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Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V
Materials Today: Proceedings xxx (xxxx) xxx
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Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V Roman Laptev ⇑, Viktor Kudiiarov, Natalia Pushilina Division for Experimental Physics, National Research Tomsk Polytechnic University, Tomsk 634050, Russia
a r t i c l e
i n f o
Article history: Received 20 May 2019 Accepted 2 July 2019 Available online xxxx Keywords: Electron beam melting Defects Titanium alloy Hydrogen Positron
a b s t r a c t The defect structure and mechanical properties of the samples manufactured from Ti-6Al-4V powder by electron beam melting (EBM) before and after hydrogenation was studied. It has been established that hydrogenation of EBM titanium alloy to a concentration of 470 ppm leads to the formation of titanium c-hydride, the volume content of which is 2.2%. A further increase in the concentration of hydrogen to 650 ppm is accompanied by a slight increase in the content of the hydride phase, while the proportion of the b phase of titanium increases (to 4.9%). When the hydrogen content is 900 ppm, the fraction of c hydride decreases and the fraction of the beta phase decreases to 4.2%, and the formation of the hydride phase d is observed. The introduction of hydrogen leads to a decrease in the wear and an increase in the hardness of EBM Ti-6Al-4V samples. This circumstance is due to the microstructure refinement under the action of hydrogen and the formation of secondary phase precipitates. The hydrogen content increases, the dislocation density and the concentration of hydrogen-vacancy complexes linearly increase, which indicates that hydrogen not only actively interacts with existing defects but also induces the formation of new defects. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
1. Introduction Nowadays additive technologies (AT) are being actively implemented in many industries [1–3]. The advantages of additive technologies over traditional methods for the production of metal products are indisputable; this is a high production rate, and the possibility of creating products of a unique geometric shape. Through the use of AT it is possible to create new generation materials with a unique set of properties [4–6]. The anisotropy of the properties, the specificity of the structure and the presence of specific defects due to the production technology in materials can have a significant impact on the interaction of 3D materials with the corrosive environment and, in particular, with hydrogen. In this regard, the study of the patterns of hydrogen interaction with metal products made with the use of AT, have not only fundamental but also practical interest. Especially these studies are relevant for metallic materials, in particular, titanium alloys, operating in a hydrogen-containing environment [7–9]. ⇑ Corresponding author. E-mail address: [email protected] (R. Laptev).
Titanium alloys are considered to be fairly resistant to chemical influence, but serious problems can arise when titanium-based alloys come into contact with hydrogen-containing media, especially at elevated temperatures. Hydrogen can cause degradation of the mechanical properties of titanium alloys. Hydrogen embrittlement is the deterioration of one or more of the mechanical characteristics of metal under the action of hydrogen. The development of hydrogen embrittlement (including delayed fracture) in titanium alloys will depend on the concentration and distribution of hydrogen in the material, external influences, the presence of defects and impurities, oxide films, etc. Today remain unexplored questions related to the influence of hydrogen on mechanical properties and defect structure of titanium alloys manufactured by electron beam melting [10–12]. 2. Material and experimental procedure 2.1. Samples preparation The samples were manufactured on an electron beam melting machine designed in Tomsk Polytechnic University. The technical
https://doi.org/10.1016/j.matpr.2019.07.101 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
Please cite this article as: R. Laptev, V. Kudiiarov and N. Pushilina, Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.101
2
R. Laptev et al. / Materials Today: Proceedings xxx (xxxx) xxx
characteristics of the machine are: accelerating voltage 40 kV, vacuum base pressure 5 10-3, build area 150 150 mm, power consumption 6 kW. The samples were manufactured from titanium Ti-6Al-4V powder produced by NORMIN (Russia). Powder with sizes ranging from 50 to 100 lm was used for samples manufacturing. The heating parameters were: beam diameter 4 mm, beam speed 16,000 mm/s, beam current 4 mA [13]. Samples were produced at melting current 15 mA. Samples with dimensions of 20 20 1 mm were used in the study. The hydrogenation of samples from the gas-phase was performed with the help of automated complex Gas Reaction Controller to the specified hydrogen concentrations. Conditions of hydrogenation: temperature 650 °C, hydrogen pressure 1 atm, hydrogenation was carried out to various concentrations (from 400 to 1000 ppm). After hydrogenation, the samples were kept in helium at a temperature of 650 °C and a pressure of 2 atm for 12 h to ensure a uniform distribution of hydrogen throughout the samples. After that, the samples were cooled at a rate of 6 °C/min to room temperature.
Fig. 1. Diffraction patterns of EBM Ti-6Al-4V samples depending on the hydrogen concentration.
2.2. Experimental procedure The microstructure of the EBM Ti-6Al-4V was investigated by scanning electron microscopy using QUANTA 200 3D (FEI Company, USA). The phase identification and structural investigations were performed by X-ray diffraction (XRD). X-ray diffraction studies were performed with CuKa radiation (1.5410 Å wavelength) using XRD-7000S diffractometer (Shimadzu, Kyoto, Japan) in Bragg-Brentano geometry from 30° to 80° with the scan speed of 10.0°/min, the sampling pitch of 0.0143°, the preset time of 42.972 s at 40 kV and 30 mA. The diffraction patterns were collected using OneSight wide-range array high-speed detector with 1280 channels. Microhardness was measured using KB30S (Pruftechnik, Ismaning, Germany) Vickers hardness testing machine with 0.5 kg load. Measurement of wear resistance and friction coefficient of samples was carried out on the instrument «High-Temperature Tribometer» (CSEM, Switzerland). The wear resistance measurement parameters were as follows: the number of turns was 2000, the indenter was tungsten carbide, the applied force was 2 N. The area of wear tracks were measured with a STIL Micromeasure 3D contactless optical profilometer. The control of the uniform distribution of hydrogen was carried out by melting in an inert gas medium on a LECO RHEN602 hydrogen analyzer. The defects were investigated by positron spectroscopy techniques on a hybrid digital complex with the system of external synchronization based on the modules of positron lifetime spectrometry (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The time resolution of the PALS module was 170 ± 7 ps, the count rate was 90 ± 30 counts/s. The count rate for the CDBS module was 116 ± 15 counts/s with the energy resolution of 1.16 ± 0.03 keV. 44Ti was used as the positron source with the activity of 0.91 MBq and positron maximum energy of 1.47 MeV. For each of the samples, two PALS spectra and one twodimensional CDBS spectrum statistically obtained 3106 and 4107, correspondingly. 3. Results and discussion Fig. 1 shows the diffraction patterns of EBM Ti-6Al-4V samples depending on the hydrogen concentration. Hydrogenation of EBM titanium alloy to a concentration of 470 ppm leads to the formation of titanium c-hydride, the volume content of which is 2.2% (Table 1). A further increase in the concentration of hydrogen to 650 ppm is accompanied by a slight increase in the content of
Table 1 The phase composition and lattice parameters of samples. Hydrogen concentration
Phase
Phase Content (vol %)
Initial EBM Ti-6Al-4V 65 ppm
Ti_hexagonal Ti_cubic
95.4 4.6
EBM Ti-6Al-4V 470 ppm
Ti_hexagonal Ti_cubic TiH_tetragonal
95.3 2.5 2.2
EBM Ti-6Al-4V 650 ppm
Ti_hexagonal Ti_cubic TiH_tetragonal
90.8 4.9 4.3
EBM Ti-6Al-4V 900 ppm
Ti_hexagonal Ti_cubic
95.8 4.2
the hydride phase, while the proportion of the b phase of titanium increases (to 4.9%). When the hydrogen content is 900 ppm, the fraction of c hydride decreases and the fraction of the beta phase decreases to 4.2%, and the formation of the hydride phase d is observed. The structure of the samples before hydrogenation represents a lamellar (a + b) structure. There is a tendency for the alpha phase to become fragmented with increasing hydrogen content (Fig. 2) according to the microstructure analysis of the EBM samples of Ti-6Al-4V using scanning microscopy. The probable mechanism of reducing the size of the alpha plates discussed by the authors in [14,15]. This phenomenon could be explained that with a higher hydrogen content, the formation of hydride and the eutectoid decomposition is much more intensive because the ease of the eutectoid decomposition is easier to occur, increasing the nucleation rate of the eutectoid lamellas and thus reducing their size. The hydrogen content used in this work is lower than the eutectoid point; therefore, it can be expected that an increase in the hydrogen content from 100 to 900 ppm contributes to the refinement of the initial structure. The results of measuring the microhardness of EBM Ti-6Al-4V samples are presented in Fig. 3. The hardness of cast and annealed samples is 320 HV0.5. The high microhardness of the EBM Ti-6Al-4V alloy, compared with the cast and annealed samples, is due to the thin lamellar structure, as well as high internal stresses in the material. It has been established that hydrogenation in the concentration range from 200 to 900 ppm increases the microhardness of EBM Ti-6Al-4V samples. Hydrogen is predominantly in the solid solution for concentrations up to 900 ppm, which is confirmed by
Please cite this article as: R. Laptev, V. Kudiiarov and N. Pushilina, Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.101
R. Laptev et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 2. The surface microstructure of the EBM Ti-6Al-4V according to SEM, depending on the hydrogen content (a 65 ppm; b 470 ppm; c 650 ppm; d 900 ppm).
samples. Thus, the wear coefficient for the EBM Ti-6Al-4V with a hydrogen concentration 500 ppm is 710–4 mm3/(Nm) the wear factor values are reduced by 20% with a further increase in the hydrogen concentration to 900 ppm. This circumstance is due to the microstructure refinement under the action of hydrogen and the formation of secondary phase precipitates. The results of the analysis of the DUAL parameters are presented in Fig. 4. Hydrogen concentration increasing leads to the S parameter increasing and the W parameter decreasing, which indicates a free volume enlargement due to defects formation [16,17]. The results of the positron spectroscopy are presented in Table 2. The component sF = 147 ± 1 ps is presented in all samples of EBM Ti-6Al-4V alloy as can be seen from Table 2. This component is associated with the positron annihilation in the delocalized state
Fig. 3. The dependence of hardness and wear rate on hydrogen content.
X-ray diffraction data (Fig. 1, Table 1). The dissolution of hydrogen and the formation of hydrides lead to a distortion of the crystal lattice and the growth of microstresses in the material, which leads to an increase in the hardness of the EBM Ti-6Al-4V alloy and hydrogenated. Another strengthening factor is the microstructure refinement under the action of hydrogen (Fig. 2). According to positron spectroscopic data, the prevailing type of defects in EBM Ti-6Al-4V are dislocations (dislocation density is (6 ± 1) 1013 m2), in addition, vacancy complexes are present in the samples (tetravacancies in order concentration 0.003 ppm) and nanoscale clusters (Ti3Al). The introduction of hydrogen (at the concentrations under consideration) leads to a decrease in the wear of EBM Ti-6Al-4V
Fig. 4. The dependence of S and W parameters on the concentration of hydrogen in EBM manufactured Ti-6Al-4V samples.
Please cite this article as: R. Laptev, V. Kudiiarov and N. Pushilina, Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.101
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Table 2 Parameters of positron annihilation in the experimental samples. State
sA (ps)
IA (%)
sB (ps)
IB (%)
kA (ns1)
kB (ns1)
sF (ps)
IF (%)
savg (ps)
Initial EBM Ti-6Al-4V EBM Ti-6Al-4V 470 ppm EBM Ti-6Al-4V 650 ppm EBM Ti-6Al-4V 900 ppm
164 ± 2 168 ± 2 167 ± 2 167 ± 2
13 56 80 82
290 ± 5 213 ± 5 212 ± 5 211 ± 5
0.06 0.19 0.21 0.23
0.11 ± 0.03 0.97 ± 0.03 3.1 ± 0.1 3.6 ± 0.1
0.002 ± 0.001 0.006 ± 0.001 0.008 ± 0.001 0.009 ± 0.001
147 ± 1 149 ± 1 149 ± 1 149 ± 1
87 40.81 19.79 17.77
149 ± 1 151 ± 1 155 ± 1 156 ± 1
Table 3 Defect concentration hydrogenation.
in
the
EBM
manufactured
Ti-6Al-4V
samples
after
State
Dislocations density Cd, 1013 m2
V-1H concentration, ppm
EBM Ti-6Al-4V 470 ppm EBM Ti-6Al-4V 650 ppm EBM Ti-6Al-4V 900 ppm
19 ± 3 62 ± 5 72 ± 5
0.015 ± 0.002 0.020 ± 0.003 0.023 ± 0.003
in the titanium lattice according to the literature data [16–18]. After hydrogenation, there are two components that are responsible for the annihilation of positrons in defects: sA = 168 ± 2 ps and sB = 212 ± 5 ps. The component sA = 168 ± 2 ps is associated with the annihilation of positrons in dislocations [19–21]. The longlived component sB = 212 ± 5 ps is responsible for the annihilation of positrons in simple hydrogen-vacancy complex V-nH (where n is the number of hydrogen atoms associated with a single vacancy), in this case, the lifetime of positrons is 7–9 ps less than the lifetime of single vacancy (sV = 222 ± 3 ps for Ti) [16,17]. It is possible to determine the concentration of detected Cd dislocations and hydrogen-vacancy complexes CV-1H using the threecomponent positron capture model [22] by the following formulas:
Cd ¼
kA
ld
C V1H ¼
;
Further increase in the concentration of hydrogen to 650 ppm is accompanied by a slight increase in the content of the hydride phase, while the proportion of the b phase of titanium increases (to 4.9%). When the hydrogen content is 900 ppm, the fraction of c hydride decreases and the fraction of the beta phase decreases to 4.2%, and the formation of the hydride phase d is observed. The introduction of hydrogen leads to a decrease in the wear and an increase in the hardness of EBM Ti-6Al-4V samples. This circumstance is due to the microstructure refinement under the action of hydrogen and the formation of secondary phase precipitates. The hydrogen content increases, the dislocation density and the concentration of hydrogen-vacancy complexes linearly increase, which indicates that hydrogen not only actively interacts with existing defects but also induces the formation of new defects.
Acknowledgement The research was funded by Russian Science Foundation No 1779-20100. References
kB
lV1H
;
where, kA, kB are the positron capture rates by dislocations and tetravacancies, respectively, and ld, lV-1H is the positron capture coefficient for dislocations and hydrogen-vacancy complexes V1H, respectively. Positron capture rate by dislocations is in the range (10-5 10-4) 2 1 m s for most metals, for titanium and its alloys this value is ld = 0.510-5 m2s1 [19,20,23]. The positron capture coefficient by hydrogen-vacancy complexes V-1H in titanium is determined earlier and it is 1.631014 s1 [16,17]. The results of the defects concentration calculations in the samples of the EBM Ti-6Al-4V alloy with different hydrogen contents are presented in Table 3. As can be seen from the Table 3, as the hydrogen content increases, the dislocation density and the concentration of hydrogen-vacancy complexes linearly increase, which indicates that hydrogen not only actively interacts with existing defects but also induces the formation of new defects. 4. Conclusions The defect structure and mechanical properties of the samples manufactured from Ti-6Al-4V powder by electron beam melting (EBM) before and after hydrogenation was studied. It has been established that: Hydrogenation of EBM titanium alloy to a concentration of 470 ppm leads to the formation of titanium c-hydride, the volume content of which is 2.2%.
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Please cite this article as: R. Laptev, V. Kudiiarov and N. Pushilina, Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.101
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V Roman Laptev ⇑, Viktor Kudiiarov, Natalia Pushilina Division for Experimental Physics, National Research Tomsk Polytechnic University, Tomsk 634050, Russia
a r t i c l e
i n f o
Article history: Received 20 May 2019 Accepted 2 July 2019 Available online xxxx Keywords: Electron beam melting Defects Titanium alloy Hydrogen Positron
a b s t r a c t The defect structure and mechanical properties of the samples manufactured from Ti-6Al-4V powder by electron beam melting (EBM) before and after hydrogenation was studied. It has been established that hydrogenation of EBM titanium alloy to a concentration of 470 ppm leads to the formation of titanium c-hydride, the volume content of which is 2.2%. A further increase in the concentration of hydrogen to 650 ppm is accompanied by a slight increase in the content of the hydride phase, while the proportion of the b phase of titanium increases (to 4.9%). When the hydrogen content is 900 ppm, the fraction of c hydride decreases and the fraction of the beta phase decreases to 4.2%, and the formation of the hydride phase d is observed. The introduction of hydrogen leads to a decrease in the wear and an increase in the hardness of EBM Ti-6Al-4V samples. This circumstance is due to the microstructure refinement under the action of hydrogen and the formation of secondary phase precipitates. The hydrogen content increases, the dislocation density and the concentration of hydrogen-vacancy complexes linearly increase, which indicates that hydrogen not only actively interacts with existing defects but also induces the formation of new defects. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
1. Introduction Nowadays additive technologies (AT) are being actively implemented in many industries [1–3]. The advantages of additive technologies over traditional methods for the production of metal products are indisputable; this is a high production rate, and the possibility of creating products of a unique geometric shape. Through the use of AT it is possible to create new generation materials with a unique set of properties [4–6]. The anisotropy of the properties, the specificity of the structure and the presence of specific defects due to the production technology in materials can have a significant impact on the interaction of 3D materials with the corrosive environment and, in particular, with hydrogen. In this regard, the study of the patterns of hydrogen interaction with metal products made with the use of AT, have not only fundamental but also practical interest. Especially these studies are relevant for metallic materials, in particular, titanium alloys, operating in a hydrogen-containing environment [7–9]. ⇑ Corresponding author. E-mail address: [email protected] (R. Laptev).
Titanium alloys are considered to be fairly resistant to chemical influence, but serious problems can arise when titanium-based alloys come into contact with hydrogen-containing media, especially at elevated temperatures. Hydrogen can cause degradation of the mechanical properties of titanium alloys. Hydrogen embrittlement is the deterioration of one or more of the mechanical characteristics of metal under the action of hydrogen. The development of hydrogen embrittlement (including delayed fracture) in titanium alloys will depend on the concentration and distribution of hydrogen in the material, external influences, the presence of defects and impurities, oxide films, etc. Today remain unexplored questions related to the influence of hydrogen on mechanical properties and defect structure of titanium alloys manufactured by electron beam melting [10–12]. 2. Material and experimental procedure 2.1. Samples preparation The samples were manufactured on an electron beam melting machine designed in Tomsk Polytechnic University. The technical
https://doi.org/10.1016/j.matpr.2019.07.101 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
Please cite this article as: R. Laptev, V. Kudiiarov and N. Pushilina, Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.101
2
R. Laptev et al. / Materials Today: Proceedings xxx (xxxx) xxx
characteristics of the machine are: accelerating voltage 40 kV, vacuum base pressure 5 10-3, build area 150 150 mm, power consumption 6 kW. The samples were manufactured from titanium Ti-6Al-4V powder produced by NORMIN (Russia). Powder with sizes ranging from 50 to 100 lm was used for samples manufacturing. The heating parameters were: beam diameter 4 mm, beam speed 16,000 mm/s, beam current 4 mA [13]. Samples were produced at melting current 15 mA. Samples with dimensions of 20 20 1 mm were used in the study. The hydrogenation of samples from the gas-phase was performed with the help of automated complex Gas Reaction Controller to the specified hydrogen concentrations. Conditions of hydrogenation: temperature 650 °C, hydrogen pressure 1 atm, hydrogenation was carried out to various concentrations (from 400 to 1000 ppm). After hydrogenation, the samples were kept in helium at a temperature of 650 °C and a pressure of 2 atm for 12 h to ensure a uniform distribution of hydrogen throughout the samples. After that, the samples were cooled at a rate of 6 °C/min to room temperature.
Fig. 1. Diffraction patterns of EBM Ti-6Al-4V samples depending on the hydrogen concentration.
2.2. Experimental procedure The microstructure of the EBM Ti-6Al-4V was investigated by scanning electron microscopy using QUANTA 200 3D (FEI Company, USA). The phase identification and structural investigations were performed by X-ray diffraction (XRD). X-ray diffraction studies were performed with CuKa radiation (1.5410 Å wavelength) using XRD-7000S diffractometer (Shimadzu, Kyoto, Japan) in Bragg-Brentano geometry from 30° to 80° with the scan speed of 10.0°/min, the sampling pitch of 0.0143°, the preset time of 42.972 s at 40 kV and 30 mA. The diffraction patterns were collected using OneSight wide-range array high-speed detector with 1280 channels. Microhardness was measured using KB30S (Pruftechnik, Ismaning, Germany) Vickers hardness testing machine with 0.5 kg load. Measurement of wear resistance and friction coefficient of samples was carried out on the instrument «High-Temperature Tribometer» (CSEM, Switzerland). The wear resistance measurement parameters were as follows: the number of turns was 2000, the indenter was tungsten carbide, the applied force was 2 N. The area of wear tracks were measured with a STIL Micromeasure 3D contactless optical profilometer. The control of the uniform distribution of hydrogen was carried out by melting in an inert gas medium on a LECO RHEN602 hydrogen analyzer. The defects were investigated by positron spectroscopy techniques on a hybrid digital complex with the system of external synchronization based on the modules of positron lifetime spectrometry (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The time resolution of the PALS module was 170 ± 7 ps, the count rate was 90 ± 30 counts/s. The count rate for the CDBS module was 116 ± 15 counts/s with the energy resolution of 1.16 ± 0.03 keV. 44Ti was used as the positron source with the activity of 0.91 MBq and positron maximum energy of 1.47 MeV. For each of the samples, two PALS spectra and one twodimensional CDBS spectrum statistically obtained 3106 and 4107, correspondingly. 3. Results and discussion Fig. 1 shows the diffraction patterns of EBM Ti-6Al-4V samples depending on the hydrogen concentration. Hydrogenation of EBM titanium alloy to a concentration of 470 ppm leads to the formation of titanium c-hydride, the volume content of which is 2.2% (Table 1). A further increase in the concentration of hydrogen to 650 ppm is accompanied by a slight increase in the content of
Table 1 The phase composition and lattice parameters of samples. Hydrogen concentration
Phase
Phase Content (vol %)
Initial EBM Ti-6Al-4V 65 ppm
Ti_hexagonal Ti_cubic
95.4 4.6
EBM Ti-6Al-4V 470 ppm
Ti_hexagonal Ti_cubic TiH_tetragonal
95.3 2.5 2.2
EBM Ti-6Al-4V 650 ppm
Ti_hexagonal Ti_cubic TiH_tetragonal
90.8 4.9 4.3
EBM Ti-6Al-4V 900 ppm
Ti_hexagonal Ti_cubic
95.8 4.2
the hydride phase, while the proportion of the b phase of titanium increases (to 4.9%). When the hydrogen content is 900 ppm, the fraction of c hydride decreases and the fraction of the beta phase decreases to 4.2%, and the formation of the hydride phase d is observed. The structure of the samples before hydrogenation represents a lamellar (a + b) structure. There is a tendency for the alpha phase to become fragmented with increasing hydrogen content (Fig. 2) according to the microstructure analysis of the EBM samples of Ti-6Al-4V using scanning microscopy. The probable mechanism of reducing the size of the alpha plates discussed by the authors in [14,15]. This phenomenon could be explained that with a higher hydrogen content, the formation of hydride and the eutectoid decomposition is much more intensive because the ease of the eutectoid decomposition is easier to occur, increasing the nucleation rate of the eutectoid lamellas and thus reducing their size. The hydrogen content used in this work is lower than the eutectoid point; therefore, it can be expected that an increase in the hydrogen content from 100 to 900 ppm contributes to the refinement of the initial structure. The results of measuring the microhardness of EBM Ti-6Al-4V samples are presented in Fig. 3. The hardness of cast and annealed samples is 320 HV0.5. The high microhardness of the EBM Ti-6Al-4V alloy, compared with the cast and annealed samples, is due to the thin lamellar structure, as well as high internal stresses in the material. It has been established that hydrogenation in the concentration range from 200 to 900 ppm increases the microhardness of EBM Ti-6Al-4V samples. Hydrogen is predominantly in the solid solution for concentrations up to 900 ppm, which is confirmed by
Please cite this article as: R. Laptev, V. Kudiiarov and N. Pushilina, Hydrogen influence on defect structure and mechanical properties of EBM Ti-6Al-4V, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.101
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Fig. 2. The surface microstructure of the EBM Ti-6Al-4V according to SEM, depending on the hydrogen content (a 65 ppm; b 470 ppm; c 650 ppm; d 900 ppm).
samples. Thus, the wear coefficient for the EBM Ti-6Al-4V with a hydrogen concentration 500 ppm is 710–4 mm3/(Nm) the wear factor values are reduced by 20% with a further increase in the hydrogen concentration to 900 ppm. This circumstance is due to the microstructure refinement under the action of hydrogen and the formation of secondary phase precipitates. The results of the analysis of the DUAL parameters are presented in Fig. 4. Hydrogen concentration increasing leads to the S parameter increasing and the W parameter decreasing, which indicates a free volume enlargement due to defects formation [16,17]. The results of the positron spectroscopy are presented in Table 2. The component sF = 147 ± 1 ps is presented in all samples of EBM Ti-6Al-4V alloy as can be seen from Table 2. This component is associated with the positron annihilation in the delocalized state
Fig. 3. The dependence of hardness and wear rate on hydrogen content.
X-ray diffraction data (Fig. 1, Table 1). The dissolution of hydrogen and the formation of hydrides lead to a distortion of the crystal lattice and the growth of microstresses in the material, which leads to an increase in the hardness of the EBM Ti-6Al-4V alloy and hydrogenated. Another strengthening factor is the microstructure refinement under the action of hydrogen (Fig. 2). According to positron spectroscopic data, the prevailing type of defects in EBM Ti-6Al-4V are dislocations (dislocation density is (6 ± 1) 1013 m2), in addition, vacancy complexes are present in the samples (tetravacancies in order concentration 0.003 ppm) and nanoscale clusters (Ti3Al). The introduction of hydrogen (at the concentrations under consideration) leads to a decrease in the wear of EBM Ti-6Al-4V
Fig. 4. The dependence of S and W parameters on the concentration of hydrogen in EBM manufactured Ti-6Al-4V samples.
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Table 2 Parameters of positron annihilation in the experimental samples. State
sA (ps)
IA (%)
sB (ps)
IB (%)
kA (ns1)
kB (ns1)
sF (ps)
IF (%)
savg (ps)
Initial EBM Ti-6Al-4V EBM Ti-6Al-4V 470 ppm EBM Ti-6Al-4V 650 ppm EBM Ti-6Al-4V 900 ppm
164 ± 2 168 ± 2 167 ± 2 167 ± 2
13 56 80 82
290 ± 5 213 ± 5 212 ± 5 211 ± 5
0.06 0.19 0.21 0.23
0.11 ± 0.03 0.97 ± 0.03 3.1 ± 0.1 3.6 ± 0.1
0.002 ± 0.001 0.006 ± 0.001 0.008 ± 0.001 0.009 ± 0.001
147 ± 1 149 ± 1 149 ± 1 149 ± 1
87 40.81 19.79 17.77
149 ± 1 151 ± 1 155 ± 1 156 ± 1
Table 3 Defect concentration hydrogenation.
in
the
EBM
manufactured
Ti-6Al-4V
samples
after
State
Dislocations density Cd, 1013 m2
V-1H concentration, ppm
EBM Ti-6Al-4V 470 ppm EBM Ti-6Al-4V 650 ppm EBM Ti-6Al-4V 900 ppm
19 ± 3 62 ± 5 72 ± 5
0.015 ± 0.002 0.020 ± 0.003 0.023 ± 0.003
in the titanium lattice according to the literature data [16–18]. After hydrogenation, there are two components that are responsible for the annihilation of positrons in defects: sA = 168 ± 2 ps and sB = 212 ± 5 ps. The component sA = 168 ± 2 ps is associated with the annihilation of positrons in dislocations [19–21]. The longlived component sB = 212 ± 5 ps is responsible for the annihilation of positrons in simple hydrogen-vacancy complex V-nH (where n is the number of hydrogen atoms associated with a single vacancy), in this case, the lifetime of positrons is 7–9 ps less than the lifetime of single vacancy (sV = 222 ± 3 ps for Ti) [16,17]. It is possible to determine the concentration of detected Cd dislocations and hydrogen-vacancy complexes CV-1H using the threecomponent positron capture model [22] by the following formulas:
Cd ¼
kA
ld
C V1H ¼
;
Further increase in the concentration of hydrogen to 650 ppm is accompanied by a slight increase in the content of the hydride phase, while the proportion of the b phase of titanium increases (to 4.9%). When the hydrogen content is 900 ppm, the fraction of c hydride decreases and the fraction of the beta phase decreases to 4.2%, and the formation of the hydride phase d is observed. The introduction of hydrogen leads to a decrease in the wear and an increase in the hardness of EBM Ti-6Al-4V samples. This circumstance is due to the microstructure refinement under the action of hydrogen and the formation of secondary phase precipitates. The hydrogen content increases, the dislocation density and the concentration of hydrogen-vacancy complexes linearly increase, which indicates that hydrogen not only actively interacts with existing defects but also induces the formation of new defects.
Acknowledgement The research was funded by Russian Science Foundation No 1779-20100. References
kB
lV1H
;
where, kA, kB are the positron capture rates by dislocations and tetravacancies, respectively, and ld, lV-1H is the positron capture coefficient for dislocations and hydrogen-vacancy complexes V1H, respectively. Positron capture rate by dislocations is in the range (10-5 10-4) 2 1 m s for most metals, for titanium and its alloys this value is ld = 0.510-5 m2s1 [19,20,23]. The positron capture coefficient by hydrogen-vacancy complexes V-1H in titanium is determined earlier and it is 1.631014 s1 [16,17]. The results of the defects concentration calculations in the samples of the EBM Ti-6Al-4V alloy with different hydrogen contents are presented in Table 3. As can be seen from the Table 3, as the hydrogen content increases, the dislocation density and the concentration of hydrogen-vacancy complexes linearly increase, which indicates that hydrogen not only actively interacts with existing defects but also induces the formation of new defects. 4. Conclusions The defect structure and mechanical properties of the samples manufactured from Ti-6Al-4V powder by electron beam melting (EBM) before and after hydrogenation was studied. It has been established that: Hydrogenation of EBM titanium alloy to a concentration of 470 ppm leads to the formation of titanium c-hydride, the volume content of which is 2.2%.
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