Synthesis of amorphous and metastable Ti₄₀Al₆₀ alloys by mechanical alloying of elemental powders

Abstract

Ti₄₀Al₆₀ amorphous and metastable alloys have been prepared by mechanical alloying (MA), under controlled milling conditions in a planetary mill. Three different quantities of kinetic energy at the collision instant have been achieved by using balls of different size, φ_b = 5, 8 and 12 mm, keeping constant all other device parameters. Assuming the collision between the balls and the vial walls to be inelastic, during the early stage of alloying, the amount of energy transferred to the trapped powder could be estimated. The experimental results show that the milling with balls of diameter φ_b = 5 or 8 mm leads to a solid-state amorphization of the Ti₄₀Al₆₀ mixture, through the attainment of a supersaturated solid solution of aluminium into α-titanium. Otherwise, the milling causes the nucleation of the A1-fcc disordered form of the TiAl intermetallic compound. The end products of MA-induced solid-state reaction (SSR) have been ascribed to the different temperature reached by the powder during each collision and to the reaction time scale for the formation of the amorphous phase, δt _a, and for the nucleation of the non-equilibrium intermetallic compound, δt _d. Differential scanning calorimetry has indicated that the crystallization of amorphous samples follows a two-step reaction. At a temperature T _c≈400 °C, the amorphous phase crystallizes into the A1 -fcc. TiAl phase having a measured heat of crystallization of 6.2 kJ(g at)⁻¹. Upon further heating, the system undergoes A1 → L1_o reordering transition with an enthalpy release of about 3.2 kJ (g at)⁻¹.