Thermionic emission microscope study of the formation of ferrite sideplates

ACTA

1298

Thermionic

emission

of the formation

METALLURGICA,

microscope

of ferrite

study

VOL.

especially

12,

during the initial stages in the evolution

sideplates”

Conclusive

their matrix

grain from locations Wood

at or near a grain

and Hellawell(5) have proposed

an alterna.te mechanism

in which the grain boundary

resolution

perature,

both

prior

sideplate

formation

evidence

for

takes place.

t>he Wood-Hellawell

obt,ained from a metallogra~hie transformation appears

mechanism

in a Cu-45 ‘A Zn alloy.

been supplemented boundaries

at which

determined grooving

prior and

to

after

The locations

a sideplates transformation

be

is not important,

however,

are given grain

ferrite were followed

were

thermal

boundaries

1.

and

by thermionic

employed

description

however,

temperatures,

where continued

that

this

at higher reaction

motion

Figures

of the grain

1 (a)-(f)

photographed isotherma’lly

(0)

used

emission micros-

is being

microscope

prepared

for pub-

basis of this form of microsdiscussed.‘lOP)

show a representat,ive

sequence

at 750°C.

The

of

of sideplates

while the 0.218 % C: alloy reacted

to conduct

and cinephotography.

events before and during the formation

boundaries (following solution annealing and grain growth at a higher temperature) seemed possible,

t4

the

of austenite proeutectoid

of the emission

in this study

copy has been previously

suggested,

to

in iron-

alloys

The positions the growth of

Hellawell(7)

then

consider

on this alloy system.

copy (using barium activation) A detailed

chemical

it was decided

of the iron-carbon

in Table

licat8ion.tg) The physical

may still be operative

applicable

ferrite sideplates

this more critical experiment

etching and were found to be essentially unchanged.c6) mechanism

and Helfaweli

equally

of proeutectoid

of /i’ grain by

by

to

on an alloy

developed

transformation

Since Wood

mechanism

The compositions

and has not

by further experiments

of the same composition.

cc

This evidence

to be incomplete, (&s7)however,

of

is very difficult

carbon alloys,(5) in which the vapor pressure problem

was

study of the /3 t

the growth

on beta brass because of the high vapor

formation

The experimental

during

to perform their

accompanying

to and

This type of experiment

sideplates and provides

of solute normally

evidently

sideplates.

pressure of zinc.

the primary path along which

of t’his problem

requires that t6he positions of the grain boundaries be continuously observed at the transformation tem-

migrates s~~~chronously with the leading edges of the the redistribution

of

sideplates.‘8’

Widmanstatten sidepfates have long been considered to develop by growth into the interior of boundary.(l-‘Q

1964

was being

arrowheads

(0

FIGS. 1. Thermionic emission micrographs of the formation of ferrite allotriomorphs and sideplates at an austenite grain boundary during isothermal transfkmation at 750°C. Fe-O.218 % C; austenitized 5 min at, 1100°C; magnification 666X. Reaction times: (a) t see; (b) t + 9Q set; (c) t + 12% set; (d) t + 142 set; (f)t + 18s sec.

in

LETTERS

Figs. l(a) and l(b) boundary parison

indicate

at which lines,

shows

THE

EDITOR

1299

the segment of the grain

sideplates

of these photographs,

as fiducial

TO

will

develop.

Com-

using twin boundaries

that

this

segment

of

the

boundary was stationary prior to the beginning of transformation in this area. Early stages in the transformation longer

are shown

arrowheads

boundary

allotriomorphs

which a sideplate “above”

cates an allotriomorph

of proeutectoid

austenite

two grain

ferrite from grain indi-

at which a sideplate has not yet

In Fig. l(d), the

The out

The shorter arrowhead sideplates

all three allotriomorphs. along

l(c). points

has grown into the austenite

the boundary.

formed.

in Fig.

in this figure

have evolved

from

Through preferential growth grain

boundary,

the

allotrio-

morphs have now grown into contact

and completely

cover this segment of the boundary.

Later stages in

the growth of the sideplates into the interior of their matrix austenite grain are shown in Figs. l(e) and l(f).

The location

accurately

of the grain boundary

traced

in Figs.

1 (c)

can still be

and

identical to that in Figs. l(a) and l(b).

l(d),

and

is

This boundary

was thus immobile from the time it was first observed, prior

to transformation,

the formation

until

of proeutectoid

of the Wood-Hellawell will develop

them, specified

thus

and move

not

of t’his mechanism,

fulfilled:

a

high

reaction

(the highest temperature

matrix grains.

emission

micrographs,

(Table

demonstrates

with

conditions

favorable

0

however,

have

been

temperature

was

used

3

GROWTH

TIME.

of

This sequence of thermionic which

is representative

observations

made reaction

that ferrite sideplates

on

all

of a three

temperatures,

form entirely by

determining random

the length of the longest

plane of polish

specimens constant

reacted for successively

this method

provide

of lengthening law.

supports the results of the thermal of the analogous transformation in

of

sideplates,

has general validity.

yield

of these

studies,

some

additional

kinetics

conducted

quite different

and 2(b).

of

results, illustrated

of these irregularly

considerable

differences

mechanisms

sideplates, and are now briefly recounted. Measurements of the rate of lengthening were previously

made

of

of ferrite

in these alloys

by

measurements

individual

ferrite

of polish,

in Figs. 2(a)

The large number of data points obtained

the validity

growth

by

true rates

on a single plane

bear

the

obtained

with a linear growth

and

achieved

on a

longer times at a data

accurate

observations were made on the kinetics and morphology of sideplate formation. These observations on

the

reasonably

sideplate

each of a series of

emission microscope

lengthening

beta brass,t6) and suggests that the original mechanism

The

and are consistent

Thermionic the

through

temperature.(12y13)

This conclusion grooving study

sideplates

5

2(b)

growth into their matrix grain, as originally proposed.

importantly

4

SEC.

Fig. 2. Typical plots of apparent sideplate length as a function of growth time. Fe-O.305 % C; austenitized 5 rnin at llOO”C, isothermally reacted at 740°C.

at which sideplates develop

1) at several

In the course

2

I

for the

studied grew into only one of two

adjoining

number

and

numbers in this alloy is about 750”C),(12)

and the sideplates

alloys

The

as most

operation

large

sideplates

synchronously

confirmed.

by Hellawell(‘)

in noticeable

that

grain boundary,

will lie at the leading edges of the

sideplates

are

by

The predictions

mechanism

only at a moving

that the boundary advancing

it was consumed

ferrite.

high

accuracies

with this technique

of

measurement shaped

plots.

in detail between

and 2(b) are representative

readily

leave little doubt

of the data;

as to The

Figs. 2(a)

the develop-

ment of a rather accurate linear growth law at a later

1300

ACTA

METALLURGICA,

VOL.

12,

1964

the length of relationship :

the indentation,

A.d= (a)

FIG. 3. (a) Group of ferrite sideplates sectioned during study of t,hree-dimensional shapes of sideplates. Be0.406% C; austonitized 30 min at 13OO”C, isothermally reacted 10 set at 700°C; magnification 500 x . (b) Gaps in ferrite sideplates. Fe-O.406 % C; austenitized 30 min at 13OO”C, isothermally reacted 4 set at 550°C. Replication electron micrograph, magnification 5000 X , enlarged 2 x .

stage in the lengthening process (Fig. 2(a)) is fairly frequently found. An explanation for the differences in the results of the two methods of measurement was found in a study of the three-dimensional shape of ferrite sideplates. On the basis of observations made on single planes of polish on an 0.41% C plain carbon steel transformed during continuous cooling, Mehl et a1.(14) have suggested that Widmanstatten ferrite crystals have the three-dimensional shape of plates. Oblak et u1.(15) examined individual crystals of Widmanstatten ferrite on mutually perpendicular planes of polish on an 0.40% C alloy steel reacted at 500°C and concluded that they are needles or rods of roughly rectangular cross-section. More detailed observations made during the present investigation by means of a multiple sectioning technique on sideplates formed at 700°C in the Fe-0.406a? C alloy produced results intermediate between those previously reported, perhaps partially because of differences in alloy This content and conditions of transformation. technique is based upon photography of selected groups of sideplates after polishing for times sufficient to remove from 0.1 to 2 ,u of metal. Knoop hardness indentations made adjacent to the sideplate groups were used to determine the distance between successive pIanes of polish, Ad, from the change in

Al,

through

the

ali:! tan 8/Z

where 19, the angle between the long faces of the indentation, is 172’ 30’. XJsing the indentations to determine the precise relative pOSi6iOnS in space of photomicrographs taken of successive sections pern~its the spatial shapes of the sideplates to be determined. Figure 3(a) shows a group of sideplates examined by this method. A cross-section of the sideplate indicated by the arrowhead, taken parallel to the broad faces of the sideplate, is shown in Fig. 4(a). This crystal is seen to approximate the plate morphology only near its base, to be of lath-like form along much of its length, and to approach needle shape near its tip. There are indications of parallel linear segments on both sides of this crystal. A portion of another sideplate, sectioned at considerably smalIer intervals, is shown in Fig. 4(b). The parallel Iinearities are more clearly shown in this illustration.

241-_

01

I

I

6

12 DISTANCE

18 FROM GRAIN

I

I

I

24

30

36

EOUNDARY,

MICRONS

“I m

9

I

0

I

5 E _ ‘8 2 :3

-31

I

34 DISTANCE

I

37

40 FROM

GRAIN

I

I

43

46

BOUNDARY,

/,

J

49

MICRONS

(b)

FIG. 4. Cross-sections of ferrite sideplates taken parallel to their broad faces from sectioning data on the group of sideplates shown in Fig. 3(a).

LETTERS

Unless shapes

the

longitudinal

of the type

axis

shown

of

TO

sideplates

in Fig.

with

4 is accurately

THE

EDITOR

alloys

1301

(including

temperatures

those of Table

by

means

1) reacted

of

parallel to and located just below the plane of polish,

microscopy

the apparent

rate of lengthening

shown that these tips are invariably

the specimen

will appear

approximately

at the surface

to increase when segments

parallel to the specimen

into the surface, the sideplate

of

and to decrease

surface grow

while segments

of

edges which make a larger angle with

respect to the surface are growing men surface.

toward

The plots of apparent

the speci-

early and intermediate

shape.

sequence

of

The detailed

curves

sideplate

of roughly

thus

operative

to the orientation

observations account

illustrated

growth

of measurement. predicted

a sideplate

and

obscured

to

by

the

linear

method

of

measurement

interpretable

longitudinal

anticipated

apparent

upon the

Measurements faces

A

morphological

detailed

sideplates

of lengthening

be studied

whose

coincident

surface. of the thickening

kinetics

ferrite

and

numerous

that

sideplates

of

sideplates

observations

indicate

of the

develop as the result of an effective interfacial barrier to growth at their broad faces. to be a predominantly containing the barrier formation

is evidently The

interfacial structure partial

accomplished

of ledges whose

structure.02)

This barrier appears

dislocation

sessile components;

parallel

“evasion” through

of the

edges have a disordered linear

segments

in the

edges of the sideplates sectioned in Figs. 4(a) and 4(b) indicate that barriers to growth, quantitatively less effective,

and perhaps qualitatively

different, are also

present at certain boundary orientations appearing in the edges of sideplates. Careful examination of the tins

of

hundreds

of

ferrite

one

are

dimension

alloys

wt. 0%Mn

Wt. % Si

0.218 0.303 0.406

0.001 0.002 0.001

0.004 0.000 0.004

-

bottom

is expressed

for valuable

to Dr. W.

discussions

tigation and for critically reviewing

during

L. Winterthe inves-

the manuscript.

ScientiJic Laboratory

E. EICHEN

Ford Motor Company

H. I. AARONSOW

Dearborn, Michigan Department of Metallurgical

G. M. POUND

Engineering Carnegie Institute of Technology Pittsburgh, Pennsylvania Ford Motor Company and

R. TRIVEDI

Carnegie Institute of Technology

well as straight-

axis is parallel to and virtually

with the specimen

is

lengthening

employed.

measurements

that

kinetics

change(12)

concomitant

of this transition-as

kinetics-requires

broad

kinetics

complex

of ferrite sideplates

forwardly

along the line

in growth

but the gradual conversion

behavior

examination

the

to evolve

this morphological

confirmed,

parabolic

began

allotriomorph

The transition

to accompany

is qualitatively from

with the experimental

line in Fig. 2(b) indicates

from a grain boundary

in

Wt. % c

_4ppreciation

These

by Figs. 2(a) and 2(b)

time at which

only

of the

for the gaps in the sideplate in Fig. 3(b).

The dashed vertical

essentially

Only when the

leading edge of such a crystal intercepts are consistent

has

during

stages of transformation(12913).

T~BLR 1. Compositions of iron-cubon

plot will

the surface can the plot assume a linear form. considerations

rounded

in two dimensions.

of shape of a

sideplate with respect to the surface. needle-shaped

electron

of polish

whereas the barriers at the broad faces are effective

parabolic

form of an individual and

planes

sideplate length

be sensitive both to the idiosyncracies particular

random

The barriers to growth at the edges of sideplates

vs. growth time constructed from observations made on a single random plane of polish should thus be an irregular

of single,

at many

replication

sidenlates

in

several

References I. H. M. HOWE, Proc. Amer. Sot. Test. Xat. 11, 262 (1911). 2. V. KRIVOBOK, Trans. Amer. Sot. Steel Treat. 7, 457 (1925). 3. C. A. DUB& H. I. AARONSON and R. F. MEHL, Rev. Metall. 55, 210 (1958). 4. H. I. AARONSON,Syrqvosium on the Mechanism Transformations in Metals, p. 47. Institute of Metals, London (1955). M. S. WOOD and A. HELLAWELL, Acta Met. 9, 428 (1961). :: H. I. AARONSON,Acta Met. 11, 219 (1963). 7. A. HELLAWELL, Acta Met. 11, 223 (1963). 8. M. BRANDRETH and A. HELLAWELL, Acta Met. 10. 174 (1962). 9. E. EICHEN, R. L. FOR~ACS and B. a. PARABIN, to be published. 10. R. D. HEIDENREICR, J. Appl. Phys. 26, 757 (1955). 11. E. ETCHEN and J. W. SPRETNAK, Trans. Amer. Sot. Metals 51, 454 (1959). 12. H. I. AARONSON,Decomposition of Austenite by Diffusional Processes, p. 387. John Wiley, New York (1962). 13. R. TRIVEDI, Ph. D. Thesis, Carnegie Institute of Technology, 1964. 14. R. F. MERL, C. S. BARRETT and D. W. SMITH, Trans. Amer. Inst. Min. (Metall.) Engrs. 105, 215 (1933). 15. J. M. OBLAB, R. H. GOODENO~ and R. F. HEHEMANN, ibid. 230, 258 (1964). * Received May 18, 1964.