Fracture micro-mechanisms in stainless steel martensite and austenite

FRACTURE

~~IeR~-~~~HAN~S~S AND U.

IN STAINLESS AUSTEN~TE~

STEEL

~A~TENSIT~

LINDBO~G~

Ductile fractures at room temperature in strips of stainless steel were examined on the 1006 A level by thin foil elect.rcm microscopy and electron diffraction. Two alloys were investigated, 18Cr 1lh’i and 20Cr 35Ni. It was observed that martensite formation occurred in connection with cracking in 18Cr 1lNi but not in 2Wr BBNi, the cryst5liite size varying between 606A and several microns. The tutomic sopsration in 18Cr 11Ni mzrtensite tended to OCCUT on random crystallographic planes while in 2OCr 3jNi austenit~e (I 11) and to some extent (100) were preferred. The difference in behavior is thought to be wlated to the degree of anisotropy of surface tension and to the marten&tic defect. struct,ure. It is suggested that the martensite transformation might accelerate stress corrosion cracking. MICROMECANISMES

DE

RUPTURE DANS L’ACIER ET AUSTENITIQUE

INOXYDABLE

MARTENSITIQUE

Les ruptures ductiles B temperature ambiante dens des languettes d’acier inoxydsble ont et8 examinees, Deux B I’tichollo 1000 A, par microscopic electroniqua sur lames minces et par diffraction Qle&onique. slliages ant &A titudies, I8Cr IlNi et 2OCr 35Ni. L’euteur a observe clue la formation da la martensite est lice ci,la fissuration dans l’acier 18Cr IlNi mais pas dens l’aeier 2OCr 35Ni, la taille des cristallites variant entre 600 A et plusieurs microns, et que la separation atomiquo clans la martensite 18Cr 1lNi tend k se produire SUP dea plans cristallographiques quelconques tandis que duns l’austenit~e 20Cr 35Ni lrs plans (I 11) et dans une certaine measure (106) sent, preferantiels. L’auteur suppose que la diffirence dens le ~o~~~ort~rnent est. r&i&e au degre d’anisotropie de la tension superfioielle et au d&ant, de structure ~~~rt~nsitiq~~0 It suggitre que la transformation martensitique poufrait accelerer la fisfislnstion par corrosion sous cont~raintes. B12DCRMIKROMECHANISMTiS

IS

ROSTFREIEM

MARTENSIT

UKD

AUSTENIT

l%ttels ~lekt,ronenm~kr~kopi~ und -beugung wurde der Brueh van rostfreiem Stahl in Streifen untersucht. Zwei Legierungen wurden gepriift: f8Cr 11% und 2QCr 35Ni. ~~~rt~ns~tbi~d~ln~erfolgte im Zusammenhang mit. Rissen in 18Cr 1INi, aber nieht in 2Wr 35% Die Kristallitgrosse variierte zwischen 600 A und einigen Mikron. Die atomare Trennung erfolgte in 18Cr 1lh’i Martensit moist’ auf belicbigen krist~llographise~en Ebcnen, withrend in 2OCr 35Ni Austenit (111) und bis zu einem gewissen Grade (106) bevorzugt waren. Das ~mters~h~edli~he Verhalten h&ngt vermut,lich mit dem Grad der Anisutropie der ~be~~cbens~annu~~g und der ~~tensitis~he~ Defektstrnktur zussmmen. Es wird vermut.et, dass die ~a~e~it~~rn~nd~~~ng R&se durch Spa~~nun~orr~~on besehleL~ni~en kann.

EXPERIMENTS

Ausbnitic steels am ductile in the absence of effects from the environment and fail only after a rather large amount of plastic deformation. Failure occurs as a resuIt of plastic instability, usually combined with cavity formation in regions of intense local&d deformation. The purpose of the present paper was to investigate the details of the fracture process in stainless steels at ambient temperature by means of transmission electron microscopy, observing the local path of the cracks and the structure of the surrc unding heavily deformed material. Particular attention was paid to the martensite transform&ion which occurs during deformation in many stainless steels and to the crystallographic aspects uf separation in austenite and deformationinduced ~kar~~~s~~~~ The experiments involved observations on foils transparent to an electron beam as well as examination of crack regions in non~,ran~ar~nt strips about 10 pm thick. The same technique was used in a previous study of fracture in a martensitic carbon steel ~~indborg and Averbaell~l~).

* Ree&nfl November 36,196;; revised January 15,1968. 7 Section for Physical Metallurgy, AB Atomenergi, Stoakhelm 43, Sweden. Presently at AB AGA, Lidingol, Sweden. ACTA

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1968

stainless steels were chosen for the investigation. One of these was an AIXI-304 type of austenitic stainless steel containing l8 per cent chromium and 11 per cent nickel. This sbeel had a tendency to partly transform into martensite and the cracks propagated in purely martensitic areas. The other material was a 20 per cent chromium, 35 per cent nickel alloy. This contained a sufficient amount of nickel to prevent the formation of ariy martensite and the structure was purely austenitic even at crack edges. Both alloys were vacuum-melted by the consumable arc method and the compositions is given by the table below. The material was received as tubes of 10 mm dia. and 0.40 mm wall thickness. The tubes were rolled flat and strips were cut out. These were further reduced by rolling to about 0. I. mm and annealed in an argon atmosphere at 1050°C for $ hr. In order to make the required thin strips the material was polished nl~e~ani~al~y a,nd finaily ele~trol~i~al~y in 10 @A perchloric-90 ‘J(, acetic acid. The resulting strips were fairly thin all over and had certain areas transparent to the electron beam. The foils were fractured at room temperature by scratching witah a needle and the regions around the main crack and subsidiary cracks were examined in a

889

Tr;oo

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Tvpe

C

Si

Mn

P

S

Cr

1vi

?;

18Cr Ni 20Cr 35Ni

0.01 0.02

0.36 0.20

0.57 0.83

0.004 0.008

0.018 0.006

18.3 19.9

10.8 35.5

0.07 0.05

electron

microscope

without

any

further

specimen preparation. Photographs of crack segments and electron diffraction patterns were then evaluated for crystallographic nique described

trace

analysis

in the previous

following

a tech-

paper.(l)

RESULTS

Dislocation

arrangement,

marten&e formation

and

cracking In the 18Cr 11Ni steel the dislocations pile-ups

along the slip planes.

other hand tangled alloy,

however,

locations

cells are prevalent.

there is an increased

in cell walls parallel

tend to form

In 20Cr 35Ni on the Even in this density

of dis-

to predominating

slip

planes (Fig. 1). Thedislocationconfigurationevidently depends

on the nickel

content

and, as suggested

by

FIG. 2. Coarse slip band intersecting 18Cr 11X.

Swann,c2) this is due to the fact that the lower-nickel

the

band.

The

boundary

alloy has a lower stacking fault energy.

in

that the tendency

S-shaped curve indicating

somewhat

to martensite

as the stacking

It also appears formation increases

fault energy decreases.

When thin strips are deformed plastically, the strain to a large extent concentrates in coarse slip bands, with a complicated

mation

the

fine structure (Fig. 2). Inside the bands

dislocations

networks. deformed

are

arranged

in

dense

irregular

A sharp transition is evident from the area in the slip band to the remaining part

of the grain which is unaffected

by the deformation.

The orientation inside and outside the band in Fig. 2 is the same, however. The distortion of the grain boundary gives an idea of the local amount of strain

is distorted

a projected

20 per cent in the [liO] direction band. Martensite

an

tends to form in the 18Cr 1lNi

defor-

when the strain is sufficiently

severe.

bands,

that no martensite

material

into

shear of about

in the middle of the

In the band of Fig. 2, however, proves

a grain boundary

has thus locally

electron

has formed withstood

diffraction

so far.

as much

The as 20

per cent or more of plastic strain without transforming. Cracks are formed in coarse slip bands in the 100 pm or so wide area affected by the needle. In 18Cr 1lNi the cracking formation

occurs

in connection

into martensite.

with local trans-

shaped region surrounding

In Fig. 3 the parallelogramthe crack has partly been

transformed

and selected area electron

diffraction

to martensite shows combined

The martensitic possible

f.c.c.

and b.c.c. patterns.

crystallite size is very fine and it is not

to discern the individual

in the messy structure.

martensitic

grains

The coarse band where mar-

tensite is formed ends abruptly

at an intersecting

line near the grain

running

through

the picture.

boundary

Near the tip there are closely

packed sets of slip lines containing dislocations

with

slip

horizontally

stacking

faults.

pile-ups of partial Occasionally

the

cracks are discontinuous as in Fig. 4. It is evident that microcracks initiate ahead of the main crack at the intersection of slip lines. The edges of the cracks tend to be rather smooth and parallel to the slip lines in this Fro. 1. Dislocation cells with concentrations along slip lines. Deformed 3% by rolling prior to thinning. 20Cr 35Ni.

rather thin portion

of the strip.

The tips are some-

times very blunt (Figs. 3 and 4).

The material below

LISDBORG:

FRACTCRE

MICRO-MECHANISTS

IN

~1A~~~NSIT~

The martensite

AND

crystallite

size varies in 18Cr 1lNi

fractures

but may be deduced

number

of

spots

in

891

ATTSTENITE

the

in each case from the

selected

area

diffraction

patterns (Fig. 5). Most single crystal orientations

give

a total of about four spots in the (110) ring and the (ZOO) ring.

A rough value of the number

contributing

to the selected

is therefore

obtained

(1 IO) and

(200)

divided

by four.

if the number

rings

same value

are

as the width

Occasionally,

much

form and grains

added

6~-1000

A.

pattern

of spots in the and

The grain diameters

this way are typically

of grains

area diffraction the

result

determined

of the transparent

larger

martensitic

with diameters

in

This is about the zone.

grains

also

up to 2 pm are ob-

served (Fig. 6). The edges of the cracks in the stainless-steel site are quite irregular and appear rounded. particularly

evident

martenThis is

with a crack

from a comparison

in 20Cr 35Ni with a stable austenitic structure (Fig. 7) in

which

many

distinguished

to the fracture. fracture

parallel

crack

segments

which give a crystallographic The

roundness

line should

and show sharp appearance

is

be determined

angles. possibly

transformation otherwise

the

by the austcnite

The difference related

be

of the martensite

reveals that the mart,ensitic

precedes rather than succeeds fracture, fracture

may

appearance

to

in fracture

differences

in

cohesion f.c.c.

properties and symmetry of the b.c.c. and It will be demonstrated by trace lattices.

analysis in the next section that the fracture directions in the stainless-steel

martensite

and thus independent

I?IQ. 3. CrIbck (C) in a deformed region, partially transformed to marten&e (M). Area, with pile-ups (I’). Diffract,ion pattern from a region near the crack tip. 18Cr IfSi.

t,ion, while fracture crystallographic

are nearly

of any crystallographic in the austenite

follows

random direcdefinite

plsnes.

the crack in Fig. 3 is displaced more or less like a solid body.

It is evident that a large amount of flow takes

place on the single slip line which forms the front of the crack. order

The crack opening

of microns,

3000-10,000

displacement

corresponding

dislocations

on the

Fig. 4 a similar behavior

to the

is of the motion

slip line.

is evident

in front

Also

of in

of the

crack. Cracks in non-transparent

parts of t,he st,rips are

usually surrounded by thin material (Figs. 5-7) in a similar way to that previously found in carbon steel.(l) The width of the thin zone is 500-1~0

A for tJhepres-

ent stainless steels, about the same value as found in carbon steel martensite but much smaller than in ferrite broken at room temperature which had a thin zone of 5000 g(l) and was thus more ductile. Electron diffract’ion shows that the transparent area at the cracks is pure austenite in the case of 20Cr 35Ni and pure martensite in the case of 18Cr 1lNi.

FIG. 4. Discodnuous crack in semi-transparent foil. Dark field. 20Cr 35Ni.

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FIG. 6. Fracture edge in martensite. The area shown is a single martensitic grain. 18Cr 11Ni.

FIG. 5. Fracture edge of a thick region. Corresponding diffraction pattern with many orientations. 100% martensite. 18Cr 11Ni.

Crystallographic

observations offracture

in 183

11Ni martensite Crystallographic

trace

analysis

requires

selected

areas of about 1 pm in diameter and each area has to be a single grain. Only large martensitic crystallites can therefore

be used.

The predominant

orientation

of these large grains was found to be (loo),

i.e. they

have the (100) plane parallel to the strip and give a (lOO)-typediffractionpattern. were also observed tions.

This is in agreement

specimens

which

Occasionally(

111) grains

but hardly ever any other orientawith findings

are homogenously

b.c.c.(l)

on steel and is

possibly a result of the deformation near a propagating crack in a strip. The original austenite did not have a pronounced texture. The crack edge, irregular as it is, was found to contain straight segments about 1000 L% in length which correspond to micro-facets on the fracture surface.

The

crystallographic

orientation

of these

FIG. 7. Fracture in a non-transparent foil and associated electron diffraction pattern. Austenite. 20Cr 35%.

893

:TE

0

i

L0

10

10

&II

Trace angle Ip (degrees)

Fra. 8.

30

b/11

Crystallographic distribution of fracture hraccs in martensite. 18ti IlKi.

plane

segments was determined by selected area electron diffraction and the resulting distribution is presented in Fig. 8. The distribution is closely uniform both for (100) grains and (111) grains. Possibly there is some increased frequency near the [OlO] direction of the (100) grains, indicating some slight preference for fract,ure on (100)~type planes. It appears possible and preferable, however, to consider the distribution as completely uniform. Crysballogmaphic observations

of fracture

in

The curves for the (211) foil orientation, on the other hand, cannot be interpretbed as resulting exclusively from fracture on planes near (ill>. The peak at’ [01x] is possibly caused by (111) fracture planes but there is an additional peak at [i20] which cannot have formed from (111). These peaks are explained, however, if (100) is postulated to be a subsidiary cleavage plane. This is the second most close-packed plane in f.c.c. metals. There are other, less close-packed, planes that could give rise to the @Of peak. However, variants of these planes should also give a number of other traces, which were not observed, and these planes are therefore disqualified. The (100) fa*mily, on the other hand, shuws traces overlapped by the observed peaks for all orientat~o~~s: @lo), (111) and (211) with the exception of [OOl](110). There are in addition a low number of observed traces in (332) grains. They also indicate {loo) fracture. It is possible to systematize the presence and absence

20Cr

Grains in the thin rim near the crack edge were found also in austenite to have only certain orientations although the bulk material lacked a strong texture. Usually the thin plane of these grains was parallel to (110) but. occasionally (211) and (111 f were also observed. These orientations accounted for about 80, 10 and 5 per cent of all grains suitable for trace analysis. This “crack texture” corresponds to the preferred orientations of rolled f.o.c. metals, in which (110) and sometimes (211) and (321) tend to lie in the plane of the sheet (BarrettS3)). A trace: analysis on the straight crack segments gave the frequency of various ~~stallographi~ directions (Fig. 9). In contrast to the 18Cr 1lNi martensite case t.here is a marked crystallographic dependence. The curves for the (110) foil orientation show peaks for the fllO] and [li2] trace directions, indicating that fractum occurs on the {ill) slip planes. There is a constant background, however, and the peaks have a finite width, implying that fracture also takes place on other planes. The (111) grains tend to fracture along [lit)] directions, again consistent with (ill)type fracture planas.

i 71l]

I

[Aj

ri31: Trace angle

:0/i] y

(degrees)

FrG. 9. Crystallographic distribution of fracture tracea in austenite. 20Cr 35Ni.

plane

ACTA

894

of peaks from each family

of fracture

METALLURGICA,

planes by con-

sidering the angle of inclination

8 between the fracture

plane and the plane of the foil.

The observed peaks all

VOL.

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away from each other in the direction or a slip-off or shear fracture

of the normal

(Modes II and III) wit,h

motion mainly in the plane of separation. method

smaller.

high concentration

of lattice defects of different types.

Thus

of dislocations

It is 61.9’

in the case of (111)

fracture

at

and 45” in the case of (1001 fracture at It is larger for all observed peaks. This

(211)[231] (llO)[~l].

simple way of systematizing interpretation

that (ill>

the results supports

and {loo}

the

are the fracture

does not distinguish

The present

correspond to large 0 (Table 1). If fracture were to occur where peaks are missing, however, 0 would be

What

is typical

of martensites

the density

material

between the modes.

cold-worked

resembles

to an extremely

TABLE

1. Fracture planes in f.c.c. and inclination angles

Trace angle (P

Foil normal 110

Separation plane

3i.3 90

111

0 22.2 39.4 90

211

Inclination an.& 0 45

100 ill 111 001 Ill 001

K.3 QO 70.5 54.7

lli 001 ill 111

61.9 66.9 90 19.5

Fracture trace observed at #J 20Cfn35Xi -.I x

solution

and in iron-carbon

martensites

-.x , pronounced peak. - , no peak observed.

is appreciable.

deviation

of 6’

It corresponds

as an average.

errors in the angular m~surements

dislocations

bouring

atoms affects the electronic

iron atoms.

carbon

Carbon occupies

atom

disturbance

The carbon

a

Cohen(4) has named the

‘*dipole

distorsion”.

Miiss-

by Gielen and Kaplowc5) suggest

that strong o-bonds

develop between iron and carbon

The

trace

The highly faulted

structure

in mart)ensite resuhs

to a standard

in a certain roughening

experimental

making any single plane less attractive

may be estimated

process.

for the

of the crystallographic

A propagat,ing from

the

cleavage

st,raight

planes.

in these previous

the weakest path for a crack.

is not

included

A comparison Averbach(

with shows

that

the

is much lower than for b.c.c., and that the

peaks are much better resolved. the same order of magnitude,

The peak width is of however.

bonds furthermore

every

might have provided

one of the (1OOj directions tensile stress.

which is under the highest

This is referred to as Snoek ordering’“)

and will t,end to further

counteract

on flOO> planes.

cleavage

The crystallographic observations indicate that fracture in stainless steel martensite occurs on random

isot,ropic

planes while in austenite separation

“amorphous”

made

in the martensites

clear that

the

crack

It is tentatively

sug-

of the lattice planes

and the Fe-C bonds that cause fracture

ou or near (11 l> planes with (100) as secondary planes of separation. Also in the previously investigated

a

will tend to strengthen

gested that it is the “roughening”

occurs principally

time

There is also a tendency for the carbon atoms t,o line up, giving dipoles in the

propagation

DISCUSSION

crack will have to

carbon atom or other defect is encountered.

The Fe-C

background

planes,

for a cleavage

direct.ion

the (100) planes that otherwise

background

the

tends t,o

bauer measurements

appears to be a real effect caused by a tendency

and

the #lOO] sites

has two nearest neighbours,

push out the two neighhours. resulting

of the

states of neigh-

of the cube edges in the iron lattice.

metal to fracture slightly off the ideal crystallographic estimates.

carbon

The presence of the defects and in particular interstitial

dislocation,

(Lindborg

The spacing

is 10-1000 A, between

tion.

at 2-3” and thus do not account for the scatter, which

scattering

up

It is evident

atoms 16 A, assuming 0.1 wt. ‘A carbon in solid solu-

deviate

The

possibly

level.

along these dipoles in the (100) directions.

The scatter around the ideal crystallographic direction

between

Each

-.-

of

that these defects somehow affect the choice of fracture

iron atoms at [000] and [loo]. x x

that

high degree.

plane for facets on the scale of 100 A.

on the middle

x x

is the

IntersGtial atoms are usually present in supersat~ated to some forty times the saturation

planes.

work

in general

studied.

“roughening”

to be nearly It should

be

does not imply

structures.

If the cohesion and the atomic bonding are affected by the defects,

it is probable

carbon steel, fracture was random in asquenched martensit!e but definite fracture planes appeared after tempering.

perties that depend

The separation may be either a normal cleavage fracture (Mode I) in which the fracture surfaces move

energy

changed. (P-N)

that, fundamental

on the atomic

bonding

pro-

are also

It is possible that, e.g., the Peierls-Nabarro

force is somehow for

attributed

dislocation

influenced. motion

The activation

in ferrite

has

been

to P-N forces and a.mounts to about 0.5 eV

at 5 kg/mm2 (ConradQ)). Measurements on martensite give a lower activation energy, 0.3-0.4 eV (Kallstrom(*)). The P-N mechanism is ~st~rbed by the defects. This evidently conforms with the present interpretation of the fracture properties. In the 20Cr 35Ni austenite (111) is evidently much weaker than other crystallographic planes in the sense that separation (normal or shear) occurs most easily along these planes. The presence of (100) fracture faoet,s in a f.c.c. material appears to be a new observation and implies that {loo) is the weakest orystallographic plane next to (111). Single-cryst4al work on aluminum alloys by Beevers and Honeycombe(g) and bv.a Price and KellyoOj showed fract-ure on coarse (1111 slip bands. Beevers and Ho~eyoombe(*) furthermore reported some (100) slip bands, but their observabion was thrown in doubt by Price and Kelly who instead suggested superposition of slip on several (1111 planes as an explanation of the bands parallel to (100). The single-crystal observations refer to features that are macroscopic in comparison with the present thin foil fracture segments. The present results suggest that (100) is in fact a plane for slip and fracture, Fracture planes and the anisotropy

of surface eaergy

Surface energy is one of the factors that determine the fracture planes. Anisotropy of the surface energy will give certain characteristic shapes to small particles in equilibrium and SundquisW) has measured the anisotropy from an analysis of particle shapes for a range of metals including a and y iron. He found in y-iron as well as in the other f.c.c. metals that the surface energy had its lowest value on (I 1I) and its second lowest one on (loo), giving a predominance of planes near t’hese in the particle surface. The surfaces were curved in the pure f.c.c. metals. All this strikingly resembles the observations of fracture planes in the present austenite, the position of the peaks, their relative size and the fact that they have a certain width. In zone-refined b.o.0. E-iron at 86O’C SundquisW) found that, the surface energy was lowest on (100) planes, second lowest on (110) and third lowest on (211). He implied, however, that traces of impurities segregating to the {lOO) surfaces may account for part of the difference between {lOO] and (110). For many metals oxygen atoms were shown to give such a lowering ofsurface energy by diffusion to the surface. It is thus not completely clear what plane would have the lowest surface energy in perfectly pure ferrite. Impurity atoms may migrate from the interior of the specimen or from the atmosphere to a crack tip. It is not clear what importance this might have on the

separation process. In the case of b.o.0. metals it is therefore questionable whether the measured surface energy is significant for the fracture llleehanism. Implications

for stress corrosion cracking

Stress corrosion cracking may be accelerated by martensite formation. If martensite is formed ahead of a stress corrosion crack there will be a possibility of considerable potential differences, acting as a driving force for further dissolution at the crack tip. The present results suggest that Inartensite will form in 304-type materials if there is any mechanical contribution to separation during stress corrosion cracking at room temperature. The potential of the martensite and austenite will differ because of the fact that the atoms have a different arrangement in the two phases and because diffusion between the phases will lead to compositional differences. The susoeptibilit3r to transgranular stress corrosion cracking increases with decreasing nickel content. This may to a large extent be due to the increasing tendency for martensite format,ion at least below about 30% Ni (coupled with the increase in the stacking fault energy). This suggestion was put forward by Edeleanu(r2’ but he later discarded the idea on the grounds that,, if a low-nickel austenitie alloy is coldworked to produce appreciable quantities of martensite, the steel becomes relatively resistant to stress corrosion cracking. It seems now, however, that this is no sufficient just,ifioation for discarding the mechanism. What is important for the stress corrosion process shouId not be the total amount of martensite but rather the establishnlent of narrow susceptible channels ahead of crack tips with a potential difference to the material away from the cracks. The tendency to martensite formation decreases as the temperature is raised. In deformed bulk specimens of 18Cr 8Ni steel no martensite is detected above about 100°C (Angeld3)) implying that less than 1% is present. The thermodynamic equilibrium temperature between austenite and martensite is considerably higher, however, and is about 370°C according to a theoretical estimate by Angel.03) IJp to this temperature martensite may possibly form locally at crack tips or corrosion ‘ctunnels” under strain and contribute to stress corrosion susceptibility in austenitic stainless steels with a moderate nickel content. CONCLUSIONS

Fracture in deformation-induced martensite in a 18Cr IlNi steel tends to follow random orystallographic planes while in a 20Cr 35Ni austenite the (111> planes are strongly preferred separation planes

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METALLURGICA,

and the (100) planes secondary separation planes. 18Cr 11X tends to have planar dislocation arrays in the form of pile-ups in slip bands. ~artensite is formed in slip bands but only after a plastic strain that exceeds 20 per cent or more, locally across the slip band. Cracking of a region does not occur until it is completely transformed into martensite. The martensite forms small crystallites with a cross section usually of 600-1000 A, although occasionally grains exceeding 1 pm are encountered. In 20Cr 35Ni the dislocations are arranged in cell walls as well as in pile-ups along slip bands. No martensite formation occurs. In both steels it appears that intersections of slip bands or deformation bands provide sites for crack nucleation. The anisotropy of surface energy appears to be important for the choice of fracture plane at least in f.c.c. metaIs. In b.c.c. the situation is more obscure due to the unce~ain effect of impurities on the fracture process. As a tentative explanation of the randomness of fracture in martensite it is suggested that the high concentrations of defects have “roughened” the lattice planes, i.e. reduced their flatness, and that Fe-C bonds

VOL.

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develop in directions that would otherwise be weak. Transgranular stress corrosion cracking in chloridecontanlinated environments is likely to be accelerated by the formation of martensite at crack tips. ACKNOWLEDGMENTS

The author is grateful to his company for permission to publish this work, to Drs. G. &&berg and R. Lagneborg for helpful hints and discussions, and to S. Ek for able assistance with the experiments.

(1966). 2. P. R. SWANX,

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