Void formation in ductile fracture of a cobalt-iron alloy

ACTA

METALLURGICA,

VOL.

9,

1961

Roger&) studied the void formation in coarse-grained copper while Puttickt2) examined the voids in finegrained copper and ingot iron. Both agreed upon the vital role of void formation in the fracture but differed vastly on the initiation sites and other aspects of voids. Because of the limited and divergent observations, it is evident that the phenomenon has not been clearly understood. This note describes the void formation in tensile tests at room temperature of a Co-Fe alloy. A close examination of the voids leads to an evaluation of the various mechanisms of void initiation and growth. The alloy under investigation has the nominal composition of 49% Co-49 % Fe-2 y0 V by weight. Although relatively high purity (>99.9 per cent) has been attained, the alloy is not completely free from oxides and other inclusions, of which most are scattered within grains. The high temperature y phase undergoes a transformation to u phase at 920°C. Rapid cooling suppresses the y + a transformation and converts y to a martensitic structure. Under certain quenching conditions,t the alloy exhibits high ductility (43-81x reduction in area at Increasing temperature fracture) with typical cup-and-cone fracture surface as FIG 5. Atomic model showing the thermal activation of the microcreep relaxation process. o pinning vacshown in Fig. 1. In all fourteen specimens, the x points of fixation other than vacancies; ancies; ductile fracture is always characterized by void . . . vacancy track behind the dislocation. At low temperatures, the vacanoy will completely pm the dislocaformation, which is undoubtedly an inherent feature tion line. The mobility of vacancies at higher temperaof the ductile fracture of this Co-Fe alloy. Specimens tures will be high enough to follow instantaneously the moving dislocation. In the intermediate temperature that have been identically annealed but slowly range, the dislocation drags behind it the vacancy furnace-cooled fracture brittly in the (100) cleavage giving rise to anelastic behaviour. plane without void formation (see Fig. 2). Thus the Another possibility to explain this discrepancy is to voids are genuine products of the ductile fracture. suppose that the diffusing entity is not a single Their formation does not involve such complications vacancy but a divacancy. as the Kirkendall effect during the annealing and the R. KAMEL dislodgment of nonmetallic inclusions in the metalloPhysics Department graphic preparation. lGculty of Science Microstructure does not seem to impose any conUwiversity of Cairo, Egypt straint upon the formation of voids. Thus far voids References have been produced in three different microstructures. 1. P. COULOMB and J. FRIEDEL, p. 555 in Dislocations and Mechanical Properties of Crystala (Ed. by J. C. FISHER, Fig. 3 displays a single martensitic structure whereas W. G. JOHNSTON, R. THOMSON and T. VREE~AND.) Wiley, Fig. 1 contains martensitic matrix and a precipitates New York (1957). 2. T. S. Ki?, Phys. Rev. 71,533 (1946); 72,41 (1947); 78,267 at the boundaries of originally y grains. There are (1948). three constituents in Fig. 4, with Mand the martensite 3. F. C. FRANK, p. 535 in Ref. 1. being two major constituents in an intimate mixture. 4. A. H. COTTRELL,Symposium on Vacancies and other Point Defects in Metals and Alloys. Monogr. Ser. Inst. Metals In almost all ductile specimens (thirteen out of No. 23, p. 14 (1958). fourteen), voids are found in layers beneath the free * Received June 6, 1960. surface within l/10 of the radius in the gage length. Equally important is the revelation that, in several specimens, voids are also found near the specimen Void formation in ductile fracture of a shoulders where deformation takes place without cobalt-iron alloy* necking. Since voids have been found in regions There have been recent reports on the formation of t The results of a detailed investigation of the mechanical voids in ductile fracture at room temperature. properties of this alloy will be reported in a later paper.

stress-

LETTERS

TO

THE

EDITOR

69

Wa. I. Voids formed in ductile fracture, which is typified by the cup-and-cone surface. Marten&tic has not been clearly revealed. Diamond dust used in polishing and 3 per cent nital in etching,

x

matrix 58.5

FIG. 2. Absence of void formation in cleavage fracture of a specimen that has been identically annealed as that in Fig. 4, but furnace-cooled.

where hydrostatic tension is neither developed (near the shoulders) nor &ppreciable (in surface layersc3)), it is concluded that the initiation of voids may be independent of hydrostatic tension. On the other hand, voids in Fig. 4 are only found in the severely deformed a phase, but almost none in the slightly deformed martensite, indicating that plastic deformation effectively promotes the void formation. Hence,

x 400

in order to be consistent with the observation in Fig. 4, the mechanism of void initiation at nonmetallic inclusions@) should include the piling up of dislocations against inclusions. Furthermore, this process provides 8 powerful method to build up stress concentration at the inclusions, which, according to Cottrell(4), is directly proportional to the number of dislocations in the piled up group. There is other evidence to

IO

ACTA

METALLURGICA,

VOL.

9,

1961

FIG. 3. Excessively grown voids in martensitic structure. Note the joining of voids x 63 to form an elongated crack in the center beneath the fracture surface.

FIG. 4. As indicated by arrows, voids are found in the severely deformed a phase, but not in the slightly deformed martensite (a”). Dark clouds in a” are precipitates of a. x 360

believe that without the participation of a stressmagnifying process, simple disintegration at an in-

magnitude larger than that of intercrystalline voids produced under creep conditions in copper and

coherent

u-brass .(@ More often large voids emerge in the central column of the neck and near the fracture

interface

is a rather inefficient

method

of

cracking.uj) As to the growth of voids, it is noted that the size of solitary voids can reach a maximum cross-sectional area of 6 x IO-4 ems which is about an order of

surface, confirming the theory that hydrostatic tension is influential in the growth of voids. In regions where hydrostatic tension is absent or small,

LETTERS

TO

THE

75O”C, vacancies

voids must grow under other type of stress such as

below

normal

without

or shear stress.

The maximum

voids and the conceivably

low mobility

under the test conditions nism by vacancy

size of the of vacancies

rule out the growth mecha-

constitute

a small

coagulation.

coagulation

migrate Direct

in gold

to dislocation

evidence

has been given

Hirschc3) for quenching

from

of

lines

vacancy

by Silcox

temperatures

and

between

910 and 960°C.

condensation.

The prollle of the fracture surface shows that voids often

71

EDITOR

portion

of

the

fracture

In

this

paper

we present

quench-hardening

studies

evidence

based

indicating

coagulation

cracks is only seen in one specimen

the annealing out of vacancies for quenching tempera-

separation

process

is inactive

of the voids.

The initiation

near voids is not observed Puttickc2).

(Fig. 3).

owing

It is therefore

Hence

to the wide of fine cracks

in this alloy as in iron by concluded

that void

for-

The mechanism

hardening

temperature

gold

for quenching

has been

established

propagating

temperature

on account

normal stress concentration

at the tip of the crack.(‘) c. w.

Westinghouse

halt

of a sudden decrease of

CHEN

Research Laboratories

of tetrahedral coagulation.(3z4)

information

showing that

the same mechanism operates over the entire quenching range

of

clustering

should

be

quenching

temperature.

vacancies

must

clustering

to take

1030-700°C. significant

then

Thus

for

this

vacancy range

of

The binding energy between be sufficiently

place;

which

large to allow

implies

a binding

energy of 0.3 eV or greater.

Pittsburgh 35, Penmylvania

In Fig. 1, the yield stress is plotted time for quenches

References 1. H. C. ROGERS,J. &fetaZe,N. Y. 10,589 (1958); Trans. Amer. Inst. Min. (Met&Z.) Engrs. 216, 498 (1960). 2. K. E. PUTTICK,Phil. Mag. 4, 964 (1959). 3. P. W. BRIDGMAN,Trans. Amer. Sm. Metals 32, 553 (1944). 4. A. H. COTPRELL,Dislocations and Plastic Plow in Crystals p. 105. Clerendon Press, Oxford (1953). 5. C. W. CHEN, J. Metals N.Y. 12, 34 (1960). 6. C. W. CHEN and E. S. MACHLIN, Trans. Amer. Inst. Min. (Metall.) Engrs. 206, 829 (1957); also, J. N. GREENWOOD, D. R. MILLERand J. W. SUITEB, Acta Met. 2, 250 (1954). 5. C. T. WANG, Applied Elasticity p. 62. McGraw-Hill, New York (1953). * Received June 22 1960.

For

all

made from

quenching

against aging

800, 750 and 700°C.

temperatures

the

yield

stress

increases to a saturation value with aging time, but no further change occurs for aging times beyond tion.

This is similar to the aging behavior

previously

for the quenching temperature

saturaobserved

of 1030”C.(5)

It was also found that the yield stress increase and the resistivity

decrease were in phase.

The above results for gold eliminate the possibility suggested

by Kimura

et al.(2), since their mechanism

was based on the existence of overaging. An additional present

Quench-hardening for relatively vacancy concentrations*

experimental

conclusion

resistivity.

The

low

vacancies density

One of the major questions concerning the nature of

structures or are absorbed

by

anneal

variation out

proportional

For example,

result supporting

is the recovery

quenching temperature

the annealing out of quenched-in vacancies is whether the vacancies coagulate into voids which may finally collapse into dislocation

of quench-

of 1030°C in

in terms

stacking faults resulting from vacancy The present work provides

crack with a void may momentarily

in gold for

tures from 1030 to 700°C.

mation does not necessarily lead to the fracture in a On the contrary, the meeting of a ductile mode. the propagation

mechanism

upon

vacancy

surface (Fig. 1). The joining of voids to produce large this joining

is the dominant

that

of

of

reaction

the

quenched-in times

with

can be explained if we assume by

migrating

to

to the quenched-in

the reaction

clusters

of

resistivity.

times were 50 and 19 hr, a

ratio of 2.6 for quenches from 600 and 700°C, respectively ; the corresponding ratio of quenched-in resitivities was 0.244.(Q The ratio of reaction times

already existing sinks. An interpretation was given by Koehler et al.ll’ on the resistivity change in quenched

mately equal to the negative two-thirds

power of the

gold in terms of the migration

ratio

for

of quenched-in

vacan-

cies and divacancies to random sinks of dislocation lines. They assumed that only single vacancies are important for quenches from 700°C. A different interpretation was given to the same results by Kimura et aZ.@). According to the latter, vacancies coagulate forming clusters when the quenching is done from above

850%;

for

quenching

from

temperatures

on the basis of the above of

the

quenched-in

model

should be approxi-

resistivities

the

two

temperatures. This gives 2.56, which agrees with the above measured ratio of 2.6. No difference in reaction time would be expected for the dislocation sink model. In conclusion : quench-hardening resulting from vacancy

coagulation

is a consistent

mechanism

for

quenching temperatures from 1030 to 700°C. Clustering of vacancies at relatively low vacancy concentration