JOURNAL OF MATERIALS SCIENCE 29 (1994) 3724-3732 Effect.of Fe content on the mechanical alloying and mechanical properties of AI-Fe alloys X. P. NIU, L. FROYEN, L. DELAEY Department of Metallurgy and Material Engineering, Kathofieke Universiteit Leuven, de Croylaan 2, B-3001 Leuven, Belgium C. PEYTOUR Direction des Etudes Mat#riaux, Renault, 8-10 Avenue Emile-Zola, F-92109 Boulogne Billancourt, France AI-Fe alloys with Fe contents ranging from 5 to 12 wt% are produced by a double mechanical alloying process (DMA) which consists of a first step of mechanical alloying (MA1) applied to elemental AI and Fe powders, with subsequent heat treatment of MA1 powders to promote the formation of AI-Fe intermetallic phases, and a second mechanical alloying step (MA2) to refine the intermetallic phase, and consolidation of the produced powders by combination of degassing and hot extrusion. The effect of Fe content on the process, as well as on the mechanical properties of the extruded alloys, has been extensively studied. The alloys produced by this process show excellent tensile strength and stiffness at room and elevated temperatures due to the strengthening of AI by intermetallics, as well as to the stabilization of the structure by inert dispersoids. 1. Introduction A1 alloys have been widely used in the engineering industry because of their attractive properties, such as light weight, high ductility, corrosion resistance and toughness. However, the conventional A1 alloys have very poor strength and stiffness at elevated temper- atures compared to steel and nickel alloys. Their applications are often restricted to the low temper- atures. Therefore, there has been a strong demand for the development of new A1 alloys with improved elevated temperatures stability. Rapid solidification processes (RSP) have been used as a technique for the production of high temperature Al alloys since the 1970s. Among the systems of interest, binary A1 Fe and A1-Fe based alloys with additions of transition elements, or some rare earth elements, via RSP have been extensively studied in the past [1 7]. In the rapid solidification process, ex- tended solid solubility of Fe in A1 up to 8.4 wt % can be obtained by suppressing the equilibrium cooling reaction. This results in a final product containing a high volume fraction of precipitate intermetallics with a size of about 1 gm. Possible application temper- atures of A1-Fe based alloys have been reported up to 315 ~ However, the properties of these RSP A1-Fe alloys decrease drastically at higher temperatures due to rapid coarsening of intermetallic phases. Since mechanical alloying (MA) was invented by J. S. Benjamin in the 1960s, it soon became an altern- ative technique for fabrication of high temperature A1 alloys. In an MA process, elemental powders or pre- alloyed powders are subjected to mechanical grinding (ball milling) in order to produce composite powders 3724 with controlled microstructures [8]. In the case of A1 alloys, inert dispersoids, such as A120 3 and A14C3 are formed during MA. These inert dispersoids, mainly situated at the grain boundaries, can stabilize micro- structures at elevated temperatures [9]. A good com- bination of strength, stiffness and structural stability at elevated temperatures has been obtained in MA A1-Ti, MA A1 Fe and A1-Mn alloys [9-11]. In addi- tion, mechanical alloying also offers an elegant possib- ility to produce nanocrystalline and amorphous A1-Fe materials [-12, 13]. An extension of the solubil- ity limit of Fe in A1, which is far beyond those levels obtained by the rapid solidification technique, is also achieved by MA [14]. Intermetallic phases, such as A13Ti, A16Mn and AIIaFe4 are hardly formed by direct MA. These inter- metallics are usually formed in subsequent thermal processes. As shown in a previous work [15], the distribution of these intermetallics in the final product is not uniform after a single MA step. The size of intermetallic phases ranges from 1 to 10 gm. As a result, the alloys show relatively low mechanical prop- erties. In order to modify the distribution of inter- metallics, a new technique, namely double mechanical alloying (DMA), has been developed [16_] and has been used to fabricate high temperature A1 Fe alloys [15, 17]. In the DMA technique, there are three main steps: 1. First, a mechanical alloying stage applied to elemental powders (called MA1 hereafter), followed by heat treatment of MA1 powders; 2. second, a mechanical alloying stage of heat trea- ted powders (called MA2 hereafter); and 0022-2461 9 1994 Chapman & Hall