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Enhancement of tensile properties by the solid solution strengthening of nitrogen in Zr-based metallic glass composites
Materials Science & Engineering A 696 (2017) 461–465
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Enhancement of tensile properties by the solid solution strengthening of nitrogen in Zr-based metallic glass composites ⁎
J.L. Chenga,b, G. Chenc, , W. Zhaoa,b, Z.Z. Wanga,b, Z.W. Zhangd,
MARK
⁎
a
School of Materials Science and Engineering, Nanjing Institute of Technology, Nanjing 211167, PR China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, PR China c Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, PR China d Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Bulk amorphous alloys Composites Mechanical characterization Plasticity
The effects of nitrogen on both the glass forming ability (GFA) and mechanical properties of Zr-based bulk metallic glasses were studied experimentally. The results show that the doping of nitrogen could induce the precipitation of N-rich Zr3Ni-type crystals, and the precipitated crystals don’t trigger heterogeneous nucleation. Therefore, the remainder molten metal contains very lower nitrogen content and keeps good glass formation ability. Based on this, using the precipitated β-Zr(Ti, Nb) acted as a solid solvent for absorbed nitrogen, the optimum mechanical properties of bulk metallic glass composites can be obtained by doping an appreciate amount of nitrogen. This finding not only gives us a clue to avoid the detriment of light interstitial impurity to glass formation, but also opens a window to adjust the mechanical properties of the composites.
1. Introduction Bulk metallic glasses (BMGs) have been studied extensively in recent years because of their unique long-range disordering structure characteristic, which endows BMGs with a combination of mechanical, chemical, and physical properties. These intriguing properties make BMGs of the most promising engineering materials [1–5]. Nevertheless, to avoid heterogeneous nucleation, most of BMGs are prepared in high vacuum systems using high-purity raw materials containing very low level impurities. The rigorous fabrication conditions for glass formation dramatically increase the manufacturing cost and severely limit the widespread applications of BMGs [6,7]. Therefore, it is vitally important to manufacture BMGs using industrial-grade raw materials and processing in conventional industrial systems. As known, oxygen and nitrogen are the most abundant elements in the Earth's atmosphere and are not easy to be excluded during the purification, melting and machining of metallic materials, because of their strong interactions with the metal elements [8–10]. The effect of oxygen on the glass formation of BMGs has been studied extensively [11–16]. Initially, oxygen was considered as a detrimental element to glass formation in BMGs [11–13]. Oxygen usually induces formation of intermetallic oxide particles [11] or metastable phases such as quasicrystalline phases [12,13], leading to a decrease in the glass formation (GFA). Recently, it was reported that proper doping of oxygen (∼0.1 at
⁎
%) could be beneficial for the GFA in some Fe- [14] and Zr-based [15,16] BMGs. Moreover, Cheng et al. [17–19] reported that the BMG composites could be obtained in the Zr-BMGs, which even contained very high oxygen level (∼4 at%). These recent results give some constructive implications to reduce the harmful of oxygen in BMGs. Compare with oxygen, only a few papers reported the effect of nitrogen on the glass formation of BMGs. Liu [20] and Cao [21] proposed that proper addition of nitrogen could facilitate the glass formation by suppressing formation of the competing crystal in Zr and Ti-based BMGs, respectively. Nevertheless, it remains unclear about effects of nitrogen on GFA and mechanical properties of BMG composites. Specially, limited work has been performed to explore the distribution of nitrogen in BMGs and their composites. In present study, the effect of nitrogen doping on the glass formation and mechanical properties was investigated for Zr-BMGs and their composites. Relevant results and underlying mechanisms related with nitrogen distribution are discussed in detail. 2. Experimental procedures Zr-based alloy ingots with a nominal composition of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (vit1) and Zr39.6Ti33.9Nb7.6Cu6.4Be12.5 (DH3) were prepared by arc-melting. To adjust nitrogen concentration in the alloys, different amounts of nitrogen, i.e., 0.5, 1 and 1.5 at%,
Corresponding authors. E-mail addresses: [email protected] (G. Chen), [email protected] (Z.W. Zhang).
http://dx.doi.org/10.1016/j.msea.2017.04.075 Received 22 December 2016; Received in revised form 18 April 2017; Accepted 19 April 2017 Available online 27 April 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 696 (2017) 461–465
J.L. Cheng et al.
needle-like crystals with the volume fraction of 14% appeared in the glass matrix, as shown in Fig. 1(b). The average length in the longitudinal and the width of needle-like crystals is 43 μm and 4 μm, respectively. The composition of the needle-like crystals is about Zr58.5Ti8.7Cu8.6Ni10.4N13.8 measured by EPMA technique. It is interesting to find that the doped nitrogen is almost segregated in the needlelike crystals, while the nitrogen content in glass matrix contains is only about 0.06% which is almost the same as N-free Vit1 alloy. Form XRD pattern, these needle-like crystals are identified as Zr3Nitype phase with orthorhombic Re3B-type structure, which doesn’t exist in the binary system. It only can be found in Zr-based alloys in the presence of oxygen or nitrogen [23]. Fig. 1(d) shows the crystallization behavior of the alloys. As shown, two alloys exhibit the similar crystallization behavior with two significantly exothermic peaks, only the (Vit1)99N1 shows slightly higher thermal parameters of Tg (glass transition temperature) and Tx (onset crystallization temperature). The first and second exothermic peaks represent the primary crystallization and eutectic crystallization, respectively. This result indicates that the nitrogen-induced precipitation of Zr3Ni-type crystals does not significantly change both the crystallization process and the stability of glass matrix. The above results clearly reveal that the doping of nitrogen induces the precipitation of N-rich Zr3Ni-type crystals, and the precipitates do not act as a heterogeneous nucleation site for crystallization. As most of the Nitrogen segregates to the precipitate, the matrix has very low nitrogen content keeping good GFA. This finding gives us a way to alleviate the harmful effects of a commonly found light interstitial impurity i.e., nitrogen, by developing appropriate metallic glass composites. It is well known that the β-Zr(Ti, Nb)/BMG composites exhibit excellent ductility reported by Hoffman et al. [24]. The Zr, Ti and Nb solid solutions have a strong affinity and a high solubility for nitrogen atom. Therefore, we expect that the β-Zr(Ti, Nb) precipitates
were added into the as-cast alloys using TiN. To ensure that TiN was melted and homogeneously dissolved in the alloys, the primary alloy ingots with Zr, Ti and TiN were prepared firstly by arc-melting. And then, the ingots were melted with other alloy elements again. The tensile samples of (DH3)100−xNx composites were prepared using the semi-solid progressive solidification (SSPS) method [22]. The microstructure and phase identification were carried out using an optical microscopy (OM) and an X-ray diffractometer (XRD), respectively. An average value of nitrogen concentration in the samples was measured using a LECO-TC436 inert gas fusion (IGF) nitrogen/ oxygen analyzer. The distribution of nitrogen in the alloys was also analyzed using electron probe microanalysis (EPMA). Thermal property measurements were conducted using a differential scanning calorimeter (DSC). At least three specimens for tensile test with a gauge length of 36 mm and 6 mm in diameter were prepared according to the ASTM E8M standard. CSM-NHT2 nano-indentation instrument was used to measure the hardness of microstructure. To ensure the results are statistically meaningful, the average value was taken from 11 times measurements for each sample. 3. Results and discussion The nitrogen contents in the samples were measured by IGF. The results show that nitrogen content in the N-free alloy is about 600 appm. For the 0.5, 1 and 1.5 at% added alloys, the nitrogen concentrations were measured to be 0.58, 1.08, and 1.59 at%, respectively. The matching between the nominal and measured values indicates that nitrogen was introduced into the alloys using the TiN. Fig. 1(a) and (b) show the microstructures of Vit1 and (Vit1)99N1 arc-melting ingots, respectively. N-free Vit1 sample exhibits a featureless glass structure, which is further confirmed by the XRD pattern (see Fig. 1(c)). While the (Vit1)99N1 shows a typical glass composite, some
Fig. 1. Optical micrographs of (Vit1)100−xNx prepared by arc-melting (a) x=0, (b)x=1, and their XRD patterns (c), DSC curves (d).
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Fig. 2. Optical micrographs of (DH3)100−xNx prepared by arc-melting (a) x=0, (b)x=1.5, and their XRD patterns (c), DSC curves (d).
Fig. 3. The elements maps of the (DH3)98.5N1.5 alloy analyzed using electron probe microanalysis.
exhibit similar microstructures which contain the coarse and spherical crystalline precipitates with the volume fraction of 58%. The crystals are homogeneously embedded in the glass matrix. The average diameter of the precipitates is approximately 43 μm. The volume
can act as a sink to absorb the nitrogen from the glass forming matrix. Based on these considerations, we further studied the effect of nitrogen on microstructure and mechanical properties of β-Zr(Ti, Nb)/BMG. As shown in Fig. 2(a) and (b), the DH3 and (DH3)98.5N1.5 alloys 463
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Fig. 4. Nano-indentation load-displacement curves (a) Engineering tensile stress–strain curves (b) of the BMG composites with different nitrogen contents.
(DH3)98.5N1.5 alloy. It can be seen that Ti, Nb and N are mainly concentrated in the β-Zr(Ti, Nb) particles while Cu is significantly enriched in glass matrix, and Zr is distributed homogeneously in the alloy. These results clearly indicate that the precipitated β-Zr(Ti, Nb) particles act as nitrogen scavengers to remove nitrogen from the remainder molten metal, which gives a high GFA for the matrix. What are the effects of this nitrogen segregation in the β-Zr(Ti, Nb) on the mechanical properties of the composites? The hardness of the alloys is measured by a nano-indentation. The results are summarized in Fig. 4(a) and Table 1. From the data, it is evident that the hardness of the glass matrix is not sensitive to the average nitrogen concentration. However the hardness of β-Zr phase increases significantly with the increase in nitrogen content. For the (DH3)98.5N1.5 alloy, the hardness of β-Zr phase is up to 7.5 GPa, which is close to the that of glass matrix. This increase should be attributed to the nitrogen segregation in the βZr phase, which leads to interstitial solid solution strengthening. This interstitial solid solution strengthening has a significant influence on the tensile mechanical properties, as show in Fig. 4(b) and Table 1. In comparison with N-free DH3 alloy, the (DH3)99.5N0.5 composite shows a substantial increase in the yield strength from 1170 to 1280 MPa without sacrificing the overall ductility. When the nitrogen content increases to 1%, the yield strength is further increased to 1370 MPa with moderate decrease in the ductility for the (DH3)99N1 composite. However, with the further increase in the nitrogen content, the (DH3)98.5N1.5 composite fails in an apparently brittle manner with 1450 MPa fracture strength. Fig. 5 shows the tensile fractographs of DH3 and (DH3)98.5N1.5 composites. The DH3 composite exhibits many
Table 1 The hardness (H), yield strength, ultimate tensile strength and total strain to failure of the DH3 alloys doped with different amounts of nitrogen are summarized. Alloys
Structure
H (GPa)
σy (MPa)
σmax (MPa)
εtot (%)
DH3
Glass matrix β-Zr phase
8.2 4.5
1170
1220
9.0
(DH3)99.5N0.5
Glass matrix β-Zr phase
8.3 5.1
1280
1330
8.7
(DH3)99N1
Glass matrix β-Zr phase
8.2 6.2
1370
1420
6.9
(DH3)98.5N1.5
Glass matrix β-Zr phase
8.3 7.5
–
1450
1.85
fraction and size of spherical crystals don’t change with nitrogen content. The spherical crystals can be identified as β-Zr(Ti, Nb) structure by XRD patterns (Fig. 2(c)). In all the samples used in the experiments, no nitrides were detected as characterized by optical microscopy and X-Ray diffraction. Fig. 2(d) shows the DSC curves for the alloys. It can be seen that all the alloys exhibit the similar the glass transition and crystallization behavior, although they have different nitrogen levels. This indicates that the composition of the glass matrices is nearly the same. Therefore, we propose that nitrogen preferentially segregates to β-Zr(Ti, Nb) phase. To provide with more evidences for our claims, EPMA measurement was carried out to analyze the composition and elements distribution, as shown in Fig. 3. The average composition of β-Zr(Ti, Nb) is Zr40.1Ti44.5Nb11.9Cu0.8N2.7 for the
Fig. 5. SEM images of the fracture surfaces for (a) DH3 and (b) (DH3)98.5N1.5 composites.
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slip bands in the β-Zr(Ti, Nb) particles and some ductile tear-off marks on the surface, in agreement with the ductile behavior of β-Zr(Ti, Nb) particles. However, the (DH3)98.5N1.5 composite shows typical brittle fracture morphology, many micro cracks cross the β-Zr(Ti, Nb) particles. This kind of interstitial solid-solution strengthening was also observed in the Ti-alloys, especially Gum metal [25,26]. Wei et al. [25] found that the tensile strength of the Gum metal increased significantly without sacrificing the ductility, when the oxygen contents increase from 0 to 1.0 at% in the Ti-Nb-Ta-Zr alloy. With a further increase in oxygen content, the ductility decreased dramatically similar to our present results.
proofreading. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
4. Conclusions
[10]
In summary, the effects of nitrogen doping in the Vit1 alloy were studied and quantified in terms of the GFA and the mechanical properties of the alloy. It was shown that the doping of nitrogen does not affect the GFA of the alloy owing to formation of N-rich Zr3Ni-type crystalline precipitates. The mechanical properties of the glass composites were also enhanced due to the hardening of the precipitates by Ndoping. These findings give a new perspective to handle a detrimental impurity i.e. Nitrogen in a way that enhances the mechanical properties without compromising GFA in a cost-effective way.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51401104, 51371062 and U1460102), the Natural Science Foundation of Jiangsu Province No. BK20140765, the Qing Lan Project, the Outstanding Scientific and Technological Innovation Team in Colleges and Universities of Jiangsu Province, the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology ASMA201403. The authors are grateful to Dr. K. Kadirvel (The Ohio State University) for careful
[24] [25] [26]
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Enhancement of tensile properties by the solid solution strengthening of nitrogen in Zr-based metallic glass composites ⁎
J.L. Chenga,b, G. Chenc, , W. Zhaoa,b, Z.Z. Wanga,b, Z.W. Zhangd,
MARK
⁎
a
School of Materials Science and Engineering, Nanjing Institute of Technology, Nanjing 211167, PR China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, PR China c Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, PR China d Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Bulk amorphous alloys Composites Mechanical characterization Plasticity
The effects of nitrogen on both the glass forming ability (GFA) and mechanical properties of Zr-based bulk metallic glasses were studied experimentally. The results show that the doping of nitrogen could induce the precipitation of N-rich Zr3Ni-type crystals, and the precipitated crystals don’t trigger heterogeneous nucleation. Therefore, the remainder molten metal contains very lower nitrogen content and keeps good glass formation ability. Based on this, using the precipitated β-Zr(Ti, Nb) acted as a solid solvent for absorbed nitrogen, the optimum mechanical properties of bulk metallic glass composites can be obtained by doping an appreciate amount of nitrogen. This finding not only gives us a clue to avoid the detriment of light interstitial impurity to glass formation, but also opens a window to adjust the mechanical properties of the composites.
1. Introduction Bulk metallic glasses (BMGs) have been studied extensively in recent years because of their unique long-range disordering structure characteristic, which endows BMGs with a combination of mechanical, chemical, and physical properties. These intriguing properties make BMGs of the most promising engineering materials [1–5]. Nevertheless, to avoid heterogeneous nucleation, most of BMGs are prepared in high vacuum systems using high-purity raw materials containing very low level impurities. The rigorous fabrication conditions for glass formation dramatically increase the manufacturing cost and severely limit the widespread applications of BMGs [6,7]. Therefore, it is vitally important to manufacture BMGs using industrial-grade raw materials and processing in conventional industrial systems. As known, oxygen and nitrogen are the most abundant elements in the Earth's atmosphere and are not easy to be excluded during the purification, melting and machining of metallic materials, because of their strong interactions with the metal elements [8–10]. The effect of oxygen on the glass formation of BMGs has been studied extensively [11–16]. Initially, oxygen was considered as a detrimental element to glass formation in BMGs [11–13]. Oxygen usually induces formation of intermetallic oxide particles [11] or metastable phases such as quasicrystalline phases [12,13], leading to a decrease in the glass formation (GFA). Recently, it was reported that proper doping of oxygen (∼0.1 at
⁎
%) could be beneficial for the GFA in some Fe- [14] and Zr-based [15,16] BMGs. Moreover, Cheng et al. [17–19] reported that the BMG composites could be obtained in the Zr-BMGs, which even contained very high oxygen level (∼4 at%). These recent results give some constructive implications to reduce the harmful of oxygen in BMGs. Compare with oxygen, only a few papers reported the effect of nitrogen on the glass formation of BMGs. Liu [20] and Cao [21] proposed that proper addition of nitrogen could facilitate the glass formation by suppressing formation of the competing crystal in Zr and Ti-based BMGs, respectively. Nevertheless, it remains unclear about effects of nitrogen on GFA and mechanical properties of BMG composites. Specially, limited work has been performed to explore the distribution of nitrogen in BMGs and their composites. In present study, the effect of nitrogen doping on the glass formation and mechanical properties was investigated for Zr-BMGs and their composites. Relevant results and underlying mechanisms related with nitrogen distribution are discussed in detail. 2. Experimental procedures Zr-based alloy ingots with a nominal composition of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (vit1) and Zr39.6Ti33.9Nb7.6Cu6.4Be12.5 (DH3) were prepared by arc-melting. To adjust nitrogen concentration in the alloys, different amounts of nitrogen, i.e., 0.5, 1 and 1.5 at%,
Corresponding authors. E-mail addresses: [email protected] (G. Chen), [email protected] (Z.W. Zhang).
http://dx.doi.org/10.1016/j.msea.2017.04.075 Received 22 December 2016; Received in revised form 18 April 2017; Accepted 19 April 2017 Available online 27 April 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 696 (2017) 461–465
J.L. Cheng et al.
needle-like crystals with the volume fraction of 14% appeared in the glass matrix, as shown in Fig. 1(b). The average length in the longitudinal and the width of needle-like crystals is 43 μm and 4 μm, respectively. The composition of the needle-like crystals is about Zr58.5Ti8.7Cu8.6Ni10.4N13.8 measured by EPMA technique. It is interesting to find that the doped nitrogen is almost segregated in the needlelike crystals, while the nitrogen content in glass matrix contains is only about 0.06% which is almost the same as N-free Vit1 alloy. Form XRD pattern, these needle-like crystals are identified as Zr3Nitype phase with orthorhombic Re3B-type structure, which doesn’t exist in the binary system. It only can be found in Zr-based alloys in the presence of oxygen or nitrogen [23]. Fig. 1(d) shows the crystallization behavior of the alloys. As shown, two alloys exhibit the similar crystallization behavior with two significantly exothermic peaks, only the (Vit1)99N1 shows slightly higher thermal parameters of Tg (glass transition temperature) and Tx (onset crystallization temperature). The first and second exothermic peaks represent the primary crystallization and eutectic crystallization, respectively. This result indicates that the nitrogen-induced precipitation of Zr3Ni-type crystals does not significantly change both the crystallization process and the stability of glass matrix. The above results clearly reveal that the doping of nitrogen induces the precipitation of N-rich Zr3Ni-type crystals, and the precipitates do not act as a heterogeneous nucleation site for crystallization. As most of the Nitrogen segregates to the precipitate, the matrix has very low nitrogen content keeping good GFA. This finding gives us a way to alleviate the harmful effects of a commonly found light interstitial impurity i.e., nitrogen, by developing appropriate metallic glass composites. It is well known that the β-Zr(Ti, Nb)/BMG composites exhibit excellent ductility reported by Hoffman et al. [24]. The Zr, Ti and Nb solid solutions have a strong affinity and a high solubility for nitrogen atom. Therefore, we expect that the β-Zr(Ti, Nb) precipitates
were added into the as-cast alloys using TiN. To ensure that TiN was melted and homogeneously dissolved in the alloys, the primary alloy ingots with Zr, Ti and TiN were prepared firstly by arc-melting. And then, the ingots were melted with other alloy elements again. The tensile samples of (DH3)100−xNx composites were prepared using the semi-solid progressive solidification (SSPS) method [22]. The microstructure and phase identification were carried out using an optical microscopy (OM) and an X-ray diffractometer (XRD), respectively. An average value of nitrogen concentration in the samples was measured using a LECO-TC436 inert gas fusion (IGF) nitrogen/ oxygen analyzer. The distribution of nitrogen in the alloys was also analyzed using electron probe microanalysis (EPMA). Thermal property measurements were conducted using a differential scanning calorimeter (DSC). At least three specimens for tensile test with a gauge length of 36 mm and 6 mm in diameter were prepared according to the ASTM E8M standard. CSM-NHT2 nano-indentation instrument was used to measure the hardness of microstructure. To ensure the results are statistically meaningful, the average value was taken from 11 times measurements for each sample. 3. Results and discussion The nitrogen contents in the samples were measured by IGF. The results show that nitrogen content in the N-free alloy is about 600 appm. For the 0.5, 1 and 1.5 at% added alloys, the nitrogen concentrations were measured to be 0.58, 1.08, and 1.59 at%, respectively. The matching between the nominal and measured values indicates that nitrogen was introduced into the alloys using the TiN. Fig. 1(a) and (b) show the microstructures of Vit1 and (Vit1)99N1 arc-melting ingots, respectively. N-free Vit1 sample exhibits a featureless glass structure, which is further confirmed by the XRD pattern (see Fig. 1(c)). While the (Vit1)99N1 shows a typical glass composite, some
Fig. 1. Optical micrographs of (Vit1)100−xNx prepared by arc-melting (a) x=0, (b)x=1, and their XRD patterns (c), DSC curves (d).
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Fig. 2. Optical micrographs of (DH3)100−xNx prepared by arc-melting (a) x=0, (b)x=1.5, and their XRD patterns (c), DSC curves (d).
Fig. 3. The elements maps of the (DH3)98.5N1.5 alloy analyzed using electron probe microanalysis.
exhibit similar microstructures which contain the coarse and spherical crystalline precipitates with the volume fraction of 58%. The crystals are homogeneously embedded in the glass matrix. The average diameter of the precipitates is approximately 43 μm. The volume
can act as a sink to absorb the nitrogen from the glass forming matrix. Based on these considerations, we further studied the effect of nitrogen on microstructure and mechanical properties of β-Zr(Ti, Nb)/BMG. As shown in Fig. 2(a) and (b), the DH3 and (DH3)98.5N1.5 alloys 463
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Fig. 4. Nano-indentation load-displacement curves (a) Engineering tensile stress–strain curves (b) of the BMG composites with different nitrogen contents.
(DH3)98.5N1.5 alloy. It can be seen that Ti, Nb and N are mainly concentrated in the β-Zr(Ti, Nb) particles while Cu is significantly enriched in glass matrix, and Zr is distributed homogeneously in the alloy. These results clearly indicate that the precipitated β-Zr(Ti, Nb) particles act as nitrogen scavengers to remove nitrogen from the remainder molten metal, which gives a high GFA for the matrix. What are the effects of this nitrogen segregation in the β-Zr(Ti, Nb) on the mechanical properties of the composites? The hardness of the alloys is measured by a nano-indentation. The results are summarized in Fig. 4(a) and Table 1. From the data, it is evident that the hardness of the glass matrix is not sensitive to the average nitrogen concentration. However the hardness of β-Zr phase increases significantly with the increase in nitrogen content. For the (DH3)98.5N1.5 alloy, the hardness of β-Zr phase is up to 7.5 GPa, which is close to the that of glass matrix. This increase should be attributed to the nitrogen segregation in the βZr phase, which leads to interstitial solid solution strengthening. This interstitial solid solution strengthening has a significant influence on the tensile mechanical properties, as show in Fig. 4(b) and Table 1. In comparison with N-free DH3 alloy, the (DH3)99.5N0.5 composite shows a substantial increase in the yield strength from 1170 to 1280 MPa without sacrificing the overall ductility. When the nitrogen content increases to 1%, the yield strength is further increased to 1370 MPa with moderate decrease in the ductility for the (DH3)99N1 composite. However, with the further increase in the nitrogen content, the (DH3)98.5N1.5 composite fails in an apparently brittle manner with 1450 MPa fracture strength. Fig. 5 shows the tensile fractographs of DH3 and (DH3)98.5N1.5 composites. The DH3 composite exhibits many
Table 1 The hardness (H), yield strength, ultimate tensile strength and total strain to failure of the DH3 alloys doped with different amounts of nitrogen are summarized. Alloys
Structure
H (GPa)
σy (MPa)
σmax (MPa)
εtot (%)
DH3
Glass matrix β-Zr phase
8.2 4.5
1170
1220
9.0
(DH3)99.5N0.5
Glass matrix β-Zr phase
8.3 5.1
1280
1330
8.7
(DH3)99N1
Glass matrix β-Zr phase
8.2 6.2
1370
1420
6.9
(DH3)98.5N1.5
Glass matrix β-Zr phase
8.3 7.5
–
1450
1.85
fraction and size of spherical crystals don’t change with nitrogen content. The spherical crystals can be identified as β-Zr(Ti, Nb) structure by XRD patterns (Fig. 2(c)). In all the samples used in the experiments, no nitrides were detected as characterized by optical microscopy and X-Ray diffraction. Fig. 2(d) shows the DSC curves for the alloys. It can be seen that all the alloys exhibit the similar the glass transition and crystallization behavior, although they have different nitrogen levels. This indicates that the composition of the glass matrices is nearly the same. Therefore, we propose that nitrogen preferentially segregates to β-Zr(Ti, Nb) phase. To provide with more evidences for our claims, EPMA measurement was carried out to analyze the composition and elements distribution, as shown in Fig. 3. The average composition of β-Zr(Ti, Nb) is Zr40.1Ti44.5Nb11.9Cu0.8N2.7 for the
Fig. 5. SEM images of the fracture surfaces for (a) DH3 and (b) (DH3)98.5N1.5 composites.
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slip bands in the β-Zr(Ti, Nb) particles and some ductile tear-off marks on the surface, in agreement with the ductile behavior of β-Zr(Ti, Nb) particles. However, the (DH3)98.5N1.5 composite shows typical brittle fracture morphology, many micro cracks cross the β-Zr(Ti, Nb) particles. This kind of interstitial solid-solution strengthening was also observed in the Ti-alloys, especially Gum metal [25,26]. Wei et al. [25] found that the tensile strength of the Gum metal increased significantly without sacrificing the ductility, when the oxygen contents increase from 0 to 1.0 at% in the Ti-Nb-Ta-Zr alloy. With a further increase in oxygen content, the ductility decreased dramatically similar to our present results.
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4. Conclusions
[10]
In summary, the effects of nitrogen doping in the Vit1 alloy were studied and quantified in terms of the GFA and the mechanical properties of the alloy. It was shown that the doping of nitrogen does not affect the GFA of the alloy owing to formation of N-rich Zr3Ni-type crystalline precipitates. The mechanical properties of the glass composites were also enhanced due to the hardening of the precipitates by Ndoping. These findings give a new perspective to handle a detrimental impurity i.e. Nitrogen in a way that enhances the mechanical properties without compromising GFA in a cost-effective way.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51401104, 51371062 and U1460102), the Natural Science Foundation of Jiangsu Province No. BK20140765, the Qing Lan Project, the Outstanding Scientific and Technological Innovation Team in Colleges and Universities of Jiangsu Province, the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology ASMA201403. The authors are grateful to Dr. K. Kadirvel (The Ohio State University) for careful
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