MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF RAPIDLY SOLIDIFIED AL-CR-CE-M (M=TRANSITION METAL) ALLOYS CONTAINING HIGH-VOLUME FRACTION OF THE ICOSAHEDRAL PHASE

Coexistent icosahedral (I) and fcc-Al phase alloys exhibiting good ben ding ductility and high tensile strength (sigma(f)) were found to form in rapidly solidified Al95.5-xCr3Co1.5Cex (x less than or equal to 3 at%) and Al(94.5-x)Cr(3)Co(1.5)Ce(1)M(x) (M=Ti, Mn, Fe, Co, Ni, Cu, Zr , Nb, Mo or Hf) alloys. The particle size and interparticle spacing of the I-phase decrease down to about 40 and 10 nm, respectively, with i ncreasing Ce content, accompanying the increase in the volume fraction of the I-phase. This microstructural change is interpreted to result from the increase in the quenching effect with increasing Ce content. The highest sigma(f) value is 1340 MPa for the Al94.5Cr3Co1.5Ce1 alloy and 1330 MPa for the Al93.5Cr3Co1.5Ce1Ti1 alloy. Even after annealing for 900 s at 673 K, the high sigma(f) values of 830 and 680 MPa are o btained for the Zr- and Fe-containing penternary alloys, respectively. The high sigma(f) values for these alloys are due to the high thermal stability of the I-phase against the grain growth and coalescence. Th e achievement of the high sigma(f) over 600 MPa and the high structura l stability allows us to expect that high-strength bulk alloys by util izing the I-phase as a strengthening medium can be prepared by extrusi on at elevated temperatures. Furthermore, it was found from the high r esolution TEM and EDX analyses that the Al base I-phase containing 7.2 at%Cr, 1.6%Cr and 2.6%Ce had a disordered structure on a short-range less than 1 nm. This is presumably because of the introduction of a hi gh density of interface between I-phase and phason-induced approximant crystalline phase. The formation of the disordered structure is thoug ht to be the reason for the good ductility which enables the achieveme nt of high sigma(f).