MA OF BRITTLE ELEMTNS.pdf

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S c r i p t a M E T A L L U R G I C A V o l . 2 1, p p . 3 0 5 - 3 1 0 , 1 9 8 7 P e r g a m o n J o u r n a l s , L t d .P r i n t e d i n t h e U . S . A . A l l r i g h t s r e s e r v e d

MECHANICAL ALLOYIN G OF BR ITTL E COMPONENTS:SILICON AND GERMANIUM

by R. M. Davis and C. C. KochD e p t. o f M a t e r i a l s S c i en c e & E n g in e e r i n g

N o r t h C a r o l i n a S t a t e U n i v e r s i t yBox 7907, R a le igh , NC 27695 -7907

( R e c e i v e d O c t o b e r 6 , 1 98 6)( R e v i s e d D e c em b e r 4, 1 9 86 )

I n t r o d u c t i o n

Mechan ica l a l lo y i ng (MA) i s a p rocess whereby a b lend o f e lemen ta l o r a l l o y powders i ss u b je c te d t o h i g h l y e n e rg e t i c c o mp re s si v e imp a c t f o r c e s re s u l t i n g i n t h e f o rma t i o n o f a c om-p o s i t e p o w d e r ( I ) . T he re s u l t a n t p o w de r d e v e lo p s t h ro u g h t h e re p e a te d c o l d -w e ld i n g a nd f r a c -t u re o f t h e p o w d e r p a r t i c l e s w i t h a f i n a l c o mp o s i t io n c o r re s p o n d in g t o t h e p e rc e n ta g e s o f t h er e s p e c t i v e c o n s t i t u e n t s i n t h e o r i g i n a l c h ar ge ( 2 - 3 ) . T h i s hi g h e n er gy b a l l m i l l i n g t e ch n i q uei s u n i qu e i n t h a t a l l o y f o r m a t i o n i s a s o l i d s t a t e p ro c es s wh ere s om e o f t h e r e s t r i c t i o n s o fe q u i l i b r i u m p h a s e f o rma t i o n may b e o ve rc ome (2 ) .

H e r e t o f o r e , m e ch an ic al a l l o y i n g h a s b e e n m a i n l y u t i l i z e d i n t h e p r o d u ct io n o f d i s p e r s i o ns t ren g t h e n e d a l l o y s ( 2 -6 ) . Wh i l e e n c o u ra g in g su cc es s h as b e e n me t i n t h i s a p p l i c a t i o n o f MA,t r u e a t om i c l e v e l a l l o y i n g a s a r e s u l t o f t h i s p ro c es s i s s t i l l a t o p i c o f m uc h d e b a te . H o w -e v e r , r e c e n t ex p e ri m e n ts s ee m t o i n d i c a t e t h a t t h e m e ch a ni ca l a l l o y i n g t e c h ni q u e c a n, i n f a c t ,

be employed to cre ate homogeneous a l l o y s .B en ja m in (2 ) ma ke s re f e re n c e t o t h e u s e o f MA i n t h e p ro d u c t i o n o f a t r u e s o l i d s o l u t i o n

o f N i -C r . N ick e l , be ing fe r r om ag ne t ic , and ch romium, were b lended and m ech an ic a l l y a l lo ye dfo r 10 -15 hou rs . The re su l ta n t powder demons t ra ted a magne t ic response id en t i c a l t o th a t o fa s i m i l a r a l l o y f o r ~ d b y m e l t i n g a nd w o r k i n g . M o r e r e c e n t l y , M c De rm o tt a nd K o ch ( 7 ) e m-p loyed mechan ica l a l lo y i ng to c re a te o rde red bcc be ta b rass f rom fcc coppe r and hcp z incpowders.

U n t i l n o w , m e c ha ni ca l a l l o y i n g h as b e en a p p l i e d t o r e l a t i v e l y d u c t i l e s ys te m s i n w h ic ht h e mec ha nis m o f r e p e at e d c o l d w e ld in g a n d f r a c t u r e h as b e e n w e l l d e f i n e d m e t a l l o g r a p h i c a l l y .Wh i le b r i t t l e c omp on en ts h av e b e e n i n t r o d u c e d i n t o t h e MA s y n t h e s i s o f n i c k e l - b a s e d s u p e r -a l l o y s ( 2 - 4 ) , i t i s n o t a p pa r en t t h a t m e c h a n ic a l a l l o y i n g i s f e a s i b l e w h en a ) l p o w d er c om -p on en ts a r e b r i t t l e . E x t e n s i v e p l a s t i c d e f o rm a t i o n d u r i n g t h e f o r m a t i o n o f t h e i n i t i a l c om -po s i tes o f t he powder componen ts has been ex pe r im en ta l l y obse rved ( 1 ,2 ) and may be es se n t ia l

t o a l l o y f o r m a t i o n by t h i s m eth od . T o e v a l u a t e t h i s p o s s i b i l i t y , s e v e r a l s ys te m s w ere s u b-jec ted to MA in wh ich bo th components we re b r i t t l e a t amb ien t tem pera tu re .

T he f o l l o w in g s umma riz es t h e i n i t i a l r e s u l t s o f a s t u d y a ime d a t u s i n g me c h a ni ca l a l l o y -ing to p roduce compos i te powders f rom e lem en ta l s i l i c o n and ge rman ium. These b r i t t l e e l e -me n ts h av e b e e n ma de t o f o rm a d i a mo n d -c u b i c s u b s t i t u t i o n a l s o l i d s o l u t i o n b y MA. E ne rg yd i s p e r s i v e x - r a y a n a l y s i s , s c an nin g e l e c t r o n m i c r o s c o py , and x - r a y d i f f r a c t i o n w ere u t i l i z e dt o c h a r a c t e r i z e t he e f f e c t s o f t h e MA p ro c es s w i t h r e s p e c t to t h i s b r i t t l e a l l o y s y s te m .

Exper imenta l Procedure

Two in i t ia l com pos i t ions were cons ide red in the S i -Ge sys tem; Ge-72 a t .% S i and S i -50a t .% Ge. The s i l i c o n and ge rman ium s ta r t in g powders we re ob ta ined f rom A l fa p roduc ts w i tha s t a t e d p u r i t y o f 9 9 .9 % a nd 9 9 .9 9 9 % , re s p e c t i v e l y . T he i n i t i a l c h arg e w as l oa d e d i n t o ac y l i n d r i c a l , s h o c k r e s i s t a n t (S 5) t o o l s t e e l v i a l ( 76 x 57 mm d i a m e t e r) a lo n g w i t h 440Cstainless steel bal ls (7.9 mmdiameter) as the mil l ing media. The vial s employed an o-ring

seal to contain the internal atmosphere and the ball-to-powder weight rat io was held constantat 5:1. Mechanical al loying was carried out with the Spe~ Industrles 8000 Mixer/Mi ll .

305

0 0 3 6 - 9 7 4 8 / 8 7 $ 3 . 0 0 + . 00

C o p y r i g h t ( c) 1 9 8 7 P e r g a m o n J o u r n a l s L t d .



3 0 6 M E C H A N I C A L A L L O Y I N G O F S i A N D G e V o l. 2 1, N o. 3

Both compositions were ball milled in both air and inert argon atmospheres. The MAprocess was carried out over a period of 8 hours with a powder sample taken for analysis at

l hour in tervals. Care was taken to insure that the samples loaded under argon were not ex-posed to air during processing and sampling to el iminate oxygen contamination. At eachsampling during the 8 hour MA run, a partial vacuum was encountered in those charges loaded

in a ir. This indicated powder oxidation had occurred during processing. No such observationwas made in the samples loaded under argon.

Since Si and Ge are isomorphous and exhibit a nearly linear composition dependence oflat t ice parameter, calculated l at t ice parameters were used to ve ri fy al loy formation andcomposition. Lattice parameters obtained were compared to those predicted by Vegard's "Law"as well as those provided by Olesinski and Abbaschian (8). X-ray diffrac tion was carriedout using copper K-alpha radiation and a nickel f i l te r in conjunction with a Debye-Scherrerpowder camera. Scanning electron microscopy was used to analyze MA powder morphology.Energy Dispersive x-ray analysis, performed by the Tracor Northern TN5500 X-ray Analyzer,was ut il ized for both qual itat ive and semi-quantitative powder characterization. EDX sampleswere mounted and polished using standard metallographic procedures which faci l i tated composi-tional determination and reduced errors to within approximately 5%.

Results and DiscussionX-ray dif fract ion of the Ge-72 at.% Si air-loaded samples revealed that af ter 7 to 8

hours of MA, a solid solution had formed. With a measured lat t ice parameter of a = 0.54921nm, deviation of this value from that of a simil ar melt-formed al loy (8) was -0.002%. Alsopresent on the x-ray pattern were di ff raction lines corresponding to SiO2 and GeO. Theseoxides were detected as early as MA=I hour and remained, although at low concentrations,throughout the course of the run.

MA in an inert argon environment yielded somewhat different results. While a Si-Geal loy was produced af te r 4 to 5 hours, the calculated lat t ice parameters indicated a sh iftin composition from approximately 37-42 wt.% Si , balance Ge, during the remaining 4-5 hoursmill ing time. Figure l demonstrates th is compositional shi f t as well as the abrupt tran-si tion from elemental Si and Ge to the al loy . Oxides of the elemental constituents werenot detected by x-ray analysis and also no partial vacuum was encountered during sampling.

At the composition corresponding to (Si-Ge) 50 at.% and MA=8 hours, the calculatedlatt ice parameter of the composite powder formed in both air and argon atmospheres corres-ponded to an alloy having a composition of 20 atomic % Si, balance Ge, i .e . , a larger thanpredicted lat t ice parameter. This deviation from Vegard's "Law" was also demonstrated af terMA=8 hours for all compositions studied with less than 72 at.% Si as shown in Figure 2.Several attempts were undertaken to try to explain th is peculiar alloying behavior whichseems to be unique to the Si-Ge system. Similar investigations (2,7,9,17) into mechanicalalloying have yielded expected resul ts in that the final composite powders possessed correctcompositions. Ini t ial screening of the start ing powders and subsequent MA of selected par-t ic le sizes indicated this anomalous alloying behavior was not a function of powder size.The presence of l ine broadening in the x-ray d iffrac tion photographs indicated that residualstress could be affecting the ab i l i ty to accurately resolve di ffr acti on lines. The powdersamples were subjected to a heat treatment designed to reduce the residual stress effectswhile avoiding temperature regimes favorable for noticeable diffusion. Subsequent x-ray

di ffract ion analysis demonstrated a marked decrease in line width but calculated la tt iceparameters closely marked pre-annealed values. Recently, Poli t is and Johnson (18) util izedmechanical al loying to synthesize amorphous alloy powders from Cu and Ti . During their in-vestigat ion they reported having to mechanically scrape the vial and balls to ensure homo-geneous composition of the f inal powder. Similar procedures were incorporated into the MAof si licon and germanium with no posit ive results realized. I t is assumed that the observedalloying behavior is caused by the reactivi ty of Si towards the mill ing media and vial walls.This leads to powder buildup and consequently a preferential loss of sil icon which has beenfound not to be simply a by-product of powder oxidation. In fac t, semi-quantitative energydispersive x-ray analysis of powder taken from the vial walls indicate higher than expected

si li con concentration levels.

Energy dispersive x-ray analysis was used to both ident ify the presence and re lat iveamounts of impur ities. In a ll samples tested, iron was detected at a maximum concentration

of 0.5 at.%. Iron contamination Is a result of vi al and mi l l ing media wear. Enhanced



V o l. 2 1, N o. 3 M E C H A N I C A L A L L O Y I N G O F S i A N D G e 3 0 7

powder homogeneity has been achieved by the replacement of the steel mi l l ing media with

transformation toughened Zr02 ba lls.

An interesting observation was made during th is study while characterizing part ic lemorphology. In every system containing br i t t l e components in which mechanical al loying has

been attempted, these being Si-Ge, Mn-Bi, and alpha quartz, numerous cases of interpart icle

necking have been observed. Figure 3 shows such necking in air-loaded Ge-72 at.% Si mechani-ca ll y alloyed for 8 hours. These necks are present as early as MA=3 hours for air-loadedsamples. Similar observations were made in argon-loaded samples where necking occurred as

early as MA=I hour. Simi lar necking observations have been made in which the in i t i a l com-ponents were duct ile (12). This seems to indicate that int er -pa rt ic le necking is a phenome-non not exclusive to br i t t l e -br i t t le MA systems. While a great majority of the observednecks demonstrate a re lat ive ly smooth surface morphology, Figure 4 shows the internal struc-ture of such a region. Rather than being a homogeneous particle, the MA powders appear tobe composed of smaller parti cles cold-welded together.

The underlying mechanisms responsible for the in te r-pa rt icl e necking require furtherstudy for resolution. I t may be that localized heating increases the pl as ti ci ty of thematerial. This possibly ini t iates an extrusion process leading to the observed necks. How-

ever, calculations aimed at predicting bulk as well as microscopic temperature rises indi -

cate otherwise. Using Schwarz's analysis (9) , which assumes bulk temperature increases asa result of localized shear of powder part ic les trapped between two coll id ing bal ls , thetemperature increase,aT, is given by

~t )½ FI]TZ (~ Ko pp Cp

where F : onV = 199.08 Mj/m2s is the dissipated energy f lux, at = 2d/v s the stress state

li fet ime , v s the longitudinal wave veloci ty, v r = 2 m/s the rela ti ve veloci ty of the ballsbefore impact, Cp the specific heat of the powder, Ko the thermal conductivi ty of thepowder, d = 0.0079 m the ball diameter, PD the powder density, and on the normal stress de-veloped by the3head-on co l l ision of two b~ll s. Considering pure Ge, with Ko = 58.6 J/m.K.s,PD : 5324 kg/m , Cp = 321.7 J/kgcK, AT = lO.l K. Applying th is method to pure Si, wi thKb = 149 J/m.K.s, pp = 2330 kg/mJ, and Cp = 677.96 J/kg.K, AT = 6.57 K. In the above calcu-lations, the normal stress was taken to be the maximum compressive stress generated by thehead-on co ll is ion of two balls. Under he given conditions, th is value can be determinedby the relation (14)

DI + Dr 2 I/3

~n = °c : 0.616 [PE~( D-- -~) ] [2]

In equation 2, E = 206.92 MPa is the elasti c modulus of the balls, P = (l .O l x I0-3)a = 7.691 x lO-4 kg the load, and a = 0.762 m/s 2 the approximate ball acceleration. Sub-

stituting these values, on is found to be 99.54 MPa.

Another method (lO), which considers heating on the microscopic level as a resul t ofsl id ing f r i ct ion, gives simi lar results. This method considers a system in which a body ismaking contact with another over a limi ted area and moving over the surface of the otherbody at constant ve loci ty. The contact area is considered to be square. Under hese con-

di ti ons, the microscopic temperature rise is given by

fW vr [ 3 ]

AT :7[.~4 ~j(K 1 + K2)

Where f = 0.6 is the fr ict ion coeff ic ient , W the load, l the half length of the side of thecontact area, J the mechanical equivalent of heat, and K , K2 the thermal conductivity of

the respective bodies. Considering Si-on-Ge wi th Kl and K2 equal to 149 and 56.2 J/m.K.s,respectively, and f = 0.6, equation 3 reduces to

AT = 1.4068 x 10 4 (~) [4 ]

Using the normal load calculated above and l = 0.025 x lO -6 m, aT = 4.32K.

These calculated values however are not consistent with measurements. Mi l ler et a l . ,using microsecond time-resolved radiometry, observed temperature increases on the order of400 to 500°C upon impaction of NaCl crystals ( l l ) . More recent ly, McDermott and Koch (12)



3 08 M E C H A N I C A L A L L O Y I N G O F S i A N D G e V o l . 2 1, N o. 3

have observed similar higher than predicted temperature r ises during phase transi tion exper i-ments.

Complex stress states in the compressed powder part ic les of fer yet another possible ex-planation for the observed necks. I t is known that some b r i t t le materials under the in-

fluence of either a pure hydrostatic stress or a hydrostatic plus a superimposed tensilestress (15-16), can demonstrate duct ile behavior. During the MA process, th is may very wellbe the case. Easterl ing and Tholen (13) have suggested a paral le l explanation. When twospherical par tic les come into contact and bonding occurs, storedelastic stress in the con-tact region can in it ia te diffusional currents that could lead to neck formation.

Summary

Mechanical alloying has been used to form a true sol id solution from br i t t le , elementalstarting Si and Ge powders. A Ge-72 at.% Si al loy was produced af te r 8 hours of ball mil l -ing with a calculated lat t ice parameter approximately equal to that o f a simi lar melt-formedal loy. Compositions in the Si-Ge system with less than 72 atomic percent Si have also beenalloyed with f ina l compositions deviating from predicted values.

In ter-part ic le necking has been observed in the b r i t t le systems Si-Ge, Mn-Bi, and inalpha-quartz af ter varying degrees of mechanical al loying.

Further work wi ll focus on following the structural evolution of the Si-Ge solid so-lution from the br i t tl e Si and Ge starting powders. Microanalytical tools including SEMand TEM wil l be used. The feasib i l i ty of MA br i t t le components opens up a wide range ofposs ib il it ies for the synthesis of new materials.

Acknowledgements

This program is supported by the Office of Naval Research, contract # N-OOO14°84-K-0253.

References

I. J. S. Benjamin and T. E. Volin, Metal l. Tran., 5, 1930, August 1974.2. J. S. Benjamin, Sci. Am., 40, 234, 1976.3. P. S. Gilman and W. D. Nix, Metall . Tran., 12A, 813, May 1981.4. G. H. Gessinger, Metall. Tran., 7A, 1203, August 1976.

5. J. S. Benjamin, Metall . Tran., l , 2943, October 1970.6. J. S. Benjamin and M. J. Bromford, Meta ll . Tran. , 7A, 1301, August 1977.7. B. T. McDermott and C. C. Koch, Scripta Metallurgica, 20, 669, 1986.8. R. W. Olesinski and G. J. Abbaschian, Bul let in of Alloy Phase Diagrams, Vol. 5, No. 2,

180, 1984.9. R. B. Schwarz and C. C. Koch, Appl. Phys. Lett. 49, 146 (1986).

lO. H. S. Carslaw and J. C. Jaeger, "Heat Conduction-i-n Solids", p 255, Oxford UniversityPress, New York, 1959.

I f . P. J. Mil ler , C. S. Coffey, and V. F. DeVost, J. Appl. Phys., 59(3), 913, February 1986.12. B. T. McDermott, unpublished result s.13. K. E. Easterling and A. R. Tholen, Phys. of S intering, Special Issue, 77, 1971.14. R. J. Roark, "Formulas fo r Stress and St ra in ", p. 275, McGraw-Hill Book Co., Inc. ,

New York, 1954.15. P. W. Bridgman, J. Applied Physics, 18, 246, February 1947.

16. P. W. BridgmaD, Phy. Review, 48, 825, November 1935.17. Pee Yew Lee, unpublished result s.18. C. Poli t is and W. L. Johnson, J. Appl. Phys., 60(3), I147, August 1986.



V o l . 2 1, N o. 3 M E C H A N I C A L A L L O Y I N G O F S i A N D G e 3 0 9

0 . 6 8

IZ. . 8 , I Q ~ ,~0.58"

( e l - o e )

i - I I -I0 . 5 4

0 . 8 31 2 3 4 5 6 7

MA T i me { hou r s )

Fig. I. Alloying behavior of argon-loaded Ge-72 at.% Si.

0 . 5 7 0 1

. . . . Measured a l t ice

- - Predicted latt ice

^ 0 . 6 8 0 4 \ \

0.550

0 , 6 4 00 1 '0 20 30 4 '0 50 6 0 7 '0 80 90 100

Atomic % Si

Fig. 2. Lattice parameter variat ion as a function of at.% Si.



3 1 0 M E C H A N I C A L A L L O Y I N G O F S i A N D G e V o l. 2 1, N o . 3

Fig. 3. In te r- pa rt ic le necking in ai r-loaded Ge-72 at.% Si (MA = 8 h)

Fig. 4. Int ernal microst ruc ture of air -loaded Ge-72 at.% Si (MA : 8 h)

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MA OF BRITTLE ELEMTNS.pdf