A COMMENT ON THE 87°C I.F. PEAK IN Fe-V-N ALLOYS

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A COMMENT ON THE 87°C I.F. PEAK IN Fe-V-N ALLOYS

Tingguo Chen, Xiaoming Xie, Ziliang Wu

To cite this version:

Tingguo Chen, Xiaoming Xie, Ziliang Wu. A COMMENT ON THE 87°C I.F. PEAK IN Fe-V-N AL- LOYS. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-233-C2-237. �10.1051/jphyscol:1987832�.

�jpa-00227136�

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JOURNAL DE PHYSIQUE

Colloque C8, supplgment au n012, Tome 48, d6cembre 1987

A COMMENT ON THE 8 7 O ~ I . F . PEAK I N Fe-V-N ALLOYS

TINGGUO CHEN, XIAOMING XIE and ZILIANG WU

Shanghai I n s t i t u t e of Metallurgy, Academy of Sciences of China, Shanghai, China 200050

Abstract

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It is shown in the present paper that the 87'~ internal friction peak in the Fe-V-N system often refered to in the literature does not behave like an s-i interaction peak. It is induced by the nitrogen atoms accupying the anion hole of a defective vanadium nitride precipitate. The effective holes capable of holding interstitials for internal friction generation must be situated in the interface between the a-Fe matrix and the defective precipitate. The 95Oc peak occurs for a precipitate with a N/V ratio of 0.7 to 0.8, corresponding to the metastable compound V,N3 of of highest hole concentration. Lattice parameter measurements show that a stoichiometric VN precipitate with ao=4.132 groduces no 95'~ peak while a defective VN

. , ,

with ao=4. 082 can give 95 C peak.

I

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INTRODUCTION

internal friction spectra in Fe-V-N alloys have been studied by Fast and Meijering /I/ and independently by Dijkstra and Sladek 121 in 1953. Both investigations used specimens of a single vanadium content (0.5% V) nitrided either in the y-range (950'~) or at 600'~ followed by a homogenization at 1 0 0 0 ~ ~ . In the internal friction spectra of quenched specimens they observed, in addition to a normal Snoek peak at 22Oc, also an abnormal peak with its peak temperature (80° to 90'~) varying with nitrogen concentration. The abnormal peak was rather broadened, signifying that there was more than one relaxation process operating. The abnormal peak was inter- preted as being due to the stress induced ordering of the nitrogen atoms bound to the octahedral interstices nearest to or next nearest to a substitutional vanadium atom. Because the specimens used in these two investigations all had been heated at high temperatures, and owing to the high affinity between vanadium and nitrogen atoms, vanadium nitride particles would unavoidably be formed in the alloy. This led Fast I31 and others 14-71 to believe that the bound nitrogen atoms neighboring a vanadium atom were located on the interface between the nitride phase and the iron matrix. However, to the present authors' aware, to all the defect models proposed by the previous investigators 11-91, complemented to the multiple diffusion paths hypothesis used to explain the relaxation processes or the basic postulate they laid for computer simulation of experimental internal friction curves, only inter- actions associated with soluble nitrogen atoms and isolated V or its complexes such as V-V, VN, etc. were considered, none had ever been dealing with an interface.

Therefore, the past studies only endeavoured to explore the substitutional - interstitial interaction nature of the 87'~ abnormal peak, which, of course, was not the original ideas of these authors.

It has been shown in the literature that the 87'~ peak was not thermodynamically stable but decayed remarkably fast even at the peak temperature. This demonstrated that the s-i interaction displayed by the 87'~ peak was much weaker than the interaction exhibited by dislocations pinned by interstitials. The latter reaction brought forth a Snoek-Koester peak known to be stable at least at the peak temperature (-22.0~~). This fact, however, was contradictory to the results found by .

Epstein et a1 1101. These authors showed that the presence of vanadium in solution would effectively suppress dislocation pinning by interstitials and thus rendered the steel non-ageing.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987832

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C8-234 JOURNAL DE PHYSIQUE

Recently Chen et a1 /11/ have re-investigated the Fe-V-N system by niriding a 1.1% V alloy at 450'~ under an 8 0 vol% H,

+

20 vol% NH3 atmosphere followed by a long time homogenization at 350'~ so as to suppress the vanadium mobility to a minimum while leaving the long range diffusion of nitrogen still permissible. The nitrogen activity in this atmosphere was low enough at the nitriding temperature so that the possibility for Fe4N formation on the specimen surface was completely eliminated /12/. It was also found essential to include the low temperature homogenization cycle in order to insure the reproducibility of the internal friction data. Our result disclosed that a high temperature peak was seen at about 2 0 0 ~ ~ while only a small trace of peak was barely observable at 90'~. The latter was presumably induced by the vanadium nitride inclusions inherited from the nitrogen rich master alloys used in vacuum melting, as has been pointed out previously /11/.

The 2 0 0 ~ ~ peak, having an activation energy of about 1.2eV was stable up to 3 0 0 ~ ~ . The binding energy between nitrogen and vanadium in solution is about 0.4eV which is slightly smaller but comparable to the dislocation-N (or C) binding energy (0.45- 0.5eV). This confirmed several authors' /6,10,13/ observation that a number of S atoms were required to trap an interstitial before it could be drifted to a dislocation sink. To date, it seems that the nature of the 87OC peak in the Fe-V-N system is still not clear and a further study of the subject seems warranted.

I1

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THE INTERNAL FRICTION DATA

Fe-0.81 at% V wire specimens of 0.76- diam. were nitrided at 4 5 0 ~ ~ under a 80 vol% H2

+

20 ~ 0 1 % NH3 atmosphere to different nitrogen concentration followed by a 10 hours vacuum anneal at 870'~ to convert all the nitrogen in solution to vanadium nitride precipitates. No internal friction peak had ever been observed in specimens with N/V(1 after precipitation anneal, indicating that in this case, all the nitrogen went to the precipitate phase. However, specimen #9, Table I, on account of its excessive nitrogen content (N/V>3), still showed a strong normal Snoek peak. Each specimen was then repeatedly reloaded with nitrogen at 450'~ plus a homogenization at 3 5 0 ~ ~ until a maximum height of the 95'~ subpeak was attained.

These maximum heights were recorded in the last row of Table I.

Typical internal friction spectra of this series of specimens are depicted in Fig.1, curve (a) was measured from specimen 1 2 listed in Table I. This specimen was only nitrided for a short time priqr to the 8 7 0 ~ ~ anneal with large amount of vanadium still remaining free in solid solution. These free vanadium atoms would be able to trap the incoming nitrogen atoms in their nearby octahedral interstices during the second time low temperature nitriding and homogenization. By computer simulation of the experimental curve (a), six subpeaks were resolved, with subpeak temperatures

Table I Maximum 95'~ subpeak height attained after 450'~ re-nitriding for specimens with different N/V ratios prior to 8 7 0 ~ ~ anneal (Fe-0.8lat%V)

Specimen {I 1 2 3 4 5 6 7 8 9

N/V before precipitation 0.14 0.33 0.48 0.54 0.69 0.83 0.99 0.95 3.25 Max ~;'(95~~)~10~ attainable 0.9 2.4 5.8 6.5 8.0 7.7 2.5 0.6 0.7

at 200°, 156', 128', 95O, 68' and 30°c respectively. The three high temperature subpeak were temporarily explained as being induced by an interstitial jumping surrounding defect complex respectively of the type V, V-N and V-2N. A detailed analysis of this subject will be reported elsewhere. The 30°c subpeak is simply the normal Snoek peak and the 95'~ subpeak was refered to in the literature as the s-i peak (with a nominal peak temperature of 8 7 O ~ at 0.77Hz). This peak will be the main object of study in the present note. The small subpeak at 68'~ comes from computer simulation and has to be assumed in order to get a best fit to the experimental curve (a), the origin of this subpeak being as yet unknown.

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TEMPERATURE ( O C ) TEMPERATURE ( O C )

Fig.1 Internal friction curves for typical re-nitridedspecimens, (a) specimen i12, (b) specimen 114, f=lHz.

Curve (b), Fig.1, was measured from spec. 14 with initial N/V=0.54. In this specimen, more vanadium had been consumed in nitride precipitation during the high temperature anneal, and consequently, perhaps not enough free vanadium atoms were available to produce the low temperature nitriding products

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the highly dispersed individual defect complexes, to make the high temperature branch of the internal friction spectrum significant (see curve (b)).

It has been known that the 95Oc peak was thermally unstable and decayed off even in the course of measurement. Specimen 1k5 which had the highest 9 5 O ~ peak ( 8 . 0 ~ 1 0 - ~ ) among all the specimens, was purposely selected for ageing at three different tem-

p e r a t u r e s 95O, 105' a n d 115'~. A f t e r each run the same specimen was reheated to

450'~ for a few minutes followed by quenching, by this the peak height was easily restored to its original value of 8 . 0 ~ 1 0 - ~ . Data for the decay of the peak were plotted in a R~[Q,'-Q~')/(Q;'-~ scale against ageing time in Fig.2. 'Q, was the initial peak height, i.e., 8.0x10- in this specimen. For the peak height at time infinity, Qkl, we took the internal friction value when the peak began to flatten off, it was here about 1x10-~. In the inset in Fig.2 the slopesof three linear curves are plotted in a semilogarithm scale against reciprocal of the ageing temperature. The slope of the curve in the inset vielded an activation energy of 0.79eV, in agreement with the acti~ation energy for nitrogen diffusion in Ci-Fe. It is shown that the 95'~ peak is induced by the jumping of nitrogen atoms in interface between the a-Fe matrix and the defective precipitate VN,, but not inside the defective precipitate.

IOOOITK

~ o l a'2-vd.4.;.a

^{3 2 5}

N I V

Fig.2 Rate of decay of the 95' peak for Fig.3 Maximum height attainable specimen #5. The inset shows the slope to 95'~ peak as a function of N/V of ageing curves plotted against reci- ratio.

procal of ageing temperatures.

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JOURNAL DE PHYSIQUE

111

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DISCUSSION

1. The precipitate phase: It can be seen in Fig.3 that in specimens with composi- tions of N/V,1 and annealed at 870°c, practically no 95'~ peak can be observed after the low temperature re-nitfiding except a strong normal Snoek peak being registered on the internal friction spectrum. This indicates that the high temperature precipitate phase consists of vanadium nitride particles of a stoichiometric composition N/V=l, the excess nitrogen still remaining in solid solution.

it is known that the stoichiometric compound VN possesses a sodium chloride structure /14/. It is also known that, when nitrogen supply is insufficient, a (nitrogen) defective compound VN, (x<l) will be formed. For x=0.75, it becomes the metastable compound V4N3 containing 25% anion holes. Actually a continuous series of defective lattice compoundsof the same type of structure can be formed, with x ranging from 1.0 to 0.7. The lattice parameters shrink continuously with increasing hole concentration from a(~~)=4.132 to a(VN o. ,)=4.072 1151. Because the mobility of the large anion particles is generally low, one would expect that these defective structures are relatively stable.

TEM observations of the thin films prepared from the specimens which had been annealed at 870'~ disclosed that all the precipitates were thin square platelets with their edge lengths ranging from 50nm to 500nm. These platelets had an orientation relationship { 1 0 0 } ~ ~ /

1

{10O)aFe with the matrix, thg poles of the platelets being parallel to the three <llO>aFe directions. The 870 C annealed specimens were then deeply etched to extract the precipitate particles by carbon replica, and electron diffraction photograms were taken in the TEM. Lattice parameters measured from these photograms revealed that, for specimens with N/V>l, a(~N)=4.132, while for specimen 85 (N/V=0.69), a(VNo.7)=4.082. Therefore, it seems evident that the existence of the defective vanadium nitride (VNx with x<l) is a necessary condition for th-e occurence of the 95'~ peak, after a low temperature reloading with free nitrogen. It also tells us that the height of the 95'~ peak is directly propor- tional to the concentration of the defective lattice in the NaCl type structure.

In Fig.3 it can be seen that the maximum height of the 95'~ peak occurs at NIVz0.7 to 0.8, in the approximity of V4N3, i.e., in the region of minimum lattice parameter or maximum concentration of nitrogen holes.

2. The defect model: An interface between the b.c.c. CGFe matrix and a f.c.c. (NaC1 structure) VN,precipitate is shown in Fi .4. The two phases bear a Bain type orientation relationship, i.e., {lOO)crFepl { 1 0 0 ) ~ ~ ~ and <I.OO>~~,I

1

<110>a~,. Pour vanadium atoms in this interface are shown in Fig.4. A nitrogen atom at 0 1 jumping through 01-T (tetrahedral interstice)-T-0, diffusion path will generate internal friction.

V aton1 Fe atom N atom

fcc N

vacuncy bcc octa.

interstice

Fig.4 Interface betweencr-Fe matrix and VN, (x<l) precipitate.

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Now, let us assume that the misfit between atoms on both sides of the interface is not large so that lattice coherency is maintained. Position 01, (Fig.4) is an octahedral interstice in the regular a-Fe lattice but at the same time an anion hole in the defectiveVN,crsytal. A nitrogen atom occupying position O1 will perhaps experience a much less amount of strain energy than it would be in a b.c.c.

ocQhedral interstice, 0,. From the experimentally determined activation energy for the 95'~ peak, 1.08eV and x0 3x10-l6 s., it can be figured out that a nitrogen atom sitting at O1 will possess a potential energy roughly 0.3eV lower than it would be at the octahedral interstices in a-Fe lattice. At the ageing temperature, thermal agitation will drive the nitrogen atom at O1 out of its 1.08eV deep potential well to the nearby a-Fe lattice 0,. Thereafter this atom will move in the a-Fe matrix by drift diffusion movement toward a dislocation sink. This is evi- denced by the slopes of the ageing curves (see inset of Fig.2) which yields an activation energy of 0.79eV. This is the activation energy for nitrogen diffusion in WFe.

IV

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