Influence of the Chemical Composition and Heat Treatment Modes on ...

ISSN 1067-8212, Russian Journal of Non-Ferrous Metals, 2018, Vol. 59, No. 2, pp. 190–199. © Allerton Press, Inc., 2018. Original Russian Text © A.V. Koltygin, V.E. Bazhenov, 2018, published in Izvestiya Vysshikh Uchebnykh Zavedenii, Tsvetnaya Metallurgiya, 2018, No. 1, pp. 64–74.

PHYSICAL METALLURGY AND HEAT TREATMENT

Influence of the Chemical Composition and Heat Treatment Modes on the Phase Composition and Mechanical Properties of the ZK51A (ML12) Alloy A. V. Koltygin* and V. E. Bazhenov** National University of Science and Technology “MISiS”, Moscow, 119049 Russia *e-mail: [email protected] **e-mail: [email protected] Received April 20, 2017; in final form, October 30, 2017; accepted for publication October 31, 2017

Abstract⎯The objects of investigation are ZK51A (ML12) alloy samples containing from 3.5 to 5.5 wt % Zn and 0.5–0.8 wt % Zr. The influence of Zn and Zr content on phase transition temperatures and the phase composition in equilibrium conditions and when using the Scheil–Gulliver solidification model is established using the calculation of phase diagrams in the Thermo-Calc program. It is shown that a significant increase in the liquidus temperature of the alloy occurs at a zirconium content in the alloy higher than 0.8– 0.9 wt %, and an increase in the melting temperature above 800°C is required, which is undesirable when using steel crucibles. The equilibrium content of alloying components in the magnesium-based solid solution at various temperatures is calculated. The microstructure of as cast and heat-treated alloys with various concentrations of alloying components is investigated using scanning electron microscopy. The distribution of Zn and Zr in a dendritic cell of the as cast and heat-treated alloy is investigated. Zinc is concentrated along the dendritic cell boundaries in the as cast state, but its concentration in their center becomes higher than along the boundaries after heat treatment (HT). Zirconium is concentrated in the center of dendritic cells. It is shown that the two-stage solutionizing mode gives the largest increment of this characteristic: 330°C, 5 h + 400°C, 5 h. The inf luence of the aging temperature (150 and 200°C) on the sample hardness is investigated. It is revealed that it is higher in the case of aging at 200°C, and its maximum is observed under holding for 8‒10 h. The HT of the alloy, including solution treatment (330°C, 5h + 400°C, 5 h) with subsequent quenching and aging (200°C, 8 h), made it possible to attain an alloy ultimate strength of 285 ± 13.5 MPa and a elongation of 11.4 ± 1%. Keywords: casting magnesium alloys, ZK51A, ML12, Mg–Zn–Zr, solidification, heat treatment, phase composition, Thermo-Calc DOI: 10.3103/S1067821218020049

INTRODUCTION The ZK51A (ML12) alloy is one of the alloys of the Mg–Zn–Zr system. It is used to fabricate simply shaped castings into sand and metallic molds. It is applied to fabricate the parts which operate for a long time at temperatures up to 200°C and for a short time at temperature up to 250°C [1, 2]. The castability of the ZK51A alloy are satisfactory. It possesses good fatigue strength and moderate corrosion resistance, which allows its use in the aircraft industry [1–4]. Nevertheless, this alloy is sensitive to the formation of microporosity and have poor weldability [3, 5]. The soundness of fabricated castings is higher than that of castings of alloys of the Mg–Al–Zn–Mn system (AZ91) [5]. Alloys similar to ZK51A are applied in various countries under grades ML12, MAG4, and MC6. The chemical composition and mechanical properties of the ZK51A alloy when compared with

analogs are presented in Table 1. Notations of heat treatment (HT) modes are noted according to those accepted in presented standards: T1 is low-temperature annealing and T5 is artificial aging [2]. Castings made of the ZK51A alloy possess high mechanical properties due to its homogeneous finegrain structure, which is provided by the presence of zirconium in the alloy [7, 8]. Zirconium is one of most efficient refiners of the magnesium grain in alloys containing no aluminum [9–13]. The causes of this phenomenon have a dual character: first, parameters of the hcp lattice of the (Zr) particles are very close to those for the magnesium solid solution (Mg), which also has the hcp lattice; therefore, the (Zr) particles act as nuclei of (Mg) [3, 14]; second, zirconium is dissolved in liquid magnesium and resticted the grain growth of the magnesium solid solution during its solidification [4, 15].

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Table 1. Chemical composition and mechanical properties of the ZK51A alloy when compared with analogs [1, 6]

Alloy

Country

Standard

Alloying components, wt % Zn

ZK51A

ML12

MAG4

МС6

United States

Russia

Great Britain

Japan

ASTM В80-97

GOST (State Standard) 2856-79

BS 2970

JIS Н5203

HT mode

σu, MPa

σ0.2, MPa

δ, %

T5 (177°C, 2 h + 220°С, 6 h)

275

180

8

T5 after welding (330°С, 2 h + 175°С, 16 h)

–

–

–

As cast

200

90

6

T1 (300°С, 4–6 h)

230

130

5

T5 (metallic mold)

245

154

7

T5 (sand mold)

230

145

5

T5

235

137

5

Zr

3.6–5.5 0.50–1.0

4.0–5.0

0.60–1.1

3.5–5.5 0.40–1.0

3.6–5.5 0.50–1.0

The as cast alloy structure is the zinc and zirconium solid solution in magnesium (Mg), along whose grain boundaries inclusions of the MgZn intermetallic phase are arranged [16]. The HT resulted in the MgZn phase and the zirconium-based solid solution (Zr) being precipitated from the supersaturated solid solution, strengthening it. Various HT modes proposed for the alloy by various standards are presented in Table 1. We studied the influence of the composition of the ZK51A (ML12) alloy on the solidification pathway, microstructure, and phase composition in the as cast state and after HT both according to recommended modes and more complex modes. EXPERIMENTAL We used pure raw materials as the charge: magnesium (99.9%1 Mg) produced by the Solikamsk magnesium plant, zinc (99.98% Zn), and master alloy Mg– 15% Zr (production of the Solikamsk pilot metallurgical plant). Melting was performed in a high-frequency induction furnace in steel crucibles under the flux based on carnallite (KCl · MgCl2). The weight of each melting was 300 g. After adding the Mg–Zr master alloy, the melt was held for 15 min at t = 760– 780°C. It was poured into a steel mold at t = 760°C. Ingots 35 mm in diameter and 140 mm in height were fabricated. Fabricated ingots were cut and the samples for the determination of hardness and metallographic sec1 Here and below, unless otherwise noted, the content of elements

is presented in wt %. RUSSIAN JOURNAL OF NON-FERROUS METALS

Table 2. Chemical composition of alloys Content of elements, wt % Alloy Mg

Zn

Zr

4.0Zn

Bal.

4.0

0.8

3.5Zn

Bal.

3.6

0.5

5.5Zn

Bal.

5.3

0.7

tions were prepared. Alloys were charged for compositions 3.5 and 5.5% limited by the zinc content. In addition, the samples of the intermediate composition were cast (charging was performed for 4.0% Zn). The zirconium content was calculated for 1%. The chemical composition of melted alloys was determined by energy dispersive X-ray spectroscopy (EDS) over the area of 1 × 1 mm (Table 2). The alloy microstructure and content of elements in phases were investigated using a Tescan Vega SBH3 scanning electron microscope (SEM) (Tescan, Czech Republic) with the Oxford EDS analysis system. The Brinell hardness was evaluated using a NEMESIS 9001 universal hardness meter produced by INNOVATEST (Netherlands). We used the following testing parameters: a ball 2.5 mm in diameter, a load of 62.5 kgf (≈61.3 kN), and a holding time under a load of 30 s. The samples were investigated in the as cast and heat-treated conditions. To determine the mechanical properties of alloys, we performed separate melting

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(a)

(b)

1000

1000

900

900

L

800

800

L + (Zr)

700

700

600

Temperature, °C

Temperature, °C

L

L + (Mg)

500 (Mg) 400 (Mg) + (Zr)

300 200 100 3.0

4.0

600

L + (Mg)

500

L + (Mg) + (Zr)

400

(Mg) + (Zr)

300 200

(Mg) + (Zr) + MgZn 3.5

L + (Mg) + (Zr)

4.5 5.0 Zn, wt % (c)

5.5

100 3.0

6.0

1000

(Mg) + (Zr) + MgZn 3.5

4.0

4.5 5.0 Zn, wt % (d)

5.5

6.0

1000 L

900

900 L 800 L + (Zr) L + (Mg) + (Zr)

700 600 500

L + (Mg)

L + (Mg) + (Zr)

(Mg)

400

(Mg) + (Zr)

300

(Mg) + (Zr) + MgZn

200 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Zr, wt %

Temperature, °C

Temperature, °C

800

L + (Zr) L + (Mg) + (Zr)

700 600

L + (Mg)

500 (Mg) 400

L + (Mg) + (Zr) (Mg) + (Zr)

300 (Mg) + (Zr) + MgZn 200 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Zr, wt %

Fig. 1. Polythermal joints of phase diagrams Mg–(3–6)% Zn–Y% Zr and Mg–X% Zn–(0.3–1.2)% Zr. X = (c) 3.5 and (d) 5.5, Y = (a) 0.4 and (b) 1.1.

according to the above-described technology in a resistance furnace using a large-capacity steel crucible and fabricating cast ingots for sample cutting according to GOST (State Standard) 1583–93. Mechanical properties were investigated using cylindrical samples 5 mm in diameter turned from cast ingots after their HT. Tensile tests were performed using an Instron 5569 universal testing machine (Instron, United States). Polythermal sections of phase diagrams were calculated using the Thermo-Calc program [17]. We used the thermodynamic database TTMG3 (Magnesium alloys database version 3) [18].

RESULTS AND DISCUSSION Equilibrium Alloy Solidification Figure 1 shows polythermal sections of the ternary phase diagram of the Mg–Zn–Zr system in the region 3–6% Zn and 0.3–1.2% Zr. The equilibrium alloy solidification at a low zirconium content starts from the primary crystals of the magnesium solid solution (Mg) at t ~ 650°C both at the minimal zinc content of 3.5% and its maximal content of 5.5% (Fig. 1a). The solidification pathway changes with an increase in the zirconium content in the alloy to 1.1% (Fig. 1b). Crystals of the zirconium-

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500

600

450

550 3, 4

1

2

2

450 Temperature, °C

Temperature, °C

1

500

400 350 300 250 200

400 3 4

350 300 250 200

150

150

100

100

50

50 0

1

2

3 4 % Zn, Zr

5

5

0

6

0.02

6

0.04 0.06 0.08 Phase weight fraction

0.10

Fig. 3. Variation in the weight fraction of phases: (1, 2) L and (5, 6) MgZn in Mg–X% Zn–(0.5–1.1)% Zr alloys and (3, 4) the (Zr) phase in Mg–(3.5–5.5)% Zn–Y% Zr alloy depending on temperature. X = (1, 5) 3.5% and (2, 6) 5.5%, Y = (3) 0.5% and (4) 1.1%.

Fig. 2. Content of (1, 2) zinc and (3, 4) zirconium in the magnesium solid solution of alloys (1, 3) Mg–3.5% Zn– 0.8% Zr and (2, 4) Mg–5.5% Zn–0.8% Zr.

based solid solution (Zr) with a very low content of dissolved components, i.e., almost pure zirconium, start to solidify from the melt first, and the liquidus temperature increases approximately to 850°C (Figs. 1a, 1b). The transition from the primary solidification of (Mg) to the primary solidification of (Zr) in alloys under consideration occurs with a zirconium content of 0.5%, irrespective of the zinc content (Figs. 1c, 1d). Then the crystals of the magnesiumbased solid solution (Mg) start to form from the melt according to the peritectic reaction, and primary zirconium crystals disappear. A decrease in the zirconium content in the alloy below 0.5% excludes the formation of primary crystals of (Zr) at any zinc content (Figs. 1c, 1d). This is undesirable because the (Zr) particles serve as nuclei of the (Mg) solid solution solidification [3, 14]. Herewith, the liquidus temperature increases to 800°C irrespective of the zinc content at the content of 0.9% Zr in the alloy. Melting at this temperature leads to the intense dissolution of iron, which enters the alloy from the walls of a steel crucible. This phenomenon causes large losses of zirconium, which forms stable phases with iron [3]. Therefore, the zirconium content in the alloy in a range of 0.5–0.9% can be considered a justified process limit for this alloy for melting in a steel crucible. The alloy ability to strengthening due to HT depends on the variation in the solubility of major alloying components in the magnesium solid solution (Mg) at various temperatures. The calculated solubility of zinc and zirconium in the solid solution (Mg) of the ZK51A alloy are presented in Fig. 2. It is seen that RUSSIAN JOURNAL OF NON-FERROUS METALS

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the maximal zinc content in the solid solution (Mg) decreases in the equilibrium state by 5%, notably, from 5.5% at a temperature close to the solidus temperature to 0.54% at t = 50°C. The zirconium content in the solid solution (Mg) is 0.8% at t = 500°C and decreases to 1.5 × 10–4% at t = 50 °C. Figure 3 shows the plots of varying the equilibrium fraction of phases depending on the temperature in Mg–Zn–Zr alloys containing 3.5–5.5% Zn and 0.5– 1.1% Zr. Zirconium (Zr) starts to precipitate from the (Mg) solid solution as the independent phase with a decrease in temperature. The amount of precipitated zirconium increases with an increase in its content in the alloy. The MgZn phase is precipitated from the solid solution at a lower temperature, and the temperature of the onset of its precipitation increases from 215 to 282°C with an increase in the zinc content from 3.5 to 5.5%; the amount of the MgZn phase increases with an increase in the content of zinc (Fig. 3). Thus, the equilibrium phase composition of the alloy at room temperature is the magnesium-based solid solution (Mg), zirconium (Zr), and the MgZn compound. The equilibrium solidus temperature of the alloy decreases from 520 to 443°C with an increase in the zinc content from 3.5 to 5.5%, irrespective of the zirconium content in the alloy (Fig. 3). Nonequilibrium Solidification Nonequilibrium solidification was considered using calculations according to the Scheil–Gulliver model in the Thermo-Calc program [19, 20]. The diffusion in the solid phase is absent according to this

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900

3, 6

Temperature, °C

800

2, 5 L + (Zr) 1, 4

700 600

1 2 3

L + (Mg) + (Zr)

500

4 5 6 L + (Mg) + (Zr) + MgZn

400 300 0

0.25 0.50 0.75 Fraction of the solid phase

1.00

Fig. 4. Temperature dependence of the fraction of the solid phase during (1–3) the equilibrium solidification and (4‒6) the nonequilibrium solidification according to the Scheil–Gulliver model of alloys (1, 4) Mg–3.5% Zn– 0.5% Zr, (2, 5) Mg–4.5% Zn–0.8% Zr, and (3, 6) Mg– 5.5% Zn–1.1% Zr.

model; therefore, the result of the calculation can disregard the actual process but give information on nonequilibrium phases and the nonequilibrium solidus temperature, which is important for determining the possible heat-treatment temperature. Figure 4 shows the temperature dependence of the fraction of the solid phase during the equilibrium and nonequilibrium solidification of alloys Mg–3.5% Zn– 0.5% Zr, Mg–4.5% Zn–0.8% Zr, and Mg–5.5% Zn– 1.1% Zr.

The nonequilibrium solidification, similarly to the equilibrium one, starts from the primary crystals of zirconium (Zr). Then the magnesium-based solid solution (Mg) is solidified. The solidification is finished at a constant temperature by the formation of the MgZn phase according to the eutectic reaction. The solidification pathway remains invariable with the variation in the Zn content in the alloy from 3.5 to 5.5%. The solidification is ended at t = 341°C. Thus, the solidus temperature is lower than in the equilibrium conditions during Scheil–Gulliver solidification by 170 and 80°C at 3.5 and 5.5% Zn, respectively. This should be taken into account in the HT of alloys in the as cast state in order to avoid the melting of the nonequilibrium eutectic phases. Starting from the above-considered peculiarities of the equilibrium and nonequilibrium solidification of alloys with the minimal (3.5Zn alloy) and maximal (5.5Zn alloy) zinc contents, we proposed several HT modes of alloys in order to reveal their influence on the alloy structure and properties (Table 3). Analysis of the Alloy Microstructure The microstructure of alloys 3.5Zn and 5.5Zn in the as cast state is presented in Fig. 5. It is seen that the alloy microstructure in the as cast state consists of dendrites of the zinc and zirconium solid solution in magnesium (Mg), as well as of bright phases arranged along the boundaries and in the center of dendritic cells. The zinc and zirconium content in the center and along the boundaries of dendritic cells in the as cast state and after HT was determined using EDS (Table 3). It was found that the zinc content in the 3.5Zn and 5.5Zn samples in the dendritic cell center is substantially lower than along its boundaries and, on the contrary, the amount of zirconium is considerably larger in the dendritic cell center. The content of zirconium in the center and along the boundaries of dendritic cells remains the same, while the fraction of zinc

Table 3. Heat treatment modes of experimental alloys and content of elements in the center and at the boundary of dendritic cells HT mode Alloy notation number

conditions

Content of elements, % in the center of the dendritic cell

at the boundary of the dendritic cell

Zn

Zr

Zn

Zr

3.5Zn

1 2 3

As cast 450°С, 5 h 450°С, 10 h

1.3 4.0 3.8

0.7 0.6 0.8

5.0 3.1 2.9

0.3 0.1 0.2

5.5Zn

4 5 6 7

As cast 330°С, 5 h 330°С, 5 h + 400°С, 5 h 330°С, 5 h + 400°С, 10 h

1.7 6.1 6.3 6.8

0.8 0.7 0.7 0.7

6.8 4.4 4.9 4.6

0.2 0.1 0.2 0.1


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MgZn MgZn

(Zr)

(Zr)

(a)

50 μm

(b)

50 μm

(c)

50 μm

(d)

50 μm

Fig. 5. Microstructure of alloys (a, c) 3.5Zn and (b, d) 5.5Zn in (a, b) the as cast and heat-treated states (Table 3) according to modes (c) 2 and (d) 6.

is larger in the dendritic cell center than at its boundary, for alloys 3.5Zn and 5.5Zn heat-treated according to modes 2 and 6, respectively. Prolongation of the HT time does not significantly affect the content of elements in the dendritic cell. The zirconium (Zr) particles most often associated with iron, silicon, manganese, and (more rarely) with other impurities are met in the center and along the boundaries of dendritic cells. Interlayers of the intermetallic phase are arranged along the cells. This is the MgZn phase according to calculations in the ThermoCalc program. EDS results show that this phase corresponds better to the Mg7Zn3 compound, which is present in the phase diagram of the Mg–Zn binary system in the high-temperature region [21]. Artificial aging without preliminary quenching at an elevated temperature is usually used in the HT mode for ZK51A alloys (Table 1). The analysis of the distribution of alloying elements in the dendritic cell of the 4.0Zn alloy heat treated according to the T1 mode showed that the HT did not eliminate the liquation of dissolved elements in the magnesium-based solid solution (Mg). Two zones of an increased concentration of alloying elements are observed—along the boundaries of dendritic cells and in their center (around the zirconium particles). The results of the RUSSIAN JOURNAL OF NON-FERROUS METALS

EDS of the distribution of alloying components inside the dendritic cell of (Mg) show that the zinc content is increased at the cell boundary and the zirconium content is increased in its center (Fig. 6). It is seen in Fig. 6 that the (Zr) inclusion contains zinc, which can be explained by the dissolution of zinc in the zirconiumbased solid solution at an elevated temperature [22]. It is evident that the HT of the alloy according to the T5 mode (Table 1) will lead to the same results as the T1 mode; therefore, we can consider the statement that [6] the grains of the magnesium solid solution are homogeneous in essence is not quite correct. Preliminary solution treatment with quenching at a temperature close to the alloy solidus with subsequent aging (T6 mode) or without aging (T4 mode) is usually performed for the maximal dissolution of alloying elements in the (Mg) magnesium solid solution in order to later attain its maximal strengthening due to the larger amount of the particles of the strengthening phase precipitated during artificial aging. The HT according to T1 and T5 modes (Table 1) increases the mechanical properties of the cast alloy mainly due to the decomposition of the supersaturated solid solution of zinc and zirconium in magnesium, which results from the nonequilibrium alloy solidification in natural formation conditions of the casting. Such HT probably does not provide maximal mechanical properties

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Hardness, HB

65

20 18

Element content, wt %

16

60

55

14 12 50

10

1

8

2

3 4 5 Solutionizing mode

6

7

2

6 4

Fig. 7. Variation in hardness of 3.5Zn and 5.5Zn alloys (Table 2) in the solutionized state according to various modes (Table 3).

1

2 0

15

30 45 60 Distance, μm

75

90

Fig. 6. (a) Dendritic cell of the magnesium solid solution and (b) distribution of elements in the alloy heat-treated according to the T1 mode. (1) Zirconium content and (2) zinc content.

of alloys; however, an increase in the HT temperature above 340°C is complicated because of the danger of melting of nonequilibrium eutectic phases forming during the nonequilibrium alloy solidification, especially for alloys with a high zinc content (Fig. 4); therefore, more complicated HT modes are not used in industrial conditions. Nevertheless, the more uniform distribution of alloying components in the alloy structure due to the elimination of microsegregation should probably provide higher mechanical properties of the alloy. This is possible with an increase in temperature and duration of HT. An increase in the HT temperature of the 3.5Zn alloy to 450°C when the amount of the MgZn intermetallic phase is minimal (Fig. 5a) leads to its complete dissolution. The alloy microstructure becomes almost single-phase with small inclusions of the Zr containing phase after 5-h holding (Fig. 5c), and further HT prolongation to 10 h almost does not affect the alloy structure and phase composition. The amount of intermetallic phase along grain boundaries in the as cast state increases upon a high zinc content (Fig. 5c), while the alloy solidus lowers (Fig. 3), which

does not allow us to perform the HT of the 5.5Zn alloy at an identically high temperature. However, alloy holding at t = 330°C decreases the amount of the eutectic intermetallic phase along grain boundaries to a minimum. The further HT, in order to accelerate the process, can be performed at a high temperature (400°C) with no concern for the melting of nonequilibrium phases; herewith, the intermetallic phase with zinc dissolves completely (Fig. 5d). It was established that an increase in HT temperature for the 5.5Zn alloy can be performed after just 5 h holding at t = 330°С, i.e., by 10°C below the nonequilibrium solidus (Fig. 4), due to a decrease in the amount of the residual intermetallic phase along grain boundaries of the solid solution (Mg). The effectiveness of the HT of alloys can be indirectly evaluated by the variation in their hardness. Figure 7 shows the results of a measurement of the sample hardness after various HT modes, including isothermal holding with subsequent quenching (Table 3). It is seen that the alloy hardness in the as cast and heat-treated states increases with an increase in the zinc content. The inhomogeneity of the alloy structure decreases during the HT, which is evidenced by the confidence range for values of hardness decreasing relatively to the as cast state. It is established (Fig. 7) that the hardness remains almost invariable relative to the as cast state (mode 1 in Table 3) after the HT of the 3.5Zn alloy at 450°C according to modes 2 and 3 (Table 3) because of the almost complete dissolution of zinc in the solid solution (Mg). An increase in hardness resulting from the HT (modes 5–7 in Table 3) at


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(a)

80

70 65 1 60

4 5 6 7

75

Hardness, HB

Hardness, HB

75

(b)

80

1 2 3

70

197

6 7

65 4 5 60

3 2

55

55

50

50 0

2

4 6 8 Aging duration, h

10

12

0

2

10

12

(c)

80

5

6

75

Hardness, HB

4 6 8 Aging duration, h

70 7 65 5 6 7

60 55 50 0

4

8 12 Aging duration, h

16

20

Fig. 8. Variation in hardness of the samples of alloys (a) 3.5Zn and (b, c) 5.5Zn (Table 2) depending on the aging time at temperatures (a, b) 150 and (c) 200°C. Aging was preceded by the HT according to modes (1)–(7) (Table 3).

a higher zinc content (the 5.5Zn alloy) relative to the as cast state (mode 4 in Table 3) is more considerable (Fig. 7). The sample strength can be increased by means of artificial aging. It was performed at t = 150 and 200°C. Herewith, the sample hardness was measured each 2 h (Fig. 8). It is seen that an increase in alloy hardness at an aging temperature of 150°C is insignificant (Figs. 8a, 8b), which evidences the absence of alloy strengthening in this case irrespective of the zinc content in it. Hardness increases at an aging temperature of 200°C larger than at 150°C (Fig. 8c), reaching the RUSSIAN JOURNAL OF NON-FERROUS METALS

maximum after 8–12 h of isothermal holding. Alloys subjected to such HT should possess higher strength. We performed mechanical tests of the 5.5Zn alloy in the as cast and heat-treated states treated according to various modes. Alloy aging was performed at t = 200°C for 8 h to attain the best strength. Results of mechanical tensile tests of the samples of the 5.5Zn alloy are presented in Fig. 9. It is seen that the maximal strength is attained for the alloy HT according to the two-stage mode, which consists of holding for 5 h at t = 330°C and subsequent quenching and aging for 8 h at t = 200°C. Herewith, the ultimate strength at a level of 285 ± 13.5 MPa at a

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350

12

σ0.2 σu

11

δ

300

9

δ, %

σ, MPa

10 250

200 8 150

7

100

6 1

2 3 Sample no.

4

Fig. 9. Results of tensile tests of the 5.5Zn alloy (1) in the as cast state and (2–4) after the HT. HT conditions: (2) 220°C, 8 h; (3) 330°C, 5 h + 200 °C, 8 h; and (4) 330°C, 5 h + 400°C, 5 h + 200°C, 8 h.

elongation of 11.4 ± 1% is attained. The further increase in HT temperature and time leads to a small decrease in strength and an insignificant increase in the elongation of the alloy. CONCLUSIONS (i) The results of calculations in the Thermo-Calc program show that the ZK51A alloy consists of the magnesium solid solution(Mg), enriched with zirconium and containing almost no impurities (Zr) phase and MgZn intermetallic phase. The actual phase composition of the alloy includes the magnesium solid solution (Mg), the (Zr) zirconium phase, and the Mg7Zn3 nonequilibrium eutectic phase. (ii) An increase in the zirconium content above 0.8–0.9% requires an increase in the melting temperature above 800°C, which is undesirable from the manufacturing viewpoint when melting the alloy in a steel crucible; therefore, such a measure is unreasonable in industrial conditions. (iii) The nonequilibrium alloy solidus temperature is 341°C, which is lower than the calculated equilibrium solidus temperature by 170 and 80°C for 3.5 and 5.5% Zn, respectively. This should be taken into account for the alloy HT in order to avoid melting of the nonequilibrium eutectic. However, it is established that an increase in the HT temperature is possible already after 5 h holding at t = 330°C (below the nonequilibrium solidus) because of a decrease in amount of the MgZn nonequilibrium intermetallic phase.

(iv) The largest increase in hardness after solution treatment with quenching is attained in the alloy with the maximal zinc content (5.5Zn) when using modes 330°C, 5 h (66.0 ± 2.1 HB), and 330°C, 5 h + 400°C, 5 h (67.2 ± 1.8 HB). The further increase in the HT time leads to a decrease in hardness after solutionizing and quenching. Subsequent alloy aging at t = 200°C for 8 h increases the maximal alloy hardness to 76.4 ± 1.3 HB. The further increase in the holding time determines a decrease in this characteristic. Aging at t = 150°C causes no significant increase in the alloy hardness. (v) The largest alloy strengthening was attained after the HT according to the two-stage mode, which consists of holding for 5 h at t = 330°C with subsequent quenching and aging for 8 h at t = 200°C. Herewith, maximal ultimate strength is at a level of 285 ± 13.5 MPa at a elongation of 11.4 ± 1%. An increase in temperature and duration of the HT does not promote an increase in the alloy strength but, on the contrary, leads to its decrease. REFERENCES 1. Hussey, B. and Wilson, J., Light Alloys, Boston: Springer, 1998. 2. Avedesian, M. and Baker, H., ASM Specialty Handbook: Magnesium and Magnesium Alloys, Ohio: ASM, 1999. 3. Polmear, I.J., Magnesium alloys and applications. Mater. Sci. Technol., 1994, vol. 10. pp. 1–16. 4. Campbell F.C., Elements of Metallurgy and Engineering Alloys, Ohio: ASM, 2008. 5. ASM International Handbook Committee. ASM Handbook, vol. 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Ohio: ASM, 1990. 6. Heat Treater’s Guide: Practices and Procedures for Nonferrous Alloys, Chandler, H., Ed., Ohio: ASM International, 1996. 7. Vinotha, D., Raghukandan, K., Pillai, U.T.S., and Pai, B.C., Grain refining mechanisms in magnesium alloys: An overview, Trans. Indian Inst. Met., 2009, vol. 62, pp. 521–532. 8. Changjiang, S., Qingyou, H., and Qijie, Z., Review of grain refinement methods for as-cast microstructure of magnesium alloy, China Foundry, 2009, vol. 6, pp. 93– 103. 9. Arroyave, R., Shin, D., and Liu, Z.K., Modification of the thermodynamic model for the Mg–Zr system, CALPHAD, 2005, vol. 29, pp. 230–238. 10. Bamberger, M., Structural refinement of cast magnesium alloys, Mater. Sci. Technol., 2001, vol. 17, pp. 15–24. 11. Lee Y.C., Dahle A.K., StJohn D.H. The role of solute in grain refinement of magnesium. Metall. Mater. Trans. A. 2000, vol. 31, pp. 2895–2906. 12. Qian, M. and Das, A., Grain refinement of magnesium alloys by zirconium: Formation of equiaxed grains, Scr. Mater., 2006, vol. 54, pp. 881–886.


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INFLUENCE OF THE CHEMICAL COMPOSITION AND HEAT TREATMENT MODES 13. Vinotha, D., Raghukandan, K., Pillai, U.T., and Pai, B.C., Grain refining mechanisms in magnesium alloys: An overview, Trans. Indian Inst. Met., 2009, vol. 62, pp. 521–532. 14. Yang, W., Liu, L., Zhang, J., Ji, S., and Fan, Z., Heterogeneous nucleation in Mg–Zr alloy under die casting condition, Mater. Lett., 2015, vol. 160, pp. 263–267. 15. St. John, D.H., Qian, M.A., Easton, M.A., Cao, P., and Hildebrand, Z., Grain refinement of magnesium alloys, Metall. Mater. Trans. A., 2005, vol. 36, pp. 1669–1679. 16. Ren, Y.P., Guo, Y., Chen, D., Li, S., Pei, W.L., and Qin, G.W., Isothermal section of Mg–Zn–Zr ternary system at 345°C, CALPHAD, 2011, vol. 35, pp. 411– 415. 17. Andersson, J.O., Helander, T., Höglund, L., Shi, P.F., and Sundman, B. Thermo-Calc and DICTRA: Computational tools for materials science, CALPHAD, 2002, vol. 26, pp. 273–312.


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18. Thermo-Calc Software TTMG3. Magnesium alloys. Database version 3. http://www.thermocalc.com/ products-services/databases/thermodynamic/ (Cited March 1, 2017). 19. Gulliver, G.H., The quantitative effect of rapid cooling upon the constitution of binary alloys, J. Inst. Met., 1913, vol. 9, pp. 120–157. 20. Scheil, E., Bemerkungen zur schichtkristallbildung, Z. Metallkd., 1942, vol. 34, pp. 70–72. 21. Liu, S., Yang, G., Xiao, L., Luo, S., and Jie, W., Effects of the growth rate on microstructures and room temperature mechanical properties of directionally solidified Mg–5.2Zn alloy, J. Miner. Met. Mater. Soc., 2016, vol. 68, no. 12, pp. 3214–3223. 22. Williams, M.E., Boettinger, W.J., and Kattner, U.R., Contribution to the Zr-rich part of the Zn–Zr phase diagram, J. Phase Equilib. Diffus., 2004, vol. 25, no. 4, pp. 355–363.

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