Fractional gas analysis procedure for estimating the contents of oxide and nitride inclusions in Alnico alloys produced from wastes of production is developed. The effect of the temperature and time of melt holding before pouring on the content of impurity oxide and nitride inclusions is considered. The content of SiO2 inclusions in Alnico mainly depends on the holding time before pouring and decreases with increasing holding time, whereas the content of Al2O3 inclusions decreases with increasing both temperature and time of holding. The content of TiN inclusions decreases with increasing holding time. As the holding temperature increases, the content of nitride inclusions decreases only with increasing holding time.Razrađena je metoda frakcijske plinske analize za vrednovanje oksidnih i dušičnih uključaka u ALNICO legurama, dobijenih iz gospodarskih otpada. Posmatran je utjecaj temperature i vremena zadržavanja kupke prije lijevanja. Sadržaj uključaka SiO2 uglavnom ovisi od vremena zadrške prije lijevanja i snižava se povećanjem tog vremena, istodobno se sadržaj uključaka Al2O3 smanjuje s povećavanjem i temperature i vremena zadrške. Sadržaj uključaka TiN smanjuje se povećavanjem vremena zadrške. S povećanjem temperature sadržaj dušičnih uključaka smanjuje se samo pri povećanju vremena zadrške prije lijevanja

A. Dalmatov

G. Burkhanov

I. Belyaev

K. Grigorovich

N. Kolchugina

S. Shibaev

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K. GRIGOROVICH, G. BURKHANOV, S. SHIBAEV, A. DALMATOV,I. BELYAEV, N. KOLCHUGINAFRACTIONAL GAS ANALYSISPROCEDURE FOR DETERMINING THE CONTENTSOF OXIDE AND NITRIDE PHASES IN ALNICO ALLOYSReceived – Prispjelo: 2008-03-18Accepted – Prihva}eno: 2009-01-21Preliminary note – Prethodno priop}enjeINTRODUCTIONProduction technology of Fe-Al-Ni-Co-Ti alloys (in-tended for Alnico permanent magnets) must providegiven compositions and maximum length of dendrites(columnar crystals) that is determined by the minimumcontent of impurities, in particular, nonmetallic inclu-sions refining the macrostructure of magnets 1, 2. Itshould be noted that the Fe-Al-Ni-Co-Ti alloys solidifywith the formation of single-phase solid solutions; theprocess is nonequilibrium 3. The reaching requiredcomposition of columnar crystals is determined by thedistribution of principal components during the direc-tional solidification, i.e., segregation processes, whichare related to the distribution coefficients of the compo-nents and cooling rate of the alloys in the temperaturerange of solidification 4. The length of columnar crys-tals is also determined by optimum contents of modify-ing alloying elements, such as S, C, Si, and minimumcontent of impurities 5, 6.At present, the existing problems, such as the re-source economy and decrease in high costs of hard mag-netic materials, determine the necessary use of wastes ofproduction. In this work, we study magnets preparedfrom Alnico wastes rather than from pure charge materi-als (this allows the prime cost of the final product to bedecreased substantially) and, thus, analyze the structureof such magnets in order to reveal regularities of the ef-fect of temperature and holding time of Alnico melts be-fore pouring on the content of oxide and nitride impurityinclusions in ingots. The aim of this study is to developan analytical procedure, namely, fractional gas analysis(FGA) technique, which allows the effect of impuritynonmetallic elements on the formation of columnarstructure in the Alnico alloys to be revealed.Earlier, we have elaborated the fractional gas analy-sis procedure that allows the content of oxygen (in theform of oxides Nd2O3 and NdxFeyOz) in Nd-Fe-B mag-nets 7 to be determined. Existing procedures for thedetermination and identification of nitride inclusions,METALURGIJA 48 (2009) 3, 187-191 187Fractional gas analysis procedure for estimating the contents of oxide and nitride inclusions in Alnico alloysproduced from wastes of production is developed. The effect of the temperature and time of melt holding be-fore pouring on the content of impurity oxide and nitride inclusions is considered. The content of SiO2 inclu-sions in Alnico mainly depends on the holding time before pouring and decreases with increasing holding time,whereas the content of Al2O3 inclusions decreases with increasing both temperature and time of holding. Thecontent of TiN inclusions decreases with increasing holding time. As the holding temperature increases, thecontent of nitride inclusions decreases only with increasing holding time.Key words: Alnico, fractional gas analysis, oxide inclusions, nitride inclusions; hard magnetic materialsPrimjena frakcijske plinske analize za odre|ivanje sadr`aja oksidnih i du{i~nih faza u ALNICO le-gurama. Razra|ena je metoda frakcijske plinske analize za vrednovanje oksidnih i du{i~nih uklju~aka u ALNICOlegurama, dobijenih iz gospodarskih otpada. Posmatran je utjecaj temperature i vremena zadr`avanja kupkeprije lijevanja. Sadr`aj uklju~aka SiO2 uglavnom ovisi od vremena zadr{ke prije lijevanja i sni`ava se pove}anjemtog vremena, istodobno se sadr`aj uklju~aka Al2O3 smanjuje s pove}avanjem i temperature i vremena zadr{ke.Sadr`aj uklju~aka TiN smanjuje se pove}avanjem vremena zadr{ke. S pove}anjem temperature sadr`aj du{i~nihuklju~aka smanjuje se samo pri pove}anju vremena zadr{ke prije lijevanja.Klju~ne rije~i: ALNICO legure, frakcionalna plinska analiza, oksidni uklju~ci, du{i~ni uklju~ci, magnetno tvrdimaterijaliISSN 0543-5846METABK 48(3) 187-191 (2009)UDC – UDK 669.14.018.24:620.153.086=111K.Grigorovich, G.Burkhanov, S.Shibaev, A.Dalmatov, N.Kolchugina,Baikov Institute of Metallurgy and Materials Science, Russian Academyof Sciences, Moscow, Russia, I.Belyaev, JSC “NPO Magneton”, Vladi-mir, Russiasuch as optical and electron microscopy, X-ray diffrac-tion and electron probe analyses, high-temperature hy-drogen extraction, and methods assuming the separationof nitrogen-containing compounds from the alloys, arelabor-consuming and do give inadequate accuracy. Thefractional gas analysis shows promise for determiningthe content of nitride phases as well.Inclusions SiO2 are the most harmful for the Alnicoalloys since the thermal expansion coefficient of the in-clusions is higher than that of the metallic matrix. Thisleads to the formation of pores in finish products. More-over, TiN inclusions hinder the grain growth. That iswhy the determination of oxygen and nitrogen species inhard-magnetic materials is effective for controlling andpredicting the properties of permanent magnets.EXPERIMENTAL PROCEDUREThe preparation procedure for Alnico alloys is de-scribed in detail in 8. The initial alloy (Alnico wastes)was melted under a calcic-aluminous slag using an openinduction furnace. Metals, such as Ti, Co, Nb, and Al,were made a part of the melt that was subsequently castin a metallic mold. Thus, we obtain a common catego-rized furnace charge (CCFC). For the investigation,samples of the alloy were prepared as follows. TheCCFC was melted and heated to temperatures of 1650,1700, and 1750 °C, held at these temperatures for 30 s(melt 1) and 120 s (melt 3), tapped to a ladle lined withfused corundum, and poured in an acid lining moldheated to 1300 °C mounted on a cooler. To prevent thecrystallization of the high-temperature  phase, the cool-ing rate of the ingots after crystallization was no lessthan 100 K/min.Table 1 shows the chemical composition of Alnicosamples under study.Table 1. Chemical composition of Alnico samples (Fe –balance)Content of components / mas. %Al Si Ti Co Ni Cu Nb6,8 0,16 4,9 34,9 13,4 2,9 0,98The segregation of alloy components was studied byelectron microprobe analysis using Camebax andMS-46 Cameca setups. The substantial segregation oftitanium and niobium and slight segregation of copper atdendrite boundaries were found. Moreover, the dendriteboundaries were found to be depleted of aluminum. Allother elements are characterized by uniform distributionin the alloy microstructure. The variations in the contentof Fe, Co, Ni, Cu, Nb, Al, and Ti in the center and atboundaries of dendritic cells were found to be 33,8-35,7;34,8-35,3; 13-14,4; 3,2-2,3; 7,91-0,75; 2,3-8,6; and6,1-4,65 mas. %, respectively.Using a Carl Zeiss scanning electron microscopeequipped with a LEO 430i energy dispersive X-raymicroanalyzer, two principal types of oxide inclusionsand a nitride phase were found in the samples understudy; these are Al2O3 and SiO2 and TiN, respectively(Figure 1).Along with fine precipitates of aluminum and siliconoxides and titanium nitride, slag drosses mainly consist-ing of aluminum oxides and Mg-containing calciumaluminosilicates were found in the Alnico samples.The FGA of Alnico samples was performed on aTC-600 LECO gas-analyzer.Fractional gas analysis is a modified reduc-ing-melting procedure realized in a graphite crucible ina carrier-gas flow at a fixed linear heating rate 9. Theanalysis is based on the difference in the thermodynamicstability of oxides (nitrides) upon heating that are princi-pal species of oxygen (nitrogen) present in a metal.Thus, we present conditions of monotonic heating of asample in a graphite crucible of an analyzer and obtaingas-evolution curves (evalogram) for oxygen and nitro-gen in the form of CO and N2, respectively. An evalo-gram is a system of maxima; each peak of the curve berelated to one or other kind of inclusions and has a char-acteristic temperature, which corresponds to the onsetand maximum of reduction (dissociation) and allows theassociated compound to be identified. Principles of ther-modynamic calculations of characteristic temperaturesof oxide inclusions in Fe-based melts are given in 10.The calculation of the temperature of titanium nitridedissociation is based on the following concept. Duringthe FGA of Alnico samples, TiN inclusions dissociatewith the formation of TiC:TiN C TiC 12 N(s) (gr) (s) 2(g)   (1)The equilibrium constant of the reaction isKa pa a( ) ,12TiC NTiN c(2)where a is the activities of corresponding substances andp N 2 is the nitrogen partial pressure in the gas-analyzerfurnace. Since the activities of substances in the stan-188 METALURGIJA 48 (2009) 3, 187-191K. GRIGOROVICH et al.: FRACTIONAL GAS ANALYSIS PROCEDURE FOR DETERMINING THE CONTENTS OF OXIDE...Figure 1. Micrographs and typical X-ray spectra of Al2O3and SiO2 oxide and TiN inclusions in the Alnicoalloys under studydard state are equal to unity, the equilibrium tempera-ture of dissociation reaction (the temperature of the on-set of dissociation during FGA) of TiN with the forma-tion of TiC is determined by only the pressure p N 2 equalto 1,875 bar:THS R pOObegN 2( )( )( ) ln111 		(3)where 	H O( )1 is the change of standard enthalpy forreaction (1), 	S O( )1 is the change of standard entropy forreaction (1); and R is the universal gas constant.Using tabulated data 11, we have calculated thetemperature of the onset of dissociation of titaniumnitride; it is 2061 K.RESULTS AND DISCUSSIONFigure 2 shows a typical curve of the oxygen extrac-tion (obtained during FGA) in the form of CO for anAlnico sample. The curve is characterized by twogas-evolution peaks that indicate the presence of oxygenin the sample in the form of two compounds, such asSiO2 (peak 1) and Al2O3 (peak 2).Figure 3 shows the typical curve of the nitrogen ex-traction during the FGA of the Alnico sample. As isseen, the evolution of nitrogen occurs at the high tem-perature; the existence of the single peak indicates thepresence of the single kind of nitrogen-containing inclu-sions.We calculated the temperature of the onset of TiNdissociation and compared it with the experimental tem-peratures of the onset of nitrogen evolution. The averageexperimental magnitude is higher than the calculatedvalue by –80 K. This difference can be explained by thefact that the tabulated data were obtained without the al-lowance for the kinetics of dissociation of the nitride inthis type of alloys.Figure 4 shows the effect of temperature and time ofholding of the Fe-Al-Ni-Co-Ti melts before the pouringon the total oxygen content. As is seen, the total oxygencontent decreases, as the temperature and time of hold-ing increase. In this case, the melt becomes more homo-geneous with respect to the oxygen inclusion content.This occurs at the expense of the removal of slag drossesfrom the metal and possible decrease of the oxygen solu-bility with increasing melt temperature.Using the FGA results, we determined changes in theoxygen content (in the form oxides) as functions of thetemperature and time of holding of the Fe-Al-Ni-Co-Timelt before pouring. These dependences are shown inFigure 5. As is seen, the content of SiO2 inclusions in theAlnico samples mainly depends on the melt temperatureand is almost independent on the holding time. In thiscase, the content of Al2O3 inclusions decreases with in-creasing both melt temperature and holding time.According to the FGA results, the total oxygen con-tent in the Alnico samples obtained after 30-s holding ofthe melt at 1750 °C is 60,8 ± 26,1 ppm; the oxygen con-tents in the form of SiO2 and Al2O3 are 17,5 ± 13,0 and40,7 ± 14,1 ppm, respectively. After 120-s holding ofthe melt at the same temperature, the total oxygen con-tent in the Alnico samples is 49,7 ± 4,8 ppm; the oxygencontents in the form of SiO2 and Al2O3 are 17,7 ± 0,4 andMETALURGIJA 48 (2009) 3, 187-191 189K. GRIGOROVICH et al.: FRACTIONAL GAS ANALYSIS PROCEDURE FOR DETERMINING THE CONTENTS OF OXIDE...Figure 2. Curve of oxygen (in the form of CO) extractionfrom Alnico alloys during FGA performed at a he-ating rate of 2,1 K/s; I(t) is the intensity (relativeunits) of CO extraction on TC-600 (LECO) setup.Figure 3. Curve of nitrogen extraction from Alnico alloysduring FGA; I(t) is the intensity (relative units) ofN2 extraction.Figure 4. Effect of the temperature and time of holding ofthe Alnico melt before casting on the total ox-ygen content30,4 ± 5,3 ppm, respectively. Thus, the prolonged hold-ing before the pouring allows the oxygen content in thealloys to be decreased at the expense of its removal fromthe melt in the form of Al2O3 inclusions.The volume fractions of oxides in the samples can becalculated using data on the oxygen content and equa-tionVyoxideoxide oxide matrixoxide OO MA(4)where oxide is the density of oxide of the given composi-tion; matrix is the density of metallic matrix (7,2 g/cm3for the Alnico alloys); AO is the atomic mass of oxygen;Moxide is the molar mass of oxide; y is the oxygen indexin the oxide formula; Ooxide is the oxygen mass fractionin the form of this oxide determined by FGA.According to the literature data 12,  SiO2 and Al O2 2are 2,8 and 4,1 g/cm3, respectively. The calculated vol-ume fractions of oxides in the Alnico alloys solidifiedafter 120-s holding at 1750 °C are = 8,55 × 10-5 ± 1,93 ×10-6 and = 1,13 × 10-4 ± 1,98 × 10-5.The volume fraction of titanium nitrides in the alloycan be calculated using equationVyTiNTiN TiN matrixTiN NN MA(5)where TiN is the density of TiN; AN is the atomicmass of nitrogen; MTiN is the molar mass of TiN; NTiN isthe nitrogen mass fraction in the form of TiN determinedby FGA. The density of TiN is 5,4 g/cm3 12.Figure 6 shows the effect of the time and temperatureof holding of the melt before pouring on the volumefraction of TiN in the Alnico samples.As the holding time increases, the volume fraction ofnitrides decreases. This is reached at the expense of themelt mixing and removal of TiN inclusions. As the hold-ing temperature increases, the decrease in the TiN con-tent takes place only during the prolonged holding. Inthis case, the melt becomes more uniform with respectto the nitrogen content. This can be related to a decreasein the nitrogen solubility with increasing melt tempera-ture.CONCLUSIONSThe structure of Alnico alloys (prepared from wastesof production) and the segregation of the principal com-ponents in them have been studied.The procedure of fractional gas analysis, which isbased on the difference in the thermodynamic stabilityof oxides, has been developed for Alnico alloys. Com-pounds SiO2 and Al2O3 were found to be principal oxideinclusion present in these alloys.As the temperature and time of holding of theFe-Al-Ni-Co-Ti melt before the pouring increase, the to-tal oxygen content decreases. In this case, the distribu-tion of oxide inclusions becomes more uniform.The results of the FGA show that the content of SiO2inclusions in the Alnico samples mainly depends on theholding time before pouring and decreases with increas-ing holding time, whereas the content of Al2O3 inclu-sions decreases with increasing both temperature andtime of holding.Nitrogen is shown to be present in the alloys in theform of TiN inclusions. The FGA procedure for the de-termination of nitride inclusions in the Alnico alloys hasbeen developed. It was shown that the content of TiN in-clusions in the Alnico alloys decreases with increasingholding time. As the holding temperature increases, thecontent of nitride inclusions decreases only with in-creasing holding time.AcknowledgementsThis work was supported by the Russian Foundation forBasic Research, project no. 08-03-12126_ofi.REFERENCES1 S. Watanabe, Y. Kamata, S. Chikamatsu, Effects of sulfuradditions on the columnar crystallization of Ti-containing190 METALURGIJA 48 (2009) 3, 187-191K. GRIGOROVICH et al.: FRACTIONAL GAS ANALYSIS PROCEDURE FOR DETERMINING THE CONTENTS OF OXIDE...Figure 5. Effect of the temperature and time of holding ofthe Alnico melt before casting on the oxygencontent in the form of oxidesFigure 6. Effect of the temperature and time of holding ofthe Alnico melt before casting on the volumefraction of TiNAlnico-type magnet alloys with high coercive force // J.Jap.Inst. Met. 29(1965)8, 782-787.2 N. Makino, J. Kimura, Effect of duplex addition of sulfurand tellurium in columnar Alnico magnet alloys containingTi // JEEE Trans. Magn., 6(1970)2, 302.3 D.A. Petrov, Disturbance of equilibrium during the crystal-lization of solid solutions // J. of physical chemistry.,XXI(1947)12, 1449-1460.4 I.I. Novikov, V.S. Zolotorevskii, Dendritic liquation in al-loys, Science, Moscow, 1966.5 P. 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Ponomarenko,Physical and Chemical Calculations of Steel-Making Pro-cesses: Tutorial for Universities, Metallurgiya, Moscow,1989.12 Mineralogy Database: http://webmineral.com.Note: The responsible translator for English Language is N. Kolchugina.METALURGIJA 48 (2009) 3, 187-191 191K. GRIGOROVICH et al.: FRACTIONAL GAS ANALYSIS PROCEDURE FOR DETERMINING THE CONTENTS OF OXIDE...

Primjena frakcijske plinske analize za određivanje sadržaja oksidnih i dušičnih faza u ALNICO legurama

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