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12
DOI: 10.17277/amt.2020.03.pp.003-007
The p-T-x-State Diagram of the Mg-Ni System
1* 12 2 Yu.V. Levinsky1 , M.M. Alymov1' 2, L.L. Rokhlin2
1 Merzhanov Institute of Structural Microkinetics and Materials Science of RAS, 8, Academician Osipyan Str., Chernogolovka, 432142, Russia; 2 Institute of Russian Academy of Sciences A.A. Baykov Metallurgy and Materials Science RAS, 49, Leninsky Prospect, Moscow, 119334, Russia
* Corresponding author: Tel.: +7 496 524 63 84. E-mail: levinsky35@mail.ru
Abstract
The analysis of equilibrium in the Mg-Ni system was carried out. Alloys on its basis are promising for use as sorbents and hydrogen storage (START). The conditions for the production and operation of such alloys imply strict control of the pressure of hydrogen. In this regard, phase equilibria in the Mg-Ni system must be considered not only depending on the composition and temperature, but also on the pressure of hydrogen.
The most complete graphical representation of the equilibrium in the Mg-Ni system is given by a three-dimensional state diagram: pressure-temperature-composition (p-T-x), the projection of the three-phase equilibrium lines of this diagram on the pressure-temperature plane (p-T-state diagram), isobaric and isothermal sections of the diagram, diagram in pMg-T coordinates.
Based on the analysis of experimental data on equilibrium in this system, the article presents the most important options of the listed types of diagrams.
The presented diagram variants can be useful in optimizing the technology and operation of sorbents and START alloys of the Mg-Ni system.
Keywords
Phase equilibria; structural diagrams; magnesium-nickel alloys; hydrogen storage.
© Yu.V. Levinsky, M.M. Alymov, L.L. Rokhlin, 2020
Introduction
Alloys of the Mg-Ni system are of interest as hydrogen sorbents and alloys of hydrogen storage rings (START) [1-17]. They have a relatively high hydrogen capacity, are convenient in terms of temperature and hydrogen pressure, and sorption and desorption parameters. In addition, some of these alloys have prospects for use as structural materials [11, 13]. In this regard, the study of phase equilibria in the Mg-Ni system with the participation of the gas phase is of interest from the point of view of improving the production technology of magnesium-thermal nickel alloys and magnesium nickelides [18].
Construction a p-T-x-state diagram
The mutual solubility in the solid state in the nickel-magnesium system is very small. The solubility of nickel in magnesium even at 500 °C is less than 0.04 %, the solubility of magnesium in nickel
at 1100 °C is less than 0.2 % (at.). Two intermetallic compounds are formed in the system: Mg2Ni (hP18) and MgNi2 (hP24, MgNi2).
The projection of the lines of maximum solubility of the magnesium-nickel system located in the three-coordinate p-T-x space on the temperature-composition plane is shown in Fig. 1 [19, 20].
The equilibrium gas over Ni-Mg alloys consists mainly of magnesium vapor. The partial vapor pressure of magnesium can be taken as the total vapor pressure even over biphasic MgNi2 + Ni alloys.
The equilibrium pressure of magnesium vapor for a wide range of concentration and temperature is presented in Table 1 [21-23]. The data of [21, 23] are in good agreement with each other, and they will be taken into account in future calculations.
In the p-T diagram of the Ni-Mg system (Fig. 2), which is the projection of the lines of three-phase equilibria located in three-dimensional p-T-x space
Ni, mass. " o
0 20 40 60 SO 100
Ni. at. °o
Fig. 1. The projection of the lines of maximum solubility in the nickel - magnesium system on the temperature - composition plane [19, 20]
Table 1
The temperature dependence constants (logPMg (Pa) = A/T + B) of the partial pressure of Mg vapor over Ni-Mg alloys
Equilibrium phases Content Ni, at. % A B Temperature range Source
Mg2Ni+MgNi2 33 - 66 8840 11.545 400 - 600 [21]
MgNi2 + Ni >66.7 10700 11.254 560 - 800 [21]
Melt 4.9 6580 9.673 583 - 691 [22]
Melt 11.3 6610 9.700 597 - 700 [22]
Melt +Mg2Ni 19.6 63.50 9.370 597 - 706 [22]
Mg2Ni + MgNi2 33.6 7810 10.664 683 - 734 [22]
MgNi2 + Ni > 66 11120 111.945 870 - 932 [22]
Melt 4.2 6730 9.81 650 - 730 [23]
Melt 17.4 6752 9.80 650 - 730 [23]
Mg2Ni + MgNi2 47.3 7850 10.65 650 - 730 [23]
Mg2Ni + Ni 70.5 10720 11.86 850 - 930 [23]
onto the pressure - temperature plane, curves 1 - 6, 6 -14, and 6 - 12 represent evaporation, boiling, and melting of magnesium.
Lines 5 - 14, 6 - 12, 7 - 13, 8 - 15, and 10 - 16 of the equilibrium of only the condensed phases are almost vertical, since in the range of Fig. 2 the pressure does not significantly affect the equilibrium temperature of the condensed phases [24].
The four-phase equilibrium Mg ^ Mg2Ni ^ ^ L ^ G takes place at point 5 (T = 506 °C, p = 16 Pa) [22]. Four curves of three-phase equilibria come out from this point: 5-6, 5-11, 5-2 and 5-7. Curves 5-6 and 5-2 of the equilibria Mg ^ L ^ G and Mg ^ Mg2Ni ^ G almost coincide with curve 1-6 of the evaporation of pure magnesium, whereas curve 5-7 of the equilibrium Mg2Ni ^ L ^ G was plotted using experimental data work [22].
Table 2
The coordinates of the four-phase equilibrium points of the nickel-magnesium system
Point numberon p-T diagram Temperature, °C Pressure, Pa Phases involved in equilibrium
5 506 16 Mg, Mg2Ni, L, G
7 760 1,3 ■ 103 Mg2Ni, MgNi2, L, G
8 1097 6,5 ■ 103 MgNi2, Ni, L, G
At point 7 (T = 760 °C, p = 1.3 • 103 Pa), the four-phase equilibrium Mg2Ni ^ MgNi2 ^ L ^ G at 600730 °C is plotted according to the data [22, 23], and the curve 7-10-8 of the equilibrium MgNi2 ^ L ^ G is plotted approximately.
Point 8 (T = 1097 °C, p = 6.5 • 103 Pa) denotes the four-phase equilibrium MgNi2 ^ Ni ^ L ^ G. Curve 4-8 of the three-phase equilibrium Mg2Ni ^ MgNi2 ^ ^ G at temperatures 850-930 °C is plotted according to the data [22, 23]. Equilibrium curve 8-9 Ni ^ L ^ G ends at low pressure at the triple point of pure nickel at a temperature of 1455 °C.
The coordinates of the key points of the four-phase equilibrium of the p-T state diagram of the nickelmagnesium system are summarized in Table 2.
The p-T phase diagram of the nickel-magnesium system shown in Fig. 2 is the basis for constructing isobaric and isothermal sections of this system at various pressure and temperature values.
Ni, mass. %
1.4 1.2
J_L
103/T, K-1
1.0 _L
0.8
c3
Cm ^ 2
eg 2 o
400
600
800 T, oC
1000
1200
0 20
40
60
80
100
1200
1000
ü
o
800
600
400
JfG Ni+G
MgNi2+G l l « n n n
' L+G I / — M gNi2+Ni
V/ 7 M g2Ni+MgNi: n ii ii
VL+ M&Ni M g2Ni ii ii ii l MgNi2 Ni ^
Mg+Mg Ni
20
40 60
Ni, at. %
100
Fig. 2. p-T state diagram of the nickel-magnesium system
Fig. 3. Isobaric section of the state diagram of the Nickel-Magnesium system at a pressure of p = 103 Pa
Examples of such cross sections are shown in Figs. 3 and 4. The isobaric section of the state diagram of the nickel-magnesium system at a pressure p = 10J Pa is shown in Fig. 3. On the p-T diagram, the isobar p = 10 intersects four three-phase equilibrium curves: 5-11, 5-7, 3-7, and 4-8. In accordance with this, in the isobaric section of Fig. 3 there are four horizontal horizons of invariant equilibria. At 516 °C (intersection of the isobar with curve 5-11), the eutectic reaction Mg ^ Mg2Ni ^ L takes place. This part of the diagram is identical with the same part of the diagram in Fig. 1. At 720 °C (the intersection of the isobar with the curve 5-7 of the p-T diagram), the liquid decomposes with the release of Mg2Ni and magnesium vapor.
At a temperature of 740 °C (the intersection of the isobar with the curve 3-7 of the p-T diagram) Mg2Ni decomposes with the release of MgNi2 and magnesium vapor. At 970 °C (the intersection of the isobar with the curve 4-8 of the p-T diagram) MgNi2 decomposes into solid Ni and gas.
0
4
3
1
0
0 20
Ni, mass. % 40 60 80
100
CM
<
M
o
¡V L 1 Nv ! L+M gNi2 -n- 11 11 '' L MgN i2
! \
! L+G 1 1 1 1 G+MgN i2 I i II 11 MgN i2+Ni
1 1 II II >
1 UG I
I I I I I I G+Ni
20
40 60
Ni, at. %
80
100
Fig. 4. Isothermal section of the state diagram of the nickel-magnesium system at a temperature of 800 °C
The isothermal section of the state diagram of the nickel-magnesium system at a temperature of 800 °C is shown in Fig. 4. The isotherm T = 800 °C intersects in the p-T diagram two curves of three-phase equilibria 4-8 and 7-10. In accordance with this, on the isothermal section there are two horizontal horizons of non-invariant equilibria. At a pressure of 70 Pa (the intersection of the isotherm with curve 4-8 in the
1.4 1.2
103/T, K-1 1.0
0.8
a
t>0 o
/10
/ s / L s * * * / / 9 /
/jf7 6 U/ ff-f-MgN . f t
/ 4 / / 5// / / MgNi2 , / ' / / / Ni
Mgf; /! ' > ' h',3 / / / / / 4/
400
600
1000
800
T, oC
Fig. 5. The pMg-J-state diagram of the Ni-Mg system at ptotal > 104 Pa
1200
p-T diagram), the MgNi2 intermetallide decomposes into a gas consisting of magnesium vapor and solid nickel. At a pressure of 3 • 10 Pa (intersection of the isotherm with curve 7-10 in the p-T diagram), a liquid containing 200 at.% Ni decomposes into gas and the intermetallic compound MgNi2. The Mg2Ni intermetallic compound is absent in the diagram in Fig. 4, since it decomposes incongruently into liquid and MgNi2 at 760 °C.
The state diagram of Ni-Mg of the first kind [25] is presented in Fig. 5. This diagram represents singlephase fields below the curve 1-6-10, indicating the equilibrium vapor pressure over solid and liquid magnesium. The diagram of Fig. 5 is easily transformed from the p-T-diagram of Fig. 2 by eliminating from the last vertical lines the invariant equilibria with the participation of only condensed phases. The single-phase fields in Fig. 5 are separated by lines of two-phase equilibria. The solid sections of lines 5-7, 3-7, and 4-8 are plotted taking into account the experimental data [4, 6], the dashed sections are obtained by extrapolating these data. The phase compositions inside single-phase fields can be calculated using the data in Table 1.
Conclusion
Different variants of a graphic representation of the equilibrium in the nickel-magnesium system are proposed for variable values of pressure, temperature, and composition. The structure of the three-dimensional spatial image of the equilibrium is represented by the projection of the lines of maximum solubility on the temperature-composition plane, the projections of the lines of non-invariant equilibria on the pressure-temperature plane (p-T-state diagram), isobaric and isothermal section of the p-T-state diagrams, and the first-order state diagram in the pMg-T coordinates.
The presented versions of the equilibrium image can be useful in optimizing the technology for production and operation of nickel-magnesium alloys.
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