Mechanical properties and microstructure of Fe₃Al intermetallics fabricated by mechanical alloying and spark plasma sintering

HE Qing(何 箐), JIA Cheng-chang(贾成厂), MENG Jie(孟 杰)

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Fabrication technology and mechanical properties of the Fe₃Al based alloys were studied by spark plasma sintering from elemental powders (Fe-30Al, volume fraction, %) and mechanically alloying powders. The mechanically alloying powders were processed by the high-energy ball milling the elemental mixture powders with the milling time of 5, 8 and 10 min, respectively. The spark plasma sintering process was performed under the pressure of 50 MPa at 1 050 ℃ for 5 min. The phase identification by X-ray diffraction presents the Fe reacts with Al completely during the processing time. The samples are nearly full density (e.g. the relative density of sinter of raw powder is 99%). The microstructure was observed by TEM. The mechanical properties were tested by three-point bending at room temperature in air. The results show that the mechanical properties are better (e.g. bend strength of  1 500 MPa ) than those of the ordinary Fe₃Al casting.

Key words: iron aluminides; mechanical alloying; spark plasma sintering; bend strength; microstructure

1 Introduction

Alloys based on intermetallic Fe₃Al have been extensively studied as candidate materials for high-temperature applications due to their remarkable high strength, good oxidation and corrosion resistance at elevated temperature. They also provide potential as low cost replacement for more expensive high temperature structural alloys containing nickel and chromium. But it has some limitations to be conquered for commercial applications, such as low ductility exhibited at low temperature and limited workability[1-3]. Recently, it has been shown that carbon may be an important alloying element to Fe₃Al-based alloys. The addition of carbon leads to improved machinability, strength and creep resistance. These improvements have been attributed to the formation of Fe₃AlC_0.5 precipitates in these carbon-containing alloys. The precipitation amount of Fe₃AlC_0.5 increases with increasing carbon content[4-7].

Conventional methods of processing Fe₃Al intermetallic, such as melting and casting, traditional powder metallurgy, have been investigated. The products prepared by melting and casting tend to form the microdefects and aliquation, which may decrease the mechanical properties of the final products. And subsequent process was difficult to prepare the sectional bar. The tradition powder metallurgy method can fabricate the products with complex shape, but the low reproducibility of tradition powder metallurgy method was a limitation for the final application. Recently, some effective processing methods have been used to improve the mechanical properties of Fe₃Al intermetallic, such as mechanical alloying (MA) and spark plasma sintering (SPS).

The MA process was developed in 1966 at International Nickel Company (INCO) as part of a program to produce a material combining oxide dispersion strengthening with gamma prime precipitation hardening in a nickel-based superalloy intended for gas turbine applications. Mechanical alloying has received much attention as a powerful tool for the fabrication of several advanced materials including equilibrium, nonequilibrium, composite and nanophase materials. The shear and impact action between the ball and ball, ball and milling vial can refine the particle size, enhance the surface activate energy and obtain the fresh powder surface. And during the subsequently consolidation process, these phenomenon can decrease sintering temperature and produce dense, fine grain and high cohesive grain boundary materials[8]. SPS was developed based on the idea of using the plasma on electric discharge machine for sintering metal and ceramics by INOUE[9]. They expected that sintering assisted by plasma could help realizing advanced materials. SPS is a rapid sintering method. There are some expected merits in SPS, such as rapid heating and cooling, impact of spark plasma, uniform temperature field and effect of electric current through the compact. And these merits conduce to increase the diffusion rate between the particles and to retard grain growth.

The aim of this study was to fabricate the Fe₃Al alloys with high density and excellent mechanical properties by MA and SPS. The process and mechanical properties were studied.

2 Experimental

Fe-30Al(volume fraction, %) raw powder was obtained by mixing Fe (purity 99.5%; 75 μm) and Al (purity 99.5%; 75 μm) for 2 h by common ball milling. A three-dimensional vibratory milling machine was used in the MA process. Stainless steel ball and vial were used with a charge ratio of 10?1. And 1-2 mL anhydrous alcohol was used as process control agent (PCA). The mixed powders were milled for 5, 8 and 10 min, respectively. During MA process, the milling machine should be halted for 5 min every 5 min working time to reduce the temperature rising.

First, the as-milled Fe-30Al powder was packed in the graphite die with the inner diameter of 20.4 mm and the inner surface covered with a thin carbon paper. And then the powder was densified by a SPS apparatus, Sumitomo Coal Mining Company Model 1050, in the vacuum of ＜10 Pa at the load of 50 MPa, and 1 050 ℃ for 5 min were selected to complete the consolidation behavior. A thermocouple placed in the middle part of the graphite die was used to control the temperature during the SPS treatment. Fig.1 shows the evolution of the SPS synthesis conditions of the SPS process with the as-milled powder. The relationship among temperature, displacement, and vacuum degree depicted in Fig.1 is nearly the same as that of other samples.

Phase analysis was made by XRD (Rigaku D/MAX-RB) and densities were measured by the Archimede’s method. The bending strength (MTS810) was measured by the three point bending method at the constant strain rate of 0.5 mm/min at ambient temperature exposed in air. The fractography feature was analyzed by SEM (Cambridge S-360). And micro- structure was observed by TEM (Hitachi H-800).

Fig.1 Evolution of SPS synthesis conditions with as-milled powder during process

3 Results and discussion

3.1 Powder properties and analysis

Fig.2 shows the SEM images of powder. The  as-milled powder slightly becomes larger than raw powder, and the powder milled for 10 min presents evidently refined and lamellar structure. This is because that the mechanical alloying process includes deformation, rupture and cold welding of powder, which conduces to a homeostasis state with the prolongation of the milling time. Iron and aluminum particles both have excellent ductibility and coalescent, so cold welding has stronger effect than rupture in the initial stage of MA.

Fig.2 SEM images of powders: (a) Raw powder; (b) Milled for 5 min; (c) Milled for 10 min

Fig.3 shows XRD patterns of the powders. The diffraction patterns show the evolution of elemental mixture powders with the prolongation of milling time. X-ray diffraction peaks are broadened with the increase of milling time. It indicates that the grains are refined and defects come into being by large local strains in the powder particles. As the milling time is increased, diffraction peaks of aluminum are noted to decrease in intensity, especially the (111) plane. This is because an iron-based solid solution is formed.

Fig.3 XRD patterns of powder milled for different time

3.2 phase analysis and density of sintered samples

Fig.4 shows the XRD patterns of the samples. It can be seen that Fe₃Al and Fe₃AlC_0.5 are formed during the short-time sintering. The forming of carbide maybe attributes to the graphite C diffusing into the samples and a little CO reacting with Fe₃Al at high temperature. The carbon comes from the mould and carbolic paper. A little residual atmosphere reacted with the carbon, and then the CO was generated. The SPS process curves (Fig.1) and the XRD patterns of the samples are nearly same, which can prove the reproducibility of the SPS process.

The densities of the samples are shown in Fig.5. All of the samples are near theoretical density (TD) materials. As the MA time is increased, the powders have finer grain size and higher surface activation energy. So the milled powders will be more easily sintered than elemental powders during the subsequent SPS process.  But from Fig.5, it can be concluded that the relative density of the sample is decreased with the increase of MA time. Maybe the increased density of defects conduces to this phenomenon.

Fig.4 XRD patterns of sintered sample with different powers

Fig.5 Relative density of sintered samples

3.3 Mechanical properties and microstructure

Fig.6 shows the TEM image of the sample with powder milled for 5 min, and the selected diffraction pattern indicates the structure and phase of the select area. The selected diffraction pattern identification shows that the area is composed of Fe₃AlC_0.5. It presents that Fe₃AlC_0.5 distributes on the grain boundary and strengthens the grain boundary of Fe₃Al.

Fig.6 TEM image of sample with powder milled for 5 min

Figs.7 and 8 show the bending strength and strain of samples with different powders. The bending strength and strain are slightly increased with the milling time before 8 min milling, but the bending strength and the strain are violently decreased with powder milled for 10 min. These trends of bending strength and strain are also related to the trend of the relative density with the increase of MA time. For the powder milled for 10 min, the higher density of defects is obtained because of the longer MA time. The defects make the bending strength and strain of this sample decrease.

Fig.7 Bending strength of samples

Fig. 8 Strain of samples

Fig.9 shows the SEM images of fracture surface of samples of the powders with various milled time. A mixed mode of brittle fracture and gliding fracture is observed, and there are obvious cleavage and dimple characters in this figure.

Fig.9 Fracture surfaces of bending samples observed by SEM: (a) Raw powder; (b) Milled for 5 min; (c) Milled for 10 min

4 Conclusions

1) During the mechanical alloying process, some atoms of Al dissolve in the lattice of Fe without the appearance of iron aluminides.

2) The nearly theoretical densities are gotten by the raw powder under the SPS at pressure of 50 MPa, sintering temperature of 1 050 ℃ for 5 min.

3) Bending strength of 1 500 MPa and strain of 10.7% are obtained with powder milled for 5 min, and the rupture mode is a mixed one of brittle fracture and gliding fracture.

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(Edited by YANG You-ping)

Corresponding author: HE Qing; Tel: +86-10-62334271; Fax: +86-10-62325983; E-mail: thqmoon@gmail.com