The distribution of tantalum in molybdenum and tungsten single crystals during electron beam floating zone melting under different crystallization conditions was investigated. The performed line chemical analysis of specimens proved creation of tantalum micro segregation, so-called growth striations, which showed themselves on concentration profiles as periodically alternating areas with increased and decreased concentrations of tantalum. The existence of these chemical inhomogeneities suggests that stationary conditions of the crystal growth were disturbed during the zone melting, i.e. fluctuations of microscopic rate of growth occurred due to convection in the melt. The determined effective distribution coefficients of tantalum in molybdenum and tungsten approach to calculated theoretical values of the equilibrium distribution coefficient

J. Drapala

J. K. Skotnicova

M. Madaj

V. M. Kirillova

English

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The effect of crystallization conditions on tantalum distribution in molybdenum and tungsten during electron beam zone melting

The distribution of tantalum in molybdenum and tungsten single crystals during electron beam fl oating zone melting
under diff erent crystallization conditions was investigated. The performed line chemical analysis of specimens
proved creation of tantalum micro segregation, so-called growth striations, which showed themselves on concentration
proĀ les as periodically alternating areas with increased and decreased concentrations of tantalum. The existence
of these chemical inhomogeneities suggests that stationary conditions of the crystal growth were disturbed
during the zone melting, i.e. fl uctuations of microscopic rate of growth occurred due to convection in the melt. The
determined eff ective distribution coeffi cients of tantalum in molybdenum and tungsten approach to calculated
theoretical values of the equilibrium distribution coeffi cient.Web of Science53221120

Skotnicová, Kateřina

Drápala, Jaromír

Madaj, Michal

Kirillova, Valentina Mikhailovna

DSpace at VSB Technical University of Ostrava

209METALURGIJA 53 (2014) 2, 209-211K. SKOTNICOVÁ, J. DRÁPALA, M. MADAJ, V. M. KIRILLOVATHE EFFECT OF CRYSTALLIZATION CONDITIONS ON TANTALUM DISTRIBUTION IN MOLYBDENUM AND TUNGSTEN DURING ELECTRON BEAM ZONE MELTINGReceived – Prispjelo: 2013-06-21Accepted – Prihvaćeno: 2013-10-30Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(2) 209-211 (2014)UDC – UDK 669.95:548.526:532.73=111K. Skot nicová, J. Drápala, M. Madaj, VŠB – Technical Un iversity of Ostrava, Faculty o f Me tallurgy a nd M aterial E ngineering, O strava – Poruba, Czech RepublicV. M. Kirillova, Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russian FederationThe distribution of tantalum in molybdenum and tungsten single crystals during electron beam fl oating zone melt-ing under diff erent crystallization conditions was investigated. The performed line chemical analysis of specimens proved creation of tantalum micro segregation, so-called growth striations, which showed themselves on concen-tration proĀ les as periodically alternating areas with increased and decreased concentrations of tantalum. The exist-ence of these chemical inhomogeneities suggests that stationar y conditions of the cr ystal growth were disturbed during the zone melting, i.e. fl uctuations of microscopic rate of growth occurred due to convection in the melt. The determined eff ective distribution c oeffi  cients of tantalum in molybdenum and tungst en approach to calculated theoretical values of the equilibrium distribution coeffi  cient.Key words: zone melting, tantalum, molybdenum, tungsten, inhomogeneities.INTRODUCTIONThe high purity , chemical homogeneity and struc-tural perfection have an essential inﬂ  uence on proper -ties of single crystals of refractory metals and their low-alloyed alloys. Thanks to a complex of physical, me-chanical and chemical properties, they represent pro-spective materials for contemporary, modern technolo-gy, especially as high-temperature structural compo-nents [1, 2]. Basic attributes of single crystals, compared to adequate polycrystalline materials, include much more stable structure, lower creep speeds, high strength, good plasticity, the high Young’s modulus, wear resist-ance and high radiation resistance. A negative property of pure single crystals of refractory metals, especially tungsten, is their low plasticity , which limits their processing technology. The plasticity can be improved, to a certain extent, by alloying these metals with other refractory metals, such as Re, Ta, Mo, or Nb, in the amount up to 5 wt.%. However , when preparing single crystals of tungsten and molybdenum-based alloys by the electron beam zone melting technology [1-7], het-erogeneous distribution of alloying elements often oc-curs, which results in structural heterogeneity and the structure sensitive properties [1, 7]. This paper deals with the effect of crystallization conditions on the tanta-lum distribution in molybdenum and tungsten single crystals during their preparation by electron beam ﬂ oat-ing zone melting. Ef fective distribution coefﬁ cients of tantalum in molybdenum and tungsten for individual conditions of crystallization and structural characteris-tics prepared single crystalline alloys are also studied.EXPERIMENTALSingle crystals of tungsten and molybdenum-based  alloys with the nominal chemical composition of Mo-1,5 at.% Ta and W-1,5 at.% Ta were grown from the melt us-ing the electron beam ﬂ oating zone melting in a vacuum applying two different zone pass rates - 1 and 3 mm/min. in RMSTC of the VŠB-Technical University of Ostrava. A polycrystalline rod of molybdenum, of 3N purity and 6 mm in diameter , was used as the starting material, in  which a longitudinal slot 1 mm wide, 0,7 mm deep and  approx. 20 cm long was milled. Little pieces of tantalum sheet enwound with the basic metal wire were inserted  into the slot. The rod prepared in this way was placed in  the electron beam zone furnace where the alloying itself  was performed applying two passes of the molten zone  along the whole length of the rod at the speed of 4 mm/min. and then the rod was gradually melted in the given  sections at speeds of 1 and 3 mm/min. At the end of each section a molten zone was solidiﬁ ed by a high speed with the objective to prevent dif fusion processes and to pre-serve the admixture concentration in the zone corre-sponding to the liquid phase. The concentration proﬁ les of tantalum in molybdenum and tungsten on longitudinal sections were obtained using the scanning electron mi-croscope JCXA  733 equipped with EDAX EDAM 3  probe. The X-ray reﬂ ection-mode topography (a modi-ﬁ ed Berg-Barret technique) and Ka and Kb radiation pro-duced by the sharp-focus X-ray tube (the focus diameter  210  METALURGIJA 53 (2014) 2, 209-211K. SKOTNICOVÁ et al.: THE EFFECT OF CRYSTALLIZATION CONDITIONS ON TANTALUM DISTRIBUTION...is 40 - 50 µm) was applied for determination of the angu-lar disorientations of sub-grains in given single crystals  [4]. The back-reﬂ ection Laue method (X-ray beam per -pendicular to the face-end section) was used for determi-nation of their crystallographic orientation. The micro-structure study was carried out using the light optical  microscope Olympus GX-51 connected with digital cam-era DP12 and the high resolution ﬁ  eld emission gun-scanning electron microscope QUANTA 450 FEG.RESULTS AND DISCUSSIONMetallographic analysisSingle crystals of refractory metals with bcc-lattice prepared using the EBFZM are characterized by a spe-ciﬁ c dislocation substructure with the size of sub-grains, which can be divided into three orders of magnitude [1]. The sub-grains of the 1st order are formed due to the growth from the seed crystal, the sub-grains of the 2nd and 3rd orders result from a polygonisation. The micro-structure of W-Ta single crystals prepared by the zone pass rate of 3 mm/min is shown in Figure 1 as an exam-ple, where the existence of sub-grains of the shape of polygons in cross section and long columnar sub-grains elongated in the direction of growth formed at crystal-lization is visible. The crystallographic orientation of all single crystals  was 16 - 20° from [100]. According to these investiga-tions the alloying of molybdenum and tungsten by tanta-lum and increasing zone pass rate resulted in increasing  of the etched pits density. It may be identiﬁ ed as the dis-location density at the grain body because it is necessary to count some more dislocations at the 1st sub-grain  boundaries, but it is impossible. The dislocation density  of these single crystals was about 10 5 to 106 cm -2. The results of the study of crystal structure by X-ray topogra-phy are summarized in Table 1. An evident tendency ex-ists of increased angular disorientation of sub-grains with the increasing zone pass rate and the fragmentation of a  substructure, where Mo-Ta single crystals are character -ized by a more perfect structure, in comparison with  W-Ta single crystals. In the area of a solidiﬁ  ed zone, a  polycrystalline structure with traces of dendritic forma-tions was observed, as well as creation of a cellular -ﬁ -brous structure in the places where an increased concen-tration of tantalum in W-Ta alloys at cell boundaries or  increased concentration of tantalum in the case of Mo-Ta alloys in central parts of cells is assumed.Table 1  Structure parameters of single crystals of tungsten- and molybdenum-based alloysSampleZone pass rateDisorientation of sub-grains1st order 2nd ordermm/min (´) (´)W-Ta 1 15-20 ~ 5W-Ta 3 20-25 ~ 5Mo-Ta 1 10-15 ~ 5Mo-Ta 3 15-20 ~ 5Chemical analysisAll the determined concentration proﬁ les of tantalum in molybdenum and tungsten for the given zone pass  rates showed an irregular sinusoidal course – see Figure  2. This concentration scatter indicates that stationary  conditions of the crystal growth were disturbed during  the zone melting, i.e. the microscopic rate of growth ﬂ uc-tuated due to convection in the melt, which is manifested by the growth striations formation and/or growth nuclei  – periodically alternating regions with a decreased and  increased concentration of an alloying element. The main reason for the formation of growth stria-tions is considered to be the natural buoyancy convec-tion inducing temperature ﬂ  uctuations in the melt. In addition, the Marangoni convection, which is bound to the existence of the melt free surface, has essential in-ﬂ uence on the occurrence of inhomogeneities in the melt. If solidiﬁ cation proceeds very slowly, the concen-tration of an admixture in the melt will be even and the concentration of an admixture in the solidifying crystal is the equilibrium distribution coefﬁ cient ko [6] multiple of the concentration in the melt (cS = ko cL). If the rate of solidiﬁ cation increases, the moving front of solidiﬁ  ca-tion will displace the admixture faster than it can difuse Figure 1  Microstructure of W-Ta single crystals prepared by the zone pass rate of 3 mm/min: a) sub-grains in central part of crystal; b) SEM-BSE image of rapid solidiĀ ed zone211METALURGIJA 53 (2014) 2, 209-211K. SKOTNICOVÁ et al.: THE EFFECT OF CRYSTALLIZATION CONDITIONS ON TANTALUM DISTRIBUTION...the increasing zone pass rate, which corresponds with the Burton, Prim and Slichter theory [6].CONCLUSIONSIt was found that the angle of disorientation of grain  boundaries in molybdenum and tungsten-based single  crystals increases with the increasing zone pass rate, and  the simultaneous increased density of dislocations. Irregu-lar concentration changes in the alloying elements con-tents, along the axis of the crystal under the given condi-tions of the crystallization, were identiﬁ  ed in individual  samples using the SEM-EDX micro-probe. These micro-segregations of alloying elements in the form of periodi-cally alternating growth bands, with the reduced and in-creased concentration of these elements, are connected  with the existence of the natural buoyancy convection in  the melt due to temperature ﬂ uctuations [1, 6]. From these experimentally measured concentration proﬁ les of tanta-lum along the rod melted gradually at the speeds of 1 and 3 mm/min, the effective distribution coefﬁ cients of tanta-lum in molybdenum and tungsten were determined using the method of a rapidly solidiﬁ ed zone.AcknowledgementsThis paper was created within the framework of the project No. CZ.1.05/2.1.00/01.0040 “Regional Material Science and Technology Centre” within the operation programme “Research and Development for Innova-tions” funded from the Structural Funds and from the state budget of the Czech Republic.REFERENCES[1]  N. P. Lyakishev, G.S. Burkhanov, Metallic single crystals, Eliz, Moscow, 2002, 311 p.[2]  E. Ragan, E. J. Dobránsky, P. Baron, T. Olejár, Metalurgija, 51 (2012) 1, 117-120.[3] S. I. Bozhko, V. G. Glebovsky, V. N. Semenov, Journal of Crystal Growth, 311 (2008) 1, 1–6.[4] V. G. Glebovsky, V.N. Semenov, International Journal of Re-fractory Metals and Hard Materials, 12 (1993) 5, 295-301.[5]  J. Liu, R. H. Zee, Journal of Crystal Growth, 163 (1996) 3, 259-65.[6]  J. Drápala, K. Skotnicová, International Journal of Mate-rials Research, 104 (2013) 1, 60-64.[7]  V. G. Glebovsky , Recrystallization in Crystal Growth: Substructure and Recrystallization, K. Sztwiertnia (ed.), InTech, Rijeka, 2012, 59-86.Note:  The responsible translator for English language is Ing. B. Škandera, Ostrava, Czech Republicinto the main melt volume and an enriched layer creates on the interface. The accumulation of admixtures in front of the crystallization front and enhancement of concentration super-cooling may invoke a considerable rise of the rate of the crystallization front shift. After occupying a certain portion of admixtures on the inter -face, their concentration in the melt decreases or returns to the initial value and the process repeats [1].Furthermore, it is obvious from the results that the solubility of tantalum in the solid and liquid phases is approximately the same, which means that the end part of the ingot was not markedly enriched (W -Ta system) or impoverished (Mo-T a system) with tantalum and from this point of view tantalum shows itself to be a convenient alloying element in molybdenum and tung-sten. The effective distribution coefﬁ cients keff of tanta-lum in molybdenum and tungsten, which describes the inﬂ uence of transport processes taking place in the re-gions close to the phase interface at the speed of crystal-lization v ≠ 0, were calculated using the method of a rapidly solidiﬁ ed zone. The principle of this method consists in the comparison of the mean concentration of admixture element xLB in a quenched zone (no diffusion processes could proceed) to the mean concentration xSB in the place just in front of the solidiﬁ ed zone when the stationary state of crystallization was achieved. The mean concentrations of tantalum used in calculations and obtained values of keff for given speeds of crystalli-zation are presented in Table 2. It is evident from the results that values kef f of tan-talum in molybdenum and tungsten converges to 1 with Figure 2  Concentration proĀ le of tantalum in molybdenum (a) and tungsten (b) single crystal prepared by the zone pass rate of 3 mm/min.Table 2  Mean concentrations of tantalum in Mo and W and obtained values of keff  for given zone pass ratesSampleZone pass rate xSB xLB keff komm/min at.% at.%Mo-Ta 1 1,55 1,35 1,141,32Mo-Ta 3 1,12 1,02 1,10W-Ta 1 1,53 1,81 0,840,79W-Ta 3 1,38 1,58 0,87

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209METALURGIJA 53 (2014) 2, 209-211K. SKOTNICOVÁ, J. DRÁPALA, M. MADAJ, V. M. KIRILLOVATHE EFFECT OF CRYSTALLIZATION CONDITIONS ON TANTALUM DISTRIBUTION IN MOLYBDENUM AND TUNGSTEN DURING ELECTRON BEAM ZONE MELTINGReceived – Prispjelo: 2013-06-21Accepted – Prihvaćeno: 2013-10-30Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(2) 209-211 (2014)UDC – UDK 669.95:548.526:532.73=111K. Skot nicová, J. Drápala, M. Madaj, VŠB – Technical Un iversity of Ostrava, Faculty o f Me tallurgy a nd M aterial E ngineering, O strava – Poruba, Czech RepublicV. M. Kirillova, Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russian FederationThe distribution of tantalum in molybdenum and tungsten single crystals during electron beam fl oating zone melt-ing under diff erent crystallization conditions was investigated. The performed line chemical analysis of specimens proved creation of tantalum micro segregation, so-called growth striations, which showed themselves on concen-tration proĀ les as periodically alternating areas with increased and decreased concentrations of tantalum. The exist-ence of these chemical inhomogeneities suggests that stationar y conditions of the cr ystal growth were disturbed during the zone melting, i.e. fl uctuations of microscopic rate of growth occurred due to convection in the melt. The determined eff ective distribution c oeffi  cients of tantalum in molybdenum and tungst en approach to calculated theoretical values of the equilibrium distribution coeffi  cient.Key words: zone melting, tantalum, molybdenum, tungsten, inhomogeneities.INTRODUCTIONThe high purity , chemical homogeneity and struc-tural perfection have an essential infl  uence on proper -ties of single crystals of refractory metals and their low-alloyed alloys. Thanks to a complex of physical, me-chanical and chemical properties, they represent pro-spective materials for contemporary, modern technolo-gy, especially as high-temperature structural compo-nents [1, 2]. Basic attributes of single crystals, compared to adequate polycrystalline materials, include much more stable structure, lower creep speeds, high strength, good plasticity, the high Young’s modulus, wear resist-ance and high radiation resistance. A negative property of pure single crystals of refractory metals, especially tungsten, is their low plasticity , which limits their processing technology. The plasticity can be improved, to a certain extent, by alloying these metals with other refractory metals, such as Re, Ta, Mo, or Nb, in the amount up to 5 wt.%. However , when preparing single crystals of tungsten and molybdenum-based alloys by the electron beam zone melting technology [1-7], het-erogeneous distribution of alloying elements often oc-curs, which results in structural heterogeneity and the structure sensitive properties [1, 7]. This paper deals with the effect of crystallization conditions on the tanta-lum distribution in molybdenum and tungsten single crystals during their preparation by electron beam fl oat-ing zone melting. Ef fective distribution coeffi cients of tantalum in molybdenum and tungsten for individual conditions of crystallization and structural characteris-tics prepared single crystalline alloys are also studied.EXPERIMENTALSingle crystals of tungsten and molybdenum-based  alloys with the nominal chemical composition of Mo-1,5 at.% Ta and W-1,5 at.% Ta were grown from the melt us-ing the electron beam fl oating zone melting in a vacuum applying two different zone pass rates - 1 and 3 mm/min. in RMSTC of the VŠB-Technical University of Ostrava. A polycrystalline rod of molybdenum, of 3N purity and 6 mm in diameter , was used as the starting material, in  which a longitudinal slot 1 mm wide, 0,7 mm deep and  approx. 20 cm long was milled. Little pieces of tantalum sheet enwound with the basic metal wire were inserted  into the slot. The rod prepared in this way was placed in  the electron beam zone furnace where the alloying itself  was performed applying two passes of the molten zone  along the whole length of the rod at the speed of 4 mm/min. and then the rod was gradually melted in the given  sections at speeds of 1 and 3 mm/min. At the end of each section a molten zone was solidifi ed by a high speed with the objective to prevent dif fusion processes and to pre-serve the admixture concentration in the zone corre-sponding to the liquid phase. The concentration profi les of tantalum in molybdenum and tungsten on longitudinal sections were obtained using the scanning electron mi-croscope JCXA  733 equipped with EDAX EDAM 3  probe. The X-ray refl ection-mode topography (a modi-fi ed Berg-Barret technique) and Ka and Kb radiation pro-duced by the sharp-focus X-ray tube (the focus diameter  210  METALURGIJA 53 (2014) 2, 209-211K. SKOTNICOVÁ et al.: THE EFFECT OF CRYSTALLIZATION CONDITIONS ON TANTALUM DISTRIBUTION...is 40 - 50 µm) was applied for determination of the angu-lar disorientations of sub-grains in given single crystals  [4]. The back-refl ection Laue method (X-ray beam per -pendicular to the face-end section) was used for determi-nation of their crystallographic orientation. The micro-structure study was carried out using the light optical  microscope Olympus GX-51 connected with digital cam-era DP12 and the high resolution fi  eld emission gun-scanning electron microscope QUANTA 450 FEG.RESULTS AND DISCUSSIONMetallographic analysisSingle crystals of refractory metals with bcc-lattice prepared using the EBFZM are characterized by a spe-cifi c dislocation substructure with the size of sub-grains, which can be divided into three orders of magnitude [1]. The sub-grains of the 1st order are formed due to the growth from the seed crystal, the sub-grains of the 2nd and 3rd orders result from a polygonisation. The micro-structure of W-Ta single crystals prepared by the zone pass rate of 3 mm/min is shown in Figure 1 as an exam-ple, where the existence of sub-grains of the shape of polygons in cross section and long columnar sub-grains elongated in the direction of growth formed at crystal-lization is visible. The crystallographic orientation of all single crystals  was 16 - 20° from [100]. According to these investiga-tions the alloying of molybdenum and tungsten by tanta-lum and increasing zone pass rate resulted in increasing  of the etched pits density. It may be identifi ed as the dis-location density at the grain body because it is necessary to count some more dislocations at the 1st sub-grain  boundaries, but it is impossible. The dislocation density  of these single crystals was about 10 5 to 106 cm -2. The results of the study of crystal structure by X-ray topogra-phy are summarized in Table 1. An evident tendency ex-ists of increased angular disorientation of sub-grains with the increasing zone pass rate and the fragmentation of a  substructure, where Mo-Ta single crystals are character -ized by a more perfect structure, in comparison with  W-Ta single crystals. In the area of a solidifi  ed zone, a  polycrystalline structure with traces of dendritic forma-tions was observed, as well as creation of a cellular -fi -brous structure in the places where an increased concen-tration of tantalum in W-Ta alloys at cell boundaries or  increased concentration of tantalum in the case of Mo-Ta alloys in central parts of cells is assumed.Table 1  Structure parameters of single crystals of tungsten- and molybdenum-based alloysSampleZone pass rateDisorientation of sub-grains1st order 2nd ordermm/min (´) (´)W-Ta 1 15-20 ~ 5W-Ta 3 20-25 ~ 5Mo-Ta 1 10-15 ~ 5Mo-Ta 3 15-20 ~ 5Chemical analysisAll the determined concentration profi les of tantalum in molybdenum and tungsten for the given zone pass  rates showed an irregular sinusoidal course – see Figure  2. This concentration scatter indicates that stationary  conditions of the crystal growth were disturbed during  the zone melting, i.e. the microscopic rate of growth fl uc-tuated due to convection in the melt, which is manifested by the growth striations formation and/or growth nuclei  – periodically alternating regions with a decreased and  increased concentration of an alloying element. The main reason for the formation of growth stria-tions is considered to be the natural buoyancy convec-tion inducing temperature fl  uctuations in the melt. In addition, the Marangoni convection, which is bound to the existence of the melt free surface, has essential in-fl uence on the occurrence of inhomogeneities in the melt. If solidifi cation proceeds very slowly, the concen-tration of an admixture in the melt will be even and the concentration of an admixture in the solidifying crystal is the equilibrium distribution coeffi cient ko [6] multiple of the concentration in the melt (cS = ko cL). If the rate of solidifi cation increases, the moving front of solidifi  ca-tion will displace the admixture faster than it can difuse Figure 1  Microstructure of W-Ta single crystals prepared by the zone pass rate of 3 mm/min: a) sub-grains in central part of crystal; b) SEM-BSE image of rapid solidiĀ ed zone211METALURGIJA 53 (2014) 2, 209-211K. SKOTNICOVÁ et al.: THE EFFECT OF CRYSTALLIZATION CONDITIONS ON TANTALUM DISTRIBUTION...the increasing zone pass rate, which corresponds with the Burton, Prim and Slichter theory [6].CONCLUSIONSIt was found that the angle of disorientation of grain  boundaries in molybdenum and tungsten-based single  crystals increases with the increasing zone pass rate, and  the simultaneous increased density of dislocations. Irregu-lar concentration changes in the alloying elements con-tents, along the axis of the crystal under the given condi-tions of the crystallization, were identifi  ed in individual  samples using the SEM-EDX micro-probe. These micro-segregations of alloying elements in the form of periodi-cally alternating growth bands, with the reduced and in-creased concentration of these elements, are connected  with the existence of the natural buoyancy convection in  the melt due to temperature fl uctuations [1, 6]. From these experimentally measured concentration profi les of tanta-lum along the rod melted gradually at the speeds of 1 and 3 mm/min, the effective distribution coeffi cients of tanta-lum in molybdenum and tungsten were determined using the method of a rapidly solidifi ed zone.AcknowledgementsThis paper was created within the framework of the project No. CZ.1.05/2.1.00/01.0040 “Regional Material Science and Technology Centre” within the operation programme “Research and Development for Innova-tions” funded from the Structural Funds and from the state budget of the Czech Republic.REFERENCES[1]  N. P. Lyakishev, G.S. Burkhanov, Metallic single crystals, Eliz, Moscow, 2002, 311 p.[2]  E. Ragan, E. J. Dobránsky, P. Baron, T. Olejár, Metalurgija, 51 (2012) 1, 117-120.[3] S. I. Bozhko, V. G. Glebovsky, V. N. Semenov, Journal of Crystal Growth, 311 (2008) 1, 1–6.[4] V. G. Glebovsky, V.N. Semenov, International Journal of Re-fractory Metals and Hard Materials, 12 (1993) 5, 295-301.[5]  J. Liu, R. H. Zee, Journal of Crystal Growth, 163 (1996) 3, 259-65.[6]  J. Drápala, K. Skotnicová, International Journal of Mate-rials Research, 104 (2013) 1, 60-64.[7]  V. G. Glebovsky , Recrystallization in Crystal Growth: Substructure and Recrystallization, K. Sztwiertnia (ed.), InTech, Rijeka, 2012, 59-86.Note:  The responsible translator for English language is Ing. B. Škandera, Ostrava, Czech Republicinto the main melt volume and an enriched layer creates on the interface. The accumulation of admixtures in front of the crystallization front and enhancement of concentration super-cooling may invoke a considerable rise of the rate of the crystallization front shift. After occupying a certain portion of admixtures on the inter -face, their concentration in the melt decreases or returns to the initial value and the process repeats [1].Furthermore, it is obvious from the results that the solubility of tantalum in the solid and liquid phases is approximately the same, which means that the end part of the ingot was not markedly enriched (W -Ta system) or impoverished (Mo-T a system) with tantalum and from this point of view tantalum shows itself to be a convenient alloying element in molybdenum and tung-sten. The effective distribution coeffi cients keff of tanta-lum in molybdenum and tungsten, which describes the infl uence of transport processes taking place in the re-gions close to the phase interface at the speed of crystal-lization v ≠ 0, were calculated using the method of a rapidly solidifi ed zone. The principle of this method consists in the comparison of the mean concentration of admixture element xLB in a quenched zone (no diffusion processes could proceed) to the mean concentration xSB in the place just in front of the solidifi ed zone when the stationary state of crystallization was achieved. The mean concentrations of tantalum used in calculations and obtained values of keff for given speeds of crystalli-zation are presented in Table 2. It is evident from the results that values kef f of tan-talum in molybdenum and tungsten converges to 1 with Figure 2  Concentration proĀ le of tantalum in molybdenum (a) and tungsten (b) single crystal prepared by the zone pass rate of 3 mm/min.Table 2  Mean concentrations of tantalum in Mo and W and obtained values of keff  for given zone pass ratesSampleZone pass rate xSB xLB keff komm/min at.% at.%Mo-Ta 1 1,55 1,35 1,141,32Mo-Ta 3 1,12 1,02 1,10W-Ta 1 1,53 1,81 0,840,79W-Ta 3 1,38 1,58 0,87

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The effect of crystallization conditions on tantalum distribution in molybdenum and tungsten during electron beam zone melting

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