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Influence of precipitation in austenite on the morphology of martensite in FeNiCoTa alloys
Scripta METALLURGICA
Vol. 8, pp. 351-356, 1974 Printed in the United States
Pergamon Press, Inc.
INFLUENCE OF PRECIPITATION IN AUSTENITE ON THE MORPHOLOGY OF MARTENSITE IN Fe-Ni-Co-Ta ALLOYS M. Laverroux and A. Pineau Centre des Mat~riaux, Ecole des Mines de Paris B.P, 114 - 91102 - Corbeil-Essonnes France
(Received February ii, 1974)
Introduction In Fe-Ni alloys many authors have shown that two main morphologies of martensite can be clearly identified. The microstructure of low Ni alloys (10 < Ni < 28 Wt pct) is currently described as lath martensite, while in high Ni alloys (Ni > 31Wt pct) lenticular martensite -also named plate-like martensite- is observed. Attempts have been made to correlate the martensite morphology with variables such as composition, temperature, and strength of austenite and/or martensite. For example, i t was suggested that the morphological transition is governed by the temperature of transformation (see e.g. Ref. 1). An increase in Nickel content leading to a decrease in Ms temperature is conducive to a transition from lath to lenticular martensite. On the other hand, the influence of the strength of austenite and martensite at the Ms temperature upon the microstructure has also been emphazised. In Fe-Ni alloys, chosen so that the austenites were paramagnetic, ferromagnetic, substitutionnal and i n t e r s t i t i a l strengthened, Davies and Magee showed that the most important variables in determining the morphology of martensite was the resistance to dislocation motion in austenite and martensite (2,3). These authors only studied short range strengthening mechanisms, such as "Invar Strengthening" and solid solution strengthening. We investigated the influence of precipitation in austenite on the morphology of martensite formed subsequently by quenching (4). In this note, a short account of these results is given for precipitation hardening of Fe-Ni-Co-Ta alloys. Experimental Results and Discussion The compositions of the alloys investigated were Fe-23Ni-8Ta (Wt%) (Alloy I) and Fe-18Ni-8Co-8Ta (Wt%) (Alloy 2). The specimens were given an austenitizing treatment at
351
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1250°C for 30 min and subsequently aged in y phase near 600°C before quenching. The a11oys were chosen so that their as-quenched microstructure was lath martensite. I~ alloy I, aging results in the precipitation of body-contered tetragonal (D022) y" particles. This type of precipitation was previously investigated in a Fe-31Ni-gTa (5,6). The precipitates composition is approximately Ni3Ta. In alloy 2, aging gives rise to the fomation of cubic (L12)Y' particles whose composition is about (Ni, Co)3 Ta. The Ms temperature of unaged specimens was determined by dilatemetry (Alloy 1 : Ms = 35°C ; alloy 2 : Ms = 320°C). In alloy I, the Ms temperature subsequent to austenite aging was also measured. The precipitation reaction which occurs in austenite gives rise to a decrease in Ni, Co and Ta contents of the remaining solid solution. The Ms temperature of an unprecipitated solid solution decreases when the content in Ni and Ta are increased. We was also confirmed that the decrease in Ni and Ta content favored the lath morphology. Thus, in alloy 1, i~ the temperature of transformation and/or composition are the most important variables in governing the martensite structure, the composition changes accompanying the precipitation reaction should always give rise to lath martensite. In alloy 2, the composition of the remaining matrix solution subsequent to an aging of austenite is not so clearly defined because o f t h e segregation of Co in precipitates. This alloy was chosen because of its higher Ms temperature in the as-quenched condition. I t was thus possible to investigate whether a morphological change could be observed in an alloy which has a higher temperature of transformation. Alloy 1 was aged at 645°C, alloy 2 at 600°C. The resulting hardness of martensite is given on FIG. 1 and 2 respectively. On FIG. 1, the variation in the Ms temperature of alloy I
FIG. 1
FIG. 2
HV
HV
~
50(] Lath
,oc ...~f/!.~..o
. ~ . . . . Ms
O~4~ct (hoym) 3O0
1
10
Lath/i /o
40(]
J
As,quenched Alloy 1 - Fe-23Ni-8Ta aged at 645°C. Hardness of martensite and Ms temperature (°C) versus aging times (t). The transition in martensite morphology is indicated by an arrow.
° ~ o
Plate °
o.//° 300 1 As.quenched i
i 10
i
100
t(hours)
Alloy 2 - Fe-18Ni-8Co-8Ta. Hardness of martensite versus aging times (t) at 600°C. The transition in martensite morphology is indicated by an arrow.
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AUSTENITE AND MORPHOLOGY OF MARTENSITE
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FIG. 3
Alloy 1, change in martensite morphology after various aging times (t} at 645°C. a) as-quenched ; b) t = 4 hours ; c) 8 hours ; d) 16 hours
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FIG. 4
a
b
c
d
Alloy 2, variation in martensite morphology subsequent to various aging times (t) at 600°C. a) as-quenched ; b) t = 16 hours ; c) and d) t = 32 hours
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plotted. An expected increase in the temperature of transformation is observed.
However this variation in Ms cannot be unambiguously used to calculate the composition of the remaining solid solution. In a similar Fe-Ni-Ta alloy, i t was found that the Ms temperature was not only dependent on composition, but that small y" particles affected the transformation temperature (4). In both alloys, the martensite morphology formed by quenching and aging for various times was observed. The microstructure of alloy 1 is given in FIG. 3. Without aging, the lath morphology typical of a r e l a t i v e l y low transformation temperature is observed (FIG. 3a). This morphology was identified for aging times up to 4 hours (FIG. 3b). Longer aging times resulted in a microstructural change. After 8 hours, large plate-like areas can be seen (FIG. 3c). The size of these areas is much bigger than that of the martensite laths. When alloy I is aged for 16 hours or 32 hours, the microstructure is almost lenticular martensite (FIG. 3d). In FIG. 3c and 3d, the mid-ribs associated with some of the martensite plates are indicated by an arrow. Transmission electron microscopy observation showed that these mid-ribs consist of a high density of thin twins. Thus, the salient features of the lenticular martensite microstructure are identified. In alloy 2, a similar change in martensite morphology was observed (FIG. 4). In this figure the
microstructure of the as-quenched alloy is shown (FIG. 4a). This structure is a
clearly defined lath morphology. The size of the laths is bigger than that of alloy 1. This morphology persisted for aging times up to 8 hours, although an important increase in hardness is measured (FIG. 2). Longer aging times resulted in a gradual microstructural change. After 16 hours, some large lenticular areas as in alloy 1 can be seen (FIG. 4b). When this alloy is aged for 32 hours or longer, the amount of areas which have a lenticular morphology is increased (FIG. 4c, d). Thus a similar behaviour to that of alloy 1 is evidenced. In both alloys, the variations in austenite hardness could not be measured. However, i t is l i k e l y that a strengthening of the austenite of the same order of magnitude than that of the martensite is induced by the precipitation reaction. In an Fe-Ni-Ta alloy of a higher Hi content and p a r t i a l l y transformed to martensite, microhardness measurements indicated that the increase in hardness of the martensite was the same as that of the austenite. Thus, these observations show unambiguously that a precipitation reaction is austenite and the resulting strengthening of austenite and martensite can drastically change the martensite morphology. These results are in agreement with those of Davies and Magee. However, i t should be emphasized that these authors used short range strengthening mechanisms while, in our study, precipitation hardening is a much longer range strengthening mechanism. Our experiments support the assumption postulated by Davies and Magee. Moreover, they indicate that, whatever i t s origin, the resistance to dislocation motion in austenite and martensite is an important variable in determining the martensite structure.
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References (1) W.S. Owen; E.A. Wilson ; T. Bell High strength materials ed V. Zackay, p. 167. John Wiley and Sons New-York 1965. (2) R.G. Davies ; C.L. MBgee, Met. Trans.,~ (1970), p. 2927. (3) R.G. Davies ; C.L. Magee, Met. Trans., 2 (1971), p. 1939. (4) M. Laverroux, Thesis Nancy 1973. (5) R. Cozar ; G. Rigaut ; A. Pineau, Scripta Met., 3 (1969), p. 883. (6) R. Cozar ; A. Pineau, Scipta Met., ~ (1973), p. 851.
Vol. 8, pp. 351-356, 1974 Printed in the United States
Pergamon Press, Inc.
INFLUENCE OF PRECIPITATION IN AUSTENITE ON THE MORPHOLOGY OF MARTENSITE IN Fe-Ni-Co-Ta ALLOYS M. Laverroux and A. Pineau Centre des Mat~riaux, Ecole des Mines de Paris B.P, 114 - 91102 - Corbeil-Essonnes France
(Received February ii, 1974)
Introduction In Fe-Ni alloys many authors have shown that two main morphologies of martensite can be clearly identified. The microstructure of low Ni alloys (10 < Ni < 28 Wt pct) is currently described as lath martensite, while in high Ni alloys (Ni > 31Wt pct) lenticular martensite -also named plate-like martensite- is observed. Attempts have been made to correlate the martensite morphology with variables such as composition, temperature, and strength of austenite and/or martensite. For example, i t was suggested that the morphological transition is governed by the temperature of transformation (see e.g. Ref. 1). An increase in Nickel content leading to a decrease in Ms temperature is conducive to a transition from lath to lenticular martensite. On the other hand, the influence of the strength of austenite and martensite at the Ms temperature upon the microstructure has also been emphazised. In Fe-Ni alloys, chosen so that the austenites were paramagnetic, ferromagnetic, substitutionnal and i n t e r s t i t i a l strengthened, Davies and Magee showed that the most important variables in determining the morphology of martensite was the resistance to dislocation motion in austenite and martensite (2,3). These authors only studied short range strengthening mechanisms, such as "Invar Strengthening" and solid solution strengthening. We investigated the influence of precipitation in austenite on the morphology of martensite formed subsequently by quenching (4). In this note, a short account of these results is given for precipitation hardening of Fe-Ni-Co-Ta alloys. Experimental Results and Discussion The compositions of the alloys investigated were Fe-23Ni-8Ta (Wt%) (Alloy I) and Fe-18Ni-8Co-8Ta (Wt%) (Alloy 2). The specimens were given an austenitizing treatment at
351
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AUSTENITE AND MORPHOLOGY OF MARTENSITE IN Fe-Ni-Co-Ta ALLOYS Vol. 8, No. 4
1250°C for 30 min and subsequently aged in y phase near 600°C before quenching. The a11oys were chosen so that their as-quenched microstructure was lath martensite. I~ alloy I, aging results in the precipitation of body-contered tetragonal (D022) y" particles. This type of precipitation was previously investigated in a Fe-31Ni-gTa (5,6). The precipitates composition is approximately Ni3Ta. In alloy 2, aging gives rise to the fomation of cubic (L12)Y' particles whose composition is about (Ni, Co)3 Ta. The Ms temperature of unaged specimens was determined by dilatemetry (Alloy 1 : Ms = 35°C ; alloy 2 : Ms = 320°C). In alloy I, the Ms temperature subsequent to austenite aging was also measured. The precipitation reaction which occurs in austenite gives rise to a decrease in Ni, Co and Ta contents of the remaining solid solution. The Ms temperature of an unprecipitated solid solution decreases when the content in Ni and Ta are increased. We was also confirmed that the decrease in Ni and Ta content favored the lath morphology. Thus, in alloy 1, i~ the temperature of transformation and/or composition are the most important variables in governing the martensite structure, the composition changes accompanying the precipitation reaction should always give rise to lath martensite. In alloy 2, the composition of the remaining matrix solution subsequent to an aging of austenite is not so clearly defined because o f t h e segregation of Co in precipitates. This alloy was chosen because of its higher Ms temperature in the as-quenched condition. I t was thus possible to investigate whether a morphological change could be observed in an alloy which has a higher temperature of transformation. Alloy 1 was aged at 645°C, alloy 2 at 600°C. The resulting hardness of martensite is given on FIG. 1 and 2 respectively. On FIG. 1, the variation in the Ms temperature of alloy I
FIG. 1
FIG. 2
HV
HV
~
50(] Lath
,oc ...~f/!.~..o
. ~ . . . . Ms
O~4~ct (hoym) 3O0
1
10
Lath/i /o
40(]
J
As,quenched Alloy 1 - Fe-23Ni-8Ta aged at 645°C. Hardness of martensite and Ms temperature (°C) versus aging times (t). The transition in martensite morphology is indicated by an arrow.
° ~ o
Plate °
o.//° 300 1 As.quenched i
i 10
i
100
t(hours)
Alloy 2 - Fe-18Ni-8Co-8Ta. Hardness of martensite versus aging times (t) at 600°C. The transition in martensite morphology is indicated by an arrow.
Vol.
8, No. 4
AUSTENITE AND MORPHOLOGY OF MARTENSITE
IN Fe-Ni-Co-Ta ALLOYS
FIG. 3
Alloy 1, change in martensite morphology after various aging times (t} at 645°C. a) as-quenched ; b) t = 4 hours ; c) 8 hours ; d) 16 hours
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FIG. 4
a
b
c
d
Alloy 2, variation in martensite morphology subsequent to various aging times (t) at 600°C. a) as-quenched ; b) t = 16 hours ; c) and d) t = 32 hours
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plotted. An expected increase in the temperature of transformation is observed.
However this variation in Ms cannot be unambiguously used to calculate the composition of the remaining solid solution. In a similar Fe-Ni-Ta alloy, i t was found that the Ms temperature was not only dependent on composition, but that small y" particles affected the transformation temperature (4). In both alloys, the martensite morphology formed by quenching and aging for various times was observed. The microstructure of alloy 1 is given in FIG. 3. Without aging, the lath morphology typical of a r e l a t i v e l y low transformation temperature is observed (FIG. 3a). This morphology was identified for aging times up to 4 hours (FIG. 3b). Longer aging times resulted in a microstructural change. After 8 hours, large plate-like areas can be seen (FIG. 3c). The size of these areas is much bigger than that of the martensite laths. When alloy I is aged for 16 hours or 32 hours, the microstructure is almost lenticular martensite (FIG. 3d). In FIG. 3c and 3d, the mid-ribs associated with some of the martensite plates are indicated by an arrow. Transmission electron microscopy observation showed that these mid-ribs consist of a high density of thin twins. Thus, the salient features of the lenticular martensite microstructure are identified. In alloy 2, a similar change in martensite morphology was observed (FIG. 4). In this figure the
microstructure of the as-quenched alloy is shown (FIG. 4a). This structure is a
clearly defined lath morphology. The size of the laths is bigger than that of alloy 1. This morphology persisted for aging times up to 8 hours, although an important increase in hardness is measured (FIG. 2). Longer aging times resulted in a gradual microstructural change. After 16 hours, some large lenticular areas as in alloy 1 can be seen (FIG. 4b). When this alloy is aged for 32 hours or longer, the amount of areas which have a lenticular morphology is increased (FIG. 4c, d). Thus a similar behaviour to that of alloy 1 is evidenced. In both alloys, the variations in austenite hardness could not be measured. However, i t is l i k e l y that a strengthening of the austenite of the same order of magnitude than that of the martensite is induced by the precipitation reaction. In an Fe-Ni-Ta alloy of a higher Hi content and p a r t i a l l y transformed to martensite, microhardness measurements indicated that the increase in hardness of the martensite was the same as that of the austenite. Thus, these observations show unambiguously that a precipitation reaction is austenite and the resulting strengthening of austenite and martensite can drastically change the martensite morphology. These results are in agreement with those of Davies and Magee. However, i t should be emphasized that these authors used short range strengthening mechanisms while, in our study, precipitation hardening is a much longer range strengthening mechanism. Our experiments support the assumption postulated by Davies and Magee. Moreover, they indicate that, whatever i t s origin, the resistance to dislocation motion in austenite and martensite is an important variable in determining the martensite structure.
356
AUSTENITE AND MORPHOLOGY OF MARTENSITE
IN Pe-Ni-Co-Ta ALLOYS
Vol.
8, No. 4
References (1) W.S. Owen; E.A. Wilson ; T. Bell High strength materials ed V. Zackay, p. 167. John Wiley and Sons New-York 1965. (2) R.G. Davies ; C.L. MBgee, Met. Trans.,~ (1970), p. 2927. (3) R.G. Davies ; C.L. Magee, Met. Trans., 2 (1971), p. 1939. (4) M. Laverroux, Thesis Nancy 1973. (5) R. Cozar ; G. Rigaut ; A. Pineau, Scripta Met., 3 (1969), p. 883. (6) R. Cozar ; A. Pineau, Scipta Met., ~ (1973), p. 851.