NanoQuasicrystalline Al-Based Alloys

F. AUDEBERT; F. PRIMA; M. GALANO; M. TOMUT; P. WARREN; I.C. STONE; B. CANTOR

Cambridge

Jornada; Nanocomposites Briefing Workshop; 2002

Over the last decade several new nanostructured Al-based alloys have been developed. These alloys can be classified in terms of the different phases (amorphous, crystalline and nanocrystalline) present in the structure: Type-1: Fine dispersion of precipitates in an Al matrix, Type-2: Mixture of amorphous and crystalline phases, Type-3: Nanoquasicrystalline precipitates embedded in an Al matrix. In order to give fundamental understanding of those classes of structures three different alloys were chosen: Al94Mn3Cu2Cr1 (type-1), Al92Fe3V3Ti2 (type-2) and Al93Fe3Cr2Ti2 (type-3). Samples of each alloys were prepared by rapid solidification using the Melt Spinning Technique. Structural characterisation was carried out by means of X-ray diffraction and transmission electron microscopy. The mechanical behaviour was evaluated by Vickers microhardness, bending tests and fractographic analysis. Also the stability and the crystallisation process were analysed by differential scanning calorimetry. The Class-1 alloy, in the melt spun state, showed a structure of precipitates localised in grain boundary and inside the micrometre sized Al grains. The structure of the Class-2 alloy presented a fine mixture of nanogranular amorphous and FCC-Al phases. Finally, the Class-3 alloy showed a homogenous distribution of nano-grains of icosahedral quasicrystals and intermetallic compound embedded in an Al matrix. The icosahedral phase had a spherical shape with an average size of 50 nm. The Class-1 alloy showed the best bending ductility but the lowest microhardness value. On the other hand, the Class-2 and Class-3 alloys showed the much higher microhardness values and good ductility. In agreement with the good bending ductility, the Class-1 alloy showed a fracture surface with significant necking. The other alloys also showed ductile fracture; however, slip bands and a localised fracture with vein pattern were observed in the fracture surface of the Class-2 alloy and the Class-3 alloy showed a high capacity to absorbed energy prior to tensile fracture. The crystallisation process of the three alloys generally showed two transformation steps, the first step in all three alloys was very sluggish and corresponds to relaxation and grain coarsening processes. The second step have different characteristics for the three alloys, with the type-3 alloy being most stable. The suitability of these alloys for production of bulk nanostructured material for structural application is discussed.