MICROSTRUCTURE AND NOVEL PROPERTIES OF NANOCRYSTALLINE AND NANOCRYSTALLINE ALLOYS PREPARED IN AL-BASED SYSTEMS BY RAPID SOLIDIFICATION

This paper aims to review our recent results on the microstructure and mechanical properties of nanocrystalline and nanoquasicrystalline all oys in Al-based system obtained by rapid solidification. The nanocryst alline alloys consist of nanoscale fcc-Al particles with a particle si ze of 3 to 5 nm embedded in an amorphous matrix. The homogeneous dispe rsion of the nanoscale fcc-Al particles caused the remarkable increase in tensile fracture strength (sigma(f)) by about 1.4 times as compare d with those for the corresponding amorphous single phase and the high est value reached as high as 1560 MPa. The high sigma(f) has been inte rpreted to result mainly from the extremely high sigma(f) of the Al pa rticles caused by the absence of internal defects, based on the result that no internal defects are observed in the nanoscale Al particles. The nanoquasicrystalline alloys are composed of spherical icosahedral (I) particles with a particle size of 10 to 50 nm surrounded by an Al layer with a thickness of about 10 nm and the volume fraction of the I -phase is as high as about 70 %. The nanoquasicrystalline base alloys exhibit high sigma(f) reaching 1350 MPa combined with good bending duc tility. The internal structure is interpreted to consist of a new none quilibrium phase with a short-range disorder and long-range I-structur e by high-resolution TEM. The formation of the unique nonequilibrium s tructure is presumed to be the origin for the achievement of high sigm a(f) and good ductility. It is also shown that the bulk I-based alloys produced by extrusion of the atomized I-based powders exhibit high si gma(f) of 500 to 850 MPa and large epsilon(f) of 6 to 25 %. These prop erties are excellent enough to demonstrate the validness of the struct ure control to nanocrystalline and nanoquasicrystalline phases.