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Tune the mechanical properties of Ti-based metallic glass composites by additions of nitrogen
Materials Science & Engineering A 694 (2017) 93–97
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Tune the mechanical properties of Ti-based metallic glass composites by additions of nitrogen ⁎
MARK
⁎
Liyuan Li, Jinshan Li , Jun Wang , Hongchao Kou State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, Shaanxi Province, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ti-based metallic glass composites Mechanical properties Solid-solution strengthening Alloy design Nitrogen
Different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the Ti48Zr20Nb12Cu5Be15 bulk metallic glass composites. Results show that addition of nitrogen can increase the yield strength of composites from 1400 to 2350 MPa due to the solid solution strengthening effect. The composites still owns certain plastic strain (above 10% when contains 14,000 ppm nitrogen) before fracture under compressive test. The reason is attribute to nitrogen shows to be a much weaker activator for the formation of brittle α-Ti phase as compared with oxygen, which make the β-Ti dendrite still softer than that of the glassy phase, thus can enhance the strength of the materials but do not deteriorate the plasticity too much. Our research show that nitrogen can be added into the Ti-based metallic glass composites in a very large range to effectively tune the suitable mechanical properties of Ti-based metallic glass composites, without worrying about the catastrophic failure in the process of deformation.
1. Introduction As a kind of advanced material with unique mechanical properties at ambient temperature, excellent rheological behavior in supercooled liquid region (SLR), and great corrosion resistance, bulk metallic glasses (BMGs) have been investigated intensively all around the world [1–3]. To overcome the fatal drawback of BMG which is their limited ductility at ambient temperatures, BMG composites (BMGCs) such as Ti48Zr20Nb12Cu5Be15 with in-situ second phase as reinforcements have been designed [4–6]. BMGCs exhibit greater room temperature ductility and excellent cryogenic plasticity and some other outstanding properties, which make them potential candidates as new-generation structural engineering materials [7–11]. In general, people believe that the microstructure and mechanical properties of BMGs and BMGCs are very sensitive to it's composition [12,13], and some constituent elements of the BMGs and BMGCs such as Ti and Zr have a strong affinity and large solubility for oxygen and some other impurity elements, which are detrimental to glass-forming ability [14,15]. Thus, BMGs and BMGCs were prepared in high vacuum system (≤10−3 Pa) using high-purity raw materials so as to control the content of impurities in a very limited concentration [16,17]. However these strict processing conditions followed with very expensive manufacturing cost will severely limit their further applications. J.L. Cheng, et al. [18,19] intentionally doped various levels of oxygen sources in the Zr60Ti20Cu5.6Ni4.4Be10 alloy and found that the solubility of oxygen
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.msea.2017.04.010 Received 15 February 2017; Received in revised form 31 March 2017; Accepted 3 April 2017 Available online 05 April 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
in β-Zr phase enhances the strength of the BMG composites without sacrificing their overall ductility when the oxygen content within 4000 ppm. This finding gives us a new clue to develop the low-cost Ti-based BMGCs with excellent mechanical properties. In our previous study, Ti48Zr20Nb12Cu5Be15 BMGCs were prepared under low vacuum [20]. We can also improve the strength of the BMGCs when the oxygen level was limited within 4500 ppm. However, nitrogen is the most abundant elements in the atmosphere. Because of strong interactions with Ti and Zr at high temperature, it is difficult to exclude nitrogen during the melting and machining process [21]. But, roles of nitrogen in Ti-based BMGCs usually been ignored. Meanwhile, metal nitriding have been popular used among the traditional tool materials such as AISI D2 tool steel [22] to get very high hardness of the upper surface layer and some other advantages. So in this study, different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the Ti48Zr20Nb12Cu5Be15 bulk metallic glass composite. The experimental results can better understand the Ti-based metallic glass composite with impurity elements, and provide an effective method to tune the mechanical properties of Ti-based metallic glass composites. 2. Experimental procedure The BMGCs with nominal composition of Ti48Zr20Nb12Cu5Be15 (Ti48) containing different nitrogen levels were produced by melting elements (> 99.95 wt% in purity) in a Ti-getter high purity argon
Materials Science & Engineering A 694 (2017) 93–97
L. Li et al.
Table 1 The average contents of oxygen and nitrogen in Ti48Zr20Nb12Cu5Be15 metallic glass composite. Number of samples
1
2
3
4
5
6
Addition of nitrogen (ppm) Average oxygen contents (ppm) Average nitrogen contents (ppm)
0 1600
1000 1700
3000 1800
5000 2200
10,000 2400
14,000 2300
120
1100
3400
5600
11,000
14,000
Fig. 2. SEM image of Ti48 BMGC with different nitrogen contents, (a) Ti48 BMGC with no nitrogen; (b) Ti48 BMGC with 14,000 ppm nitrogen.
Fig. 1. (a) XRD patterns for the Ti48Zr20Nb12Cu5Be15 metallic glass composites with different amounts of nitrogen; (b) crystal lattice parameters of the β-Ti(Zr, Nb) phase dendrite.
atmosphere. Different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the alloys using TiN powder (99.5% in purity). The primary alloy ingots with Ti and TiN were prepared firstly by arc-melting to ensure that the TiN was melted and formed Ti solid solution. This Ti solid solution was melted with other alloy elements by arc-melting again. The ingots are melted four times to ensure the homogeneity of components. Cylindrical specimens with 3 mm in diameter and 50 mm in length were got by copper mould casting. Average levels of nitrogen and oxygen in the samples were measured using a LECO 600 series Gas analyzer. Phase structure of the BMGCs was tested by XRD (DX-2700) with Cu Kα radiation, in the angular range between 20° and 80° (2θ). Diffraction data were collected at room temperature, the voltage and the current were 40 kV and 30 mA, respectively. Rietveld method [23] based on the XRD patterns was used for qualitative and quantitative phase analyses of crystal lattice parameters of the dendrite. The onset glass transition temperature and crystallization temperature of matrix were characterized by differential scanning calorimetry (DSC, NEZTCH 402) at a constant heating rate of 20 K/min. Uniaxial compressive tests were performed on SANS CMT5105 universal testing machine at ambient temperature
Fig. 3. DSC curve of Ti48Zr20Nb12Cu5Be15 bulk metallic glass composite with different nitrogen contents (heating rate: 20 K/min). Glass transition temperature (Tg) and onset crystallization temperature (Tx) are defined. Table 2 DSC data obtained at a heating rate of 20 K/min. Tg, Tx are defined as the onset glass transition and crystallization temperatures, respectively. ΔTx=Tx-Tg. Addition of nitrogen (ppm)
0
1000
3000
6000
10,000
14,000
Tg (°C) Tx (°C) ΔTx (°C)
382 449 67
383 451 68
384 450 66
385 450 65
384 447 67
386 451 65
using specimens of 3 mm in diameter and 6 mm in length, the strain rate is 1×10−4 s−1. Morphology observations and fracture surfaces were analyzed by SEM (VEGA2LMH). The sample with high nitrogen
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Materials Science & Engineering A 694 (2017) 93–97
L. Li et al.
Fig. 4. (a) A TEM bright-field image for the Ti48 BMGC with 14,000 ppm nitrogen level; (b) the corresponding selected area diffraction pattern (SADP) for the β-Ti dendritic second phase (region 1); (c) the SADP for the glass matrix in region 2.
Fig. 5. Mechanical properties of Ti48 BMGCs with different nitrogen contents at the rate of 1×10−4 s−1 at ambient temperature.
level were investigated by transmission electron microscopy (TEM) performed on Tecnai G2 F30. The hardness of the glass matrix and dendrites was measured by TI 950 Tribolndenter nanomechanical test instrument.
3. Results and discussion 3.1. Composition and microstructure of Ti-based metallic glass composites The average contents of oxygen and nitrogen in the alloys are shown in Table 1. It shows that oxygen exists in the BMGCs is limited within 1600–2400 ppm, hence, the influence of mechanical properties caused by oxygen can be ignored. And the average N content of Ti48 metallic glass composite is just a little higher than the N we added. So controlling the weight of TiN powder added into the alloy can be a useful way to get the BMG composites with different nitrogen contents. Fig. 1(a) displays the XRD patterns obtained from as-cast rods of the nitrogen-contained Ti48 BMGCs. The β-Ti solid solution with a bcc structure superimposed on a broad diffuse-scattering peak characteristic of amorphous structure. No α-Ti peaks or other crystalline peaks can be identified from the XRD pattern even the nitrogen contents up to 14,000 ppm. The main peaks shift toward lower angles slightly with nitrogen level increasing, which is due to the lattice distortion of the β-
Fig. 6. (a) Lateral surfaces of fracture samples and (b) Fracture morphology for the Ti48 BMGC with 14,000 ppm nitrogen level.
Ti phase dendrite caused by the solid-solution of nitrogen. Fig. 1(b) presents the crystal lattice parameters of β-Ti phase dendrite with different amount of nitrogen. The SEM secondary electron images are shown in Fig. 2. It shows that the dendritic second phases are homogeneously embedded in the amorphous matrix. The composites with different nitrogen contents 95
Materials Science & Engineering A 694 (2017) 93–97
L. Li et al.
butes to the sharply increase in the yield strength of the composites. Previous studies show that the Ti48Zr20Nb12Cu5Be15 BMGCs with 7500 ppm oxygen show the brittle fracture, and the yield strength is 2100 MPa [20]. However The yield strength of the composite with 14,000 ppm nitrogen is 2350 MPa. According to the rule of mixture, yield strengths of the crystalline phase with 14,000 ppm nitrogen is higher than the crystalline with 7500 ppm oxygen. In addition, the Ti48 metallic glass composite obtained much higher strength by adding 14,000 ppm nitrogen still exhibits 10% macroscopic plasticity. Fig. 6(a) is the SEM images of the broken sample after deformation, the BMG composite containing 14,000 ppm nitrogen follows shear failure with fracture angles of less than 45°. The stepwise sliding indicated by black arrows in Fig. 6(b) indicates the interaction between dendrites and shear bands [30]. The fracture surface partially covered by melting layer as also seen for Vit 1 [31], which appear to have undergone extensive melting and resolidification showing evidence for enormous heat dissipation during surface separation. The plastic dimple fracture, and lots of shear bands are the evidences of the excellent plasticity and tougheness in the composites [32]. The plasticity and toughness [33–35] of the composites can be attributed to the nitrided β phase dendrites with higher shear modulus and hardness still keep the ability of forming multiple shear bands and hinder crack propagation. Fig. 7 shows the load and displacement curve (P–h curve) from the glass matrix and dendrites. The hardness of glass matrix remained unchanged, whereas that of dendrite increased rapidly from N=0 to N=14,000 ppm for solid solution strengthening. But the hardness of the dendrite with 14,000 ppm nitrogen is still lower than that of glass matrix. The higher hardness of β phase still keep the ability to arrest the shear bands in the glass matrix after yielding. Oxygen is known to possess a higher strengthening effect than nitrogen in some β-Ti alloys [36]. As the results of DSC and TEM, nitrogen was shown to be a weaker activator of α formation than oxygen. No β-Ti→α-Ti transformation which leads to the brittle fracture of the composite even the nitrogen level is very high. So nitrogen can be added into the Ti-based metallic glass composites in a very large range to get the suitable mechanical properties we want. Besides, larger composition of error of N during the manufacturing process can be permitted without worrying about the catastrophic failure in the process of deformation.
Fig. 7. Indentation load–displacement curves for the glass matrix and dendrites with different nitrogen level.
have similar volume fraction and the size of the dendrites. So the microstructures of Ti48 metallic glass composite will not been changed even the composites were added with 14,000 ppm nitrogen. DSC heating experiments were taken to analyze the changes in the glassy matrix. The DSC curves and thermodynamic parameters of the Ti48 composites are shown in Fig. 3 and Table 2. The glass transition temperatures (Tg) and crystallization onset temperatures (Tx) [24–26] were determined to be negative changes with the increase of nitrogen contents, followed by the supercooled liquid region (ΔTx=Tx−Tg) with a temperature interval above 50 °C. This result means that the glassforming ability (GFA) of the matrix does not change even the nitrogen level achieve 14,000 ppm. In Ti48 metallic glass composite, oxygen over 4500 ppm will sharply decrease the GFA of matrix and the sharply decrease of GFA will lead to some tiny α-Ti with a hcp structure separates out from the interface between the dendritic second phases and the glassy matrix [20]. The result of Ti48 BMGC with high nitrogen level detected by TEM (shown in Fig. 4) can also provide the evidence of no β-Ti→α-Ti transformation even the nitrogen level is very high. 3.2. Mechanical properties and the fracture morphology of Ti-based metallic glass composites
4. Conclusion The engineering compressive stress-strain curves are shown in Fig. 5. Compared with the nitrogen-free Ti48 BMGCs, the composite added with 3000 ppm nitrogen shows a substantial increase in the yield strength from 1400 MPa to 1850 MPa with the macroscopic plasticity at fracture strains keeping more than 30%. The yield strength of bulk metallic glass composites follows a rule of mixtures [27]: σcomposite=fcrystalline σcrystalline+fglass σglass. Where fcrystalline and fglass and σcrystalline and σglass are the volume fractions and yield strength of the crystalline and glassy phases, respectively. The yield strength of amorphous matrix at the ambient temperature (T0) can be determined by the glass transition temperature (Tg) and molar volume(V) [28]: σy≈50 (Tg-T0)/V. Since the DSC results show the Tg of the Ti48 metallic glass composites with different nitrogen levels were determined to be the same value (384 ± 2 °C), the nitrogen dissolved in amorphous matrix will no change the yield strength of the glassy phase. As the result displayed in Fig. 2, the composites with different nitrogen contents show the similar volume fraction of the crystalline phase. Thus, the change of the yield strength of the composites is only correlated with yield strengths of the crystalline phase. Similarly the solid solution of nitrogen can increase the yield strength of β-Ti alloy was proved by some previous related studies [29]. In this work, lattice distortion detected from XRD (Fig. 2) and hardening of dendrite tested by nanoindentation (Fig. 7) can prove that the yield strength of dendrite is increasing with the nitrogen added. Therefore, the solidsolution hardening effect of nitrogen in the crystalline phase contri-
The Ti48 metallic glass composite show a substantial increase in the yield strength and a decrease in the ductility with the addition of N. The plasticity of Ti48 metallic glasses composite containing 14,000 ppm nitrogen is still above 10%. The results suggest that the yield strength of glass phase is still higher than the yield strength of β-Ti phase when the nitrogen level is very high. Solid-solution hardening effect of N may decrease the ability of β-Ti phase to arrest the shear in the glass matrix. But the embrittlement of Ti-based metallic glass composites is caused by the brittle tiny α-Ti crystals exist in the interface between the dendrite and matrix. Nitrogen was shown to be a weaker activator of α formation than oxygen. An ultimate yield strength up to 2350 MPa was reached, and the yield strength and plasticity of the composites can be tailored by adding different amount of nitrogen into the Ti-based metallic glass composites.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (3102015ZY085), the fund of the State Key Laboratory of Solidification Processing in NWPU (No. 103-QP-2014), and the Program of Introducing Talents of Discipline to Universities (B08040).
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97
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Tune the mechanical properties of Ti-based metallic glass composites by additions of nitrogen ⁎
MARK
⁎
Liyuan Li, Jinshan Li , Jun Wang , Hongchao Kou State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, Shaanxi Province, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ti-based metallic glass composites Mechanical properties Solid-solution strengthening Alloy design Nitrogen
Different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the Ti48Zr20Nb12Cu5Be15 bulk metallic glass composites. Results show that addition of nitrogen can increase the yield strength of composites from 1400 to 2350 MPa due to the solid solution strengthening effect. The composites still owns certain plastic strain (above 10% when contains 14,000 ppm nitrogen) before fracture under compressive test. The reason is attribute to nitrogen shows to be a much weaker activator for the formation of brittle α-Ti phase as compared with oxygen, which make the β-Ti dendrite still softer than that of the glassy phase, thus can enhance the strength of the materials but do not deteriorate the plasticity too much. Our research show that nitrogen can be added into the Ti-based metallic glass composites in a very large range to effectively tune the suitable mechanical properties of Ti-based metallic glass composites, without worrying about the catastrophic failure in the process of deformation.
1. Introduction As a kind of advanced material with unique mechanical properties at ambient temperature, excellent rheological behavior in supercooled liquid region (SLR), and great corrosion resistance, bulk metallic glasses (BMGs) have been investigated intensively all around the world [1–3]. To overcome the fatal drawback of BMG which is their limited ductility at ambient temperatures, BMG composites (BMGCs) such as Ti48Zr20Nb12Cu5Be15 with in-situ second phase as reinforcements have been designed [4–6]. BMGCs exhibit greater room temperature ductility and excellent cryogenic plasticity and some other outstanding properties, which make them potential candidates as new-generation structural engineering materials [7–11]. In general, people believe that the microstructure and mechanical properties of BMGs and BMGCs are very sensitive to it's composition [12,13], and some constituent elements of the BMGs and BMGCs such as Ti and Zr have a strong affinity and large solubility for oxygen and some other impurity elements, which are detrimental to glass-forming ability [14,15]. Thus, BMGs and BMGCs were prepared in high vacuum system (≤10−3 Pa) using high-purity raw materials so as to control the content of impurities in a very limited concentration [16,17]. However these strict processing conditions followed with very expensive manufacturing cost will severely limit their further applications. J.L. Cheng, et al. [18,19] intentionally doped various levels of oxygen sources in the Zr60Ti20Cu5.6Ni4.4Be10 alloy and found that the solubility of oxygen
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.msea.2017.04.010 Received 15 February 2017; Received in revised form 31 March 2017; Accepted 3 April 2017 Available online 05 April 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
in β-Zr phase enhances the strength of the BMG composites without sacrificing their overall ductility when the oxygen content within 4000 ppm. This finding gives us a new clue to develop the low-cost Ti-based BMGCs with excellent mechanical properties. In our previous study, Ti48Zr20Nb12Cu5Be15 BMGCs were prepared under low vacuum [20]. We can also improve the strength of the BMGCs when the oxygen level was limited within 4500 ppm. However, nitrogen is the most abundant elements in the atmosphere. Because of strong interactions with Ti and Zr at high temperature, it is difficult to exclude nitrogen during the melting and machining process [21]. But, roles of nitrogen in Ti-based BMGCs usually been ignored. Meanwhile, metal nitriding have been popular used among the traditional tool materials such as AISI D2 tool steel [22] to get very high hardness of the upper surface layer and some other advantages. So in this study, different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the Ti48Zr20Nb12Cu5Be15 bulk metallic glass composite. The experimental results can better understand the Ti-based metallic glass composite with impurity elements, and provide an effective method to tune the mechanical properties of Ti-based metallic glass composites. 2. Experimental procedure The BMGCs with nominal composition of Ti48Zr20Nb12Cu5Be15 (Ti48) containing different nitrogen levels were produced by melting elements (> 99.95 wt% in purity) in a Ti-getter high purity argon
Materials Science & Engineering A 694 (2017) 93–97
L. Li et al.
Table 1 The average contents of oxygen and nitrogen in Ti48Zr20Nb12Cu5Be15 metallic glass composite. Number of samples
1
2
3
4
5
6
Addition of nitrogen (ppm) Average oxygen contents (ppm) Average nitrogen contents (ppm)
0 1600
1000 1700
3000 1800
5000 2200
10,000 2400
14,000 2300
120
1100
3400
5600
11,000
14,000
Fig. 2. SEM image of Ti48 BMGC with different nitrogen contents, (a) Ti48 BMGC with no nitrogen; (b) Ti48 BMGC with 14,000 ppm nitrogen.
Fig. 1. (a) XRD patterns for the Ti48Zr20Nb12Cu5Be15 metallic glass composites with different amounts of nitrogen; (b) crystal lattice parameters of the β-Ti(Zr, Nb) phase dendrite.
atmosphere. Different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the alloys using TiN powder (99.5% in purity). The primary alloy ingots with Ti and TiN were prepared firstly by arc-melting to ensure that the TiN was melted and formed Ti solid solution. This Ti solid solution was melted with other alloy elements by arc-melting again. The ingots are melted four times to ensure the homogeneity of components. Cylindrical specimens with 3 mm in diameter and 50 mm in length were got by copper mould casting. Average levels of nitrogen and oxygen in the samples were measured using a LECO 600 series Gas analyzer. Phase structure of the BMGCs was tested by XRD (DX-2700) with Cu Kα radiation, in the angular range between 20° and 80° (2θ). Diffraction data were collected at room temperature, the voltage and the current were 40 kV and 30 mA, respectively. Rietveld method [23] based on the XRD patterns was used for qualitative and quantitative phase analyses of crystal lattice parameters of the dendrite. The onset glass transition temperature and crystallization temperature of matrix were characterized by differential scanning calorimetry (DSC, NEZTCH 402) at a constant heating rate of 20 K/min. Uniaxial compressive tests were performed on SANS CMT5105 universal testing machine at ambient temperature
Fig. 3. DSC curve of Ti48Zr20Nb12Cu5Be15 bulk metallic glass composite with different nitrogen contents (heating rate: 20 K/min). Glass transition temperature (Tg) and onset crystallization temperature (Tx) are defined. Table 2 DSC data obtained at a heating rate of 20 K/min. Tg, Tx are defined as the onset glass transition and crystallization temperatures, respectively. ΔTx=Tx-Tg. Addition of nitrogen (ppm)
0
1000
3000
6000
10,000
14,000
Tg (°C) Tx (°C) ΔTx (°C)
382 449 67
383 451 68
384 450 66
385 450 65
384 447 67
386 451 65
using specimens of 3 mm in diameter and 6 mm in length, the strain rate is 1×10−4 s−1. Morphology observations and fracture surfaces were analyzed by SEM (VEGA2LMH). The sample with high nitrogen
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Fig. 4. (a) A TEM bright-field image for the Ti48 BMGC with 14,000 ppm nitrogen level; (b) the corresponding selected area diffraction pattern (SADP) for the β-Ti dendritic second phase (region 1); (c) the SADP for the glass matrix in region 2.
Fig. 5. Mechanical properties of Ti48 BMGCs with different nitrogen contents at the rate of 1×10−4 s−1 at ambient temperature.
level were investigated by transmission electron microscopy (TEM) performed on Tecnai G2 F30. The hardness of the glass matrix and dendrites was measured by TI 950 Tribolndenter nanomechanical test instrument.
3. Results and discussion 3.1. Composition and microstructure of Ti-based metallic glass composites The average contents of oxygen and nitrogen in the alloys are shown in Table 1. It shows that oxygen exists in the BMGCs is limited within 1600–2400 ppm, hence, the influence of mechanical properties caused by oxygen can be ignored. And the average N content of Ti48 metallic glass composite is just a little higher than the N we added. So controlling the weight of TiN powder added into the alloy can be a useful way to get the BMG composites with different nitrogen contents. Fig. 1(a) displays the XRD patterns obtained from as-cast rods of the nitrogen-contained Ti48 BMGCs. The β-Ti solid solution with a bcc structure superimposed on a broad diffuse-scattering peak characteristic of amorphous structure. No α-Ti peaks or other crystalline peaks can be identified from the XRD pattern even the nitrogen contents up to 14,000 ppm. The main peaks shift toward lower angles slightly with nitrogen level increasing, which is due to the lattice distortion of the β-
Fig. 6. (a) Lateral surfaces of fracture samples and (b) Fracture morphology for the Ti48 BMGC with 14,000 ppm nitrogen level.
Ti phase dendrite caused by the solid-solution of nitrogen. Fig. 1(b) presents the crystal lattice parameters of β-Ti phase dendrite with different amount of nitrogen. The SEM secondary electron images are shown in Fig. 2. It shows that the dendritic second phases are homogeneously embedded in the amorphous matrix. The composites with different nitrogen contents 95
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butes to the sharply increase in the yield strength of the composites. Previous studies show that the Ti48Zr20Nb12Cu5Be15 BMGCs with 7500 ppm oxygen show the brittle fracture, and the yield strength is 2100 MPa [20]. However The yield strength of the composite with 14,000 ppm nitrogen is 2350 MPa. According to the rule of mixture, yield strengths of the crystalline phase with 14,000 ppm nitrogen is higher than the crystalline with 7500 ppm oxygen. In addition, the Ti48 metallic glass composite obtained much higher strength by adding 14,000 ppm nitrogen still exhibits 10% macroscopic plasticity. Fig. 6(a) is the SEM images of the broken sample after deformation, the BMG composite containing 14,000 ppm nitrogen follows shear failure with fracture angles of less than 45°. The stepwise sliding indicated by black arrows in Fig. 6(b) indicates the interaction between dendrites and shear bands [30]. The fracture surface partially covered by melting layer as also seen for Vit 1 [31], which appear to have undergone extensive melting and resolidification showing evidence for enormous heat dissipation during surface separation. The plastic dimple fracture, and lots of shear bands are the evidences of the excellent plasticity and tougheness in the composites [32]. The plasticity and toughness [33–35] of the composites can be attributed to the nitrided β phase dendrites with higher shear modulus and hardness still keep the ability of forming multiple shear bands and hinder crack propagation. Fig. 7 shows the load and displacement curve (P–h curve) from the glass matrix and dendrites. The hardness of glass matrix remained unchanged, whereas that of dendrite increased rapidly from N=0 to N=14,000 ppm for solid solution strengthening. But the hardness of the dendrite with 14,000 ppm nitrogen is still lower than that of glass matrix. The higher hardness of β phase still keep the ability to arrest the shear bands in the glass matrix after yielding. Oxygen is known to possess a higher strengthening effect than nitrogen in some β-Ti alloys [36]. As the results of DSC and TEM, nitrogen was shown to be a weaker activator of α formation than oxygen. No β-Ti→α-Ti transformation which leads to the brittle fracture of the composite even the nitrogen level is very high. So nitrogen can be added into the Ti-based metallic glass composites in a very large range to get the suitable mechanical properties we want. Besides, larger composition of error of N during the manufacturing process can be permitted without worrying about the catastrophic failure in the process of deformation.
Fig. 7. Indentation load–displacement curves for the glass matrix and dendrites with different nitrogen level.
have similar volume fraction and the size of the dendrites. So the microstructures of Ti48 metallic glass composite will not been changed even the composites were added with 14,000 ppm nitrogen. DSC heating experiments were taken to analyze the changes in the glassy matrix. The DSC curves and thermodynamic parameters of the Ti48 composites are shown in Fig. 3 and Table 2. The glass transition temperatures (Tg) and crystallization onset temperatures (Tx) [24–26] were determined to be negative changes with the increase of nitrogen contents, followed by the supercooled liquid region (ΔTx=Tx−Tg) with a temperature interval above 50 °C. This result means that the glassforming ability (GFA) of the matrix does not change even the nitrogen level achieve 14,000 ppm. In Ti48 metallic glass composite, oxygen over 4500 ppm will sharply decrease the GFA of matrix and the sharply decrease of GFA will lead to some tiny α-Ti with a hcp structure separates out from the interface between the dendritic second phases and the glassy matrix [20]. The result of Ti48 BMGC with high nitrogen level detected by TEM (shown in Fig. 4) can also provide the evidence of no β-Ti→α-Ti transformation even the nitrogen level is very high. 3.2. Mechanical properties and the fracture morphology of Ti-based metallic glass composites
4. Conclusion The engineering compressive stress-strain curves are shown in Fig. 5. Compared with the nitrogen-free Ti48 BMGCs, the composite added with 3000 ppm nitrogen shows a substantial increase in the yield strength from 1400 MPa to 1850 MPa with the macroscopic plasticity at fracture strains keeping more than 30%. The yield strength of bulk metallic glass composites follows a rule of mixtures [27]: σcomposite=fcrystalline σcrystalline+fglass σglass. Where fcrystalline and fglass and σcrystalline and σglass are the volume fractions and yield strength of the crystalline and glassy phases, respectively. The yield strength of amorphous matrix at the ambient temperature (T0) can be determined by the glass transition temperature (Tg) and molar volume(V) [28]: σy≈50 (Tg-T0)/V. Since the DSC results show the Tg of the Ti48 metallic glass composites with different nitrogen levels were determined to be the same value (384 ± 2 °C), the nitrogen dissolved in amorphous matrix will no change the yield strength of the glassy phase. As the result displayed in Fig. 2, the composites with different nitrogen contents show the similar volume fraction of the crystalline phase. Thus, the change of the yield strength of the composites is only correlated with yield strengths of the crystalline phase. Similarly the solid solution of nitrogen can increase the yield strength of β-Ti alloy was proved by some previous related studies [29]. In this work, lattice distortion detected from XRD (Fig. 2) and hardening of dendrite tested by nanoindentation (Fig. 7) can prove that the yield strength of dendrite is increasing with the nitrogen added. Therefore, the solidsolution hardening effect of nitrogen in the crystalline phase contri-
The Ti48 metallic glass composite show a substantial increase in the yield strength and a decrease in the ductility with the addition of N. The plasticity of Ti48 metallic glasses composite containing 14,000 ppm nitrogen is still above 10%. The results suggest that the yield strength of glass phase is still higher than the yield strength of β-Ti phase when the nitrogen level is very high. Solid-solution hardening effect of N may decrease the ability of β-Ti phase to arrest the shear in the glass matrix. But the embrittlement of Ti-based metallic glass composites is caused by the brittle tiny α-Ti crystals exist in the interface between the dendrite and matrix. Nitrogen was shown to be a weaker activator of α formation than oxygen. An ultimate yield strength up to 2350 MPa was reached, and the yield strength and plasticity of the composites can be tailored by adding different amount of nitrogen into the Ti-based metallic glass composites.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (3102015ZY085), the fund of the State Key Laboratory of Solidification Processing in NWPU (No. 103-QP-2014), and the Program of Introducing Talents of Discipline to Universities (B08040).
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