₃Ga₁₇+0.2 at%B alloy were annealed at 800℃for 2 min under NH₃ atmosphere(as named nitriding) in order to study the effect of BN precipitates on promoting abnormal grain growth.Figure 5shows microstructure of nitrided Fe₈₃Ga₁₇+0.2 at%B alloy sheets.Many precipitate particles with diameter of1-2μm are observed in nitrided sheets,but these particles distribute unevenly,as shown in Fig.5a.From EDS profiles and composition of precipitates shown in Fig.5b,it is found that compared with that in the as-directional solidified alloy,obvious changes in morphology,size,composition and distribution of the B-rich precipitates are observed in the nitrided sheets.The nitrogen content in the nitrided sheets is determined by oxygen-nitrogenhydrogen determinator (ONH 2000),and the concentration reaches 0.02 wt%,which is much higher than that of nonnitrided specimen.Therefore,it is considered that introducing BN precipitates in Fe₈₃Ga₁₇+0.2 at%B alloy sheets could be achieved by nitriding annealing at 800℃for 2 min under NH₃ atmosphere.

Fig.3 IPF images and ODF plots (φ₂=45°) of final annealed sheets:a,b Fe₈₃Ga₁₇+0.5 at%B alloy and c,d Fe₈₃Ga₁₇+0.5 at%NbC alloy(grain boundaries:white lines being 0°-15°and black lines being≥15°;TD transverse direction)

Fig.4 SEM images of final annealed sheets:a Fe₈₃Ga₁₇+0.5 at%B and b Fe₈₃Ga₁₇+0.5 at%NbC

In order to develop the abnormal grain growth of Goss grains,the nitrided Fe₈₃Ga₁₇+0.2 at%B alloy sheets were heated from 900 to 1080℃at rate of 0.25℃·min^-1and then underwent a high-temperature annealing at1250℃for 4 h under flow Ar/H₂ mixed atmosphere.Figure 6 shows the microstructure and texture of the final annealed sheet.Some large grains are observed from the optical image,as shown in Fig.6a,indicating the significant abnormal grain growth.IPF image and corresponding ODF plot indicate the abnormal grain growth of Goss grains,which results in a sharp Goss texture in the final annealed sheet,as shown in Fig.6b,c.

Fig.5 a SEM images of nitrided Fe₈₃Ga₁₇+0.2 at%B alloy sheets and b EDS profiles of precipitates (inset being enlarge image of a B-rich precipitate and chemical composition)

Fig.6 a OM image,b IPF image and c ODF plot (φ₂=45°) of final annealed Fe₈₃Ga₁₇+0.2 at%B alloy sheets

Unlike the work of Na et al. [ 28] ,where inpidual solid NbC particles (less than 10μm in diameter) were added to the melt and stirred sufficiently to be uniformly dispersed in the Fe-Ga matrix while melting,the endogenous precipitates as inhibition phase were employed in our studies.In systems with precipitated particles,it is important to precisely control the content and size of particles that precipitate from a homogeneous alloy matrix.Precipitation behavior of NbC phase in Fe-Ga and Fe-Ga-Al alloy sheets has been investigated in our previous works [ 29, 30] ,and the content and size of precipitated particle were discussed.In addition to content and size of particles,the thermal stability and hardness of precipitates also have great influence on the inhibition force for promoting abnormal grain growth during the recrystallization annealing processes.Table 1 gives some properties of the precipitates used in this work.It is seen that the decomposition temperature of Fe₂B phase is around 950℃,much lower than those of NbC and BN precipitates phases,indicating a low thermal stability for Fe₂B precipitates.So,Fe₂B precipitates are easy to coarsen and decompose at high temperature during the continuous heating process,resulting in a decrease or even disappearance of inhibition force.Consequently,after the final annealing,abnormal grain growth is not observed to occur in the B-alloyed Fe₈₃Ga₁₇ alloy sheets.By contrast,for NbC and BN precipitates,the thermal stability and hardness of particles are both better than those of Fe₂B precipitates,as shown in Table 1,which could lead to strong inhibition force.As a result,the abnormal grain growth and Goss texture are both achieved in NbC-doped and nitrided B-doped Fe₈₃Ga₁₇alloy sheets.

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Table 1 Some properties of precipitates used in this work

Fig.7 Magnetostrictive curves of final annealed sheets

Figure 7 shows magnetostrictive curves of NbC-doped and nitrided B-doped Fe_s3Ga₁₇ alloy sheets after hightemperature annealing.Because of the preferred Goss texture,the saturation magnetostriction of 2.05×10^-4 is observed in the Fe₈₃Ga₁₇+0.5 at%NbC alloy sheets.For the nitrided B-doped Fe₈₃Ga₁₇ alloy sheets,the magnetostriction of 1.81×10^-4 is a little smaller than that of NbC-doped alloy sheets,which could be attributed to the slight deviation from ideal Goss orientation in B-doped alloy sheets,as shown in Fig.6c.Moreover,there is little hysteresis in the magnetostriction vs.magnetic field,resulting in an almost reversible magnetostriction.

4 Conclusion

After the continuous heating and high-temperature annealing,the Fe₈₃Ga₁₇+0.5 at%B alloy sheets do not occur significant abnormal grain growth.Correspondingly,textures of{111}<112>and{100}<001>in addition to Goss texture are obtained in the final annealed sheets.By contrast,after the same annealing processes,the size of{110}textured grains is very large in the final annealed Fe₈₃Ga₁₇+0.5 at%NbC alloy sheets due to the abnormal grain growth,resulting in a sharp Goss texture.BN precipitates were introduced into Fe₈₃Ga₁₇+0.2 at%B alloy sheets by nitriding annealing at 800℃for 2 min under NH₃ atmosphere.The inhibition force is improved by introducing BN precipitate particles,which promotes the abnormal grain growth and leads to a strong Goss texture.

During the recrystallization annealing process,Fe₂B precipitates is easy to coarsen and decompose at high temperature due to the low thermal stability,resulting in a decrease or even disappearance of inhibition force.By contrast,for NbC and BN precipitates,the thermal stability and hardness of particles are both better than those of Fe₂B precipitates,leading to strong inhibition force.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos.51501006 and51271019).

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