- Email: [email protected]
Microstructural characteristics and in situ reinforcement in NbC-doped Fe81Ga19 magnetostrictive alloys
Materials and Design 88 (2015) 1342–1346
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Microstructural characteristics and in situ reinforcement in NbC-doped Fe81Ga19 magnetostrictive alloys Aili Sun, Jinghua Liu ⁎, Chengbao Jiang ⁎ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 31 December 2014 Received in revised form 22 August 2015 Accepted 30 August 2015 Available online 3 September 2015 Keywords: Magnetostriction NbC Grain refinement Particulate reinforced composites Secondary-phase reinforcement
a b s t r a c t The microstructural phase distribution and the origin of mechanical enhancement of NbC-doped Fe–Ga magnetostrictive alloys are not very clear. In this study, a detailed investigation into the microstructural characteristics and their effects on the mechanical properties has been performed on NbC-doped Fe81Ga19 alloys. Increasing NbC causes remarkable grain refinement of the Fe-Ga alloy matrix. Rod-like NbC particles precipitating in Fe–Ga grain matrix and NbC dendrite clusters segregating at Fe–Ga grain boundaries are observed. As a result, NbCparticle / Fe81Ga19 and (NbCparticle + NbCdendrite) / Fe81Ga19 magnetostrictive composites are obtained. The mechanical properties are found to be significantly improved by combined effects of grain-refinement strengthening and precipitate reinforcement, generating a maximal tensile strength of 620 MPa and an elongation ratio of 0.9%. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Magnetostriction is a magnetoelastic phenomenon in which a magnetic material changes its dimension upon the magnetizing as a function of the applied magnetic field. Magnetic materials with large magnetostriction are expected to be used in applications of sensors, actuators, positioning devices, and damping devices [1,2]. Among various magnetostrictive materials, the rare-earth-free Fe–Ga (Galfenol) alloys possess great advantage in practical applications due to its high magnetostriction, low saturation field, and low costs [3,4]. A large magnetostriction up to 400 ppm with a low saturation field was reported in Fe–Ga bulk single crystals along the [001] direction [5]. In recent years, to develop rolled Fe–Ga sheets with desired highly-orientated textures shows a growing tendency [6–8]. In these textured sheets, the large magnetostriction along the specific crystallographic orientations can be obtained and, at the same time the eddy current loss found in bulk state can be suppressed efficiently. However, Fe–Ga alloys still have their drawback in terms of mechanical properties. The binary Fe–Ga alloys usually consist of coarse grains and the grains have an intrinsic brittleness due to the strong p–d covalent bonding between Ga and Fe atoms [9,10]. Relatively low fracture strength and ductility are thus observed during the intergranular and/ or brittle fractures [10,11]. These make Fe–Ga alloys cannot bear high and cyclic loading in use as a key material. Besides, during the rolling process for thin sheets serious edge cracks along the grain boundaries
⁎ Corresponding authors. E-mail address: [email protected] (J. Liu), [email protected] (C. Jiang).
http://dx.doi.org/10.1016/j.matdes.2015.08.150 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
procreate as well, showing a low deforming ability [12]. In order to solve these issues, ternary additives, such as NbC, ZrC and MoC carbides, have been added into the Fe–Ga alloys [10,13]. The mechanical properties can be thus enhanced in different degrees. However, there are few reports on the detailed microstructural phase distribution of carbidedoped Fe–Ga alloys and on the relationship between microstructures and mechanical properties. In this study, in order to in depth characterize the microstructural phase distribution of carbide-doped Fe–Ga alloys and to better understand the microstructural origin of strengthening mechanism, Fe81Ga19 alloy was chosen as the base alloy, into which NbC powders have been added with percentage proportions of x = 0, 0.5 and 1.0. The experimental results reveal the co-existence of grain refinement of Fe–Ga matrix, rod-like NbC precipitate particles in Fe–Ga grains, and/or NbC dendrite clusters at Fe–Ga grain boundaries. It is interesting to obtain NbCparticle / Fe81Ga19 and (NbCparticle + NbCdendrite) / Fe81Ga19 magnetostrictive composites, showing different behaviors of mechanical enhancements. 2. Experimental procedures (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloy ingots with a diameter of 65 mm were prepared by arc melting Fe (99.99 wt.%), Ga (99.99 wt.%) and NbC powers under argon atmosphere. Each ingot was re-melted four times to assure the homogeneity. These ingots were annealed at 1373 K for 3 h and then slowly cooled to room temperature. Room-temperature structures of the alloy ingots and starting NbC powders were detected using Rigaku D/Max2200 X-ray diffractometer with Cu–Kα radiation. Microstructures of samples, morphology of NbC powers and fractographs of the tensile samples were investigated
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
by optical microscope (OM) and scanning electron microscopy (SEM) with energy-dispersive spectrum (EDS), and transmission electron microscope (TEM). The average grain size was calculated by using image analysis software Image-Pro Plus. The lattice constant was calculated by fitting the XRD data using Jade-software. Magnetostrictions of the samples with dimensions of 14.00 × 7.00 × 6.00 mm were measured using the standard strain gauge. Five tensile samples of each alloy with a gauge length 26 mm were machined by wire-electrode cutting from the center of ingots. The tensile samples were subjected to room-temperature tensile test until fracture of the sample, using a computer-controlled Instron-8801 testing machine with a strain rate of 4.8 × 10−5 s−1. The engineering stress–strain data were presented in this study. 3. Results and discussion Fig. 1(a) displays the XRD patterns of the (Fe81Ga19)100 − x(NbC)x alloys. The main phase of Fe81Ga19 alloy shows a typical A2 structure, similar to the previous results in Fe–Ga alloys [6,13,14]. The lattice constant of A2 main phase shows a clear increasing tendency with increasing NbC addition, as shown in Fig. 1(b). This gradual increase gives evidence that Nb and/or C atoms enter into the Fe–Ga matrix as interstitial C and/ or larger substitutional Nb atoms, which further implies that the doped NbC phase decomposes to free atoms during the arc melting at high temperatures. At the same time, the fcc NbC phase exists in the alloys as the diffraction peaks of fcc NbC phase can be clearly observed, as shown in Inset of Fig. 1(a). The increasing intensity of fcc peaks means increasing amount of NbC phase in the alloys. The XRD patterns of starting NbC powders are given in Fig. 1(a) for a comparison. The starting NbC powders show a single fcc phase and it is coherent with the fcc NbC phase in NbC-doped Fe81Ga19 alloys. Fig. 2 shows the microstructural images of the (Fe81Ga19)100 − x (NbC)x alloys. In NbC-free Fe81Ga19 alloy, the coarse equiaxed grains with an average size of 650 μm can be observed (Figs. 2(a) and 3). Adding NbC powders results in a remarkable grain refinement of Fe81Ga19 alloys to 380 μm for x = 0.5 and 180 μm for x = 1.0 (Figs. 2(a) and 3). Fig. 3 shows a linear relation between the grain size and NbC content. For the alloy with x = 0.5, homogeneous equiaxed grains with straight boundaries are seen (Fig. 2(b)). When x was increased to 1.0 (Fig. 2(c)), the refined grains show a quite different morphology. A kind of rose-like morphology appears with destabilized and winding grain boundaries. Besides, it is clear that a secondary phase exists at the grain boundaries. Backscattered electron SEM images are further shown in Fig. 2(d), (e) and (f). In Fig. 2(d), the binary Fe–Ga matrix has a single phase. In Fig. 2(e) and (f), a secondary phase with a strong back-scattering
1343
and two different morphologies can be seen after adding NbC powders. EDS analysis (not shown) further identified these phases as NbC phase, which is in good agreement with the XRD analysis (Fig. 1(a)). For x = 0.5, high-density rod-like NbC particles, with a dimension of about 0.5 × 3 μm, disperse in the grain matrix (Fig. 2(e)). These NbC particles come from the precipitation of Nb and C atoms from the Nb/Ccontaining Fe–Ga solid solution, which is similar to the NbC precipitates reported in other NbC-added Fe-based alloys [15,16]. Except for the rodlike NbC precipitates within the grain matrix, in the alloy with x = 1.0 (Fig. 2(f)) NbC dendrite clusters segregate at grain boundaries, which is similar to the case in Fe–Mn–Si–Cr–Ni–C shape memory alloy [17]. These dendrite clusters are quite different from the shape (spherical particles) and size (about 1 μm) of the starting NbC powders (Inset of Fig. 2(d)). They are thus deduced to originate from the growth up of the NbC primary phase during the solidification. As a result, we obtain NbCparticle / Fe81Ga19 (x = 0.5) and (NbCparticle + NbCdendrite) / Fe81Ga19 (x = 1) magnetostrictive composites in NbC-doped Fe81Ga19 alloy. Based on the distribution characteristics of matrix microstructure and NbC phase in NbC-doped Fe81Ga19 alloy, here we discuss the grain-refinement behavior of Fe–Ga matrix. The increasing number of grains is firstly ascribed to the heterogeneous nucleation of the Fe–Ga grains on NbC nuclei that form at high-temperature melt, similar to the case of NbC nuclei in Ni–Nb–C superalloys [18]. In this study, the increasing NbC doping provides increasing number of nuclei. At the same time, the NbC dendrite clusters existing in front of the solid/liquid interfaces lower the grain growth rates by blocking the solute transports. This may result in constitutional undercooling and a resultant destabilization of solid/liquid interfaces [19,20], which can be evidenced by the rose-like grains seen in Fig. 2(c) and (f). It is reasonable that more nucleations in the residual liquid occur and further produce more grains. These factors make collective contributions to the grain refinement of Fe–Ga matrix. In this section, the magnetostrictive and mechanical properties of these grain-refined composite alloys were subsequently presented, as shown in Fig. 4. The magnetostrictions were measured on different positions of each sample, as shown in Fig. 4(a) and (b). The fielddependent magnetostriction of NbC-added alloys saturates at about 1.3 kOe, similar to the NbC-free Fe–Ga alloy Fig. 4(a). The average magnetostriction of the alloys with x = 0 and 0.5 are about 48 and 47 ppm, respectively. A larger magnetostriction of 64 ppm is obtained in the alloy with x = 1.0. It is interesting to observe an improvement of the magnetostriction Fe–Ga alloys by adding a non-magnetic NbC phase. The stress–strain curves were measured on several samples of each alloy, as shown in Fig. 4(c), (d) and (e). The maximal fracture stress (Rm) of binary Fe81Ga19 alloy is less than 200 MPa. The corresponding
Fig. 1. (a) XRD patterns and (b) lattice constants of the (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. Inset of (a) shows the XRD patterns between 32° and 42°.
1344
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
Fig. 2. (a–c) Optical micrographs of (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. (d–f) Backscattered electron SEM images of (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. Inset to (d) shows the morphology of starting NbC powders.
fracture mode is a typical intergranular fracture along the boundaries of coarse grains (Inset I of Fig. 4(e)). After doping NbC, the Rm increases remarkably. A maximal Rm is reached at about 620 MPa in the alloy with x = 0.5, which is three times as large as NbC-free Fe81Ga19 alloy. When the doping addition of NbC was increased to 1.0, a Rm of 540 MPa is obtained. At the same time, the elongation ratio εm is also improved from maximal value of 0.2% for x = 0 to 0.90% and 0.75% for x = 0.5 and 1.0 respectively, which indicates that a better ductility is gained in Fe–Ga alloy under the NbC doping. Mixed modes of intergranular and
Fig. 3. NbC content dependence of grain size of Fe–Ga matrix.
transgranular fractures can be seen in both NbC-doped alloys (inset II for x = 0.5 and inset III for x = 1.0 of Fig. 4(d) and (e)), which gives evidence of the enhancement of the mechanical properties in NbC-added Fe81Ga19 alloys. However, one can see both the ductility and strength of the alloy with x = 1.0 are slightly decreased again relative to the alloy with x = 0.5, as shown in Fig. 4(d) and (e). For the mechanical enhancements, the primary contribution originates from the significant grain refinement of Fe–Ga matrix, as shown in Figs. 2 and 3. The decreasing size of grains strengthens the alloys, as described by the Hall–Petch relation [21,22]. According to the Hall– Petch relation, the decrease in grain size (shown in Fig. 3) would result in a distinct increase in fracture strength. High-density grain boundaries are produced owing to the grain refinement, which prevent the initiating of cracking by dispersing the stress and energy [22]. Both the strength and ductility would be improved simultaneously in these grain-refined alloys. With the smallest grain size, in contrast, the lower ductility and strength in the alloy with x = 1.0 relative to the alloy with x = 0.5 implies that NbC phase may impose a negative influence on the mechanical properties. The high-resolution fracture surfaces of the two NbC-doped Fe81Ga19 alloys with x = 0.5 and 1.0 are further analyzed, as depicted in Fig. 5. Again, the mixed modes of intergranular and transgranular fractures (Insets of Fig. 4) are observed. Fig. 5(a) shows an intergranular fracture surface of a grain boundary in the alloy with x = 0.5, on which large numbers of rod-like NbC particles (white arrows) with a diameter of 1 μm and lengths of 1 ~ 3 μm appear along or perpendicular to the surface. A NbCparticle / Fe81Ga19 composite is observed in this alloy. In addition, some particles were peeled off from the surface due to high stress during the tensile testing. These dispersed rod-like particles will act as connective tissues and enhance the cohesion of interface between
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
1345
Fig. 4. (a–b) Magnetostriction plots of the (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. (c–e) Tensile stress–strain and the data of the (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys.
neighboring grains. Fig. 5(b) shows a transgranular fracture surface of the alloy with x = 0.5. The rod-like NbC particles act as fibers embedded in the grain matrix (white arrows). These NbC phase particles correspond to the secondary phase observed within grains in Fig. 2(e) and (f). Some ruptures of NbC rods are also observed. Further, many dimples and tearing edges can be found on the surface (black arrows). These results indicate that the strength is enhanced and the plastic deformation proceeds after the elastic deformation of the composite, which is in good agreement with the yield behavior shown in Fig. 4(c). In Fig. 5(c), the transgranular fracture can be also seen for the alloy with x = 1.0. One can further see the dispersed NbC precipitate particles in the grain matrix, corresponding to the particles in Fig. 2(f). Simultaneously, on an intergranular fracture surface of grain boundary
(Fig. 5(d)), NbC dendritic grains are clearly observed, corresponding to the ones at the grain boundaries shown in Fig. 2(f). The dendrites consist of 1st-order dendrite trunk, 2nd-order and even 3rd-order arms (white arrows in Fig. 5(d)), which forms a coarse dendrite cluster with well-developed dendritic-arm network. The morphology is similar to the branched Cu–Al–Ti oxide secondary phase in Cu8Al–Ti alumina composites [23]. A hybrid (NbCparticle + NbCdendrite) / Fe81Ga19 composite is formed in this alloy. From the stress–strain curves shown in Fig. 4(c), the ductility and strength of the alloy with x = 1.0 are significantly higher than that of the Fe–Ga binary alloy, however, are obviously lower than that of the alloy with x = 0.5. With the smallest grain size and grain-inner precipitate phase in this alloy, the lower Rm and εm provide evidence of the worsening of the coarse dendrite cluster on
Fig. 5. Fractographs of (Fe81Ga19)100 − x(NbC)x alloys with x = 0.5 (a, b) and 1.0 (c, d).
1346
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
NbC-doped Fe–Ga alloys. The tensile strength and ductility are remarkably improved with a maximal stress of 620 MPa and elongation ratio of 0.9% in (Fe81Ga19)99.5(NbC)0.5 alloy. The strengthening mechanism is related to the combination of grain-refinement strengthening and grain-inner precipitate particle reinforcement. This study provides a deep understanding on the microstructural distribution and strengthening mechanism of carbide-doped Fe–Ga magnetostrictive alloys. Acknowledgements This work was supported by the National Basic Research Program of China (2012CB619404) and the National Natural Science Foundation of China (Grant No. 51331001). References
Fig. 6. TEM image of the NbC precipitate phase in (Fe81Ga19)99.5(NbC)0.5.
mechanical properties. The coarse dendrite clusters at grain boundaries reduce the cohesion of the grain boundaries and counteract the mechanical enhancement of the NbC-doped Fe81Ga19 alloy with x = 1.0. The mechanical improvements by NbC precipitate phase are also confirmed by TEM analysis on the tensile specimen after tensile testing. Fig. 6 shows the TEM image of a sample taken from the position near the fracture section in NbCparticle / Fe81Ga19 composite (x = 0.5). A rod-like NbC precipitate is observed in the grain matrix, corresponding to the precipitates in Figs. 2 and 5. It can be seen that the precipitate is surrounded by high-density dislocation networks accompanied by a remarkable contrast of stress fields, which reflects the interactions between precipitate and dislocations [24,25]. During the tensile testing, the dislocations were initiated at the mismatched (Fe–Ga)A2/NbC interface and were pinned by the non-deforming precipitate phase. The deformation and cracking of grain matrix were thus retarded by the dispersed high-strength precipitates. The combined effects of grainrefinement strengthening and grain-inner precipitate particle reinforcement together resulted in the enhancement in mechanical properties of the NbC-doped Fe81Ga19 magnetostrictive alloys. It should be further pointed out that, the nano-scale precipitates may produce local lattice strains via mismatch at NbC/Fe–Ga interface, which would provide in situ stress to the Fe–Ga matrix. This atomlevel pre-stress may improve the magnetostriction (shown in Fig. 4(a)), which is similar to the case of Fe–Ga alloys under pre-stress [7] and deserves further study. 4. Conclusions In this work, a detailed investigation into the microstructural characteristics and their effects on the mechanical properties of NbC-doped Fe81Ga19 alloys has been performed. A grain refinement of the Fe–Ga matrix is obtained owing to the heterogeneous nucleation of grains on NbC nuclei. At the same time, high-density rod-like NbC particles precipitating in Fe–Ga matrix and NbC dendrite clusters segregating at the grain boundaries are observed. The NbCparticle / Fe81Ga19 and (NbC particle + NbCcluster) / Fe81Ga 19 composites are formed in the
[1] P.R. Downey, A.B. Flatau, Magnetoelastic bending of Galfenol for sensor applications, J. Appl. Phys. 97 (2005) 10R505. [2] A.G. Olabi, A. Grunwald, Design and application of magnetostrictive materials, Mater. Des. 29 (2008) 469–483. [3] S. Guruswamy, N. Srisukhumbowornchai, A.E. Clark, J.B. Restorff, M. Wun-Fogle, Strong, ductile, and low-field-magnetostrictive alloys based on Fe–Ga, Scr. Mater. 43 (2000) 239. [4] A.E. Clark, M. Wun-Fogle, J.B. Restorff, T.A. Lograsso, J.R. Cullen, Effect of quenching on the magnetostriction on Fe1 − xGax (0.13 b × b 0.21), IEEE Trans. Magn. 37 (2001) 2678. [5] A.E. Clark, K.B. Hathaway, M. Wun-Fogle, J.B. Restorff, T.A. Lograsso, V.M. Keppens, G. Petculescu, R.A. Taylor, Extraordinary magnetoelasticity and lattice softening in bcc Fe–Ga alloys, J. Appl. Phys. 93 (2003) 8621–8623. [6] A.L. Sun, J.H. Liu, C.B. Jiang, Recrystallization, texture evolution, and magnetostriction behavior of rolled (Fe81Ga19)98B2 sheets during low-to-high temperature heat treatments, J. Mater. Sci. 49 (2014) 4565. [7] J.H. Li, X.X. Gao, J. Zhu, X.Q. Bao, T. Xia, M.C. Zhang, Ductility, texture and large magnetostriction of Fe–Ga-based sheets, Scr. Mater. 63 (2010) 246–249. [8] S.-M. Na, A.B. Flatau, Single grain growth and large magnetostriction in secondarily recrystallized Fe–Ga thin sheet with sharp goss (011)[100] orientation, Scr. Mater. 66 (2012) 307–310. [9] R.A. Kellogg, A.M. Russell, T.A. Lograsso, A.B. Flatau, A.E. Clark, M. Wun-Fogle, Mechanical properties of magnetostrictive iron–gallium alloys, Proc. SPIE 5053 (2003) 534. [10] S.M. Na, A.B. Flatau, Deformation behavior and magnetostriction of polycrystalline Fe–Ga–X (X = B, C, Mn, Mo, Nb, NbC) alloys, J. Appl. Phys. 103 (2008) 07D304. [11] J. Li, X. Gao, J. Zhu, J. Li, M. Zhang, Ductility enhancement and magnetostriction of polycrystalline Fe–Ga based alloys, J. Alloys Compd. 484 (2009) 203–206. [12] L.M. Cheng, A.E. Nolting, B. Voyzelle, C. Galvani, Deformation behavior of polycrystalline Galfenol at elevated temperatures, Proc. SPIE 6526 (2007) N6526. [13] T. Takahashi, T. Okazaki, Y. Furuya, Improvement in the mechanical strength of magnetostrictive (Fe–Ga–Al)–X–C (X = Zr, Nb and Mo) alloys by carbide precipitation, Scr. Mater. 61 (2009) 5–7. [14] C. Bormio-Nunes, C.T. dos Santos, I.F. Leandro, R.S. Turtelli, R. Grossinger, M. Atif, Improved magnetostriction of Fe72Ga28 boron doped alloys, J. Appl. Phys. 109 (2011) 07A934. [15] W.M. Rainforth, M.P. Black, R.L. Higginson, E.J. Palmiere, C.M. Sellars, I. Prabst, P. Warbichler, F. Hofer, Precipitation of NbC in a model austenitic steel, Acta Mater. 50 (2002) 735–747. [16] F. Perrard, P. Donnadieu, A. Deschamps, P. Barges, TEM study of NbC heterogeneous precipitation in ferrite, Philos. Mag. 86 (2006) 4271–4284. [17] W. Zhang, N. Li, Y.H. Wen, S.K. Huang, Grain refinement and carbide precipitation in Fe–Mn–Si–Cr–Ni–C shape memory alloy fabricated by equal channel angular pressing, Mater. Sci. Technol. 26 (2010) 691–694. [18] M. Mizumoto, S. Sasaki, T. Ohgai, A. Kagawa, Development of new additive for grain refinement of austenitic stainless steel, Int. J. Cast Met. Res. 21 (2008) 49–55. [19] M. Easton, D. StJohn, Grain refinement of aluminum alloys: part II. Confirmation of, and a mechanism for, the solute paradigm, Metall. Mater. Trans. A 30 (1999) 1625–1633. [20] S.S. Li, B. Tang, D.B. Zeng, Effects and mechanism of Ca on refinement of AZ91D alloy, J. Alloys Compd. 437 (2007) 317–321. [21] S.S. Zhou, K.K. Deng, J.C. Li, S.J. Shang, W. Liang, J.F. Fan, Effects of volume ratio on the microstructure and mechanical properties of particle reinforced magnesium matrix composite, Mater. Des. 63 (2014) 672–677. [22] N. Hansen, Hall–Petch relation and boundary strengthening, Scr. Mater. 51 (2004) 801–806. [23] C. Krüger, A. Mortensen, Infiltrated Cu8Al–Ti alumina composites, Compos. A: Appl. Sci. Manuf. 66 (2014) 1–15. [24] M. Vogelsang, R.J. Arsenault, R.M. Fisher, An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites, MTA 17 (1986) 379–389. [25] T.A. Khraishi, L. Yan, Y.-L. Shen, Dynamic simulations of the interaction between dislocations and dilute particle concentrations in metal–matrix composites (MMCs), Int. J. Plast. 20 (2004) 1039–1057.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Microstructural characteristics and in situ reinforcement in NbC-doped Fe81Ga19 magnetostrictive alloys Aili Sun, Jinghua Liu ⁎, Chengbao Jiang ⁎ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 31 December 2014 Received in revised form 22 August 2015 Accepted 30 August 2015 Available online 3 September 2015 Keywords: Magnetostriction NbC Grain refinement Particulate reinforced composites Secondary-phase reinforcement
a b s t r a c t The microstructural phase distribution and the origin of mechanical enhancement of NbC-doped Fe–Ga magnetostrictive alloys are not very clear. In this study, a detailed investigation into the microstructural characteristics and their effects on the mechanical properties has been performed on NbC-doped Fe81Ga19 alloys. Increasing NbC causes remarkable grain refinement of the Fe-Ga alloy matrix. Rod-like NbC particles precipitating in Fe–Ga grain matrix and NbC dendrite clusters segregating at Fe–Ga grain boundaries are observed. As a result, NbCparticle / Fe81Ga19 and (NbCparticle + NbCdendrite) / Fe81Ga19 magnetostrictive composites are obtained. The mechanical properties are found to be significantly improved by combined effects of grain-refinement strengthening and precipitate reinforcement, generating a maximal tensile strength of 620 MPa and an elongation ratio of 0.9%. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Magnetostriction is a magnetoelastic phenomenon in which a magnetic material changes its dimension upon the magnetizing as a function of the applied magnetic field. Magnetic materials with large magnetostriction are expected to be used in applications of sensors, actuators, positioning devices, and damping devices [1,2]. Among various magnetostrictive materials, the rare-earth-free Fe–Ga (Galfenol) alloys possess great advantage in practical applications due to its high magnetostriction, low saturation field, and low costs [3,4]. A large magnetostriction up to 400 ppm with a low saturation field was reported in Fe–Ga bulk single crystals along the [001] direction [5]. In recent years, to develop rolled Fe–Ga sheets with desired highly-orientated textures shows a growing tendency [6–8]. In these textured sheets, the large magnetostriction along the specific crystallographic orientations can be obtained and, at the same time the eddy current loss found in bulk state can be suppressed efficiently. However, Fe–Ga alloys still have their drawback in terms of mechanical properties. The binary Fe–Ga alloys usually consist of coarse grains and the grains have an intrinsic brittleness due to the strong p–d covalent bonding between Ga and Fe atoms [9,10]. Relatively low fracture strength and ductility are thus observed during the intergranular and/ or brittle fractures [10,11]. These make Fe–Ga alloys cannot bear high and cyclic loading in use as a key material. Besides, during the rolling process for thin sheets serious edge cracks along the grain boundaries
⁎ Corresponding authors. E-mail address: [email protected] (J. Liu), [email protected] (C. Jiang).
http://dx.doi.org/10.1016/j.matdes.2015.08.150 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
procreate as well, showing a low deforming ability [12]. In order to solve these issues, ternary additives, such as NbC, ZrC and MoC carbides, have been added into the Fe–Ga alloys [10,13]. The mechanical properties can be thus enhanced in different degrees. However, there are few reports on the detailed microstructural phase distribution of carbidedoped Fe–Ga alloys and on the relationship between microstructures and mechanical properties. In this study, in order to in depth characterize the microstructural phase distribution of carbide-doped Fe–Ga alloys and to better understand the microstructural origin of strengthening mechanism, Fe81Ga19 alloy was chosen as the base alloy, into which NbC powders have been added with percentage proportions of x = 0, 0.5 and 1.0. The experimental results reveal the co-existence of grain refinement of Fe–Ga matrix, rod-like NbC precipitate particles in Fe–Ga grains, and/or NbC dendrite clusters at Fe–Ga grain boundaries. It is interesting to obtain NbCparticle / Fe81Ga19 and (NbCparticle + NbCdendrite) / Fe81Ga19 magnetostrictive composites, showing different behaviors of mechanical enhancements. 2. Experimental procedures (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloy ingots with a diameter of 65 mm were prepared by arc melting Fe (99.99 wt.%), Ga (99.99 wt.%) and NbC powers under argon atmosphere. Each ingot was re-melted four times to assure the homogeneity. These ingots were annealed at 1373 K for 3 h and then slowly cooled to room temperature. Room-temperature structures of the alloy ingots and starting NbC powders were detected using Rigaku D/Max2200 X-ray diffractometer with Cu–Kα radiation. Microstructures of samples, morphology of NbC powers and fractographs of the tensile samples were investigated
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
by optical microscope (OM) and scanning electron microscopy (SEM) with energy-dispersive spectrum (EDS), and transmission electron microscope (TEM). The average grain size was calculated by using image analysis software Image-Pro Plus. The lattice constant was calculated by fitting the XRD data using Jade-software. Magnetostrictions of the samples with dimensions of 14.00 × 7.00 × 6.00 mm were measured using the standard strain gauge. Five tensile samples of each alloy with a gauge length 26 mm were machined by wire-electrode cutting from the center of ingots. The tensile samples were subjected to room-temperature tensile test until fracture of the sample, using a computer-controlled Instron-8801 testing machine with a strain rate of 4.8 × 10−5 s−1. The engineering stress–strain data were presented in this study. 3. Results and discussion Fig. 1(a) displays the XRD patterns of the (Fe81Ga19)100 − x(NbC)x alloys. The main phase of Fe81Ga19 alloy shows a typical A2 structure, similar to the previous results in Fe–Ga alloys [6,13,14]. The lattice constant of A2 main phase shows a clear increasing tendency with increasing NbC addition, as shown in Fig. 1(b). This gradual increase gives evidence that Nb and/or C atoms enter into the Fe–Ga matrix as interstitial C and/ or larger substitutional Nb atoms, which further implies that the doped NbC phase decomposes to free atoms during the arc melting at high temperatures. At the same time, the fcc NbC phase exists in the alloys as the diffraction peaks of fcc NbC phase can be clearly observed, as shown in Inset of Fig. 1(a). The increasing intensity of fcc peaks means increasing amount of NbC phase in the alloys. The XRD patterns of starting NbC powders are given in Fig. 1(a) for a comparison. The starting NbC powders show a single fcc phase and it is coherent with the fcc NbC phase in NbC-doped Fe81Ga19 alloys. Fig. 2 shows the microstructural images of the (Fe81Ga19)100 − x (NbC)x alloys. In NbC-free Fe81Ga19 alloy, the coarse equiaxed grains with an average size of 650 μm can be observed (Figs. 2(a) and 3). Adding NbC powders results in a remarkable grain refinement of Fe81Ga19 alloys to 380 μm for x = 0.5 and 180 μm for x = 1.0 (Figs. 2(a) and 3). Fig. 3 shows a linear relation between the grain size and NbC content. For the alloy with x = 0.5, homogeneous equiaxed grains with straight boundaries are seen (Fig. 2(b)). When x was increased to 1.0 (Fig. 2(c)), the refined grains show a quite different morphology. A kind of rose-like morphology appears with destabilized and winding grain boundaries. Besides, it is clear that a secondary phase exists at the grain boundaries. Backscattered electron SEM images are further shown in Fig. 2(d), (e) and (f). In Fig. 2(d), the binary Fe–Ga matrix has a single phase. In Fig. 2(e) and (f), a secondary phase with a strong back-scattering
1343
and two different morphologies can be seen after adding NbC powders. EDS analysis (not shown) further identified these phases as NbC phase, which is in good agreement with the XRD analysis (Fig. 1(a)). For x = 0.5, high-density rod-like NbC particles, with a dimension of about 0.5 × 3 μm, disperse in the grain matrix (Fig. 2(e)). These NbC particles come from the precipitation of Nb and C atoms from the Nb/Ccontaining Fe–Ga solid solution, which is similar to the NbC precipitates reported in other NbC-added Fe-based alloys [15,16]. Except for the rodlike NbC precipitates within the grain matrix, in the alloy with x = 1.0 (Fig. 2(f)) NbC dendrite clusters segregate at grain boundaries, which is similar to the case in Fe–Mn–Si–Cr–Ni–C shape memory alloy [17]. These dendrite clusters are quite different from the shape (spherical particles) and size (about 1 μm) of the starting NbC powders (Inset of Fig. 2(d)). They are thus deduced to originate from the growth up of the NbC primary phase during the solidification. As a result, we obtain NbCparticle / Fe81Ga19 (x = 0.5) and (NbCparticle + NbCdendrite) / Fe81Ga19 (x = 1) magnetostrictive composites in NbC-doped Fe81Ga19 alloy. Based on the distribution characteristics of matrix microstructure and NbC phase in NbC-doped Fe81Ga19 alloy, here we discuss the grain-refinement behavior of Fe–Ga matrix. The increasing number of grains is firstly ascribed to the heterogeneous nucleation of the Fe–Ga grains on NbC nuclei that form at high-temperature melt, similar to the case of NbC nuclei in Ni–Nb–C superalloys [18]. In this study, the increasing NbC doping provides increasing number of nuclei. At the same time, the NbC dendrite clusters existing in front of the solid/liquid interfaces lower the grain growth rates by blocking the solute transports. This may result in constitutional undercooling and a resultant destabilization of solid/liquid interfaces [19,20], which can be evidenced by the rose-like grains seen in Fig. 2(c) and (f). It is reasonable that more nucleations in the residual liquid occur and further produce more grains. These factors make collective contributions to the grain refinement of Fe–Ga matrix. In this section, the magnetostrictive and mechanical properties of these grain-refined composite alloys were subsequently presented, as shown in Fig. 4. The magnetostrictions were measured on different positions of each sample, as shown in Fig. 4(a) and (b). The fielddependent magnetostriction of NbC-added alloys saturates at about 1.3 kOe, similar to the NbC-free Fe–Ga alloy Fig. 4(a). The average magnetostriction of the alloys with x = 0 and 0.5 are about 48 and 47 ppm, respectively. A larger magnetostriction of 64 ppm is obtained in the alloy with x = 1.0. It is interesting to observe an improvement of the magnetostriction Fe–Ga alloys by adding a non-magnetic NbC phase. The stress–strain curves were measured on several samples of each alloy, as shown in Fig. 4(c), (d) and (e). The maximal fracture stress (Rm) of binary Fe81Ga19 alloy is less than 200 MPa. The corresponding
Fig. 1. (a) XRD patterns and (b) lattice constants of the (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. Inset of (a) shows the XRD patterns between 32° and 42°.
1344
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
Fig. 2. (a–c) Optical micrographs of (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. (d–f) Backscattered electron SEM images of (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. Inset to (d) shows the morphology of starting NbC powders.
fracture mode is a typical intergranular fracture along the boundaries of coarse grains (Inset I of Fig. 4(e)). After doping NbC, the Rm increases remarkably. A maximal Rm is reached at about 620 MPa in the alloy with x = 0.5, which is three times as large as NbC-free Fe81Ga19 alloy. When the doping addition of NbC was increased to 1.0, a Rm of 540 MPa is obtained. At the same time, the elongation ratio εm is also improved from maximal value of 0.2% for x = 0 to 0.90% and 0.75% for x = 0.5 and 1.0 respectively, which indicates that a better ductility is gained in Fe–Ga alloy under the NbC doping. Mixed modes of intergranular and
Fig. 3. NbC content dependence of grain size of Fe–Ga matrix.
transgranular fractures can be seen in both NbC-doped alloys (inset II for x = 0.5 and inset III for x = 1.0 of Fig. 4(d) and (e)), which gives evidence of the enhancement of the mechanical properties in NbC-added Fe81Ga19 alloys. However, one can see both the ductility and strength of the alloy with x = 1.0 are slightly decreased again relative to the alloy with x = 0.5, as shown in Fig. 4(d) and (e). For the mechanical enhancements, the primary contribution originates from the significant grain refinement of Fe–Ga matrix, as shown in Figs. 2 and 3. The decreasing size of grains strengthens the alloys, as described by the Hall–Petch relation [21,22]. According to the Hall– Petch relation, the decrease in grain size (shown in Fig. 3) would result in a distinct increase in fracture strength. High-density grain boundaries are produced owing to the grain refinement, which prevent the initiating of cracking by dispersing the stress and energy [22]. Both the strength and ductility would be improved simultaneously in these grain-refined alloys. With the smallest grain size, in contrast, the lower ductility and strength in the alloy with x = 1.0 relative to the alloy with x = 0.5 implies that NbC phase may impose a negative influence on the mechanical properties. The high-resolution fracture surfaces of the two NbC-doped Fe81Ga19 alloys with x = 0.5 and 1.0 are further analyzed, as depicted in Fig. 5. Again, the mixed modes of intergranular and transgranular fractures (Insets of Fig. 4) are observed. Fig. 5(a) shows an intergranular fracture surface of a grain boundary in the alloy with x = 0.5, on which large numbers of rod-like NbC particles (white arrows) with a diameter of 1 μm and lengths of 1 ~ 3 μm appear along or perpendicular to the surface. A NbCparticle / Fe81Ga19 composite is observed in this alloy. In addition, some particles were peeled off from the surface due to high stress during the tensile testing. These dispersed rod-like particles will act as connective tissues and enhance the cohesion of interface between
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
1345
Fig. 4. (a–b) Magnetostriction plots of the (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys. (c–e) Tensile stress–strain and the data of the (Fe81Ga19)100 − x(NbC)x (x = 0, 0.5, and 1.0) alloys.
neighboring grains. Fig. 5(b) shows a transgranular fracture surface of the alloy with x = 0.5. The rod-like NbC particles act as fibers embedded in the grain matrix (white arrows). These NbC phase particles correspond to the secondary phase observed within grains in Fig. 2(e) and (f). Some ruptures of NbC rods are also observed. Further, many dimples and tearing edges can be found on the surface (black arrows). These results indicate that the strength is enhanced and the plastic deformation proceeds after the elastic deformation of the composite, which is in good agreement with the yield behavior shown in Fig. 4(c). In Fig. 5(c), the transgranular fracture can be also seen for the alloy with x = 1.0. One can further see the dispersed NbC precipitate particles in the grain matrix, corresponding to the particles in Fig. 2(f). Simultaneously, on an intergranular fracture surface of grain boundary
(Fig. 5(d)), NbC dendritic grains are clearly observed, corresponding to the ones at the grain boundaries shown in Fig. 2(f). The dendrites consist of 1st-order dendrite trunk, 2nd-order and even 3rd-order arms (white arrows in Fig. 5(d)), which forms a coarse dendrite cluster with well-developed dendritic-arm network. The morphology is similar to the branched Cu–Al–Ti oxide secondary phase in Cu8Al–Ti alumina composites [23]. A hybrid (NbCparticle + NbCdendrite) / Fe81Ga19 composite is formed in this alloy. From the stress–strain curves shown in Fig. 4(c), the ductility and strength of the alloy with x = 1.0 are significantly higher than that of the Fe–Ga binary alloy, however, are obviously lower than that of the alloy with x = 0.5. With the smallest grain size and grain-inner precipitate phase in this alloy, the lower Rm and εm provide evidence of the worsening of the coarse dendrite cluster on
Fig. 5. Fractographs of (Fe81Ga19)100 − x(NbC)x alloys with x = 0.5 (a, b) and 1.0 (c, d).
1346
A. Sun et al. / Materials and Design 88 (2015) 1342–1346
NbC-doped Fe–Ga alloys. The tensile strength and ductility are remarkably improved with a maximal stress of 620 MPa and elongation ratio of 0.9% in (Fe81Ga19)99.5(NbC)0.5 alloy. The strengthening mechanism is related to the combination of grain-refinement strengthening and grain-inner precipitate particle reinforcement. This study provides a deep understanding on the microstructural distribution and strengthening mechanism of carbide-doped Fe–Ga magnetostrictive alloys. Acknowledgements This work was supported by the National Basic Research Program of China (2012CB619404) and the National Natural Science Foundation of China (Grant No. 51331001). References
Fig. 6. TEM image of the NbC precipitate phase in (Fe81Ga19)99.5(NbC)0.5.
mechanical properties. The coarse dendrite clusters at grain boundaries reduce the cohesion of the grain boundaries and counteract the mechanical enhancement of the NbC-doped Fe81Ga19 alloy with x = 1.0. The mechanical improvements by NbC precipitate phase are also confirmed by TEM analysis on the tensile specimen after tensile testing. Fig. 6 shows the TEM image of a sample taken from the position near the fracture section in NbCparticle / Fe81Ga19 composite (x = 0.5). A rod-like NbC precipitate is observed in the grain matrix, corresponding to the precipitates in Figs. 2 and 5. It can be seen that the precipitate is surrounded by high-density dislocation networks accompanied by a remarkable contrast of stress fields, which reflects the interactions between precipitate and dislocations [24,25]. During the tensile testing, the dislocations were initiated at the mismatched (Fe–Ga)A2/NbC interface and were pinned by the non-deforming precipitate phase. The deformation and cracking of grain matrix were thus retarded by the dispersed high-strength precipitates. The combined effects of grainrefinement strengthening and grain-inner precipitate particle reinforcement together resulted in the enhancement in mechanical properties of the NbC-doped Fe81Ga19 magnetostrictive alloys. It should be further pointed out that, the nano-scale precipitates may produce local lattice strains via mismatch at NbC/Fe–Ga interface, which would provide in situ stress to the Fe–Ga matrix. This atomlevel pre-stress may improve the magnetostriction (shown in Fig. 4(a)), which is similar to the case of Fe–Ga alloys under pre-stress [7] and deserves further study. 4. Conclusions In this work, a detailed investigation into the microstructural characteristics and their effects on the mechanical properties of NbC-doped Fe81Ga19 alloys has been performed. A grain refinement of the Fe–Ga matrix is obtained owing to the heterogeneous nucleation of grains on NbC nuclei. At the same time, high-density rod-like NbC particles precipitating in Fe–Ga matrix and NbC dendrite clusters segregating at the grain boundaries are observed. The NbCparticle / Fe81Ga19 and (NbC particle + NbCcluster) / Fe81Ga 19 composites are formed in the
[1] P.R. Downey, A.B. Flatau, Magnetoelastic bending of Galfenol for sensor applications, J. Appl. Phys. 97 (2005) 10R505. [2] A.G. Olabi, A. Grunwald, Design and application of magnetostrictive materials, Mater. Des. 29 (2008) 469–483. [3] S. Guruswamy, N. Srisukhumbowornchai, A.E. Clark, J.B. Restorff, M. Wun-Fogle, Strong, ductile, and low-field-magnetostrictive alloys based on Fe–Ga, Scr. Mater. 43 (2000) 239. [4] A.E. Clark, M. Wun-Fogle, J.B. Restorff, T.A. Lograsso, J.R. Cullen, Effect of quenching on the magnetostriction on Fe1 − xGax (0.13 b × b 0.21), IEEE Trans. Magn. 37 (2001) 2678. [5] A.E. Clark, K.B. Hathaway, M. Wun-Fogle, J.B. Restorff, T.A. Lograsso, V.M. Keppens, G. Petculescu, R.A. Taylor, Extraordinary magnetoelasticity and lattice softening in bcc Fe–Ga alloys, J. Appl. Phys. 93 (2003) 8621–8623. [6] A.L. Sun, J.H. Liu, C.B. Jiang, Recrystallization, texture evolution, and magnetostriction behavior of rolled (Fe81Ga19)98B2 sheets during low-to-high temperature heat treatments, J. Mater. Sci. 49 (2014) 4565. [7] J.H. Li, X.X. Gao, J. Zhu, X.Q. Bao, T. Xia, M.C. Zhang, Ductility, texture and large magnetostriction of Fe–Ga-based sheets, Scr. Mater. 63 (2010) 246–249. [8] S.-M. Na, A.B. Flatau, Single grain growth and large magnetostriction in secondarily recrystallized Fe–Ga thin sheet with sharp goss (011)[100] orientation, Scr. Mater. 66 (2012) 307–310. [9] R.A. Kellogg, A.M. Russell, T.A. Lograsso, A.B. Flatau, A.E. Clark, M. Wun-Fogle, Mechanical properties of magnetostrictive iron–gallium alloys, Proc. SPIE 5053 (2003) 534. [10] S.M. Na, A.B. Flatau, Deformation behavior and magnetostriction of polycrystalline Fe–Ga–X (X = B, C, Mn, Mo, Nb, NbC) alloys, J. Appl. Phys. 103 (2008) 07D304. [11] J. Li, X. Gao, J. Zhu, J. Li, M. Zhang, Ductility enhancement and magnetostriction of polycrystalline Fe–Ga based alloys, J. Alloys Compd. 484 (2009) 203–206. [12] L.M. Cheng, A.E. Nolting, B. Voyzelle, C. Galvani, Deformation behavior of polycrystalline Galfenol at elevated temperatures, Proc. SPIE 6526 (2007) N6526. [13] T. Takahashi, T. Okazaki, Y. Furuya, Improvement in the mechanical strength of magnetostrictive (Fe–Ga–Al)–X–C (X = Zr, Nb and Mo) alloys by carbide precipitation, Scr. Mater. 61 (2009) 5–7. [14] C. Bormio-Nunes, C.T. dos Santos, I.F. Leandro, R.S. Turtelli, R. Grossinger, M. Atif, Improved magnetostriction of Fe72Ga28 boron doped alloys, J. Appl. Phys. 109 (2011) 07A934. [15] W.M. Rainforth, M.P. Black, R.L. Higginson, E.J. Palmiere, C.M. Sellars, I. Prabst, P. Warbichler, F. Hofer, Precipitation of NbC in a model austenitic steel, Acta Mater. 50 (2002) 735–747. [16] F. Perrard, P. Donnadieu, A. Deschamps, P. Barges, TEM study of NbC heterogeneous precipitation in ferrite, Philos. Mag. 86 (2006) 4271–4284. [17] W. Zhang, N. Li, Y.H. Wen, S.K. Huang, Grain refinement and carbide precipitation in Fe–Mn–Si–Cr–Ni–C shape memory alloy fabricated by equal channel angular pressing, Mater. Sci. Technol. 26 (2010) 691–694. [18] M. Mizumoto, S. Sasaki, T. Ohgai, A. Kagawa, Development of new additive for grain refinement of austenitic stainless steel, Int. J. Cast Met. Res. 21 (2008) 49–55. [19] M. Easton, D. StJohn, Grain refinement of aluminum alloys: part II. Confirmation of, and a mechanism for, the solute paradigm, Metall. Mater. Trans. A 30 (1999) 1625–1633. [20] S.S. Li, B. Tang, D.B. Zeng, Effects and mechanism of Ca on refinement of AZ91D alloy, J. Alloys Compd. 437 (2007) 317–321. [21] S.S. Zhou, K.K. Deng, J.C. Li, S.J. Shang, W. Liang, J.F. Fan, Effects of volume ratio on the microstructure and mechanical properties of particle reinforced magnesium matrix composite, Mater. Des. 63 (2014) 672–677. [22] N. Hansen, Hall–Petch relation and boundary strengthening, Scr. Mater. 51 (2004) 801–806. [23] C. Krüger, A. Mortensen, Infiltrated Cu8Al–Ti alumina composites, Compos. A: Appl. Sci. Manuf. 66 (2014) 1–15. [24] M. Vogelsang, R.J. Arsenault, R.M. Fisher, An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites, MTA 17 (1986) 379–389. [25] T.A. Khraishi, L. Yan, Y.-L. Shen, Dynamic simulations of the interaction between dislocations and dilute particle concentrations in metal–matrix composites (MMCs), Int. J. Plast. 20 (2004) 1039–1057.