Seminar in Computational Thermodynamics & Kinetics with Thermo-Calc Software Madrid, 1-2 June, 2010

1. Seminar in Computational Thermodynamics & Kinetics
with Thermo-Calc Software
Madrid, 1-2 June, 2010
Computational Thermodynamics
applied to powder metallurgy
Dr. Tomás Gómez-Acebo

2. Outline
• The fundamentals: assessments
• Composition tuning of a high speed steel
• Development of master alloys for powder
metallurgy
• Life of Gas Turbine coatings

3. THE FUNDAMENTALS:
ASSESSMENTS
T. Gómez-Acebo, "Thermodynamic Assessment of the Ag-Zn System", CALPHAD
22 (2), 203-220 (1998)
doi:10.1016/S0364-5916(98)00024-8

4. Thermodynamic assessment
• Review of literature
data:
– Phase diagram:
compositions, T, solu
bility…
– Chemical thermo:
, a, cp, H, …
– Crystallography
• Thermodynamic
model of each phase
• Reproduce
experimental data
The natural way of understanding thermodynamic models

7. Thermodynamic models: Gibbs energy
E
Gm xAg GAg x Zn GZn RT xAg ln xAg x Zn ln x Zn Gm
Gref (mechanical Sid (configurational
mixture) entropy)
Gid (ideal solution)
Excess Gibbs energy: Redlich-Kister polynomials
2
E 0 1 2
Gm xAg x Zn LAg,Zn LAg,Zn xAg x Zn LAg,Zn xAg x Zn 
Each phase is modelled separatedly

8. Summary of Ag-Zn assessment
• 50 literature sources (papers)
• 700 experimental points
• Model 7 phases: L, , , , , ,
– Liquid: 0L=a+bT; 1L=a
– (fcc): 0L=a+bT; 1L=0
– (bcc): 0L=a+bT; 1L=a
– (hcp-Zn): 0G; 0L=a
– (hcp): 0G; 0L=a+bT; 1L=a; 2L=a
– (Zn)1(Ag,Zn)2: 0G; 0L=a
– (Ag,Zn)2(Ag)2(Ag,Zn)3(Ag,Zn)6: 0G

9. COMPOSITION TUNING OF A
HIGH SPEED STEEL
V. Trabadelo, S. Giménez, T. Gómez-Acebo and I. Iturriza, “Critical assessment of
Computational Thermodynamics in the alloy design of PM high speed steels”.
Scripta Materialia, 53 (3) 287-292 (2005). doi:10.1016/j.scriptamat.2005.04.017

10. Sintering of high speed steels
• Complex chemistry:
– Fe-Cr-Mo-Co-V-W-C-N
– High C content: carbides
• Optimum Sintering Temperature (OST)
– Effect of C, N and
sintering atmosphere
– L+fcc+carbides
– Avoid cementite
– Liquid phase sintering

11. M2 HSS: validation of database (SSOL2)
Experimental DT analysis Reported OST

12. M35MHV: identification of the stable
phases
All phases in SSOL Only observed phases: L, bcc ( ),
Including gas, MC=(Mo,W)C fcc ( and MX), M6C, M3C, Fe2MoC ( )

13. M35MHV: effect of C and N
Fe–1.80C–4.0Cr–5.4Mo–5.5Co–0.035N–4.2V–
6.0W–0.06O, with C additions, sintered in
90N2-9H2-1CH4
100 ppm N 7000 ppm N
Narrow sintering window Wider sintering window

14. M42HVIG vs M35MHV
• New experimental HSS: M42HVIG
HSS C N* O* Cr Co Mo V W Si Fe
M35MHV 1.82 350 600 4.00 5.50 5.40 4.20 6.00 - Bal.
M42HVIG 1.48 221 484 4.08 8.50 10.1 5.29 - 0.41 Bal.
wt.-%, * ppm
• Reasonable to consider the same set of
phases, rejecting the remaining phases:
– L, bcc ( ), fcc ( and
MX), M6C, M3C, Fe2MoC ( )

15. M42HVIG with 1.1 wt.% N
Sintered in 90N2-9H2-1CH4
• Discrepancies in Solidus
temperature:
– Calculated: 1136 ºC
– Experimental: 1156 ºC
• Correct prediction of
microstructure (sintering at
OST=1210 ºC): carbides
Measured N: 1.14 wt.%

16. Change in carbide morphology with C
content
M42HVIG + 0.4% C M42HVIG + 0.7% C
• Higher C content -> change in morphology of
bright carbides
• Evolution from cubic M6C to hexagonal M2C

17. Carbides composition in M42HVIG

18. Recalculated diagram for M42HVIG
• Solidus temperature:
– Calculated old: 1136 ºC
– Calculated new: 1158 ºC
– Experimental: 1156 ºC
• 1: intersection liquidus/M2C
• 2: peritectic
L+M6C = M2C
• 1 to 2: increase in C content:
– Increases M2C, not C in fcc
– No reduction in Tsol
• Mo stabilizes M2C

19. Recalculate phase diagram for M35MHV
considering M2C
Original isopleth New isopleth, with M2C

20. Why is M2C observed in M42HVIG and
not in M35MHV?
• Mo and W have a similar role in
HSS: formation of M6C carbide
• Equivalent Mo content:
M Mo
we (Mo) w(Mo) w( W )
MW
• Driving force for precipitation of
M2C at 1150 ºC
• Constant equivalent Mo:
we(Mo) = 10%
• High positive value: less stable

21. Conclusions
• For well-known systems: calculations with
few phases
• Computer-aided design of HSS: accurate
selection of phases involved
• Sintering behaviour of well-studied systems
should not be automatically extrapolated for
new compositions

22. DEVELOPMENT OF MASTER
ALLOYS FOR P/M
T. Gómez-Acebo, M. Sarasola and F. Castro, “Systematic search of low melting
point alloys in the Fe-Cr-Mn-Mo-C system”. Calphad, 27 (3) 325-334 (2003).
doi:10.1016/j.calphad.2003.12.001

23. Master alloys
• Pre-alloyed powders added to promote
densification
• In liquid phase sintering: liquid formation at
“low” temperatures
• Enhances diffusion of chemical elements
• Alloy design: systematic search of low
melting point alloys
• Study of liquidus surface, liquidus
monovariant lines

24. Projections of liquidus monovariant
lines
“Top” view
“Side” view

25. Ternary Al-Mg-Zn
Red arrow:
lowest eutectic
temperature
Liquidus surface: Two projections of the liquidus monovariant
projection onto the lines of the Al-Mg-Zn system onto
composition axis. temperature-composition planes. Minimum
liquidus temperature: 338 ºC for 3.97Al-
49.0Mg-47.0Zn (in wt-%).

26. Ternary Al-Mg-Zn
0
340ºC
HEAT FLOW (W/g)
-1 -phase
and
MgZn
-2
Mg
-3
250 300 350 400 450
T (ºC)
DSC analysis of an SEM micrograph of the Al-Mg-Zn
experimentally obtained alloy alloy with minimum liquidus
with composition close to that temperature, showing the
with minimum liquidus identified phases.
temperature.

27. Ternary Al-Cu-Mg
Projections of the liquidus monovariant lines.
Liquidus surface
0
Minimum liquidus temperature: 425 ºC for
428ºC
32.5Al-4.29Cu-63.2Mg (in wt-%).
HEAT FLOW (W/g)
-1
AlMg- hcp(Mg)
-2 Q-phase
-3
350 400 450 500
T (ºC)
DSC analysis alloy with
composition close to that with
minimum liquidus temperature.

28. Quaternary Al-Cu-Mg-Zn
Projections of the liquidus monovariant lines onto the temperature-
composition planes for part of the quaternary Al-Cu-Mg-Zn system. Cu
additions to the ternary do not reduce the liquidus temperature of the Al-Mg-
Zn eutectic.

29. Ternary Fe-Mn-C

30. Ternary Fe-Mn-C

31. Ternary Fe-Mn-C
DSC and TG analyses of an Optical micrograph of the C-Fe-Mn
experimentally obtained alloy with alloy with minimum liquidus
composition close to that with minimum temperature, showing eutectic
liquidus temperature structure of fcc+M3C.

32. Quinary C-Cr-Fe-Mn-Mo system
• Quaternary C-Fe-
Mn-Mo system.
• “1”: eutectic with
lowest T:
– 1309 K (1036 ºC)
– Fe-4C-21Mn-
10Mo
• Quinary C-Cr-Fe-Mn-Mo
system.
• Cr additions to the
quaternary do not
reduce the liquidus
temperature of the
eutectic.

33. Note on calculation of liquidus
monovariant lines in multicomponent
systems
• With Thermo-Calc, currently a 5-dimension diagram
can be calculated.
– The first two axis variables can be any property
considered as a condition (i.e. composition of two
components)
– The other axes have to be potentials (temperature
and activity of the other components).
• The calculation proceeds when the diagram is
calculated starting from an invariant point.
• Extremely sensitive to starting point of calculation.

34. Binary Mn-Ni
• Intermediate phases not included in databases

35. Conclusions
• Calculation like those presented here allow
the systematic search of liquid phases in the
whole composition range.
• Projections onto a temperature vs
composition plane allow easy identification of
multicomponent eutectic points.
• Experimentally obtained alloys in the Al-Mg-
Zn, Al-Cu-Mg and Fe-Mn-C ternary systems
have allowed verification of the theoretical
predictions for the eutectic temperatures.

36. LIFE ESTIMATION OF GAS
TURBINE OVERLAY COATINGS
T. Gómez-Acebo, B. Navarcorena and F. Castro, “Interdiffusion in multiphase, Al-
Co-Cr-Ni-Ti diffusion couples”. Journal of Phase Equilibria and Diffusion, 25 (3) 237-
251 (2004). http://dx.doi.org/10.1007/s11669-004-0112-y

37. Introduction
• GT blades: coatings of
oxidation-resistant alloys:
– MCrAlY: M=Ni,Co,Fe
– Pt-Aluminides
• Life of the coating: loss of
oxidation resistance

38. Introduction
• Coating: -fcc + -B2
– : bond coat (diffusion)
– : Al reservoir
• Loss of oxidation resistance: Al
– Oxidation: growth of oxide layer
– Spallation: loss of oxide layer
– Inward diffusion of Al
– Outward diffusion of Ni etc:
depletion of .

39. Objectives
• Diffusion in ternary and multicomponent Al-
Co-Cr-Ni-Ti alloys
• Review of thermodynamic and kinetic data
• Lifetime estimation of MCrAlY coatings

40. Materials and experimental procedure
Alloy Preparation Diffusion couples
• Mixture of high-purity metals: • Al-Co-Cr /
Al, Co, Cr, Ni, Ti. + /
• Uniaxially pressed at 400 MPa. • Al-Co-Ni / + ’
• Furnace melt at Tliq+200 K in Ar. • Al-Co-Cr-Ni + /
• Homogenisation 3h, 1100 ºC in • Al-Co-Cr-Ni-Ti + /
Ar. + / +Ni3Ti
• Diffusion annealing: 1100 ºC,
24-72 h
• Diffusion profiles: EDAX

41. Thermodynamic description
• TCNI1 database [N. Dupin and B. Sundman, "A thermodynamic
database for Ni-base superalloys", Scan. J. Metall., 30, 184-192
(2001)].
• All binaries assessed
• Assessed ternaries:
– Al-Co-Ni
– Al-Cr-Ni
– Al-Cr-Ti
– Al-Ni-Ti
– Cr-Ni-Ti
• Non-assessed ternaries:
– Al-Co-Cr
– Al-Co-Ti
– Co-Cr-Ni
– Co-Cr-Ti
– Co-Ni-Ti

42. Thermodynamic data of Al-Co-Cr
Calculations from
the three binaries
(no ternary
parameters)
Experimental data
[K. Ishikawa et al,
"Phase equilibria
and stability of the
BCC aluminide in
the Co-Cr-Al
system", Ber.
Bunsenges. Phys.
Chem., 102, 1206-
1210 (1998)].
Unrealistic data for
solvus line / +

43. Al-Co-Cr alloys
Co-5.0Al-25.7Cr
f =0.06 (meas.)
f =0.03 (calc.)
Co-6.0Al-27.9Cr
f =0.24 (meas.)
f =0.23 (calc.)
 Calculations from the binaries
(no ternary parameters)
 Good agreement for +
region Co-7.7Al-32.0Cr
 GT29: a commercial MCrAlY f =0.59 (meas.)
coating: Co-6Al-29Cr-[0.5Y] f =0.51 (calc.)

44. Kinetic description
• Ni-database [C. E. Campbell, W. J. Boettinger, and U. R.
Kattner, "Development of a diffusion mobility database for Ni-
base superalloys", Acta Mat., 50, 775-792 (2002)].
• Assessed sub-systems:
– Al-Cr  Non assessed sub-systems:
– Al-Ni • Al-Co
• Co-Cr
– Al-Ti
• Co-Ti
– Co-Ni • Cr-Ti
– Cr-Ni • Other ternary sub-systems
– Ni-Ti
– Al-Cr-Ni  Diffusion only in -fcc phase
– Al-Ni-Ti

45. Kinetic description (fcc phase)
M i0 Qi 1 Qi*
• Atomic mobilities: Mi exp exp
RT RT RT RT
Qi* Qi RT ln( M i0 )
• Redlich-Kister polynomials:
Qi* x j Qi j xp x j k
Aipj ( x p x j )k
j p j p k
Ti Ni
• Accepted approximations: QAl QAl
Cr Cr
QCo Q Ni
Ti Co
QCo QCo
Ti Ni
QCr QCr
Al Co Ti Ni
QTi QTi QTi QTi
Cr Cr
QTi QAl
Ti Ni
Q Ni Q Ni
Co Fe
QAl 5QFe

46. Diffusion in Al-Co-Cr ( / couples)
C1: Co-4.2Al / Co-8.9Cr C2: Co-4.0Al / Co-14.1Cr
1100 ºC, 72 h 1100 ºC, 72 h

47. Diffusion in Al-Co-Cr ( + / couples)
C3: Co-8.2Al / Co-11.1Cr C4: Co-9.1Al / Co-17.0Cr C5:Co-10.0Al / Co-30.0Cr
1100 ºC, 72 h 1100 ºC, 72 h 1100 ºC, 72 h

48. Diffusion in Al-Co-Cr ( + / couples)
Original interface
C4: Co-9.1Al / Co-17.0Cr
Regression of phase + <

49. Diffusion in Al-Co-Ni ( / + ’ couples)
C6: Ni-5Al-30Co / Ni-10Al-23.3Co
1100 ºC, 48 h
> + ’
Original interface

50. Diffusion in Al-Co-Cr-Ni ( + / couples)
C9: Co-5Al-25.7Cr / Ni-6.5Al-40.8Co
1100 ºC, 72 h
+ <
C10: Co-6Al-27.9Cr / Ni-5.5Al-39.8Co
1100 ºC, 72 h
+ <
C11: Co-7.7Al-32Cr / Ni-5Al-38Co
1100 ºC, 72 h
+ <

51. Diffusion in Al-Co-Cr-Ni ( + / couples)
C9: Co-5Al-25.7Cr C10: Co-6Al-27.9Cr C11: Co-7.7Al-32Cr
/ Ni-6.5Al-40.8Co / Ni-5.5Al-39.8Co / Ni-5Al-38Co

52. Diffusion in Al-Co-Cr-Ni ( + / couples)
C9: Co-5Al-25.7Cr
/ Ni-6.5Al-40.8Co
C10: Co-6Al-27.9Cr
/ Ni-5.5Al-39.8Co
Regression of phase
C11: Co-7.7Al-32Cr
/ Ni-5Al-38Co

53. Diffusion in Al-Co-Cr-Ni-Ti ( + / )
Original interface
C13: Co-6.5Al-28Cr / Ni-5.5Cr-8.9Ti-1.4Al C12: Co-6.7Al-29.4Cr / Ni-20Cr-6Ti
1100 °C, 72 h 1100 °C, 72 h
+ < + ’] + < + Ni3Ti]

54. Diffusion in Al-Co-Cr-Ni-Ti ( + / )
C13: Co-6.5Al-28Cr /
Ni-5.5Cr-8.9Ti-1.4Al
1100 ºC, 72 h
+ < + ’]
C12: Co-6.7Al-29.4Cr
/ Ni-20Cr-6Ti
1100 ºC, 72 h
+ < + Ni3Ti]

55. Lifetime estimation of GT coatings
• Al loss is due to three factors:
– Interdiffusion
– Oxidation
– Spallation (only in discontinuous
operation)
• Highest rate: interdiffusion
• Possible criteria for lifetime estimation of the coating:
– Loss of phase in the coating
– Depletion of phase in the coating surface
– Depletion of Al in the coating surface: formation of a stable
oxide

56. Lifetime estimation of GT coatings
Depletion of -phase
1150
GT29 100/200 m 1100
Temperature (ºC)
1050
CMSX-4 4 mm 1000
950
900 100 m
200 m
850
1 month 1 year 10 years
800
1.E+05 1.E+06 1.E+07 1.E+08 1.E+09
time (s)

57. Conclusions
• Review of thermodynamic and kinetic data
– Al-Co-Cr: good predictions from the binaries
– Kinetic data needed for Co
• Analysis of the diffusion paths:
– Good prediction for Al
– Good prediction for depletion of -phase
– Other elements (Cr, Ni): not satisfactory
predictions
• Lifetime estimation of the coatings:
interdiffusion

58. Future CALPHAD conferences