Glass Forming Ability And Crystallization Of Al-X-Ce Amorphous Alloys

In this paper, the glass-forming ability (GFA) of the Al-Ni-Mg based amorphous alloys, the crystallization behaviors of Al-Mg-Ce and Al-TM-RE amorphous alloys and the structure relaxation of Al₈₃Ni₁₀Ce₇ amorphous alloy have been systematically investigated by using the X-ray diffractometer (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It has been found that the proper addition of Mg can promote the formation of nanoglassy alloy in the Al-Ni-Ce system in a relative wide composition range (Al₈₂Ni_10-xMg_xCe₈, 4≤x≤5; Al₈₃Ni_10-xMg_xCe₇, 2≤x≤4 and Al₈₄Ni_10-xMg_xCe₆, 2≤x≤4). The origin of the formation of nanoglassy alloy can be interpreted in terms of the small negative heat of mixing between Mg and other constituents.The investigation of GFA in the Al₈₄Ni_10-xMg_xCe₆ amorphous alloys indicated that the GFA should be in direct proportion to |ΔH×S_σ/k_B | and in inverse proportion to (T_l-T_m). Thus, based on this assumption, a new criterion is proposed to beΩ= |ΔH×S_σ/k_B|/(T_l-T_m) to evaluate the GFA. Here,ΔH, S_σ, k_B, T_m and T_l represent the mixing enthalpy, mismatch entropy, Boltsmann constant, the initial melting point and the final melting point, respectively.The crystallizations of Al₈₂Mg₁₀Ce₈ and Al₈₄Mg₁₀Ce₆ amorphous alloys indicate that a non-equilibrium phase Al₉₂Ce₈ precipitates in the first crystallization stage, which is quite different from the crystallization of common Al-based amorphous alloy, characteristic of primary crystallization of fcc-Al or eutectic crystallization of fcc-Al and compounds. With the Ni additions to the Al₈₂Mg₁₀Ce₈ and Al₈₄Mg₁₀Ce₆ amorphous alloys, primary crystallization of fcc-Al occurs in the Al₈₄Mg₇Ni₃Ce₆ and Al₈₄Mg₈Ni₂Ce₆ amorphous alloys.The crystallization of Al₈₂Mg₁₀Ce₈ amorphous alloy consists of two stages. A metastable phase Al₉₂Ce₈ precipitates in the first crystallization stage. With increasing temperature, the metastable phase Al₉₂Ce₈ is beginning to decompose into fcc-Al and Al₄Ce phases in the second crystallization stage; and at the same time, Al₁₂Mg₁₇ phase pricipitates from the amorphous matrix. It should be noted that an unknown metastable phase emerges in the second crystallization stage. The final crystalline precipitations consist of fcc-Al, Al₁₂Mg₁₇ and Al₄Ce phases. The crystallization process of Al₈₄Mg₁₀Ce₆ amorphous alloy is simmlar to that of Al₈₂Mg₁₀Ce₈ amorphous alloy, which can be summarized as follow: amorphous→metastable phase Al₉₂Ce₈ + amorphous→(fcc-Al+Al₄Ce) + Al₁₂Mg₁₇. The only difference between the crystallizations of Al₈₂Mg₁₀Ce₈ and Al₈₄Mg₁₀Ce₆ amorphous alloys is that no unknown metastable phase appears in the second crystallization stage of Al₈₄Mg₁₀Ce₆ amorphous alloy.A small addition of Si can transform a eutectic crystallization (Al₈₅Ni₁₀Ce₅) to a primary crystallization (Al₈₃Ni₁₀Si₂Ce₅); while, a small addition of Co can transform a primary crystallization (Al₈₇Ni₇Nd₆) to a eutectic crystallization (Al₈₇Ni₅Co₂Nd₆). Based on the Kissinger method, the activation energy for crystallization of the Al₈₃Ni₁₀Ce₅, Al₈₅Ni₁₀Ce₅, Al(87)Ni₇Nd₆ and Al₈₇Ni₅Co₂Nd₆ amorphous alloys are determined to be 191, 290, 166 and 221 kJ/mol, respectively. It can be found that the activation energy for crystallization decreases with Si addition; while, the activation energy for crystallization increases with Co addition. Namely, the thermal stablity decreases with Si addition; while, the thermal stablity increases with Co addition.According to the DSC scan, it is interesting to find that the second exothermic peak gradually decayed with increasing the pre-annealing temperature from 473K to 533K; while, with further increasing the pre-annealing temperature up to 553K, the second peak gradually enhanced. This behavior suggests a change in the amorphous structure upon relaxation. Continuous heating crystallization and isothermal crystallization exhibit different crystallization mechanisms of the present alloy. Fcc-Al and a metastable phase precipitate simultaneously in the first crystallization stage during continuous heating; however, only a metastable phase precipitates during isothermal annealing below glass transition temperature (Tg).