Optical micrography examinations indicated that the as-quenched microstructure of alloys A (0Cr) through D (6Cr) was essentially γ phase with annealing twins. A typical example is shown in Figure 4.1a. The micrograph indicates that Cr could be completely dissolved within the γ matrix at 1200℃ for Cr ≦ 6%. However, when the Cr content was increased up to 8%, some precipitates were observed to be formed on the grain boundaries, as illustrated in Figure 4.1b. TEM examinations indicated that the precipitates on the grain boundaries were (Fe,Mn,Cr)7C3 carbides； these precipitates were similar to those observed by the present researchers in the as-quenched Fe-(7.1-9.1) wt.%Al -(29.2-30.2) wt.%Mn -(6.6-9.1) wt.%Cr-(0.94-1.07) wt.%C alloys [16,17]. TEM examinations also revealed that a high density of fine k′
carbides was present within the γ matrix in all the alloys. The fine k′
carbides having an L′12 (ordered f.c.c.) structure were formed by spinodal decomposition during quenching. An example is shown in Figures 4.1c and 1d. This result is similar to that obtained for the as-quenched Fe-9 wt.%Al-(28-30) wt.%Mn-(1.8-2) wt.%C alloys [20,21]. Accordingly, besides the presence of the coarse (Fe,Mn,Cr)7C3 carbides on the grain boundaries in alloy E (8Cr), the as-quenched microstructure of alloys
Figure 4.1(a)
Figure 4.1(b)
Figure 4.1(c)
Figure 4.1(d)
Figure 4.1 Optical micrographs of present alloys: (a) alloy D (6Cr), and (b)
alloy E (8Cr). Transmission electron micrographs of alloy D
(6Cr): (c) (100)k′ DF, and (d) a selected area diffraction pattern
taken from a mixed region covering the γ matrix and fine k′
carbides. The zone axis is [001] (hkl: γ matrix; hkl: k′ carbide).
A (0Cr) through E (8Cr) was γ phase containing fine k′ carbides.
Figure 4.2 shows the potentiodynamic polarization curves of the five alloys measured in 3.5% NaCl solution. A broad passive region can be clearly observed in the curves of all the alloys except in the curve of the alloy not containing Cr. In addition, the width of the passive region increased as the Cr content increased from 3 to 6%, and the width decreased as the Cr content increased further up to 8%. The characteristic electrochemical parameters extracted from the polarization curves are listed in Table 4.1. As the Cr content changed, the corrosion potential (Ecorr) of the alloys varied from -846 to -538 mV. Alloy D (6Cr) exhibited the noblest Ecorr (-538 mV). Similarly, as the Cr content increased from 3 to 6%, the pitting potential (Epp) drastically increased from -223 to -25 mV. However, when the Cr content was increased further up to 8%, Epp became more negative (-412 mV). This indicated that alloy D (6Cr) had the highest resistivity to pitting damage. Figures 4.3a-d show the Auger depth profiles of the passive film formed on alloys A (0Cr), B (3Cr), D(6Cr), and E(8Cr), respectively. As can be clearly seen in these figures, broad peaks of Cr, Al, and O were observed at a depth of 0-2mm in alloys B(3Cr), D(6Cr), and E (8Cr) alloys. The presence of a layer of Cr and Al oxides in the passive film may play an important role in improving
Figure 4.2 Potentiodynamic polarization curves for present five alloys
measured in 3.5% NaCl solution.
Figure 4.3 AES depth profiles of passive film of present alloys: (a) A (0Cr),
(b) B (3Cr), (c) D (6Cr), and (d) E (8Cr).
the corrosion resistance characteristics of alloys B (3Cr) through D (6Cr).
However, the formation of the coarse Cr-rich (Fe,Mn,Cr)7C3 carbides resulted in a drastic decrease in the Ecorr and Epp values of alloy E (8Cr).
As expected, the experimental results presented above were similar to those obtained for the as-quenched Fe-(7.1-9.1) wt.%Al-(29.2-30.2) wt.%Mn-(0-9.1) wt.%Cr-(0.94-1.07) wt.%C alloys [18, 19].
The tensile properties of the five as-quenched alloys investigated in the present study are also listed in Table 4.1. It can be clearly seen from the table that the UTS, YS, and elongation of alloy A (0Cr) were 1080 MPa, 868 MPa, and 55.5%, respectively. As the Cr content increased from 3 to 6%, the tensile strength slightly increased. Alloy D (6Cr) possessed the highest UTS (1122 MPa) and YS (902 MPa) with a good elongation of 36.5%. Owing to the high density of fine k′ carbides within the γ matrix, alloys A (0Cr) through D (6Cr) in the as-quenched condition could possess a remarkable combination of strength and ductility. This result is similar to that reported for the Fe -9 wt.%Al-28 wt.%Mn-1.8 wt.%C alloy [21]. However, when the Cr content was increased up to 8%, both the strength and elongation drastically reduced because of the formation of the coarse (Fe,Mn,Cr)7C3 carbides on the grain boundaries.
Table 4.1 Electrochemical parameters extracted from polarization curves and mechanical properties of the present five alloys.
Electrochemical parameters from polarization curves Mechanical properties Alloy
Ecorr (mV) Ecr (mV) Epp (mV) Ip (A·cm-2) UTS (MPa) YS (MPa) El (%)
A (0Cr) -846 — — — 1080 868 55.5
B (3Cr) -710 -572 -223 1.5E-05 1092 876 47.2
C (5Cr) -571 -524 -65 8.19E-06 1102 882 39.1
D (6Cr) -538 -490 -25 9.12E-07 1122 902 36.5
E (8Cr) -746 -652 -412 3.52E-06 984 835 22.6
Ecorr, corrosion potential; Ecr, critical potential for active-passive transition;
Epp, pitting potential; Ip, passive current density, minimum value.
On the basis of the above observations, three important experimental results are obtained as discussed below. (I) A comparison of the results of this study with those of previous studies, revealed that the yield strength of alloy D (6Cr) was not only superior to that (410~550 MPa) of the as-quenched Fe-(7.8-10) wt.%Al -(28-34) wt.%Mn-(0-2.8) wt.%Cr-(0-1.75) wt.%M (M=Nb+V+Mo+W)-(0.85-1.3) wt.%C alloys but also comparable to that (665~1094 MPa) of the aged alloys [3-9, 11-15]. In addition, the Ecorr
and Epp values of the alloys measured in 3.5% NaCl solution in previous studies were in the range of -920 to -789 mV and -500 to -240 mV, respectively [11-15]. Owing to the presence of a layer of Al and Cr oxides in the passive film, the Ecorr and Epp values of alloy D (6Cr) drastically increased to -538mv and -25mV, respectively. Therefore, it was concluded that the as-quenched alloy D (6Cr) possessed a better combination of high-strength, high-ductility, and moderate corrosion resistance. (II) The corrosion resistances of the present alloys in 3.5%
NaCl solution were similar to those of the as-quenched Fe-(7.1-9.1) wt.%Al-(29.2-30.2) wt.%Mn-(3-9.1) wt.%Cr-(0.94-1.07) wt.%C alloys [16,17]. This implied that the presence of the fine k′ carbides within the γ matrix did not considerably affect the electrochemical behavior of the alloy in 3.5% NaCl solution. (III) The FeAlMnC alloy system was initially
developed with the intention of replacing the conventional FeNiCr stainless steels because of its high strength, good toughness, low density, and low cost. However, previous studies revealed that the FeAlMnC alloys exhibited poor corrosion resistance in aqueous environments
[11-15]. It is interesting to note that the characteristics of alloy D (6Cr) are similar to those of the conventional martensitic stainless steels, conventional martensitic stainless steels are commonly used for manufacturing components that require a combination of high strength and moderate corrosion resistance [22]. AISI 410 (12%Cr-0.10%C) is an example of martensitic stainless steels [22]. Therefore, in the following description, we will compare the properties of AISI 410 and alloy D (6Cr).
The high strength and moderate corrosion resistance of the AISI 410 can be achieved by carrying out heat treatment, including austenitizing, air-quenching and tempering treatments [22]. After being tempered at a temperature between 250 and 593℃, the UTS, YS, and elongation were found to be 827~1337 MPa, 724~1089 MPa, and 20~17%, respectively [22]. Additionally, the Ecorr and Epp values of AISI 410 measured in 3.5%
NaCl solution ranged from -675 to -312 mV and -250 to -100 mV, respectively [23-25]. The mechanical strength of alloy D (6Cr) was comparable to that of AISI 410, and alloy D exhibited better elongation
than AISI 410. In addition, the Epp value (-25 mV) of alloy D (6Cr) was much higher than that (-250~-100 mV) of AISI 410. This might be attributed to the fact that although AISI 410 might have a Cr content of up to about 12%, a large amount of coarse Cr-rich (Fe,Cr)23C6 carbides precipitate within its martensitic matrix during tempering. The precipitation causes the Cr concentration, which is dependent on the Cr-rich carbides, to reduce considerably, and hence, the boundaries between the carbides and the martensitic matrix act as locations for nucleation and as anodic positions for pitting corrosion [26, 27]. Therefore, the pitting potential of the tempered AISI 410 decreases considerably. In contrast to the tempered AISI 410, there was no evidence of the formation of chromium carbides in the as-quenched alloy D (6Cr). Furthermore, the average particle size of the fine k′ carbides was only about 22 nm. In addition, the fine k′ carbide had the same crystal structure and a similar lattice parameter as the γ matrix, therefore, the interface between the k′ carbide and the γ matrix was completely coherent. Consequently, even at a considerably low Cr content, the pitting potential of alloy D (6Cr) was noticeably higher than that of the tempered AISI 410.
4-4
Conclusions
In this study, a new austenitic Fe-9 wt.%Al-28 wt.%Mn-6 wt.%Cr-1.8 wt.%C alloy is developed. The as-quenched alloy possesses a remarkable combination of high-strength, high-ductility, and moderate corrosion resistance, which is attributed to the presence of fine k′ carbides formed coherently within the γ matrix during quenching and to a layer of Cr and Al oxides in the passive film. The pitting potential Epp (-25 mV) of alloy D (6Cr) measured in 3.5% NaCl solution is noticeably higher than that of the aged FeAlMnC alloys(-500~-240 mV) and the tempered AISI 410 martensitic stainless steel(-250~-100 mV). In addition, the tensile strength of alloy D is comparable to that of the aged FeAlMnC alloys and AISI 410.
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