Structural aspects of polytype multilayer phases in metals and alloys

Scripta METALLURGICA

Vol. 17, pp. 699-704, 1983 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

STRUCTURAL ASPECTS OF I~LYTYPE HULTILAYER PHASES IN ~ T A L S AND ALLOYS B. I.NIKOLI N Institute of Metal P~ysics,Academ~ of Science of the Uk~SSH, Kiev,USSR (Received December 20, 1982) (Revised April i, 1983)

Polytypism phenomenon was first discovered by Be,,mb~er in 1912 for SiC and then was found in -w_ny other lamella2 semiconductor and molecular cr-gstals~ ZnS, Cdloetc./~/.Identical structure of close-packed atomic planes and their diff~Tent stacking order in various pclyt~pes is the structural criterion of polytype modifications of the same material. As a result the unit cell parameters in a close-packed laye~ coincide (a = b), but the parameter along a normal for layers is different: c = Nd (N is the number of close-packed layers in the unit cell, d is the interplanar ~istance between these Is~ers). Among more tha~ 150 pol~types of SiC and Zn6 there are such unit cells having up to i00 or more the close-packed planes /I/. As the parameter c>> a,b such structures are often designated as the long-pe~iod ones. The c~ystalline structures with N = 10-20 in the unit cells are formed also in metal alloys in g~owth i~om the melt or after high temperatume annealing These are the intermetallic compounds of the AnB m %l~pe (Laves phases, an%-iphase domain struotumes) which consist of diffa~ent packing of identical atomic is~e~s and they may be considered as equilibrium p o l ~ y p e struq%ltEes also. necentl~ the metastable m-rtensite phases with multilayer (long period) cryst~lline lattices which sue unusual for metals and alloys were found after martensitic ~ans~ormations in metal alloys with low stacking-fault enerKy /2,3/. Their ,,nit cells have sometimes up to I00 or more the identical closepacked planes and in X-ray photographs of such single crystals the reflections from parallel planes of the matrix fcc s ~ u c t u m e and martensite NR 'one coincide. As the corresponding interplane distances are equal or nearly equal (dlll foo = doool NR and d220 foe = dll0 NR ), both the interatomic distances of close-packed planes and intervals between the atoms of neig~bou~ing close-packed planes end fcc --~ NR transformation are not changed whereas the stacking order is chan6ed. This requirement agrees with structural po~vt~pism criterion /I/, therefore a conclusion was drawn /@/, that the lon~-period ma~tensite phases of close-packed s~uctlu~es in the metal alloys m~y be considered as m ~ _ tensite pol~t~pes. Now we consider in short the crystalline s~uotu~e of pol~type martensite phases and the metal alloys in which they are formed. _ Feuba.sed all~x~.~ During quenc"b/~g Fe-Mn (Mn>lO.O~) alloys alloye~ with CarPOrt /~/, n i ~ o g e n />/ ann copper /6/ to a ~empeza%u~e lowe~ than 20 C, the m~r~ensite $ (2H~ a n d ~' (~SR~) phases are formed from fcc y-phase (Table~). ~x~n increasing concen~ation ~f these elements the following sequence of s~uctl/2es is observed! 2H ---~ 18R 1 --~ Zcc Concentration boundaries of these s~uctumes are shifted depending upon m a ~ i x -phase pretreatment (multiple phase ~-~ ~ transformations, nlastle ~ e ~ n ~ m . e ~ . /. ~ne . ~ i .~artensite crystals a~e like #h~n plates to be o~iented parallel

!~

o o?~eo~a~

~-,~-Uzcc p~anes. ~u~n~ zo~ation oz the l ~ l phase (Zoo ~ z a ~ )

ane ~ne ~nverse Vransformat~on ~nto ~"-phase (18H1 --~fcc) some physical 699 0056- 9748/83/060699-06503.00/0 Copyright (c) 1983 Pergamon Press Ltd.

700

POLYTYPE ~RILTILAYER PHASES

Vol.

17,

No. 6

p~operties (electrical resistance, paramagnetic susceptibility, volume, strength and plastic characterlsticsJ are changed in a stepwise manner. Therefore, these transformations a~e first-order. In structural aspect, the fcc lattice of t~e ~--phase and the two marteneite 2H and 18R 1 structures represent .~he po~ytypes in Fe-based alloys as their ,injt cells cGnslst D~6var~ous suacKxng Oridentical atomic close-packed planes snowed by auomic ~ nets. Co-based allo~s.-With regard to the crystalline structure of martensite, the binary cobalt alloys may be divided into two g~oups: 1. the alloys with unlimited solubilit~ of alloying element (Co-Ni, Co-Mn, Co-Cr), in which t h e / J ~ (fcc .~ hcp) m-rtensite transformation takes place and the ~ - p h a s e as well as pure Co has the 2H (hop) lattice! 2. the alloys with limited solubility of alloying element (Co-Cu, Co-Ti, Co-Nb, Co-Ta) in which the various polytype multilayar NR structures are formed during the ma~tensitic transformation (Table 1). According to /2/, the multiAayet polytype ma~tensite phases with long-period unit cells (.unusual for metals and alloys) occur with the number of closeTpacK^ed layers acnxevxnog va-ues oI more then i00. For example, the c~'-phase_tGo+lu.u~ uu alloyJ IS cna~ac~erlsea by a rhombohedral lattice (space g/cup R3m) in the "n~t cell of which there are about 126 layers with parameters: a ~ 0,2526 nm, c = 25.9230 nm in the hexagonal axes and a = 8.6~20 nm and ~ = 1 42' in rhombohedral ones /8,9/. Therefore in Y-ray photographs of such structures the diffraction patterns with large differences in intensit~ are observed (Fig.l). Nith increasin8 alloyed element concentration, the sequence of multilayer phases 2H --~ ~R --~ fcc is observed in metastable phase diagram, the NR structures involve quite a number of multilayer polytype structures. In Co-A1 alloys (Fig2) for e ~ l e , the multilayar m-ytensite phases have four latticesIat leas~ | 2H, 4~R, 84R and 126R /4,10/, but in Co-Nb - 15RI, 126R and 2H /ll/. t is for all the cobalt systems that the phase transformation does not take place in the alloys having a still ~ e a t e r content of alloyed element, the initial fcc lattice phase instead being maintained. The crystalline lattices of polytype structures occu~rlng in Co-, Fe-Nnbased alloys are not perfect but involve a large density ( X ~ 0.i) of random stacking faults. As a result, in X-ray photographs the diffraction patterns a~e ver~ much brgadened end continuous diffusion intensity strips appear betwean them (Yig.l). In some alloys several multilayer phases a~e formed on tensitlc transformation. Therefore, as a result of extrapolation of extended diffraction patterns, the continuous diffusion strips of weakly expressed maxims being analogous with two-dimensional diffraction are observed in X-ray photographs of the single crystals. The polytype phase crystals involve thin plates oriented in parallel to(lll)fc c planes and some of theLw properties are changed durin~ formation. Gu-. A~-° Au-based allo~s.- The ma~tensitic transformation in b~n~_'r~ CU-, AS-, Au-based alloys (except Cu-Si, in which the ~ - p h a s e has a nonordered fcc la~tlce) takes place in the ~ - p h a s e /12/, which has an ordered boo lattice. The ma~tensite innon-ferrous alloys f o x e d upon quenchln~ without deformation has predominant two lattlces: 2H and 9R (Table i). Due to atomic ordering,howeve~, in the martenslte phase and the difference between at~mlc radii of the alloy components the 2H ma~tensit structure is distorted to a monoclinic one and the c period of the phases with rhombohedral symmetr~ is doubled, whereby the 9R lattice becomes the 18Rl structure. Besides these two structures in Cu-al-Ni alloys, under plastic deformation a number of other close-packed maztenalte phases with multilayer lattices (fo~ example, 18Ro, 90R ) are formed directly from the matrix ~ - p h a s e o~ f~om martensite (marte~site ~ martensite transformation) /13, i@). The martensite phases with multilayer lattices are formed also in Ti-Ni alloys of near equiatomic composition.Thus, the ma~tensite phases in non-for.rous alloys aye characterised b~ the close-packed s t r u c ~ e s having more complex close-pacKed atomic planes (H-nets, T-nets) with structural polytypism cna~ao~erls~-Acs. ~nererore they may oe considered also as polytype ma~tensite modifications. The fo~mat-lon of maytensite polytypes in the metal alloys results f ~ o m f i r s t orde~ phase transformations in the solid stste and considered as one dimensional polymo~phism /i/.

Vol. 17, No. 6

POLYTYPE MULTILAYER PHASES

TABLE 1 Crystalline Lattices from M a ~ e ~ % e

i

lsys~en

..... 1 I

Co~osition

I i

~ 15. cu-~Z leRz

8. Co-~e

2H

ZO. Co-~ 11. C o A l

12. Co-Nb

i~. Co-Sn

~i.on

10.O~LI zz.O-l~.~1 z.~.o-z4.o.~l ZFm± Z6. c~-al-al 1~.o~&1 ~.~ 18aI -"-"as --~ -"~.~ 17. cu..al-~: za.o'l.al

0-1.0%~e

o6

1.0-5.5~e 0-?.O~u 8.O-lO.O~Ou O-~.O~T~ ~.o-6.o~±

~ d,. di' ~ o~'

~S 18. C~-Sa 2H 126R1 19. Cu-Za ~ z~:~ z

0-~.0%AI z~.O-6.0~Al

~' o~

2.fl 1~ 1

~.o~.o~z ~.o-6.~/~z 4..0-.6.0~A1 O-2.0%ab 5.o-6.om~ro 0-~.0~ma

oc~ z~..:m2 ~ ~ ~C~ Z~aR1 21. A~-al ~ 2H ~-" 1.SR1 2.2. A¢-Ccl oc' 3.26~]. ~ 2S

a~.og,ca .6.0~a

5.0-12.0%Ta O-~.O~Sn 5.O-Z2.0~Sn

o6' 1261{1 ~ ~ o~ BaR1

~.8-5.~Y=61 4..5-~.8~Si ~5.0-55.0~i

5.o-za.o~sn

~'

5.0-12.0~8n

oC' I~4R 1

5.o.z2.o~.,

~'

~, O=J+.0%~b

13. Co-~a

Compo=~-

I I

z. Pe-an zS.o-25.o~a~ ~ 2. ~e-m~-c zo.o-2o.o~ 6' o.2~-o.eo~c 3. ~e-Mn-a ZO.O-aO.OYa~ 6 O.lO-O.3O~a ~. ~e-Ua-cu : ] o . o - ~ . o ~ ~' z.o-5.o~c= 5. Co-a~ o.o-~o.~a~ oc e. co-an o-as.o~an ~c 7. Co..Cr o-35.o~'~ o~

9. Co-Cu

in H e t ~ Allo~s

~mse ~l;~=e~W STs~en .

701

zo~R1 z56a z

l~Je

I

~'ioe

~' 9H ~' IeR~ r~' F='

o~ ea ~ ' zea:. ~j zaa~ jS~ 9o~ ~'

5 . ~ Ha

20. A~-G~

2~. Cu-Si

24. mi~Ni

~.O~Sa ?..5.0~n ~8.0~Zn ~.O~Z: ~o.o~z= ~O.O~Zn ~5.0 a t . ~ a

~6.o ~.o 6.2.5 7.16

,.~.~,a ,, :.m~.a ~d~.l ~f.al

~ ~.O~Ccl

~8~ ~ j~' 91' ~

i~ 2B ~C Y~+9~ 9~

9R+]I~T ~' ~O

.#~ r; ,,SJ 99. T~ ?..K g.,' ~ 9~ ~' ~" 9R ~c 2H ~ 9B

~5.o-55.oa~.~ml oE 9~ ~5.0-55.0=~.%Ni ~

18R

702

POLYTYPE MULTILAYER PHASES

Vol. 17, No. 6

Fig.la. Rotating Y-ray photograph of single crystal Co+I0.0~C~ after quenching f~om^1200 C and cooling to -196~C. dJ-m~tensite witJ1 126Rl lattice.

Fig.lb. Rotating X-ray pho~og~aph of sin61e cz~stal Co+12.0~Cu afte~ water quenching f~om i~00 C and cooling to -196vC. ~<' -martensite with disozdered sequence of close-packed planes.

~ ,00

i i

|

~

~

!

Fig.2. The Co-rich p a ~ of the Co-A1 ~iag~am. M=l-ma2tensite point~ on wate~ quenching, M . . - o n aging at 700~ for lb.

~

FCC

~ \

the boundary of eolu-

i

-.o

bility of A1 in Co.

I" I Co

t

4

6

8 Ae, w'l;%

Vol. 17, No. 6

POLYTYPE MULTILAYER PHASES

703

Slio menh.ni~m and formation orincioles of the tahiti!aver m~ten~ite ~ - The fcc lattice transfo~min~ into different close-packed m u l t i la~er ma~tenslte structures Qfcc ~ ~ ) and as well the foe r 2H transformatlon in Co may be represented by me-n~ of simple periodic slip d u e to the movement of partial di~locations a/~6~ ~ 112> in {iii~÷~. planes of the m a ~ i x phase or by a mare complex process involving ordered ~VVdeformation producing s~acklng faults. As the c~ystallo~aP~7 of the partial dislocations in the fcc lattice is Enown~ in o~de~ to find the transformation a dis~ibution of periodlcal slips is necessary. According to such a mechanism the 9R and 18R~ structures a~e fol~ed f~om the fcc lattice by means of slips with one slip period: M = 3 a n d M = 6 for the fcc ---~ 9R and the fcc ~ 18R 1 ~ansformations, respectively /2/. The formation of 48R, 8~R, 126R, I~4R and other multilayer phases with superlong-period la%-tlces can not be represented by means of the slip with one sllp period as a difTe~ent dis~ibu~lonof diffTaction maxima in intensi~ w o u l d be observed in X-~a~ photographs than those found in ~ig.l /2/. There£o~e to fo~m such st~uctu~es:~mo~e complex slip period is needed involvin6 two or more subpe~o~ss H = ~ ÷ Ho + ~ T... However these periods for the above s~uctu~es have n o t be~n dete1~mi~ed as a sequence of plane alternation A,B and C in the ,,-~t cell of these s~uctu~es has not been found. The lattice rearrangement net produced by using the periodical shi£ts in fcc lattice ma~ be applied also to ~he bcc --~ ~ ~ansfo~mation by in~oducing an interstitial hypothetical fcc lattice into bcc ---~ fcc ~ NR net fo~mally. For the first stage of bcc --~ fcc ~ n ~ f o r m a t i o n a cr~stallogeomet~ical rearrangement mechanism /15/, developed well for steels, may be used, and the second stage m~7 be done by shearing the close-packed planes {lllJ fcc as considered a b o v e . The initial m a ~ I x has an essential influence on the multilaye~ structure appea~ing in ~ne cobalt and iron man6anese alloys. In the allo~s with low alloying element concen~ation (so as to be in the homogeneous solubili~ range ~he fcc --~ hcp t~aulafo~mation takes place, but aging in the /3-range or for the multiple fcc ~m~ hcp t~anafo~mations do not change the 2H ma~tensite lattice. On the cont1~l-y, in the alloys where the solubili~ limit of alloying element is exceeded, depending upon the temperature and agin6 time in the range the various multilaye~ polytype lattices ere formed above the ~f tempera%~1~e du~ing coolin~ after aging /16/. As evidenced by these results~ the size of the particles of the equilibrium phase which a~e p~ecipitated on heatin~ and du~in~ quench4 ng and theL~ distribution in the matrix a~e decisive in the fo~marion of the verious multilaye~ structures. According to /2/ these particles and other clustel~-%~pe conc~a~ratic~ nonhomogeneities have inb~bitox~ action on She sllp in the fcc ---~ ~R t~ansforma%-lon whereby the m~itilejer structtt~es a~e fo~med. In reali%~, in homogenous alloys with low alloying element concen~a~ion~ for the fcc --~ hcp t~ansfo~mation the shifts must occu~ on every second la~e~ ( i i I ) ~ c as the elastic stresses a~isin6 from the alloyin6 elements a~e negli@ib~. ~ith hi~her concentrations of alloyed elements a~ising durin~ quenching or aging, stresses in the matrix ere p~oduced which have inhlbito~y action in part of the slips. As a result, instead of the hcp (2H) s ~ u c ~ u re the various multila~er structures needing the smalle~ amount of ~he slip a r e fo~med. The multila~er ma~tensite phases a~e not formed in the whole of the cobalt, iron-manganese and non-fexTous alloys in which the ma~tensitic t~ans~o~ma~ion takes place. It is found /2/ that the following c~dit/ons a~e necessary fo~ the appearance of such s~uctu~es~ i. Low stacking fault energy ( ~ < ~0 ~j/m2); 2. Limited solubili~ of the alloyed element in the fcc lattice of the ma~-~ix phase and inhomogenei%~ of the solid solution du~ing the ma~ten~itic transformation. The a ~ l y s i s of published data and the stud~ of the crystalline lattices led to the conclusion that these conditions a~e fulfilled for three ~ o u p s of the alloys given in Table 1 in which the multila~er ma~tensite phases are formed.

704

POLYTYP~ MULTILAYER PHASES

Vol. 17, No. 6

E~FEEENCIS i. R.Ve~ma and P.Kzishna. Polymo~phism and Polyt~pism in Crystals, John-Wiley and Sons~ Ino.pNew Yo~k-LondonrS~dney, (1968). 2. B.I.Nikolln, ~iz. Met. Metalloved.,45, ii0 (1978). 3. L.I.Lyssak and B.I.Nikolin, Doklad~ AN SSSR, 153, 812 (1963). 4. B.I.Nikolin and N.N.Schevchenko, Sc~ipta Met., 14, 467 (1979). 5. B.I.Nikolin and Yu.N.Makogon, Fiz. Met. Netalloved., 39, 154 (i~75). 6. B.I.Nikolin and Yu.N.Nakogon, Netallofizika iss., 7~j iO3 (1978). 7. B.I.Nikolin, L.I.Lyssak and Yu.N.Nakogon, Yiz. Met. metalloved., 32, 871 (1971). 8. B.I.Nikolin, Doklady AN SSSR, 229, 837 (1976). 9. B.I.Nikolin, Fiz. ~et. Metalloved., @3, 591 (1976). i0. B.I.Nikolin and N.N.Schevchenko, Doklad~ AN SSSR, 249, 856 (1.979)~. ii. B.I.Nikolin and N.N.Schevchenko, Doklad~ AN SSBR, 261, 1 3 ~ (1981). 12. H.Warlimont and L.Delaey~ Progress in Hate~lals ~clence, Pe:ergamohPress, Oxford-New Yawk-Toronto-Sydney, v.18 (197~). _ 13. K.Otsuka, H.Sakamoto and K.Shlmlzu, Ac~a Iot.j 27, 585 (197.9).. 14. V.V.Nar~ynov and L.G.Khandros, Doklady AN SS~, 25~, i ~ 9 (1977). 15. G.V.Ku.vdyumov and G.Saks__Z. P~Tsic, 6%~ 38~ (1930). 16. B.I.Nikolin, Doklady AN SSSR, 233, 587 (1977).