Phase Stability in Mechanically Alloyed Mg-Ni System Studied by Experiments and Thermodynamic Calculations

doi:10.1007/s40195-015-0287-8

Abstract

Abstract:

In this study, microstructural evolution of Mg-Ni alloy during mechanical alloying (MA) was investigated. Also, a thermodynamic approach was utilized to predict the most stable phases formed in Mg-Ni alloy after MA. The phase composition and microstructural properties of Mg-Ni alloy were assessed by X-ray diffractometry, high-resolution field emission scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that ball milling of magnesium and nickel powder mixture for 70 h yields nanostructural Mg₂Ni compound with an average grain size of ~20 nm. Thermodynamic calculations revealed that in the composition ranges of 0.0 < X _Mg < 0.03 (at.%) and 0.97 < X _Mg < 1, there is no driving force for amorphous phase formation. In the composition range of 0.07 < X _Mg < 0.93, the change of Gibbs free energy for amorphous phase formation was more negative than solid solution. While for X _Mg=0.66 (nominal composition of Mg₂Ni intermetallic phase), the change of Gibbs free energy for intermetallic phase was found to be more negative than both amorphous and solid solution phases indicating that Mg₂Ni intermetallic compound is the most stable phase, in agreement with the experimental observations.

Key words: Mechanical alloying, Nanocrystalline, Hydrogen storage material, Mg2Ni, Thermodynamics

Mohammad Hossein Enayati, Fathallah Karimzadeh, Soheil Sabooni, Majid Jafari. Phase Stability in Mechanically Alloyed Mg-Ni System Studied by Experiments and Thermodynamic Calculations[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(8): 1002-1007.

Figures/Tables 7

Fig. 1 SEM images of as-received magnesium a, nickel b powder particles

Table 1 Ball milling parameters for producing Mg2Ni compound

Parameter	Value
Ball-to-powder mass ratio	10:1
Time (h)	70
Rotation speed (rpm)	500
Atmosphere	Argon
Stearic acid (wt%)	1
Packing ratio (vol%)	30

Table 2 Parameters for thermodynamic calculation at 298 K

Element	V (cm³/mol)	N _ws (10²³ e/cm³)	Φ ^* (V)	P (kJ/(V² cm))	Q (kJ/V)	K (GPa)	G (GPa)
Mg	14	1.6	3.45	12.3	115.6	45	17
Ni	6.6	5.36	5.2	12.3	115.6	180	76

Fig. 2 XRD patterns of magnesium and nickel powder mixture; 0 h-milled a, 20 h-milled b, 50 h-milled c, 70 h-milled d

Fig. 3 SEM image of powder particles a; high-resolution TEM images of Mg2Ni b, c

Fig. 4 Components of enthalpy change related to the formation of solid solution (ΔH elastic and ΔH chemical), amorphous phase (ΔH amorphous) and Mg2Ni intermetallic phase

Fig. 5 Gibbs free energy changes for formation of solid solution, amorphous intermetallics phases in Mg-Ni system

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