Continuous observation of isothermal martensite formation in Fe-Ni-Mn alloys

Acu mernll. Vol. 32, No. 3, pp. 407-413, 1984 Printed in Great Britain. All rights reserved

Copyright

OOOI-6160/84 53.00 + 0.00 C 1984 Pcrgamon Press Ltd

CONTINUOUS OBSERVATION OF ISOTHERMAL MARTENSITE FORMATION IN Fe-Ni-Mn ALLOYS S. KAJIWARA National Research Institute for Metals, 2-3-12 Nakameguro, Meguro-ku, Tokyo 153, Japan (Received 2 May 1983) Abstract-The isothermal formation of martensite in Fe-23Ni-3.9Mn alloy was continuously observed at low temperature, using a cold stage of optical microscope. Various aspects of the dynamic features of the isothermal transformation have been found; such as (1) the isolated martensite nucleation, (2) the intensive slip in austenite concurrent with martensite formation, and (3) the lack of the autocatalytic effect in the nucleation events. It seems that these characteristic features are important factors for the mode of the martensitic transformation to be isothermal. R&sum&-Nous

avons observe continiiment la formation isothetme de la martensite dans un alliage Fe-23%Ni-3,9%Mn a basse temperature grace au Porte-objet refroidi d’un microscope optique, nous avons observe divers aspects de la transformation isotherme, tels que (1) la germination de la martensite isolte, (2) le glissement intensif dans I’austenite au tours de la formation de la martensite, et (3) l’absence d’effet autocatalytique dans les phenomenes de germination. 11semble que ces traits caractlristiques soient des facteurs importants pour que le mode de la transformation martensitique soit isothenne. ~faaaung-Der Verlauf der isothermen Bildung von Martensit in Fe-23Ni-3,9Mn wurde bei niedriger Temperatur im opt&hen Mikroskop mit Kiihltisch verfolgt. Verschiedene Einzelheiten des dynamischen isothermen Prozesses wurden gefunden, etwa (1) die isolierte Keimbildung des Martensits. (2) die ausgepriigte Gleitung im Austenit, welche die Martensitbildung begleitet, und (3) das Fehlen des autokatalytischen Effektes bei den Keimbildungsvorgingen. Es scheint, als ob diese charakteristischen Einzelheiten wichtig sind, damit die martensitische Umwandlung isotherm ablaufen kann.

1. INTRODUCTION

Isothermal martensitic transformation at low temperatures drew much attention in the 1950’s, because it was thought that its kinetics study may elucidate the nucleation process of martensitic transformation [l-5]. Much effort has been so far made to test if the proposed nucleation models can explain satisfactorily the observed isothermal transformation kinetics, especially, the activation energy for martensite nucleation [5-111. However, most of the experimental work was limited to the measurement of the transformation rate and no extensive studies were made on the crystallographic aspects of the isothermal martensitic transformation. There was little discussion on the validity of the nucleation models from the crystallographic viewpoint. Recently the present author studied in detail the morphology and crystallography of the isothermal martensitic transformation of Fe-Ni-Mn alloys [12] and reached a conclusion that the invariant plane strain condition on the habit plane is operative from a very early stage of the transformation, which is in contradiction with a current nucleation model proposed by Olson and Cohen [lo]. Among interesting results in the previous work [12], the following features may be important to find the mechanism of the isothermal formation of martensite. (I) Large single martensite plates, in the usual sense, do not exist: a 407

martensite plate which appears to be a large single plate on the surface relief actually consists of small fragmentary plates with the same orient_tion. These platelets form a macroscopic habit of (225) as schematically shown in Fig. 1. The habit plane of the martensite platelet is close to (112), though it is considerably scattered. (2) The self-accommodating formation of several variants of the martensite plates is not observed. (3) Intensive slip occurs in austensite on the slip piane which is_closest to-the habit plane, namely, on (111) for the (225) and (112) habits. This slip plane, the macroscopic habit plane and the habit plane of the martensite platelet have a common zone axis as seen in Fig. 1. In the present work continuous observation by optical microscope has been made on the isothermal formation of martensite in Fe-Ni-Mn alloys. This kind of dynamic observation may give important clues to find the basic difference in the formation mode of martensite between the isothermal and athermal transformation. The results are discussed in relation to the proposed rate controlling mechanisms of the isothermal martensitic transformation.

2. EXPERIMENTAL

An Fe-23.OSNi-3.85Mn (wt%) alloy was prepared by arc-melting in argon. The starting materials were

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3. EXPERIMENTAL

Fig. I. Schematic picture of morphology of isothermal martensite. A large number of martensite platelets constitute an apparently large plate. decarburized pure iron plates produced from “electric” iron, high-purity nickel and manganese (99.99%). Chemical analysis showed that interstitial impurities of carbon and oxygen in this alloy are present at less than 30 ppm. A rod-shaped ingot with 40 mm length and 10 mm diameter was homogenized in an evacuated silica capsule at 1373 K for 1 week. After removing a layer of 0.5 mm from the surface, the ingot was cold rolled to 0.35 mm thickness. Specimens with 0.35 x 3 x 20 mm were austenized at 1523 K for 20 min in evacuated capsules and quenched in water without breaking the capsules. In this heat treatment the specimens were wrapped in a foil of Fe40 Mn alloy to prevent the sublimation of Mn atoms. After the heat treatment, a layer of 0.1 mm was removed from the specimen surface by chemical polishing because the composition of Mn near the surface may have been changed during the heat treatment. A high austenizing temperature was necessary to produce large grains. A very simple cold stage was used for optical microscope observation at low temperatures. This stage consists of a cylindrical Dewar vessel with the optically flat bottom. The Specimens were placed on the bottom of the vessel and the observation was made from the bottom side. In order to cool the specimen rapidly to the temperature of isothermal run, normal pentan cooled close to its freezing point (143 K) was poured into the vessel and the temperature of the bath was kept nearly constant by contacting with a small beaker filled with liquid nitrogen. The temperature of the specimen was measured with a copper-constantan thermocouple soldered to the specimen.

RESULTS

Figure 2( l)-( 16) show a series of photographs taken at nearly constant temperature close to the nose-point in the TTT diagram of this alloy. The temperature of the specimen at the time of photographing is shown in Fig. 3, where small vertical lines on the abscissa indicates the moment when each photograph in Fig. 2 was taken. At the end of the isothermal run, the specimen was up-quenched to room temperature to prevent further transformation. As seen Fig. 2(l), the central area in this series of the photos is a large grain and no martensite plates except a faint one (indicated by arrow) were formed during cooling. The following interesting features are noted in this series of the micrographs. (1) The martensite nucleates at isolated plates. Martensite plates indicated by single arrows in Fig. 2--(2), (3), (4), (6), (8), (9) and (12) are evidently such examples. These martensite plates have an appearance of the “W-shape, suggesting the formation of a pair of martensite plates. However, one branch of the “P-shape corresponds to the intensive slip in austenite which occurred with the martensite formation. This will be shown later. (2) These martensite plates grow at later stages, forming many parallel branches. These branches also correspond to intensive slip lines in austenite and the direction of slip lines is parallel to the branches. Such growth is seen in the pair of photos (2)-(3), (3~(4), (5)-(6), (6)-(7), (9)-(10), (12-13) and (13-14) in Fig. 2, respectively. The martensite plates concerned are indicated by single and double arrows. The growth of martensite was jerky and with a very high speed. The direction of the growth of the martensite plate is usually in one direction; that is, the growth direction is in an acute angle with the slip line growth direction. (3) The martensite plates with the same macroscopic habit form side by side. The formation of these martensite plates does not occur in one time: each martensite plate forms one after another. Since it was observed that some of these martensite plates have nucleated at isolated places within the crystal, we can reasonably assume that the nucleation sites for the other martensite plates were also isolated places. A stereographic analysis of the martensite plates in Fig. 2 is shown in Fig. 4. The austenite orientation of the region concerned was determined from four directions of slip lines observed, using a method developed by Takeuchi et al. [13]. It was found by successive removal of the surface that martensite plates other than four variants with the macroscopic habits R,, R2, R, and & are surface martensite. The surface traces H,, H,, H, and H4, of these four variants are shown in Fig. 2(16). The martensite platelets with {112} habit planes and the particular slip planes in autensite, both of which are closely associated with the respective macroscopic habit planes (see Fig. 1), are also shown in Fig. 4 by Ii,, li2, liJ, 3i, and s,, s,, S,, S,, respectively. (Their surface

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traces and slip line directions are denoted by 11,. 11:. h,, h,, and S,, S, Sz. S,. respectively.) The surface traces of the martensite platelets were obtained from etched patterns such as shown later in Fig. 5(c). Although four variants of martensite were observed, the majority of the martensite plates formed belong to the variant with A, or n4 habit plane. and furthermore it is noted that martensite plates with the I?., habit have formed in the initial stage [Fig. 2(3H5)]. followed by those with R, [Fig. 2(6)-(IO)]. and at later stages both the variants have appeared concurrently [Fig. 2(1 I)-(15)]. Such a formation se-

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quence is quite characteristic of the isothermal martensitic transformation. Figure 5 shows an area identical to the framed region in Fig. 2( 16) after the specimen was upquenched to room temperature: (a) being surface relief. and (b) and (c) etched patterns. Layers of 5 and 20 pm have been removed from the specimen surface for Fig. 5(b) and (c), respectively. Some of the martensite plates seen in Fig. 5(a) and (b) disappear completely in (c). These are surface martensite. The other martensite plates become fragmentary in (c). These small plates correspond to the ( 1121martensite

Fig. 2( l-8).

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Fig. 2. Series of micrographs taken in situ during the isothermal run at 140 K. showing the nucleation and growth of martensite plates. Time (min) elapsed after quenching to 140 K was as follows; (1) 0, (2) I. (3) 2. (4) 3. (5) 4. (6) 7. (7) 8. (8) 9. (9) IO. (IO) 12. (II) 14. (12) 16. (13) 18. (14) 20. (15) 25 and (16) 30. which constitute a (225) macroscopic habit as shown in Fig. 1. The tendancy of the coalescence of the { 112) platelets into a large martensite plate is greater near the specimen surface. which indicates that the growth of the platelets is much easier near the surface. It should be noted that the martensite platelets with Ti, and A, habits are seen in (c) even at places where no corresponding plates are observed in (a) and (b). Some of these platelets are isolated. This fact shows that martensite can nucleate interior the platelets

specimen and yet does not grow to reach the spedmen surface in some cases. By comparing identical martensite plates in Figs 2 and 5, we can prove the earlier statement that the apparent “V”-shape of a freshly nucleated martensite plate and many parallel branches produced in the subsequent growth are merely due to the intensive slip in austenite associated with the martensite formation As typical examples. we take two martensite plates indicated by arrows in Fig. 2( II). The identical

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2732

250-

: =I ‘0

200-

I i

I

: t c”

163-148-143-1

IIIIC

0

/‘II

5

Time

Fig. 3.

c

1

,

20

30

8

IO (mlnl

Temperamre of the specimen during photographing the series of micrographs

shown

in Fig. 2.

plates in Fig. 5 are also shown by short arrows, and it is evident in this figure that there exist no martensite variants corresponding to the branches of the martensite plates in Fig. 2(11). 4. DISCL’SSION Figure 6(a), (b) and (c) show a schematic representation of the formation step of an apparently large martensite plate with the j225) habit. which is deduced from the present observations: (a) a martensite platelet nucleates with intensive slip in austenite. (b)

at a later stage a chain of the platelets are formed almost instantaneously, and (c) at further later these platelets grow and coalesce into a large plate. The intensive slip in austenite produced with the martensite formation is caused to accommodate the shape strain of the transforming martensite plate as concluded in the previous work by detailed crystallographic analysis (121. This conclusion is supported by the “V”-shape appearance of a freshly nucleated martensite plate mentioned in the preceding section, for the observed “V-shape resembles very much the surface relief of a pair of selfaccommodating martensite plates and this similarity means that the intensive slip occurred in austenite plays an identical role to the variant formation in accommodating the shape strain. The occurrence of intensive slip in autensite only in one side of a martensite plate with the 1225) macroscopic habit. which was listed as one of the unexplained problems in the previous work [12], is now understandable when we consider the formation process of a large unified martensite plate; that is, as shown in Fig. 6. a band of intensive slip lines are produced for each platelet in the same way as for the first one. It is rather surprising that the martensite nucleation at an isolated place such as schematically shown in Fig. 6(a) occurs not only in the initial stage of

Too

Fig. 4. Stereographic projection of habit planes and associated slip planes observed in Fig. 2. Pole P is specimen normal H,.h,and S, (i = 1. 2. 3. 4) are the observed surface traces of macroscopic habit plane. martensite platelet and slip plane, respectively. Part of the great circle representing plane normals which have the zone axis parallel to the observed surface trace is shown for each martensite plate. 4M

321

H

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chain reaction. there is a definite distance between the neighbouring platelets. These facts give serious doubt to the well known assumption in the martensite kinetic study that a large number of martensite plates are formed from an existing martensite plate. (This is called as the autocatalytic effect.) This assumption has been so far considered to be a priori valid. If we take the autocatalytic parameter to be 2 x 10’” per cm’ of martensite according to the Pati and Cohen’s estimate [7], the number of newly formed martensite plates per one existing plate would be 8. which is quite unrealistic in view of the present observations. The lack of this autocatalytic effect in the martensite formation is the most characteristic kinetic feature for the isothermal martensitic transformation. This is considered to be due to that the shape strain of a

Fig. 5. Micrographs corresponding to the framed region m Fig. 2( 16). taken after up-quenching lo room temperature: (a) surface relief. (b) and (c) etched patterns after removing layers of 5 and 20 ltm from the surface. respectively.

isothermal run but also in the later stages where man) martensite plates have already been formed. Very few martensite plates were observed to have nucleated from the immediate neighbour of the existing plates. Electron microscopic observations in the previous work [12] showed that. even in the case of Fig. 6(b) where a group of martensite platelets form by the

Fig. 6. Schematic representation of the formation step of a large martensite plate with the 1225j macroscopic habit plane.

transforming martensite plate is accommodated by slip in austenite. On the other hand. in the case of athermal martensitic transformation, the shape strain of the martensite plates is usually self-accommodated by formation of several variants of martensite, and in some ferrous alloys the autocatalytic effect is extremely prominent and a burst type transformation occurs. Such difference in the mode of the accommodation of the shape strain of the martensite plate is probably due to a difference in the yield stress of austenite; that is. the yield stress for the alloys undergoing isothermal martensitic transformation is much smaller and consequently the plastic accommodation is much easier. Actually the present author measured the yield stresses of austenitic specimens of Fe-37.5Ni. Fe-34Ni-0.4C and Fe-26Ni-3.6Mn to be 480. 660 and 220 MPa at 77 K, respectively. The alloy contents of the former two are close to those of Fe-Ni and Fe-Ni-C alloys exhibiting typical burst type transformation while that of the latter close to the alloy exhibiting isothermal transformation. Although there is no autocatalytic effect in the usual sense, an accelerated stage is observed on the isothermal transformaton curve in a certain condition [4, 6, 141. This fact may be explained by that dislocations produced by the martensite formation can contribute to activate potential nuclei of martensite in the neighbouring region, because the plastic accommodation of the shape strain will be made easier by the existence of such mobile dislocations unless there is an appreciable work hardening. From the above discussion it seems that the plastic accommodation of the shape strain of the martensite plate is an important necessary condition for the transformation to be isothermal. This is in accordance with the proposition made by the present author in the previous work [12] that the rate controlling factor for isothermal martensitic transformation is thermally activated motion of dislocations in austenite to accommodate the shape strain of the transforming martensite plate. The evidence for that the dislocation motion in the austenite at low temperature is a thermally activated one is provided by a remarkable temperature dependence of the yield

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stress of the austenitic specimen observed in Fe-26Ni-3.6Mn [ 141.If the plastic accommodation in austenite is achieved by athermal motion of dislocations, the transformation will be athermal even in the case where the self-accommodating formation of martensite variants does not occur. Such an example is seen in an Fe-8Cr-1.1 C alloy [15, 161. Besides the plastic deformation in austenite to accommodate the shape strain, two other deformations are involved in the martensitic nucleation, that is, the “lattice deformation” which generates a martensite lattice from austenite, and the “lattice invariant deformation” which produces a strain-free interface between the two phases. These deformations could be possible rate limiting steps for the isothermal martensitic transformation. Raghavan and Cohen [6] proposed that the rate limiting step is the isothermal formation of dislocation loops to accomplish the lattice invariant deformation. (The original concept of this mechanism was presented by Kaufman and Cohen [S].) On the other hand, Olson and Cohen [lo, 111 concluded recently that the most probable rate limitting step is a thermally activated motion of partial dislocations bounding the semicoherent embryo which is assumed to form in their nucleation model. The latter proposal is equivalent to saying that the lattice deformation is the rate controlling, for the formation of these partials and their subsequent movement constitute an important part of the lattice deformation in their nucleation model. Both of the proposed mechanisms, however, can not explain the following experimental facts found very recently by the present author [ 141.(1) The nucleation rate of isothermal martensite under applied stress is drastically increased when the stress exceeds a certain level. (2) This critical stress level coincides with the yield stress of austenite if the chemical driving energy is not very large, and consequently when the yield stress is increased by refinement of the grain size, the critical stress becomes higher. Such effect of the applied stress is explained straightforwardly by the present author’s mechanism. 5. SUMMARY

Continuous observation of the isothermal mation of martensite in Fe-23.05Ni-3.85Mn

forhas

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been made at 140 K, using a cold stage of optical microscope. The results are summarized as follows, (1) The formation step of an apparently large martensite plate with the {225} habit has been clarified. (2) The martensite nucleates at isolated places not only at the early stage of the isothermal run but also at later stages where many martensite plates have been already formed. (3) The martensite plates with the same macroscopic habit form side by side, and each of them has nucleated one after another at some separated places. (4) The autocatalytic effect, in the usual sense, in the martensitic nucleation is hardly observed. (5) The present dynamic observations support the rate controlling mechanism proposed by the present author for the isothermal martensitic transformation.

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