Aluminium alloys are most widely used light alloys for structural components. The highest strength of wrought aluminum alloy is around 600 MPa reported for Al-Cu-Li based alloys. However, Al-TM-RE amorphous alloys and its composite with nanocrystals were reported to have tensile strengths exceeding 1 GPa [1]. This strength is really remarkable compared to the conventional wrought alloys, but it can be achieved only in melt-spun ribbons. For structural applications, it is essential to achieve such a high strength in bulk materials, and this is the motivation of this research to explore the process to produce nanocomposite microstructure in aluminum alloys by using mechanical alloying and subsequent consolidation processes. In our recent work, we have demonstrated that it is possible to obtain high strength nanocrystalline Al-Fe alloys by mechanical alloying (MA) and spark plasma sintering (SPS) as described in the subsequent section [2]. However, since the bulk nanocomposites were processed by SPS, the bulk size was limited to a small dimension of o10 mm ×10 mm. Because of this size limitation, the mechanical properties were tested only in the compression mode. Therefore, we propose to carry on further research on the development of bulk nanocrystalline Al-TM alloys with larger dimensions by using MA and hot extrusion processes. There are no restrictions in scaling up the mechanical alloying process as it has already been used to produce MA7000 alloys industrially. Extrusion does not give any restrictions on forming bars of large dimensions. After scaling up the preliminarily investigated nanocrystalline Al-Fe alloys, we will investigate standard mechanical properties, like tensile yield strength, ultimate tensile strength and creep properties. The goal of this work is to test the feasibility of processing Al-TM nanocomposites with tensile strength of 1000 MPa at room temperature and 500 MPa at 350 ºC with satisfactory ductility.
We have selected Al-Fe binary alloy as it was known that Fe gives effective solid solution strength if solved in an Al by a non-equilibrium process. Another reason for the selection of Fe was its sluggish diffusivity in Al, which would lead to high temperature tolerance. Pure Al and Fe powders were used as starting materials for the mechanical alloying (MA) of Al-5at.%Fe powder. Ball milling was carried out at room temperature in an process control agent using a planetary ball mill. The powders were consolidated using a SPS machine.


Fig. 1 Compressive stress strain curves of mechanically milled and spark plasma sintered Al-Fe alloy measured at various temperatures.

Fig. 2 Elevated temperature strength of a Al-5at.%Fe alloy comparing with other commercial and ultrafine-grained alloys.

We have succeeded to develop an ultrahigh strength “bulk” nanocrystalline Al-Fe alloy with a compressive strength of 1.2 GPa and plastic strain up to 15 % at ambient temperature, and 500 MPa at 350°C as shown in Figure 1. Note that tensile properties have not been investigated yet because of the size restriction of the sample. Figure 2 shows the temperature dependence of the compression strength of the sample in comparison with the tensile strength reported for various other aluminum alloys. Its room temperature strength is more than two times higher than the highest strength wrought aluminum alloy. The notable feature is the high temprature strenth. Although the data is obtained in compression mode, this shows promising feature of the nanocomposite Al-Fe alloys processed by MA and SPS.


Figure 3 (Right): Typical Micro-/Nano-structure of a sintered sample showing 1000 MPa in yield strength (a) backscattered electron SEM image, (b) bright field TEM image, (c) micro-beam diffraction pattern taken from a coarse grained phase with black contrast, (d) selected area diffraction pattern taken from a nanocrystalline region, (e) dark field TEM image using {111} reflection of a-Al and (f) elemental map of Fe. Note that (e) and (f) were taken from the region surrounded by solid line in (a).

Figure 3 shows typical microstructure of the sintered sample with yield strength above 1000 MPa. Backscattered electron SEM image (Figure 3 (a)) confirms the presence of second phase with white contrast, which was found to be Fe13Fe4 phase grains. Dark contrasted regions were characterized to be a-Al and the gray contrasted regions found to be composed of nanocrystals of a-Fe and Al6Fe phases as shown in Figure 3 (c) and (d). The nanocrystalline grains have grain size of about 80 nm. Energy filtered image of Fe corresponding to the nano-grained region (Fig. 3 (d)) shows that a large amount of Al6Fe phase is dispersed in the matrix phase as indicated by the SAED pattern.


Figure 4 Variation in strength at room temperature as a function of inverse grain size.

Figure 4 summarizes the yield strength as a function of inverse grain size for various aluminum alloys. The strength of the present Al-Fe based alloy showed remarkable grain refinement effect down to 100 nm, although the Hall-Petch relationship of the pure Al break down only below 500 nm. Ultra-high strength above 1000 MPa can be achieved from the sample with grain size below 80 nm when more than 10 % of Al6Fe phase is contained in its volume fraction.

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References
[1] Y.H. Kim, A. Inoue and T. Masumoto, Mater. Trans. JIM 31 (1990) 747-749.
[2] T. T. Sasaki, T. Mukai and K. Hono, Scripta Mater, 57 (2007) 189-192.

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Related Publications

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Microstructure and mechanical properties of bulk nanocrystalline Al-Fe alloy processed by mechanical alloying and spark plasma sintering
T. T. Sasaki, T. Ohkubo, and K. Hono, Acta Mater (2009), submitted.

Bulk nanocrystalline Al₈₅Ni₁₀La₅ alloy fabricated by spark plasma sintering of atomized powders
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High-strength bulk nanocrystalline Al-Fe alloy processed by mechanical alloying and spark plasma sintering
T. T. Sasaki, T. Mukai and K. Hono, Scripta Mater. 57, 189 - 192 (2007).