AMORPHIZATION OF Nb-TM SYSTEMS BY MECHANICAL ALLOYING OF THE PURE METALS

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AMORPHIZATION OF Nb-TM SYSTEMS BY

MECHANICAL ALLOYING OF THE PURE METALS

M. Schänzer, H. Mehrer

To cite this version:

M. Schänzer, H. Mehrer. AMORPHIZATION OF Nb-TM SYSTEMS BY MECHANICAL ALLOY- ING OF THE PURE METALS. Journal de Physique Colloques, 1990, 51 (C4), pp.C4-87-C4-93.

�10.1051/jphyscol:1990410�. �jpa-00230770�

COLLOQUE DE PHYSIQUE

Colloque C4, suppl6ment au n014, Tome 51, 15 juillet 1990

AMORPHIZATION OF Nb-TM SYSTEMS BY MECHANICAL ALLOYING OF THE PURE METALS

M. SCHANZER and H. MEHRER

Institut fiir Metallforschung, Universitat Miinster, Wilhelm-Klemm-Strasse 10, 0-4400 Miinster, F.R.G.

Abstract

-

Binar powder r n i x t ~ ~ r e s of crystalline niobium and late transition metals

( T M = C u

Ni

c o , K . ~ n . ~ r ) were amorphized i n a planetary ball mill. The development o f the microstructure during the amorphization process was observed by optical microscopy. T h e amorphous Nb-TM alloys were characterized by X - r a y diffractometry

DSc

and

DTA.

Glass forming ranges were determined and in the case of C d b compared with the corresponding values for quenched amor hous alloys and with theoretical predictions. The crystallization behaviour war studied for

CO-

and ~ e & b alloys as well.

1

-

I N T R O D U C T I O N

Mechanical alloying

MA)

can lead t o amorphization o f binary mixtures of crystalline powders. I n the literature this has been studie

d

extensively for mixtures contaming t i t a n i i ~ m group metals as one constituent. Although the system N i - N b was the first for which vitrification by

MA

was observed

111.

up t o now no systematic

investigation for mixtures containing

Nb

and a late transition metal are available.

T h e amorphization reaction is usually driven by the negative enthalpy o f mixin of the pure constituents.

Enthalpies

of

m i x i n g calculated from hLiedemats model

/2/

(see

Fig1

or G!bbls free enerwies of mixing calculated by more sophisticated methods (e.g.

CALPHAD

calculations

f )

or N i - N b and

COAL ¹³¹⁾

^indicate

Fig.1

-

Enthalpies of mixin for binary systems T N l x N b r ~ o - ~ calculated from Miedema's model as function o f comporition.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990410

C4-88 COLLOQUE DE PHYSIQUE

that vitrificatio~.~ ~1po1.1

MA

can be expected in further Nb-Thl systems. T h e fairly rapid diffusion of Cu,

Ni, CO,

Fe and

Cr

i n crystalline

Nb

(Fig.2) indicates that the kinetic conditions for amorphization may be favourable as well

/4/.

The fact that the phase diagrams of the mentioned systems (except for Cu-Nb) reveal intermetallic phases

/5/

renders amorphization even more probable. I n the present paper we present some results of a systematic investigation about the amorphization o f

Nb-TM

systems (TM=Cu,Ni,Co.Fe.Mn,Cr) by mechanical alloying and about the crystallization behaviour o f the a m o r p h o ~ ~ s C O - 4 b and Fe-Nb powders.

-

^{T I K}

2500 2000 1500

- :T,' 2740K

\

V)

C(

E

-

\ 0

18"-

0.4 0.5 0.6 0.7

Fig.

2 -

Arrhenius diagram of self- and impurity diffusion in N b according t o

/a/.

2

-

EXPERIMENTAL METHODS

Crystalline powders with typical sizes between 5 and 24 p m and purities between 99,8 and 99.95 at.%

were purchased from Cerac Inc.. Powder mixtures of N b and a single

TM

were mechanically alloyed under argon atmosphere by operating a planetary ball mill a t room temperature. The investigated compositions are shown in the following iist:

X-ray analysis was carried out in reflection geometry in a powder diffractometer using M d a radiation and a monochromator. Additional information about the morphology and microstructure of the powders and their temporal development was obtained with the aid o f optical microscopy. For this purpose the powders weie embedded in synthetic resin and then polished, so that the particles were cut. Subsequently, the samples were etched t o reveal the microstructure.

T h e crystallization of the C o - N b alloys was studied by differential scannin%calorimetry

(DSc)

using copper pans. The F e - N b powders were examined i n differential thermal analysis

(

T A ) experiments because their crystallization temperatures lie above 7250C.

3

-

RESULTS AND

DISCUSSION

-

Amorphization in T M - N b systems

Fig. 3 shows X - r a y diffraction traces o f T M - N b powders o f nearly equiatomic compositions after milling times between 50 and 100 hours. Amorphization is observed in all systems investigated except for C u - N b where the crystalline Bragg peaks remain even after 100 hours of milling and more..

23

/

d e g r e e

Fig.3 - X - r a y diffraction traces (Mo-Kci radiation) of TMxNbloo-, powders w i t h nearly equiatomic compositions prepared by mechanical alloying after long milling times.

Optical micrographs (Fig.4) and X-ray diffraction patterns ( F i g s ) of

CO;:N~?~

powder samples after various milling times reiveal the temporal development of the amorphization. F t o m the micrographs i t can be seen that the powder particles which originaily varied in shape and size have become almost spherical with diameters closely around l O 0 ~ t m after long milling times. Fig.4 further shows t h a t the amorphization process is completed after a milfing time of 50 hours.

Or!

the contrary the gradual disappearance of the residual Braf? peaks in the X--ray patterns of Fig.5 indicate that full amorphization is already achieved after 30 hours of m ~ ~ l ~ n g . This discrepancy between the microscopic and X-ray data can be resolved by closer inspection of Fig.4:

The micro raphs appear t o demonstrate that arnorphization proceeds from the surface region towards the centre of t t e particles. This finding makes i t obvious that X--rays with a typical penetration depth of about

10

t o 20 p m cannot detect the remaining ciystallites in the inner parts o f the particles.

In summary, it can be concluded t h a t for C o - N b amorphization appears t o be completed .after 50 hours of milling. Similar mi!ling times are necessary for the full amorphization o f F e - N b and Ni-Nb whereas C r - N b and M n - N b totally amorphize only after about 100 hours of milling.

CLI-NIO

which according t o Fig.1 has the smaiiest enthaipy of mixing and no intermetallic compounds i n its equilibrium phase diagram cannot be amorphized at all.

C4-90 COLLOQUE DE

PHYSIQUE

Fig.4

-

Optical micro raph of thvee

( l h 10 h 2 0 1 50 h).

mechanicall!! alloyed

C O ; ~ N ~ ? ~

powder samples after different milling times

2 3 / d e g r e e

Fig.5 -X-ray diffraction traces (h.4o-Ksrad;ation) for Co7sNb25 after different milling times.

-

Glass forming ranges

X-ray diffractometry o f the various compositions after sufficiently long milling times was used to estimate the glass forming ranges which are presented in Fig.6. The vitrification ranges for Cr-Nb and hln-Nb are within narrower lim~ts than those of C o a b and Fe-Nb. The glass forming range of Ni-Nb according t o Petzoldt et al.

/6/ has been added.

x i a t % TM

Fig.6

-

Glass forming ranges of mechanically alloyed T M - N b systems estimated by X-ray analysis (Ni-Nb according t o

161).

In Fig.7 the glass forming range of mechanically alloyed C o q b powders is compared with the vitrification range observed in splat cooling experiments /7/. I t is obvious that in the latter case vitrification is restricted to a narrower region around the deep eutectics a t near equiatomic compositions of the equilibrium phase diagram.

A comparison with theoretical predictions for the C o a b system is shown in Fig.7 as well. This comparison is based partly on CALPHAD calculations of the Gibbs free energies of the various crystalline and of the amorphous phase at 5800C

/3/.

Since amorphiration is indeed observed in our experiments i t is clear that the formation of crystalline equilibrium compounds must be somehow frustrated by kinetic constraints. Assuming a metastable equilibrium between the amorphous phase and either the Nb rich or the CO rich terminal solid solution a range for the single phase glassy state is obtained which is narrower than the observed glass forming range (see Fig.7). Usin the concept of "polymorphic phase diagrams" /9.10/ pol\fmorphic vitrification of the terminal crystalltne soli8 solutions is on the other hand possible m a composition range which is wider than the observed glass forming range (see Fig.7).

Using thermodynamic criteria Johnson 9/ has pointed out that similar to superheating also increasing chemical disorder can lead t o an ultimate instabi

f

ity limit of a crystalline solid. The effect of chemical disorder has been

.

treated in a simple model by Egami and Waseda /11/ for a random binary solution containing atoms of two different atomic volumes

VA

and

VB.

According to their approach a solution of small

A

atoms (large

B

atoms) in a matrix of large

B

atoms (small A atoms) becomes topologically unstable wher, the concentration of small (large) atoms reaches a critical composition of

respectively. If one assumes that an amorphous phase is formed whenever the competing solid solution is unstable the Egami criterion provides a further estimate for the ranFe of glass formation. Using X

=

0.07 for the unknown constant values of X between 0.07 and 0.09 were

determined

by Lion and Chen /12/j and the atomic volumes of CO and

l % !

b

(VCo

= 6.7 cm3 mol-l, VNb = 10.87 cm2 mol-1) we get the limits shown in Fig.7.

It

is obvious that none of the "theoretical predictions" is in full agreement wlth the experimental results. For the Egami criterion this is hardly surprising in view of the simplicity of the model which takes into account only atomic size mismatch but no electronic effects. The "CALPHAD predictions" refer to the tem erature of 580", for which the Gibbs free energies have been calculated whereas our ball milling experiments, aehough the ball mill is operated at room temperature. can involve very non-isothermal conditions. Furthermore. Fecht et al.

/13,14/ have shown that the heavy deformation durin the ball milling process reduces the grain size of metals to nanometer scale. The enthalpy stored in nanocrystAine materials in particular in the form of grain boundaries can be a significant amount of the enthalpy of fusion. Stored enthalpies of about

2

kJ mol-l have been measured by thermal analysis of Nb samples. Such values are substantial and should be included into a thermodynamic description of the ball milling process.

COLLOQUE DE PHYSIQUE

A t o m i c P e r c c n l C o b n l l C o

Fig.7 - Glass forming range of the CO--Nb system estimated by X - r a y analysis and compared with other l t x p e r i m e ~ t a l /7/ and theoretical results

/ 3 / .

For comparison the equl!ibrium phase diagram is also shown.

-

Crystallization behaviour

T h e C o a b and Fe-Nb alloys were examined with

DSc

and

DTA

measurements. respectively. Crystallization t e m p e r a t ~ ~ r e s

T,

(taken as peak temperatures) were determined with a heating rate o f 10 K / m i n .

Tx

values of C o - N b are shown in Fig.8 and compared t o corresponding data of rapidl!. quenched alloys /S/. Contrary t o the present

T,

v a l ~ ~ e s . the latter crystallizatiorl temperatures were measured with a heatln rate of 20 K / m i n as onset temperatures. Nevertheless, the good agreement between both sets of data c a n t e seen in Fig.8. Fig.9 shows the crystallization temperatures of F e - N b investigated by

DTA.

For this system no i~alues are available in the literature.

S p l a t c o o l i n g

550 ^{1 l}

l

0 10 20 30 40 50 60 70 80 90 100

Fig.8 -

Cr!tstailization temperatl~re

T,

of mechanrcally alloyed

CO,N~~O!,-~

as a function of composition compared t o splat cooling data /g/.

I l

Fe-Nb

# f

i t ? ^{f f}

v ,

.

^i.Peak

+

.

^2.Peak

,

^I

Fig.9 -Crystallization temperatures Tx of mechanically alloyed FexNbfoo-x

as a

function of composition determined from DTA measurements.

ACKNOWLEDGEMENTS

We are grateful to Dr. F. Sornmer and Prof. B. Predel (MP1 fur Metallforschung. Stuttgart. FRG) for their help with the DTA measurements. to Dr. N.A. Stolwijk for helpful comments and crlticai reading of the manuscript.

and to cand.phys. Th.

Sell

for the X-ray pattern of the NiS0Nbs0 powder. We also acknowledge valuable discussions with Dr. H. J. Fecht and Professor \Rd. L. Johnson.

REFERENCES

1

l/

Koch: C.C.. Cavin. O.B.. MC Kamey, C.G. and Scarbrough J.O. Appl. Phys. Lett 43 (1983) 1017.

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1 0 0

(1980

1.

2

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/

4 /

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ress

in Materials Science 3 (1986) 81.

/ l O / Johnson. W.L.

~ e c l t

H. J J. Less-Common Metals (1988) 63.

/11/

Egami, T., Waseda. Y., J. Non-Cryst. Sol. @(1984) 113.

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Rev.

831 (1986) 8238.

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/14/ Fecht H.J. Hellstern, E. Fu Z.. Johnson, W.L., f\/letall. Trans., in print.