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Effect of Al substitution for Ga on the mechanical properties of directional solidified Fe-Ga alloys
Author’s Accepted Manuscript Effect of Al substitution for Ga on the mechanical properties of directional solidified Fe-Ga alloys Yangyang Liu, Jiheng Li, Xuexu Gao
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S0304-8853(16)31092-7 http://dx.doi.org/10.1016/j.jmmm.2016.09.072 MAGMA61853
To appear in: Journal of Magnetism and Magnetic Materials Received date: 14 June 2016 Revised date: 9 August 2016 Accepted date: 12 September 2016 Cite this article as: Yangyang Liu, Jiheng Li and Xuexu Gao, Effect of Al substitution for Ga on the mechanical properties of directional solidified Fe-Ga a l l o y s , Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.09.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Al substitution for Ga on the mechanical properties of directional solidified Fe-Ga alloys Yangyang Liu, Jiheng Li, Xuexu Gao* State Key Laboratory of Advanced Metals and Materials, University of Science and Technology Beijing 100083, China Abstract: Alloys of Fe82Ga18-xAlx (x=0, 4.5, 6, 9, 12, 13.5) were prepared by directional solidification technique and exhibited a <001> preferred orientation along the axis of alloy rods. The saturation magnetostriction value of the Fe82Ga13.5Al4.5 alloy was 247 ppm under no pre-stress. The tensile properties of alloys of Fe82Ga18-xAlx at room temperature were investigated. The results showed that tensile ductility of binary Fe-Ga alloy was significantly improved with Al addition. The fracture elongation of the Fe82Ga18 alloy was only 1.3%, while that of the Fe82Ga9Al9 alloy increased up to 16.5%. Addition of Al increased the strength of grain boundary and cleavage, resulting in the enhancement of tensile ductility of the Fe-Ga-Al alloys. Analysis of deformation microstructure showed that a great number of deformation twins formed in the Fe-Ga-Al alloys, which were thought to be the source of serrated yielding in the stress-strain curves. The effect of Al content in the Fe-Ga-Al alloys on tensile ductility was also studied by the analysis of deformation twins. It indicated that the joint effect of slip and twinning was beneficial to obtain the best ductility in the Fe82Ga9Al9 alloy.
Keywords: Fe-Ga alloy, Fe-Ga-Al alloy, tensile property, twinning.
＊corresponding author. Tel.:+86 10 62334343; fax: +86 10 62334327; e-mail: [email protected]
1. Introduction Fe-Ga alloys, as well known, are a kind of functional metal materials with high magnetostrictive strain. It has been reported that the maximum saturation magnetostriction (3/2)λ100 in single crystal Fe-Ga alloys can reach up to 400 ppm, which is largely higher than that in traditional magnetostrictive alloys [1-4]. Compare to Terfenol-D, Fe-Ga alloys exhibit high strength and toughness. The unique combination of magnetostrictive and mechanical properties in Fe-Ga alloys makes them attractive for sensors, actuators and transducers applications. Although single crystal Fe-Ga alloys are quite tough, polycrystalline Fe-Ga alloys can suffer from intergranular fracture due to weak grain boundaries [5-7]. Recently, studies on Fe-Ga alloys have shown that addition of small amounts of alloying elements leads to improvement of grain boundary strength and then enhancement in the mechanical strength and ductility [8-12]. The unique combination property of Fe-Ga alloys makes them a promising magnetostrictive material, but the significant eddy current losses originating from high conductivity of alloys limit their operation at high frequencies. Rolling them into thin sheets has been thought to be an economical and effective way to reduce the eddy current losses. Na et al. investigated the influence of alloying additions of B, C, Mn, Mo, Nb and NbC on the rollability and deformation behavior of Fe-Ga alloys [8]. As reported by Na et al., binary and C-added Fe-Ga alloys were easily fractured during hot rolling due to the cracks formed along grain boundaries [8]. Alloying additions of B, Mo, Nb and NbC into Fe-Ga alloys changed the fracture mode from an intergranular to a transgranular fracture mode with cleavage and then improved the roll ability. High-quality rolled sheets were obtained by alloying additions except Mn, which was detrimental to improvement in ductility of Fe-Ga alloys. Furthermore, alloying addition of Cr has been shown an effective alloying element to improve the ductility of Fe-Ga alloys [6]. Nolting et al. conducted tensile testing on binary and alloyed Galfenol rolled sheets with the thickness of 4~5 mm [12]. At room temperature, the binary Galfenol rolled sheets had ultimate tensile strength of 455 MPa and little or no plasticity occurs prior to fracture. All the binary Galfenol specimens showed intergranular fracture mode. The addition of Al increased both strength and ductility and changed fracture mode to transgranular fracture. The elongation at fracture of alloys with Al additions was up to 5.8%. On the other hand, Yuan et al. explored the
Goss texture development in rolled columnar-grained Fe-Ga alloys during secondary recrystallization [13, 14]. It was revealed that the <100> orientation from initial directional solidified Fe-Ga alloys can be inherited by the rolled sheets, resulting in a larger magnetostriction. In our previous work, the <100> oriented and columnar-grained Fe-Ga-Al alloys have been prepared by directional solidification technique and the magnetostriction was up to 250 ppm under no pre-stress. This work examined the tensile properties of directional solidified Fe-Ga and Fe-Ga-Al alloys at room temperature. The effect and mechanism of Al substitution for Ga on the mechanical properties of Fe-Ga alloys were mainly studied, which could provide theoretic foundation for improving the roll ability of Fe-Ga based alloys. 2. Experimental Alloys of Fe82Ga18-xAlx (x=0, 4.5, 6, 9, 12, 13.5) were prepared from pure Fe (99.9 wt. %), pure Ga (99.99 wt. %) and pure Al (99.9 wt. %) and were grown by the directional solidification process at a growth rate of 12 mm/min [15]. Ga and Al losses were considered as 2 wt. % and 0.5 wt. %, respectively. The alloy rods were obtained with the dimension of 36×36×200 mm. Then specimens for tensile test were machined by electrical discharge machining. Before tensile testing, specimens were annealed at 1100 oC for 1h and then 730 oC for 3h followed by furnace cooling. Tensile tests were conducted in the DDL50 electronic universal testing machine at room temperature. The tensile stress was applied along the axial direction of alloy rods and the tensile rate was controlled at constant 0.6 mm/min. The specimens were polished and etched with a solution of 10% HNO3 and 90% C2H5OH at room temperature. The crystallographic orientation of specimens was detected by electron backscattered diffraction (EBSD) and the pole figure and inverse pole figure were obtained by analyzing EBSD patterns. The fracture surface of tensile specimens was observed by scanning electron microscope (SEM). An optical microscope was employed to observe the microstructure of tensile specimens. 3. Results and discussion Fig. 1(a) shows the optical photograph taken from the longitudinal section of Fe82Ga4.5Al13.5 alloy rod, where the arrow indicates the rod axis. It can be seen that the alloy is composed of coarse columnar grains with the diameter of 1~2 mm. The growth direction of columnar grains slightly deviates from the axis of alloy rod. The crystallographic orientation of columnar grains
was examined by EBSD and the inverse pole figure corresponding to the rod direction (RD) is shown in Fig. 1(b). The columnar grains orientation along RD is consistent with the [001] crystallographic direction, indicating a <001> preferred orientation along the axis of alloy rod. The magnetostrictive properties of alloys of Fe82Ga18-xAlx with no pre-stress were measured using the JDAW-2011 magnetostriction measurement equipment. The magnetostriction values at room temperature were obtained along the RD using resistance strain gauges with the gauge area of 2.8 mm×2.0 mm (base area of 6.4 mm×3.5 mm). Fig. 2 shows the magnetostriction versus applied magnetic field for alloys of Fe82Ga18-xAlx. The saturation magnetostriction value of the Fe82Ga13.5Al4.5 alloy was 247 ppm under no pre-stress. Increasing Al substitution for Ga to 6 at. %, the saturation magnetostriction still maintained a high level. When Al addition is more than 9 at. %, the saturation magnetostriction decreased sharply, but still maintained a level of ~200 ppm. Even with 13.5 at. % Al substitution for Ga in the Fe82Ga18 alloy, the saturation magnetostriction value still reached up to 199 ppm. This value is higher than that of the single crystal Fe82Al18 alloys [16]. Fig. 3 shows the stress-strain curves for alloys of Fe82Ga18-xAlx obtained from tensile tests at room temperature. It can be seen that the ductility of Fe-Ga alloy was significantly improved with Al addition. Substitution of 4.5 at. %Al for Ga significantly increased the fracture elongation from 1.3% to 12.3% in the Fe82Ga18 alloy. Compared to binary Fe-Ga alloy, the Fe-Ga-Al alloys showed an improvement in ductility, notably the Fe82Ga9Al9 alloy showed the greatest fracture elongation of 16.5%. The fracture surfaces for alloys of Fe82Ga18-xAlx are shown in Fig. 4. The cleavage steps and river patterns clearly seen in the fracture surfaces indicated a cleavage fracture mechanism. Both binary Fe-Ga alloy and ternary Fe-Ga-Al alloys exhibited transgranular fracture mode. The brittle fracture along grain boundary is prone to occur in as-cast binary Fe-Ga alloy, since the weak grain boundary. Other works have shown that there was only elastic deformation and no obvious yield can be observed in tensile testing for as-cast binary Fe-Ga alloy [6, 12]. Compared to the as-cast, binary Fe-Ga alloy from directional solidification method shows a certain plastic deformation and changes the fracture mode from intergranular to transgranular fracture, as shown in Fig. 4 (a). For directional solidified binary Fe-Ga alloy, the improvement of ductility is mainly due to the coarse columnar grains grown along tensile testing direction. The
formation of the coarse columnar grains reduces weak grain boundaries in the direction perpendicular to tensile testing direction, causing the change of fracture mode from intergranular to transgranlunar. Li et al. have pointed out that both the elements of B and Cr improved the ductility of Fe83Ga17 alloy, but the mechanisms were completely different [6]. The element of B tended to segregate at the grain boundaries. The segregation decreased the grain boundary energy and increased the grain boundary cohesion. Furthermore, it was beneficial to resist the crack propagation that addition of B refined grains of Fe-Ga alloy and increased the area of grain boundaries. Therefore, B improved the ductility of Fe-Ga alloy. Differing from the element of B, the improvement in ductility of Fe-Ga alloy with Cr addition was attributed to the solution strengthening. That Cr dissolved in the Fe-Ga matrix played a role of solution strengthening both improved the strength of grain boundary and cleavage. It has been observed that there was no interfacial segregation of Al in the Fe-Ga-Al alloys [17]. Similar to the element of Cr, Al dissolved in the Fe-Ga matrix. It seems that the solution of Al could also strengthen the grain boundary and cleavage. Fig. 5 shows the ultimate tensile strength and yield strength for alloys of Fe82Ga18-xAlx obtained from tensile tests at room temperature. In order to ensure accuracy of the data, several tensile tests were carried out for each of the alloys. Although there was a certain dispersion of the data, which was probably caused by the difference of microstructure such as shrinkage cavity and porosity, the trend in the ultimate tensile strength and yield strength was still clear. With the addition of Al, the ultimate tensile strength of Fe-Ga alloy increased, while the yield strength decreased. The more difference between the ultimate tensile strength and yield strength represents the better ductility and toughness in the Fe-Ga-Al alloys. McKamey et al. pointed out that the beneficial effect of Cr on ductility of the Fe3Al alloy was attributed to the solution softening and thus the easier slip behavior of dislocations [18]. Similarly, the decrease of yield strength in the Fe-Ga-Al alloys originates from solution softening of Al atoms. Both Ga and Al can be dissolved in Fe matrix and have similar solubility. Ga dissolved in Fe matrix will cause a certain degree of lattice distortion, since the difference in size between Ga and Fe atom. Lattice distortion increases the resistance to dislocation slip and makes it more difficult to carry out. So that the yield strength of Fe-Ga alloy increased, accompanied with ductility loss. The size of Al atom is smaller than that
of Ga atom. Al substitution for Ga tends to contract the lattice, reducing the degree of lattice distortion in Fe matrix. Therefore, the yield strength decreased and the ductility was improved in the Fe-Ga-Al alloys. It is worth noting that all the alloys in the tensile tests exhibited serrated yielding in the stress-strain curves, which was accompanied by an audible sound. Discontinuous yielding was also observed by Kellogg et al. during tensile tests of single crystal Fe-Ga alloy [5]. The authors thought that the discontinuous yielding resulted from twinning, kind band formation or stress-induced phase transformation. In addition, Nolting et al. also reported that discontinuous yielding was observed during tensile tests of Fe-Ga alloys with C, Cr and Al additions [12]. They hypothesized that the formation and arrest of secondary cracks was the source of the discontinuous yielding. Fig. 6 shows the optical photographs of the cross-section near fracture surface in the Fe-Ga-Al specimens. A high density of deformation bands were observed in almost all the grains. The crystallographic orientation relationships between the deformation bands and the matrix were detected by EBSD system. The deformation microstructure of the Fe82Ga13.5Al4.5 specimen and the corresponding orientation map are shown in Fig. 7 (a). There were distinctly differences between the orientations of the deformation bands and the matrix and the misorientation of the two crystallographic orientations was 60o <111>. Fig. 7 (b) shows the pole figure of the deformation bands and the matrix. More than three coincident points corresponding to the deformation bands and the matrix were observed in the pole figure, respectively, as indicated by dotted circle in Fig. 7 (b). These results indicate that the deformation bands and the matrix exhibited a twinning orientation of {112} <111>. Deformation twinning occurred in the Fe-Ga-Al specimens during the tensile tests and high densities of deformation twins were observed. In addition to the twins parallel to each other, some cross twins also occurred, suggesting that there was another different twinning system in the grains. Overall, the deformation twinning was thought to be the source of serrated yielding in the stress-strain curves of alloys of Fe82Ga18-xAlx. Slip and twinning are the two fundamental modes by which metals and alloys can deform plastically. For bcc metals and alloys, twinning occurs when specimens are tested at low temperature or high strain rate. In that case slip is impeded, twinning is prone to occur. On one hand, a great number of deformation twins can provide great plastic deformation. On the other
hand, the grain orientation can be greatly adjusted by twinning. New orientation of grain is beneficial to the activation of new slip or twinning system, so that plastic deformation is promoted. Kellogg et al. investigated the tensile properties along two different crystallographic directions of single crystal Fe-Ga alloys at room temperature [5]. The {112} <111> slip occurred when Fe-Ga alloy was tested along [100] direction. The critical shear stress for {112} <111> slip was greater than that for {110} <111> slip, which occurred when tests along [110] direction. Therefore, twinning is more prone to occur in tensile tests along [100] direction for Fe-Ga based alloys. Fig. 8 shows the fracture elongation and volume fraction of deformation twins in the Fe-Ga-Al specimens corresponding to those in Fig. 6, respectively. The volume fraction of deformation twins was estimated through the area percentage of deformation twins in the entire cross-section of specimens. With Al content increased, the volume fraction of deformation twins tended to decrease. The volume fraction of deformation twins was 59.89% in the Fe82Ga13.5Al4.5 alloy, while the fraction decreased to 21.44% in the Fe82Ga4.5Al13.5 alloy. This tendency was also reflected in the stress-strain curves, where the serration reduced with Al content increased. The elongation at fracture is determined by the joint effect of slip and twinning. It was seen that the fracture elongation of the Fe82Ga4.5Al13.5 alloy still maintained at a level similar to the Fe82Ga13.5Al4.5 alloy, although the volume fraction of deformation twins decreased by 63%. It was revealed that the slip made a significant contribution to the fracture elongation of alloys with high Al content. Notably, the maximum elongation at fracture of 16.5% was observed in the Fe82Ga9Al9 alloy. It seems that the alloy concentration is beneficial to obtain the best joint effect of slip and twinning. 4. Conclusions The tensile ductility of Fe-Ga alloy was significantly improved with Al addition. Substitution of 4.5 at. %Al for Ga greatly increased the fracture elongation from 1.3% to 12.3% in the Fe82Ga18 alloy. While the Fe-Ga-Al alloys showed improvement in ductility compared to binary Fe-Ga alloy, the Fe82Ga9Al9 alloy showed the greatest fracture elongation of 16.5%. Alloys of Fe82Ga18-xAlx in the tensile tests exhibited serrated yielding in the stress-strain curves. The observation of deformation microstructure showed that a great number of deformation twins formed in the Fe-Ga-Al alloys, which were thought to be the source of serrated yielding in the stress-strain curves.
The plastic deformation during tensile tests is determined by the joint effect of slip and twinning. With Al content increased, the volume fraction of deformation twins tended to decrease, but meanwhile the slip was promoted. Consequently, the fracture elongation of the Fe82Ga4.5Al13.5 alloy still maintained at a level similar to the Fe82Ga13.5Al4.5 alloy, although the volume fraction of deformation twins decreased by 63%.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 51271019, 51501006) and State Key Laboratory for Advanced Metals and Materials (2014Z-10, 2014Z-19)
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Figure captions Fig. 1 (a) Optical photograph taken from the longitudinal section of Fe82Ga4.5Al13.5 alloy rod, where the arrow indicates the rod axis; (b) Inverse pole figure corresponding to the rod direction of the Fe82Ga4.5Al13.5 alloy. Fig. 2 The λ-H curves for alloys of Fe82Ga18-xAlx under no pre-stress, the inset shows the variation of the saturation magnetostriction observed on varying the Al content in the alloys. Fig. 3 Stress-strain curves for alloys of Fe82Ga18-xAlx. Fig. 4 Fracture surface morphology of alloys: (a) Fe82Ga18, (b) Fe82Ga13.5Al4.5, (c) Fe82Ga12Al6, (d) Fe82Ga9Al9, (e) Fe82Ga6Al12, (f) Fe82Ga4.5Al13.5. Fig. 5 Ultimate tensile strength and yield strength at room temperature for alloys of Fe82Ga18-xAlx. Fig. 6 Optical micrograph of the cross-section near fracture surface for the alloys: (a) Fe82Ga13.5Al4.5, (b) Fe82Ga12Al6, (c) Fe82Ga9Al9, (d) Fe82Ga6Al12, (e) Fe82Ga4.5Al13.5. Fig. 7 The orientation relationship of the deformation bands and matrix in the Fe82Ga13.5Al4.5`alloy: (a) orientation map; (b) {112} pole figure. Fig. 8 Volume fraction of deformation twins () and elongation at fracture () for alloys of Fe82Ga18-xAlx.
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Highlights
Tensile ductility of directional solidified Fe-Ga alloys was significantly improved with Al addition.
The fracture elongation of binary Fe82Ga18 alloy was only 1.3% at room temperature.
The fracture elongation of Fe82Ga9Al9 alloy was 16.5% at room temperature.
A great number of deformation twins formed in the Fe-Ga-Al alloys during tensile tests at room temperature.
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PII: DOI: Reference:
S0304-8853(16)31092-7 http://dx.doi.org/10.1016/j.jmmm.2016.09.072 MAGMA61853
To appear in: Journal of Magnetism and Magnetic Materials Received date: 14 June 2016 Revised date: 9 August 2016 Accepted date: 12 September 2016 Cite this article as: Yangyang Liu, Jiheng Li and Xuexu Gao, Effect of Al substitution for Ga on the mechanical properties of directional solidified Fe-Ga a l l o y s , Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.09.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Al substitution for Ga on the mechanical properties of directional solidified Fe-Ga alloys Yangyang Liu, Jiheng Li, Xuexu Gao* State Key Laboratory of Advanced Metals and Materials, University of Science and Technology Beijing 100083, China Abstract: Alloys of Fe82Ga18-xAlx (x=0, 4.5, 6, 9, 12, 13.5) were prepared by directional solidification technique and exhibited a <001> preferred orientation along the axis of alloy rods. The saturation magnetostriction value of the Fe82Ga13.5Al4.5 alloy was 247 ppm under no pre-stress. The tensile properties of alloys of Fe82Ga18-xAlx at room temperature were investigated. The results showed that tensile ductility of binary Fe-Ga alloy was significantly improved with Al addition. The fracture elongation of the Fe82Ga18 alloy was only 1.3%, while that of the Fe82Ga9Al9 alloy increased up to 16.5%. Addition of Al increased the strength of grain boundary and cleavage, resulting in the enhancement of tensile ductility of the Fe-Ga-Al alloys. Analysis of deformation microstructure showed that a great number of deformation twins formed in the Fe-Ga-Al alloys, which were thought to be the source of serrated yielding in the stress-strain curves. The effect of Al content in the Fe-Ga-Al alloys on tensile ductility was also studied by the analysis of deformation twins. It indicated that the joint effect of slip and twinning was beneficial to obtain the best ductility in the Fe82Ga9Al9 alloy.
Keywords: Fe-Ga alloy, Fe-Ga-Al alloy, tensile property, twinning.
＊corresponding author. Tel.:+86 10 62334343; fax: +86 10 62334327; e-mail: [email protected]
1. Introduction Fe-Ga alloys, as well known, are a kind of functional metal materials with high magnetostrictive strain. It has been reported that the maximum saturation magnetostriction (3/2)λ100 in single crystal Fe-Ga alloys can reach up to 400 ppm, which is largely higher than that in traditional magnetostrictive alloys [1-4]. Compare to Terfenol-D, Fe-Ga alloys exhibit high strength and toughness. The unique combination of magnetostrictive and mechanical properties in Fe-Ga alloys makes them attractive for sensors, actuators and transducers applications. Although single crystal Fe-Ga alloys are quite tough, polycrystalline Fe-Ga alloys can suffer from intergranular fracture due to weak grain boundaries [5-7]. Recently, studies on Fe-Ga alloys have shown that addition of small amounts of alloying elements leads to improvement of grain boundary strength and then enhancement in the mechanical strength and ductility [8-12]. The unique combination property of Fe-Ga alloys makes them a promising magnetostrictive material, but the significant eddy current losses originating from high conductivity of alloys limit their operation at high frequencies. Rolling them into thin sheets has been thought to be an economical and effective way to reduce the eddy current losses. Na et al. investigated the influence of alloying additions of B, C, Mn, Mo, Nb and NbC on the rollability and deformation behavior of Fe-Ga alloys [8]. As reported by Na et al., binary and C-added Fe-Ga alloys were easily fractured during hot rolling due to the cracks formed along grain boundaries [8]. Alloying additions of B, Mo, Nb and NbC into Fe-Ga alloys changed the fracture mode from an intergranular to a transgranular fracture mode with cleavage and then improved the roll ability. High-quality rolled sheets were obtained by alloying additions except Mn, which was detrimental to improvement in ductility of Fe-Ga alloys. Furthermore, alloying addition of Cr has been shown an effective alloying element to improve the ductility of Fe-Ga alloys [6]. Nolting et al. conducted tensile testing on binary and alloyed Galfenol rolled sheets with the thickness of 4~5 mm [12]. At room temperature, the binary Galfenol rolled sheets had ultimate tensile strength of 455 MPa and little or no plasticity occurs prior to fracture. All the binary Galfenol specimens showed intergranular fracture mode. The addition of Al increased both strength and ductility and changed fracture mode to transgranular fracture. The elongation at fracture of alloys with Al additions was up to 5.8%. On the other hand, Yuan et al. explored the
Goss texture development in rolled columnar-grained Fe-Ga alloys during secondary recrystallization [13, 14]. It was revealed that the <100> orientation from initial directional solidified Fe-Ga alloys can be inherited by the rolled sheets, resulting in a larger magnetostriction. In our previous work, the <100> oriented and columnar-grained Fe-Ga-Al alloys have been prepared by directional solidification technique and the magnetostriction was up to 250 ppm under no pre-stress. This work examined the tensile properties of directional solidified Fe-Ga and Fe-Ga-Al alloys at room temperature. The effect and mechanism of Al substitution for Ga on the mechanical properties of Fe-Ga alloys were mainly studied, which could provide theoretic foundation for improving the roll ability of Fe-Ga based alloys. 2. Experimental Alloys of Fe82Ga18-xAlx (x=0, 4.5, 6, 9, 12, 13.5) were prepared from pure Fe (99.9 wt. %), pure Ga (99.99 wt. %) and pure Al (99.9 wt. %) and were grown by the directional solidification process at a growth rate of 12 mm/min [15]. Ga and Al losses were considered as 2 wt. % and 0.5 wt. %, respectively. The alloy rods were obtained with the dimension of 36×36×200 mm. Then specimens for tensile test were machined by electrical discharge machining. Before tensile testing, specimens were annealed at 1100 oC for 1h and then 730 oC for 3h followed by furnace cooling. Tensile tests were conducted in the DDL50 electronic universal testing machine at room temperature. The tensile stress was applied along the axial direction of alloy rods and the tensile rate was controlled at constant 0.6 mm/min. The specimens were polished and etched with a solution of 10% HNO3 and 90% C2H5OH at room temperature. The crystallographic orientation of specimens was detected by electron backscattered diffraction (EBSD) and the pole figure and inverse pole figure were obtained by analyzing EBSD patterns. The fracture surface of tensile specimens was observed by scanning electron microscope (SEM). An optical microscope was employed to observe the microstructure of tensile specimens. 3. Results and discussion Fig. 1(a) shows the optical photograph taken from the longitudinal section of Fe82Ga4.5Al13.5 alloy rod, where the arrow indicates the rod axis. It can be seen that the alloy is composed of coarse columnar grains with the diameter of 1~2 mm. The growth direction of columnar grains slightly deviates from the axis of alloy rod. The crystallographic orientation of columnar grains
was examined by EBSD and the inverse pole figure corresponding to the rod direction (RD) is shown in Fig. 1(b). The columnar grains orientation along RD is consistent with the [001] crystallographic direction, indicating a <001> preferred orientation along the axis of alloy rod. The magnetostrictive properties of alloys of Fe82Ga18-xAlx with no pre-stress were measured using the JDAW-2011 magnetostriction measurement equipment. The magnetostriction values at room temperature were obtained along the RD using resistance strain gauges with the gauge area of 2.8 mm×2.0 mm (base area of 6.4 mm×3.5 mm). Fig. 2 shows the magnetostriction versus applied magnetic field for alloys of Fe82Ga18-xAlx. The saturation magnetostriction value of the Fe82Ga13.5Al4.5 alloy was 247 ppm under no pre-stress. Increasing Al substitution for Ga to 6 at. %, the saturation magnetostriction still maintained a high level. When Al addition is more than 9 at. %, the saturation magnetostriction decreased sharply, but still maintained a level of ~200 ppm. Even with 13.5 at. % Al substitution for Ga in the Fe82Ga18 alloy, the saturation magnetostriction value still reached up to 199 ppm. This value is higher than that of the single crystal Fe82Al18 alloys [16]. Fig. 3 shows the stress-strain curves for alloys of Fe82Ga18-xAlx obtained from tensile tests at room temperature. It can be seen that the ductility of Fe-Ga alloy was significantly improved with Al addition. Substitution of 4.5 at. %Al for Ga significantly increased the fracture elongation from 1.3% to 12.3% in the Fe82Ga18 alloy. Compared to binary Fe-Ga alloy, the Fe-Ga-Al alloys showed an improvement in ductility, notably the Fe82Ga9Al9 alloy showed the greatest fracture elongation of 16.5%. The fracture surfaces for alloys of Fe82Ga18-xAlx are shown in Fig. 4. The cleavage steps and river patterns clearly seen in the fracture surfaces indicated a cleavage fracture mechanism. Both binary Fe-Ga alloy and ternary Fe-Ga-Al alloys exhibited transgranular fracture mode. The brittle fracture along grain boundary is prone to occur in as-cast binary Fe-Ga alloy, since the weak grain boundary. Other works have shown that there was only elastic deformation and no obvious yield can be observed in tensile testing for as-cast binary Fe-Ga alloy [6, 12]. Compared to the as-cast, binary Fe-Ga alloy from directional solidification method shows a certain plastic deformation and changes the fracture mode from intergranular to transgranular fracture, as shown in Fig. 4 (a). For directional solidified binary Fe-Ga alloy, the improvement of ductility is mainly due to the coarse columnar grains grown along tensile testing direction. The
formation of the coarse columnar grains reduces weak grain boundaries in the direction perpendicular to tensile testing direction, causing the change of fracture mode from intergranular to transgranlunar. Li et al. have pointed out that both the elements of B and Cr improved the ductility of Fe83Ga17 alloy, but the mechanisms were completely different [6]. The element of B tended to segregate at the grain boundaries. The segregation decreased the grain boundary energy and increased the grain boundary cohesion. Furthermore, it was beneficial to resist the crack propagation that addition of B refined grains of Fe-Ga alloy and increased the area of grain boundaries. Therefore, B improved the ductility of Fe-Ga alloy. Differing from the element of B, the improvement in ductility of Fe-Ga alloy with Cr addition was attributed to the solution strengthening. That Cr dissolved in the Fe-Ga matrix played a role of solution strengthening both improved the strength of grain boundary and cleavage. It has been observed that there was no interfacial segregation of Al in the Fe-Ga-Al alloys [17]. Similar to the element of Cr, Al dissolved in the Fe-Ga matrix. It seems that the solution of Al could also strengthen the grain boundary and cleavage. Fig. 5 shows the ultimate tensile strength and yield strength for alloys of Fe82Ga18-xAlx obtained from tensile tests at room temperature. In order to ensure accuracy of the data, several tensile tests were carried out for each of the alloys. Although there was a certain dispersion of the data, which was probably caused by the difference of microstructure such as shrinkage cavity and porosity, the trend in the ultimate tensile strength and yield strength was still clear. With the addition of Al, the ultimate tensile strength of Fe-Ga alloy increased, while the yield strength decreased. The more difference between the ultimate tensile strength and yield strength represents the better ductility and toughness in the Fe-Ga-Al alloys. McKamey et al. pointed out that the beneficial effect of Cr on ductility of the Fe3Al alloy was attributed to the solution softening and thus the easier slip behavior of dislocations [18]. Similarly, the decrease of yield strength in the Fe-Ga-Al alloys originates from solution softening of Al atoms. Both Ga and Al can be dissolved in Fe matrix and have similar solubility. Ga dissolved in Fe matrix will cause a certain degree of lattice distortion, since the difference in size between Ga and Fe atom. Lattice distortion increases the resistance to dislocation slip and makes it more difficult to carry out. So that the yield strength of Fe-Ga alloy increased, accompanied with ductility loss. The size of Al atom is smaller than that
of Ga atom. Al substitution for Ga tends to contract the lattice, reducing the degree of lattice distortion in Fe matrix. Therefore, the yield strength decreased and the ductility was improved in the Fe-Ga-Al alloys. It is worth noting that all the alloys in the tensile tests exhibited serrated yielding in the stress-strain curves, which was accompanied by an audible sound. Discontinuous yielding was also observed by Kellogg et al. during tensile tests of single crystal Fe-Ga alloy [5]. The authors thought that the discontinuous yielding resulted from twinning, kind band formation or stress-induced phase transformation. In addition, Nolting et al. also reported that discontinuous yielding was observed during tensile tests of Fe-Ga alloys with C, Cr and Al additions [12]. They hypothesized that the formation and arrest of secondary cracks was the source of the discontinuous yielding. Fig. 6 shows the optical photographs of the cross-section near fracture surface in the Fe-Ga-Al specimens. A high density of deformation bands were observed in almost all the grains. The crystallographic orientation relationships between the deformation bands and the matrix were detected by EBSD system. The deformation microstructure of the Fe82Ga13.5Al4.5 specimen and the corresponding orientation map are shown in Fig. 7 (a). There were distinctly differences between the orientations of the deformation bands and the matrix and the misorientation of the two crystallographic orientations was 60o <111>. Fig. 7 (b) shows the pole figure of the deformation bands and the matrix. More than three coincident points corresponding to the deformation bands and the matrix were observed in the pole figure, respectively, as indicated by dotted circle in Fig. 7 (b). These results indicate that the deformation bands and the matrix exhibited a twinning orientation of {112} <111>. Deformation twinning occurred in the Fe-Ga-Al specimens during the tensile tests and high densities of deformation twins were observed. In addition to the twins parallel to each other, some cross twins also occurred, suggesting that there was another different twinning system in the grains. Overall, the deformation twinning was thought to be the source of serrated yielding in the stress-strain curves of alloys of Fe82Ga18-xAlx. Slip and twinning are the two fundamental modes by which metals and alloys can deform plastically. For bcc metals and alloys, twinning occurs when specimens are tested at low temperature or high strain rate. In that case slip is impeded, twinning is prone to occur. On one hand, a great number of deformation twins can provide great plastic deformation. On the other
hand, the grain orientation can be greatly adjusted by twinning. New orientation of grain is beneficial to the activation of new slip or twinning system, so that plastic deformation is promoted. Kellogg et al. investigated the tensile properties along two different crystallographic directions of single crystal Fe-Ga alloys at room temperature [5]. The {112} <111> slip occurred when Fe-Ga alloy was tested along [100] direction. The critical shear stress for {112} <111> slip was greater than that for {110} <111> slip, which occurred when tests along [110] direction. Therefore, twinning is more prone to occur in tensile tests along [100] direction for Fe-Ga based alloys. Fig. 8 shows the fracture elongation and volume fraction of deformation twins in the Fe-Ga-Al specimens corresponding to those in Fig. 6, respectively. The volume fraction of deformation twins was estimated through the area percentage of deformation twins in the entire cross-section of specimens. With Al content increased, the volume fraction of deformation twins tended to decrease. The volume fraction of deformation twins was 59.89% in the Fe82Ga13.5Al4.5 alloy, while the fraction decreased to 21.44% in the Fe82Ga4.5Al13.5 alloy. This tendency was also reflected in the stress-strain curves, where the serration reduced with Al content increased. The elongation at fracture is determined by the joint effect of slip and twinning. It was seen that the fracture elongation of the Fe82Ga4.5Al13.5 alloy still maintained at a level similar to the Fe82Ga13.5Al4.5 alloy, although the volume fraction of deformation twins decreased by 63%. It was revealed that the slip made a significant contribution to the fracture elongation of alloys with high Al content. Notably, the maximum elongation at fracture of 16.5% was observed in the Fe82Ga9Al9 alloy. It seems that the alloy concentration is beneficial to obtain the best joint effect of slip and twinning. 4. Conclusions The tensile ductility of Fe-Ga alloy was significantly improved with Al addition. Substitution of 4.5 at. %Al for Ga greatly increased the fracture elongation from 1.3% to 12.3% in the Fe82Ga18 alloy. While the Fe-Ga-Al alloys showed improvement in ductility compared to binary Fe-Ga alloy, the Fe82Ga9Al9 alloy showed the greatest fracture elongation of 16.5%. Alloys of Fe82Ga18-xAlx in the tensile tests exhibited serrated yielding in the stress-strain curves. The observation of deformation microstructure showed that a great number of deformation twins formed in the Fe-Ga-Al alloys, which were thought to be the source of serrated yielding in the stress-strain curves.
The plastic deformation during tensile tests is determined by the joint effect of slip and twinning. With Al content increased, the volume fraction of deformation twins tended to decrease, but meanwhile the slip was promoted. Consequently, the fracture elongation of the Fe82Ga4.5Al13.5 alloy still maintained at a level similar to the Fe82Ga13.5Al4.5 alloy, although the volume fraction of deformation twins decreased by 63%.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 51271019, 51501006) and State Key Laboratory for Advanced Metals and Materials (2014Z-10, 2014Z-19)
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Figure captions Fig. 1 (a) Optical photograph taken from the longitudinal section of Fe82Ga4.5Al13.5 alloy rod, where the arrow indicates the rod axis; (b) Inverse pole figure corresponding to the rod direction of the Fe82Ga4.5Al13.5 alloy. Fig. 2 The λ-H curves for alloys of Fe82Ga18-xAlx under no pre-stress, the inset shows the variation of the saturation magnetostriction observed on varying the Al content in the alloys. Fig. 3 Stress-strain curves for alloys of Fe82Ga18-xAlx. Fig. 4 Fracture surface morphology of alloys: (a) Fe82Ga18, (b) Fe82Ga13.5Al4.5, (c) Fe82Ga12Al6, (d) Fe82Ga9Al9, (e) Fe82Ga6Al12, (f) Fe82Ga4.5Al13.5. Fig. 5 Ultimate tensile strength and yield strength at room temperature for alloys of Fe82Ga18-xAlx. Fig. 6 Optical micrograph of the cross-section near fracture surface for the alloys: (a) Fe82Ga13.5Al4.5, (b) Fe82Ga12Al6, (c) Fe82Ga9Al9, (d) Fe82Ga6Al12, (e) Fe82Ga4.5Al13.5. Fig. 7 The orientation relationship of the deformation bands and matrix in the Fe82Ga13.5Al4.5`alloy: (a) orientation map; (b) {112} pole figure. Fig. 8 Volume fraction of deformation twins () and elongation at fracture () for alloys of Fe82Ga18-xAlx.
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Highlights
Tensile ductility of directional solidified Fe-Ga alloys was significantly improved with Al addition.
The fracture elongation of binary Fe82Ga18 alloy was only 1.3% at room temperature.
The fracture elongation of Fe82Ga9Al9 alloy was 16.5% at room temperature.
A great number of deformation twins formed in the Fe-Ga-Al alloys during tensile tests at room temperature.