On the nature of the martensite to austenite reverse transformation

LETTERS

On the nature of the martensite austenite reverse transformation*

TO

to

The reverse transformation of martensite to austenite in an Fe + 32.5% Ni alloy has been studied in detail and the main results of this investigation will be briefly described. Austenite single crystals of this alloy were first quenched just below M, = -90”C,(1) whereby about 60 vol.% of the specimen transformed to martensite. The structure of the specimen is shown light optically in Fig. 1 using the Pepperhoff-technique(2) (the specimen is polished and TiO, is subsequently evaporated onto the polished surface) by which martensite appears dark and austenite light. Then the martensite was transformed back to austenite by heating the specimen with different heating rates. Using high heating rates, from lOO”C/lO min to lOO”C/sec, the reverse transformation was found to be an athermal process which always started at about A, = 300-310°C and finished at about A, = 400420°C. These temperature data agree with those which have already been reported by G. Krauss and M. Cohen.(l) The sequence of structure changes during heating is shown in Figs. 2(a-g), where the same specimen area is observed after subsequent heatings to increasing temperatures : at 345°C the reverse transformation has started in small regions along the austenite-

FIG.

t These austenite

THE

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EDITOR

martensite interfaces, which can be detected by the formation of surface relief in these regions [see Figs. 2(a) and 2(b)]. The different martensite and austenite regions in the same area as in Fig. 2(a) are also observed by the Pepperhoff-technique in Fig. 2(c). At 380°C the reverse transformation has proceeded by the development of small austenite plates differently orientated within each martensite crystal.? These austenite plates also produce surface relief [Fig. 2(d)] and may be made clearly visible by the Pepperhoff-technique’2) [Fig. 2(e)]. After heating to 420°C the rest of the martensite has undergone reversal, which again is detected by the appearance of surface relief [Fig. 2(f)]. No contrast is seen anymore on a TiO,-coated surface, which means that at 420°C the reverse transformation has been completed. The formation of differently orientated austenite plates within a single martensite crystal [Fig. 2(e)] suggests that the reverse transformation does not necessarily reproduce the austenite lattice in its original orientation. This suggestion is directly confirmed by etching the specimen after total reverse transformation, in 33% HNO, + 33% HCl + 33% H,O by which it is shown that the original single crystal of austenite now contains polycrystalline regions [Fig. 2(g)]. Using transmission electron microscopy it was observed that the reversed austenite (A,) crystals contain dense dislocation networks as described by

1. Fe + 32.55’& Ni quenched to -97°C. Structure developed by Pepperhoff-technique’2’ consists of martensite (dark) and retained au&mite (light). x 100 plates were also observed

by G. Krauss

during a preliminary

study in this laboratory.

402

ACTA

METALLURGICA,

VOL.

15,

FIG. Z(a).Same specimen as in Fig. 1 after heating to 345°C. produced

surface relief at austenitelmartensite

G. Kraussc3) and a few stacking faults along {lll}a,. The orientation relationships measured between the martensite and the reversed austenite crystalsc4) were found to be close to the orientation relationships of the austenite-martensite transformation.(5-7) This indicates that the crystallographic nature of the reverse transformation is basically the same as that of the austenite to martensite transformations and that it basically can be described by the Baindistortion.(*) The habit planes of the reversed austenite plates were measured to be close to {llO}planes of the martensite lattice.

FIG. 2(b). Selected area of the same specimen as in Fig. 2(a). x350

1967

Reverse transformation interfaces. X 100

has

These crystallographic data could quantitatively be explained by an application of the crystallographic theory of martensitic transformations(gslO) using the Wechsler-Lieberman-Read version and assuming that the lattice invariant shear of the transformation takes place along {lll}Al-planes of the reversed austenite. This proves that the investigated reverse transformation which occurred at high heating rates below 420°C is a pure martensitic transformation. With a low heating rate of e.g. 0.3O/min several different processes occur during the heating treatment. This is demonstrated in Fig. 3, which is a plot of the

FIG. 2(c). Same specimen as in Fig. 2(b). Structure developed after slight polishing by Pepperhoff-technique. x 350

LETTERS

TO

FIG. 2(d). Same specimen ELS in Fig. 2(c) after second heating to 380°C. Reverse transformation has produced surface relief within the martensite crystals. x 350

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EDITOR

FIG. 2(g). Same specimen EMin Fig. 2(f). H,O, etch. x 350

measured

temperature

specimen containing going transformation pure made

austenite

difference

martensite:,

AT

HCl-HNO,-

between

a

and therefore under-

processes during heating, and a

specimen.

in a differential

The

measurements

calorimeter

designed

were by

0.

Krisement.(ll) There are three effects I, II and III visible in Fig. 3, two of which (I and III) absorb heat and one of which (II) produces

heat.

The temperature

ranges in which

these effects occur are roughly indicated where the AT-curve

FIG. 2(e). Same specimen &a in Fig. 2(d). Structure developed after slight polishing by Pepperhoff-technique. x 350

FIQ. 2(f). Same specimen MJin Fig. 2(e) after third heating to 420°C. Reverse transformation has produced surface relief. X 350 $ These specimens were quenched in liquid nitrogen,

by the points

crosses the line AT = 0. Actually

these temperature ranges overlap. Light opticai inspection of the specimens showed that the phase changes during process I are analogous to those already demonstrated in Figs. 2(a-e) : the temperature of the reacting specimen decreases slowly when the initial reverse transformation takes place in small at regions along austenitelmartensite interfaces;

FIQ. 3. Calorimetrically

measured temperature difference AT between a specimen containing martensite and a pure austenitic specimen during heating. arbitrary units lsktPT is ca1ibrated in

whereby about 80 vol.%

of the specimen transformed

to martensite.

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METALLURGICA,

VOL.

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1967

FIG. 4(b). Same specimen as in Fig. 4(a) after second heating in the calorimeter to 498°C. x 200

Fra. 4(e). Fe + 32.5% Ni, quenched to -196”C, heated in the calorimeter to 473°C. Structure developed by Pepperhoff-technique. x 200

paring Fig. 4(b) with Fig. 4(a) (e.g. at the area marked higher

temperatures

the

burst-like

reverse austenite plates produces

formation

of

sudden temperature

Above

about

martensite

has

400°C

stopped

almost

undergone

this temperature

the

and a noticable

be observed (This

anymore

is almost

reverse

of the original transformation.

transformation

change in structure

is

can not

until 475°C had been reached.

the temperature

reverse transformation at about 550°C.

60%

reverse

austenitelaustenite-boundarycl,.)

of the ferrite Above

475%

+ the

starts again and is completed

This second reverse transformation

was found to have the features of a diffusion controlled nucleation

and growth process : (1) it was not possible

to demonstrate

the formation

isothermal measurement tion

that the sphere-like

of the newly reversed

is that of diffusion controlled

drops.

At

by an arrow) it is noticed phology

continued

during

of surface

reliefs,

(2)

showed that this transformatime periods

of the order of

30 hr at 500°C and (3) the morphology

of reversed

austenite regions is that of diffusion controlled growing particles.

The last statement

is demonstrated

in Figs.

austenite

mor-

in Fig. 4(b)

growth processes.

As no pronounced change in structure could be noticed during process II, only indirect evidence was obtained about the nature of this process. was found to take place isothermally which

was first heated

than showed

continuously

isothermally

up to 44O”C,

a heat production

60 hr at 414°C) which indicates diffusion controlled.

The process

(e.g. a specimen during

that the process

is

As the amount of heat developed

in the specimen during isothermal

treatment

went up

to 60 cal per 36 g weight of specimen, the annihilation of any lattice defects is ruled out and the origin of process

II has to be sought

in the field of chemical

decompositions. As the specimen is in the two-phase region (austenite + ferrite) the effect most likely to occur

is the

diffusion

martensite-austenite Ni-content

in

a

of

Fe

interfaces. certain

and

Ni

across

the

By this process the

martensite

region

along

this treatment the structure of the specimen is the same as that observed just after process I had occurred.

the interphases is lowered, the martensite is stabilized and reverse transformation does not continue until the temperature is raised above 475’C. In summary it can be stated that in the alloy

The same area of specimen is shown in Fig. 4(b) after the specimen had been heated a second time with the same heating rate, but now up to about 500°C. Com-

investigated the reverse transformation takes place at temperatures between 300 and 420°C athermally and martensitically if the specimen is heated using high

4(a) and 4(b). Figure 4(a) shows the specimen just after process II has finished at about 475°C. After

LETTERS

heating about

rates. 300°C

The reverse transformation in small

martensite interfaces.

regions

along

within the martensite

of reverse

crystals.

austenite

plates

At low heating rates,

part of the reverse transformation

takes place marten-

in the same manner as just described.

simultaneously the

starts at

the austenite/

It continues at higher tempera-

tures by the formations

sitically

TO

But

occurring diffusion of Fe and Ni across interfaces

austenitelmartensite

seems

to

be

THE

405

EDITOR

gradient

or an electric

for the migration

vations

and does not continue

is raised above

time the reverse transformationis

475°C.

This

of the literature

material

in the

Thanks support

condition,

was

The

i.e. 2 hr at 1060°C and

average

0.055 mm

dia.

grain and

size

its

of

this

composition

Ni C Si Mn P S Fe Cr -------18.74 9.43 0.07 0.59 1.52 0.013 0.017 Bal

Electron

transmission

micrographs

obtained

from

thinned foils of type 304 stainless steel after solution

near future. Bottcher

used in this study was type

(wt. ‘A) is shown as follows:

and a more detailed

will be published

obser-

may also

stainless steel sheet (0.030 in. thick) in

quenched.

a diffusion controlled

Full results of this investigation

that solid particles

The starting material 304 austenitic

process. discussion

The present

micrographic

move in a solid under the influence of grain boundary

which has not yet reversed. is interrupted

evidence and liquid

migration.

water

until the temperature

with electron

which indicate

a solution-treated

transformation

Experimental

inclusions(4) in a solid has been obtained. note is concerned

strong enough to stabilize the rest of the martensite By this effect the reverse

field.

of gaseous inclusions(3)

are due to Miss H. for experimental of this investigation

schungsgemeinschaft

Ibach

assistance.

and Mr. W.

heat

The financial

precipitation

or other

boundaries.

The

by the Deutsche

is gratefully

For-

acknowledged. H. KESSLER w.

l?ITSC!H

treatment

annealed

revealed

absence

material

phases

was

of

carbide

at the

then

grain

isothermally

for 2 hr at 9OO“C and water quenched,

order to precipitate Specimens solution

the

second

in

carbides at the grain boundaries.

were next electrolytically

of 700 ml methanol,

polished

in a

100 ml butylcellosolve,

137 ml distilled water and 62 ml perchloric acid (70 %).

Max- Plan&-Imtitut fiir Eisenforschung Diisaeldorf, Deutachland

The specimens were then etched by means of a slight agitation in a solution of 10 ml nitric, 10 ml acetic and

References and M. COHEN, Trans. Am. Inst. Min. metall. 1. G. KRAUSS Engrs 224, 1212 (1962). 2. W. PEPPERJOFF and H. B~HLER, Arch. Eisenhutten Wee. 34, 839 (1963). 3. G. KRAUSS, Acta Met. 11, 499 (1963). 4. H. KESSLER and W. PITSCH, Acta Met. 13, 871 (1963). 5. Z. NISHIYAMA, Sci. Rep. TGhoku Univ. 25, 637 (1934/35). 6. G. WASSERNANN, Mitt. K.- Wihklm. Inst. Eieenforsch. D&weld. 17. 149 (1935). 7. A. KODHEND~R~ER z&d G. OTTO, Arch. Eisenhiitten wea. 30, 227 (1959). 8. E. C. BAIN, Trans. Am. Inst. Min. metall. Engrs 70, 25 (1924). 9. M. S. WECHSLER, D. S. LIEBERMAN and T. A. READ, J. Metals 5, 1603 (1953). 10. J. S. BOWLES and J. K. MACKENZIE, Acta Met. 2,129,224 (1954). 11. F. WEVER, 0. KRISEMENT and H. SCHADLER, Forschungsberichte des Wirtehqfts- und Verkehrsministeriums Nordrhein- Westfalen Nr. 459 (1957). * Received August 2, 1966.

15 ml hydrochloric

acids.

Replication

by transmission

electron microscopy

in the following

manner.

for examination was carried out

A cellulose acetate primary

replica of the specimen was shadow cast with platinum at an angle of 30” to its surface. normal replica

to the

shadowed

was dissolved

Carbon was deposited

surface

in acetone

film with the platinum

attached.

used to obtain the transmission The electron grain

micrograph

boundary

which

as carbides

The rrghost” positions

the carbon

This film was then micrographs.

of Fig. 1 shows a single contains

particles in a similar-treated identifiedt5)

and the primary leaving

of

particles;

such

type 304 alloy have been composition

(CrFe),,C,.

of the carbide particles are also

evident in what appears to be the initial position of the grain boundary prior to its migration during the 900°C anneal.

The one-to-one

correspondence

between

the ghost and actual positions of the carbide particles

The movement The migration through

of solid particles

in a solid*

of slightly soluble spherical particles

a solid under the influence of a temperature

gradient has been treated theoretically by Shewmon.(l) Tiller@) hasderivedequationsfor themigrationof liquid particles in a solid under the influence of a temperature

is clearly evident in Fig. 1. That the effect shown in Fig. 1 is not an artefact, e.g. due to slippage of the replica during its preparation, is indicated by the electron micrograph of Fig. 2. The latter figure shows the junction of three grain boundaries which have apparently migrated from right to left (in Fig. 2).

At least two former positions

of each