The plastic deformation of ordered FeCo and Fe3 Al alloys

THE

PLASTIC

DEFORMATION

OF ORDERED

N. S. STOLOFFt

FeCo and Fe,Al ALLOYS*

and R. G. DAVIES1

The effects of long range order on yielding, strain hardening, deformation modes and ductility are examined for an equimolar FeCo alloy cont.&&g 2 O/_V, and for FesAl. As with other ordered systems, these alloys exhibit a peak in yield stress at a critical degree of order. This behaviour is rationalized in terms of a mechanism involving a transition from the interaction of unit dislocations with a partially ordered lattice to the interaction of superdislocations with a nearly perfectly ordered lattice. Ordering is shown to lower the room temperature yield stress of FeCo-V markedly but to have little influence on the strength of Fe&l. Changes in strain hardening rate upon ordering are much smaller for these alloys than for C&Au type alloys. Ordering suppresses wavy glide in FeCo-V and, to a smaller extent, in Fe&l and changes in ductility of these alloys with order c&n be correlated directly with the slip modes; no relation is observed between ductility and strain hardening or yield stress. LA DEFORMATION PLASTIQUE D’ALLIAGES FeCo ET Fe,Al ORDONNES L’effet de l’ordre ti grande distance sur la limite Blastique, le durcissement, les modes de deformation et la, ductilite, a et& examine pour un slliage 49 % Fe, 49 % Co contenant 2 % de vanadium, et pour Fe,Al. Comme avec les autres systemes ordonnes, ces alliages montrant un maximum de limite elastique B un degre d’ordre critique. Ce comportement est explique par un mecanisme qui fait intervenir une transition de la dislocation unit&e avec un r&au partiellement ordonne it l’interaction de superdislocations aver un reseau &pproximativement ordonne d’une fsqon parfait%. On montre que l’ordre abaisse fortement la limite Blastique B I’ambiante du Fe&V mais It une faible influence sur la resistance du Fe,Al. Les changements dans la vitesse du durcissement de deformation avee l’ordre sont plus faibles avec ces alliages qu’avec les alliages du type Cu,Au. L’ordre empIche le glissement ondulant dans le FeCo-V et, dans une plus basse mesure, dans le Fe,Al, et les changements de ductilite de ces rtlliagesavec l’ordre peuvent etre mis en correlation directement avec le mode de glissement; on n’a observe aucune relation entre la ductilite et le durcissement de deformation ou la limite Blastique. DlE PLASTISCHE ~ERFORMU~G VON GEORDNETEN FeCo- UND Fetal-LEGIE~U~GEN An einer gquimolaren FeCo-Legierung mit 2 % V und an Fe,AI wurden die F,infiiisse der Fernordnung auf FlieDen, Verfestigung, Verformimgsmechanismus und Dehnbsrkeit untersucht. Wie andere geordnete Systeme zeigen such diese Legienmgen sin Maximum der FlieDspctnnung bei einem kritischen Ordnungsgrad. Dieses Verhslten wird zurtickgefiihrt auf einen Mechanismus, nach dem die Wechselwirkung zwischen normalversetzungen und einem teilweise geordneten Gitter tibergeht in eine Wechselwirkuug zwischen tiberstrukturversetzungen und einem fast vollkommen geordneten Gitter. Die Ordnung erniedrigt die FlieSspannung van FeCo_V bei Raumtemperatur betrhchtlich, hat jedoch wenig Emflu aud die Festigkeit van Fe,Al. Die Anderungen der Verfestig~g~es~hw~digke~t nach der Ordnungseinstellung sind bei diesen Legierungen vie1 kleiner als bei Legierungen vom Typ Cu,Au. Die Ordmmg unterdriiokt welliges Gleiten in FeCo-V und -etwas schwiicher- in Fe,Al. Versnderungen der Dehnbarkeit dieser Legierungen mit der Ordnung kiinnen direkt mit den Gleitvorgilngen in Besiehung gebracht werden. Zwischen der Debnbarkeit und der Verfestigung oder der FlieRspannung wird kein Zusammenhang beobachtet.

The strength of long-range ordered alloys depends upon the degree of order, S,fl-s) and for those systems in which stable antiphase domains exist the strength may also depend upon the domain size, s.(*s7) A peak in the flow stress or hardness occurring with changes in the degree of order is manifested in many alloys whether or not a domain structure exists, and appears to be characteristic of alloys which can be disordered below the melting point. For some materials, for example Fe&l, there have been conflicting reports concerning the degree of order at which the peak is observed.(2-4) Also, no explanation has been provided for the observation that for Cu~Au,(l) and according to one report,c2) for Fe&l, a peak is observed at T,, the critical temperature for ordering, while for other alloys the peak is observed at an intermediate degree * Received August 5, 1963; revised October 4, 1963. t Scientific Laboratory, Ford Motor Company, Dearborn, Michig&n. ACTA METALLURGICA

VOL. 12, MAY

1964

473

of order below Tc.(5*6) The interest in the flow stress of ordered alloys has resulted in a relative scarcity of information on other aspects of deformation behavior, such as strain hardening rate, deformation modes and ductility. Ordering of CuaAu type superlattices leads to a large increase in strain hardening rate,(6-9) but only a small effect has been reported for FeaA.l.(4) Long range order has been thought to promote brittleness in Ni3Mn(6) and FeCo(i*) but the reasons for this have not been made sufficiently clear. Moreover, Chenol) has questioned the link between brittleness and order in FeCo. The authors have recently noted an increase in ductility upon ordering for the h.c.p. alloy Hg,Cd,(iQ and this appeared to be a consequence of cross glide induced by ordering, which is opposite to the reported effects of order on glide in Cu,Au and NiaMn. (la) In view of the questions raised by the earlier work on ordering, the purpose of this investigation was to exslmine in detail the effects

ACTA

474

METALLURGICA,

of long range order on the deformation behavior

of two alloys, Fe&l

and fracture

and FeCo, in which the

VOL.

12, 1964

respectively

under an argon atmosphere

1OOO’C in air to

8 in. dia.

rod.

degree of order can be varied between zero and one by

specimens

suitable thermal treatment.

length were prepared from the FeCo-V

This offers an advantage

over alloys such as CusAu and Mg,Cd which order by

and

a nucleation

samples

in which the available range of order is only about 0.8 to 1. The experi-

mental

and growth reaction

data on yield

deformation

stress, strain hardening

modes and ductility

rate,

for FeCo and Fe&l

ductility

Cylindrical

high were prepared

are feasible among alloys of

It is desirable

the nature

Equimolar

of two interpenetrating

AB alloys of the B2 strucare composed

simple cubic lattices.

the sites on one sublattice while the other sublattice is lowered

of the

chosen for the

ture, of which FeCo is representative,

temperature

samples,

for Fe&

studies.

&in. dia. x 0.4 in.

in addition to several Fine grained

(average

diameter

~0.005

in. for FeCo-V

in. for Fe&l),

were prepared in both alloys by annealing at 875°C for 4 hr. Ordering treatments were carried out just prior to testing as follows:

to review briefly

ordering processes in the superlattices present study.

compression

metallographic

tensile samples used for fracture studies. and No.015

structure.

for flow stress

Several

for

compression

structures

any general correlations

measurements.

were prepared

will be related to previous results in the literature for

differing

tensile

with a 0.125 in. gage dia. and 0.78 in. gage

several ordered alloys, in order to determine

whether

and rolled at

Threaded

are occupied contains

through

All of

by A atoms

B atoms.

As the

T,, a homogeneous

second order reaction occurs, with a continuous

varia-

one set of FeCo-V temperatures

samples was quenched from various

in the range

brine to allow determination ties at room temperature of order and a second heated

875 to 550°C into

as a function set of FeCo-V

to 800°C and slow cooled;

sequently

of the degree samples

samples were examined

for evidence

of a second

Repre-

metallographically

phase;

none

was found.

tion in the degree of order from 0 to 1. In the case of

Since the flow stress and strain hardening

binary FeCo, for which T, is near 72O”C,(li’ the equilib-

quenched Fe,Al have been reported previously,(4)

rium degree of order at temperature

one set of samples,

should be retained

by rapid quenching to room temperature, experimental point.

evidence

although no

seems to be available

No stable domain structure is expected

well-annealed

on this in the

state, since four sublattices are necessary

cooled

prior

rate of only

to be tested in the range 400 to

6OO”C, was required slow

was

these were sub-

tested in the range 600 to 800°C.

sentative

iced

of the mechanical proper-

for this material. to testing

These

to produce

were

complete

order and a large domain size, then allowed to remain at the test temperature

for at least 15 min prior to

to form the triple points required

for a stable struc-

testing to produce the equilibrium

ture.04)

is reported

the test temperature. Specimens were strained in an Instron machine

The

extremely

pure

binary

alloy

brittle, but the addition

to be

of 2% V improves

the ductility without altering the phase relations;(11J5)

a crosshead

rate of 0.005 in/min

also V has been reported to slow the ordering kinetics

0.01 in/min

of the pure binary alloy so that quenched

were carried

should be easier to achieve.06)

Therefore

in disorder the ternary

degree of order for

for Fe,Al.

Elevated

for FeCo-V, temperature

out in air by heating

at and

tests

in a resistance

furnace. The variation

in the degree of order with tempera-

alloy, containing 2% V, was chosen for study. Fe&l, at temperatures above T,, i.e., 550”C,(17)

ture for Fe&l

possesses

B2

extent

techniques.“‘)

has recently been established by X-ray In the case of FeCo-V it was necessary

permitted

by the composition.

below

to determine

whether or not V changed the ordering

type

order

to the

maximum

Upon

cooling

T,, the DO, structure is formed which is made up of eight b.c.c. unit cells and may be thought to be

behavior,

composed

obtained

of four interpenetrating

A continuous

variation

f.c.c.

in the degree

sublattices.

of DO,

order

and

a similar

carried out for this material.

X-ray investigation was The degree of order was

on a sample quenched

various annealing temperatures;

into iced brine from the results are shown

from 0 to 1 is obtained upon cooling below T,, and can

in Fig. 1. T, is about 72O”C, and at 550°C the maxi-

be retained

mum degree of order obtainable

by quenching

to room

temperature.o’)

A domain structure has been observed directly by thin film electron microscopy and inferred from X-ray studies.(17) EXPERIMENTAL An

alloy

containing

PROCEDURE

50 at.% Fe, 48 at.% Co and

2 at.% V, and an Fe-24.6

at.% Al alloy were melted

is reached.

in this alloy, S = 0.92,

This curve, which is in agreement

at-temperature with theoretical

with

specific heat measurements(lg) and predictions(20y21) for binary FeCo (also

shown in Fig. 1) indicates that T, is unaffected

by the

presence of 2% V, and that the equilibrium degree of order or complete disorder can be retained by rapid quenching.

STOLOFF

DAVIES:

AND

DEFORMATION

OF

ORDERED

FeCo

Dienes (Theoretical) Cowley (Theoretical Present

Ref. 21 20 heat)

Ref. 19

( X-rays

Investigations

1

800

700 TEMPERATURE,%

600

“500

47.5

Fe,Al

1 Ref.

( Specific

Koya & Sato

AND

in long-range order parameter, S, with temperature for FeC

FIG. 1. The variation

Fe Co-V Tested at 25°C (Quench Temperatures

0

1

2

I

I

I

I

4

6

8

IO

I

I2

TOTAL STRAIN,

2. Stress-strain curves of FeCo-V

FIG.

EXPERIMENTAL

in flow stress with degree of order

can be determined

both at elevated temperatures

The

temperatures

room

and

as a function of quench temperatemperature FeCo-V

tensile

samples

stress-strain

quenched

1

I

I

20

22

24

PERCENT

strength

The two alloys chosen for study have the advantage

of several

I

I8

strain are plotted

that the variation

tures .

I

16

Fig. 2, and derived

RESULTS

flow stresses

curves

I

I4

at 25’C, as a function of quench temperature.

A. The effect of long range order on the yield and

at room temperature

indicated in “C)

from

in the range 875 to 550°C are shown in

values of the flow stress for 0.1% in Fig. 3. A moderate

was observed

with decreasing

increase in quench

tem-

perature until the degree of order reached

0.2, when

an extremely

occurred.

rapid

decrease

in strength

Note that the fully ordered material yields at 31,000 psi, compared to an average of about 50,000 psi for disordered samples, demonstrating that the strength of fully ordered material is quite low. When tests were performed at elevated temperatures, see Fig. 4, a peak

was

observed

near

S = 0.4,

and

a rapid

METALLURGICA,

ACTA

476

I

.9I

.7 !

.8 I

.6.5 4.20 I111111

VOL.

DEGREE

OF LONG

12,

1964

RANGE

ORDER,

S.

60

t

:

Flow Stress

0 * ‘g 50

for 0.1%

Strain

Fe Co-V Tested

at

25OC

600

500

700 QUENCH

Fm.

3. The dependence

40

of the room temperature temperature.

.8

.9

I

I

800 TEMPERATURE,

.7

.6 .5 4.2 0

I

, I ,,,,I

900

c&J

“C flow strew of FeCo-V

DEGREE

OF LONG

RANGE

on quench

ORDER.

‘: 300 x .:: 25 z k! zok z i

15 -

600

500

FIG. 4. The dependence

decrease

in

consequence

strength of diffusion

ing at elevated

was

noted

controlled

above

800

700 TEMPERATURE,Y of the flow stress of FeCo-V

T,,

processes

as a occurr-

temperatures.

The effect of the degree of order on the flow stress of Fe,Al is illustrated in Figs. 5 and 6; a peak in flow stress of quenched material was observed at S = 0.4 as is shown in Fig. 5, which is a replot of data from a previous investigation.c4) Similarly, Lawley et oZ.t3)

have observed a peak in flow stress for quenched samples near S = 0.4. For elevated temperature tests,

900

on test temperature.

Fig. 6, the peak occurred

at S = 0.5.

temperature

peak

extending

tests

the

was

As in the room much

broader,

over a wider range of S, than for FeCo-V

but qualitatively

the shapes of the curves for the two

materials were similar. The elevated temperature data of Kaysert2) for Fe,Al indicated 8 peak in flow stress at 54O”C, which was labeled

as T,.

However

more

recent worko’) has established T, as 55O”C, so that the peak should have been placed at S = 0.3 by Kayser.

STOLOFF

DAVIES:

.4ND

DEFORMATION

Degree

OF

of Long Ronge Order,

ORDERED

FeCo

AND

Fe,AI

477

S.

Fe3AI Tested Flow

25 ‘C Stress

for 0.2%

Stroin

i’c

i\

I

I

OUENCH FIG.

5.

I

I

500

600

I

400

I

TEMPERATURE,OC

The dependence of the room temperature flow stress of Fe,Al on quench temperature.

DEGREE

OF LONG

RANGE

ORDER,

S

0

I

50

I

400

I

5oc

600

TEMPERATURE,X FIG.

B.

6. The dependence of the flow stress of Fe,Al on test temperature.

The effect of long range order on the rate of

The authors have recently reported that for Fe&l

strain hardening

maximum

Strain hardening data for quenched FeCo-V, expressed as the difference in flow stress at the indicated strain intervals (i.e., o, refers to the stress at 1% strain, etc.) are presented in Fig. 7, and the maximum increase in strain hardening upon ordering can be seen to be quite small, of the order of 20%.

ordering

increase

in rate of strain hardening

is about 4Oo/o,(4) while for the h.c.p.

lattice Mg,Cdo2)

(DO,, structure),

the upon

super-

there is no effect of

order at room temperature. Cu,Au type superlattices on the other hand are known to strain harden much more rapidly (~1000/0 greater rate) in the ordered condition.(6-Q) Therefore at least at room temperature

ACTA

478

METALLURGICA, .8 I

.7 ,

.6.5?120

VO_L. DEGREEOF

12,

1964

LONG RANGE ORDER,

S.,

1 I 1’,‘, Fe Co-V

0

400’

’ 500

FIG.

Tested at 25°C

I 600

t

I

700 800 OUENCH TEMPERATURE ,*C

I 900

7. The variation of strain hardening rate of FeCo-V with quench temperrtture

(expressed as differeuces of flow stresses at indicated plastic strains).

Cu,Au type alloys (CusAu, N&Fe, Ni,iHn) exhibit the largest differences in strain hardening bebween the ordered and disordered conditions. C. The effect of long-range order on ~e~or~~~Qn modes and ductility

A striking feature of the effect of order on the deformation behavior of FeCo-V was the observation of a marked change from wavy glide to pIanar glide, as shown in Fig. 8. A change in mode with increasing order was first observed near S = 0.2, corresponding to the position of the maximum flow stress for quenched samples, and continued until S exceeded

0.6. The disordered material appeared to slip on all planes of the (111) zone, as is characteristic of pure Fe, while ordering tended to confine slip to a single set of planes. The energy of an antiphase boundary produced by glide is postulated to be lowest on {llo},(s’ so that (110) presumably are the operative slip planes in the ordered condition. A trend to planar glide upon ordering was noted also for Fe&l, see Fig. 9, although the effect was not as pronounced as for FeCo-V. ~~rcinkowski and Brown have previously reported no effect of order on slip in Fe,Al.(l*) Ordering of FeCo-V produced a sharp drop in

FIG. 8. The effect of long range order on the slip mode of F&o-V, (a)disordered, (b)ordered.

compressed 4 % at 25”C, x 140.

STOLOFF

DAVIES:

AND

DEFORMATION

OF

ORDERED

FeCo

AND

Fe,Al

479

(b) FIG. 9. The effect of long range order on the slip nnode of Fe,Al, (a) disordered, (b) ordered.

ductility,

while little effect was noted for FesAl.

variation in ductility ture (degree

of FeCo-V

of order)

The

with quench tempera-

is plotted

in Fig.

10.

While

compressed

4%

at 25”C,

/I-CuZn,(5) NisMn,(@ CU,AU(~,~~)and MgsCd 02) represent four superlattice

types,

nucleation

and growth

disordered material necked after 15% uniform elonga-

reactions.

The generalization

tion, the fully ordered

whether or not stable domains

uniform elongation

condition

exhibited

only 5%

and no necking was observed.

is confirmed. FeCo-V,

DISCUSSION

A

summary

of

of flow

FesAl and FeCo-V,

the

experimentally

stress

with

degree

determined of order

for

in the literature, appears

in Table 1. The systems analyzed,

which include also .8

.9

I

I

.7

I

maximum

observed

degree of order.

strength

for

Fe&l,

, I11111

I

1

I

1

600

700

800

900

10. The effeot of long-range

order on the ductility of FeCo-V.

for ordering,

and at elevated

tures, and for ,B-CuZn a peak is observed

.6 -5 4.2 0 DEGREE OF LONG RANGE ORDER,

S.

of the

For CusAu a peak is

at T,, the critical temperature

both at room temperature

QUENCH TEMPERATURE,%

3

exist in the structure

tests on these alloys reveal a peak as a function quenched-in

Fe Co-V Tested ot 25°C

FIG.

that a peak is observed

NisMn and Mg,Cd occurs at an intermediate

0

500

by

ordering

not depend on thermal effects since room temperature

as well as for several other alloys

for which data are available

The

which order ‘either

or by homogeneous

degree of order, in the range 0.2 5 S < 0.7, and does

1. Variation in $0~ stress with degree of order variations

x 70.

temperaat S = 0.9

480

ACTA

METALLURGICA,

VOL.

12,

1964

TABLE 1. Effect of order on flow stress

Alloy

Structure

Fe&l* F&o-V* j!?-CuZn’S~ NiSMn’6’ Cu,Au”p= Mg,Cd”2’

Degree of order at peak in flow stress At temp. Quenched

T, 555°C 720 465 480 390 153

DO, 132 R2 Ll, Ll, DO,9

Domains

0.4 0.2 -

0.5 0.4 0.9 -

homogeneous homogeneous homogeneous nucleation and growth nucleation and growth nucleation and growth

yes IlO

no

0.7 at T, below T,

at T, below T,

Type of ordering

yes yes

yes

* Present work

in elevated

temperature

tests

(disorder

cannot

be

Theories of order strengthening in

the

past

by

predicts

a peak in flow stress at some intermediate

temperature

retained by quenching). Ardley,o)

have been presented Sumino,(23)

Flinn,(24)

(degree of order), but since there can be

little or no diffusion room temperature,

in quenched

samples

Rudman(25) and Marcinkowski

and Miller.c6) Ardleyo)

25°C. Therefore

considered

in yield strength

bute to the strength of FesAl, FeCo-V,

that the variation

with

tested

at

the effect should not appear near

this mechanism

is unlikely

to contri-

CusAu, Mg,Cd

degree of order observed for Cu,Au was a consequence

or NiaMn, all of which exhibit a peak in flow stress in

of two processes:

the quenched

temperatures giving

mechanism

increment

from higher

the degree of short range order increases,

rise to increased

cutting r -

(1) as T, is approached strength

through

first proposed

the bond

by Fisher;c2Q the

in stress due to this process

is given

by

o/b where r is the applied stress, o is the ordering

condition.

Sumino(23) has suggested that the stress field of an edge dislocation range ordered

can induce regions.

the alignment

of short

It was considered

that long

the development

of stress-

range order can restrict

oriented short range order so that a maximum

in the

energy and b the Burgers vector, (2) as T, is approached

dislocation

at T,.

from

This mechanism

lower

temperatures

and

the degree

of order

locking

would

changes from 1 to 0.8, a restraining force is exerted on

for

gliding

reveals no discontinuity

superdislocations

(two unit dislocations

nected by a strip of antiphase boundary) bonds

are created

perfectly

by

a superdislocation

long range ordered lattice.

discontinuous

change

and a peak is to be expected cussed a similar mechanism, However, dislocation

in a not

At T, there is a

in the degree

S = 0.8 to 0 as is characteristic

of order

here.

Rudman(25)

applied to data for FesAl.

due to small deviations

from perfect long-

restore the lattice.

Ardley-Rudman

mechanism

applied to systems such as Fe&l geneously,

A different

cannot

be

which order homo-

rate

that of

was proposed

as the diffusion

by Flinn(24)

temperature allows

rises,

or Fe&l,

probably The

dislocations

Another mechanism

short range order was suggested

kowski and Millerc6) who proposed flow stress observed interaction

between

can keep pace with dislocation

apply to a homogeneously or FeCo-V

and

massive

this mechanism ordered

cannot

superlattice

such

in which there are no distinct

regions of short range order. The

theories

generally range

outlined

applicable

of experimental

Table 1. However,

above

flow stress above

do not seem

to be

in their present form to the wide observations

the concept

summarized

We shall consider

first suggested by Marcinkowski

in

of short range order account of changes in

T,, and will be retained

to

motion,

0.7 is due to the

superdislocations However

which

by Marcin-

that the peak in

in NisMn at S g

short range order. as Fe&l

well

however

in flow stress at T, for either

discussion.

enabling the domain boundaries to migrate more easily through the lattice. The Flinn model therefore

work

and no change at T, was noted in

earlier work on Mg,Cd.02) involves

work equally

present

an

climb, dragging domain boundary with them, thereby leading to geometrical inhibition of slip. At still higher temperatures diffusion is so rapid that atomic rearrangement

FeCo-V

alloys.

hardening provides an adequate

structure at T,.

mechanism

suggested

increasing

should Further-

since for these there is no abrupt disconti-

nuity in the dislocation who

dis-

the concept of a restraining force on a super-

always, on the average, the

from

of first order reactions

range order is not clear since a superdislocation more,

con-

since wrong

quenched

force is to be anticipated

in this

also a factor that was et CXZ.(~~) i.e. the varia-

tion in spacing between dislocation

pairs comprising

a

superdislocation as the degree of order changes, to explain variations in flow stress below T,. Dislocation pairs have been observed in CU,AU,‘~~) Ni,Mn(s) workers.

and Fe,Al(2s) by Marcinkowski and coSuperdislocations also have been observed

STOLOFF

in t!?-CuZn by Hall unit dislocations

superdislocations

connected

The energy

proportional

DEFORMATION

and in FeCo by Marcinkowski.c30)

As pointed out previously, boundary.

DAVIES:

AND

consist of

by a strip of antiphase

of the boundary,

which

is

to the ordering energy EOR,t31) depends

on the degree of order through

the relation

E,,

OF

ORDERED

The spacing, dislocation elasticity length

sociate

is so low that the superdislocations

into

their

constituent

ordinary

which then can glide independently.

theory.

a low degree of long range order, say S = 0.1, gliding unit dislocations creating

leave antiphase boundary

trails, thus

wrong bonds which gives rise to hardening

similar to that which occurs range order. associate

above

T, due to short

As S increases unit dislocations

in pairs

dislocations

(superdislocations).

gliding

in an ordered

tend to

Since

matrix

of superdislocations

increases.

the flow stress is to be expected tion of dislocations

A drop in

when a large propor-

are gliding in pairs, and the flow

therefore

are led

to

the

conclusion

that

for

materials in which the degree of order can vary continuously from 0 to 1 that a peak in yield strength will be observed at some intermediate

degree of order.

For

the case of Cu,Au, in which the degree of order changes discontinuously strength

from 0 to 0.8 at T,, the maximum

should

Ard1ey.o)

be observed

We may consider

at

T,,

as shown

the situation

as a special case of the mechanism

by

in CusAu

outlined here, since

T, for Cu,Au represents the position of the transition from unit dislocations The relation flow

stress

estimated

to superdislocations.

between

and

the

the position

dislocation

for the particular

of the peak in

structure

case of FeCo-V

can

be

by calcu-

lating the variation in energy of the antiphase boundary connecting location.

the unit dislocations

Marcinkowski

of a superdis-

and Fishert2s) have predicted

that ordering of B2 superlattices

should cause a change

from wavy glide on many planes of the (111) zone to planar

glide

along

(110).

As

was

shown

earlier,

ordered FeCo does deform by planar glide, presumably along (110).

Flinn (24) has derived

an expression

for

the energy of a aa0 (110) type antiphase boundary

in

an ideal B2 superlattice: El10 B2

where E,,

_ -

is the ordering

4Eo,S2

(1)

___ nowi

energy, given

kTC, S by7

the degree of order and a, is the lattice parameter.

is

between

toss sin2 8 + ~ l-v

[

where G is the shear modulus,

ordinary

dis-

e

(2)

1

b is the Burgers vector,

v is Poisson’s ratio, and 6 = 90” for screw dislocations and 0” for edge dislocations.

At equilibrium,

equal the surface tension of the antiphase given

by equation

(l),

i.e., combining

F, must boundary

equations

(1)

and (2):

ao22/2Gb2 sin2 8 +

r =

25i-kT,S2

create no

stress will continue to decrease with increasing order. We

En = i g

super-

wrong bonds, the strength will begin to decrease as the proportion

acting

of mixed character is given by:

dis-

dislocations

of a super-

by means of isotropic

The mutual repulsive force per unit

=

In materials with

481

Fe,Al

r, of the unit dislocations

of dislocation

locations

AND

can be calculated

SzE&S,“). When S is low, the energy of the connecting boundary

FeCo

The results

of applying

equation

degrees of order in FeCo-V superdislocations acter.

1

co528

~ l-v

(3)

(3) for various

are shown in Fig. 11 for

of pure edge and pure screw char-

(The effect of order on G is likely to be quite

small and was therefore neglected.)

For fully ordered

material,

S = 1, the spacing for edge type superdis-

locations

is about

resolution

for

60 A (near the practical

most

electron

decreases the spacing gradually S reaches 0.3, beyond

becomes

ently.

Since

quenched probably

in flow

at S g

become

exceeds value.

elevated temperatures sociation

in

when the separation

of

1000 A, which appears at a

0.4, in samples tested at

(see Fig. 4), and this may be a

of thermal

activation

aiding in the dis-

of superdislocations.

Additional provided

stress is observed

The peak is observed

higher degree of order, Sr consequence

At large separations

0.2, the superdislocations

dissociated

the unit dislocations to be a reasonable

S

should be able to glide independ-

a peak

FeCo-V

As

larger until

which a very rapid increase in

spacing with decreasing S occurs. each unit dislocation

limit of

microscopes).

support for the proposed mechanism

by the change in slip mode of FeCo-V

was with

degree of order.

As S increased from zero a decrease

in the proportion

of wavy glide wasnotednears

An increasing proportion

= 0.2.

of planar glide was observed

until S g 0.6, beyond which no change in slip mode could be detected. This is indicative of an increase in the proportion of superdislocations with increasing order from S = 0.2 to 0.6, since the strip of antiphase boundary connecting the unit dislocations will tend to lie along (IlO}. The position of the flow stress maximum

of Fe,Al

can be accounted for by a procedure similar to that used above for FeCo-V, provided that the difference

482

ACTA

METALLURGICA,

Fe Co-V

VOL.

12,

1964

\

DEGREE

OF LONG

RANGE

ORDER,

S.

FIQ. 11. The variation in the spacing of dislocations comprising a superdislocation with the degree of long range order.

NUMBER

i

OF DISLOCATION

2

3

J

I

o,[ili]

FIG. 12. Superlattice dislocation in a DO, superlattice (Fe&l). Marcinkowski and Brown.“”

After

STOLOFP

AND

DAVIES:

DEFORMATION

OF

Long Range Order

ORDERED

Short

FeCo

AND

Fe,AI

483

Range Order

I

I

I

I

t

0.7

0.8

0.9

I

I.!

flTc

Fig. 13. Schematic variation of flow stresses with quenched in order for Fe,Al,FeCo-V andCu,Au.

in nature of the superdislocations in the two alloys is taken into account. Marcinkowski and Brownos) have shown that the superdislocation in the DO, type of ordered lattice must consist of four unit dislocations with Burgers vectors &a (111) referred to the unit b.c.c. cell, as shown in Fig. 12, where NNAPB is nearest neighbor antiphase boundary and NNNAPB is next nearest neighbor antiphase boundary. They calculated values for T and r1 of 610 A and 260 A respectively for a pure screw superlattice dislocation, using a value of 56 ergs/em for the NNNAPB energy and 42 erg~lcm for the NNAPB energy. To obtain the quantitative dependence of r and rl with degree of order is difficult, but it is possible to show when the superdislo~tion is fikely to dissociate by making certain assumptions. The DO, type of order depends for its existence upon next nearest neighbor interactions which depend upon the degree of DO, order, while the nearest neighbor interaction depends upon the degree of B2 type of order, which does not change upon disordering the DO, lattice. Thus on decreasing the DO, type of order, the superdislocation may be expected to split into two other superdislocations connected by NNAPB; the main effect of changing the DO, order is therefore to change the spacing between dislocations 2 and 3 which are connected by NNNAPB.(32) Neglecting interactions between dislocations other than 2 and 3, their spacing as a function of degree of DO, order can be calculated, as for FeCo-V, by using the values of NNNAPB energy and spacing for the fully ordered condition as calculated by Marcinkowski and Brown.(f8) The largest errors in

neglecting the other dislocations will be when the dislocations are closest together (i.e. S is large) and will diminish as X decreases. The variation of the spacing of screw dislocations 2 and 3 with degree of DO, order calculated as indicated above is shown in Fig. 11 together with the calculated valuesforFeC~V. The dissociation of superdislocations, whether of screw or edge character, again is assumed to occur near the position of the flow stress maximum. Ex~rimenta~y a peak is observed at S = 0.4 for quenched samples of Fe,A1 and at S = 0.5 for elevated temperature tests, corresponding to a spacing of 500-1~0 8. Again the location of the peak at slightly higher degree of order at elevated temperatures suggests that thermal activation can aid in the dissociation of sup~rdisloeations. The broadness of the peak in FesAl relative to FeCo-V can be related to the greater range of X over which As shown in unit dislocations can exist in FesAl. Fig. 11, the equilibrium spacing of unit dislocations for a given degree of order is greater for FeaAl than FeCo-V, so that the probability that one dislocation of the pair can cross-slip away or be trapped by some obstacle is greater for Fe&l. The second dislocation then must create antiphase boundary as deformation continues, so that the strength remains elevated at higher degrees of order than is the case for FeCo-V. A schematic diagram summarizing the predicted variation in room temperature flow stress with changes in the quenched-in degree of order for C&Au, FeCo-V and Fe&l is presented in Fig. 13. For quench temperatures, T,above and just below T,the flow variation is a manifestation of the stress to move unit dislocations,

ACTA

484

METALLURGICA,

VOL.

12,

TABLE 2. Effects of order on room temperature

Alloy

1964

mechanical

% Change upon ordering Yield Strain Uniform stress hard. elong.

structure

properties

Ordered

Slip mode disordered

planar planar and wavy planar planar wavy

wavy wavy cross-slip cross-slip planar

-

FeCo-V * Fe&* Ni,Mn16) Cu,Au”’ Mg,Cd”2’

B2

-50% 0 $10 -30 -50

+20% +40 + 100 $-loo 0

and increases with decreasing temperature.

The stress

*Present

to

move

DO, Ll* Ll, DO,,

-7.5% 0 -65 + 300

work

superdislocations

for

quench

tempera-

increases known

in strength

upon ordering,t6)

that ordering

tures below T, is assumed constant, so that changes in

and domains grow with great difficulty;

the flow stress are a reflection

ordered

proportion of superdislocations temperatures. probability

of an increase

in the

probably

represents

This applies also to CusAu, since the

retaining

considerable

dissociation

is greater

forX=O.SthanforX=l. T, order by a nucleation with an equilibrium areas)

from

from above

and growth

reaction,

but

two phase region

(ordered

plus

existing

above

For these materials

the cutting

of ordered

domains

the surrounding

the fully

a mechanism

short

ordered involving

range

ordered

from

observed

unit

to superdislocations

in Ni3Mnc6) and probably

short

Ductility reduces Ni,Mn,(@

of FeCo-V

ductility

of Mg3Cd.02) restricted

by

matrix

ordering

in Fe,Al

a

the ductility

of ordered

large domains

(except

significantly

the order-induced

to

changes

in

rate and ductility

for FeCo-V

and Fe,Al,

as

is made between the room

the

strength

It

must

be

Ni,Mn

slip

which orders very

of

FeCo-V, effect

Long range order Mg3Cdo2)

on the

remembered,

strength

however,

and of that

can

stress

boundaries

or by

reducing

the

slip systems to below five, the

number required to accommodate of shape without

crack

arbitrary

formation

(Taylor

criterion(34)). Planar (110) glide in ordered FeCo-V, metals,

can

give

rise

to

twelve

as in all b.c.c.

independent

arise from stress concentrations

criterion

slip

fied. Therefore the brittleness of ordered FeCo-V

can be satismay

at blocked glide bands,

as has been shown directly for MgO by Johnston Parker.(36) vicinity

If the cleavage

of the grain boundary

stress is exceeded

and

in the

before deformation

can

occur to relieve the stress, the material will fracture with little ductility. The ductility vs. quench tempera-

short range order in

ture curve for FeCo-V, Fig. 8, shows a rapid drop in ductility as the degree of order increases. Similarly

and this may give rise to the to fully ordered material.

the glide behavior changes rapidly from wavy to planar in nature as the degree of order increases. For

retain considerable

the quenched condition, high strength relative

cross

for Ni,Mn,

quenched Fe,Al retains B2 type order, so that a direct comparison between ordered and fully disordered material is not possible. FeCo-V, Cu,Au and Mg,Cd undoubtedly

changes

of

either by permitting

to build up at the ends of slip bands grain

number of independent minimum

restriction

modes, inde-

systems,‘35) so that the Taylor

CU,AU’~~~~)and has little Fe,Al.

by

The

deformation

of fully ordered material with

sluggishly) and disordered material. reduces

can be correlated

of other factors such as yield stress or strain

well as for several alloys for which data appear in the properties

systems

pendent

2. The mechanical properties of fully ordered materials

temperature

by

occurs in Mg,Cd

blocked

The comparison

(see

with the operative

concentrations

strain hardening

that cross slip

in FeCo-V

directly

of these alloys.

as percentages)

ordering

has been

rate.

Table 2 summarizes

10) and

(see Fig. 9), and is enhanced

to brittleness

literature.

an

ordering in Mg3Cd.(12) Therefore it appears likely that

contribute

strength,

and

condition

(see Fig.

It is noteworthy

hardening

(expressed

structure

order,

has little effect on FesAl, and increases the

proposed in this paper also contributes

yield

range

data of Table 2 show that ordering sharply

the ductility

also, it is difficult to determine whether the mechanism the behavior

and Miller

non-equilibrium

Fig. 8) and in Ni3Mn,03) is only partially restricted by

determined

in flow stress with order.(6112) Although

transition

a

increase in strength relative to thedisordered

is severely

by unit dislocations

appears to account for the experimentally changes

therefore the

by Marcinkowski

is to be expected.

NisM.n(s3) and Mg3Cd,02) upon cooling

region.

tested

with decreasing quench

of superdislocation

disordered

Ni,Mn

but it is well

is very sluggish in this system

is the sole material

listed in Table

2 which

S 2 0.6 no further

change in glide character

can be

STOLOFF

noted,

DAVIES:

AND

DEFORMATION

and indeed little further loss in ductility

condition

glide occurs on (0001) and to a much smaller extent on (10iO)02);

this gives rise at most

to four inde-

pendent slip systems, all of which permit deformation to the basal plane only,

Groves and Kelly.(35) applied

achieved

stress

grain

{lOil} fo;ir

compatibility

be

Upon

and (1122) glide also become

opera-

secondary

modes.

Glide on {lOil}

additional

independent for extension

basal plane.

cannot

to appear early

these too do not provide independent

out by

under the action of

process, as was observed.02)

tive as sign’?:ant provides

as pointed

Therefore

and cracks will be expected

in the deformation ordering,

However, systems,

(1122)

systems,

but

normal to the

glide can provide

including

extension

five

normal

the basal plane, so that a general deformation

to

becomes

possible.(35) SUMMARY

FeCo-V

AND

and Fe&l

an intermediate

temperatures

function

of

stress

involving

of order

be

a transition

temperature. rationalized

hardening

tested

The

by

at

as a

peak

in

a mechanism

from the glide of unit disloca-

tions to superdislocations. on strain

whether

or at room temperature

quenching can

CONCLUSIONS

exhibit a peak in yield stress at

degree

elevated yield

Ordering haslittle influence

in FeCo-V

and Fe,Al,

unlike

CusAu alloys which exhibit a 100% increase in strain hardening

rate in the ordered condition.

is suppressed

by order in FeCo-V

by order in Fe&l. lated

with

supported

FeCo

AND

Fe,Al

485

1. G. W. ARDLEY, Acta Met. 3, 525 (1955).

For the case of MgsCd, in the disordered

an

ORDERED

REFERENCES

can

be observed.

parallel

OF

the

by experimental

of

these

can be corre-

alloys,

and

287 (1961). 4. R. G. DAVIES and N. S. STOLOFF, Acta Met. 11,1187 (1963). 5. N. BROWN, Phil. Mag. 4, 693 (1959). 6. M. J. MARCINKOWSKI and D. S. MILLER, Phil. Mag. 6, 871 (1961). 7. W. D. BIGGS and T. BROOM, Phil. Mug. 45, 246 (1954). 8. P. FLINN, Strengthening Mechanisms in Solids. ASM, Met,als Park, Ohio (1962). 9. A. E. VIDOZ, D. P. LAZAREVIC and R. W. CAHN, Aeta Met. 11, 17 (1963). 10. W. C. ELLIS and E. S. GREINER, Trans. Amer. Sot. Metals. 29, 415 (1941). 11. C. W. CHEN, J. A&. Phys. 32, 3488 (1961). 12. R. G. DAVIES and N. S. STOLOFF, Trans. Amer. Inst. Min. (Metall.) Engrs. to be published. 13. T. TAOKA, K. YASUKOCHI and R. HONDA, Mechanical Properties of Intermetallic Compounds. Wiley, New York (1960). 14. H. LIPSON, Progr. Met. Phys. 2, Interscience, New York (1950). 15. C. W. CHEN and G. W. WIENER, J. Appl. Phys. 30, 199s (1959). 16. R. W. FOUNTAIN and J. F. LIBSC~, Trans. Amer. Inst. Min. (Met&Z.) Enqrs. 197, 349 (1953). 17. R. G. DAVIES, J. Phys. Chem. Solids 24, 985 (1963). 18. M. J. MARCINKOWSKI and N. BROWN, Acta Met. 9, 764 (1961). 19. S. KAYA and H. SATO, Proc. Phys. Math. SOL, Japan 25, 261 (1943). 20. J. COWLEY, Phys. Rev. 77, 669 (1950). 21. G. J. DIENES, Acta Met. 3, 544 (1955). 22. G. C. KUCZYNSKI and M. DOYAMA, Acta Met. 3,415 (1955). 23. K. SUMINO. Sci. Rep. Res. Inst., Tohoku Univ. AlO, 283

(1959). 24. P. A. FLINN, Trans.

glide

but is little changed

These observations ductility

Wavy

2. F. X. KAYSER, Ford Motor Co., WADC Techn. Rept. 57-298, part I (1957). 3. A. LAWLEY, E. A. VIDOZ and R. W. CAHN, Acta Met. 9,

25. 26. 27.

are 28.

data for other alloys.

29.

ACKNOWLEDGMENTS

The authors

are grateful

to B. Kovacs

Lezius for assistance with the experimental to Drs. M. J. Marcinkowski

and D. K. program,

and R. M. Fisher of U.S.

Steel Corporation for providing a copy of their paper prior to publication, and to Drs. T. L. Johnston, G. F. Bolling and H. Sato for many helpful discussions critical review of the manuscript.

and

30. 31. 32. 33. 34. 35.

36.

Amer. Inst. Min. (Met&Z.) Engrs. 218, 145 (1960). P. S. RUDMAN, Acta Met. 10, 253 (1962). J. C. FISHER, Acta Met. 2, 9 (1954). M. J. MARCINKOWSKI, N. BROWN and R. M. FISHER, Acta Met. 9, 129 (1961). M. J. MARCINKOWSKI and R. M. FISHER, J. AppZ. Phys. 34, 2135 (1963). D. HULL, Electron Microscopy and Strength of Crystals, p. 439. Wiley, New York (1963). M. J. MARCINKOWSKI, private communication. M. J. MARCINKOWSKI, Electron Microscopy and Strength of Crystals. Wiley, New York (1963). H. SATO, private communication. A. PAOLETTI and F. P. R~CCI, J. Appl. Phys. 34, 1571 (1963). G. I. TAYLOR, J. Inst. Met. 62, 307 (1938). G. W. GROVES and A. KELLY, Phil. Mug. 9, 877 (1963). T. L. JOHNSTON and E. R. PARKER, Fracture of Solids. Interscience, New York (1963).