The influence of Cr alloying on microstructures of Fe-Al-Mn-Cr alloys

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The inﬂuence of Cr alloying on microstructures

of Fe–Al–Mn–Cr alloys

J.W. Lee

a,*

, C.C. Wu

b

, T.F. Liu

a

a_{Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road,}

300 Hsinchu, Taiwan, Republic of China

b_{Department of Mechanical Engineering, Southern Taiwan University of Technology, Tainan, Taiwan, Republic of China}

Received 3 November 2003; received in revised form 23 February 2004; accepted 25 February 2004

Abstract

The eﬀects of increasing chromium content on the phase transformations in Fe–Al–Mn–Cr alloys have been investigated by means of transmission electron microscopy and energy-dispersive X-ray spectrometry. The experimental results revealed that increasing the chromium addition would expand both the A12a-Mn and DO3phase-ﬁeld regions.

Keywords: Fe–Al–Mn–Cr alloys; Phase transformations; TEM; Aging; Precipitation

1. Introduction

For economic and strategic considerations, Fe–Al– Mn alloys with aluminum and manganese as substitutes for chromium and nickel used in conventional stainless steels have been widely investigated by many workers [1–3]. On the basis of their studies, it is generally con-cluded that the Fe–Al–Mn alloys possess a good oxi-dation resistance at high temperatures; however, the corrosion resistance of the Fe–Al–Mn alloys was infe-rior to that of the conventional stainless steels. It was reported that the addition of chromium to the Fe–Al– Mn alloys could signiﬁcantly improve the corrosion resistance [4,5]. Additionally, iron aluminides also at-tracted much interest due to their superior characteris-tics at high temperature. But, applications have been restricted by their embrittlement at ambient tempera-ture. So far, chromium was found to be the most eﬀec-tive alloying element to improve their environmental embrittlement [6,7]. One possible reason that has been proposed is that alloying of chromium into Fe–Al alloys would suppress the formation of the brittle ordered DO3

phase. However, up to now, little information con-cerning the microstructures of the Fe–Al–Mn–Cr alloys

has been reported. We ﬁrst performed transformation electron microscopy observations on the phase trans-formations of an Fe–9.1wt.%Al–29.9wt.%Mn–2.9wt.% Cr alloy [8–10]. Based on our previous studies, the phase transformation sequence of the Fe–9.1Al–29.9Mn– 2.9Cr alloy with increasing aging temperature was found to be (ferrite + DO3) + austeniteﬁ (ferrite + DO3+

A12a-Mn) + austeniteﬁ (ferrite + A12a-Mn)+ austenite

ﬁ (ferrite + A13 b-Mn) + austenite ﬁ ferrite + austenite, in which the transitions occurred at temperatures

be-tween 410 and 430C, 450 and 500 C, 520 and 550 C

and 800 and 850C in sequence. Extending the previous

work, it is interesting to study successively the eﬀects of the higher chromium contents on the microstructural developments of Fe–Al–Mn–Cr alloys. Therefore, the purpose of this work is an attempt to examine the microstructural developments of both the Fe–8.0Al– 29.7Mn–5.0Cr and Fe–8.1Al–29.8Mn–10.0Cr alloys

aged at temperatures ranging form 350 to 850C.

2. Experimental procedure

Two alloys of compositions Fe–8.0wt.%Al–29.7wt.% Mn–5.0wt.%Cr (designated as 5Cr) and Fe–8.1wt.%Al– 29.8wt.%Mn–10.0wt.%Cr (designated as 10Cr) alloys were prepared in a vacuum induction furnace. In addi-tion to contain 5.0 and 10 wt.% chromium, respectively,

*

Corresponding author. Tel.: 571-2121x55363; fax: +88-63-571-3987.

E-mail address:davidlee@cc.nctu.edu.tw(J.W. Lee).

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both the alloys were composed of similar compositions. After being homogenized, slices sectioned from the

in-gots were subsequently solution heat-treated at 1050C

for 2 h and then quenched into room temperature water. The aging processes were performed at temperatures

ranging from 350 to 850C for various times in a

vac-uum heat-treatment furnace. Electron microscopy was performed on a JEOL-2000FX scanning transmission electron microscopy operating at 200 kV. This micro-scope was equipped with a Link ISIS 300 energy-dis-persive X-ray spectrometer (EDS) for chemical analysis.

3. Results and discussion

Optical microscopy examinations exhibited that the as-quenched microstructure of the 5Cr alloy was the mixture of the continuous ferrite (a) phase and the dis-crete austenite (c) phase, which is similar to that of the Fe–9.1Al–29.9Mn–2.9Cr alloy in our previous study [8]. As the chromium content was increased from 5 to 10 wt.%, the as-quenched microstructure of the 10Cr alloy

was changed from duplex (aþ c) phases to single a

phase. Transmission electron microscopy examinations also indicated that when the as-quenched 5Cr alloy was heat-treated by subsequent aging processes, no precipi-tates were ever found within the austenite phase. Therefore, besides the phase transformations in the 10Cr alloy, only the microstructural changes inside the ferrite grains in the 5Cr alloy are discussed in the present study. Fig. 1(a) is a bright-ﬁeld (BF) electron micrograph of the 10Cr alloy aged at 350C for 2 h, revealing that ﬁne precipitates were formed within the interior ferrite grain;

moreover, not only fine precipitates but also some irregular-shaped precipitates could be observed along grain boundary. Fig. 1(b), a selected-area diffraction pattern (SADP) taken from the mixed region covering the fine precipitates and the surrounding ferrite matrix in Fig. 1(a) indicates that the fine precipitates have an

ordered DO3structure with lattice parameter a¼ 0:578

nm. Fig. 1(c) is a SADP taken from an area covering ﬁne precipitates and irregularly shaped precipitates formed on the grain boundary in Fig. 1(a). As compared to Fig. 1(b), in addition to the ordered DO3phase, another kind

of precipitate could also be detected. From analyzing this SADP, the lattice parameter of the other precipitate was determined to be of 0.888 nm and the orientation relationship between the precipitate and the ferrite ma-trix was cubic to cubic. Compared with our previous study in the Fe–9.1Al–29.9Mn–2.9Cr alloy [9], the pre-cipitates are of A12a-Mn with a complex bcc structure.

Fig. 1(d) is a (1 1 1)DO3 dark-ﬁeld (DF) electron

micrograph, demonstrating that ﬁne DO3 precipitates

were formed not only within the interior ferrite grain but also along grain boundary. However, the (0 1 1)a-Mn DF electron micrograph shows that these irregularly shaped a-Mn precipitates were only formed along grain boundary, as illustrated in Fig. 1(e). With progressing aging time at the same temperature, no evidence of other

precipitates than that of the DO3 phase and the a-Mn

could be found. Fig. 2(a) and (b) is the (1 1 1)DO3 and

(0 1 1)a-Mn DF electron micrographs indicating that in

addition to grown DO3 particles, a-Mn precipitates

started to appear inside the ferrite grains. After pro-longed aging time to reach equilibrium condition at 350 C, the microstructure of the alloy was still a mixture of

Fig. 1. Electron micrographs of the 10Cr alloy aged at 350C for 2 h: (a) BF; (b) and (c) two SADPs taken from the ferrite grain interior and grain boundary in (a), respectively. The zone axes are [0 1 1]. (hklF¼ ferrite, hkl ¼ DO3phase, hkl¼ a-Mn) (d) and (e) are (1 1 1)DO3and (0 1 1)a-Mn DF,

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(a + DO3+ a-Mn) phases. Transmission electron

microscopy examinations also revealed that the micro-structural change within the ferrite grains in the 5Cr

alloy aged at 350 C was similar to that in the 10Cr

alloy. However, this result is diﬀerent from that reported by the present workers in the Fe–9.1Al–29.9Mn–2.9Cr

alloy, in which only DO3phase was observed within the

ferrite matrix at aging temperatures ranging from 350 to 410C [8].

Fig. 3(a) and (b) is the (1 1 1)DO3and (0 1 1)a-Mn DF

electron micrographs of the 10Cr alloy aged at 450 C

for short time, revealing that the extremely ﬁne DO3and

a-Mn precipitates were formed almost simultaneously within the ferrite matrix. When the 10Cr alloy was aged

at 450C for 48 h, the DO3and a-Mn precipitates have

grown in shapes of sphere and polygon, respectively, as shown in Fig. 4(a) and (b). Moreover, it is clearly seen in Fig. 4(a) that the DO3phase was also formed at regions

surrounding the polygonal-shaped a-Mn precipitates. It is demonstrated that at the aging temperature of 450C, the microstructure of the 5Cr alloy was similar to that of the 10Cr alloy, but the amount of the DO3phase in Fig.

5 was obviously less than that in Fig. 4(a).

When the 10Cr alloy was aged at 550 C for short

time, in addition to the a-Mn precipitates, no evidence

of the DO3 phase could be found within the ferrite

grains, as shown in Fig. 6(a), a BF electron micrograph. By contrast, the a-Mn precipitates were formed more Fig. 2. Electron micrographs of the 10Cr alloy aged at 350C for 240 h: (a) (1 1 1)DO3DF, and (b) (0 1 1)a-Mn DF.

Fig. 3. Electron micrographs of the 10Cr alloy aged at 450C for 1 h: (a) (1 1 1)DO3DF and (b) (0 1 1)a-Mn DF.

Fig. 4. Electron micrographs of the 10Cr alloy aged at 450C for 48 h: (a) (1 1 1)DO3DF and (b) (0 1 1)a-Mn DF.

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rapidly inside the internal ferrite grains at a temperature higher than 350C (i.e. 450, 550 C). After being aged at 550 C for 24 h, it is clearly demonstrated in Fig. 6(b)

that the DO3phase would be formed not only within the

ferrite grains but also around the a-Mn precipitates. Nevertheless, it is apparent that the amount of the DO3

phase in Fig. 6(b) is much less than that in Fig. 4(a). On

the contrary, even though the 5Cr was aged at 550 C

for a long period of time, in addition to the presence of

well-grown a-Mn precipitates, the DO3 phase could be

detected neither within the ferrite grains nor around the periphery of the a-Mn precipitates. Transmission

elec-tron microscopy examinations revealed that the DO3

phase existing in the 10Cr alloy could not be observed

above 600 C and the microstructure in equilibrium

stage at 650 C was a mixture of (a þ a-Mn) phases,

which could be preserved up to a temperature between

700 and 750C. In the 5Cr alloy; however, the (a þ

a-Mn) phases could only be maintained up to a lower

temperature between 600 and 650 C. When the 10Cr

and 5Cr alloys were aged at temperatures above the

existing temperatures of (aþ a-Mn) phases, namely 750

and 650 C, respectively, another kind of precipitate

with Widmanst€atten morphology could be observed

within the ferrite matrix in both the alloys. A typical example is shown in Fig. 7(a). Fig. 7(b) is an SADP taken from an area covering the needle-like precipitate and its surrounding ferrite matrix in Fig. 7(a), indicating

that the precipitates with Widmanst€atten morphology

are of A13 b-Mn having a simple cubic structure with

lattice parameter a¼ 0:630 nm [10]. Transmission

elec-tron microscopy examinations also exhibited that the

stable microstructure of (aþ b-Mn) phases in both the

alloys could be observed at aging temperature up to

somewhere between 800 and 850 C. However, when

both the alloys were aged at temperatures above 850C,

no evidence of any precipitate could be detected within the ferrite grains. This feature is consistent with that observed in both the as-quenched alloys.

Fig. 5. (1 1 1)DO3 DF electron micrograph of the 5Cr alloy aged at

450C for 48 h.

Fig. 6. Electron micrographs of the 10Cr alloy aged at 550C: (a) BF, after 2 h and (b) (1 1 1)DO3DF, after 24 h.

Fig. 7. Electron micrographs of the 10Cr alloy aged at 750C for 6 h: (a) BF; (b) an SADP taken from an area covering the needle-like precipitate and its surrounding ferrite matrix in (a). The zone axis is [0 0 1]. (hkl¼ b-Mn.)

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Based on the above observations, some experimental results are discussed below. When the two present alloys containing 5.0 and 10 wt.% chromium were aged at 350 C, the stable microstructure was a mixture of (a +

DO3+ a-Mn) phases, rather than (a + DO3) phases as

observed in the Fe–9.1Al–29.9Mn–2.9Cr alloy aged in the

temperature range 350–410C in our previous study [8].

Comparing the present 10Cr and 5Cr alloys with the previous Fe–9.1Al–29.9Mn–2.9Cr alloys, it is also worthwhile to note that not only the a-Mn but also the

DO3phase could be preserved up to higher temperatures

in the alloy containing a higher chromium content [9]. Conversely, it was reported that alloying of chromium into Fe–Al alloys would suppress the formation of the

DO3phase so that the environment embrittlement of Fe–

Al alloys would be reduced [6]. In order to clarify these features, a STEM-EDS study was performed. The quantitative analyses of at least 10 diﬀerent EDS proﬁles exhibited that the average chemical composition of the

a-Mn in the 10Cr alloy aged at 450 C was Fe–

(4.3 ± 0.4)Al–(41.2 ± 0.3)Mn–(20.1 ± 0.5)Cr (percent by weight). It is obviously seen that not only the manganese but also the chromium content of the a-Mn is much greater than that of the ferrite matrix in the as-quenched condition, and the reverse result is obtained for the alu-minum content. Therefore, it is suggested that the in-crease of the chromium content in the Fe–Al–Mn–Cr alloys would pronouncedly expand the a-Mn ﬁeld region. Additionally, as the two present alloys were aged at

lower temperature of 350C, the diﬀusion of Mn atom

was severely retarded and the formation of the a-Mn became very difficult. Since grain boundaries acted as a more effective preferential diffusion path for Mn atoms than the ferrite grain interior, the precipitation of a-Mn occurred first along grain boundaries by heterogeneous nucleation and then proceeded toward the ferrite grains interior after long-time aging at 350 C. Since the dif-fusion rate of Mn atom is sensitive to temperature change, the precipitation of a-Mn became rapid at

higher temperatures (i.e. 450, 550 C), as mentioned

above. Nevertheless, it is reasonable to predict that with

raising aging temperature to 550 C, the solubility of

aluminum in the ferrite matrix increases in the 10Cr

alloy. This inhibits the formation of the Al-rich DO3

phase within the ferrite matrix during the early stage of

isothermal aging at 550 C. But along with the

precipi-tation of Mn-rich a-Mn, the surrounding matrix of the a-Mn would be enriched in aluminum, which would

enhance the precipitation of Al-rich DO3 phase in the

vicinity of the a-Mn. Also, the amount of the DO3phase

is gradually less with increasing aging temperature, which is consistent with our experimental results. Therefore, it is proposed that the addition of chromium to the Fe–Al–Mn alloys would enhance the formations

of both the a-Mn and the DO3phases. This feature has

never been found by other workers before.

4. Conclusions

1. When the present alloys were aged at temperatures ranging from 350 to 1050C, the phase transformation sequence within the ferrite grains was found to be

(a + DO3+ a-Mn)fi (a þ a-Mn) fi (a + b-Mn) fi a.

2. Increasing chromium content in the Fe–Al–Mn–Cr alloys would enhance the formations of both the

a-Mn and the DO3phases and expand both phase-ﬁeld

regions.

Acknowledgements

The author is pleased to acknowledge the ﬁnancial support of this research by the National Science Coun-cil, Republic of China under Grant NSC91-2216-E-009-019.

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