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Solubility of Nitrogen in Molten Fe-V Alloy*

By Harue WADA **

Synopsis

The solubility Qf lIitrogen in liquid valladium alld iron-vanadium alloys

has bew measured in the tem/Jerature range 1 800 10 2 200 C over wide

comllosition range using levitation-melting method.

The solub-ility oj nitrogen in liquid vanadium was very high and de­

creased with increasing temjJerature. In the iron-valladium alloys, it was

increased with increasing vanadium conlenl alld decreased with increasillg

temlJeta ture. The departure jrom Sieverts' law oj this system was caused

~y the curious jact that the aclivi~y coef}iciellt was decreased with increasing

nilrogen partial/mssure.

Valnes oj the partial molar hea l oj so/ulioll Qf lIitrogw Jl~ ~ and interac­

tion coef}1cient elr) were deterlllined as the junctions oj composition alld

temjJerature, res/Jecliveiy. The values oj JiJ~ in iron-va lladium alloys were

more negative than those in iron-chromiulIl alloys. Entha/jJY coef}icient

h~ ') was estimated as - 2.26 kca//mol.

I. Introduction

T ransition me ta ls of earl y g roups, such as titanium a nd vanadi um, espec ia ll y in liquid state have high so lu bi lity of nitrogen . This may indicate tha t the so lubility has a correla tion with the num ber of 3d or 4d electro ns, as po in ted out previously. l ),Z) The solubili­ty o f nitrogen in molten iron increases strongly with thc addition of these e lem ents, so-call ed good a bso rbers, as a lso indicated in our previous work . The p resent work is d ea ling with the solubility of nitrogen in m olten vanadium and iron-vanadium a ll oys by levita­ti on-melting, as the seri es of \\'orkl),Z' by the use of same technique.

The levitation-melting method has fo llowing ad­vantages:

( I ) It m akes pos 'ible to measure the p roperty of active m eta ls even in the molten sta te , without any contamin a tion by crucible.

(2) It is suita ble fOI- the determina tion of the tem­perature depend ence o f a property, because it is pos­sible to operate over the wide temperature range.

(3) The equilibrium is so rapid ly atta ined that the experiments are speed y and continuous. Further­more, the so lidifying and cooling procedures are quite easy.3)

The solubility of n itrogen in molten iron-vanadium system was measured by Evans and Pehlke,4) El T aye b and Par lee. 5 ) The solubility of nitrogen in m olten iron, increase b y the addition of vanadium and the inte raction parameter has been reported as 0.0934) or 0.0945 ) at I 600°C. H owever, there a re som e diffi­culties in measuremen ts a t higher temper a tures using crucibles, and the results have not shown an y sati sfac­tory agreem ent with one another.

In th e present work, the m easu rem ents are m ad e in the temperature range I 800 0 to 2 200°C and th e com­position range 3.2 to 100 %V . T he e ffect of vanadium on the so lubility of nitrogen a nd the heat of so lution of ni trogen will a lso be di scussed .

II. Experimental

The equipments for levita tio n-melting and the ex­perimen ta l proced u res are the same as reported in the previous pap ers.l),2) T he mo lten specimens were mainta i ned a t a constant tempera ture for abou t 80 sec.

The ma terials were prepa red as follows: the pure iro n which was prepa red by vacuum melting electro­lyt ic iron a nd d eoxidi zed with carbon, (the specimen !! in th e previou. paper ;J) its oxygen content was abou t 40 ppm ) was again a rc-melted in an a rgon a tmosphere toge ther with a necessary amount of high purity vanadium (99.84%) supplied by Ore met. T hen the ingo ts were remelted in vacuum by applying electron-beam. T he composit ions of the iron-vanadi­um a lloys were d etermined by chemical analys is a ft er the melting. The impurities in vanadium used a re shown in T a ble I .

Table I. Chem ical composit ion of vanadi um

Element C H o N Fe Cr N i

wt% 0 . 008 0 .0096 0.060 0 .007 0.044 0.005 0.027

Th e compOS itIOns of the atm osphere gas selected were 1.1 % N 2- Ar, 5. I %N z- Ar, 20.6 % N 2- AI-, and 50.1 %N2- Ar. Since som e d evia tion from Sieverts' law was expec ted fOI- h igh vanadium a lloys, the ex­peri ments we re made under various nitrogen pres­sures.

The form a tion of the vanadium ni tride took p lace when the hig h va nadium alloys w ere levitation-melted in some hig her nitrogen partial pressures. In this case, the molten specimen was solidifi ed in the levita ­ti on co il as soon as the nitride formed . The condition was carefull y chosen to avoid the formation o f the n itrid e because i t should resul t in an incorrect value for the nit rogen so lubili ty. The microscopic observa tion of the solidified specimen en a ble us to judge whether the nitride has formed or not. Microstructures o f two specimens with and withou t the nitride formation during levitation-m elting are shown in Photo 1. The difference be tween these two microstructures is ob vious. Specimen (a ) w hich solidified in the levita tion coil

* Origina ll y pu blished in J. j alJan I l1 st. M etals, 33 (1969 ), 720 in J apanese. English version received M ay 23, 1969. * * National R esearch I ns titute fo r M etals, Nakameguro, l eguro-ku, T okyo 153 . Now at M assachu e t ts Institute of Technology,

Cambridge, M ass .

Research Article ( 399 )

( 400 ) Transactions lSIJ, Vol. 9 , 1969

a fter the forma ti on of nitrid e reveals la rge gra ins o f the nitride precipita tes, ",herea sp ecimen (b) which so lidi­fi ed in the copper mold shows a solidification struc ture of the sma ll g ra in size .

The tem pera ture measurem ent was m ad e b y the TCP- IIA type two-co lor eye p yrometer through a quartz window. The accuracy was within ± lO°C.

The copper mold 11 was used throughout the p re­sent work. Th e content o r nitrogen in th e sp ecimen solidified in thi s m old was d e termined by the Kje ldahl method. The d ecomposition of th e a ll oyed specimen was a lmost complete with th e p e rchlori c acid , a nd the pure vanadium was decomposed by the sulfuri c acid , potassium sulfate and cupric sulfa te.

Pho to. I . (a) l\ I ic rostructure or a specimen with ni t ri de

p recip it ation dur in g lev itation-melting. (Fe-

67.60 ° V a lloy)

(b) l\'Iicrostr uc tu re ora sp ec imen without ni tride precip it ation d u rin g lev ita tion-melting . ( r e-

67.7 °0 V a lloy)

III. R esults an d D iscussion

1. E xperimental R esults

The solubility or nitrogen was plo tted against tem­pera ture in Figs . l , 2, and 3, where the measured values were converted into the solubility under I a tm. nitro­gen pressure, without any correction for the d evia tion from Sieverts' law. Fig. I shows the results obtained

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fo r th e specimens o r lower vanadium content than 50.Gwt % V . The solubility in iron-vanadium alloys in creases drasti call y with in creas ing vanadium content. The tem pera ture de pend ence of the solubility is nega­ti ve a nd becomes more negative with increasing vanadi­um content. At 22 .2 % V , th e solubility calculated from th e results under the different partial pressures of ni t rogen a re no t in agreem ent w ith one another, which indi cates the poss ibility of d evia tion from Sieverts' law .

Fig . 2 shows the solubility for the a lloys having higher vanadium content conve rted into 1 a tm. nitro­gen pressure. The solubility increases with increasing vanadium content a nd d ecrease with in creas ing tem­pe ra ture, and usua lly shows the d evia ti on from Sieverts' la w.

As shown in Fig . 3, th e solubility of nitrogen is ve ry hi g h in the molte n pure vanadium . It reaches to abollt 1 8wt%~ a t 2 OOO°C under 1 a tm. pressure o f nitrogen , whi ch is calcul a ted from m easurem ents car­ri ed o ut in the 5. 1 % N 2- Ar gas, a nd d ecreases with

6.0

5.0

4. 0

2 ' 30 ~ .

"

1.0

~I

3 4.7 ~o V,~ • • .~_ ---....~ --. .... @,

I~ iii .... • p ,\ , = 0.051 a tm 0.--.... Iii .... ® P'" = 0.206 atm --""1 @ p.\ , = 0.501 at m ~. " p.\ , = 1.00 a tm

22~ • ' -~"'---~

®~ • • '::--~ ·.,--=.-------12.7%V 5~,o--. J-"~-'-'~_" ()-" __

o - 0-- 3.20 ?'o V - =0=,,= ,,= 0--0-

1800 1900 2000 2100

- T e mpe ra ture (OC )

Fig. I . So lubi lity of n itrogen in liqu id Fe- V system a t I a tm

n itrogen p re"su lT

2

* ~ 10

Fig. 2.

P,- o-:GI I -;; tnTI P, = 0.051 a tm p \ = 0.206 a tm

o t o t

1800 1900 2000 2100 ---Tempe ra t ur e (T )

Solubili ty of ni troge n in liqu id Fe- \ ' sys tem at I a tm

n itrogen p ressure

. ,

,-

to.

....

•

z ~

"

30~--------------------------'

20

10

1900

P, = 0.011 aIm I • P, = 0.05 1 aIm ® 1\ , = 0.206 a t m

2000 2100 --- Temperature ee)

2200

Fig. 3. Solu bility of nitrogen in liquid pure V at 1 atm

nitrogen pressure

i nCl-easing temperature.

2. D eviationfi'om Sieverls' Law in H igh Vanadium Alloys

The solubility of nitrogen in each iron- vanadium a lloy was approximated hy a straight line, and averaged solubilities at certain temperatures were determined. R elations bet ween sq uare root of the ni trogen partial pressure and the solu bi liti es in 12.7 % V alloy at I 800°, I 900°, 2 000°, and 2 I oooe are shown in Fig. 4. Thi s a lloy obeys th e Sieverts' law. The relationship be­tween square roo t o f the nitrogen partial pressure and the so lu bi liti es in iron-vanadium alloys and pure va­

nadium a t 2 000 e is shown in Fig . 5. It is shown that the pure vanadiu m and the iron-vanadium a lloys ha ving vanad ium conten t more than 22 .2 % V do not obey the Sieverts ' law. The activity coeffi cient of nitrogen in these alloys decreases with increasing nitro­gen partial pressure as might be expected from Figs. I , 2, and 3. amely, the eq uilibrium partial mola r free energy of solution of nitrogen d ecreases with in­creasing nitrogen partial pressure. Such decrease in the activity coeffici ent of nitrogen a in the case of iron-vanadium a lloy has not been obse rved in the solu­bility of nitrogen of other good absorbe rs. Although the cause of thi s d eviation is not cla rified yet, the change in the interac tion energy of .tJ- l"-f interaction, or Y - l"-f interaction, or both of them , in the molten a lloy may be considered. In order to d escribe the general feature of thi s d evia tion , it will be necessary to d eterm ine the precise values of the nitrogen solu­bility in 3d- or 4d- transition m etal s.

The dev ia tions from Sieverts ' law at 2 ooooe are summarized in Fig. 6. The dissolution of nitrogen into metal is generally expressed as

1/2 N 2 (gas) =; t: (in m etal) . .. .. .. ........ ( I )

The eq uili brium constant K is given as

O~N .j~ k - I C - . (2) - -lPN, .. ..... .............. .. .

where J.~ is the actIvIty coe ffi cient of nitrogen. The standard state of the activity coefficient is the infinite dilute solution of nitrogen. Therefore, the extrapola­

tion of the experimental value of log(%l"-f /-IP N,) to

Transactions ISIJ, Vol. 9, 1969 ( 401 )

0.4

0.3

2 1

~ 0.2 ::

0.5 - - .;--p; (atm )

1.0

Fig. 4. Rela tionship between -I PN , and \V t Oo~ for 12.7 ° 0 V­F e a lloy

8.0

7.0

6.0

5.0 z l ~ 4.0

:::

3.0

2.0

1.0

Pure V

76.3%V

67.7%V

o P, ,= O.Oll atm • p ,.= 0.051 atm ® P, ,= 0.206 atm

22.2%V ______ 12.7%V

o 0.2 0.3 0.4 0.5 0.6 0.7 ---./P;. (atm )

Fig. 5. R elationship between -IP~ , and wtO o ~ for va rioLi s F e- V alloys at 2 000 C

PN,= O _---- ____ ~'- Pure V

0 ---- -------~. -76. 3%V 1.0 :-:-= .:-::-:: __ --- -- . -67.7%V

__ -- • - 50.6%V

~I~ b.O

..s 0 .

- 5.9 4%V - 1.0 3.20%V

P " 0.011 atm • PN• = 0.051 atm ® PN; = 0.206 atm o PN1 = 0.501 atm () P N!:;;; 1.00 atrn

o 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 - - wt% l'{

Fig. 6. D eparture of nitrogen from Sieverts' law at 2 OOO°C

-I PN,=O gives the logarithm of the equilibrium con­stant a t infinite dilution.

3. Interaction between Vanadium and Nitrogen

The averaged solubilities for I 800°, I 900°, 2 000°, and 2 1000e which are obtained from the straight lines shown in Fig. I are plotted against the vanadium content as shown in Fig. 7. The cu rve obtained is

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( 402 J Transactions ISIJ, Vol. 9, 1969

downward-convexed one, and the solubility increases with increas ing vanadium content. The form of thi s curve is quite simila r to that obtained by Pehlke and ElJiott6) at I 606°C.

The interaction coe ffi cients e\J) were calculated by the Eq. (3) at I 800 0

, I 9000, 20000

, and 2100°C and are plotted against I j T in Fig. 8.

e\J) = d~V [log % ....2 (pure Fe)

- log %tT (Fe- V a lloy)] .. . .... .. .. .... (3)

The values of e\J) determined by Eva ns and Pehlke, 4) EI Tayeb and Parlee,5) Ch ipman and Corrigan 7 ) and Maekawa and akagawa8 ) at lower tem peratures than the present work are also shown in Fig . 8. The I'esult of the present work is reasonablly consistent , especia ll y in low temperature range, with those calculated by Chipman and Corrigan, who have estimated by the empiri ca l eq uation from the value of eR') = - 0. 1 0(6) at I 600°C. In higher temperature range, however , the agreements between the present results and thc extrapolated values from the results of Evans and Pehlke,4) and EI Taye b and Parlee5 ) seem more rea­sonable.

The interaction pa rameter e\J) is expressed with two terms, entha lpy coe ffi cient h\J) and entropy co­efficient S\J) as follows7 ) :

( I') _ hR') eN - 4.575 T

S~I') 4.575 . ..... .... .... .... (4)

The straight Iinc shown in Fig . 8 is expressed as follows:

e\J) = - 4~5~~OT - ~:~~~ .. ... ... .. .. .... .. (5)

The enthalpy coefficient h\J) is obtained from Eqs . (4) and (5) as !z<{ )=- 2.26 kcal jmol. This value is more negative than that determined by E vans and Pehlke,4) h\!') = - I. 7 kcaljmol. These values are a lso plotted in Fig. 9.

The pa rtial molar heat of solution of nitrogen JH:N

are shown in Fi g. 9 as a function of the vanadium content. The value of JH:N is determined from the temperature coeffi cient of the 10g%tT in the composi­tion range where Sieverts' law is obeyed , and in the highet' vanadium concentration range it is determined from th e temperature d ependence of the log K a t PN,= O, which is obta ined from Fig. 6.

JHN LlS N log K = 2.3RT - 2. 3R ............... (6)

The value of LlH:N for iron-chromium a lloy is a lso shown in Fig. 9 for comparison.

The LlBN in the iron-vanadium a lloy shows more larger d eviation from the straight line connecting the values of lEi'N of each terminal meta l than that in the iron-chromium alloy. The JHN cha nges drasti cal­ly with vanadium content even in the dilute range of vanadium. The result that the JBN has more nega­tive value in the iron-vanadium alloy than the iron­chromium alloy may correspond to the fact that the

Research Article

number of 3d-electrons of vanadium is smaller than that of chromium. As pointed out previously,2) the effect of a lloy ing element on the solubility of nitrogen in liquid iron may have so me relationship with its number of d-elec trons. It may be suggested that a short-

4.0

3.0

1.0

Present work (J 1800·C • 1900·C ., 20WC

Z 0 21WC

Peh lke and Elliott ------- 1606·C

20 30 40 50 - wt%V

Fig. 7. Solubi lity ofn it rogcn in liquid Fc- V sys tem at I atm

nitrogen pressure

Maekawa and Nakagawa

- 0.1

~ :.".

"" / -

o

1-0.05

U~~zwada ......-;;;:-------- .

------,

I ~ This s tudy

- 6

-5

o 4.0

.-., Evans and Peh lke 0- - -<0 El Tayeb and Par lee

[ ---- Chipman and Corrigan

5.0 l i T X 10'

Fig. 8. T emperature dependence of e\J)

l

/ /'

.---·F~-=-C-;:---·-· ___ /

;. --r'" Peh I ke a nd e P r esent work / Ell iott 0 El Tayeb and Pad ee

10 20 30 40 50 60 70 80 90 100 - - wt %V

Fig. 9. H eat of solution of nitrogen in liquid Fe- V system

..

"

~ .'

)

"'.

+

range ordering ex ists between vanadium and nitrogen in iron-vanadium-n itrogen system due to the remark­ab le difference in the affinities between Fe-~ and V-~.

IV. Conclusion

Th e solubili ty of nitrogen in molten vanadium and iron-vanad ium a lloys has been measured over the tem­peratu re range I 800° to 2 200°C by the levitation­melting m ethod.

The solubility of ni trogen in pure liquid vanadium was very high and decreased with increasing tempera­ture. In the iron-vanad ium alloys, solubili ty of nitro­gen was increased with in creasing vanadium content and decreased with increasing temperature. T he departure from Sieverts' law of thi s system was notice­able, because the act ivity coeflicien t was d ecreased with increasing nitrogen partial pressure.

Values of the partial molar heat of solution of nitro­gen J H N and the interaction eoe flleient e'ii) were d etermined as the functions of composition and tem­perature, respectively. The values of J H N in iron­vanadium alloys were more negative than those in

Transactions lSI], Vol. 9, 1969 ( 403 )

iron-chromium alloys. From the results obtained, it is suggested that the bonding between y and ~ is more attractive than that between F e and ~ or Cr and ~ .

This may be owing partiall y to th e fact that the num­ber of 3d-electrons of vanadium is smaller than those of ch rom ium and iron .

REFERENCES

I) I-i. Wada, K . Gunji , ancl T. Wada: TrailS. l SI} , 8 ( 1968), 323.

2) H. Wada, K. Gunj i, and T. Wada: TrailS. I SIj, 8 (1968), 329.

3) D. W. Gomersall , A. McLean , and R. G. Ward: T rans. Met. Soc. A I ME, 242 (1968), 1309.

4) D. B. Evans and R. D. Pehlke : TrailS. Met. Soc. A IME, 233 (1965), 1620.

5) N. M. E I T eya b a ncl N. A. D. Pa rlee: Trans. iv/et . Soc. A IME, 227 ( 1963 ),929.

6) R . D. Pehlke and J. Ell iott : Trans. Met. Soc. A I ME, 218

(1960), 1088. 7) J. Ch ipman and D. A. Corrigan: Trans. Met. Soc. A I ME,

233 (1965), 1249 . 8) S. Mackawa and Y. :'\akagawa : T elsu-to-Hagane, 46 ( 1960),

972.

Research Article