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Growth of chromium-vanadium single crystals by floating-zone technique
24!
Journal of Crystal Growth 57 (1982) 241—244 North-Holland Publishing Company
GROWTH OF CHROMIUM-VANADIUM SINGLE CRYSTALS BY FLOATING-ZONE TECHNIQUE P.C. CAMARGO
~,
J.A. EHLERT and F.R. BROTZEN
Materials Science Laboratory, Rice University, Hotsston, Texas 77001, USA
Received 17 March 1981; manuscript received in final form 7 December 1981
Single crystals of chromium-rich vanadium alloys were prepared by the floating-zone method using a combination of resistance and RF-induction heating. Single crystals having a diameter of 1 cm and a length of up to 5 cm were successfully grown.
1. Introduction The high vapor pressure of chromium and vanadium makes it difficult to grow single crystals of these metals and their alloys by conventional means. This difficulty is aggravated by the need for relatively large crystals which are required for such tasks as the measurement of elastic constants [1]. We have succeeded in developing a method for growing chromium-rich vanadium crystals of 1 cm diameter by a modified floating-zone technique. This technique combines the elimination of impurities by zone refining with the growth of relatively large and homogeneous crystals. Refining of polycrystalline chromium by a zone -melting technique was studied thoroughly by Bigot and his coworkers [2—41.Single crystals of chromium and of certain chromium alloys have been prepared by arc-zone melting by Carlson et al. [5]. The properties of the high-purity chromium crystal produced by Battelle Memorial Institute in Columbus, Ohio, were examined by Crutchley and Reid [6]. More recently, Igaki et al. [7] successfully zone-refined chromium in a hydrogen atmosphere by using RF-induction heating. Their procedure resulted in specimens composed of several columnar crystals which required mechanical separation. *
Present address: Universidade Federal de São Carlos, 13560 São Carlos, SP, Brazil.
The technique described below produces consistently satisfactory crystals of chromium and chromium alloys. It permits the use of conventional RF zone-refining equipment which has to undergo only relatively simple modifications.
2. Procedure Chromium-rich crystals containing up to 2% vanadium were produced in our laboratory for the purpose of studying certain antiferromagnetic properties. The starting materials were Marz-grade chromium powder (— 18 + 35 mesh), 99.996% pure, acquired from Materials Research Corporation, Orangeburg, New York, and 99.7% pure granulated vanadium obtained from Alpha Division, Ventron Corporation, Danvers, Massachusetts. The powders were melted in a model V4 Materials Research Corporation arc melter. This unit has a water-cooled copper hearth which contains a series of grooves to hold the charge. The hearth is enclosed by a water-cooled stainless-steel jacket which permits melting to take place in an inert atmosphere. A movable tungsten electrode, about 3/16 inch in diameter, provides the DC arc. Prior to the melting operation, the unit was pumped out by a mechanical vacuum pump and flushed several times with argon. The argon atmosphere was static during the melting and had been gettered by melting titanium in one of the grooves
0022-0248/82/0000—0000/$02.75 © 1982 North-Holland
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P. C. Carnargo et al.
/
Growth of chromium—vanadium single crystals
in the copper hearth to remove traces of oxygen, nitrogen and water vapor. Titanium pickup was avoided by starting melting of the chromium alloys only after the titanium had cooled to hearth temperature. In melting the alloys, homogeneity of the final product was one of the primary concerns. To this end, the chromium powder was spread in a groove of semicircular cross section and the vanadium granules were buried in it as evenly as possible. The mixture was melted numerous times, having been turned over after each pass. Thereafter, the ingots were broken into small pieces and remelted to enhance compositional uniformity. The ingots then had a nearly cylindrical shape which facilitated the subsequent zone-melting operation. The arc-melted ingots were approximately 1 cm in diameter and 10 cm long, weighing about 50 g each, The ingots were transferred to the zone melter and carefully aligned between two tungsten electrodes, fig. 1. The system was pumped down by an oil diffusion pump to 2 X 10~Torr and kept at this pressure for about 12 h. Thereafter, helium containing 8.5% hydrogen was admitted to the system until the pressure reached 620 Torr at room temperature. The specimen was then heated by ,,Electrode
passing a current of 600 A through it. After the temperature distribution along the length of the specimen had stabilized, both ends of the specimen were welded to the tungsten electrodes by RF induction heating. Because of the difference in cross section between the tungsten electrodes and the specimen, resistance heating did not produce a uniform temperature throughout the specimen, reaching about 1500°C in the middle section and somewhat higher temperatures at the ends. This temperature distribution, however, proved to be adequate for the subsequent zone melting by induction. The actual crystal-growing and refining process was carried out by using simultaneously resistance heating and an RF-induced molten zone, about 0.6 cm in width. A photograph of the focusing watercooled induction coil is given in fig. 2. Zone melting was started about 2 cm from the top of the specimen and proceeded by moving the RF coil downward at a speed of 5 cm/h. Since the geometry of the starting samples was usually quite irregular, it became necessary to regulate the RF power manually in order to maintain the molten-zone dimensions as constant as possible. At the same time, the lower part of the sample holder had to be moved up or down whenever an unexpected change in cross-section of the molten zone dictated such an adjustment.
“Expansion joint
—W Rod
~OOQ.~ R.F.
,0000,
Coil~ Sample
—~
—W Rod Electrode\
I
‘-Movoble Cu base
Fig. I. Zone-melting device combining RF induction and resistance heating.
Fig. 2. Focusing coil for zone melting of high ‘~.apor-prcssurc metals and alloys.
P.C. Camargo et a!.
/
Growth of chromium-vanadium single crystals
243
3. Discussion and conclusions
One chromium crystal and two chromium-rich alloy crystals containing 0.67% and 1.5% vanadium were produced. No attempt was made here to grow the crystals with a predetermined orientation. The monocrystalline nature of the specimens was checked through Laue back-reflection patterns, which consisted of sharp and well-defined reflection spots, as shown in fig. 3 for the 1.5 vanadium—chromium crystal. Metallography revealed no noticeable precipitates. The composition of the specimens was determined by microprobe analysis with a relative accuracy of ±0.05%. Since the nominal compositions of the crystals had been 0.5% vanadium for the 0.67% sample and 1.5% vanadium for the 1.5% sample, the loss of chro-
The development of the above procedure for the production of chromium and chromium— vanadium alloy crystals was governed by the need to overcome the difficulties inherent in the melting of these materials. The vapor pressure of chromium is very high, viz., I kPa at the melting point (1890°C),precluding the use of vacuum melting. The need for a gaseous atmosphere in the system eliminates the conventional and relatively simple technique of electron-beam melting. The alternative is the use of RF-induction heating to create a moving molten zone in the specimen. In order to maintain a desirably narrow and stable zone, proper design of the focusing coil calls for the inner edge of the concentrator plate to be as close to the specimen as possible, fig. 2. There is, however, a tendency for vaporized metal from the molten zone to condense at the inner edge of the water-cooled concentrator. This results in the gradual growth of “dendritic” metal crystals from the concentrator toward the sample and, ultimately, in a contact between focusing coil and molten zone. When the small dendrites touch the molten zone, the liquid metal is drawn toward the dendrites by surface tension and the crystalgrowing process is obstructed. On the other hand, when the sample is heated by resistance the power level of the RF required to create the molten zone is greatly diminished, so that a concentrator with a relatively large (2.3 cm) inside diameter can be used without the risk of an unduly wide molten zone. “Dendritic” growth still occurs, but the large diameter of the inner edge of the concentrator keeps these dendrites from touching the zone during the time of the melting cycle. Resistance heating also tends to stabilize the molten zone because it makes possible a more uniform temperature distribution across the sample [8]. Above all, the absence of radial temperature gradients across the molten zone prevents the formation of columnar crystals. The need for using a hydrogen-bearing atmosphere arose because of the tendency of chromium to form the very stable oxide Cr203, whose melt-
mium during the melting operation was somewhat larger than that of vanadium,
ing point is 2400°C.The formation of these oxides on the surface of the sample leads to nuclei of
_______________________________________________
Fig. 3. Laue back-reflection pattern of the 1.5% vanadium— chromium alloy crystal.
244
P. C. Camargo et a!.
/
Growth of chromium—vanadium single crystals
solidification and interferes with the production of single crystals. An additional advantage of the presence of hydrogen in the atmosphere is that it tends to avoid arcing [9]. Furthermore, the presence of hydrogen in zone-refining atmospheres was found by Bigot [2—4]to be imperative for the production of high-purity chromium. We found that a gas mixture of 8.5% hydrogen in helium is as effective as pure hydrogen and, for safety reasons, more desirable. Alloy crystals produced by the method described here are homogeneous, and microprobe analysis revealed no radial concentration gradients. In summary, the resistance heating played a fundamental role in the success of growing large single crystals of chromium and chromium alloys because it promoted a planar solid—liquid interface and a low temperature gradient in the solid. These are two highly desirable conditions for the formation of single crystals in the floating-zone process [8].
Acknowledgements The authors wish to express their appreciation to the National Science Foundation for the sup-
port of this research as well as to the Brazilian Comissão Nacional de Pesquisas for the fellowship of one of the authors (P.C.C.). The help of Dr. C.E. Anderson, Rice University, Materials Science Laboratory, is also gratefully acknowledged.
References [I] S.B. Palmer and E.W. Lee, Phil. Mag. 24 (1971) 311. [2] J. Bigot and S. Talbot-Besnard, Compt. Rend. (Paris) 261 (1965) 121. [3] J. Bigot, Mem. Sci. Rev. Met. 65 (1968) 61. [4] J. Bigot, Ann. Chim. 5 (1970) 397. [5] ON. Carlson, F.A. Schmidt and W.M. Paulson, Trans. ASM 57 (1965) 356. [6] D.E. Crutchley and C.N. Reid. in: High-Temperature Materials, 6th Plansee Seminar, Reutte. 1968, Ed. F. Benesovsky, p. 57. [7] K. Igaki, M. Isshiki and K. Yakushiji, Trans. Japan Inst Metals 20 (1979) 641. [8] J.E. Murphy and MM. Wong, J. Less-Common Metals 40 (1975) 65.
[9] R.C.J. Draper, in: Crystal Growth, Ed. BR. Pamplin (Pergamon, New York, 1975) p. 510.
Journal of Crystal Growth 57 (1982) 241—244 North-Holland Publishing Company
GROWTH OF CHROMIUM-VANADIUM SINGLE CRYSTALS BY FLOATING-ZONE TECHNIQUE P.C. CAMARGO
~,
J.A. EHLERT and F.R. BROTZEN
Materials Science Laboratory, Rice University, Hotsston, Texas 77001, USA
Received 17 March 1981; manuscript received in final form 7 December 1981
Single crystals of chromium-rich vanadium alloys were prepared by the floating-zone method using a combination of resistance and RF-induction heating. Single crystals having a diameter of 1 cm and a length of up to 5 cm were successfully grown.
1. Introduction The high vapor pressure of chromium and vanadium makes it difficult to grow single crystals of these metals and their alloys by conventional means. This difficulty is aggravated by the need for relatively large crystals which are required for such tasks as the measurement of elastic constants [1]. We have succeeded in developing a method for growing chromium-rich vanadium crystals of 1 cm diameter by a modified floating-zone technique. This technique combines the elimination of impurities by zone refining with the growth of relatively large and homogeneous crystals. Refining of polycrystalline chromium by a zone -melting technique was studied thoroughly by Bigot and his coworkers [2—41.Single crystals of chromium and of certain chromium alloys have been prepared by arc-zone melting by Carlson et al. [5]. The properties of the high-purity chromium crystal produced by Battelle Memorial Institute in Columbus, Ohio, were examined by Crutchley and Reid [6]. More recently, Igaki et al. [7] successfully zone-refined chromium in a hydrogen atmosphere by using RF-induction heating. Their procedure resulted in specimens composed of several columnar crystals which required mechanical separation. *
Present address: Universidade Federal de São Carlos, 13560 São Carlos, SP, Brazil.
The technique described below produces consistently satisfactory crystals of chromium and chromium alloys. It permits the use of conventional RF zone-refining equipment which has to undergo only relatively simple modifications.
2. Procedure Chromium-rich crystals containing up to 2% vanadium were produced in our laboratory for the purpose of studying certain antiferromagnetic properties. The starting materials were Marz-grade chromium powder (— 18 + 35 mesh), 99.996% pure, acquired from Materials Research Corporation, Orangeburg, New York, and 99.7% pure granulated vanadium obtained from Alpha Division, Ventron Corporation, Danvers, Massachusetts. The powders were melted in a model V4 Materials Research Corporation arc melter. This unit has a water-cooled copper hearth which contains a series of grooves to hold the charge. The hearth is enclosed by a water-cooled stainless-steel jacket which permits melting to take place in an inert atmosphere. A movable tungsten electrode, about 3/16 inch in diameter, provides the DC arc. Prior to the melting operation, the unit was pumped out by a mechanical vacuum pump and flushed several times with argon. The argon atmosphere was static during the melting and had been gettered by melting titanium in one of the grooves
0022-0248/82/0000—0000/$02.75 © 1982 North-Holland
242
P. C. Carnargo et al.
/
Growth of chromium—vanadium single crystals
in the copper hearth to remove traces of oxygen, nitrogen and water vapor. Titanium pickup was avoided by starting melting of the chromium alloys only after the titanium had cooled to hearth temperature. In melting the alloys, homogeneity of the final product was one of the primary concerns. To this end, the chromium powder was spread in a groove of semicircular cross section and the vanadium granules were buried in it as evenly as possible. The mixture was melted numerous times, having been turned over after each pass. Thereafter, the ingots were broken into small pieces and remelted to enhance compositional uniformity. The ingots then had a nearly cylindrical shape which facilitated the subsequent zone-melting operation. The arc-melted ingots were approximately 1 cm in diameter and 10 cm long, weighing about 50 g each, The ingots were transferred to the zone melter and carefully aligned between two tungsten electrodes, fig. 1. The system was pumped down by an oil diffusion pump to 2 X 10~Torr and kept at this pressure for about 12 h. Thereafter, helium containing 8.5% hydrogen was admitted to the system until the pressure reached 620 Torr at room temperature. The specimen was then heated by ,,Electrode
passing a current of 600 A through it. After the temperature distribution along the length of the specimen had stabilized, both ends of the specimen were welded to the tungsten electrodes by RF induction heating. Because of the difference in cross section between the tungsten electrodes and the specimen, resistance heating did not produce a uniform temperature throughout the specimen, reaching about 1500°C in the middle section and somewhat higher temperatures at the ends. This temperature distribution, however, proved to be adequate for the subsequent zone melting by induction. The actual crystal-growing and refining process was carried out by using simultaneously resistance heating and an RF-induced molten zone, about 0.6 cm in width. A photograph of the focusing watercooled induction coil is given in fig. 2. Zone melting was started about 2 cm from the top of the specimen and proceeded by moving the RF coil downward at a speed of 5 cm/h. Since the geometry of the starting samples was usually quite irregular, it became necessary to regulate the RF power manually in order to maintain the molten-zone dimensions as constant as possible. At the same time, the lower part of the sample holder had to be moved up or down whenever an unexpected change in cross-section of the molten zone dictated such an adjustment.
“Expansion joint
—W Rod
~OOQ.~ R.F.
,0000,
Coil~ Sample
—~
—W Rod Electrode\
I
‘-Movoble Cu base
Fig. I. Zone-melting device combining RF induction and resistance heating.
Fig. 2. Focusing coil for zone melting of high ‘~.apor-prcssurc metals and alloys.
P.C. Camargo et a!.
/
Growth of chromium-vanadium single crystals
243
3. Discussion and conclusions
One chromium crystal and two chromium-rich alloy crystals containing 0.67% and 1.5% vanadium were produced. No attempt was made here to grow the crystals with a predetermined orientation. The monocrystalline nature of the specimens was checked through Laue back-reflection patterns, which consisted of sharp and well-defined reflection spots, as shown in fig. 3 for the 1.5 vanadium—chromium crystal. Metallography revealed no noticeable precipitates. The composition of the specimens was determined by microprobe analysis with a relative accuracy of ±0.05%. Since the nominal compositions of the crystals had been 0.5% vanadium for the 0.67% sample and 1.5% vanadium for the 1.5% sample, the loss of chro-
The development of the above procedure for the production of chromium and chromium— vanadium alloy crystals was governed by the need to overcome the difficulties inherent in the melting of these materials. The vapor pressure of chromium is very high, viz., I kPa at the melting point (1890°C),precluding the use of vacuum melting. The need for a gaseous atmosphere in the system eliminates the conventional and relatively simple technique of electron-beam melting. The alternative is the use of RF-induction heating to create a moving molten zone in the specimen. In order to maintain a desirably narrow and stable zone, proper design of the focusing coil calls for the inner edge of the concentrator plate to be as close to the specimen as possible, fig. 2. There is, however, a tendency for vaporized metal from the molten zone to condense at the inner edge of the water-cooled concentrator. This results in the gradual growth of “dendritic” metal crystals from the concentrator toward the sample and, ultimately, in a contact between focusing coil and molten zone. When the small dendrites touch the molten zone, the liquid metal is drawn toward the dendrites by surface tension and the crystalgrowing process is obstructed. On the other hand, when the sample is heated by resistance the power level of the RF required to create the molten zone is greatly diminished, so that a concentrator with a relatively large (2.3 cm) inside diameter can be used without the risk of an unduly wide molten zone. “Dendritic” growth still occurs, but the large diameter of the inner edge of the concentrator keeps these dendrites from touching the zone during the time of the melting cycle. Resistance heating also tends to stabilize the molten zone because it makes possible a more uniform temperature distribution across the sample [8]. Above all, the absence of radial temperature gradients across the molten zone prevents the formation of columnar crystals. The need for using a hydrogen-bearing atmosphere arose because of the tendency of chromium to form the very stable oxide Cr203, whose melt-
mium during the melting operation was somewhat larger than that of vanadium,
ing point is 2400°C.The formation of these oxides on the surface of the sample leads to nuclei of
_______________________________________________
Fig. 3. Laue back-reflection pattern of the 1.5% vanadium— chromium alloy crystal.
244
P. C. Camargo et a!.
/
Growth of chromium—vanadium single crystals
solidification and interferes with the production of single crystals. An additional advantage of the presence of hydrogen in the atmosphere is that it tends to avoid arcing [9]. Furthermore, the presence of hydrogen in zone-refining atmospheres was found by Bigot [2—4]to be imperative for the production of high-purity chromium. We found that a gas mixture of 8.5% hydrogen in helium is as effective as pure hydrogen and, for safety reasons, more desirable. Alloy crystals produced by the method described here are homogeneous, and microprobe analysis revealed no radial concentration gradients. In summary, the resistance heating played a fundamental role in the success of growing large single crystals of chromium and chromium alloys because it promoted a planar solid—liquid interface and a low temperature gradient in the solid. These are two highly desirable conditions for the formation of single crystals in the floating-zone process [8].
Acknowledgements The authors wish to express their appreciation to the National Science Foundation for the sup-
port of this research as well as to the Brazilian Comissão Nacional de Pesquisas for the fellowship of one of the authors (P.C.C.). The help of Dr. C.E. Anderson, Rice University, Materials Science Laboratory, is also gratefully acknowledged.
References [I] S.B. Palmer and E.W. Lee, Phil. Mag. 24 (1971) 311. [2] J. Bigot and S. Talbot-Besnard, Compt. Rend. (Paris) 261 (1965) 121. [3] J. Bigot, Mem. Sci. Rev. Met. 65 (1968) 61. [4] J. Bigot, Ann. Chim. 5 (1970) 397. [5] ON. Carlson, F.A. Schmidt and W.M. Paulson, Trans. ASM 57 (1965) 356. [6] D.E. Crutchley and C.N. Reid. in: High-Temperature Materials, 6th Plansee Seminar, Reutte. 1968, Ed. F. Benesovsky, p. 57. [7] K. Igaki, M. Isshiki and K. Yakushiji, Trans. Japan Inst Metals 20 (1979) 641. [8] J.E. Murphy and MM. Wong, J. Less-Common Metals 40 (1975) 65.
[9] R.C.J. Draper, in: Crystal Growth, Ed. BR. Pamplin (Pergamon, New York, 1975) p. 510.