A novel high-strength, high-ductility and high-corrosion-resistance FeAlMnC low-density alloy

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Viewpoint Paper

A novel high-strength, high-ductility and high-corrosion-resistance

FeAlMnC low-density alloy

Po-Chih Chen, Chuen-Guang Chao and Tzeng-Feng Liu

⇑

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC Available online 26 October 2012

Abstract—The as-quenched Fe–8.68 wt.% Al–30.5 wt.% Mn–1.85 wt.% C alloy is plasma-nitrided at 500°C for 8 h. The nitrided layer obtained is 40 lm thick and composed predominantly of AlN, with a small amount of Fe4N. The resultant surface hardness

(1860 Hv), substrate hardness (550 Hv), ductility (33.6%) and corrosion resistance in 3.5% NaCl solution in the present nitrided alloy are far superior to those obtained previously in optimally nitrided high-strength alloy steels, as well as martensitic and precip-itation-hardening stainless steels.

Keywords: Spinodal decomposition; Plasma nitriding; Corrrosion resistance; Microhardness; FeAlMnC alloy

Introduction

The austenitic Fe–Al–Mn–C quarternary alloys have been attracting tremendous attention because of their excellent combinations of high strength and high ductility and because they have no need for expensive strategic alloying elements (e.g. Cr, Ni, Mo). Moreover, due to the high aluminum content, the density of the alloys is 13% lower than conventional steels[1]. Previous studies have shown that the as-quenched microstructure of Fe–(7.8–10) wt.% Al–(28–30) wt.% Mn–(0.8–1.3) wt.% C alloys was single-phase austenite (c)[1–5]. An optimal combination of strength and ductility could be obtained for the Fe–Al–Mn–C alloys aged at 550°C for 16 h

[2,3]. Under these aging conditions, a high density of ﬁne (Fe,Mn)3AlC carbides (j0-carbides) having an L012

(or-dered face-centered cubic, fcc) structure precipitated coherently within c matrix without any grain boundary precipitates. After optimal aging, the ultimate tensile strength (UTS), yield strength (YS) and elongation (El) of the Fe–Al–Mn–C alloys could reach 1130–1250 MPa, 1080–1120 MPa and 33–31%, respectively[2,3].

Recently, we investigated the as-quenched micro-structure of the Fe–9.8 wt.% Al–29 wt.% Mn–(1.45– 2.05) wt.% C alloys, and found that an extremely high density of nano-sized j0_{-carbides was formed within c}

matrix by spinodal decomposition during quenching

[6]. This is quite diﬀerent from that observed in the austenitic Fe–Al–Mn–C (C 6 1.3 wt.%) alloys, in which ﬁne j0_{-carbides could only be observed in aged alloys.}

Due to the pre-existing nano-sized j0_{-carbides, the aging}

temperature and time required to attain the optimal combination of strength and ductility are, respectively, much lower and less than those of the previous Fe– Al–Mn–C (C 6 1.3 wt.%) alloys. For example, with al-most equivalent elongation, the Fe–9 wt.% Al–28 wt.% Mn–1.8 wt.% C alloy aged at 450°C for 12 h can possess yield strength 28% higher than that of the optimally aged Fe–Al–Mn–C (C 6 1.3 wt.%) alloys [7].

Although the austenitic Fe–Al–Mn–C alloys could possess excellent combinations of strength and ductility, the corrosion resistance of the alloys was insuﬃcient for applications in aggressive environments [4,5]. Plasma nitriding was widely utilized to improve surface hard-ness and corrosion resistance of metallic materials [8– 19]. However, to date, little information concerning the plasma nitriding treatment for Fe–Al–Mn–C alloys has been reported in the literature. The main purpose of this work is to investigate the characteristics of an Fe–8.68 wt.% Al–30.5 wt.% Mn–1.85 wt.% C alloy after plasma nitriding at 500°C for 8 h.

The Fe–8.68 wt.% Al–30.5 wt.% Mn–1.85 wt.% C al-loy was prepared in an air induction furnace. After being homogenized at 1150°C for 6 h, the ingot was hot-rolled to a 6 mm thick plate. The plate was subsequently solu-tion heat-treated at 1200°C for 2 h and then quenched

⇑Corresponding author. Tel.: +886 3 5131288; fax: +886 3 5713987; e-mail:tﬂiu@cc.nctu.edu.tw

Available online at www.sciencedirect.com

Scripta Materialia 68 (2013) 380–383

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into room-temperature water. The specimen was pol-ished using SiC papers to 2400 grit before plasma nitrid-ing. The plasma nitriding process was performed at 500°C for 8 h using an atmosphere of 50% N2and 50%

H2under a pressure of 130 Pa. X-ray diﬀraction (XRD)

was carried out using a Bruker D8 with Cu-Karadiation.

The nitrogen concentration and microhardness of the ni-trided alloy were determined by using glow discharge spectrometer (GDS) and Vicker’s indenter at 100 gf, respectively. Potentiodynamic polarization curves were measured in 3.5% NaCl solution at 25°C with a scan rate of 2 mV s1. A saturated calomel electrode (SCE) and a platinum wire were used as reference and auxiliary elec-trodes, respectively. Tensile tests were carried out at room temperature with an Instron 8501 tensile testing machine at a strain rate of 6.7 104s1.

Figure 1a is a transmission electron microscopy (TEM) (1 0 0)j0 dark-ﬁeld image and the corresponding

diﬀraction pattern of the as-quenched alloy, revealing that an extremely high density of nano-sized j0_-carbides

can be observed within c matrix and the nano-sized j0

-carbides were formed by spinodal decomposition during quenching[6,7]. By using a LECO 2000 image analyzer, the average size and volume fraction of the j0_-carbides

were determined to be10 nm and 38%, respectively. A

detailed investigation indicated that when the as-quenched alloy was aged at 500°C for 8 h, the alloy could possess an excellent combination of strength and ductility with the UTS, YS and El being 1402 MPa, 1298 MPa and 34.5%, respectively. For achieving the eﬀects of aging and nitriding simultaneously, the plasma nitriding was ﬁxed at 500°C for 8 h with various processing pressures and gas compositions. The experiments indicated that the working pressure of 130 Pa with a gas composition of 50% N2and 50% H2could give rise to the best plasma

nitriding results.Figure 1b is a cross-sectional scanning electron microscopy (SEM) image of the nitrided alloy, showing that the thickness of the nitrided layer is 40 lm. The grain boundaries of the substrate are clearly revealed by the nital etchant, while the nitrided layer re-mains intact. Moreover, the boundary between nitrided layer and substrate is obscure. Figure 1c shows the XRD result for the nitrided alloy, revealing that besides c diﬀraction peaks, diﬀraction peaks belonging to AlN and Fe4N can also be detected. Both AlN and Fe4N have

an fcc structure with lattice parameters of 4.06 nm and 3.79 nm, respectively[20,21]. Moreover, the intensity of the AlN diﬀraction peaks is much higher than that of Fe4N phase, indicating that the nitrided layer is

com-posed predominantly of AlN phase with a signiﬁcantly less amount of Fe4N phase. Furthermore, the XRD peaks

are fairly broadened, which may be due to the large amount of nitrogen incorporated in these phases [11– 12,15–19]. Figure 2a shows the nitrogen concentration as a function of depth, revealing that at the outmost sur-face, the nitrogen concentration is as high as 20 wt.% (48 at.%). The nitrogen concentration gradually de-creases with increasing depth. Figure 2b shows the microhardness of the nitrided alloy as a function of depth. The surface microhardness is extremely high (1860 Hv), and gradually decreases with increasing depth until the substrate value is 550 Hv. Tensile test indicated that UTS, YS and El of the nitrided alloy were 1388 MPa, 1286 MPa and 33.6%, respectively, which are comparable to those obtained for the same alloy aged at 500°C for 8 h. By slightly tilting the specimen, the fracture and free surfaces could be observed simultaneously, as illustrated inFigure 2c. High density of dimples can be seen within the austenite + j0_{-carbides matrix, and no microvoids}

or microcracks are observed in the vicinity of the interface between nitrided layer and substrate. Obviously, the sub-strate remains ductile and the nitrided layer itself is very compact with good adhesion to the substrate.

Potentiodynamic polarization curves for as-quenched and plasma nitrided alloys in 3.5% NaCl solution are shown in Figure 3a. Evidently, for the untreated alloy (curve I), there is no apparent passivation region. The cor-rosion current density (icorr) and corrosion potential

(Ecorr) are 2 106A cm2and790 mV, respectively.

However, an obvious passivation region can be observed for the nitrided alloy (curve II), and icorr is evidently

reduced by about three orders of magnitude to 6 1010A cm2 and Ecorr is drastically improved to

+50 mV. Moreover, the values of the pitting corrosion current density (ip) and pitting potential (Epit) for the

ni-trided alloy are 2 107A cm2and +2030 mV, respec-tively. Apparently, plasma nitriding has resulted in a pronounced enhancement in corrosion resistance.Figure

Figure 1. (a) TEM (100)j0dark-ﬁeld image and corresponding

diﬀrac-tion pattern (hkl: c, hkl: j0_{-carbide) of the as-quenched alloy. (b) SEM}

image of the present nitrided alloy (etched in 5% nital). (c) X-ray diﬀraction pattern for the present nitrided alloy.

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3b and c shows SEM images of the corroded surfaces, indicating that during polarization the grain boundaries and c matrix of the untreated alloy were severely attacked, while only a few very small (0.3 lm) corrosion pits (as indicated with arrows inFig. 3c) were formed for the ni-trided alloy.

That the nitrided layer of the present nitrided alloy is composed predominantly AlN with a small amount of Fe4N is a remarkable feature. For many industrial

appli-cations requiring high strength, high wear resistance and high corrosion resistance, the nitrided low-Cr (Cr < 1.2 wt.%) alloy steels (e.g. AISI 4140, 4340 and 5140) and high-Cr (Cr > 12 wt.%) martensitic stainless steels (e.g. AISI 410) as well as precipitation-hardening (PH) stain-less steels (e.g. AISI 17-4PH) were widely used. Accord-ing to extensive previous studies, the optimal nitridAccord-ing conditions for the low-Cr steels were 520–550°C for 4–6 h[8–10], while those for high-Cr stainless steels were 400–480°C for 2–20 h[11–19]. The nitrided layer formed in these body-centered cubic (bcc) steels is mainly com-posed of Fe3N (hexagonal close packed, hcp) and Fe4N

(fcc), without or with a trace of CrN (fcc)[8–19]. After optimal nitriding treatment, the surface microhardness of the low-Cr alloy steels and high-Cr stainless steels were between 890–940 Hv and 1000–1350 Hv, respectively, which are far lower than 1860 Hv obtained in the present nitrided alloy. The primary reason is that due to AlN

for-mation in the present nitrided alloy, nitrogen concentra-tion near the surface can reach 20 wt.%, whereas the sur-face nitrogen concentrations of the optimally nitrided low-Cr alloy steels and high-Cr stainless steels were 5.7–10 wt.% and 10–15 wt.%, respectively [17,22–25]. The hardness of the nitrides generally increases with increasing nitrogen concentration. For instance, the hardness of AlN is 25.7 GPa[26], which is much higher than that of Fe3N (11.2–12.4 GPa), and Fe4N (8.6–

11.2 GPa)[22,27]. It is worthwhile to emphasize here that the substrate hardness (550 Hv) of the present nitrided al-loy is also much higher than 210–400 Hv obtained in the optimally nitrided high-strength alloy steels and stainless steels[9–16,18]. The reason is that prior to nitriding, these steels need to temper at 15°C above the nitriding temper-ature[28], and then nitrided at the optimal temperature for a long duration. This would deteriorate the substrate hardness drastically [23]. Detailed comparisons of sur-face hardness and substrate hardness are listed inTable 1. The most important indicators for evaluating the cor-rosion resistance of metallic materials are icorr, ip, Ecorr

and Epit; lower current densities and higher potentials

indicate better corrosion resistance [8–19]. Table 1lists the values of icorr, ip, Ecorrand Epit obtained using the

same SCE in 3.5% NaCl solution at room temperature for the present nitrided alloy, and the previous results for the optimally nitrided low-Cr alloy steels (including

Figure 2. (a) Nitrogen concentration proﬁle measured by GDS of the present nitrided alloy. (b) Hardness proﬁle of the present nitrided alloy. (c) SEM image of the present nitrided alloy after tensile test.

Figure 3. (a) Polarization curves for the present untreated and nitrided alloys in 3.5% NaCl solution. (b) and (c) SEM images of the corroded surfaces for the present untreated and nitrided alloys, respectively.

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4140, 4340 and 5140), 410 martensitic stainless steels and 17-4PH stainless steels. For comparison, the results for optimally plasma nitrided lower-strength austenitic stainless steels (AISI 304 and 316) are also listed inTable 1. Evidently, under the same testing conditions, the icorr

and ipof the present alloy are two or three orders of

mag-nitude lower, while the values of Ecorrand Epitare

signif-icantly higher than those of the alloy steels and stainless steels, indicating that the present nitrided alloy has far superior corrosion resistance in 3.5% NaCl solution. Moreover, the size of the surface corrosion pits of the present nitrided alloy is only about 0.3 lm (Fig. 3c), which is much smaller than that (10–200 lm) observed in optimally nitrided alloy steels and stainless steels un-der similar polarization tests [8–9,12–14]. The lower ip

value results in smaller corrosion pits [12,14], which is in good agreement with the experimental results shown in Table 1. Another important criterion for evaluating the pitting resistance is the diﬀerence between Epitand

Ecorr, namely DE = Epit Ecorr[29]. InTable 1, the DE

value for optimally nitrided alloy steels and stainless steels is between +270 and +1330 mV, while that for the present nitrided alloy is +1980 mV, which again demonstrates the superior characteristics of the present nitrided alloy, presumably due to the high nitrogen con-centration at surface[17].

Another feature of the present study is that after etching the boundary between nitrided layer and sub-strate was obscure (Fig. 1b) and no microvoids or cracks could be detected between the nitrided layer and substrate of the fractured surface (Fig. 2c). This is attributed to the fact that both AlN and Fe4N phases

have the same fcc crystal structure as the c matrix and j0_{-carbides with very similar lattice parameters, which}

may result in excellent adhesion between nitrided layer and substrate.

The as-quenched microstructure of the present alloy is ductile c phase containing an extremely high density of nano-sized j0_{-carbides formed through spinodal}

decom-position during quenching. The as-quenched alloy is plasma-nitrided at 500°C for 8 h, resulting in the eﬀects of aging and nitriding simultaneously. Furthermore, the resultant 40 lm thick nitrided layer is composed predom-inantly of AlN, the nitrogen concentration at surface is extremely high up to 20 wt.%. Consequently, the surface microhardness (1860 Hv), substrate hardness (550 Hv), ductility (33.6%) and corrosion resistance in 3.5% NaCl solution of the present nitrided alloy are far superior to those obtained previously for the optimally nitrided high-strength alloy steels as well as martensitic and pre-cipitation-hardening stainless steels.

Acknowledgements

This work was supported by the National Science Council, Taiwan (NSC-100-2221-E-009-053-MY3).

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Alloy Polarization test results in 3.5% NaCl solution Hardness (Hv)

Icorr(A/cm 2

) Ip(A/cm

2

) Ecorr(mV) Epit(mV) DE(mV) Surface Substrate

Alloy steels 8 108_{4 10}7 ₄₁₀6_{9 10}6 ₄₀₀₂₀₀ _+500+800 _+770+1000 ₈₉₀₉₄₀ ₂₇₅₃₂₀ 410 (MSS) 6 108_{6 10}7 ₈₁₀5_{8 10}4 ₂₂₀₃₀ _+50+600 _+270+630 _11501204 ₂₁₀₂₆₂ 17-4PH(SS) 4.1 106_{9 10}6 ₉₁₀6_{1.3 10}5 ₂₀₈₂₀₇ _+700+715 _+907+923 _11601167 ₃₆₀₄₀₀ 304 (SS) 1.46 108_{1 10}7 ₄₁₀7_{2 10}6 ₃₀₀₉₈ _+125+400 _+425+498 _10001200 ₂₂₀₂₅₀ 316 (SS) 1 107_{3.5 10}7 ₁₁₀5_{8 10}5 _33083.8 _+600+1200 _+683.8+1330 ₁₃₅₀ ₂₂₀ Present alloy 6 1010 ₂₁₀7 ₊₅₀ ₊₂₀₃₀ ₊₁₉₈₀ ₁₈₆₀ ₅₅₀