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Investigation the effect of B4C addition on properties of TZM alloy prepared by spark plasma sintering
Int. Journal of Refractory Metals and Hard Materials 58 (2016) 182–188
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Investigation the effect of B4C addition on properties of TZM alloy prepared by spark plasma sintering Baris Yavas, Gultekin Goller ⁎ Istanbul Technical University, Department of Metallurgical and Materials Engineering, 34469, Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 7 March 2016 Received in revised form 21 April 2016 Accepted 29 April 2016 Available online 30 April 2016 Keywords: Molybdenum TZM alloy Spark plasma sintering Refractory metals Oxidation
a b s t r a c t TZM alloy is one of the most important molybdenum (Mo) based alloy which has a nominal composition containing 0.5–0.8 wt.% titanium (Ti), 0.08–0.1 wt.% zirconium (Zr) and 0.016–0.02 wt.% carbon (C). It is a possible candidate for high temperature applications in a variety of industries. However, the rapid oxidation of TZM alloys at high temperature in air is considered to be one of the drawback. In this study, TZM alloys with additions of 0– 5 wt.% B4C were prepared by spark plasma sintering (SPS) at 1420 °C utilizing 40 MPa pressure for 5 min under vacuum. The effects of B4C addition on oxidation, densification behavior, microstructure, and mechanical properties were investigated. The TZM alloy with 5 wt.% B4C have exhibited an approximately 66% reduction in mass loss under normal atmospheric conditions in oxidation tests made at 1000 °C for 60 min. And an increase from 1.9 GPa to 7.8 GPa has been determined in hardness of the alloy. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Pure molybdenum is one of the most important refractory metals due to its high melting point, good thermal conductivity and low thermal expansion coefficient needed for many high temperature applications [1]. Since recrystallization temperature of Mo based alloys are higher than the pure one in most applications done above 1000 °C materials selection is made in the favor of Mo based alloys [2]. The beginning of recrystallization can be inhibited by the addition of fine particles dispersed such as carbides [3]. The fine particles added in to the TZM alloy form precipitates TiC and ZrC in the grain boundaries of molybdenum. These precipitates contribute to solid-solution strengthening and behave as inhibitors in recrystallization [4–5]. Homogeneously dispersed carbides increase the recrystallization temperature of pure molybdenum approximately 500 °C. Besides the recrystallization temperature, higher creep and tensile strength are obtained by TZM alloy at temperatures above 1000 °C [4]. They are highly desirable for high temperature applications in military, aerospace and nuclear industries [6]. The major drawback of the Mo and Mo based alloys are low oxidation resistance at elevated temperatures [4]. TZM alloy can be used in air or oxidizing atmosphere without any restriction below 400 °C. However, mass gain is rapidly increased between 400 and 650 °C due to the formation of oxidation products such as MoO2 and other oxides (MoOZ), where 2 ≤ z b 3. Rapid vaporization of MoO3 results in mass loss and increase in oxidation rates above 650 °C. A volatile and non-protective
⁎ Corresponding author.
http://dx.doi.org/10.1016/j.ijrmhm.2016.04.020 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
MoO3 causes to unusable and damaged structure restricting its potential for widespread use [4,6–9]. Thus, in order to use TZM alloy at elevated temperatures some precautions should be carried out. Generally, alloying and coating techniques are used to protect TZM alloy. High melting point and oxidizing problem restrict production methods for TZM alloy. Powder metallurgy and vacuum arc melting (VAR) are suitable techniques to prepare them. Desired properties could not be achieved by VAR method in single step due to the formation of segregation. Stirring is not possible during VAR process, so segregation of alloying elements (Ti, Zr and C) is inevitable. Therefore, it is required to crush and re-melt the alloy several times which bring time and energy consuming processes to TZM alloy production [8,10–12]. Furthermore, to give the TZM alloy a final shape as a product hot forging or extrusion is needed after VAR process. Grain coarsening is also another drawback for this technique [1]. In this study, both type of additive and production method selected to produce TZM alloy were different than literature. B4C was added into pre-alloyed TZM powder in different amounts in order to enhance the mechanical and oxidation resistance. The earlier studies on the oxidation protection of Mo generally consist of MoSi2 coating or doping with another metal. The results of boron modified MoSi2 coatings include the favorable effect on oxidation by formation of MoB, Mo2B and Mo5SiB2 phases [16–19]. These results created a motivation to choose B4C as an additive for TZM due to formation of Mo2B during sintering. SPS has been utilized both to overcome the difficulties faced in VAR. SPS has been used as a sintering method to densify TZM alloys at relatively lower temperatures for shorter holding times compared to the conventional ones. A pulsed direct current passes through the graphite punch rods and dies simultaneously with a uniaxial pressure to sinter
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powders or pre-alloyed powders in SPS sintering. During the process grain coarsening can be suppressed by rapid heating and the densification of is accelerated at higher temperatures [13–15].
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rate of 10 °C/min then exposed to 60 min dwell time and cooled to RT inside the furnace. 3. Results and discussion
2. Experimental procedure
3.1. Thermodynamic calculation of possible compounds by FactSage©
Pre-alloyed TZM (H.C. Starck Corp.) and B 4C (H.C. Starck Corp. Grade HS) powders were used as starting materials (Table 1). Particle sizes of TZM and B4C powders were measured by laser particle sizer (Malvern Mastersizer 2000). Average particle size of TZM and B4C powders were obtained as 20.3 ± 0.06 μm and 1.78 ± 0.04 μm, respectively. The morphology of the TZM powder and the particle size distribution were given in Fig. 1(a) and (b). Although in the SEM image the particle sizes were seemed to be around 2 μm, the average particle size distribution was determined to be around 20 μm. This could be due to the cold welding of particles in the preparation of pre-alloyed powders. Possible phases which could form during sintering process was calculated by FactSage© software. Temperature and B4C amounts were taken as parameters for determining the possible phases. Before the experiments a blank run test (without powder) was made to investigate the expansion amount of graphite punch rods for the shrinkage calibration. The raw materials were weighed in the quantities calculated and mixed by Turbula 8 h in order to get a homogeneous powder mixture. A graphite die with 50 mm inner diameter was filled with the powder mixture and then spark plasma sintering process (SPS-7.40 MK-VII, SPS Syntex Inc) was conducted. Two graphitic sheets were placed in between the punches – the powder and the die – the powder for easy removal and better conductivity. The powder was sintered at 1420 °C for 300 s with a heating rate of 100 °C/min. A uniaxial pressure of 40 MPa and a pulsed direct current (12 ms/on, 2 ms/off) were applied during the entire SPS process under vacuum. The temperature was measured by an optical pyrometer (Chino, IR-AH) and sintering was conducted under current controlled mode by monitoring the shrinkage behavior during SPS process. The sintering temperature was taken to be the temperature where the shrinkage was constant. Sintered specimens were 50 mm in diameter and approximately 4 mm thickness. Surfaces of the specimens were cleaned by sandblasting for to remove the graphite sheets. Then, the characterizations were performed. The bulk density of the specimens were determined by Archimedes' method. The crystalline phases were identified by X-ray diffractometry (XRD; MiniFlex, Rigaku Corp.) in the 2θ range of 10–90° with Cu–Kα radiation (λ = 1.54 Å). Vickers hardness (Hv) values were gathered (VHMOT, Leica Corp.) under a load of 9.8 N for 12 s and their average values were taken after 20 indentations. The microstructural characterization was carried out by scanning electron microscopy (FESEM; JSM 7000F, JEOL Ltd.). Energy dispersive spectroscopy (EDS) was used to investigate the surface composition. Oxidation behaviors of the specimens were studied in air condition at 600, 800 and 1000 °C. The samples were placed into alumina crucible and put in a laboratory muffle furnace after surface area of them were measured in order to calculate mass change in mg/cm2. The samples were weighed before and after heat treatment. They were heated from room temperature (RT) to determined temperature by a heating
Thermodynamic calculation was performed by FactSage© software which is able to calculate free energy of the system and give a result of chemical equations by a wide integrated database. Depending the temperature, the reaction products was calculated over the temperature range of 100–2000 °C for addition of 5 wt.% B4C in Fig. 2(a). Mo2B phase was observed in all temperatures. The amount of Mo was reduced slightly by the increase in temperature up to 1225 °C. Then, a sudden decrease in the amount of Mo was seen and some impurities was formed such as Ti, Zr, C and B which resulted in a decrease in the purity of Mo. Moreover, Mo2C formed and its amount was increased by rising temperature. Added to this, TiC and ZrC phases were found in a complex compound over the temperature of 1250 °C. The effect of B4C amount on the reaction products was examined in Fig. 2(b) at constant temperature of 1420 °C equal to sintering temperature. The amount of Mo2B and Mo2C increased due to increment in source of B and C by addition of B4C, while amount of Mo decreased. By the way, there is no noticeable change in amount of the complex compound comprising of TiC and ZrC. All of these information were taken into consideration in evaluation of the results.
Table 1 Amount of B4C addition, density and Vickers hardness of the sintered samples. Code
B4C (wt.%) addition
Density (g/cm3)
Hardness (GPa)
TZM–0B4C TZM–0.5B4C TZM–1B4C TZM–2B4C TZM–5B4C
0 0.5 1 2 5
9.93 ± 0.02 9.82 ± 0.01 9.72 ± 0.04 9.58 ± 0.0.3 9.41 ± 0.02
1.9 ± 0.03 2.1 ± 0.02 3.4 ± 0.04 5.2 ± 0.02 7.8 ± 0.01
3.2. Densification behavior and crystalline phases Displacement of the punch rods due to the shrinkage of the powders during the SPS process was measured in micron sensitivity. The data were used to determine the starting and ending temperature of the densification. Ending temperature of the densification also referred as the sintering temperature of samples. Fig. 3 shows the densification behavior of the samples at 800–1420 °C and isothermal displacement at 1420 °C for 300 s. The shrinkage of monolithic TZM powders started at 1020 °C and stopped at 1420 °C. The addition of B4C increased the starting temperature of the shrinkage. A significant effect of 5 wt.% B4C on the starting temperature of shrinkage from 1020 °C to 1160 °C was shown in Fig. 3. However, the effect of B4C addition on the ending temperature of shrinkage was not remarkable. The reason for this increase could be due to formation of Mo2B phase in the structure. Sintering of transition metal borides (MxBy) have been reported in previous studies. The authors reported that pressure assistance, sintering additives and prolonged holds during sintering for transition metal borides were required. It is well known that the preparation of dense metal borides was difficult because of their low self-diffusion coefficient and strong covalent bonding [20–23]. Moreover, Mo2B crystallizes in tetragonal structure with type of Al2Cu (space groupI4/mcm) and illustrates ultra-incompressible behavior with high bulk modulus [24–26]. Table 1 shows the variations in the densities of the samples as a function of their composition. The relative densities were not calculated because it was not possible to determine the amount of phases formed during the sintering [3]. A density of 9.93 g/cm3 was measured for monolithic TZM (theoretical density: 10.16 g/cm3) which was equal to relative density of 97.7%. The density values decreased by an increase in the amount of B4C. With the addition of 5 wt.% B4C into TZM the density was decreased from 9.93 g/cm3 to 9.41 g/cm3. This reduction in density values by addition of B4C could be associated with the formation of Mo2B phase having 9.26 g/cm3 theoretical density. The X-ray diffraction analyses of the monolithic TZM and TZM–B4C specimens with variable amount of B4C were shown in Fig. 4. The characteristic peaks of molybdenum (JCPDS: 42-1120) were observed for all compositions. The peaks for α-Mo2C (JCPDS: 31-0871) started to be apparent by the addition of 1 wt.% B4C, while Mo2B phase (JCPDS: 25-0561) could be detected after 2 wt.% B4C addition. Almost all of the
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Fig. 1. SEM image of the pre-alloyed TZM powder (a), particle size distribution of the powder (b).
Fig. 2. FactSage results; effect of temperature at constant 5 wt.% B4C addition (a), effect of B4C addition at 1420 °C (b).
Fig. 3. Relationship between displacement, temperature and the time dependence of isothermal displacement of samples.
Fig. 4. XRD patterns of sintered TZM, TZM–0.5B4C, TZM–1B4C, TZM–2B4C and TZM–5B4C samples.
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Fig. 5. SEM analysis of TZM (a), TZM–0.5B4C (b), TZM–1B4C (c), TZM–2B4C (d) and TZM–5B4C (e).
characteristic peaks of Mo2B and Mo2C were determined for TZM–5B4C specimen. Moreover, the intensities of the peaks also showed that the amount of Mo2B was increased by the addition of B4C, even though the pure Mo was decreased in the structure. These XRD results verified the thermodynamic calculations. However, TiC and ZrC phases calculated were not detected by XRD. This can be due to low amount of Ti, Zr and C in the pre-alloyed TZM powder [27].
3.3. Microstructural observation of polished surfaces SEM images of the polished samples were given in Fig. 5. Different phases in the microstructure identified by back scattered electron (BSE) images and their elemental compositions were obtained by EDS. Additive free TZM specimen shown in Fig. 5(a) consisted of Mo matrix with TiC, ZrC and Mo2C precipitates. Mo matrix did not displayed a single contrast in BSE images probably because of the some additive elements enter to lattice of Mo in different quantity. Conversion of pure Mo to the structure including a few amount of other alloying elements was predicted before the microstructural investigation according to thermodynamic approach carried out by FactSage© software. Majumdar et al. investigated the preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot
isostatic pressing, and they determined hexagonal (Mo0·54Ti0.46)C and orthorhombic (Mo 0·72 Ti 0.28)C as a complex molybdenum– titanium carbides in Mo matrix which are qualified thermodynamically feasible [12]. Moreover, Mo2B phase started to be observed by addition of B4C as well (Fig. 5(b–e)). Amount of Mo2B in the microstructure was increased by increasing in the amount of B4C addition. By the way, TiC and ZrC generally were detected together as a complex compound. Analysis results were in agreement with thermodynamic calculations. 3.4. Evaluation of oxidation behavior Fig. 6 shows the mass change per surface area of the samples versus exposure temperature for 60 min. A slight increase in mass was observed for the all samples at 600 °C. Mass gain at 600 °C probably due to formation of oxidation products such as MoO2 and other oxides (MoOZ), where 2 b z b 3. Smolik et al. examined the oxidation of TZM alloy in air, and they reported that mass gain could be seen under 650 °C because of MoO2 and other non-stoichiometric oxides located at the surface [7]. Mass loss started to increase over 600 °C for the all compositions, while it decreased by addition of B4C. Table 2 shows the mass change values of specimens during oxidation tests for all composition and temperature. When mass loss of monolithic TZM and TZM–5B4C specimens were compared at 1000 °C, there was an approximately 66.3% decrease in mass loss by addition of 5% B4C. Yang et al. investigated the effect of La addition into TZM alloy on oxidation behavior. They evaluated the decrease in mass loss as an enhancement in oxidation resistance [6]. It is assumed that formation of Mo2B by addition of B4C into TZM alloy could provide a resistance to oxidation according to specific mass change results. Views of the samples after oxidation test were given in Fig. 7 in order to compare with each other. Surface of the samples oxidized at 600 °C included some oxide products but there was a not significant change Table 2 Mass changes of the specimens after oxidation test at 600–1000 °C. Specific mass change (mg/cm2) Holding temperature (°C) TZM–0B C TZM–0.5B C TZM–1B C TZM–2B C TZM–5B C 4 4 4 4 4
Fig. 6. Mass changes of the samples after oxidation test at 600–1000 °C.
600 800 1000
0.063 −6.543 −7.420
0.054 −6.508 −6.851
0.048 −4.083 −5.369
0.039 −2.596 −4.999
0.020 −0.662 −2.504
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Fig. 7. View of the samples after oxidation test at 600–1000 °C.
in structure of the samples. A glassy structure was observed on the surface of the samples above the temperature of 800 °C due to melting of MoO3 (melting point: 795 °C) and MoO3–MoO2 eutectic (778 °C) [7]. Evaporation of volatile oxides caused to both mass loss and deformation of structure with increasing temperature. These effects of the oxidation product were decreased by addition of B4C into TZM. The structure of the TZM–5B4C sample was almost fully protected with contribution of Mo2B phase at 800 and 1000 °C. Fig. 8 shows the SEM images of the surface morphology of the samples oxidized at 600, 800 and 1000 °C. Oxide products were observed on the surface of the samples. However, surface morphology of the samples oxidized at 600 °C (Fig. 8(a)) was different from the ones oxidized at 800 and 1000 °C (Fig. 8(b–c)). Differences could derive from formation of the different oxide types of Mo on the surface. The oxides caused to a slight increase in mass was in type of MoOz (2 b z b 3) were occurred at 600 °C. However, mass loss was observed by formation of MoO3 over this temperature. Lamellar type structure was shown on the surface at 800 and 1000 °C, indicating that this morphology belongs to MoO3.
Yang et al. also observed lamellar slices on oxidation tests of TZM and TZM–La [6]. The lamellae particles composed of volatile MoO3 were more dense for monolithic TZM when compare to TZM–5B4C. Formation of less MoO3 on surface by increasing amount of B4C addition demonstrated the enhancement of oxidation resistance. Increase in MoO3 was also determined by calculating ratio of O and Mo atoms using EDS analysis results. Fig. 9 shows the results of EDS analysis of monolithic TZM and TZM–5B4C after oxidation tests at 800 °C and 1000 °C. O/Mo atomic ratios were 4.71 and 2.69 for monolithic TZM and 5 wt.% B4C addition, respectively at 800 °C. In addition, ratios were 5.25 and 2.75 after 1000 °C. The results showed that oxygen content on the surface of the samples decreased by B4C addition, and this brought about improvement in oxidations resistance. 3.5. Mechanical properties Table 1 summarizes the effect of B4C addition on the Vickers hardness of TZM alloy. Theoretical hardness of TZM was given as 2 GPa in
Fig. 8. SEM images of the surface morphology of oxidized samples at 600 °C (a), 800 °C (b) and 1000 °C (c).
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Fig. 9. EDS analysis results of the surface of samples oxidized at different temperatures TZM–0B4C at 800 °C (a), TZM–5B4C at 800 °C (b), TZM–0B4C at 1000 °C (c), TZM–5B4C at 1000 °C (d).
the literature [28]. In present study, the hardness of monolithic TZM measured as 1.9 GPa was compatible with literature. The values increased by addition of B 4 C into TZM. The hardness value was increased from 1.9 GPa to 7.8 GPa with addition of 5 wt.% B 4C. Increment in hardness could arise from the formation of Mo2 B (theoretical value: ~ 23 GPa) phases in the structure during sintering process. 4. Conclusions Pre-alloyed TZM powders with variable amount of B4C between 0 and 5 wt.% were produced by spark plasma sintering. The sintering was performed at 1420 °C under 40 MPa for 5 min. The results showed that the oxidation resistance of TZM was improved by formation Mo2B formed as a result of reaction between Mo and B4C during sintering process. Mass loss after the oxidation test at 1000 °C for 60 min was decreased ~ 66% by addition 5 wt.% B4C into TZM. In addition, Vickers hardness was increased from 1.9 to 7.8 GPa in the same amount of B4C addition. Consequently, addition of B4C into TZM caused to reaction sintering resulted with formation of Mo2B phases in the structure, and this result brings about enhancement of the oxidation resistance and increase in the hardness.
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Investigation the effect of B4C addition on properties of TZM alloy prepared by spark plasma sintering Baris Yavas, Gultekin Goller ⁎ Istanbul Technical University, Department of Metallurgical and Materials Engineering, 34469, Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 7 March 2016 Received in revised form 21 April 2016 Accepted 29 April 2016 Available online 30 April 2016 Keywords: Molybdenum TZM alloy Spark plasma sintering Refractory metals Oxidation
a b s t r a c t TZM alloy is one of the most important molybdenum (Mo) based alloy which has a nominal composition containing 0.5–0.8 wt.% titanium (Ti), 0.08–0.1 wt.% zirconium (Zr) and 0.016–0.02 wt.% carbon (C). It is a possible candidate for high temperature applications in a variety of industries. However, the rapid oxidation of TZM alloys at high temperature in air is considered to be one of the drawback. In this study, TZM alloys with additions of 0– 5 wt.% B4C were prepared by spark plasma sintering (SPS) at 1420 °C utilizing 40 MPa pressure for 5 min under vacuum. The effects of B4C addition on oxidation, densification behavior, microstructure, and mechanical properties were investigated. The TZM alloy with 5 wt.% B4C have exhibited an approximately 66% reduction in mass loss under normal atmospheric conditions in oxidation tests made at 1000 °C for 60 min. And an increase from 1.9 GPa to 7.8 GPa has been determined in hardness of the alloy. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Pure molybdenum is one of the most important refractory metals due to its high melting point, good thermal conductivity and low thermal expansion coefficient needed for many high temperature applications [1]. Since recrystallization temperature of Mo based alloys are higher than the pure one in most applications done above 1000 °C materials selection is made in the favor of Mo based alloys [2]. The beginning of recrystallization can be inhibited by the addition of fine particles dispersed such as carbides [3]. The fine particles added in to the TZM alloy form precipitates TiC and ZrC in the grain boundaries of molybdenum. These precipitates contribute to solid-solution strengthening and behave as inhibitors in recrystallization [4–5]. Homogeneously dispersed carbides increase the recrystallization temperature of pure molybdenum approximately 500 °C. Besides the recrystallization temperature, higher creep and tensile strength are obtained by TZM alloy at temperatures above 1000 °C [4]. They are highly desirable for high temperature applications in military, aerospace and nuclear industries [6]. The major drawback of the Mo and Mo based alloys are low oxidation resistance at elevated temperatures [4]. TZM alloy can be used in air or oxidizing atmosphere without any restriction below 400 °C. However, mass gain is rapidly increased between 400 and 650 °C due to the formation of oxidation products such as MoO2 and other oxides (MoOZ), where 2 ≤ z b 3. Rapid vaporization of MoO3 results in mass loss and increase in oxidation rates above 650 °C. A volatile and non-protective
⁎ Corresponding author.
http://dx.doi.org/10.1016/j.ijrmhm.2016.04.020 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
MoO3 causes to unusable and damaged structure restricting its potential for widespread use [4,6–9]. Thus, in order to use TZM alloy at elevated temperatures some precautions should be carried out. Generally, alloying and coating techniques are used to protect TZM alloy. High melting point and oxidizing problem restrict production methods for TZM alloy. Powder metallurgy and vacuum arc melting (VAR) are suitable techniques to prepare them. Desired properties could not be achieved by VAR method in single step due to the formation of segregation. Stirring is not possible during VAR process, so segregation of alloying elements (Ti, Zr and C) is inevitable. Therefore, it is required to crush and re-melt the alloy several times which bring time and energy consuming processes to TZM alloy production [8,10–12]. Furthermore, to give the TZM alloy a final shape as a product hot forging or extrusion is needed after VAR process. Grain coarsening is also another drawback for this technique [1]. In this study, both type of additive and production method selected to produce TZM alloy were different than literature. B4C was added into pre-alloyed TZM powder in different amounts in order to enhance the mechanical and oxidation resistance. The earlier studies on the oxidation protection of Mo generally consist of MoSi2 coating or doping with another metal. The results of boron modified MoSi2 coatings include the favorable effect on oxidation by formation of MoB, Mo2B and Mo5SiB2 phases [16–19]. These results created a motivation to choose B4C as an additive for TZM due to formation of Mo2B during sintering. SPS has been utilized both to overcome the difficulties faced in VAR. SPS has been used as a sintering method to densify TZM alloys at relatively lower temperatures for shorter holding times compared to the conventional ones. A pulsed direct current passes through the graphite punch rods and dies simultaneously with a uniaxial pressure to sinter
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powders or pre-alloyed powders in SPS sintering. During the process grain coarsening can be suppressed by rapid heating and the densification of is accelerated at higher temperatures [13–15].
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rate of 10 °C/min then exposed to 60 min dwell time and cooled to RT inside the furnace. 3. Results and discussion
2. Experimental procedure
3.1. Thermodynamic calculation of possible compounds by FactSage©
Pre-alloyed TZM (H.C. Starck Corp.) and B 4C (H.C. Starck Corp. Grade HS) powders were used as starting materials (Table 1). Particle sizes of TZM and B4C powders were measured by laser particle sizer (Malvern Mastersizer 2000). Average particle size of TZM and B4C powders were obtained as 20.3 ± 0.06 μm and 1.78 ± 0.04 μm, respectively. The morphology of the TZM powder and the particle size distribution were given in Fig. 1(a) and (b). Although in the SEM image the particle sizes were seemed to be around 2 μm, the average particle size distribution was determined to be around 20 μm. This could be due to the cold welding of particles in the preparation of pre-alloyed powders. Possible phases which could form during sintering process was calculated by FactSage© software. Temperature and B4C amounts were taken as parameters for determining the possible phases. Before the experiments a blank run test (without powder) was made to investigate the expansion amount of graphite punch rods for the shrinkage calibration. The raw materials were weighed in the quantities calculated and mixed by Turbula 8 h in order to get a homogeneous powder mixture. A graphite die with 50 mm inner diameter was filled with the powder mixture and then spark plasma sintering process (SPS-7.40 MK-VII, SPS Syntex Inc) was conducted. Two graphitic sheets were placed in between the punches – the powder and the die – the powder for easy removal and better conductivity. The powder was sintered at 1420 °C for 300 s with a heating rate of 100 °C/min. A uniaxial pressure of 40 MPa and a pulsed direct current (12 ms/on, 2 ms/off) were applied during the entire SPS process under vacuum. The temperature was measured by an optical pyrometer (Chino, IR-AH) and sintering was conducted under current controlled mode by monitoring the shrinkage behavior during SPS process. The sintering temperature was taken to be the temperature where the shrinkage was constant. Sintered specimens were 50 mm in diameter and approximately 4 mm thickness. Surfaces of the specimens were cleaned by sandblasting for to remove the graphite sheets. Then, the characterizations were performed. The bulk density of the specimens were determined by Archimedes' method. The crystalline phases were identified by X-ray diffractometry (XRD; MiniFlex, Rigaku Corp.) in the 2θ range of 10–90° with Cu–Kα radiation (λ = 1.54 Å). Vickers hardness (Hv) values were gathered (VHMOT, Leica Corp.) under a load of 9.8 N for 12 s and their average values were taken after 20 indentations. The microstructural characterization was carried out by scanning electron microscopy (FESEM; JSM 7000F, JEOL Ltd.). Energy dispersive spectroscopy (EDS) was used to investigate the surface composition. Oxidation behaviors of the specimens were studied in air condition at 600, 800 and 1000 °C. The samples were placed into alumina crucible and put in a laboratory muffle furnace after surface area of them were measured in order to calculate mass change in mg/cm2. The samples were weighed before and after heat treatment. They were heated from room temperature (RT) to determined temperature by a heating
Thermodynamic calculation was performed by FactSage© software which is able to calculate free energy of the system and give a result of chemical equations by a wide integrated database. Depending the temperature, the reaction products was calculated over the temperature range of 100–2000 °C for addition of 5 wt.% B4C in Fig. 2(a). Mo2B phase was observed in all temperatures. The amount of Mo was reduced slightly by the increase in temperature up to 1225 °C. Then, a sudden decrease in the amount of Mo was seen and some impurities was formed such as Ti, Zr, C and B which resulted in a decrease in the purity of Mo. Moreover, Mo2C formed and its amount was increased by rising temperature. Added to this, TiC and ZrC phases were found in a complex compound over the temperature of 1250 °C. The effect of B4C amount on the reaction products was examined in Fig. 2(b) at constant temperature of 1420 °C equal to sintering temperature. The amount of Mo2B and Mo2C increased due to increment in source of B and C by addition of B4C, while amount of Mo decreased. By the way, there is no noticeable change in amount of the complex compound comprising of TiC and ZrC. All of these information were taken into consideration in evaluation of the results.
Table 1 Amount of B4C addition, density and Vickers hardness of the sintered samples. Code
B4C (wt.%) addition
Density (g/cm3)
Hardness (GPa)
TZM–0B4C TZM–0.5B4C TZM–1B4C TZM–2B4C TZM–5B4C
0 0.5 1 2 5
9.93 ± 0.02 9.82 ± 0.01 9.72 ± 0.04 9.58 ± 0.0.3 9.41 ± 0.02
1.9 ± 0.03 2.1 ± 0.02 3.4 ± 0.04 5.2 ± 0.02 7.8 ± 0.01
3.2. Densification behavior and crystalline phases Displacement of the punch rods due to the shrinkage of the powders during the SPS process was measured in micron sensitivity. The data were used to determine the starting and ending temperature of the densification. Ending temperature of the densification also referred as the sintering temperature of samples. Fig. 3 shows the densification behavior of the samples at 800–1420 °C and isothermal displacement at 1420 °C for 300 s. The shrinkage of monolithic TZM powders started at 1020 °C and stopped at 1420 °C. The addition of B4C increased the starting temperature of the shrinkage. A significant effect of 5 wt.% B4C on the starting temperature of shrinkage from 1020 °C to 1160 °C was shown in Fig. 3. However, the effect of B4C addition on the ending temperature of shrinkage was not remarkable. The reason for this increase could be due to formation of Mo2B phase in the structure. Sintering of transition metal borides (MxBy) have been reported in previous studies. The authors reported that pressure assistance, sintering additives and prolonged holds during sintering for transition metal borides were required. It is well known that the preparation of dense metal borides was difficult because of their low self-diffusion coefficient and strong covalent bonding [20–23]. Moreover, Mo2B crystallizes in tetragonal structure with type of Al2Cu (space groupI4/mcm) and illustrates ultra-incompressible behavior with high bulk modulus [24–26]. Table 1 shows the variations in the densities of the samples as a function of their composition. The relative densities were not calculated because it was not possible to determine the amount of phases formed during the sintering [3]. A density of 9.93 g/cm3 was measured for monolithic TZM (theoretical density: 10.16 g/cm3) which was equal to relative density of 97.7%. The density values decreased by an increase in the amount of B4C. With the addition of 5 wt.% B4C into TZM the density was decreased from 9.93 g/cm3 to 9.41 g/cm3. This reduction in density values by addition of B4C could be associated with the formation of Mo2B phase having 9.26 g/cm3 theoretical density. The X-ray diffraction analyses of the monolithic TZM and TZM–B4C specimens with variable amount of B4C were shown in Fig. 4. The characteristic peaks of molybdenum (JCPDS: 42-1120) were observed for all compositions. The peaks for α-Mo2C (JCPDS: 31-0871) started to be apparent by the addition of 1 wt.% B4C, while Mo2B phase (JCPDS: 25-0561) could be detected after 2 wt.% B4C addition. Almost all of the
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Fig. 1. SEM image of the pre-alloyed TZM powder (a), particle size distribution of the powder (b).
Fig. 2. FactSage results; effect of temperature at constant 5 wt.% B4C addition (a), effect of B4C addition at 1420 °C (b).
Fig. 3. Relationship between displacement, temperature and the time dependence of isothermal displacement of samples.
Fig. 4. XRD patterns of sintered TZM, TZM–0.5B4C, TZM–1B4C, TZM–2B4C and TZM–5B4C samples.
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Fig. 5. SEM analysis of TZM (a), TZM–0.5B4C (b), TZM–1B4C (c), TZM–2B4C (d) and TZM–5B4C (e).
characteristic peaks of Mo2B and Mo2C were determined for TZM–5B4C specimen. Moreover, the intensities of the peaks also showed that the amount of Mo2B was increased by the addition of B4C, even though the pure Mo was decreased in the structure. These XRD results verified the thermodynamic calculations. However, TiC and ZrC phases calculated were not detected by XRD. This can be due to low amount of Ti, Zr and C in the pre-alloyed TZM powder [27].
3.3. Microstructural observation of polished surfaces SEM images of the polished samples were given in Fig. 5. Different phases in the microstructure identified by back scattered electron (BSE) images and their elemental compositions were obtained by EDS. Additive free TZM specimen shown in Fig. 5(a) consisted of Mo matrix with TiC, ZrC and Mo2C precipitates. Mo matrix did not displayed a single contrast in BSE images probably because of the some additive elements enter to lattice of Mo in different quantity. Conversion of pure Mo to the structure including a few amount of other alloying elements was predicted before the microstructural investigation according to thermodynamic approach carried out by FactSage© software. Majumdar et al. investigated the preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot
isostatic pressing, and they determined hexagonal (Mo0·54Ti0.46)C and orthorhombic (Mo 0·72 Ti 0.28)C as a complex molybdenum– titanium carbides in Mo matrix which are qualified thermodynamically feasible [12]. Moreover, Mo2B phase started to be observed by addition of B4C as well (Fig. 5(b–e)). Amount of Mo2B in the microstructure was increased by increasing in the amount of B4C addition. By the way, TiC and ZrC generally were detected together as a complex compound. Analysis results were in agreement with thermodynamic calculations. 3.4. Evaluation of oxidation behavior Fig. 6 shows the mass change per surface area of the samples versus exposure temperature for 60 min. A slight increase in mass was observed for the all samples at 600 °C. Mass gain at 600 °C probably due to formation of oxidation products such as MoO2 and other oxides (MoOZ), where 2 b z b 3. Smolik et al. examined the oxidation of TZM alloy in air, and they reported that mass gain could be seen under 650 °C because of MoO2 and other non-stoichiometric oxides located at the surface [7]. Mass loss started to increase over 600 °C for the all compositions, while it decreased by addition of B4C. Table 2 shows the mass change values of specimens during oxidation tests for all composition and temperature. When mass loss of monolithic TZM and TZM–5B4C specimens were compared at 1000 °C, there was an approximately 66.3% decrease in mass loss by addition of 5% B4C. Yang et al. investigated the effect of La addition into TZM alloy on oxidation behavior. They evaluated the decrease in mass loss as an enhancement in oxidation resistance [6]. It is assumed that formation of Mo2B by addition of B4C into TZM alloy could provide a resistance to oxidation according to specific mass change results. Views of the samples after oxidation test were given in Fig. 7 in order to compare with each other. Surface of the samples oxidized at 600 °C included some oxide products but there was a not significant change Table 2 Mass changes of the specimens after oxidation test at 600–1000 °C. Specific mass change (mg/cm2) Holding temperature (°C) TZM–0B C TZM–0.5B C TZM–1B C TZM–2B C TZM–5B C 4 4 4 4 4
Fig. 6. Mass changes of the samples after oxidation test at 600–1000 °C.
600 800 1000
0.063 −6.543 −7.420
0.054 −6.508 −6.851
0.048 −4.083 −5.369
0.039 −2.596 −4.999
0.020 −0.662 −2.504
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Fig. 7. View of the samples after oxidation test at 600–1000 °C.
in structure of the samples. A glassy structure was observed on the surface of the samples above the temperature of 800 °C due to melting of MoO3 (melting point: 795 °C) and MoO3–MoO2 eutectic (778 °C) [7]. Evaporation of volatile oxides caused to both mass loss and deformation of structure with increasing temperature. These effects of the oxidation product were decreased by addition of B4C into TZM. The structure of the TZM–5B4C sample was almost fully protected with contribution of Mo2B phase at 800 and 1000 °C. Fig. 8 shows the SEM images of the surface morphology of the samples oxidized at 600, 800 and 1000 °C. Oxide products were observed on the surface of the samples. However, surface morphology of the samples oxidized at 600 °C (Fig. 8(a)) was different from the ones oxidized at 800 and 1000 °C (Fig. 8(b–c)). Differences could derive from formation of the different oxide types of Mo on the surface. The oxides caused to a slight increase in mass was in type of MoOz (2 b z b 3) were occurred at 600 °C. However, mass loss was observed by formation of MoO3 over this temperature. Lamellar type structure was shown on the surface at 800 and 1000 °C, indicating that this morphology belongs to MoO3.
Yang et al. also observed lamellar slices on oxidation tests of TZM and TZM–La [6]. The lamellae particles composed of volatile MoO3 were more dense for monolithic TZM when compare to TZM–5B4C. Formation of less MoO3 on surface by increasing amount of B4C addition demonstrated the enhancement of oxidation resistance. Increase in MoO3 was also determined by calculating ratio of O and Mo atoms using EDS analysis results. Fig. 9 shows the results of EDS analysis of monolithic TZM and TZM–5B4C after oxidation tests at 800 °C and 1000 °C. O/Mo atomic ratios were 4.71 and 2.69 for monolithic TZM and 5 wt.% B4C addition, respectively at 800 °C. In addition, ratios were 5.25 and 2.75 after 1000 °C. The results showed that oxygen content on the surface of the samples decreased by B4C addition, and this brought about improvement in oxidations resistance. 3.5. Mechanical properties Table 1 summarizes the effect of B4C addition on the Vickers hardness of TZM alloy. Theoretical hardness of TZM was given as 2 GPa in
Fig. 8. SEM images of the surface morphology of oxidized samples at 600 °C (a), 800 °C (b) and 1000 °C (c).
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Fig. 9. EDS analysis results of the surface of samples oxidized at different temperatures TZM–0B4C at 800 °C (a), TZM–5B4C at 800 °C (b), TZM–0B4C at 1000 °C (c), TZM–5B4C at 1000 °C (d).
the literature [28]. In present study, the hardness of monolithic TZM measured as 1.9 GPa was compatible with literature. The values increased by addition of B 4 C into TZM. The hardness value was increased from 1.9 GPa to 7.8 GPa with addition of 5 wt.% B 4C. Increment in hardness could arise from the formation of Mo2 B (theoretical value: ~ 23 GPa) phases in the structure during sintering process. 4. Conclusions Pre-alloyed TZM powders with variable amount of B4C between 0 and 5 wt.% were produced by spark plasma sintering. The sintering was performed at 1420 °C under 40 MPa for 5 min. The results showed that the oxidation resistance of TZM was improved by formation Mo2B formed as a result of reaction between Mo and B4C during sintering process. Mass loss after the oxidation test at 1000 °C for 60 min was decreased ~ 66% by addition 5 wt.% B4C into TZM. In addition, Vickers hardness was increased from 1.9 to 7.8 GPa in the same amount of B4C addition. Consequently, addition of B4C into TZM caused to reaction sintering resulted with formation of Mo2B phases in the structure, and this result brings about enhancement of the oxidation resistance and increase in the hardness.
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