行政院國家科學委員會補助專題研究計畫成果報告
PHASE TRANSFORMATIONS IN FE-9.0AL-30.0MN-4.0CR
ALLOY
計畫類別：個別型計畫
整合型計畫
計畫編號：NSC90-2216-E009-043
執行期間：90 年 08 月 01 日至 91 年 07 月 31 日
計畫主持人：劉增豐
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執行單位：
國立交通大學材料科學及工程學系
中 華 民 國 91 年 12 月 17 日
行政院國家科學委員會補助專題研究計劃成果報告
PHASE TRANSFORMATIONS IN FE-9.0AL-30.0MN-4.0CR ALLOY
鐵-9.0 鋁-30.0 錳-4.0 鉻合金相變化
計劃編號：NSC90-2216-E009-043
執行期限：90 年 08 月 01 日至 91 年 07 月 31 日
主持人：劉增豐 國立交通大學材料科學及工程學系
1.中文摘要 經由 OM、SEM、TEM 及 X-ray 來分 析 Fe-9.0Al-30.0Mn(wt.%) 合 金 的 相 變 化，在淬火狀態下為肥立鐵及沃斯田鐵的 混合相。且不論在任何溫度的時效處理 後，在沃斯田鐵基地內都不會發現析出 物，不過隨著溫度的提升在肥立鐵的晶粒 內卻可發現許多不同的析出物。在肥立鐵 晶粒內隨著溫度提高完整的相變化過程 為(α + A12 α-Mn + D03) → (α + A12 α-Mn) → (α + A13 β-Mn) → α。 關鍵字：鐵鋁錳鉻合金、相變化、D03,A12 α-Mn and A13 β-Mn AbstractPhase transformations in the Fe- 9.0 wt. % Al -30.0 wt. % Mn alloy with 4.0 Cr has been investigated by means of optical microscopy, scanning transmission electron microscopy and energy-dispersive X-ray spectrometey. In the as-quenched condition, the microstructure in the Fe- 9.0 wt. % Al- 30.0 wt. % Mn-4.0 wt. % Cr alloy was (ferrite) + (austenite) duplex phases. When the alloy was aged at various temperatures, no precipitate could be found within the austenite matrix. However, different kinds of precipitates could be observed within the ferrite grains with increasing the aging temperature. In the alloy, the phase transformation sequence in the ferrite grains as the aging temperature increased were found to be (α + A12 α-Mn + D03) → (α + A12 α-Mn) → (α + A13
β-Mn) → α.
Keywords: Fe-Al-Mn-Cr alloy, phase transformation, D03, A12 α-Mn and A13
β-Mn
2. INTRODUCTION
Recently, a considerable amount of interest has focused on developing Fe-Al-Mn alloys as the substitutes for conventional stainless steels [1-9]. Results concerning corrosion resistance and high temperature oxidation resistance have been given in several publications [3,4,5]. In order to improve the corrosion resistance and high temperature oxidation resistance, chromium or silicon has been added to the Fe-Al-Mn alloys [6-9]. Whereas, the phase transformations in a Fe-9.1 wt.% Al-29.9 wt.% Mn-2.9 wt.% Cr alloy have been studied principally by the present workers
[10-12]
. After solution heat-treated at 980℃ for 1 hour and then quenched into room-temperature water rapidly, the microstructure of the Fe-9.1wt.% Al-29.9wt.% Mn-2.9wt.% Cr alloy was the mixture of the （austenite）+（ferrite） phases. When the alloy was aged at temperatures ranging from 350℃ to 560℃ for moderate times, no precipitates could be detected within the austenite phase. However, there are three kinds of precipitates, namely D03 phase, A12 -Mn,
and A13 -Mn, could be observed within the ferrite phase. The D03 phase has an
ordered face-centered cubic structure with lattice parameter a=0.5785nm. The orientation relationship between the D03
phase and ferrite matrix was (002) D03
//(002) and [110] D03 //[110] [10]. The A12
-Mn precipitate has a body-centered cubic structure with lattice parameter a=0.8889nm. The orientation relationship between the A12 -Mn and ferrite matrix
was (110)-Mn//(110), [001]-Mn//[001]. It is noted that the A12 -Mn precipitates formed within the ferrite matrix has a dohecahedral shape, which is bounded by six sets of {110} interface planes [11]. The A13 -Mn precipitate has a simple cubic structure with lattice parameter a=0.63nm. The orientation relationship between the A13 -Mn and ferrite matrix was determined to be (001)-Mn//(001) and
[120]-Mn//[100]. The axis/angle pair of
rotation between the A13 -Mn and the ferrite matrix is <100>/26°[12].
Up to date, most examinations were focused on the Fe-Al-Mn alloy with Cr≦ 2.9 wt%. Little information concerning the Fe-Al-Mn alloy with higher chromium content has been provided. Therefore, the purpose of the present study is an attempt to examine the effect of the higher chromium content on the microstructural developments of the Fe-9wt.% Al-30.0wt.% Mn alloy by using optical microscopy, scanning transmission electron microscopy and energy-dispersive X-ray spectrometry.
3. EXPERIMENTAL PROCEDURE (A) Alloy Preparation
The alloy, Fe-9.0wt.% Al-30.0wt.% Mn with 4.0 wt.% Cr was prepared in an air induction furnace by using electrolytic iron (99.5%), electrolytic aluminum (99.7%), electrolytic manganese (99.9%), electrolytic chromium (99.9%). After being homogenized at 1150℃ for 24 hours under a protective argon atmosphere, the ingots were hot-forged and then cold-rolled to a final thickness of 2.0 mm. The sheets were subsequently solution heat-treated at 1050 ℃ for 2 hours in a vacuum furnace and quenched into room-temperature water rapidly. The aging processes were performed at the temperatures ranging from 350℃ to 950℃ for various times in a vacuum furnace and then quenched into water rapidly.
(B) Microstructure Observations and
Diffraction Analyses
(1) Optical Microscopy (OM)
The specimens were prepared by the following steps: sectioning, mounting, rough polishing, fine polishing and finally etching with 40% nitric acid, 40% glycerine and 20% hydrochloric acid.
(2) Transmission Electron Microscopy (TEM) Specimens for electron microscopy were prepared by means of a double-jet electropolisher with an electrolyte of 13%perchloric acid , 35% acetic acid and 52% ethanol. Electron microscopy was performed on a JEOL-2000FX scanning transmission electron microscope (STEM) operating at 200 kV. This microscope was equipped with a Link ISIS 300 energy-dispersive X-ray spectrometer (EDS) for chemical analysis. Quantitative analysis of elemental concentrations for Fe, Mn, Al and Cr were made with the aid of a Cliff-Lorimer Ratio Thin Section method.
4. RESULTS
Figure 1(a) is an optical micrograph of the alloy after being solution heat-treated at 1050℃ for 2 hours and then quenched. It reveals a two-phase microstructure. The small grains (i.e., marked as A) with annealing twins are austenite (γ) phase and the large grains are of the ferrite (α) phase (i.e., marked as F). Figure 1(b) is a bright-field (BF) electron micrograph of the as-quenched alloy. Figures 1(c) and 1(d) are two selected-area diffraction patterns (SADPs) taken from the grains marked “A” and “F” in Figure 1(b), respectively. It indicates that the microstructure of the alloy in the as-quenched condition was a mixture of austenite and ferrite phases [2,6-12].
Figure 2(a) is a BF electron micrograph of the alloy aged at 350℃ for 48 hours, revealing that fine precipitates were formed within the ferrite matrix. Figures 2(b) is an SADP taken from an area covering the fine precipitates and their surrounding matrix. It is seen in this SADP that in addition to the reflection spots of the disordered ferrite phase [10-12], the
diffraction pattern also consist of two sets of reflection reciprocal lattices. It is found that one set was of the D03 phase having an
ordered B.C.C. structure with lattice parameter a=0.5785nm [10] and another was of the A12 α-Mn having a complexed B.C.C. structure with lattice parameter a=0.8889nm [11]. Figure 2(c) is a (111) D03
dark-field (DF) electron micrograph, showing that a high density of D03
precipitates was formed within the ferrite matrix. Figure 2(d), an (110) A12 α-Mn DF electron micrograph of the same area as Figure 2(a), clearly revealing that the presence of the fine A12 α-Mn particles.
When the alloy was aged at 450℃, cuboidal-shaped particles were always observed within the ferrite phase, as illustrated in Figure 3(a). Figures 3(b) demonstrates SADP taken from the particle marked as “D” in Figure 3(a). It is found that the cuboidal-shaped particles are also of the A12 α-Mn [11]. Figure 3(c) is an SADP taken from an area covering the particle marked as “D” and its surrounding matrix, clearly reveal that in addition to the reflection spots of the A12 α-Mn and ferrite phase, another kind of reflection spots could be observed in the SADP. It is found that the reflection spots belong to the D03
phase. Figures 3(d) and (e) are (110) A12 α-Mn and ( 1 11) D03 DF electron
micrographs, showing the presence of the A12 α-Mn particles and the D03 precipitates,
respectively. It is clear in Figure 3(e) that the D03 precipitates were formed not only
within the ferrite matrix with a spherical-shaped but also at the regions surrounding the A12 α-Mn particles. This result indicates that the microstructure of the ferrite grains at 450 ℃ was still a mixture of (α + A12 α-Mn + D03) phases.
Transmission electron microscopy examinations of thin foils indicated that the (α + A12 α-Mn + D03) phases could be
preserved up to 500℃. However, when the aging temperature was increased to 550℃, no evidence of the D03 precipitates could be
detected within the ferrite phase, as shown
in Figure 4(a). Figure 4(b) is a (110) A12
α-Mn DF electron micrograph, revealing that the morphology of the A12 α-Mn particles was gradually changed from cuboidal to irregular-shaped.
When the alloy was aged at 650℃, the A12 α-Mn particles disappeared and plate-like precipitates started to occur within the ferrite grains, as illustrated in Figure 5(a). Figure 5(b) shows a BF electron micrograph of the alloy aged at 650 ℃ for 6 hours. Figures 5(c) was performed to include only the precipitate marked as “P” in Figure 5(b). The crystal structure of the precipitate was determined to be a simple cubic structure with a lattice parameter a = 0.630 nm, which is consistent with that of A13 β-Mn [12]. Figures 4(d) demonstrates SADP taken from the precipitate marked as “P” and its contiguous region, displaying that the orientation relationship between the A13 β-Mn and the ferrite matrix was (012)β-Mn//(001)α,(02 1 )β-Mn// (010)α,
(100)β-Mn//(100)α. Figure 4(e), a (001) A13
β-Mn DF electron micrograph, shows the presence of the plate-like A13 β-Mn precipitates. Accordingly, the microstructure of the ferrite grains at 650℃ was a mixture of (α + A13 β-Mn) phases.
Progressively higher temperature aging and quenching experiments indicated that the (α + A13 β-Mn) phases were present up to 800℃. However, when the alloy was aged at 850℃ for 2 hours and then quenched, no evidence of any precipitates could be observed within the ferrite grains. This indicates that the microstructure within the ferrite grains present at 850℃ or above is a single ferrite phase.
Based on the above observations, the phase transformation sequence in the ferrite grains was found to be (α + A12 α-Mn + D03) → (α + A12 α-Mn) → (α + A13
β-Mn) → α.
On the basis of the observations, it is clear that the phase transition sequence in the ferrite grains was found to be (α + A12 α-Mn + D03) → (α + A12 α-Mn) → (α +
A13 β-Mn), in which the precipitation of A12 α-Mn could be observed in the present alloy aged at temperatures ranging from 350℃ to 550℃. In the past, however, the A12 α-Mn precipitates have never found in the Fe-Mn-Al system alloys. In addition, a kind of ordered D03 precipitates could be
detected in the ferrite grains at aging temperatures ranging from 350℃ to 450℃, the coexistence of A12 α-Mn and ordered D03 precipitates have also never been found
in the Fe-Mn-Al system alloys by other workers. In order to clarify these observations, an STEM-EDS study was undertaken. The average weight percentages of alloying elements examined by analyzing at least ten different EDS spectra of each phase as listed in Table 1. It is seen in Table 1 that the manganese and chromium contents of the A12 α-Mn precipitates is much greater than those of the ferrite matrix in the as-quenched condition, and the reverse result is obtained for the concentration of aluminum. Therefore, it is reasonable to believe that the addition of 4.0 wt% chromium to the Fe-9Al-30Mn alloy would cause the formation of (Cr , Mn )-riched A12 α-Mn precipitates and the surrounding ferrite matrix of A12 α-Mn precipitates would be enriched in aluminum. Form the previous studies [10], it is concluded that the higher aluminum concentration in the Fe-Mn-Al alloys would enhance the formation of order D03 phase. Thus, the enrichment of
aluminum content in the ferrite phase would enhance the formation of Al-riched D03
phase at the region contiguous to the A12 α-Mn precipitates.
Furthermore, it is well-known that the solubility of aluminum in the ferrite phase increases with increasing the aging temperature. For the reason, it is therefore suggested that since the solubility of aluminum in the ferrite phase would increase as in increasing the aging
temperature from 450 ℃ to 550 ℃ , the Al-riched D03 phase would be absent in the
ferrite matrix.
6. CONCLUSIONS
The phase transformations in Fe- 9.0 wt. % Al –30.0 wt. % Mn alloy with 4.0 wt. % Cr have been investigated in the present study.
(1) The phase transitions sequence of the present alloy was found to be (α + A12 α-Mn + D03) → (α + A12
α-Mn) → (α + A13 β-Mn) → α. (2) Adding 4.0 wt% chromium to the
Fe-Al-Mn alloys would enhance the precipitations of the A12 α-Mn phases. Along with the precipitation of Al-lacked A12 α-Mn, the Al-riched D03 precipitates were
formed in the vicinity of the A12 α-Mn precipitates at temperatures between 350℃ and 450℃.
(3) When the aging temperature was increased to 550℃, the absenc3e of the D03 phase contiguous to the A12
α-Mn would be found, which was attributed to the result of the higher aluminum solubility in the ferrite matrix at higher temperature.
7. ACKNOWLEDGEMENTS
The authors are pleased to acknowledge the financial support of this research by the National Science Council, Republic of China under Grant NSC90-2216-E-009-043.
8. REFERENCES
1. J. Charles and A. Berghean, Met Prog, 5 71 (1981).
2. P. Tomaszewicz and G.R. Wallwark, Corr., 40 152 (1984).
3. Helmut Erhart, Rosie wang and Robert A. Rapp, Oxidation of Metals, 21, Nos. 1/2 (1984).
4. Rosie Wang, Mark J. Straszheim and Robert A. Rapp, Oxidation of Metals, 21, Nos. 1/2 (1984).
5. P.R.S. Jackson and G.R. Wallwork, oxidation of Metals, 21, Nos. 3/4 (1984).
6. J. C. Garcia, N. Rosas and R.J. Rioja: Met. Prog., Aug. (1982) 47~50.
7. C. J. Altstetter, A.P. Bentley, J.W. Fourie and A.N. Kirkbride: Mater. Sci. Eng., 82 (1986) 13~25.
8. R. Wang and F.H. Beck: Met. Prog., Mar. (1983) 72~76.
9. D.J. Schmatz: Trans. ASM, 52 (1960) 898~910.
10. T. F. Liu and C. M. Wan, Scripta Metallurgical, Vol. 19, pp. (1985) 805~810.
11. T. F. Liu and J. C. Tasy, Scripta Metallurgical, Vol. 21, pp. (1987) 1213~1218.
12. T. F. Liu and C. M. Wan, Scripta Metallurgical, Vol. 23, pp. (1989) 1243~1248.
Table 1. Chemical Compositions of the Phase Revealed in Alloy A by Energy-Dispersive X-ray Spectrometer (EDS) Chemical Composition (Wt.%) Heat Treatment Phase Al Mn Cr Fe γ 6.10 31.78 2.24 59.88 As-quench ed α 8.79 27.13 6.06 58.02 α 5.94 20.19 3.18 70.69 D03 9.13 24.68 5.09 61.1 450℃ 48hrs aging A12α- Mn 5.26 48.21 17.26 39.27 α 9.56 24.95 8.62 56.87 800℃ 2hrs aging A13 β- Mn 4.11 36.85 13.21 45.83 1(a) 1(b) 1(c) 1(d)
Fig.1 (a) An optical micrograph of the as-quenched alloy A. Electron micrographs of the as-quenched alloy A (b) BF, (c) SADP taken from the grain marked “A” in (b). The foil normal is [011]. (d) SADP taken from the grain marked as “F” in (b). The foil normal is [011].
2(a) 2(b)
2(c) 2(d)
Fig.2 Electron micrographs of the alloy A aged at 350℃ for 48 hours. (a) BF, (b) an SADP taken from an area covering the fine particles and their surrounding matrix. The foil normal is [110]. (hkl = ferrite matrix; hkl = D03 ; hkla = α-Mn). (c) (111) D03 DF,
3(a) 3(b)
3(c) 3(d)
3(e)
Fig.3 Electron micrographs of the alloy A aged at 450℃ for 48 hours. (a) BF, (b) SADP taken from the particle marked as “D” in (a). The foil normal is [001]. (c) SADP taken from the area covering the particle marked as “D” in (a) and its surrounding matrix. The foil normal is [100]. (hkl = ferrite matrix; hkl = D03; hkla
= α-Mn). (d) (110) A12 α-Mn DF, (e) (111) D03 DF.
4(a) 4(b)
Fig.4 Electron micrographs of the alloy A aged at 550℃ for 24 hours. (a) an SADP taken from a region containing an A12 α-Mn particle and its surrounding matrix, (b) (110) A12 α-Mn DF.
5(a) 5(b)
5(c) 5(d)
5(e)
Fig.5 (a) An optical micrograph of the alloy A aged at 650℃ for 6 hours. (b) BF, (c) SADP taken from the particle marked as “P” in (b). The foil normal is [001] (d) SADP taken from the precipitate marked as “P” and its contiguous region, The zone axes is [001]α and [012] β-Mn,. (hklb =β-Mn).
