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Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering
Int. Journal of Refractory Metals and Hard Materials 29 (2011) 141–145
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Short Communication
Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering E. Ahmadi a, M. Malekzadeh a,⁎, S.K. Sadrnezhaad a,b a b
Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran Center of Excellence for Advanced Processes of Production of Materials, Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 11 March 2010 Accepted 3 September 2010 Keywords: TZM Sintering Mechanical alloying Nanostructure
a b s t r a c t Mechanical milling proceeded by sintering was used to synthesize nanostructured temperature-resistant TZM alloy. Milling under Ar for different times (1, 2, 3, 5, 10, 15, 20, 25, and 30 h) and sintering at 1500, 1600 and 1700 °C for 30, 45, 60 and 90 min resulted in increasing of low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC with a size of ~ 65 nm in the matrix near LEGBs. Morphology and grain size of the products were determined from scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns, almost precisely. Optimum density of nanostructured TZM alloy ~ 9.95 ± 0.01 g/cm3 was achieved by sintering at 1700 °C for 90 min. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction TZM alloy shows unique properties like creep strength, resistance to recrystallization and softening as compared to the elemental molybdenum, high corrosion/erosion resistance, low thermal expansion, good heat-conduction, exceptional high-temperature stiffness and strength, thermal compatibility with other materials such as copper and Ti–6Al–4V alloy and stable physical and mechanical properties over a wide range of temperatures [1–10]. Nominal composition of TZM is Mo–0.5Ti–0.08Zr–0.02C (wt.%) [1,2]. It is applicable in nuclear and aerospace industries for making hightemperature dies, fusion-reactor diverter components and missile combustion chambers [1–6,8]. TZM alloy can be fabricated either by powder metallurgy (PM) or by vacuum arc melting (VAR) and electron beam melting (EBM) methods [1,4,5,8,11–13]. Information on production and characterization of nanostructured TZM alloy is very limited. Dispersion of nano-sized carbides and its effects on mechanical and physical behavior of TZM alloy are non-existent. Abe et al. have tried to disperse the nano-sized particles in 9Cr–3 W–3Co–VNb and 9Cr– 2 W–VNbTi heat resisting steels [14,15]. Optimum creep strength combined with adequate solid solution strengthening has been acquired by this dispersion process. It has been shown that the grain growth and the structural failure can be inhibited by formation of fine titanium and zirconium carbides at the grain boundaries of the TZM alloys [16–18]. Low-angle boundaries (LABs) have been shown
⁎ Corresponding author. Tel.: +98 21 66165215; fax: +98 21 66005717. E-mail address: [email protected] (M. Malekzadeh). 0263-4368/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2010.09.003
to display improved physical, mechanical and chemical properties relative to the high-angle boundaries (HABs) existing generally in the samples [19–21]. Mechanical alloying proceeded by sintering of elemental powders was employed in this research to extend low-angle grain boundaries, to increase the dispersion of carbides within the LABs and production of nanostructured TZM alloy. Effect of the milling time as well as the sintering time and temperature on crystallite size was rigorously investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization of the produced samples. 2. Experimental procedure Molybdenum, titanium and zirconium powders with a minimum purity of 99.99% and particle sizes of less than 2 μm were mixed with highly pure activated carbon powder having a particle size of 44 μm and then dry-milled in a planetary ball mill (PM400, Germany) under an Ar atmosphere for 1, 2, 3, 5, 10, 15, 20, 25 and 30 h. High purity of 99.999% argon gas was used in the ball-milling steps. The milling cup was made of stainless steel. It contained the mixture of stainless steel balls and powder with a ball to powder weight ratio (BPR) of about 10:1. To prevent iron contamination due to the stainless steel balls and milling jar during the milling step, a pre-milling for about 100 h was accomplished. Tablets sizing ø13 × 5 mm were made of nanostructured and microstructured powders by compaction at 300 MPa pressure. The tablets were heated up with a heating rate of 5 °C/min to 1500, 1600 and 1700 °C and isothermally sintered under purified Ar for 30, 45, 60 and 90 minutes. The etching solution was 1 g CrO3 (BDH) +1 ml H2SO4 + 500 ml water and the etching time was about 30 s. The Archimedean method was used to measure the density of
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the sintered samples. Fe total of milled powders was determined by atomic absorption spectroscopy (AAS, Perkin Elmer 3110). Morphology, particle size and microstructure of the sintered specimens were studied by X-ray diffraction (XRD, Powder Metallurgy 9920/50, Philips, Holland) having Cu Kα radiation of 0.15405 nm wavelength and scanning electron microscope (SEM, VEGA, TESCAN Czech Republic).
3. Results and discussion The XRD patterns of the mechanically alloyed Mo–0.5Ti–0.08Zr– 0.02C (wt.%) powders are given in Fig. 1. As is seen, the XRD graphs exhibit three well-defined sharp peaks, altogether. Among these peaks, the tallest one confirms the existence of the elemental molybdenum and two fairly short ones verify titanium and zirconium carbides. These peaks indicate that there is no significant amount of crystalline species like titanium and zirconium. The XRD patterns given in Fig. 1 indicate that by increasing the milling time, the carbide peaks broaden until they resemble a nearly amorphous phase at t = 30 h. Fig. 2 shows the SEM images of the as-received and the dry-milled TZM powder after 30 h. As is illustrated in Fig. 2 and determined by the wall to wall tool of a scanning electron microscope, the average particle size is about 1.5 μm for as-received Mo powder and ~60 nm for 30 h dry-milled under argon powder, moreover ~500 nm for agglomerated clusters of milled powders. TZM powders may be contaminated with materials such as iron eroded from the milling media of stainless steel. Fe total of milled powders was determined by atomic absorption spectroscopy (AAS). The iron content of milled powders is presented in Fig. 3. It can be observed that iron contamination increases continuously from 100 ppm for 5 h to about 400 ppm for 30 h of milling time. The maximum iron content of powders milled for 30 h is approximately 400 ppm, however. The effect of milling time on the average particle size of the TZM powder is shown in Fig. 4. Patently, it can be seen that the average particle size of the TZM powder becomes lower than 100 nm, after milling for 30 h. Synthesis and preparation of temperature-resistant TZM alloy by mechanical milling proceeded by sintering route have also been reported by other researchers, and their results showed that formation of nano-carbides during mechanical alloying leads to refine the particle size of the TZM alloy [22,23]. Density change of the microstructured and the nanostructured TZM alloys is shown against
Fig. 1. XRD patterns of the TZM powders ball-milled under argon for 1, 2, 3, 5, 10, 15, 20, 25 and 30 h.
Fig. 2. SEM image of TZM powder mixture (a) as-received and (b) dry-milled for 30 h.
the sintering time and temperature in Fig. 5. These plots demonstrate remarkable differences between nanostructured and microstructured samples. Higher densities are observed at higher sintering temperatures and longer sintering times (e.g. 1700 °C and 90 min). Greater densities of the nanostructured tablets can be attributed to the higher sintering temperatures. Hence, any change from microstructured to nanostructured TZM, results in a denser tablet. Optimum density can
Fig. 3. Iron concentration of TZM milled powders with respect to the milling time.
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Fig. 6. SEM image of TZM alloy sintered by as-received powders at 1700 °C for 90 min.
Fig. 4. Average particle size (nm) versus milling time for TZM powder mixtures.
be obtained at lower temperatures in comparison with other results, as well [1,11,23]. In addition, at lower temperatures nano-sized powder compacts offer equivalent densification because of not having enough temperature for sintering processes. By increasing the temperature up to the 1700 °C, densification of TZM compact tablets becomes increased in comparison with the others. Density of nanostructured TZM alloy ~ 9.95 ± 0.01 gr/cm3 was achieved by sintering at 1700 °C for 90 min. However, greater densities of nanostructured tablets can be attributed to the higher sintering temperatures. This is included in the text. The sintered microstructures were examined by scanning electron microscopy (SEM). The SEM images of microstructured and nanostructured TZM alloys for 90 min sintered at 1700 °C are given in Figs. 6 and 7, respectively. Fig. 6 shows that TiC and ZrC carbides are distributed mainly in the grain boundaries (GBs). In Fig. 7, the SEM images indicate the carbides observed in the inter-grains near the low-energy grain boundaries (LEGBs) of the sample. The mean size of the TiC and the ZrC carbides being about 65 nm is much smaller than that of the Mo, as shown in Fig. 7b. A typical EDX spectrum of TiC and ZrC is shown in Fig. 8. A great amount of TiC and ZrC carbides appeared in the EDX spectrum. This quantitative metallography confirms the presents of carbides. Majumdar, et al. [1,2] deduced that the carbides shift the recrystallization temperature of Mo to a higher temperature region. So, TiC and ZrC considered as the second phase particles are able to raise the recrystallization temperature and prevent the grain growth in the molybdenum matrix. They thus improve the creep properties of the sample [1,13,16,23]. In addition, Sanders, et al. deduced that prevalence
Fig. 5. Density of the microstructured and the nanostructured TZM alloy against the sintering time and temperature.
of low-energy grain boundaries (LEGBs) inhibit dislocation activity caused by small grain sizes and is responsible for low strain rates and higher creep resistance [18]. From the results of this research, it is clear that during the reduction of grain size to the nanocrystalline range, the TiC and ZrC carbides disperse into the inter-grains and into the lowenergy grain boundaries of the TZM alloy (see Fig. 7a). Significant differences are, therefore, observed in the SEM morphology image. A low-angle grain boundary is created in the nanostructured TZM alloy. On the other hand, the maximum relative density of the nanostructured TZM alloy prepared by mechanical milling and sintering was around 0.975 (9.95/10.16). A little porosity plays a great role in some special applications. For example, the presence of pores noticeably increases the resistance to thermal stress. Porous materials have low thermal expansion coefficient in comparison to the non-porous materials [24,25]. In this paper, increasing the low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC near them during mechanical
Fig. 7. SEM images of nanostructured TZM alloy sintered by milled powders at 1700 °C for 90 min, (a) both regular grain boundaries (GBs) and low-angle grain boundaries (LAGBs) and (b) carbides.
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Fig. 8. EDX spectrum of nanostructured TZM alloy sintered by milled powders at 1700 °C for 90 min, showing: (a) TiC and (b) ZrC carbides.
alloying seem, therefore, to improve the mechanical and physical properties of the TZM. Around 2.5% porosity conduces, for example, better thermal properties. 4. Conclusion Production of nanostructured temperature-resistant TZM alloy from elemental powders was carried out by mechanical milling/ alloying (MA) and then sintering. The XRD peaks pointed out by increasing the mechanical alloying/milling time and the carbide peaks broadened and nearly became amorphous after ~30 h were observed. The average particle size of nano-powders (~60 nm) was obtained after 30 h milling/alloying procedure. SEM morphologies showed that the specimens sintered at 1700 °C for 90 min had higher density (~9.95 g/cm3) with respect to the other specimens. Modification of the microstructure to the nanostructure caused higher density at lower temperatures in the TZM alloy. The SEM images approved creation of low-angle grain boundaries (LAGBs) in nanostructured TZM and indicated that TiC and ZrC were distributed mainly near the low-energy grain boundaries (LEGB) of the sample. The mean size of the TiC and the ZrC precipitates being about 65 nm was much smaller than that of the grain size of the elemental Mo. Acknowledgment The authors would like to thank the Materials and Energy Research Center (MERC) for its supports.
References [1] Majumdar S, Sharma IG, Raveendra S, Samajdar I, Bhargava P, Tewari R. A study on preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot isostatic pressing, and its characterization. Mater Chem Phys 2009;113:562–6. [2] Majumdar S, Sharma IG, Raveendra S, Samajdar I, Bhargava P. In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM. Mater Sci Eng, A 2008;492:211–7. [3] Filacchioni G, Casagrande E, De Angelis U, De Santis G, Ferrara D. Effects of strain rate on tensile properties of TZM and Mo–5%Re. J Nucl Mater 2002;307–311:705–9. [4] Byun TS, Li M, Cockeram BV, Snead LL. Deformation and fracture properties in neutron irradiated pure Mo and Mo alloys. J Nucl Mater 2008;376:240–6. [5] Cockeram BV. The mechanical properties and fracture mechanisms of wrought low carbon arc cast (LCAC), molybdenum–0.5pct titanium–0.1pct zirconium (TZM), and oxide dispersion strengthened (ODS) molybdenum flat products. Mater Sci Eng, A 2006;418:120–36. [6] Cockeram BV, Smith RW, Snead LL. The influence of fast neutron irradiation and irradiation temperature on the tensile properties of wrought LCAC and TZM molybdenum. J Nucl Mater 2005;346:145–64. [7] Alur AP, Kumar KS. Monotonic and cyclic crack growth response of a Mo–Si–B alloy. Acta Mater 2006;54:385–400. [8] Dietmar Fister, Murg, Wolfgang Wiezoreck and Laufenberg, “Molybdenum powder mixture for TZM”; Uniter State Patent, Jul. 25 (1995), Patent Number 5435829. [9] Dixon DC. Zirconia-TZM brazed joints. J Mater Sci Lett 1994;13:472-472. [10] Chan HY, Liaw DW, Shiue RK. The microstructural observation of brazing Ti–6Al–4V and TZM using the BAg-8 braze alloy. Int J Refract Met Hard Mater 2004;22:27–33. [11] Nersisyan HH, Lee JH, Won CW. The synthesis of nanostructured molybdenum under self-propagating high-temperature synthesis mode. Mater Chem Phys 2005;89:283–8. [12] Sharma IG, Chakraborty SP, Suri AK. Preparation of TZM alloy by aluminothermic smelting and its characterization. J Alloy Comp 2005;393:122–8. [13] Fan J, Lu M, Cheng H, Tian J, Huang B. Effect of alloying elements Ti, Zr on the property and microstructure of molybdenum. Int J Refract Met Hard Mater 2009;27:78–82.
E. Ahmadi et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 141–145 [14] Ohtsuka S, Ukai S, Fujiwara M. Nano-mesoscopic structural control in 9CrODS ferritic/martensitic steels. J Nucl Mater 2006;351:241–6. [15] Abe F, Taneike M, Sawada K. Alloy design of creep resistant 9Cr steel using a dispersion of nano-sized carbonitrides. Int J Press Vessels Pip 2007;84:3–12. [16] Chakraborty SP, Banerjee S, Kulwant Singh, Sharma IG, Grover AK, Suri AK. Studies on the development of protective coating on TZM alloy and its subsequent characterization. J Mater Process Technol 2008;207:240–7. [17] Kang DH, Park SS, Oh Yoon S, Kim Nack J. Effect of nano-particles on the creep resistance of Mg–Sn based alloys. Mater Sci Eng, A 2007;449–451:318–21. [18] Meyers MA, Mishra A, Benson DJ. Mechanical properties of nanocrystalline materials. Prog Mater Sci 2006;51:427–556. [19] Alexandreanu Bogdan, Sencer Bulent H, Thaveeprungsriporn Visit, Was Gary S. The effect of grain boundary character distribution on the high temperature deformation behavior of Ni–16Cr–9Fe alloys. Acta Mater 2003;51:3831–48.
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[20] Visit ThaveeprungsripornWas Gary S. The role of coincidence-site-lattice boundaries in creep of Ni–16Cr–9Fe at 360 °C. Metall Mater Trans A 1997;28:2101–12. [21] Was Gary S. Visit Thaveeprungsriporn, and Douglas C. Crawford, “Grain boundary misorientation effects on creep and cracking in Ni-based Alloys”. JOM 1998;50: 44–9 6. [22] Lohse BH, Calka A, Wexler D. Synthesis of TiC by controlled ball milling of titanium and carbon. J Mater Sci 2007;42:669–75. [23] Majumdar S, Raveendra S, Samajdar I, Bhargava P, Sharma IG. Densification and grain growth during isothermal sintering of Mo and mechanically alloyed Mo– TZM. Acta Mater 2009;57:4158–68. [24] Bakunov VS. High temperature creep of refractory ceramics, distinctive features of the process. Refract Ind Ceram 1994;35:392–4. [25] Bakunov VS. High temperature creep in refractory ceramics. Porous Mater, Refract Ind Ceram 1994;35:317–21.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Short Communication
Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering E. Ahmadi a, M. Malekzadeh a,⁎, S.K. Sadrnezhaad a,b a b
Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran Center of Excellence for Advanced Processes of Production of Materials, Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 11 March 2010 Accepted 3 September 2010 Keywords: TZM Sintering Mechanical alloying Nanostructure
a b s t r a c t Mechanical milling proceeded by sintering was used to synthesize nanostructured temperature-resistant TZM alloy. Milling under Ar for different times (1, 2, 3, 5, 10, 15, 20, 25, and 30 h) and sintering at 1500, 1600 and 1700 °C for 30, 45, 60 and 90 min resulted in increasing of low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC with a size of ~ 65 nm in the matrix near LEGBs. Morphology and grain size of the products were determined from scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns, almost precisely. Optimum density of nanostructured TZM alloy ~ 9.95 ± 0.01 g/cm3 was achieved by sintering at 1700 °C for 90 min. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction TZM alloy shows unique properties like creep strength, resistance to recrystallization and softening as compared to the elemental molybdenum, high corrosion/erosion resistance, low thermal expansion, good heat-conduction, exceptional high-temperature stiffness and strength, thermal compatibility with other materials such as copper and Ti–6Al–4V alloy and stable physical and mechanical properties over a wide range of temperatures [1–10]. Nominal composition of TZM is Mo–0.5Ti–0.08Zr–0.02C (wt.%) [1,2]. It is applicable in nuclear and aerospace industries for making hightemperature dies, fusion-reactor diverter components and missile combustion chambers [1–6,8]. TZM alloy can be fabricated either by powder metallurgy (PM) or by vacuum arc melting (VAR) and electron beam melting (EBM) methods [1,4,5,8,11–13]. Information on production and characterization of nanostructured TZM alloy is very limited. Dispersion of nano-sized carbides and its effects on mechanical and physical behavior of TZM alloy are non-existent. Abe et al. have tried to disperse the nano-sized particles in 9Cr–3 W–3Co–VNb and 9Cr– 2 W–VNbTi heat resisting steels [14,15]. Optimum creep strength combined with adequate solid solution strengthening has been acquired by this dispersion process. It has been shown that the grain growth and the structural failure can be inhibited by formation of fine titanium and zirconium carbides at the grain boundaries of the TZM alloys [16–18]. Low-angle boundaries (LABs) have been shown
⁎ Corresponding author. Tel.: +98 21 66165215; fax: +98 21 66005717. E-mail address: [email protected] (M. Malekzadeh). 0263-4368/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2010.09.003
to display improved physical, mechanical and chemical properties relative to the high-angle boundaries (HABs) existing generally in the samples [19–21]. Mechanical alloying proceeded by sintering of elemental powders was employed in this research to extend low-angle grain boundaries, to increase the dispersion of carbides within the LABs and production of nanostructured TZM alloy. Effect of the milling time as well as the sintering time and temperature on crystallite size was rigorously investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization of the produced samples. 2. Experimental procedure Molybdenum, titanium and zirconium powders with a minimum purity of 99.99% and particle sizes of less than 2 μm were mixed with highly pure activated carbon powder having a particle size of 44 μm and then dry-milled in a planetary ball mill (PM400, Germany) under an Ar atmosphere for 1, 2, 3, 5, 10, 15, 20, 25 and 30 h. High purity of 99.999% argon gas was used in the ball-milling steps. The milling cup was made of stainless steel. It contained the mixture of stainless steel balls and powder with a ball to powder weight ratio (BPR) of about 10:1. To prevent iron contamination due to the stainless steel balls and milling jar during the milling step, a pre-milling for about 100 h was accomplished. Tablets sizing ø13 × 5 mm were made of nanostructured and microstructured powders by compaction at 300 MPa pressure. The tablets were heated up with a heating rate of 5 °C/min to 1500, 1600 and 1700 °C and isothermally sintered under purified Ar for 30, 45, 60 and 90 minutes. The etching solution was 1 g CrO3 (BDH) +1 ml H2SO4 + 500 ml water and the etching time was about 30 s. The Archimedean method was used to measure the density of
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the sintered samples. Fe total of milled powders was determined by atomic absorption spectroscopy (AAS, Perkin Elmer 3110). Morphology, particle size and microstructure of the sintered specimens were studied by X-ray diffraction (XRD, Powder Metallurgy 9920/50, Philips, Holland) having Cu Kα radiation of 0.15405 nm wavelength and scanning electron microscope (SEM, VEGA, TESCAN Czech Republic).
3. Results and discussion The XRD patterns of the mechanically alloyed Mo–0.5Ti–0.08Zr– 0.02C (wt.%) powders are given in Fig. 1. As is seen, the XRD graphs exhibit three well-defined sharp peaks, altogether. Among these peaks, the tallest one confirms the existence of the elemental molybdenum and two fairly short ones verify titanium and zirconium carbides. These peaks indicate that there is no significant amount of crystalline species like titanium and zirconium. The XRD patterns given in Fig. 1 indicate that by increasing the milling time, the carbide peaks broaden until they resemble a nearly amorphous phase at t = 30 h. Fig. 2 shows the SEM images of the as-received and the dry-milled TZM powder after 30 h. As is illustrated in Fig. 2 and determined by the wall to wall tool of a scanning electron microscope, the average particle size is about 1.5 μm for as-received Mo powder and ~60 nm for 30 h dry-milled under argon powder, moreover ~500 nm for agglomerated clusters of milled powders. TZM powders may be contaminated with materials such as iron eroded from the milling media of stainless steel. Fe total of milled powders was determined by atomic absorption spectroscopy (AAS). The iron content of milled powders is presented in Fig. 3. It can be observed that iron contamination increases continuously from 100 ppm for 5 h to about 400 ppm for 30 h of milling time. The maximum iron content of powders milled for 30 h is approximately 400 ppm, however. The effect of milling time on the average particle size of the TZM powder is shown in Fig. 4. Patently, it can be seen that the average particle size of the TZM powder becomes lower than 100 nm, after milling for 30 h. Synthesis and preparation of temperature-resistant TZM alloy by mechanical milling proceeded by sintering route have also been reported by other researchers, and their results showed that formation of nano-carbides during mechanical alloying leads to refine the particle size of the TZM alloy [22,23]. Density change of the microstructured and the nanostructured TZM alloys is shown against
Fig. 1. XRD patterns of the TZM powders ball-milled under argon for 1, 2, 3, 5, 10, 15, 20, 25 and 30 h.
Fig. 2. SEM image of TZM powder mixture (a) as-received and (b) dry-milled for 30 h.
the sintering time and temperature in Fig. 5. These plots demonstrate remarkable differences between nanostructured and microstructured samples. Higher densities are observed at higher sintering temperatures and longer sintering times (e.g. 1700 °C and 90 min). Greater densities of the nanostructured tablets can be attributed to the higher sintering temperatures. Hence, any change from microstructured to nanostructured TZM, results in a denser tablet. Optimum density can
Fig. 3. Iron concentration of TZM milled powders with respect to the milling time.
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Fig. 6. SEM image of TZM alloy sintered by as-received powders at 1700 °C for 90 min.
Fig. 4. Average particle size (nm) versus milling time for TZM powder mixtures.
be obtained at lower temperatures in comparison with other results, as well [1,11,23]. In addition, at lower temperatures nano-sized powder compacts offer equivalent densification because of not having enough temperature for sintering processes. By increasing the temperature up to the 1700 °C, densification of TZM compact tablets becomes increased in comparison with the others. Density of nanostructured TZM alloy ~ 9.95 ± 0.01 gr/cm3 was achieved by sintering at 1700 °C for 90 min. However, greater densities of nanostructured tablets can be attributed to the higher sintering temperatures. This is included in the text. The sintered microstructures were examined by scanning electron microscopy (SEM). The SEM images of microstructured and nanostructured TZM alloys for 90 min sintered at 1700 °C are given in Figs. 6 and 7, respectively. Fig. 6 shows that TiC and ZrC carbides are distributed mainly in the grain boundaries (GBs). In Fig. 7, the SEM images indicate the carbides observed in the inter-grains near the low-energy grain boundaries (LEGBs) of the sample. The mean size of the TiC and the ZrC carbides being about 65 nm is much smaller than that of the Mo, as shown in Fig. 7b. A typical EDX spectrum of TiC and ZrC is shown in Fig. 8. A great amount of TiC and ZrC carbides appeared in the EDX spectrum. This quantitative metallography confirms the presents of carbides. Majumdar, et al. [1,2] deduced that the carbides shift the recrystallization temperature of Mo to a higher temperature region. So, TiC and ZrC considered as the second phase particles are able to raise the recrystallization temperature and prevent the grain growth in the molybdenum matrix. They thus improve the creep properties of the sample [1,13,16,23]. In addition, Sanders, et al. deduced that prevalence
Fig. 5. Density of the microstructured and the nanostructured TZM alloy against the sintering time and temperature.
of low-energy grain boundaries (LEGBs) inhibit dislocation activity caused by small grain sizes and is responsible for low strain rates and higher creep resistance [18]. From the results of this research, it is clear that during the reduction of grain size to the nanocrystalline range, the TiC and ZrC carbides disperse into the inter-grains and into the lowenergy grain boundaries of the TZM alloy (see Fig. 7a). Significant differences are, therefore, observed in the SEM morphology image. A low-angle grain boundary is created in the nanostructured TZM alloy. On the other hand, the maximum relative density of the nanostructured TZM alloy prepared by mechanical milling and sintering was around 0.975 (9.95/10.16). A little porosity plays a great role in some special applications. For example, the presence of pores noticeably increases the resistance to thermal stress. Porous materials have low thermal expansion coefficient in comparison to the non-porous materials [24,25]. In this paper, increasing the low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC near them during mechanical
Fig. 7. SEM images of nanostructured TZM alloy sintered by milled powders at 1700 °C for 90 min, (a) both regular grain boundaries (GBs) and low-angle grain boundaries (LAGBs) and (b) carbides.
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Fig. 8. EDX spectrum of nanostructured TZM alloy sintered by milled powders at 1700 °C for 90 min, showing: (a) TiC and (b) ZrC carbides.
alloying seem, therefore, to improve the mechanical and physical properties of the TZM. Around 2.5% porosity conduces, for example, better thermal properties. 4. Conclusion Production of nanostructured temperature-resistant TZM alloy from elemental powders was carried out by mechanical milling/ alloying (MA) and then sintering. The XRD peaks pointed out by increasing the mechanical alloying/milling time and the carbide peaks broadened and nearly became amorphous after ~30 h were observed. The average particle size of nano-powders (~60 nm) was obtained after 30 h milling/alloying procedure. SEM morphologies showed that the specimens sintered at 1700 °C for 90 min had higher density (~9.95 g/cm3) with respect to the other specimens. Modification of the microstructure to the nanostructure caused higher density at lower temperatures in the TZM alloy. The SEM images approved creation of low-angle grain boundaries (LAGBs) in nanostructured TZM and indicated that TiC and ZrC were distributed mainly near the low-energy grain boundaries (LEGB) of the sample. The mean size of the TiC and the ZrC precipitates being about 65 nm was much smaller than that of the grain size of the elemental Mo. Acknowledgment The authors would like to thank the Materials and Energy Research Center (MERC) for its supports.
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