Ferritic alloys strengthened by β′ phase and nanosized oxide

Lin Zhang a,n, Xuanhui Qu a, Rafi-ud din b, Mingli Qin a, Yue Wang a

a Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR Chinab Materials Division, PINSTECH, P.O. Box, Nilore, Islamabad, Pakistan

a r t i c l e i n f o

Article history:Received 20 November 2013Accepted 30 November 2013Available online 7 December 2013

Keywords:Metals and alloysNanoparticlesElectron microscopyMicrostructure

a b s t r a c t

Ferritic alloys, strengthened by precipitating dispersed NiAl intermetallic compound (β′) and nanosizedoxides, were fabricated by mechanical alloying route. The particle size evolution, chemistry, andinterfacial structure of the nanoprecipitates were investigated. The near spherical β′ phase with volumefraction of 17.9% and an average diameter of 124 nm was obtained. The oxide nanoparticles, with aparticle size of 3–8 nm, were uniformly distributed in α-Fe matrix. Moreover, both the β′ phase andnanosized oxides were found to be coherent or semi-coherent with the matrix. The β/β′ Fe-based ODSalloys can be considered as a potential candidate for the replacement of γ/γ′ Ni-base ODS alloys.

& 2013 Published by Elsevier B.V.

1. Introduction

Ferritic alloys, based on 9–12 wt% Cr–steel, exhibit lower thermalexpansion, higher thermal conductivity, good oxidation resistance,and lower material costs. These advantages of ferritic alloys haverendered them very attractive for their use in high-temperatureapplications, such as the fabrication of heat exchangers in advancedpowder plant or the structural materials in nuclear reactors [1–2].However, the lack of high temperature creep strength of these alloyswith increasing service temperature necessitates the furtherimprovement of their mechanical properties [3].

The use of second-phase nanoparticles is one of the mostimportant methods to extend the high-temperature strength limitof ferritic alloys [4–5]. On the one hand, body-centered cubic(bcc)-based ordered B2-type NiAl precipitates (β′) are effectiveintermetallic strengthening species [6]. β′ phase demonstratesobvious strengthening effect by virtue of the creation of antiphaseboundaries or coherent strains that effectively impede dislocationmotion [7]. The conventional way of synthesizing an alloy dopedwith oxide particles involves the high-energy ball milling with theaddition of Y2O3 [8]. In the present work, less thermodynamicallystable Fe2O3 and YH2 were used as raw materials, which can becompletely dissolved during ball milling [9], resulting in theformation of Y2O3 nanoparticles thorough the mechano-chemistry reaction between YH2 and Fe2O3.

The utilization of combination of above two kinds of strength-ening phases points out a new direction in the design of novel β/β′Fe-base ODS alloy. It is expected that the intermediate tempera-ture strength can be enhanced by employing the β′phase, while

nanosized oxides are more effective at elevated temperature.Moreover, β/β′ Fe-base ODS alloy has low density and low cost,which is a potential candidate for the replacement of γ/γ′ Ni-baseODS alloys. β/β′ Fe-base ODS alloys are fabricated via mechanicalalloying. The chemical composition, particle size, and interfacialstructure of above two kinds of precipitates have been character-ized. This information aids in designing the alloy compositions andthe microstructure optimization.

2. Experimental

Ferritic ODS alloys with the composition of Fe-6.5Al-11.4Ni-8.8Cr-3.4Mo-0.4YH2-(0.17~0.35)Fe2O3 (wt. %) were designed. Firstly,the powder mixture was mechanical alloyed in a high energyplanetary ball mill at a rotation speed of 300–350 rpm with theball/powder weight ratio of 10:1 in Ar atmosphere for 5 min–48 h.Secondly, the MA powder was consolidated by spark plasmasintering (SPS) at 970 1C with the pressure of 50 MPa. Finally, thespecimens were solution treated at 1200 1C for 1 h and aged at700 1C for 48 h. The phase constituents of the alloys were identifiedby a Rigaku D/max-RB12 X-Ray diffract meter with Cu Kα radiation.The oxygen content of the powder was analyzed by using a LECOTN-114 nitrogen-oxygen analyzer. The microstructure of the alloywas examined by JSM-7001F field emission scanning electronmicroscopy. JEM-2010 transmission electron microscopy (TEM)was employed to observe the morphology of the nanoprecipitates.

3. Results and discussion

Fig. 1(a) depicts the XRD patterns of the powder mixture milledfor various periods of time. In case of the powder milled for 5 min,

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Materials Letters

0167-577X/$ - see front matter & 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.matlet.2013.11.127

n Corresponding author.E-mail address: zhanglincsu@163.com (L. Zhang).

Materials Letters 117 (2014) 286–289

the peaks of YH2, Fe2O3 and Al are detected. The intensity of YH2,Fe2O3 and Al diffraction peaks decrease with increasing millingtime. Only the width broadening of the α-Fe diffraction peak hasbeen observed for the powder milled for 20 h, implying thecomplete dissolution or decomposition of Fe2O3 and YH2 in α-Fematrix. One important factor for the refinement of the oxides isthe reduction of excess oxygen contents effectively. The excessoxygen contents are determined by subtracting the amount ofoxygen contained in Y2O3 from the total oxygen contents. Fig. 1(b) shows the average particle size and extra oxygen contents ofthe MA powder as a function of milling time. Particle size of theMA powder increases with increasing milling time. After millingfor 20 h, the dramatic refinement of particle size is observed dueto the disintegration of powder particles. With the disintegrationof powder particles, the extra oxygen contents of the 0.35% Fe2O3-added powder increases sharply due to the absorption of impurityoxygen at solid/gas interface [10]. In the case of powder with theaddition of 0.17% Fe2O3, the extra oxygen contents increasesslightly and the final extra oxygen contents are much lower thanthat of the 0.35% Fe2O3-added powder.

Fig. 2 displays the microstructure of the heat treated β/β′ Fe-based ODS alloys. Fig. 2(a) clearly indicates the presence ofsmall amount of irregular white precipitates (marked by W) inthe microstructure. Additionally, a large quantity of near sphericaldark phase has precipitated in a body-centered cubic (BCC) α-Fematrix. By combining the XRD result, the dark precipitates areNiAl-type phase with the B2 structure (designated as β′). Thevolume fraction of β′ phase is measured to be 17.9%, and thenumber density of β′ phase is 7.0�1019 m�3. β′ phase has anaverage particle size of 124 nm. The high magnification micro-structure of the alloy demonstrates the formation of β/β′ two-phase microstructure (inset in Fig. 2(a)). It is also evident in Fig. 2(b) that the β/β′ Fe-based ODS alloys are consisted of α-ferrite, NiAl(β′ phase), Al95Fe4Cr, and Mo.

Table 1 lists the average chemical compositions of variousphases observed in Fig. 2(a). The contents of Al (23.16 at%) and

Ni (20.43 at%) in the black precipitates, marked by D, are roughlythe same and corresponds to the β′ precipitates. The black phaseshows high contents of Al (38.22 at%). The contents of Mo in theirregular white phase (W) is as high as 11.18 at%. In accordancewith the XRD results, the white phase is identified to be theundissolved Mo. The most of Mo remains in the matrix with only asmall amount is partitioned into the NiAl precipitates. The solidsolution of Mo into α-Fe matrix reduces the lattice misfit betweenβ′ precipitates and α-Fe matrix, resulting in the stabilization of thespherical morphology of β′ phase.

Fig. 3 shows the dark field TEM images of β/β′ Fe-based ODSalloy and the corresponding selected area diffraction pattern.Fig. 3(a) clearly reveals that near spherical β′ phase has precipi-tated from the matrix. The particle size of β′ phase is in the rangeof 52–161 nm. Fig. 3(b) shows the corresponding selected areaelectron diffraction pattern. β/β′ Fe-based ODS alloys are micro-structurally analogous to classical Ni-based γ/γ′ superalloys con-taining coherent ordered intermetallic precipitates (Ni3Al) in adisordered solid-solution matrix (γ-Ni) [11]. The characteristics ofphase boundary between the α-Fe and β′ phase have been studiedby HRTEM, as shown in Fig. 3(c). The central part corresponds tothe nanoparticle/matrix interface region showing dark contrast, asindicated by the dotted white curve. Fig. 3(d) indicates the highmagnification image of the interfacial domain. The atomic planesacross boundary of α-Fe and β′ phase have displayed the con-tinuity, implying the presence of a certain degree of coherency.

Fig. 1. XRD patterns of the powder mixture milled for various periods of time, and the effect of milling time on particle size and excess oxygen contents of the powder.

Fig. 2. SEM images of the heat treated alloy (a) and the corresponding XRD pattern (b).

Table 1Chemical composition of various phases observed in Fig. 2(a) (at%).

Domain Ni Al Cr Mo Fe

α-ferrite matrix 7.86 6.56 6.53 2.42 74.84β′ phase (marked by D) 20.43 23.16 8.33 – 49.88Black phase (marked by B) 3.67 38.22 12.26 2.28 43.57White phase (marked by W) 5.65 15.6 10.95 11.18 56.62

L. Zhang et al. / Materials Letters 117 (2014) 286–289 287

Fig. 3. (a) The ark field TEM images of β/β′ Fe-based ODS alloys and corresponding selected area diffraction pattern (b), HRTEM image (c), and the FFT image (d).

Fig. 4. The bright field TEM image of β/β′ Fe-based ODS alloys (a) and the HRTEM image of nanosized oxide (b), as well as the EDS result (c).

L. Zhang et al. / Materials Letters 117 (2014) 286–289288

The measured lattice parameters for the ferritic matrix (αα) andthe β′ precipitate (αβ′) are 2.0381 Å and 2.0362 Å, respectively, for,which corresponds to a lattice misfit (δ¼ αα �αβ′j j

αα) of 0.09%. The

small lattice misfit brings about the formation of coherent inter-facial structure, contributing to the improved stability of high-density β′ precipitate during prolonged aging. The high latticecoherency of β′ precipitation can accommodate the extra energyneeded to create the antiphase boundaries, and making it easierfor dislocation to cut through the nanoscale precipitates. Fig. 3(d)shows the inverse fast Fourier transform (FFT) image, which isreconstructed by using the diffraction spots with scattering vectorsparallel to the interface, as shown in the inset of Fig. 3(d).

Fig. 4(a) displays the TEM image of β/β′ Fe-based ODS alloys.A high density of oxide nanoparticles distribute uniformly in thematrix. It is clearly demonstrated that the size of these oxideparticles is in the range of 3–8 nm with irregular shape. Thenumber density of these nanosized oxide nanoparticles is found tobe 1.7�1024 m�3. The detailed interfacial structure of the oxidenanoparticle is displayed in Fig. 4(b). The plane distance ismeasured to be 2.61 Å and 1.89 Å, respectively, which correspondsto (200) and (220) planes of Y2O3 oxides. The lattice continuitybetween the oxides and surrounding matrix is also clearlyobserved, suggesting that oxide nanoparticles remain coherentlywithin a-Fe. The reduced interfacial energy favors the homoge-neous nucleation of the oxides and accounts for the enhancedstability of these nanoparticles. Fig. 4(c) shows the EDS analysis ofthe oxide particle. It is obvious that the oxide is composed of Y andO. Hence, in accordance with the XRD and EDS results, these oxideparticles are confirmed to be Y2O3 oxides.

4. Conclusions

The β/β′ Fe-based ODS alloys were successfully fabricated bymechanical alloying. The β/β′ Fe-based ODS alloys were strengthened

by a high volume fraction (17.9%) of β′ intermetallic compound with amean precipitate size of 124 nm and a high density of nanosizedoxide particles with a precipitate size of below 8 nm. Both NiAl-typeintermetallic phase and nanosized oxides were found to be coherentor partially coherent with the matrix. It is expected that β/β′ Fe-basedODS alloy has high strength without significantly loss in the ductility.In short, the β/β′ Fe-based ODS alloys can be considered as a potentialcandidate for the replacement of γ/γ′ Ni-base ODS alloys.

Acknowledgments

The research was financially supported by National NatureScience Foundation of China (51104007) and Beijing NaturalScience Foundation (2132046).

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