On the nature of the α → γ transformation in iron: A study of whiskers

ON

THE

OF THE a+ y TRANSFORMATION A STUDY OF WHISKERS*

NATURE

R. P. ZERWEKHt$

and

C.

M.

IN

IRON:

WAYMANt

Observations of the CL- y transformation in iron whiskers suggest that the transformation can occur In whiskers less than 50~ in width, a well-defined martensitically rather then by nucleation-and-growth. These features were successfully predicted by the shape change and c( --t y habit plane were observed. Superheating of the whiskers ~8s observed, and there phenomenological theory of martensite formation. were indications that the a + y transformation can be nucleated by structural imperfections. The well-defined tc + y transformation shape change was not observed for large whiskers, or for small It is suggested that “normal” whiskers which were contaminated, deformed, br previously transformed. nucleation-and-growth transformations can oacur martensitically if the degree of superheating is high enough. LA

NATURE

DE

LA

x +

TRANSFORMATION DE

BARBES

DE

y DANS

LE

FER:

UNE

ETUDE

FER

L’observation de la transformation cc + y d8ns des berbes de fer suggere que la transformation peut se produire par voie martensitique plutbt que par un processus de germination et croissance. Dans des barbes de largeur inferieure 8 50 ,u, on a observe un changement de forme bien defini et un plan d’habitat TV+ y: ces aspects sont correctement p&dim par la theorie phenomenologique de la formation de la martensite. On 8 observe une surchauffe dans les barbes, et il est vraisemblable que la transformation La modification de forme bien definie lors de la GC-+ y peut dtre amorcee par des defauts de structure. transformation CI - y n’a pas Bte observee pour les grandes barbes, ou pour de petites barbes qui avaient Bte contaminees, deformees, ou transformees ,prealablement. Les auteurs suggerent que les transformations “normales” par germination et croissance peuvent se produire par voie martensitique si le degre de surchauffe est suffisant. DAS

WESEN

DER

cc -+

y-UMWANDLUNG AN

VON

EISEN;

EINE

UNTERSUCHUNG

FADENKRISTALLEN

Beobachtungen der CI + y-Umwandlung vo Eisen-Whiskern weisen darauf hin, da6 die Umwandlung eher martensitisch 81s durch Keimbildung un 3 Wachstum vor sich geht. In Whiskern von weniger als 50~ Dicke wurde eine wohldefinierte Gest$anderung und eine cc - y -Habitusebene beobachtet. Diese Eigenschaften wurden von der philnomenologischen Theorie der Martensitbildung erfolgreich vorhergesagt. Uberhitzung der Whisker wurdg beobachtet. und es gab Anzeichenda fur, da9 die x ---f yUmwandhmg durch Baufehler 81s Keime her orgerufen werden kann. Die wohldefinierte Formanderung trat nicht auf bei gro5en Fadenkristal ‘ren sowie bei kleinen Fadenkristallen, die ober&chlich Es wird die verunreinigt oder verformt waren oder die bereits eine Umwandlung durchlaufen hatten. Vorstellung vertreten, da6 eine Umwandlung, die “normalerweise” durch Keimbildung und Wachstum verliiuft, bei geniigender Uberhitzung such m8rtensitisch vor sich gehen kann.

INTRODUCTION

Compared which

has

siderably

to the y -+ a transformation been

rather

widely

investigated,

less effort has been devoted

transform&on the purpose

which

occurs

upon

and kinetic

study

analysis.

It was

The

out on b.c.c.

which are known

onic

some

characteristics

and

was carried

investigators(1-3)

to be

have

suggested

that

be diffusion controlled

and may be shear-like (marten-

the

cycle.

the

in polycrystalline

microscopy,

filmed the

in specimens

specimens

obtained

showed

by thermi-

the y -+ a (and

Eichen

and Spretnak

for

shearing

(martensitic)

moves.

substantial

as a result of an u --f y +

between

and

manner, surface

a transformation

proposed

distinguishing

transformations

iron may not

exhibited

criterion

of high

made a frame-by-frame

Their motion pictures,

emission

rumpling

single crystals of simple geometry having fairly high purity and relatively low dislocation density. Past

and Spretnako)

a interface

a -+ y) interface to move in a discontinuous

of

y -+ a transformation

Eichen

of the y +

purity iron and subsequently

to the u -3 y

heating.

the a + y transformation. iron whiskers,

motion

aon-

of the present work to investigate

of the crystallographic

sitic) in nature.

in iron,

that a valid cooperative

nucleation-and-growth

is the manner in which the interface

It was suggested

case of the former _would

that the interface advance

in the

discontinuously

Received April 24, 1964; revised June 29, 1964. * Several prints of a 16 mm film showing the a -+ y trensformation characteristics of iron whiskers were made, ‘and are available on a loan basis from the Department of Mining. z;tJlgy and Petroleum Engineering of the University of

with time, while for the latter, the interface would propagate smoothly. This criterion was not entirely

t Department of Mining, Metallurgy, Engineering, University of Illinois. Urbana. $ Presently at: Metallurgy Department, versity, Ames, Iowa.

(“whiskers”)

ACTA

METALLTJRGICA

VOL.

acceptable

and Petroleum Illinois. Iowa State Cni-

13, FEBRUARY

1965

(see the discussion following

Transformations

in

Ref. 1.) single

crystals

of iron have led to some interesting and

as yet uninterpreted that transformed 99

filamentary

observations.

iron whiskers

Brenner(4) noticed (grown

in the b.c.c.

ACTA

100

METALLURCICA,

condition) exhibited severe “kinking”* and distortion as a result of the b.c.o. to f.c.c. transformation. However, his specimens were evidently examined at room temperature, and the (possible) additional effects of the y -+ a phase change on cooling could not be separated from those of the a -+ y transformation. The distortions were attributed to localized transformation and it was pointed out that superheating above the a -+y transformation temperature may have occurred. The features of the a -+ y interface in single crystal whiskers can be observed by transforming a specimen in a temperature gradient, thus causing the gamma phase to consume the alpha phase by the motion of a single boundary. If only a part of the whisker is transformed, the phase interface can be preserved after cooling to room temperature, The retained interface, together with the known geometry of the whiskers, easily permits a two-surface analysis of the interface. Such a procedure was followed. EXPERIMENTAL

PROCEDURE

The iron whiskers used in this investigation were grown in the b.c.c. condition by the halogen reduction method of l3renner(s) which was modified for larger yields by Wayman. Many whiskers with a vast range of sizes were obtained in a single boat, but for most of the experiments, whiskers having a maximum width of ~50 ,I.Jand up to 5 mm in length were used. An upper size limit was imposed because the smaller specimens exhibited decidedly better surfaces (absence of growth steps) and are known to be more nearly dislooation-free. Only whiskers having (100) growth axes and (100) f aces were used. (100) whiskers are usually square or rectangular in cross-section, the orthogonal faoes which are parallel to the growth axis being (100) planes. Specimens were mounted in a (modified Unitron model HHS-2) hot stage in order to observe the a --t y transformation. The hot stage was placed on an inverted metallograph. The whiskers were attached to a hypodermic needle with “Sauereisen” cement, and the needle in turn was secured to an adjustable rod which led into the stage proper through a vacuum seal, A 1 cm x 0.5 cm ribbon-type, tantalum, resistance heating element was used. Specimens were cantilevered to avoid constraints and stresses (other than the whisker weight itself), and the free ends were placed about 40 p from the heating element. After the specimens were positioned, the stage was evacuated to approximately 1O-6 torr, flushed with * “Kinking” in the present context refers to the devietion of the whisker axis from its original growth position 88 a result of trmu3formation.

VOL.

13,

1965

an argon-15% hydrogen atmosphere (reducing), and re-evacuated. The argon-hydrogen mixture was again introduced into the system and ma~ta~ed at a pressure slightly greater than one atmosphere during the heating runs. During the course of the work, it became apparent that the atmosphere surrounding the specimen during heating was important, so comparative runs were also made under a dynamic vacuum. Controlled heating was accomplished by driving a variac with a motor whose speed was reduced by gears (1000: 1). Because of the small size of the specimens and the desirability for no constraints and for no plastic deformation, it was not possible to place thermocouples on the specimens during runs. However, an approximate heating rate was determined by spot welding a thermocouple to the middle of a typical specimen. A heating rate of approximately l’C/sec was used for most of the observations. The most extreme temperature gradient involved was estima~d to be &.4X/p. This was determined by melting the end of a whisker (l536’C) and at the same time noting the distance from the liquid-solid interface to the cc-+ y interface ; the ~mperatu~ of the latter was assumed to be 910°C. Several unsuccessful attempts were made to measure the temperature of the a -+ y interface by means of a pyrometer. interftbce moved from the heated end The a-+y of the specimen towards the cooler end as long as the heater current was continuously increased. In general cases, as soon as the a -+y transformation was observed, the control mechanism was reversed so that the y --f a transformation could be observed. The filament current was switched off when more rapid cooling was desired. Motion pictures during several runs were taken so that the interface movement could be analyzed. A 16 mm Pathe camera loaded with Kodak “Plus-X” film was used at 16 frameslsec. To compare the behavior of polycrystalline specimens to that of the whiskers, some specimens of zone-refined iron were swaged to 1.5 mm in d_iameter and annealed in a static vacuum at 900°C to a final grain size of N40 p. The wires were then thinned by chemical polishing in a 1: 1 solution of concentrated HsPO, and 30% HsO, to a final diameter of about 40 ,u. These specimens were then mounted in the stage and transformed in the same manner as the whiskers. OBSERVATIONS

The whiskers less than 50 p in width had smoother surfaces and exhibited more reproducibly the salient features of the a -+ y transformation than did the

ZERWEKH

ANI)

x ---, y TRANSFORMATION

WAYMAN:

whiskers.

confined and

Hence,

to the smaller

the

appearance

of

characteristic

features

The surface

upheavals

most

observations

whiskers.

Marked

surface

the motion

of the u + y transformation. parallel

of the interface

can be seen.

In by

is due to the enlargement

film.)

2(a) and 2(b) (photographed

Figures

temperature

after one tc -

observed

kinking,

amount

of kinking

and

y 4

in

of the 16 mm at room

GCcycle) also show the

it is to be noted

7.3 8.5 x.4 5.5 6.8 5.6 6 8 8.4

and the

(The graininess

the photograph

8

1

when the y and u phases were in equikinking

B, degrees

2 3 4 5 6 7 8 9 10

to the

of a single interface. Figure 1 shows a (LX.+ y) whisker filmed at high

The advancement

well-defined

that

the

is very small when the whiskers

Fro. 2(a). Whisker after experiencingone CI+y - c( transformation cycle. The method of measuring the kinking angle b from the (100) whisker axes is shown. x 120. 4

Specimen

were

transformed

temperature librium.

TABLE 1. Kink angle B for (100) iron whiskers tioasured as shown in Fig. 2(a)

were

upheavals

were usually

101

WHISKERS

kinking

planar int,erface lvhich separated the two phases. occurred most cases the a - y transformation partially

IRON

Prc:. 2(b). Adjarcnt face of whisker shown in Fig. 2(a). Sota the nearly perpendicular interface and the very small amount of kinking. This whisker was rectangular in cross-section. x 120.

FIG. 1. A pitrtially transformed whisker as shown by enlargement of two motion picture frames (each a few seconds apart). Note the planar a - y interface, the par~allel markings behind the interface, and the well- x 90. defined kinking of the whisker.

larger

IN

are

viewed

(Fig. 2(b)).

on

two

of

the

four

orthogonal

faces

The angle of kinking /? (defined as shown

in Fig. 2(a) due to the transformation

was measured

from the (100) whisker axis, and is given in Table 1 for a number of whiskers. made

by analyzing

These measurements

were

the films taken at-temperature.

FIG. 3. Transformed (c( - y - a) whisker showing curvature due to the y - E transformation upon cooling. x 120.

102

aCTA

METALLURGICA,

FIG. 4. Fine striations behind the Q -+ y interface which formed during the a --f y transformation. Photographed at room temperature after one cc - y - a cycle. x 800.

However, the kinking was not reversible in that the angle p measured at room temperature after one 0: + y + u cycle was not significantly different from that obtained by analyzing the films taken during the a 4 y transformation. Although the kinking was not reversible, some specimens acquired a noticeable curvature during the y -+ a transformation on cooling as shown in Fig. 3. The a -+ y interface was observed to move in a decidedly discontinuous manner. Once the transformation was initiated at the heated end of the specimen, growth of the new phase would be rapid; about one-fourth of the whisker transformed almost immediately (specimens were typicahy 5 mm long). During this early stage of the transformation, it was difficult to observe in detail the interface because of the rapidity of its movement. However, after the initial stage the boundary movement slowed down, and in many cases stopped temporarily. Thereafter, the interface would move with jumps of various lengths. The interface would sometimes stop again for several seconds, or move along at a barely discernible rate, speed up momentarily, and then slow down again, etc. This unpredictable behavior made flhning difficult. In general, it appeared that as the interface moved farther away from the heat source, the transformation assumed a more regular course. The interface could be made to move completely from one end of a 5 mm whisker to the other. In addition to the typical a + y transformation behavior just described, some whiskers showed evidence of pronounced superheating prior to transformation A region far along the whisker length was observed to transform before the region nearest the heat source. The new phase (yf would then rapidly

VOL.

13,

1965

consume the superheated region by the Ba&wu~d movement of an interface towards the hotter end. In several cases the specimens became crontaminated, due to insuficient evacuation of the heating stage before the introduction of the reducing atmosphere. Upon subsequent heating, the specimen would oxidize. If the run was continued, the transformation was very difficult to observe, even if the oxidation was slight. Little or no kinking was observed in the oxidized whiskers, and the a + y interface was so ill-defined that it was almost impossible to see. In two eases where oxidation occurred evidence of the a -+ y transformation was so slight that the whisker melted before any obvious change was noted. It was thought that the cantilevered placement of the whiskers in the heating stage might affect the transformation behavior. Two runs were made with the whiskers in a vertical position. The a-y transformation resulted in the same characteristics as before, so positioning geometry had no apparent effect. Figure 4 is a high magnification photograph (oil immersions of the interface in a rectangular whisker (37 ,u x 13 p) taken at room temperature after one a --f y + a cycle. The sharpness of the interface is to be noted as well as the fine parallel striations behind the interface which resulted from the CL -+ y transformation. These features were observed in all of the smaller (<50 cl) specimens. Figures 4 and 5(a) show that the transformation kinking does not always begin sharply at the interface. In some of the film strips, it was noted that the parallel striations behind the interface were not always perfectly parallel to the interface itself. By etching through several specimens with 2% Nital, it was verified that the sharp a + y interface extends through the volume of the whiskers and is therefore not a surface effect.

FIG. 5(a). Transformed (a - y -+ a) whisker showing diffuse region of interface (lower part) and kinking behind interface. Oblique illumination. x 450.

AND

ZERWEKH

dc +y

WAYMAN:

TRANSFORMATION

IN

IRON

size which were plastically through the a +

103

WHISKERS

deformed

y transformation

before heating

temperature.

Figure 7 shows a large (200 ,U wide) whisker which The underwent the a -+ y -+ a transformations. a ---f y

interface

was

quite

mation

kinking

was

absent.

parallel

striations

transformation

which

irregular

formed

were observed

and the transformed polycrystalline.

The

of the large whiskers, effect, a dislocation

behind

non-planar

transforgroups

during

region appeared

absence of transformation

and

However,

of

the y -+ a

the interface, to be obviously

interface

and

the

kinking were characteristic

which

suggests

either a size-

density effect, or both.

FIG. 5(b). Same specimen &s shown in Fig. 5(a), but Note the recrystallization and polyadjacent face. x 450. crystallinity behind the interface.

The trace of the interface plane on two of the four whisker

faces

well-defined. The more

diffuse

perpendicular typical

(opposite

faces)

was

sometimes

less

Figures 5(a) and 5(b) show this feature. trace

example).

well-defined

was always

to the whisker In some

interface

shown in Fig. 5(a).

the one nearly

axis (Figure

5(b) is a

cases, a portion

was partially

of the

obliterated,

as

Figure 6 shows the displacement

of surface growth steps by the a --+ y transformation. The effects of cycling through the transformation temperature were noted for several of the smaller (40 1~) whiskers. After the specimens experienced one a -+ y + decidedly

a cycle,

different

The kinking

the a -

y transformation

was

from that in the virgin whiskers.

behavior

was essentially

absent,

there

was no well-defined interface plane, and the appearance of parallel surface striations was not observed. Similar behavior

was noted

for virgin whiskers

of the same

FIG. 7. Composite photograph showing two adjacent faces of a 200 p whisker which underwent an a + y -+ Q transformation cycle. The irregular a - y interface can be seen. Note the parallel markings in the former y grains which were due to the y + a transformation.

The transformation characteristics of the polycrystalline wires were not obvious because the round cross-section

imposed

limitations

on the observations

which could be made at the necessary cation.

However,

high magnifi-

at least in the grains nearest the

heat source,

the a -+ y transformation occurred by the motion of a single interface throughout each grain. A shape change was also observed in the wires, but was not nearly as obvious as that in the small whiskers because of the differently oriented grains. FIG. 6. Transformed (z - y - c() whiskers showing the displacement of growth steps due to the a --+ y transformation across the phase boundary. x 450.

Although nary

the observations

in nature,

it would

must be taken as prelimiappear

that

the

a -+ y

ACTA

104

Pm. 8. c( -

METALLURGICA,

y habit plane normals for numerous whiskers, permuted to common unit triangle.

transformation of the 40 ~1wires of zone-refined iron is fundamentally similar to that of the smell whiskers. CRYSTALLOGRAPHY TRANSFORMATION

OF THE a IN WHISKERS

y

In all of the small ((50 p wide) specimens, a planer cc--+ y interface was observed after the initial rapid transformation stage. This habit plane was determined relative to the (parent) b.c.c. phase by means of a two-surface analysis and the poles fell into the grouping as shown in Fig. 8, in which all normals have been permuted to a common unit triangle. The habit plane normals, although clearly irrational, were close to (Oll)b*. There were three outstanding deviations from the rather close clustering near {Oll}b but these were’rationalized on the basis of specimen oxidation or specimen contact with the The habit plane normals are heating element. accurate to within 2” and are clearly not {Oll}b. Analytically, the crystallography of the a + y transformation in pure iron is similar to the f.c.c. to b.c.c. t,ransformation but the procedure is reversed. That is, there is an “inverse” Bain strain and lattice For analysis in terms of the correspondence.(7) phenomenological theory of martensite formation@J’) the input data are the lattice parameters of the u and v phases at the transformation temperature, the lattice (Bain) correspondence, and an assumed plane and direction for the lattice invariant or “inhomogeneous”(lO) shear, which the theory requires to insure that the parent-martensite interface is on the average distortionless. The predicted quantities are the interface (habit) plane, the lattice orientation relationship, and the direction and magnitude of the surface upheaval (shape deformation). The formal analysis can be carried out with the lattice invariant * Subscripts b and f pertain to the b.c.c. and f.c.c. lattices respectively.

VOL.

13,

1965

shear occurring in either the parent or the product phase, but it is m&thematically more convenient to employ the former. The lattice correspondence then enables the planes and directions to be referred to the product, and vice-versa. In this analysis, the plane of the lattice invariant shear, referred to the product (f.c.c.) phase, was assumed to be {lll}f. This choice was made since the close-packed {lll}f plane is a physically realistic plane of shear in the f.c.c. structure. Shear directions in the close-packed plane were taken as (1 lO)f, (II2)f, or any direction between. these two. The vector result of shear in alternating (112)f directions (i.e., Shockley partial dislocations) can lead to a (11O)f shear direction, or to an irrational shear direction. That is, there can be an infinite number of apparent shear directions for shear on (11 l>f, depending upon the particular (112)f components.f Graphical analysis@JO)of the CI- y transformation, assuming it to be martensitic, showed that a predicted habit plane in good agreement with the observed habit plane would result by taking the direction for the lattice invariant shear to be between (11O)f and (112)f (shear plane {lll}f). Two lattice invariant shear systems which gave good habit plane agreement were (Oll)b[122], and (011)~[133]b (nearest rational directions were taken so that the analysis and computations would be easier to carry out). The theoretically predicted habit planes for the b.c.c. to f.c.c. transformation in iron are given in Fig. 9. The a- and y- iron lattice parameters, assuming that the transformation occurred at 916”C, were taken from Pearson;(11) these are a,(y) = 3.6394kX, a,(a) = 2.8985kX.

FIQ. 9. Predicted a + y habit planes. A: results from the lattice invariant shear (01l)b [13%]b. B: results from the lattice invariant shear (0ll)b [122]b. t The theory does not distinguish between slip, twinning or faulting as the physical mode of the lattice invariant shear. However, because of the high transformation temperature, it is unlikely that twinning would be the mode of the lattice invariant deformation.

ZERWEKH

The crystallographic with the lattice which

gave

analysis

invariant

the

employing

the

lations.@)

(This

WAYMAN

AND

best matrix

was carried

shear system

habit

plane

algebra

was

done

a -+ y TRANSFORMATION

:

further

method

because

for

the

IRON

in Fig. 2(a))

by

calcu-

graphical

106

WHISKERS

8.5”.

with the experimentally

(011)4122]b

agreement

shown

IN

This compares

measured

favorably

values as given in

Table 1. On the whisker faces orthogonal

to the ones

from which the angle @ was measured, the theory predicts a kinking of about three degrees, whereas

method is relatively inaccurate for determining the orientation relationship and the magnitude of the

values from one to two degrees were measured. It would thus appear that the observed crystallo-

shape deformation.)

graphic

The predictions

were as follows :

the Habit plane normal

results can be adequately

phenomenological DISCUSSION

0.009083

Pl =

The observation most

Orientation relationship 0.54”

from

[lOl]f

from

(111)r

The

the

0.543599]b.

habit plane is the same as that shown orientation

relationship

planes

and

orientation

relationship.*

orientation mentally

relationship since

close-packed

Unfortunately, could

be

such an X-ray

of the y phase behind

obtained

experiwould

where recrystal-

the interface

would

have occurred before an exposure could be completed. The angle of the lattice compares about

favorably

mation

in Fe-Ni

invariant

with

7’ for the f.c.c.

shear u = 6.83”

comparable

to b.c.c.

alloys.

shape deformation@)

a

martensitic

The matrix

angle

of

transfor-

expressing

the

is

where I is the identity direction

of

the

is the habit consideration, (Oll)b[l2& the whisker

matrix,

shape

of displacement

to stabilize

any one position

hotter

for a reasonable

Moreover,

it

y interface

in

period of time.

It

the cc -

initiated

at some imperfection

or easy nucleation

site, and then

advanced

hotter

the

into

Considering

the

mation

whisker.

there are likely

(nucleation

sites), the trans-

must in general search for some point at

to nucleate.

specimen

of

that in small whiskers

to be few imperfections which

region

The extreme

is the most

likely

(hot)

end of the

place for the transfor-

to start, but in the case where the transfor-

mation began further along the whisker, a structural defect

may have existed

nucleation

at this point,

and then “backwards”

causing

movement

the

of the

interface. u + y crystallography Kinetic such

as

criteria

for

discontinuous have

martensitic motion

not in general

of

transformations the

survived

two-phase the test of

time. However, the geometrical characteristics, particular the shape deformation,(i3) do serve ml determines

deformation,

d,

is

the

distinguish

the

nucleation-and-growth

of the shape strain, and pl’

plane normal. i e., a lattice

the

of superheating

would appear that the a + y transformation

interface

p, = 1 + m&p,’ magnitude

into

The amount

was not possible

formation

no lattice

measurement

have to be made at high temperature lization

predicts

of the cc and y phases are approximately the Kurdjumov-Sachso2) i.e., essentially

parallel,

the in a

the heat source.

moved

of a reference filament in a pyrometer.

-0.292986,

close-packed

not nearest then

out,

to initiate

of the small size of the whisker in relation to the size

in Fig. 9, and agrees well with the mean experimental

directions

was observed

interface

a+y

was one of the

As pointed

at the interface proper could not be measured because

Direction of shape deformation

that

of superheating

region of the whisker.

u = 6.83”

The theoretical

RESULTS

to be made.

region of the whisker

Angle of lattice invariant shear

d, = [0.786549,

important

a + y transformation

(Oll)b 2.16”

(lil]b

OF

The role of imperfections

0.796286 1 b

(

one.

for by

crystallography

theory. -0.604854

The predicted

accounted

martensite

For the case under invariant shear on

the theoretical matrix P, predicts that axis would be kinked (the angle /3, as

* However, for the f.c.c. -+ b.c.t. transformation in ran alloys, the nearly prtrellel {Oil}-{ Ill} planes are in the. same unit triangle as the habit plane; this is not the case here.

diffusion

martensitic controlled.

transformation

transformations

from classical

transformations That

in to

which

are

is to say, a ma_rtensitic

is characterized

by

the

change

in

shape of a transformed region. In this connection, the a + y transformation observed in the small (<50

p) whiskers is without qualification

martensitic.

Clearly the observed u + y transformation is not a massive transformation, since by deEnition(l4) a massive transformation

results in no shape change.

106

ACTA

METALLURGICA,

The unique CI-+ y habit plane and shape deformation which

are predicted

martensitic observed a

by

formation

the geometrical

are in agreement

experimentally.

martensitic

theory

CL-+ y

with those

transformation

even

of

more

attractive. the a -+ y interface single-interface

(see Fig. 4) observed

raise an interesting

martensitic

occurs in AuCd,05)

point.

transformation

as the interface

behind In a

such

as

moves from one

end of the crystal to the other, there are no observable traces

(i.e., parallel

present

striations)

case, the striations

their nature is not clear. mation

a relatively

temperature

left behind.

are clearly Despite

but

the high transfor-

the markings

are unlikely

potent,ial

a ---f y (martensitic) tally

more

hand

heterogeneously

by nucleating

same time.

transformation

complex

be that the

is crystallographi-

than thought.

now be consi.dered. As Fig. 8 shows,

This point

A comparison

mation

iron (as observed

will

of

the

the experimentally

determined

It may be that the observed involved

two or more variants

If so this would explain the sharp

there

variants

are four of

the

crystallographically

may

mean

observed

a few degrees of (11O)f;

with (hkl) and (hkl) etc. habits

habit

these are

which form

Or, the situation may be similar to that

recently observedo6)

for parallel martensite

(Fe-l %C)

such plates form side by side,

record.

If either

of these

the present crystallographic as naive,

would be more complex.

cases

is

analysis must

since the shape deformation Such a “composite”

explain

the observed

shown in Table 1. Nevertheless,

of the

as

shape

the a -

y

could be superheated, might

superheating

is small

“normally”

but it is felt

was

substantial

and

by nucleation

irregular,

to occur

to the usual case where the

transformation

and growth.

When the small whiskers transformed, initially

by the

it was not possible

to cause the a + y transformation

proceeds was

character-

exhibited

attained

although

This irregularity non-uniform

may

have

temperature

the interface

straightened

out

shape deformation by It

been due to an initial

distribution

in the region of

the whisker nearest the heating element. The absence of kinking in the larger whiskers may also be due to their higher imperfection content, and of more potential

nuclei per unit volume.

in the contaminated specimens may be explained considering the perfection of whiskers in general.

could not be super-

Unfortunately,

as compared

the

If the larger whiskers

to those

superheating

that

because of their size

to measure the degree of superheating, enough

than

work larger

the transformation

be similar

smaller whiskers. the

of

that the larger whiskers

heated as were the smaller ones.

that

degree

greater

in previous

to the heating element

hence the likelihood characteristics of a well-defined

Also,

(with sixteen times the vo!ume)

istics

the

after a small length of the whisker had transformed.

shape

scatter

the observed

change in the whiskers serves to classify transformation as martensitic.

The absence

behaved

This variation

that

is considerably

It is probable

the

When

Transformation

However,

were examined ; this would minimize

superheating.

deformations.

the surface relief resembles the peaks and grooves in

is probable

previously.

martensitically,

might

whiskers.

be due to the geometry

It

plates of the Same variant which have different shape

deformation

p)

attained with the present stage and the

specimens

relative

In other words, the (hkl), (MCZ), (hZk), and (hlk). parallel striations may be due to stacks of martensite

be regarded

(~200

wires about 40 p in diameter

investigated.

specimens

itself.

a -+ y transformation

a phonograph

work the transfor-

like the smaller whiskers.

in behavior

to the interface

applicable,

by Eichen a significant

present heating stage and the size of the specimens

attained

(plates)

in the

in bulk poly-

for example

in the present

larger

polycrystalline

small

side by side.

occurs

and deJong12)) reveals

Also.

somewhat

out earlier that in some of the film

plane, all within

average out to

behavior of the smaller whiskers differed from

strips, the striations appeared to be not quite parallel

equivalent

The a-ty more or less

at many points at the

to that which

and Spretnak’n

It was pointed

since

defects

of the a + y transformation

small whiskers

superheating

striations

proceed

many small regions might effectively

a -+ y habit planes although irrational fell near (11O)b.

of the habit plane.

additional

nuclei.

In such a case the shape change of the

crystalline

to

do not favor

it may

can introduce

can

that

of the interface

when surface contami-

transformation then

in

comparatively

the diffusion of impuri-

transformation

the observed

velocity

“finds”

sites. However,

ties into the lattice

difference.

On the other

whisker

nation occurs (i.e., oxidation)

represent traces of the interface as revealed by thermal The sharpness of the markings as well as grooving. grooving.

that the a + y transformation

perfect

zero for the specimen as a whole.

In the

revealed

1965

few nucleation

and

The parallel striations

13,

would be expected

of

This makes the proposal

VOL.

formation

transformation

If such is the casts t.he trans-

could nucleate at many points at the same

time. and if martensitic. mations would average

the various shape deforout bo zero. The same

ZERWEKH

AND

WAYMAN:

a -+ y TRANSFORMATION

explanation may apply to the absence of a shape deformation in polycrystalline specimens of iron. The parallel surface markings which developed in the y grains of a large (200 ~1) whisker may indicate a martensitic nature for the y + Mtransformation, but little more can be said at this time. It is not known to what extent the observed markings (Fig. 7) correlate with the surface roughening in polycrystalline iron(lp2) due to cycling through the transformation. The question of driving force for a martensitic a -+ y transformation is an important one. In the it is known that “pure” light of recent findings (1711s) iron will transform martensitically if quenched rapidly enough. Bibby and Parr(?) report that the M, temperature for Fe-O.O017%C is 75O’C and suggest that for pure iron the iK, temperature would probably lie between 800 and 900°C. The driving force would then be 100 Cal/mole or less, about one-third of the value. On the other hand, if previously accepted presently accepted thermodynamic values are correct, the maximum driving force for the u + y transformation in iron is N -18 Cal/mole and occurs at 1130°C. However because of the peculiar variation of AFar<’ with temperature, a driving force of N -12 Cal/mole is available at 1000°C (this value increases to zero at 1392°C the equilibrium temperature of the y + 6 transformation and then becomes positive) so little is to be gained by extensive superheating. Evidently an order of magnitude difference between the driving forces for the a -+ y and y -+ a transformations exists and it is therefore desirable to rationalize the apparent occurrence of a martensitic y + a transformation. Since the strain energies (i.e., lattice deformations) for the a + y and y -+ CItransformations are essentially the same, it may be that the single interface cc-+ y transformation observed in iron whiskers does not involve the large constraints and surface energy typical of platelike martensite of the y -+ a transformation. The lower values of the elastic constants at high temperatures are probably also significant. It appears reasonable that the superheating at the extreme end of the whiskers was substantial enough to initiate the cc-+ y transformation martensitically. That is, the superheating overcomes the nucleation barrier. This is the most important since the activation energy for the growth of martensitic phases is

IN

IRON

107

WHISKERS

effectively zero.(lO) Once the transformation has nucleated and the initial interface created, the new phase can grow at the expense of the old with enhanced facility, since at least, no new interface is required (in regions farthest from the heat source where presumably the superheating would be lower). If the u -+ y transformation involves different variants (side by side) of martensite as previously suggested, it may be that the initial transformation is triggered by the superheating and that subsequent plates are formed autocatalytically. The observed transformation behavior favors the suggestion that superheating causes a “normal” nucleation and growth transformation to occur If so, similar behavior may be martensitically. observed in other cases, i.e., Ti and Zr. ACKNOWLEDGMENTS

This work was supported by the Air Force Office of Scientific Research and the Atomic Energy Commission through the Materials Research Laboratory at the University of Illinois. Discussions with Drs. J. W. Christian and T. A. Read are appreciated. REFERENCES SPRETNAK, Trans. Amer. Sot. Met. 51. 454 (19591. 2. M. DEJONQ, Ph.D. Thesis, Amsterdam (1960). 3. V. BHARTJCHA et al., Trans. Amer. Inst. Min. (Metall.) Engrs. 221, 498 (1961). 4. S. S. BRENNER. in Growth and Perfection of Cwatals 1. E. EICHEN and J. W.

p. S. i: C. 7. E.

185. John Wiley, New York (1958)“. ” ” S. BRENNER, Acta Met. 4, 62 (1956). M. WAYMAN, J. AppZ. Phys. 32, 1844 (1961). C. BAIN, Trans. Amer. Inst. Min. (Met&.) Engrs. 70, 25 (1924). 8. J. S. BOWLES and J. K. MACKENZIE, Acta Met. 2, 129, 138, 224 (1954). 9. M. S. WECHSLER et. al. Trans. Amer. Inst. Min. (Metall.)

Engrs. 197, 1503 (1953).

10. D. S. LIEBERMAN, Acta Met. 8, 680 (1958). 11. W. B. PEARSON, A Handbook of Lattice Spacings and Structures of Metals and Alloys. Pergamon Press, New York (1958). 12. G. V. KURDJUMOV and G. SACHS, 2. Physik 64,325 (1930). 13. J. W. CHRISTIAN, in Decomposition of Austenite by Diffusional Processes, p. 371. Interscience, New York (1962). 14. M. COHEN et al., Perspectives in Materials Research, p. 315. Office of Naval Research, U.S. Navy (1961). D. S. LIEBERMAN et al., J. Appl. Phys. 28, 532 (1957). :;: A. J. MORTON and J. S. BOWLES, Acta Met. 12,629 (1964). 17. C. J. ALTSTETTER and C. M. WAYMAN, Acta Met. 10,

992 (1962).

18. M. J. BIBBY and J. G. PARR, J. 1Ton St. Inst.

(1964). 19. L. KAUFMAN and M. COHEN, Progr. Met. (1958). Pergamon Press, Oxford.

202, 100

Phys.

7, 165