Mechanical properties and structure of brazed joints of casting nickel alloy JS26VI: Part 1.
Functional properties of components from nickel HTA are improved by application in the process of casting with directional solidification. Oriented solidification of the cast metal prevents occurrence of grain boundaries perpendicular to direction of external load action.
Mechanical properties of alloy JS26 with polycrystalline structure were investigated in work . In [4, 5] dependence of tensile strength and relative elongation of cast alloy upon crystallographic orientation of macrograins in specimens of casts is shown. Maximum ultimate strength value of alloy JS26NK, equal to 1200-1253 MPa at 20 [degrees]C, corresponded to direction <111>, whereby relative elongation of the metal constituted about 10 %. Specimens of the alloys, growth axle projections of which corresponded to central part of a stereographic triangle, had minimum strength values (780-840 MPa) at relative elongation 14.5-20.0 %. Disturbance of the specimens relative each other in this case did not exceed 12 degrees. So, mechanical properties of alloy JS26, microstructure of which corresponds to central area of a stereographic triangle, are close to those of a polycrystalline alloy with equiaxial grains. Presented data served as a basis for comparing with mechanical properties of the alloy JS26VI brazed joints (BJ), which had relative elongation at room temperature 7-13 %.
Main requirement to nickel HTA is thermal stability of their structure at temperature of the item operation, determined by high-temperature strength of solid matrix solution, low rate of coagulation and dissolution of main strengthening [gamma]'-phase, and kinetics of carbide reactions. Heat stability is connected with alloying complex of the alloy, i.e. composition and quantity of the components, especially of those having low coefficients of diffusion. Niobium and vanadium like hafnium and tantalum, being distributed among the matrix, [gamma]'-phase and carbide phases, limit diffusion processes in the base high-temperature system Ni-Cr-Co-W-Mo-Ti-Al and increase energy of atomic bonds. Vanadium, which enters into composition of JS26, is the weakest y'-forming element. Its role mainly consists in increasing solubility of refractory components in the matrix solution, which enables retarding of diffusion processes in nickel alloys [6-7].
Resistance to coagulation and dissolution of [gamma]'-phase in brazing of HTA are determined by alloying complex of the seam metal, in connection with which chemical composition of the low-temperature brazing alloy and the filler are factors, which determine level of the compound heat stability. Reserve for increasing functional reliability of BJ is optimization of chemical composition of the used filler, presence in its composition of elements with high melting point and low coefficients of diffusion, and final heat treatment of the item being renovated.
Level of high-temperature strength, technological ductility and fatigue resistance of cast nickel alloys are determined by amount and shape of carbide phases in the matrix solution. Carbide particles, precipitating along crystallite boundaries, prevent intergrain slippage at high values of temperature and stress and increase creep resistance and long-term strength of the metal. At the same time, they decrease ductility and durability of metal.
At relatively slow cooling of casts or components, being renovated by brazing, carbides Me23C6 of complex shape (mainly on the basis of chromium) form in the cast metal, which are the source of origination of cracks and stipulate reduction of ductility and fracture toughness at low temperature. In the process of solidification concentration of internal stresses occurs due to difference of thermal coefficients of linear expansion of the matrix and carbide phases, which also reduces resistance to fatigue. At low content of carbon in cast metal (< 0.02 %) carbides acquire orbicular shape instead of that of hieroglyphs, which enables growth of HTA ductility .
Isothermal brazing at high (> 1180 [degrees]C) temperature in vacuum is an effective method for repairing components of hot duct of state-of-the-art turbine engines and installations. Selection of rational systems of brazing alloys, where silicon and boron in combination with powder fillers from multicomponent nickel HTA (of Rene-142, JS6U and JS32 type) are present as depressants, allow in combination with a finishing vacuum heat treatment renovating single and complex components of turbine nozzle guide vanes and nozzle doors, which underwent in the process of their operation heat-fatigue fracture, etc.
Materials and methodology of the experiment. For producing of the alloy JS26VI BJ by the method of resistance isothermal brazing in vacuum, perfabricated cast plates of 6 mm thickness, having size 45 x 100 mm, were used. The plates were produced with equiaxial solidification. After the plates were split into thinner billets of 2.6-2.8 mm thickness, they were ground and subjected to vacuum annealing at temperature 1220 [degrees]C for 1 h. Pressure of residual gases in the chamber constituted not more than 5.6-[10.sup.-3] Pa. For producing BZ by the method of resistance brazing 12 x 21 mm plates were used. Filling of gaps, having length about 10 mm and width 550-600 [micro]m, which were made in the 13 x 60 mm billets by means of spark cutting, was also used. Incised billets were blown by powder SiC and repeatedly annealed.
The plates had arbitrary structure of growth. Prevalent orientation of dendrites in the solidified metal is stipulated by accompanying heat dissipation during pouring into the mould. In formation of BJ factor of dendrite growth orientation in the billet was not taken into account, and butting of the plates, which constituted future joint, was arbitrary. So, BJ was produced from the metal of equiaxial solidification.
Composition from a low-melting component on the basis of system Ni-Co-Cr-Al-2.5 % B (#1) and powders of cast HTA Rene-142 and JS6U was used as a basic brazing alloy. Traditional application of the Rene-142 alloy powder as a filler is stipulated by the fact that tungsten, tantalum and rhenium enabled as the main alloying additives reduction of diffusion mobility of components in the melt and increase of the interatomic bond energy in the seam metal. Application of the JS6U alloy powder as a filler of the brazing alloy allowed bringing nearer chemical composition of the brazing mixture to that of the basis.
Experience of work with BJ, produced with application of boron-containing brazing alloys, containing powder Ni-12 % Si as a depressant, and with repair of the aviation turbine engine (AGTE) doors from casting alloy VJL12U showed doubtless advantage of the former brazing alloys . Being in solid solution of the seam metal, silicon exerts influence on the shape of carbide phases being precipitated and suppresses formation of primary carbides of <<Chinese font>> type, reduces dissolution temperature of carbide phases in the matrix, enables precipitation of finer carbide fractions, and disperses carbide phase within the volume of a polycrystal and over boundaries of solidified grains of the seam metal.
For isothermal brazing of alloy JS26 several compositions of brazes, containing additionally commercial brazing alloy NS12 as an additive, and compositions without this alloy were considered. The main filler in brazing mixtures was powder of alloy Rene-142 (Figure 1): 40 % #1 + 60 % Rene-142; 20 % #1 + 20 % NS12 + 60 % Rene-142; 25 % #1 + 15 % HS12 + 60 % Rene-142; 40 % #1 + 30 % Rene-142 + 30 % JS6U.
The brazing alloy was applied on contacting surfaces of the billets, and they were compressed at the force 30 N or the slot was coated, pressing brazing mixture inside. Technological process of the brazing was invariable, and stepwise condition of heating was used .
Maximum temperature of brazing in different experiments was 1210-1230 [degrees]C. Depending upon temperature, duration of isothermal seasoning at [T.sub.max] was 30-10 min; as temperature increased, time of brazing reduced. Brazed specimens were subjected to the final heat treatment. Optimization of vacuum annealing conditions was the most crucial task.
In addition to estimating mechanical properties of BJ, short-term and long-term strength of the base Metal--alloy JS26VI--was investigated in the state of supply and after two options of heat treatment of BJ specimens. Both options included homogenization annealing of BJ at 1160 [degrees]C for 2 h, which allowed redistributing alloying elements in the solution and enabled dissolution of course primary carbide phases and equalization of sizes and shape of the strengthening [gamma]'-phase; and ageing at temperature 1050 (2 h) and 900 [degrees]C (3 h), causing additional precipitation of sub-dispersed [gamma]'-phase from solid phase, volume share of which determines strength and ductility of HTA.
Experimental results. Results of mechanical tests of the alloy JS26 showed that heat treatment of the basic alloy according to the conditions, close to those of heat treatment of the BJ metal (annealing at 1220 [degrees]C, 1 h + 1160 [degrees]C, 2 h + 900 [degrees]C, 3 h) increased a little tensile and yield strength of the alloy, ensuring mechanical properties, characteristic of JS26 of equiaxial solidification (Table 1) .
[FIGURE 1 OMITTED]
Influence of different thermophysical conditions of brazing [T.sub.br] = 1220 [degrees]C, 15 min and 1230 [degrees]C, 10 min can be seen in Figure 2, where results of statistical processing of BJ mechanical tensile test data, obtained with application of complex brazing alloy with 20 % NS12, are compared. Increase of brazing temperature by 10 [degrees]C caused reduction of ductility and strength values of the seam metal and fusion line of BJ of alloy JS26.
More than 70 % of specimens, produced at [T.sub.br] = = 1230 [degrees]C, had strength 450-550 MPa and zero ductility. At the same time, 85 % of specimens produced at [T.sub.br] = 1220 [degrees]C, 15 min, combined satisfactory strength (650-800 MPa) with relative elongation (3-13 %). Q-factor of brazed joints equaled Q = 83-110 %.
Increase of the brazing temperature by 10 [degrees]C unambiguously caused insignificant increase of yield strength (more alloyed solid solution) and reduction of ductility and ultimate strength of the seam metal. Resistance brazing of plates at 1220 [degrees]C within 15 min brought much better result in comparison with seasoning for 10 min at 1230 [degrees]C (Figure 2).
Increase of the brazing process temperature did not guarantee improvement of BJ functional characteristics. In case of brazing at 1220 [degrees]C metal of the seam preserved density of its structure. At a higher brazing temperature inclination increased to sweating of eutectic component of the solidified brazing alloy from the seam metal in high-temperature homogenization annealing (1160 [degrees]C, 2 h), which caused loss of the seam strength.
For BJ formed at 1230 [degrees]C, 10 min, two modes of final annealing were used: two- (1160 [degrees]C, 2 h + 1050 [degrees]C, 2 h) and single-stage (1080 [degrees]C, 2 h) ones. Single-stage annealing brought worse results concerning ductility of BJ for both options of brazing alloys. Fracture occurred below yield point. At the same time, a portion of BJ, annealed according to two-stage scheme, demonstrated satisfactory ductility--relative elongation constituted 2.3-8.0 %. So, brazing temperature 1230 [degrees]C for the brazing alloy with 20 wt.% NS12 is a threshold one. Selective sweating of low-melting brazing alloy fraction from the seam metal causes occurrence of porosity in it and, accordingly, reduction of strength and ductility of the BJ metal.
Figure 2. Statistical curves of distribution of strength values of JS26VI alloy BJ produced with application of brazing alloy #1 + 60 % Rene-142 with addition of 20 % NS12 at [T.sub.br] = 1220 [degrees]C, 15 min (1) and 1230 [degrees]C, 10 min (2): N--number of specimens
[FIGURE 2 OMITTED]
Strength of BJ, produced with application of different brazing alloy systems, was 685-771 MPa ([??]t = = 721 MPa). Stability is achieved of high strength values of BJ, produced with application of a complex brazing alloy with 20 % NS12, in comparison with the joints, produced with application of the base brazing alloy 40 % #1 + 60 % Rene-142 (see Figure 2). The base brazing alloy does not ensure reserve of ductility for the joints, subjected to brittle tension fracture. Such effect is connected with migration of boron to the fusion line at brazing temperature. Accumulation of boron near the fusion line is the main reason of this negative phenomenon.
High density of short-term strength values of metal of BJ, produced with application in the brazing composition of powder NS12, attracted our attention. Main result consists in the same level of strength of the joints, determined by structural state of the metal being brazed, i.e. by conditions of final heat treatment of BJ. In BJ, produced with application of complex brazing alloy with 20 % NS12, fracture occurred, as a rule, over the base metal or over the fusion line, whereby relative elongation of the joint specimens equaled 7-18 %.
According to the adopted technology, final annealing after isothermal brazing and quick cooling consisted of two stages: 1160 [degrees]C, 2 h + 1050 [degrees]C, 2 h. Mechanical properties of the base metal after mentioned heat treatment are given in Table 1 and correspond to the results of tensile tests of BJ. Coincidence of yield strength values of BJ and base metals is close to the ideal one.
Strengthening of the matrix is ensured due to precipitation in solid solution of intermetallic phases of a complex chemical composition. As far as ultimate strength is concerned, here enters into competition structure of grain boundaries of the metal being brazed and morphology of the phases being precipitated in the thermal diffusion interaction (chemical erosion) zone.
Influence of the final ageing conditions on physical-chemical properties of the BJ metal can be seen in Figure 3. The main result consists in the fact that all BJ, annealed at 900 [degrees]C, 3 h, had higher level of strength and somewhat lower ductility in comparison with the specimens, annealed at 1050 [degrees]C, 2 h.
Another result consists in the fact that BJ, produced with application of a complex brazing alloy containing 20 % NS12, are characterized by high technological ductility at room temperature and more intensive work needed for fracture of the specimens in tensile tests.
[FIGURE 3 OMITTED]
BJ of alloys VJL12U and JS6U, produced by the same brazing alloys under similar thermophysical conditions, had [[sigma].sub.t] by 100 MPa higher than that of the joints of alloy JS26VI (Table 1) . Seam metal chemistry of joints of these alloys is approximately the same (due to identity of the brazing mixtures). So, strength of the BJ seam metal of alloy JS26 should be higher than strength of the metal to be brazed, which is registered in a real experiment in tensile tests.
The alloy JS26 BJ fracture sites became defects of base metal structures near the fusion line. Occurring in the base metal cracks near carbide particles or in very carbides propagate in the seam metal over grain boundaries, which have excessive precipitations of carbide phases.
As it follows from Figure 3, final ageing performed at 1050 [degrees]C, 2 h and 900 [degrees]C, 3 h, guaranteed, approximately, the same level of yield and tensile strength of metal of BJ, for production of which a complex brazing alloy with silicon was used. At the same time, ageing at 900 [degrees]C, 3 h embrittled BJ metal on the basis of the base brazing alloy. Annealing at 1050 [degrees]C, 2 h allowed producing BJ, fracture of which occurred over the base metal (elongation 6.5-8.5 %) at average strength 756 MPa.
Statistical processing data of test results of the array of JS26VI alloy specimens, produced by composite brazing alloys without silicon and containing as a low-melting component commercial brazing alloy NS12, are given in Figure 4. Presented dependences generalize results of mechanical tensile tests of BJ metal specimens of equiaxial solidification after different options of final heat treatment.
Higher ductility at 20 [degrees]C had BJ, for formation of which a complex brazing alloy was used; 50 % of specimens had yield strength 600-650 MPa and ultimate strength 650-700 MPa.
Sharp peak of the yield strength curve (Figure 4) confirms good quality and stability of technological process of the brazing. In this case (more than 50 % of specimens) BJ are able to deform plastically at 20 [degrees]C up to final fracture.
The diagram clearly registers ability of BJ to withstand a certain plastic strain, because on presented curves significant difference (up to 100 MPa) between values [[sigma].sub.t] and [[sigma].sub.0.2] of tested BJ is presented. Blurred maximum of curve [[sigma].sub.t] in comparison with curve [[sigma].sub.0.2] indicates presence of technological deviations in the process of brazing. Probably, there was difference in granulometric composition of the brazing alloy or in the conditions of the brazing mixtures producing in brazing of different specimens. This may cause either reduction of BJ strength, or ensure for them maximum possible value of Q-factor.
Microstructure peculiarities of fracture. Interconnection between mechanical properties of BJ and microstructure of used brazing alloys was investigated by fractograms of tested specimens, which reflected most accurately character of BJ fracture in loading.
[FIGURE 4 OMITTED]
Example of brittle fracture of a BJ specimen of JS26 alloy at [[sigma].sub.t] = 460 MPa and [epsilon] = 0 %, produced by brazing with boron-containing brazing alloy # 1 with a filler from 30 wt.% Rene-142 + 30 wt.% JS6U at 1220 [degrees]C, 15 min, is given in Figure 5, a, b. In BJ fracture porosity is discovered, which weakens section of the specimen. This became the reason of low BJ tensile strength. Fracture occurred as a result of confluence of discontinuity flaws in plastic zone before apex of the crack. Fracture mechanism of the joints is a normal tear .
Fracture pattern of the BJ specimen with relative elongation [epsilon] = 7.5 % is illustrated in Figure 5, c. Brazing alloy with 20 % NS12 was used in resistance brazing of this specimen. Failure of the base metal was combined with that of the seam metal in the fracture. Crack in the BJ specimen occurred on the surface (near the defect) of the base alloy, when such brazing alloy was used, and caused final fracture of the joint in the place of the base metal transition into the seam.
Similar picture of BJ fracture was obtained in another experiment (brazing with complex brazing alloy at 1220 [degrees]C, 20 min; annealing at 1160 [degrees]C, 2 h + ageing at 900 [degrees]C, 3 h). Elongation in tensioning of the specimen constituted 10.8 % (Figure 5, d). BJ fracture was initiated in the base metal near the fusion line and transferred into the seam metal at final stage of fracturing.
Mixed character of fracture of BJ, produced by a complex brazing alloy, in tension indicates that main crack passed not only over the seam metal, but also touched volumes of the metal being brazed (Figure 5, g, h). In the fracture islands of failure, adjacent to the base metal (Figure 5, e) or the fusion line (Figure 5, f), are present. The same pattern of failure was detected in the BJ specimen with 13 % elongation. In the fracture of this specimen ([[sigma].sub.t] = 762.6 MPa) metal of the brazed seam (20 % NS12 + 20 % #1 + 60 % Rene-142) underwent tough fracture at the microlevel.
Fractures of the BJ specimens demonstrate different amount of carbide phase, detected over grain boundaries in brazed seams without and with addition of 20 % of silicon-containing brazing alloy. Higher amount of carbides in the seam matrix is observed in case of a traditional brazing alloy without silicon. Carbide particles, located over grain boundaries, enable transfer of plastic deformation from a grain to a grain in loading, contributing to uniform plastic flow of the polycrystalline aggregate.
[FIGURE 5 OMITTED]
Fractograms of the BJ fracture surface confirm conclusion that the higher is elongation of a BJ, the higher is its strength. Toughness of a joint is determined by ability of the material for plastic deformation.
Long-term strength. According to GOST 10145081 long-term (50 h) strength of JS26VI alloy (of equiaxial solidification) should constitute at the test temperature 900 [degrees]C not less than 422 MPa (on average 450 MPa), whereby 100-hour long-term strength constitutes not less than 373 MPa (on average 402 MPa).
In testing of specimens of the base metal JS26VI at 900 [degrees]C 50-hour long-term strength should be achieved at the level of applied stresses 420-425 MPa .
To BJ specimens twice lower stress (196 MPa) was applied than to specimens from the base metal, and main influence on difference in properties exerted chemical composition of the brazing alloy (Table 2).
In the course of long-tern strength tests in this work BJ were used with a fixed gap of 600-800 mm width. In this way long-term strength of solidified metal of the BJ developed seam was determined.
Long-term strength tests were performed at temperature 900 and 950 [degrees]C. Results of BJ tests, obtained in two experiments, are given in Table 2.
Working life of BJ, produced by a brazing alloy without addition of NS12, is noticeably higher than that of BJ, produced with application of a complex brazing alloy. Long-term strength of BJ, produced with addition of NS12, was characterized by high stability of results.
Two-stage heat treatment of all specimens included homogenization annealing at 1160 [degrees]C, 2 h, and at the final stage--ageing.
Conditions of ageing did not exert significant influence on long-term strength of BJ. Satisfactory working life was achieved in the specimens of BJ, which were subjected to final ageing at 900 [degrees]C for 3 h. Time till fracture of BJ of JS26VI alloy constituted 1.0-1.5 h at 900 [degrees]C and stress 196 MPa (20 kg/[mm.sup.2]) for a complex brazing alloy with 20 % NS12. Working life of the only specimen constituted 4 h 20 min in case of the gap brazing by the base brazing alloy 40 % #1 + 60 % Rene-142.
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[7.] Baburina, E.V., Dolzhansky, Yu.M., Lomberg, B.S. et al. (1987) Structure stability of heat-resistant nickel alloys and its increase by optimal alloying. Ibid., 5, 62-63.
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[9.] Malashenko, I.S., Kurenkova, V.V., Belyavin, A.F. et al. (2006) Short-term strength and microstructure of brazed joints of alloy VJL12U produced using boron-containing brazing alloy with addition of silicon. Advances in Electrometallurgy, 4, 23-38.
[10.] Malashenko, I.S., Kurenkova, V.V., Belyavin, A.F. et al. (2006) Strength and physical metallurgy of brazed joints of cast nickel alloy ChS70VI. Ibid., 1, 19-20.
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I.S. MALASHENKO (1), V.V. KURENKOVA (1), E.V. ONOPRIENKO (2), V.V. TROKHIMCHENKO (2), A.F. BELYAVIN (1) and L.V. CHERVYAKOVA (2)
(1) RC <<Pratt and Whitney>>, Kiev, Ukraine
(2) E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
Table 1. Results of mechanical tensile tests of specimens of alloy JS26 after different conditions of heat treatment Specimen Conditions of heat treatment No. G1 Initial G2 G3 1210 [degrees]N, 1 h + 1160 [degrees]N, 2 h + 1050 [degrees]N, 2 h G4 G5 1210 [degrees]N, 1 h + 1160 [degrees]N, 2 h + 900 [degrees]N, 3 h G6 Specimen Cross-section [[sigma].sub. [[sigma].sub.t], [delta], of specimen, 0.2], MPa MPa % [mm.sup.2] G1 5.33 644.0 773.0 16.2 G2 5.21 659.0 738.0 8.5 G3 5.17 578.7 719.0 13.2 G4 5.19 567.0 705.0 8.5 G5 5.27 670.0 774.4 13.2 G6 5.17 683.0 872.8 8.5 Table 2. BJ working life of JS26VI alloy at 900 [degrees]C produced by brazing of gaps up to 600 mm width at temperature 1225 [degrees]C for 20 min after different conditions of ageing Specimen No. Type of brazing alloy 0G1 Base metal 0G0 0G2 ZG4 20 % #1 + 20 % NS12 + 60 % Rene-142 ZG1 ZG2 ZG3 ZG6 ZG9 40 % #1 + 60 % Rene-142 ZG5 ZG7 Specimen No. Final heat treatment of BJ before test 0G1 1220 [degrees]C, 1 h + 1050 [degrees]C, 4 h 0G0 0G2 ZG4 1160 [degrees]C, 2 h + 1050 [degrees]C, 2 h ZG1 1160 [degrees]C, 2 h + 900 [degrees]C, 4 h ZG2 ZG3 ZG6 ZG9 1160 [degrees]C, 2 h + 1050 [degrees]C, 2 h ZG5 ZG7 1160 [degrees]C, 2 h + 900 [degrees]C, 4 h Specimen No. [[sigma].sub.t] x [tau], min [epsilon], % [9.08.sup.-1], MPa 0G1 45 70 0.75 0G0 40 115 1.28 0G2 35 300 3.3 ZG4 20 105 1.4 ZG1 20 60 0.7 ZG2 20 90 0.4 ZG3 20 90 2.1 ZG6 20 70 2.6 ZG9 20 70 0.37 ZG5 20 260 1.5 ZG7 20 75 0
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|Author:||Malashenko, I.S.; Kurenkova, V.V.; Onoprienko, E.V.; Trokhimchenko, V.V.; Belyavin, A.F.; Chervyakov|
|Publication:||Advances in Electrometallurgy|
|Date:||Jan 1, 2007|
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