Crystallization kinetics of amorphous magnesium-rich magnesium-copper and magnesium-nickel alloys

Abstract

Amorphous magnesium-rich alloys Mg y X 1Ϫy (X ϭ Ni or Cu and 0.82 Ͻ y Ͻ 0.89) have been produced by melt spinning. The crystallization kinetics of these alloys have been determined by in situ X-ray diffraction (XRD) and isothermal and isochronal differential scanning calorimetry (DSC) combined with ex situ XRD. Microstructure analysis has been performed by means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Crystallization of the Mg-Cu alloys at high temperature takes place in two steps: primary crystallization of Mg, followed by simultaneous crystallization of the remaining amorphous phase to Mg and Mg 2 Cu. Crystallization of the Mg-Cu alloys at low temperatures takes place in one step: eutectic crystallization of Mg and Mg 2 Cu. Crystallization of the Mg-Ni alloys for a Mg content, y Ͼ 0.85, takes place in two steps: primary crystallization of Mg and of a metastable phase (Mg ϳ5.5 Ni, with Mg content y ϭ 0.85), followed by the decomposition of Mg ϳ5.5 Ni. Crystallization of the Mg-Ni alloys for a Mg content y Ͻ 0.85 predominantly takes place in one step: eutectic crystallization of Mg and Mg 2 Ni. Within the experimental window applied (i.e., 356 K Ͻ T Ͻ 520 K and 0.82 Ͻ y Ͻ 0.89), composition dependence of the crystallization sequence in the Mg-Cu alloys and temperature dependence of the crystallization sequence in the MgNi alloys has not been observed.

Calculation of metastable free-energy diagrams and glass formation in the Mg–Cu–Y alloy and its boundary binaries using the Miedema model

“…The upper limit of the amorphization range increases to 34 at.% Cu if the amorphous phase is assumed to have chemical short-range order, but comparison with the experimental amorphization range suggests that chemical short-range order is not well developed in the amorphous alloy. Crystallization of the amorphous alloys will produce a mixture of the stable Mg and CuMg 2 phases [36,37].…”

A thermodynamic study of the hydrogenation of the pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound

“…From them, the formation enthalpy at 0 K of Mg 6 Pd 0.5 Ni 0.5 has been calculated as À11.5 kJ/mol atoms, a result which confirms the tendency to destabilization introduced by the substitution of Pd by Ni (D f H 0 0 (Mg 6 Pd) ¼ À21.8 kJ/mol atoms). It should be remembered that Mg 6 Ni is isomorphous to Mg 6 Pd but it is a metastable phase only formed either by low-temperature annealing of amorphous melt-spun ribbons [20] or under very high hydrogen pressures [17]. The enthalpy change of reaction (1) can be calculated from the enthalpy of formation of the several phases entering in reactions (2)-(5).…”

“…Previous investigations have shown that Mg 6 Ni compound, isomorphous to Mg 6 Pd, can be prepared in a metastable state by low-temperature annealing of melt-spun amorphous ribbons [20]. Mg 6 Ni transforms into a two-phase alloy Mg þ Mg 2 Ni above 473 K. Hydrogenation properties of metastable Mg 6 Ni and related compounds have been reported [21][22][23][24].…”

Phase Diagrams and Glass Formation in Metallic Systems

“…Glass formation in the Cu-Mg system has been extensively studied in the past [36,37] and a glass forming range (GFR) appears around the eutectic between hcp-Mg and CuMg 2 phases. The Cu-Mg equilibrium phase diagram has been assessed by several authors and the most recent thermodynamic description, carried out by Liang et al [38] as part of the modelling of the ternary Cu-Mg-Zn system, has been taken as a starting point.…”