FORMATION OF NANOGRANULAR AMORPHOUS PHASE IN RAPIDLY SOLIDIFIED AL-TI-M (M=V, FE, CO OR NI) ALLOYS AND THEIR MECHANICAL STRENGTH

A nanogranular mixed structure consisting of amorphous and fcc-Al phas es was formed in malt-spun Al-Ti-M (M=V Fe, Co or Ni) alloys containin g more than 92 at% Al. The composition range of the nanogranular struc ture is the widest for M=Fe and Co,followed by Ni and then V. The high est Al concentration for formation of the nanogranular amorphous phase reaches 94% Al for the Fe-and Co containing alloys. The amorphous and Al phases are homogeneously coexistent. The grain sizes of the amorph ous and Al phases are about 11 and 10 nm, respectively,for Al93Ti4Fe3 and increase in the order of Fe < Co < Ni < V. However, their grain si zes keep small values of 8 and 25 nm, respectively, even for Al93Ti4V3 . These nanogranular alloys exhibit high tensile strength (sigma(f)) e xceeding 1000 MPa and high Vickers hardness (H-v) above 290 and the hi ghest sigma f and H-v are 1320 MPa and 450, respectively,for the Al93T i4Fe3 alloy. The sigma f and H-v change in the order of Fe > Ni > Co > V. This order does not completely agree with that for the grain sizes of the amorphous and Al phases because of the difference in their vol ume fractions. The high mechanical strength of the nanogranular alloys is due to the refinement effect of the amorphous and Al grains and th e disordering-induced strengthening effect of the amorphous phase. The high formation tendency of the amorphous phase is presumably due to t he increase in the stability of the supercooled liquid against crystal lization caused by the difficulty of atomic diffusivity resulting from the strong attractive interaction among the constituent elements. The synthesis of the nanogranular amorphous and Al phases with high mecha nical strength in the Al-rich alloys is important for future developme nt of a new type of high specific strength material. (C) 1998 Acta Met allurgica Inc.