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Properties at elevated temperature and recrystallization of molybdenum doped with potassium, silicon and aluminum
International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13 www.elsevier.com/locate/ijrmhm
Properties at elevated temperature and recrystallization of molybdenum doped with potassium, silicon and aluminum Yong Wang a
a,*
, Gao Jiacheng a, Chen Gongming a, Weiqin Li b, Yonggui Zhou b, Wei Zhang b
College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China b Zigong Cemented Carbide Co. Ltd., Zigong, Sichuan 643011, China Received 8 December 2006; accepted 23 January 2007
Abstract Potassium, silicon and aluminum (AKS) were doped in molybdenum by solid–liquid mixing, drying, and reducing. Rods and wires were prepared by pressing, sintering, swaging and drawing. Ten-millimeter diameter as-swaged rods were subjected to mechanical measurement. Their tensile properties at 973, 1273 and 1623 K were investigated and compared with that of undoped samples. The ultimate tensile strength (rb) and yield strength (rs) were increased by dopants at 973 K and 1273 K while necking area (w) and elongation (d) were decreased. However, all of the strength and ductility properties were increased at 1623 K when compared with the undoped samples. Strength and ductility properties dropped simultaneously at 1623 K as compared with those at lower temperatures. Correspondingly, fractography analysis on pure molybdenum rods indicated that the breaking path varied from intragranular to intergranular. Primary and secondary recrystallization temperatures were determined by differential scanning calorimetry (DSC) to be 1655 and 1746 K respectively. Furthermore, some dopant particles containing potassium, silicon, and aluminum were detected by SEM and EDX in as-drawn wires of 1 mm diameter. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Molybdenum; AKS; Elevated temperatures properties; Recrystallization
1. Introduction Molybdenum has many attractive properties such as high strength at elevated temperature, low thermal expansion coefficient, and high thermal and electrical conductivity [1]. It is a widely used high temperature metal. In spite of its promising properties its usage has been limited because of the severe intergranular embrittlement after totally recrystallizing [2]. The recrystallization temperature of pure molybdenum is 1023 K [3]. One of the more effective methods of increasing the recrystallization temperature of molybdenum is adding dopants. There are two groups of dopants: oxides of rare
*
Corresponding author. Tel./fax: +86 23 6510 2466. E-mail address: [email protected] (Y. Wang).
0263-4368/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.01.009
earth elements such as La2O3 [4]; and potassium incorporated with silicon and aluminum [5]. The performance of potassium in molybdenum is thought to be the same as it is in tungsten. The potassium alumosilicates will decompose during sintering and some of the doping additives will evaporate. Due to the insolubility of potassium in tungsten and molybdenum, the residual potassium contents are related to the formation of spherical potassium filled bubbles [6,7]. However, some second phase particles apart from potassium pores were also found in K–Si–Al doped molybdenum [8]. In this work, potassium, silicon, and aluminum were doped into molybdenum dioxide. Rods with a diameter of 20 mm were made by drying, reducing, hydrostatically pressing, and sintering. They were then swaged into 10 mm diameter rods. At this stage, their mechanical properties were measured and compared with the mechanical
10
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13
properties of pure molybdenum. Also, some new evidence was provided to support the idea that second phase particles with silicon and aluminum existed in doped products. 2. Materials and methods 2.1. Preparing of doped and control samples For sample preparation, the solid–liquid mixing method was used in the present work. K2SiO3, Al(NO3)3 and KCl was dissolved in distilled water. The solution was then mixed with MoO2 powders. The nominal amounts of doping elements by weight content with respect to molybdenum are listed in Table 1. The mixture was dried and then reduced in a hydrogen atmosphere. The reduced powders were hydrostatically pressed into 20 mm diameter rods. These rods were sintered at temperatures above 2273 K for several hours in a vacuum furnace and it was found that their relative density was higher than 99%. The as-sintered materials were swaged into 10 mm diameter rods. Rod samples were prepared at this stage to measure their mechanical properties at elevated temperatures. The remaining rods were further swaged to 3 mm in diameter, tempered, and then successively drawn into 1 mm diameter wires. Pure molybdenum was used as a control group. The processes were similar to those used for doped groups with the exception of the doping and drying procedures. Table 1 Nominal amounts of doped elements (ppm) K
Si
Al
1600
600
100
2.2. Analyzing method Tensile properties of the swaged materials with the diameter of 10 mm were measured at 973, 1273, and 1623 K using a Gleeble-1500D thermal simulation instrument. The fracture graphs of undoped rods were observed using a KYKY-1000B scanning electron microscope. Recrystallization temperature measurements of the doped wire with 1 mm diameter were conducted by DSC using a Netzsch 449 C simultaneous thermal analyzer. The heating rate was 10 K/min. Drawn directional microstructure of DSC samples was observed using an Olympus optic microscope. In order to observe the bubbles and particles, the doped wire was annealed at 1673 K for 1 h and then broken along the drawing direction. The fractography was investigated using a scanning electron microscope (SEM) and energy dispersive X-ray detector (EDX). 3. Results and discussion 3.1. Mechanical properties at elevated temperatures Figs. 1 and 2 give the stress-strain curves of different samples at different temperatures. This data is also summarized in Table 2. The mechanical properties of molybdenum, especially the strengths, were improved by doping with potassium, aluminum and silicon. At each temperature, the tensile strengths and yield strengths of doped rods are significantly higher than those of undoped samples. Also, the improvement of tensile and yield strengths induced by doping increased progressively with the increase in measuring temperature. At 1623 K, the tensile strength of the doped
Fig. 1. Stress–strain curves of pure molybdenum at (a) 973 K, (b) 1273 K and (c) 1623 K.
Fig. 2. Stress–strain curves of doped molybdenum at (a) 973 K, (b) 1273 K and (c) 1623 K.
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13 Table 2 Mechanical properties at elevated temperatures Sample
Properties
973 (K)
1273 (K)
1623 (K)
Pure Mo
rb (MPa) rs (MPa) w (%) d (%)
293 136 86 16
258 76 94 18
53 44 31 13
Doped Mo
rb (MPa) rs (MPa) w (%) d (%)
413 247 56 9
369 141 61 19
231 130 49 13
materials is more than fourfold and yield strength more than threefold compared to those of their undoped counterparts. At 973 and 1273 K, the improvements of strength were accompanied by a drop in ductility, but both properties were improved simultaneously at the highest temperature. It should be noted that the strength and ductility decrease simultaneously at 1623 K. Fig. 3 displays the fractography of the undoped samples at 1273 and 1623 K. At 1273 K, dimples were the main microstructure on the fracture surface, which implied a ductile fracture. At 1623 K, brittle fracture was fulfilled by intergranular separation as shown in Fig. 3b. The change in fracture mechanism
11
was the reason for the simultaneous drop of strength and ductility. The hardening phases in doped molybdenum are bubbles which resulted from the vaporization of potassium [7] and second phase particles that contain silicon [8] or both silicon and aluminum [9]. Most of the bubbles and particles are distributed on grain boundaries with some of them being distributed intragrains. Those in grains hinder the movement of dislocations and so the doped materials were strengthened and embrittled at 973 and 1273 K. At 1623 K, the rods deform and fracture mainly by sliding and cracking along grain boundaries, thus resulting in a significant drop in the strength and ductility of the undoped materials. At this temperature, the bubbles and particles restrained the sliding of grain boundaries and helped to significantly strengthen the material. The loss of ductility of the doped samples is much less than that of the undoped ones, so that the former is more ductile than the latter at 1623 K. 3.2. Recrystallization temperature of doped sample Fig. 4 shows the DSC curve of the doped wire. Two exothermic peaks were detected at 1655 and 1746 K
Fig. 3. Fractography of undoped molybdenum rod at (a) 1273 K and (b) 1623 K.
Fig. 4. DSC spectrum of doped molybdenum.
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Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13
Fig. 5. Microstructure of doped sample after DSC analysis.
respectively. The sample was sintered over 2273 K, and the dopants experienced decomposition. Therefore, there are no allotropic or any other phase transformations that occur in this heating process. These two peaks correspond to primary and secondary recrystallization, respectively. Fig. 5 displays the microstructure of the sample after DSC analysis. The grain size is quite inhomogeneous. Some grains are abnormally larger than others, which was the result of secondary recrystallization. After primary recrystallization, the grains are elongated and interlocked with each other. During secondary recrystallization, grains grow larger and equiaxial. However, secondary recrystallization did not occur homogeneously. That should be owing to the fact that the bubbles and particles on the grain boundaries prevent their movement as they do in primary recrystallization, and only those with very little or no bubbles and particles can move to form some coarse grains. The annealed wire was broken along the drawing direction, and the fracture surface is intergranular as shown in Fig. 6. The doped wire remains brittle even after recrystallization when it is loaded perpendicularly to the drawing direction. Dopant distribution can be seen on the grain boundary. The row of bubbles indicated by an arrow are potassium bubbles as discussed by other authors [5,9]. Silicon and aluminum were detected, as well as potassium in a larger distribution (spectrum1 in Fig. 6a) by EDX. It is a
partially decomposed aluminosilicate particle similar to that which has been found by other authors [8,9]. The secondary recrystallization is a nucleation and growth procedure, and the nucleation occurs by the coalescence mechanism of neighboring coincident (low misorientation) primary grains [10]. Therefore, the secondary recrystallization is also a thermally activated process as well as the primary one. Its driving force comes mainly from the reduction of grain boundary area where there is much disorder and many lattice defects. During primary recrystallization many dislocations and vacancies disappear. Crystal materials become more ordered by primary and secondary recrystallization, which lower the entropy of the material. The change of Gibbs free energy, DG, is given by: DG ¼ DH T DS;
ð1Þ
in which, DH is the change in enthalpy, T is absolute temperature, DS is the change in entropy. Since the entropy decreases during both recrystallizations, the second term on the right side is negative. Thus, DH must be negative to make the free energy decrease, which is necessary for an irreversible process. On the DSC curve, DH is the area defined by a peak and the baseline. Therefore, primary and secondary recrystallizations both give exothermal peaks. 4. Conclusions The strength of molybdenum was increased by doping with potassium, silicon and aluminum. Especially at 1623 K, the tensile strength of doped material was increased more than quadrupled, and the yield strength more than tripled in comparison to the undoped material. At the lower temperatures, the increase in strength is accompanied by a decrease in ductility, but both of them were enhanced by the dopant at 1623 K. Therefore, the use of doped molybdenum is much more preferable at elevated temperatures. The ductility increased as the temperature increased from 973 to 1273 K, but dropped at 1623 K. Fractography investigation using SEM illustrated that the mechanism of fracture changed at 1623 K. The fracture of molybdenum was transgranular and the microstructure of the fracture
Fig. 6. Fractography of annealed wire (a) and EDX spectrum of a dopant particle (b).
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13
face was dimples at 1273 K. However, intergranular break occurred at 1623 K, and grains were found on the fracture face. Two exothermal peaks, corresponding to primary and secondary recrystallization respectively, appeared on the DSC spectrum of the doped sample. After primary recrystallization, the grains were elongated and interlocked with each other. Some grains grew up and became equiaxial due to secondary recrystallization. The temperatures of primary and secondary recrystallization were found to be 1655 and 1746 K respectively. Some dopant particles containing silicon, aluminum, and potassium were found by EDX in as-drawn 1 mm doped molybdenum wire. These particles and potassium bubbles inhibited the migration of dislocations and grain boundaries, and so increased the mechanical properties and recrystallization temperatures of molybdenum. References [1] Freeman RR. Properties and application of commercial molybdenum and molybdenum alloys. In: Harwood JJ, editor. The metal molybdenum. Cleveland: ASM; 1958. p. 10–1.
13
[2] Hiraoka Y. Significant effect of carbon content in the low-temperature fracture behavior of molybdenum. Mater Trans 1990;31:861–4. [3] Matucha KH. Materials science and technology. Structure and properties of nonferrous alloys, vol. 8. Weinheim: Wiley-VCH; 1995. p. 600. [4] Myoung KY, Yutaka H, Ju C. Recrystallization of molybdenum wire doped with lanthanum oxide. Int J Refract Met Hard Mater 1995;13: 221–7. [5] Yoo MK, Hiraoka Y, Choi J. Recrystallization of molybdenum wire doped with potassium silicate. Metall Mater Trans A 1995;25:801–16. [6] Schade P. Bubble evolution and effects during tungsten processing. Int J Refract Met Hard Mater 2002;20:301–9. [7] Iorio LE, Bewlay BP, Larsen M. Dopant particle characterization and bubble evolution in aluminium–potassium–silicon doped molybdenum wire. Metall Mater Trans A 2002;33:3349–56. [8] Xianliang J, Pingan J. The second phase particles in doped molybdenum sheets. J Mater Sci Lett 1990;9:763–4. [9] Iorio LE, Bewlay BP, Larsen M. Analysis of AKS and lanthanadoped molybdenum wire. Int J Refract Met Hard Mater 2006;24: 306–10. [10] Myoung KY, Yutaka H, Ju C. Secondary recrystallization of molybdenum sheet doped with CaO and MgO under hydrogen atmosphere. Int J Refract Met Hard Mater 1995;13:195–203.
Properties at elevated temperature and recrystallization of molybdenum doped with potassium, silicon and aluminum Yong Wang a
a,*
, Gao Jiacheng a, Chen Gongming a, Weiqin Li b, Yonggui Zhou b, Wei Zhang b
College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China b Zigong Cemented Carbide Co. Ltd., Zigong, Sichuan 643011, China Received 8 December 2006; accepted 23 January 2007
Abstract Potassium, silicon and aluminum (AKS) were doped in molybdenum by solid–liquid mixing, drying, and reducing. Rods and wires were prepared by pressing, sintering, swaging and drawing. Ten-millimeter diameter as-swaged rods were subjected to mechanical measurement. Their tensile properties at 973, 1273 and 1623 K were investigated and compared with that of undoped samples. The ultimate tensile strength (rb) and yield strength (rs) were increased by dopants at 973 K and 1273 K while necking area (w) and elongation (d) were decreased. However, all of the strength and ductility properties were increased at 1623 K when compared with the undoped samples. Strength and ductility properties dropped simultaneously at 1623 K as compared with those at lower temperatures. Correspondingly, fractography analysis on pure molybdenum rods indicated that the breaking path varied from intragranular to intergranular. Primary and secondary recrystallization temperatures were determined by differential scanning calorimetry (DSC) to be 1655 and 1746 K respectively. Furthermore, some dopant particles containing potassium, silicon, and aluminum were detected by SEM and EDX in as-drawn wires of 1 mm diameter. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Molybdenum; AKS; Elevated temperatures properties; Recrystallization
1. Introduction Molybdenum has many attractive properties such as high strength at elevated temperature, low thermal expansion coefficient, and high thermal and electrical conductivity [1]. It is a widely used high temperature metal. In spite of its promising properties its usage has been limited because of the severe intergranular embrittlement after totally recrystallizing [2]. The recrystallization temperature of pure molybdenum is 1023 K [3]. One of the more effective methods of increasing the recrystallization temperature of molybdenum is adding dopants. There are two groups of dopants: oxides of rare
*
Corresponding author. Tel./fax: +86 23 6510 2466. E-mail address: [email protected] (Y. Wang).
0263-4368/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.01.009
earth elements such as La2O3 [4]; and potassium incorporated with silicon and aluminum [5]. The performance of potassium in molybdenum is thought to be the same as it is in tungsten. The potassium alumosilicates will decompose during sintering and some of the doping additives will evaporate. Due to the insolubility of potassium in tungsten and molybdenum, the residual potassium contents are related to the formation of spherical potassium filled bubbles [6,7]. However, some second phase particles apart from potassium pores were also found in K–Si–Al doped molybdenum [8]. In this work, potassium, silicon, and aluminum were doped into molybdenum dioxide. Rods with a diameter of 20 mm were made by drying, reducing, hydrostatically pressing, and sintering. They were then swaged into 10 mm diameter rods. At this stage, their mechanical properties were measured and compared with the mechanical
10
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13
properties of pure molybdenum. Also, some new evidence was provided to support the idea that second phase particles with silicon and aluminum existed in doped products. 2. Materials and methods 2.1. Preparing of doped and control samples For sample preparation, the solid–liquid mixing method was used in the present work. K2SiO3, Al(NO3)3 and KCl was dissolved in distilled water. The solution was then mixed with MoO2 powders. The nominal amounts of doping elements by weight content with respect to molybdenum are listed in Table 1. The mixture was dried and then reduced in a hydrogen atmosphere. The reduced powders were hydrostatically pressed into 20 mm diameter rods. These rods were sintered at temperatures above 2273 K for several hours in a vacuum furnace and it was found that their relative density was higher than 99%. The as-sintered materials were swaged into 10 mm diameter rods. Rod samples were prepared at this stage to measure their mechanical properties at elevated temperatures. The remaining rods were further swaged to 3 mm in diameter, tempered, and then successively drawn into 1 mm diameter wires. Pure molybdenum was used as a control group. The processes were similar to those used for doped groups with the exception of the doping and drying procedures. Table 1 Nominal amounts of doped elements (ppm) K
Si
Al
1600
600
100
2.2. Analyzing method Tensile properties of the swaged materials with the diameter of 10 mm were measured at 973, 1273, and 1623 K using a Gleeble-1500D thermal simulation instrument. The fracture graphs of undoped rods were observed using a KYKY-1000B scanning electron microscope. Recrystallization temperature measurements of the doped wire with 1 mm diameter were conducted by DSC using a Netzsch 449 C simultaneous thermal analyzer. The heating rate was 10 K/min. Drawn directional microstructure of DSC samples was observed using an Olympus optic microscope. In order to observe the bubbles and particles, the doped wire was annealed at 1673 K for 1 h and then broken along the drawing direction. The fractography was investigated using a scanning electron microscope (SEM) and energy dispersive X-ray detector (EDX). 3. Results and discussion 3.1. Mechanical properties at elevated temperatures Figs. 1 and 2 give the stress-strain curves of different samples at different temperatures. This data is also summarized in Table 2. The mechanical properties of molybdenum, especially the strengths, were improved by doping with potassium, aluminum and silicon. At each temperature, the tensile strengths and yield strengths of doped rods are significantly higher than those of undoped samples. Also, the improvement of tensile and yield strengths induced by doping increased progressively with the increase in measuring temperature. At 1623 K, the tensile strength of the doped
Fig. 1. Stress–strain curves of pure molybdenum at (a) 973 K, (b) 1273 K and (c) 1623 K.
Fig. 2. Stress–strain curves of doped molybdenum at (a) 973 K, (b) 1273 K and (c) 1623 K.
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13 Table 2 Mechanical properties at elevated temperatures Sample
Properties
973 (K)
1273 (K)
1623 (K)
Pure Mo
rb (MPa) rs (MPa) w (%) d (%)
293 136 86 16
258 76 94 18
53 44 31 13
Doped Mo
rb (MPa) rs (MPa) w (%) d (%)
413 247 56 9
369 141 61 19
231 130 49 13
materials is more than fourfold and yield strength more than threefold compared to those of their undoped counterparts. At 973 and 1273 K, the improvements of strength were accompanied by a drop in ductility, but both properties were improved simultaneously at the highest temperature. It should be noted that the strength and ductility decrease simultaneously at 1623 K. Fig. 3 displays the fractography of the undoped samples at 1273 and 1623 K. At 1273 K, dimples were the main microstructure on the fracture surface, which implied a ductile fracture. At 1623 K, brittle fracture was fulfilled by intergranular separation as shown in Fig. 3b. The change in fracture mechanism
11
was the reason for the simultaneous drop of strength and ductility. The hardening phases in doped molybdenum are bubbles which resulted from the vaporization of potassium [7] and second phase particles that contain silicon [8] or both silicon and aluminum [9]. Most of the bubbles and particles are distributed on grain boundaries with some of them being distributed intragrains. Those in grains hinder the movement of dislocations and so the doped materials were strengthened and embrittled at 973 and 1273 K. At 1623 K, the rods deform and fracture mainly by sliding and cracking along grain boundaries, thus resulting in a significant drop in the strength and ductility of the undoped materials. At this temperature, the bubbles and particles restrained the sliding of grain boundaries and helped to significantly strengthen the material. The loss of ductility of the doped samples is much less than that of the undoped ones, so that the former is more ductile than the latter at 1623 K. 3.2. Recrystallization temperature of doped sample Fig. 4 shows the DSC curve of the doped wire. Two exothermic peaks were detected at 1655 and 1746 K
Fig. 3. Fractography of undoped molybdenum rod at (a) 1273 K and (b) 1623 K.
Fig. 4. DSC spectrum of doped molybdenum.
12
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13
Fig. 5. Microstructure of doped sample after DSC analysis.
respectively. The sample was sintered over 2273 K, and the dopants experienced decomposition. Therefore, there are no allotropic or any other phase transformations that occur in this heating process. These two peaks correspond to primary and secondary recrystallization, respectively. Fig. 5 displays the microstructure of the sample after DSC analysis. The grain size is quite inhomogeneous. Some grains are abnormally larger than others, which was the result of secondary recrystallization. After primary recrystallization, the grains are elongated and interlocked with each other. During secondary recrystallization, grains grow larger and equiaxial. However, secondary recrystallization did not occur homogeneously. That should be owing to the fact that the bubbles and particles on the grain boundaries prevent their movement as they do in primary recrystallization, and only those with very little or no bubbles and particles can move to form some coarse grains. The annealed wire was broken along the drawing direction, and the fracture surface is intergranular as shown in Fig. 6. The doped wire remains brittle even after recrystallization when it is loaded perpendicularly to the drawing direction. Dopant distribution can be seen on the grain boundary. The row of bubbles indicated by an arrow are potassium bubbles as discussed by other authors [5,9]. Silicon and aluminum were detected, as well as potassium in a larger distribution (spectrum1 in Fig. 6a) by EDX. It is a
partially decomposed aluminosilicate particle similar to that which has been found by other authors [8,9]. The secondary recrystallization is a nucleation and growth procedure, and the nucleation occurs by the coalescence mechanism of neighboring coincident (low misorientation) primary grains [10]. Therefore, the secondary recrystallization is also a thermally activated process as well as the primary one. Its driving force comes mainly from the reduction of grain boundary area where there is much disorder and many lattice defects. During primary recrystallization many dislocations and vacancies disappear. Crystal materials become more ordered by primary and secondary recrystallization, which lower the entropy of the material. The change of Gibbs free energy, DG, is given by: DG ¼ DH T DS;
ð1Þ
in which, DH is the change in enthalpy, T is absolute temperature, DS is the change in entropy. Since the entropy decreases during both recrystallizations, the second term on the right side is negative. Thus, DH must be negative to make the free energy decrease, which is necessary for an irreversible process. On the DSC curve, DH is the area defined by a peak and the baseline. Therefore, primary and secondary recrystallizations both give exothermal peaks. 4. Conclusions The strength of molybdenum was increased by doping with potassium, silicon and aluminum. Especially at 1623 K, the tensile strength of doped material was increased more than quadrupled, and the yield strength more than tripled in comparison to the undoped material. At the lower temperatures, the increase in strength is accompanied by a decrease in ductility, but both of them were enhanced by the dopant at 1623 K. Therefore, the use of doped molybdenum is much more preferable at elevated temperatures. The ductility increased as the temperature increased from 973 to 1273 K, but dropped at 1623 K. Fractography investigation using SEM illustrated that the mechanism of fracture changed at 1623 K. The fracture of molybdenum was transgranular and the microstructure of the fracture
Fig. 6. Fractography of annealed wire (a) and EDX spectrum of a dopant particle (b).
Y. Wang et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 9–13
face was dimples at 1273 K. However, intergranular break occurred at 1623 K, and grains were found on the fracture face. Two exothermal peaks, corresponding to primary and secondary recrystallization respectively, appeared on the DSC spectrum of the doped sample. After primary recrystallization, the grains were elongated and interlocked with each other. Some grains grew up and became equiaxial due to secondary recrystallization. The temperatures of primary and secondary recrystallization were found to be 1655 and 1746 K respectively. Some dopant particles containing silicon, aluminum, and potassium were found by EDX in as-drawn 1 mm doped molybdenum wire. These particles and potassium bubbles inhibited the migration of dislocations and grain boundaries, and so increased the mechanical properties and recrystallization temperatures of molybdenum. References [1] Freeman RR. Properties and application of commercial molybdenum and molybdenum alloys. In: Harwood JJ, editor. The metal molybdenum. Cleveland: ASM; 1958. p. 10–1.
13
[2] Hiraoka Y. Significant effect of carbon content in the low-temperature fracture behavior of molybdenum. Mater Trans 1990;31:861–4. [3] Matucha KH. Materials science and technology. Structure and properties of nonferrous alloys, vol. 8. Weinheim: Wiley-VCH; 1995. p. 600. [4] Myoung KY, Yutaka H, Ju C. Recrystallization of molybdenum wire doped with lanthanum oxide. Int J Refract Met Hard Mater 1995;13: 221–7. [5] Yoo MK, Hiraoka Y, Choi J. Recrystallization of molybdenum wire doped with potassium silicate. Metall Mater Trans A 1995;25:801–16. [6] Schade P. Bubble evolution and effects during tungsten processing. Int J Refract Met Hard Mater 2002;20:301–9. [7] Iorio LE, Bewlay BP, Larsen M. Dopant particle characterization and bubble evolution in aluminium–potassium–silicon doped molybdenum wire. Metall Mater Trans A 2002;33:3349–56. [8] Xianliang J, Pingan J. The second phase particles in doped molybdenum sheets. J Mater Sci Lett 1990;9:763–4. [9] Iorio LE, Bewlay BP, Larsen M. Analysis of AKS and lanthanadoped molybdenum wire. Int J Refract Met Hard Mater 2006;24: 306–10. [10] Myoung KY, Yutaka H, Ju C. Secondary recrystallization of molybdenum sheet doped with CaO and MgO under hydrogen atmosphere. Int J Refract Met Hard Mater 1995;13:195–203.