Nanoindentation behavior of nanostructured bulk (Fe,Cr)Al and (Fe,Cr)Al-Al2O3 nanocomposites

Nanoindentation behavior of nanostructured bulk (Fe,Cr)Al and (Fe,Cr)Al-Al2O3 nanocomposites

Journal of Alloys and Compounds 792 (2019) 348e356 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 792 (2019) 348e356

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Nanoindentation behavior of nanostructured bulk (Fe,Cr)Al and (Fe,Cr) Al-Al2O3 nanocomposites F. Sourani a, b, M.H. Enayati a, *, X. Zhou c, S. Wang c, A.H.W. Ngan b a

Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran Department of Mechanical Engineering, University of Hong Kong, Hong Kong c Department of Physics, Southern University of Science and Technology, Shenzhen, Guangdong, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 February 2019 Received in revised form 30 March 2019 Accepted 1 April 2019 Available online 5 April 2019

The purpose of the present study was to design a new generation of FeAl-based intermetallic compounds with improved mechanical and oxidation properties. Synthesis of powders of FeAl, (Fe,Cr)Al and (Fe,Cr)Al in situ composites with 5 and 10% Al2O3 contents was reported elsewhere. In this paper the sintering behavior is reported. FeAl, (Fe,Cr)Al and (Fe,Cr)Al - Al2O3 powders were sintered by a hot-pressing method at 1600  C and 5.5 GPa for 15 min to produce bulk specimens for mechanical and microstructural characterization. Microstructural characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of theses nanocomposites disclosed uniform distribution of Al2O3 nano-particles. XRD analysis indicated the presence of FeAl, (Fe,Cr)Al and Al2O3 phases. The presence of nano-sized Al2O3 particles effectively retarded grain growth by pinning the grain boundary during hot pressing. Nanoindentation tests indicated that with increasing content of Cr and Al2O3 reinforcement, the hardness and Young's modulus increased, and the fracture toughness reaches a high value of 19.6 ± 0.4 MPa m1/2 for the (Fe,Cr)Al-10%Al2O3 composition, with a hardness value of 21.7 ± 1.8 GPa. Nanoindentation creep tests at room temperature showed that composites with Al2O3 nanoparticles had higher creep rate than FeAl and (Fe,Cr)Al. It was found that improvements in the mechanical properties of FeAl intermetallic compound can be achieved by adding Cr and uniform distribution of Al2O3 nanoparticles. © 2019 Elsevier B.V. All rights reserved.

Keywords: (Fe,Cr)Al-10%Al2O3 nanocomposite Microstructure Mechanical properties High pressure consolidation Nanoindentation

1. Introduction There has been a great deal of interest in studying whether and how alloying elements can improve the mechanical, tribological, electrochemical and other properties of the FeAl intermetallic compound. Alonso et al. [1] indicated that alloying elements of Si, V, Co, Cr, Mn, Cu, Zn, and Ni exhibit high solubility in FeAl, whereas solubility is more limited for Zr, Nb, and Ta. Zamanzadeh et al. [2] studied the strength of the compound with the formation of solid solutions of Cr, Mo, Ti and V at various temperatures, and found that the increase in the yield stress depends on the temperature and that Cr at high temperatures did not exhibit a significant effect on the yield strength, but V, Mo, and Ti led to a greater increase in yield strength. Risanti et al. [3] found that Nb is an effective element in increasing the tensile strength of FeAl at high temperatures via the formation of Fe2Nb and other precipitates that increase strength

* Corresponding author. E-mail address: [email protected] (M.H. Enayati). https://doi.org/10.1016/j.jallcom.2019.04.012 0925-8388/© 2019 Elsevier B.V. All rights reserved.

and prohibit the grain growth, although they can also aggravate the room-temperature brittleness of FeAl. Klein and Baker [4] showed that the addition of 5% Cr to Fe-40Al increased the yield strength and ultimate strength, and reduced the grain size from 28 mm in the Cr-free samples to 5 mm in Fe-40A1-5Cr samples. They also reported that at a strain rate  1  104 s1, the ductility of Fe-40Al5Cr increased mildly by about 2% compared with the sample without Cr, and on increasing strain rate the ductility also increased, with the highest ductility of about 7% achieved at the strain rate of 1s1. Nevertheless, the increase in strength with the presence of Cr observed by these researchers was actually greater than the values reported by Schneibel et al. [5]. Klein and Baker [4] also reported a large number of CrAl2 secondary particles in their samples, while such particles were not observed by Schneibel et al. Sharma et al. [6] reported that the wear rate of the Fe-28Al composition was improved by Cr addition. Since reaching to the submicron grain size is very important for improving the mechanical properties of the FeAl intermetallic compound, achieving this grain size and inhibiting grain growth in a sintering process is one of the key concerns. In this connection,

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Enayati et al. [7] have successfully synthesized FeAl nanostructures by using the mechanical alloying (MA) method and optimizing milling parameters such as time and speed of milling and the number of balls. The grain size of the powder obtained was about 40 nm, and after heat treatment for 1 h at 800  C, the milled powder did not exhibit significant grain growth. Other reports along a similar direction include: NiAl intermetallic with a 20 nm grain size and hardness of ~1035 HV synthesized by 60 h of MA milling [8], (Fe,Ti)3Al with a grain size of about 20 nm and hardness of ~1050 HV synthesized by milling for 100 h [9], TiAl with a grain size of 50 nm and hardness of ~1190 HV by milling for 60 h [10], and (Ni, Fe)3Al with ~5 nm grain size and hardness of ~1170 HV by milling for 80 h [11]. In addition to monolithic intermetallics, intermetallic composites reinforced by ceramic phases have also been synthesized by the MA route. Khodaei et al. [12] fabricated Fe3Al-30 vol% Al2O3 nanocomposites by MA of two types of powder mixtures, namely, (i) Fe2O3, Al and Fe powders with the Al2O3 nanoparticles produced in situ in the Fe3Al matrix during milling, as well as (ii) a powder mixture containing directly the Al2O3 in addition to Fe-Al. The product obtained from the first, in situ method was more finegrained with a higher hardness of 538 HV and three-point bend strength of 173 MPa, compared with the direct method which produced a 490 HV hardness and 43 MPa three-point bending strength. Rafiei et al. [13] succeeded in the synthesis of (Fe,Ti)3AlAl2O3 nanocomposites using MA of Fe, Al and TiO2 powders, during which Al initially reacted with TiO2 to produce Ti crystals and an Al2O3 amorphous phase, and then Ti and Al diffused into the Fe network to form an Fe(Al,Ti) solid solution. After long milling for about 100 h, (Fe,Ti)3Al nanocrystals of ~10 nm in size were produced. In a further study, Rafei et al. [14] reported that the grain size and hardness of (Fe,Ti)3Al were approximately 12 nm and 1050 HV respectively, while those of Fe3Al was about 35 nm and 700 HV. Aghili et al. [15] synthesized an (Fe, Cr)3Al-Al2O3 nanocomposite with a grain size of ~30 nm after over 100 h of milling. They reported [16] the hardness of this nanocomposite to be ~1140 HV, against the 935HV for monolithic (Fe, Cr)3Al. Karnowski et al. [17] also synthesized an FeAl-TiN nanocomposite by mixing Al-Ti-Fe powders and then hot-pressed in 8 GPa pressure. The product exhibited a high hardness and density of about 97% of the theoretical value. FeAl composites with Al2O3 or Fe3AlC reinforcing phase produced by MA of powders and hot pressing were also investigated by Issushi et al. [18]. Abu-Oqail et al. [19] showed the effect of GNPs coated Ag on microstructure and mechanical properties of Cu-Fe dual-matrix nanocomposite. They reported that the hardness of Cu-20%Fe/0.6%GNPs nanocomposite improved by 12.5% compared to Cu-20%Fe dual-matrix composite and the wear rate reduced by 81.9% compared to Cu-20%Fe dual-matrix composite. Fathy et al. [20] indicated that increasing GNPs volume fraction improves compressive strength, hardness and antifriction properties of Al-10%Al2O3/1.4%GNPs composites. Wagih et al. [21] investigated the effect of GNPs content on thermal and mechanical properties of hybrid Cu-Al2O3/GNPs coated Ag nanocomposite. Their results indicated that compressive strength and hardness significantly improved by increasing GNPs and the coefficient of thermal expansion and electrical conductivity reduced. Abu-Oqai et al. [22,23] showed that the presence of ZrO2 nanoparticles improves mechanical properties of Cu-ZrO2 nanocomposites. Melaibari et al. [24] found that the tensile strength and elongation of Al-5vol.%SiC nanocomposite follow an exponential behavior with the number of passes in accumulative roll bonding (ARB) process. Wagih et al. [25] investigated Al-4%SiC nanocomposite. The tensile strength for ARBed Al and Al-4%SiC nanocomposite after nine passes was 3.19 and 4.09 times greater than that for annealed Al 1050. Wagih et al. [26,27] also showed that the

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young's modulus, yield strength, and hardness of the AleAl2O3 and CueAl2O3 nanocomposite were better compared to that for pure aluminum and Cu. Further, Wagih et al. [28] found that the addition of Al2O3 to Al can help to improve the compressibility behavior. The above survey indicates that the powder metallurgy method of MA is an effective method for synthesizing intermetallic based materials, and strategies including decreasing the grain size of the particles, forming secondary phases during the synthesis and adding suitable alloying elements can improve properties such as strength, hardness, oxidation and creep resistance of the product. Sintering the MA-synthesized powder materials is also a useful method for manufacturing bulk products from powders [29]. In our previous paper [30] we report the successful synthesis of powders of FeAl, (Fe,Cr)Al and (Fe,Cr)Al composites with 5 and 10% Al2O3 contents. Here, we aim to study the microstructures and mechanical properties of sintered products made by hot pressing these powders together. 2. Materials and method In this work, MA continued for 100 h as our previous work [30] showed that MA for 100 h is the appropriate time for achieving the minimum grain size. MAed powder was subsequently sintered at high pressures (P) of 4e5.5 GPa and high temperatures (T) of 900e1600  C for 15e30 min High-pressure experiment was carried out in a 6  10 MN cubic press. Pressure and temperature calibration have previously been described in Ref. [31]. In each experiment, the powder mixture was compacted in to a cylindrical pellet of 10 mm in diameter and 5 mm in height. A molybdenum container was used to surround the sample and prevent possible contamination. The sample was then put in a high-pressure cell assembly for high P-T experiments. More experimental detail can be found elsewhere [32]. Density of the hot-pressed specimens was measured using immersion technique as specified in ASTMB311 and the relative density is reported as the ratio of the measured density to the theoretical density. Phase constituents were detected by using X-ray diffractometer (XRD) with a copper target and its Ka radiation, and Energydispersive X-ray spectroscopy (EDS). For characterization of microstructure and texture a field-emission scanning electron microscope (SEM) (LEO 1530) with an Electron Backscattered Diffraction (EBSD) analyzer, and a transmission electron microscope (TEM) (FEI Tecnai G220) were applied. Focused-ion beam FIB (FEI Quanta 200) milling was used to produce TEM foils from the bulk samples. Nanohardness measurements with a maximum load of 20e200 mN a load controlled mode were performed by using an Agilent G200 nanoindenter with a Berkovich tip. The nanoindentation tests were carried out in a 5  5 array and were repeated 10 times, and before the nanoindentation tests, all specimens were ground and then polished with 0.5 mm diamond paste to produce a smooth surface. 3. Results and discussion 3.1. Microstructure and phase analysis Fig. 1 shows the XRD patterns of the FeAl, (Fe,Cr)Al and (Fe,Cr) Al-Al2O3 powders ball-milled for 100 h, and the samples sintered by hot pressing of powders at 1600  C under 5.5 GPa for 15 min. The ball-milled powders and sintered samples mostly consist of three phases: (1) the FeAl intermetallic phase; (2) the (Fe,Cr)Al matrix phase; and (3) the Al2O3 phase. Comparing the FeAl XRD patterns for the hot-pressed samples with their ball-milled powder counterparts, one can conclude that the sintering step leads to an obvious grain growth. Also it can be noticed that, for the three

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Fig. 1. XRD patterns of the FeAl, (Fe,Cr)Al and (Fe,Cr)Al-Al2O3 powders ball-milled for 100 h, and samples sintered (S) by hot pressing of powders at 1600  C.

materials studied, hot pressing does not result in the formation of new products other than the phases in the powders. In Fig. 1, the positions of diffraction peaks in the as-milled and sintered samples are the same while the peak widths decreased as a result of grain growth process during sintering. It should be noted that positions of XRD peaks for (Fe,Cr)Al are at higher angels with respect to those for FeAl compound. The XRD peaks were analyzed using the WilliamsoneHall method [33], and Table 1 shows the calculated crystallite size and internal strain of the various samples.

Table 2 Bulk densities of hot-pressed samples. %Composites

Density (gr/cm3)

Relative Density

FeAl (Fe, Cr)Al (Fe,Cr)Al-5%volAl2O3 (Fe,Cr)Al-10%volAl2O3

5.56 6.04 5.96 5.85

99.2 99.5 99.7 99.8

Table 1 Grain size and internal strain for different materials after milling for 100 h and subsequent sintering. FeAl

(Fe, Cr)Al

(Fe, Cr)Al-Al2O3 Al2O3

Grain size (nm) Internal strain (%)

(Fe, Cr)Al

Before Sintering

After Sintering

Before Sintering

After Sintering

Before Sintering

After Sintering

Before Sintering

After Sintering

54 0.88

1613 0.59

30 1.78

1125 0.74

29 1.9

100 0.83

18 2.6

70 0.43

Fig. 2. Polished surfaces of the hot-pressed samples: (a) FeAl (b) (Fe,Cr)Al (c) (Fe,Cr)Al-5%Al2O3 (d) (Fe,Cr)Al-10%Al2O3.

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Fig. 3. SEM, EDS and elemental mapping images of hot -pressed (a) (Fe,Cr)Al-5%Al2O3 (b) (Fe,Cr)Al-10%Al2O3.

The sintering process evidently caused grain growth and reduction in internal stain. Among the three types of materials, the sintering induced grain growth is least significant in (Fe,Cr)Al-Al2O3, probably because the presence of nanometric Al2O3 particles can effectively retard grain growth by pinning grain boundaries during sintering [34]. The relative densities for all hot-pressed samples are listed in Table 2. High relative densities are achieved for all composites. The result here demonstrates that hot pressing of MA powder is an effective fabrication technique for densification of Al2O3 reinforced Fe-Al based intermetallics. SEM images of polished and etched surfaces of the hot-pressed

samples of FeAl, (Fe,Cr)Al, (Fe,Cr)Al-5%Al2O3 and (Fe,Cr)Al-10% Al2O3 are shown in Fig. 2. (Fe,Cr)Al grains in these samples are equiaxed with sizes ranging from 1 to 3 mm, while some grains have elongated shapes with lengths varying between 4 and 6 mm. Fig. 3 shows the results of EDS analysis of the different phases in the hotpressed (Fe,Cr)Al-Al2O3 samples. The darker phase in the SEM images is Al2O3, and the gray phase is the (Fe,Cr)Al grains. The absence of significant residual porosity, as observed from the SEM micrographs in Figs. 2 and 3, confirms that the samples were densified to >99% of their theoretical density. The EDS results in Fig. 3 show that the particles consist mostly of Al and O with a ratio close to Al2O3. The uniform distribution of Al2O3 nanoparticles in the

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Fig. 3. (continued).

microstructure without the formation of new phases is consequence of ball milling process (Fig. 4).

3.2. Mechanical property measurements Nanoindentation technique was used to determine the loading-

unloading characteristics of as-milled powder particles and hotpressed samples under a maximum load of 20 mN and withdrawing the indenter from the specimen over a period of 20 s. The nanohardness of powder particles is higher than that of the bulk material. The results of the nanoindentation tests are summarized in Fig. 5. The nanohardness of the bulk materials and powder

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Fig. 4. Bright-field TEM micrographs of hot-pressed samples (a) (Fe,Cr)Al-5%Al2O3 and (b) (Fe,Cr)Al-10%Al2O3 showing Al2O3 nanoparticles within the grains and coarser particles located at boundaries.

Fig. 5. Load-indentation depth curve, comparison between powder particles milled for 100 h (P) and hot-pressed samples (S).

particles were ~11e21 GPa and ~30 GPa, respectively. The high nanohardness of the particles can be attributed to the smaller grain size of powders compared to the bulk sample, as seen in Table 1. Table 3 shows the nanohardness, Young's modulus and fracture toughness of different hot-pressed samples. The nanohardness and modulus of (Fe,Cr)Al-10 %vol Al2O3 were the highest due to the high hardness of Al2O3 nanoparticles. Also the (Fe,Cr)Al-10%vol Al2O3

nanocomposite had the highest density which can explain its highest nanohardness compared to the other hot-pressed samples as seen in Fig. 5. The results of nanohardness test also indicate that addition of Cr as a substitution element has significantly improved Young's modulus and hardness of FeAl intermetallic. The size and distribution of the reinforcement particles are important for achieving high hardness.

Table 3 Nanohardness (H), Young's modulus (E) and fracture toughness of hot-pressed samples. Composites

Modulus (GPa)

Nanohardness (GPa)

Fracture toughness (MPa.m1/2)

FeAl (Fe, Cr)Al (Fe,Cr)Al-5%volAl2O3 (Fe,Cr)Al-10%volAl2O3

337.1 ± 18.6 404.8 ± 31.4 554.3 ± 17.8 607.1 ± 27.7

11.1 ± 1.0 14.6 ± 1.2 18.3 ± 1.4 21.7 ± 1.8

12.8 ± 0.5 15.3 ± 0.7 18.4 ± 0.4 19.6 ± 0.4

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Fig. 6 shows the typical curves of Young's modulus and nanohardness at different maximum loads for hot-pressed (Fe,Cr)Al. As seen, Young's modulus (E) and nanohardness (H) are dependent on the load and as the indentation load increases from 20 mN to 200 mN, E and H decrease from 404.8 to 221.3 GPa, and from 24.6 to 6.5 GPa, respectively. Fracture toughness variations are mainly attributed to Al2O3 nanoparticles. KIC was measured by Eq (1) using the length of the indentation-induced crack, considering that the type of crack geometry is the median/radial:

KIC ¼ a

 n   E P H C 1:5

(1)

where c is half of the total crack span around the indentation created by load P, in a material with Young's modulus E (GPa) and

hardness H (GPa), and a~ 0.016 [35]. For each measurement of crack length (2c) as shown in Fig. 7a, the test was repeated 5 times, and the standard deviation was about 1 mm. The maximum load to create a crack was different in different samples. The high loads of 30, 40 and 50 kgf create large cracks needed for accurate measurement on FeAl, (Fe,Cr)Al and (Fe,Cr)Al-Al2O3, respectively. No cracks were seen in lower loads. The fracture toughness of FeAl, (Fe,Cr)Al, (Fe,Cr)Al-5%volAl2O3, and (Fe,Cr)Al-10%volAl2O3, were 12.8, 15.3, 18.4, and 19.6 MPa m1/2, respectively. In nanocomposites with nano reinforcement particles, the fracture mode is mainly the transgranular fracture mode. Some possible mechanisms that can improve the fracture toughness of composites include: 1) crack bridging e the Al2O3 nanoparticles may bridge the two surfaces of cracks and hinder further propagation of the crack as seen in Fig. 7b; 2) pull-out mechanism nanoparticles are pulled out from the matrix and this slows down

Fig. 6. Variation of E and H of hot-pressed (Fe,Cr)Al obtained during indentation to maximum loads of 20, 30, 40, 50 and 200 mN.

Fig. 7. (a) A Vickers hardness indent on the (Fe,Cr)Al hot-pressed sample, (b) Propagation of crack during indentation of (Fe,Cr)Al e 10% vol Al2O3 nanocomposite, (c) particle pullout and crack bridging, (d) crack deflection and microcracks.

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crack propagation by the interfacial friction between the Al2O3 nanoparticles and the (Fe,Cr)Al matrix as seen in Fig. 7c; 3) crack deflection - when cracks arrive at the uniformly distributed Al2O3 nanoparticles, they change their direction which increases the crack propagation path; 4) microcracks formation (Fig. 7d). Creep behavior was evaluated during load-hold at 200 mN in nanoindentation tests. Each nanoindentation test was repeated for 10 times to obtain averaged results. As seen in Fig. 8a, the maximum indentation depth decreases with the addition of Cr and Al2O3 nanoparticles at the same load level. The creep displacement during load-hold is plotted in Fig. 8b as a function of time. The (Fe,Cr)Al e 10%vol Al2O3 showed the lowest creep deformation while the FeAl exhibited the highest creep deformation. The higher creep resistance of (Fe,Cr)Al-Al2O3 compared to FeAl is due possibly to: (1) Cr strengthening mechanisms [36], i.e. solid solution and precipitation hardening, and (2) hindering dislocations movements due to the presence of Al2O3 nanoparticles.

4. Conclusion Hot-pressing (5.5 GPa, 1600  C) method was used to consolidate mechanical alloyed FeAl and (Fe,Cr)Al-Al2O3 powders. In all samples, hot pressing resulted in a high relative density of >99%. The microstructural studies revealed a uniform distribution of the Al2O3 nanoparticles in the matrix. It was found that by adding Cr and dispersion of Al2O3 reinforcement phase in the FeAl matrix, the nanohardness of the samples increased from 19 GPa for FeAl to 45 GPa for (Fe,Cr)Al-10 vol% Al2O3 nanocomposite, and Young's modulus increased from 337 to 607 GPa respectively. The fracture toughness had a direct relationship with the addition of Cr and Al2O3 and reached 19.6 MPam1/2 for (Fe, Cr) Al-10 vol % Al2O3 nanocomposite. Nanoindentation creep tests revealed that the creep resistance also increases with the addition of Cr and Al2O3.

Fig. 8. (a) Indentation load-depth curves and (b) Creep time-depth curves of the hotpressed samples.

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