o5-thermodynamic and thermal analysis of mg – cast alloys

University of Ljubljana Faculty of material science and engineering Department for materials and metallurgy

Thermodynamic and thermal analysis of Mg – cast alloys

Jožef Medved, Primož Jožef Medved, Primož Mrvar, Maja VončinaMrvar, Maja Vončina

[email protected]

University of Ljubljana, Faculty of natural science and engineeringDepartment of materials and metallurgy

Aškerčeva 12, Ljubljana, Slovenia

TOFA 2010, Porto, 12-16. september


Topics:

-Thermodynamic calculation

Purpose: Complex investigation of Mg - alloys

-Thermodynamic calculation

-Thermal analyses (ETA, DSC, TG)

- Non-metallic inclusions characterization, mechanical properties

- High temperature corrosion resistance



IsoplethIsopleth phase phase diagram Mgdiagram Mg––AlAl––MMnn at at 6,28 6,28 massmass.% Al .% Al 0,3056 mass. % 0,3056 mass. % MnMn, , 0,0294 mass. % 0,0294 mass. % SiSi and and 0,0032 mass. % 0,0032 mass. % FeFe..

AL11MN4#1 STATUS ENTEREDNumber of moles 8,3785E-03, Mass 2,8853E-01Mass fractions:AL 5,74581E-01 MG 0,00000E+00 MN 4,25419E-01 ZN 0,00000E+00

HCP_A3#1STATUS ENTEREDNumber of moles 9,9162E-01, Mass 2,4262E+01Mass fractions:MG 9,44096E-01 ZN 2,02378E-03 AL 5,38805E-02 MN 2,55276E-07



A new eequipment with the measuring cell for »in situ« thermal analyses of Mg and Al alloys.

Measuring

card

PC

Measuring cell


University of Ljubljana - NTF - Department for materials and metallurgy

Experimental - ETA


Cooling curves and microstructures of Mg and alloy MgAl4

5,7 K/s5,7 K/s

400

500

600

700

dT/d

t [K

/s]

438 °C511 °C604 °C

T [°

C]

-4

0

5,7 K/s

0 200 400 600 800 1000300

Cooling curve (T)

t [s]

-4

Cooling rate (dT/dt)



Cooling curve and isopletphase diagram of AM50

-0,5

0,0

0,5

1,0

200

300

400

500

600

700

TS=426 °C

TE2

=431 °C

TE=511,2 °C

TL=611,4 °C

T [°

C]

dT

/dt [

°C/s

]


-1,00 100 200 300 400 500 600 700

0

100

t [s]


Cooling curve, microstructure and phase diagram of AM60

-0,5

0,0

0,5

1,0

400

500

600

700

TS=428,1 °C

TE2

=432,9 °C

TE=509,4 °C

TL=607,6 °C

T [°

C]

dT

/dt [

°C/s

]


-1,00 100 200 300 400 500 600 700

300

t [s]


Cooling curve and phase diagram of AZ91

-1

0

1

0 100 200 300 400 500 600 700

300

400

500

600

700

TE=510,7 °C

TS=410,8 °C

TE2

=424,1 °C

TL=581,4 °C

T [°

C]

dT

/dt [

°C/s

]

1

Cooling curve of AE42


0 100 200 300 400 500 600 700

t [s]

-2

-1

0

0 100 200 300 400 500 600 700

300

400

500

600

700

TE=598,2 °C

TS=566,5 °C

TL=619,3 °C

T [°

C]

t [s]

dT

/dt [

°C/s

]


Crucibles material: platinum. Heating and cooling speed: 10 K/min.Max. temp.: 720 °CAtmosphere: vacuum, nitrogen.

DSC mesurements of Mg-alloys



STA heating curve of theAM50 alloy

L → αMg


L → (αMg+Mg17Al12)


STA cooling curve of the AM60 alloy

L → αMg

L → (αMg+Mg17Al12)



DSC analysis of solidification of Mg- alloys



DSC analysis as melt control method

350

360

370

(J/g

)

Comparis on of melting Enthalpy

1. Hydro

280

290

300

310

320

330

340

1 2 3 4

Ent

halp

y(J

/g)

Material

2. Elektron

3. Mimet

4. Gold River



T [°

C]

AM 60

1: αMg 2: Al4Mn 3: Tekoče 4: Al11Mn4 5: Mg2Si 6: Al8Mn5 7: Cu2Al 9: Mg17Al12

Conclusion – AM60

mass. % Al

0 500 1000 1500

200

300

400

500

600

700

800 TS=569 °C

Cooling curve (T)

dT/d

t [K

/s]

TE=418 °CT

L=610 °C

T [°

C]

t [s]

-4

0




Non-metallic inclusions characterization

Zap. Št. Spojine Temperatura razpada ∆G(kJ/mol)

1 2MgO*2Al2O3*5SiO2 600 -9389 2 3CaO*Al2O3*3SiO2 600 -6985 3 3CaO*MgO*3SiO2 600 -4644 4 K2O*4SiO2 592 -4661 5 CaO*Al2O3*2SiO2 600 -4488 6 2CaO*Al2O3*SiO2 600 -4253 7 2CaO*MgO*2SiO2 27 -3940 8 CaO*Al2O3*SiO2 600 -3478 9 CaO*MgO*2SiO2 600 -3249 10 MgO*Al2O3 600 -2422 11 2CaO*SiO2 600 -2370 12 2MgO*SiO2 600 -2311 13 CaO*MgO*SiO2 27 -2296 14 2CaO*Fe2O3 27 -2196 15 MnO*Al2O3 27 -2183 16 Al2O3 600 -1756 17 MgO*SiO2 600 -1643 18 MgO*MoO 600 -1592

Stehiometriccompositions of inclusions and Gibbs

∆ 18 MgO*MoO3 600 -1592 19 CaO*MoO3 27 -1579 20 CaO*MgO 600 -1336 21 MgCO3 540 -1177 22 SiO2 600 -967 23 CaCl2 600 -915 24 MoO3 600 -844 25 MgCl2 600 -748 26 MgO 600 -674 27 CaO 600 -653 28 MoO2 600 -653 29 CaS 600 -541 30 KCl 600 -529 31 NaCl 600 -495 32 FeCl 600 -474 33 MgS 600 -407 34 SiS2 600 -312

free energies (∆G) .



1)

2)

Microstructure of AM60 alloy in a HDC as-cast sate; a) primary dendriteαMg (1), inter-metallic compound Mg17Al12 (2) and Al4Mn (3), b) Macrophoto of specimen.

3)2 mm

9 mm



Kem. el

At. % Mas. %

O 76,767 65,161 Mg 1,013 1,306 Si 21,583 32,156

SEM micro photo of SiO2 particle in AM 60 alloy.

Probable source of SiO 2 particles:- Silicon-thermal reduction: T >1200°C…..4MgO + Si → Mg2SiO2 + 2Mg- Mg2SiO2 can not be reduce by Si under 1500°C.- If MgO in magnesium silicate is replaced by CaO (dolomite can be used):

2CaMgO2 + Si → 2Mg + Ca2SO4



Kem. el. At. % Mas. % O 37,052 24,600 Mg 52,349 52,798 Al 2,342 2,622 Ca 5,525 9,188 Mo 2,681 10,675

SEM micro photo of inclusion on base of Mo, Ca, Mg and O.

Probable source of inclusion:-Possible compounds are: MgO⋅MoO3, CaO⋅MoO3, MoO3 in MoO2

-According to the lowest value of ∆G, inclusion can be compound CaO⋅MoO3 +MgO.



High temperature corrosion resistance

Although magnesium alloys are used at room temperatures, they can be putted out to the high temperatures and oxidizing atmospheres in different stages of processing, such as: overheating of charge, melting, pouring, heat treatment and mechanical processing, recycling etc. The results are undesirable effects that change chemical properties and deteriorate structural properties of surface layers. For this reason, the knowledge of oxidation of Mg-alloys at different temperatures is important for optimization of technological processes.

Protective oxide layer The growth of “oxide flowers”



Experimental work

Cylindrical samples with diameter 4.0 mm and height 2,0 mm were prepared. The samples were searched by TG on STA 449 Jupiter Instrument (NETZSCH). The samples were heated in protective gas (nitrogen) till oxidation temperatures (200, 400, 450 and 500˚C), with the heating speed of 20 K/min. The samples were then held in oxygen atmosphere for 12 h. Then cooling with 20 atmosphere for 12 h. Then cooling with 20 K/min followed till room temperature.

Alloy Mg(mass %)

Al(mass %)

Si(mass %)

Mn(mass %)

Zn(mass %)

AE42 Ost. 4,2 0,03 0,29 0,15 1,5 RE

AM50 94,7 4,7 - 0,40 0,12

AM60 93,7 5,84 0,028 0,33 0,05

AZ91 Ost. 8,5-9,5 - 0,17-0,4 0,45-0,9

Chemical composition of tested alloy



Results

450 °C

200 °C

TG curves and structures for AE42 alloy



450 °C

400 °C

TG curves and structures for AZ91 alloy


450 °C

400 °C

TG curves and structures for AM50 alloy



450 °C

400 °C

TG curves and structures for AM60 alloy



Mass changes for all samples at 200 °C



450 °CMass changes for all samples at 450 °C

∆mAM50 = 99,55591 + 0,00222 × t

∆mAE42 = 99,70671 + 7,42442×10-4 × t

∆mAM60 = 99,26979 + 0,00712 × t - 1,12601×10-5 × t2 + 1,26182×10-8 × t3

∆mAZ91 = 121,55408 - 0,74693 × t + 0,0083 × t2 - 3,16601×10-5 × t3 + 5,24557×10-8 × t4 -

3,22361×10-11 × t5 TOFA 2010, Porto, 12-16. september

a)

b)

700

800 TS=569 °C

TE=418 °CT

L=610 °C

0

Calculated polythermal phase diagram Mg-Al-Mn (a) and b) binary phase diagram Al-Mg.

0 500 1000 1500

200

300

400

500

600

Cooling curve (T)

dT/d

t [K

/s]

T [°

C]

t [s]

-4


Microstructure and cooling curve of Mg-alloy at 5,7 K/s cooling rate


The results of heating and cooling DSC curves indicate the course of melting and solidification of Mg-Al alloys. The addition of Al decreases liquidus temperature. DSC curves show that alloys with 5 or more mass % of Al have another peak of solidification. Optical microscopy reveals that this is due to cooling with nonequilibrium eutectic crystallization;

It has been proved that on all examined alloys during heating at oxidation temperature, a thin oxide layer appears which has a “protective nature”. The temperature, a thin oxide layer appears which has a “protective nature”. The alloys that have eutectic in microstructure are protected by the oxide layer up to the first incipient fusion (equilibrium up to 437˚C, practically up to 400 °C). In alloys without eutectic the protective oxide layer exists up to higher temperatures.

It is evident that the most stable alloy at high temperatures is AE 42 and the most unstable is AZ 91. Alloys with bigger Al addition have lower corrosion stability.



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o5-thermodynamic and thermal analysis of mg – cast alloys

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