Formation, thermal stability and electrical resistivity of quasicrystalline phase in rapidly quenched Al-Cr alloys

Abstract

The icosahedral quasicrystal has been found to appear in a wide composition range from about 5 to 16 at % Cr in rapidly quenched Al-Cr alloys, but the formation of the quasicrystal line single phase was limited only in the vicinity of about 15.5at% Cr. Analytical solute concentrations in the quasicrystalline phase are not always constant and increase continuously from 9.0 to 15.4 at% Cr with increasing nominal solute concentration from 6 to 15.4%. The quasicrystal can be approximately formulated to be Al₁₁ Cr₂ with a maximum deviation of about 6% Cr from the stoichiometric ratio to lower concentration side. Vickers hardness and electrical resistivity increase gradually with increasing chromium content and rapidly at about 14.5% Cr, and their values of Al_84.6Cr_15.4 quasicrystal are 710 DPN, 2.38μΩm at 4.2 K, and 2.72μΩm at 293 K. On the other hand, the onset transformation temperature of quasicrystal to crystalline phase,T _t, and the heat of transformation, ΔH _t show maximum values of 644 K and 1805 J mol⁻¹ at 14.5% Cr and decrease to 625 K and 550 J mol⁻¹ at 15.4% Cr. Al_84.6Cr_15.4 quasicrystal trans forms at two stages to a stable orthorhombic Al₁₁Cr₂ compound through a metastable intermediate phase with unidentified structure, while the quasicrystal + Al structure in Al-Cr alloys containing less than 15% Cr changes directly to stable phases of compounds and aluminium. The distinct difference in transformation behaviour of the quasicrystal is thought to be the reason for the abrupt changes inT _t and ΔH _t at a composition between 14.5 and 15.4% Cr.