Kinetics of the fcc to hcp phase transformation and the formation of martensite in pure cobalt

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Vol. 32,No. 10, pp. 1671..1676,1995 Copyright 0 1995 Elsevier Science Ltd printed in the USA. All rights reserved 0956-716X/95 $9.50 + .OO

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KINETICS OF THE FCC TO HCP PHASE TRANSFORMATION AND THE FORMATION OF MARTENSITE IN PURE COBALT Ji-Cheng Zhao and Michael R. Notis Department of Materials Science and Engineering Lehigh University, Bethlehem, PA 180153195 USA (Received August 23,1994) (Revised December 21,1994)

Introduction The fee to hcp (a-+&) phase transformation in pure cobalt has been studied extensively by monitoring the cooling transformation-start temperature which was found to vary from about 426 “C down to about 330 “C, depending on experimental conditions [l-3]. However, the overall transformation kinetics is not clear with regards to how the cooling rate, impurity concentration, and grain size influence the transformation process; and as to how many martensitic phases may exist. Recently, some general characteristics for the trend in the variation of cooling transformation start temperature with cooling rate have been emphasized [4,5]. These characteristics hold for all metallic elements and alloys. In this paper, we employed them to analyze the transformation kinetics observed in pure cobalt.

Cooling

Transformation

Kinetics

of Cobalt

A number of systematic investigations [ 1, 6-101 can be found in the literature concerning the variation of a+~ transformation temperature (T,) with cooling rate (f); they are summarized in Figs. 1 and 2. At very low cooling rates, T, should be asymptotic to the alE equilibrium temperature which is widely identified as 422 “C [2,3,1 I]. With increasing cooling rate, T, decreases until it levels off at some cooling rate (-10 “C/s in Fig. l), producing a plateau in the corresponding T,-? diagram. At higher cooling rates (>-lo0 “C/s in Fig. l), T, yields to another plateau; and, as the cooling rate exceeds about 5~10~ “C/s, T, again yields to a third plateau, as shown in Fig. 1. The first plateau at -390 “C is for the fee to E martensite transformation (discussed in more detail below) which demonstrates that the corresponding martensitic transformation-start temperature, M:, is independent of cooling rate. Except for this plateau, each of the other two plateaux should also signify a separate martensitic transformation [4,5]. In this connection, there should exist at least three kinds of martensite (including E martensite) in pure cobalt. We denote the martensite of the second and the third plateau as E’ martensite and E” martensite, respectively. A 9R martensite has already been identified by Kajiwara et al. [12,13] in rapidly quenched ultra-fine particles of pure cobalt. Based on this result, the E” martensite might be expected to have the 9R structure. Kajiwara et al. found no other martensite, except for the E martensite and the 9R

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FCC TO HCP TRANSFORMATION

Bibby8 Parr Hashimoto

-_- ____-_________ *..

Vol. 32, No. 10

0 Mitzayev 8 Wyanov A Mitzayev & Schastlivtsev

fcdhcp equilibrium temperature

.

“‘7--i&

& martensite

y*\;,+ E’ martensite

0 E” martensite 300’ 10-1

. . ......I

100

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10’

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102

. . ......’

..‘...I

lo3

10’

.

..‘...I

lo5

. . ‘-

106

Cooling Rate, “C/s Fig.

1 The transformation (start) temperature vs cooling showing the overall cooling transformation kinetics

4601

4

. .....*.

........

__.....

rate (Tt-*) diagram for pure cobalt for the formation of different products.

. .......

. _.‘.‘.I

.

._‘._.I

. . ---

Cobalt

m

..,\, -------

fcc/hcp equilibriumtemperature

0

0

Hashimoto

0 Troiano 8 Tokich -

300’ 10’3

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-*.....*

10-l

......’ loo

. ....-.’ 10’

.

......, . .......’ 103 1oa

...-

10’

Cooling Rate, “C/s

Fig. 2 The transformation (start) temperature vs cooling showing the difference in kinetics for the formation

rate Cr,-@ diagram for pure cobalt of hcp phase in different specimens.

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FCC TO HCP TRANSFORMATION

martensite. We will explain transformation kinetics.

their

observation

1673

after discussing

the effect

of grain

size on the

Figure 2 compares the experimental results of three investigations concerning E phase formation [ 1,7,10]. The curved lines in Fig. 2 are conjectured in order to explain the effect of grain size and impurities on the transformation kinetics. It is clear that the cooling rate range (i.e., the plateau range) for the formation of E martensite varies from specimen to specimen, but the Mg remains nearly unchanged. This phenomenon is true for martensitic transformations in a wide variety of materials [4,5], i.e., grain size often alters the cooling rate range for the formation of each product, but very often it has only a trivial effect on the M,. Similarly, impurities often shift the formation range of each product to the lower cooling rate region, but they have no or only a trivial effect on the M, [4,5]. In this way, even at the same cooling rate, different products can be obtained in different specimens of different grain size and impurity concentration. For example, at a cooling rate of 1x 10’ “C/s, Troiano and Tokich [l] obtained a E martensite (Fig. 2); whereas, at the same cooling rate, both Bibby and Parr [6] and Mirzayev et al. [8,9] obtained the E’ martensite (Fig. 1). Unfortunately, the two latter groups did not provide crystal structure analysis of their specimens. The E’ martensite probably has a 4H (double hcp) structure, but this point needs to be confirmed experimentally in the future. Sometimes, metastable products which are formed in specimens of a certain grain size can be suppressed (i.e. cannot be formed) in specimens of other grain size. There is no systematic investigation available for pure cobalt, but there is evidence available for pure iron and steels [4], as shown in Fig. 3 for zone-refined pure iron [14]. The massive ferrite and bainitic ferrite which are formed in specimens with 7pm grain size cannot be formed in specimens with 150 pm grain size. This phenomenon may help to explain why only two kinds of martensite (E and 9R) was observed in the ultra-fine particles of Kajiwara et al. The absence of the other E’ martensite in their particles may be due to one or both of the following reasons: the grain size (particle size) is not appropriate for the formation of this martensite; or, the cooling rates they used (determined by only three kinds of gas or gas mixtures) were not in the formation range of this martensite. This explanation can be justified by the fact that a 4H martensite which was confirmed to exist in Co-Fe alloys [ 151 was also not found in their ultra-fine Co-Fe particles [13]. In addition, Mirzayev and Ul’yanov [8] and Mirzayev and Schastlivtsev [9] had already found a dramatic difference in morphology for the A

V y

grain size, brn

Bainitic ferrite

600XL_

_~_____X____x___x-x~__-___

Lath martensite x__-__--__*_x_LI_

soo-

G00O

I

I

I

I

I

IA

10

20

30

40

50

60

Cooling rate, K S-’ Fig. 3

v

I

I

I

I

100

180

260

300

4 0

x103

The transformation (start) temperature vs cooling rate diagram (T,-? diagram) for zone-refined pure iron showing the grain size effect on the kinetics for the formation of different products.

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transformation product formed at the second and the third plateau in Fig. 1, which is also an indication of the existence of three different kinds of martensite in pure cobalt. As an interesting comparison, the bee to hcp transformation in pure Ti and pure Zr also display several plateaux [.5&j. Each of these plateaux corresponds to a specific martensite [5,8]. At a cooling rate lower than the threshold (-10 “C/s in Fig. 1) of the first plateau, the transformation product in pure cobalt is the low temperature equilibrium phase (E). The transformation takes place by nucleation at and growth (mostly) along grain boundaries of the fee phase. The resultant microstructure is usually called an allotriomorph (or grain boundary E). The first plateau at -390 “C can be a massive transformation or a martensitic transformation. The plateau itself conveys no information to tell the difference because the crystal structure of the product phase formed in this first plateau region is the *same as that of the equilibrium phase (E). Both transformation modes exhibit a plateau in T,-T diagrams in terms of overall transformation kinetics [4,5]. However, the transformation mechanisms are different. It is believed that interface movement is the controlling factor of a massive transformation [16] and that localized diffusion takes place at the transformation interfaces; in this connection a massive transformation is a (weakly) thermally activated process. Martensitic transformation, however, involves only a shear (or displacive) mode and is not a thermally activated process. Therefore, it is possible to differentiate the two transformation modes by analyzing the morphology and the interface structure of the transformation product. If this plateau is a massive transformation, the morphology of the E phase should exhibits “massive” patches of grains that are surrounded by irregular boundaries consisting of a mixture of planar and curving sections [17]; there should be no surface relief effect. If the plateau is a martensitic transformation, E martensite usually exhibits a lath-like or plate (needle)-like morphology, the E/cx interfaces should be coherent, and usually exhibits a specific crystallographic relationship. Moreover, there is usually observable surface relief effect. For many alloys and pure elements, the first plateau is found to be a massive transformation [SJ. For instance, the first plateau in the T,-T diagram (Fig. 3) of pure iron is confirmed to be a massive transformation [4,14]. The situation for pure cobalt seems different in that most researchers [ 1,2,7,10,18-291 believe that the a+& transformation is martensitic, mostly due to a consideration that the fee (a) to hcp (E) transformation involves only a change in the stacking sequence of the close packed planes of both phases. Many of these investigations [ 18-291 are in agreement that the transformation is martensitic, although discrepancy exists concerning the exact mechanism of transformation. The cooling rates used in these studies may not be in the corresponding range of the first plateau (recall that the cooling rate range depends on the grain size and the impurity concentration); thus all the evidence provided by these studies is indirect. It is, however, reasonable to designate the first plateau in Figs. 1 and 2 as E martensite. For an alloy (not for a pure element) at cooling rates lower than the threshold of the first plateau, the transformation takes place by nucleation of a new phase particle with a composition different from that of the original phase. Such a process involves long-range diffusion. Because longrange diffusion is time dependent, it should be cooling rate dependent as well. In other words, the transformation temperature in this cooling rate range should also be cooling rate dependent. The fact that M, and the massive transformation-start temperature, M,, are (nearly) independent of cooling rate implies that long-range diffusion no longer plays a dominant role in these types of transformations. In this connection, the threshold of the plateau should signify the onset cooling rate for a massive

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or a martensitic transformation. The situation becomes complicated for pure elements (like cobalt) because there is no long-range diffusion at any cooling rate. Thus, massive transformation in pure elements cannot be defined by the composition-invariant nature but can only be defined by its mechanism (interface controlled) and by its resultant morphology. In this situation, the threshold of the first plateau may not be the clear-cut onset for a massive transformation. The cooling a+& transformation-start temperature is often indiscriminately referred as “M,” in the literature [2,3,7,10]. This is an inappropriate practice. It is clear from Fig. 1 that at low cooling rates (c-10 “C/s) the transformation is not martensitic and the product is simply grain boundary E. Only when the cooling rates are high enough can martensite be produced and the plateau temperatures are the real M, points. At low cooling rates, the a+& transformation-start temperature can vary all the way from the equilibrium temperature of 422 “C down to the first plateau temperature of -390 “C [5], i.e., any temperature between 422 and 390 “C is possible, and accounts for the wide variation of “M,” temperature observed in different investigations.

Summary The experimental data on the kinetics of the fee to hcp phase transformation in pure cobalt have been summarized (in Figs. 1 and 2) and analyzed in conjunction with the recent finding of a metastable 9R martensite structure by Kajiwara et al. [12,13]. From the observed transformation kinetics, it is predicted that there should exist another martensitic structure for cobalt in addition to the E martensite and the 9R (E”) martensite, i.e., there are three kinds of martensite. Each martensite has a separate M, temperature, which is identified as 39Ok5 “C (E), 370+8 “C (E’) and 330215 “C (E”), respectively. The absence of this unidentified E’ martensite in the ultra-fine particles of Kajiwara et al. does not exclude its existence in other forms of pure cobalt. Grain size and impurity effects on the transformation kinetics are also discussed and used to explain the experimental observations in both bulk and ultra-fine particle cobalt.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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