An investigation was made on supercooling phenomena in Al-Cr, Al-Mn and Al-Zr binary alloys, which have a strong tendency to form metastable solid solutions by rapid cooling of these molten alloys, by means of an improved technique of thermal analysis and microscopic observation. The results obtained were as follows:
(1) The square of the degree of supercooling was proportional to cooling rate in the solidification of the alloys of which the solute concentration was less than the maximum solid solubility (C_e) at the equilibrium state. In case of solute concentration more than C_e, this relation was satisfied under the conditions in which the primary crystals were metastable solid solutions.
(2) The minimum cooling rate to form the metastable solid solutions containing solute atoms above C_e increased a in the following sequence: 120°C/sec for Al-Mn, 180°C/sec for Al-Cr and 190°C/sec for Al-Zr alloys, and when the cooling rate exceeded each minimum value the concentration of supersaturatedly dissolved solute and supercooling rose with cooling rate.
(3) A phase relation which gives the primary crystallization ranges of the metastable solid solutions (C>C_e), solid solutions (C<C_e), intermetallic compounds and pure solvent can be expressed by the three factors: composition (C), cooling rate (v) and supercooling (ΔT), that is, by C-v-ΔT non-equilibrium phase diagram.
(4) A relation between supercooling (ΔT) and dendrite spacing (λ in μ) can be shown by
logλ=2nlogΔT+A, where n=0.32 and A is a constant.