Computational Thermodynamics

applied to powder metallurgy

Dr. Tomás Gómez-Acebo

Seminar in Computational Thermodynamics & Kinetics

with Thermo-Calc Software

Madrid, 1-2 June, 2010

• The fundamentals: assessments

• Composition tuning of a high speed steel

• Development of master alloys for powder

metallurgy

• Life of Gas Turbine coatings

Outline

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

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

Thermodynamic models: Gibbs energy

G x G x G RT x x x x Gm

E

mAg Ag Zn Zn Ag Ag Zn Znln ln

E

mG x x L L x x L x xAg Zn Ag,Zn Ag,Zn Ag Zn Ag,Zn Ag Zn

0 1 22

Gref (mechanical

mixture)

Sid (configurational

entropy)

Gid (ideal solution)

Excess Gibbs energy: Redlich-Kister polynomials

Each phase is modelled separatedly

• 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

Summary of Ag-Zn assessment

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

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

M2 HSS: validation of database (SSOL2)

Experimental DT analysis Reported OST

M35MHV: identification of the stable

phases

All phases in SSOL

Including gas, MC=(Mo,W)C

Only observed phases: L, bcc ( ),

fcc ( and MX), M6C, M3C, Fe2MoC ( )

M35MHV: effect of C and NFe–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

Narrow sintering window

7000 ppm N

Wider sintering window

M42HVIG vs M35MHV

• New experimental HSS: M42HVIG

• Reasonable to consider the same set of

phases, rejecting the remaining phases:

– L, bcc ( ), fcc ( and

MX), M6C, M3C, Fe2MoC ( )

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

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.%

• Higher C content -> change in morphology of

bright carbides

• Evolution from cubic M6C to hexagonal M2C

Change in carbide morphology with C

content

M42HVIG + 0.4% C M42HVIG + 0.7% C

Carbides composition in M42HVIG

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

Recalculate phase diagram for M35MHV

considering M2C

Original isopleth New isopleth, with M2C

W

Mo)W()Mo()Mo(M

Mwwwe

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:

• Driving force for precipitation of

M2C at 1150 ºC

• Constant equivalent Mo:

we(Mo) = 10%

• High positive value: less stable

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

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

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

Projections of liquidus monovariant

lines

“Top” view

“Side” view

Ternary Al-Mg-Zn

Liquidus surface: projection onto the composition axis.

Two projections of the liquidus monovariant lines of the Al-Mg-Zn system onto temperature-composition planes. Minimum liquidus temperature: 338 ºC for 3.97Al-49.0Mg-47.0Zn (in wt-%).

Red arrow: lowest eutectic temperature

Ternary Al-Mg-Zn

SEM micrograph of the Al-Mg-Zn alloy with minimum liquidus temperature, showing the identified phases.

Mg

-phase

and MgZn

DSC analysis of an experimentally obtained alloy with composition close to that with minimum liquidus temperature.

-3

-2

-1

0

250 300 350 400 450

T (ºC)

HE

AT

FL

OW

(W

/g)

340ºC

Liquidus surfaceProjections of the liquidus monovariant lines. Minimum liquidus temperature: 425 ºC for 32.5Al-4.29Cu-63.2Mg (in wt-%).

Ternary Al-Cu-Mg

-3

-2

-1

0

350 400 450 500

T (ºC)

HE

AT

FL

OW

(W

/g) 428ºC

AlMg- hcp(Mg)

Q-phase

DSC analysis alloy with composition close to that with minimum liquidus temperature.

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.

Ternary Fe-Mn-C

Ternary Fe-Mn-C

Ternary Fe-Mn-C

DSC and TG analyses of an experimentally obtained alloy with composition close to that with minimum liquidus temperature

Optical micrograph of the C-Fe-Mn alloy with minimum liquidus temperature, showing eutectic structure of fcc+M3C.

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.

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.

Binary Mn-Ni

• Intermediate phases not included in databases

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.

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

• GT blades: coatings of

oxidation-resistant alloys:

– MCrAlY: M=Ni,Co,Fe

– Pt-Aluminides

• Life of the coating: loss of

oxidation resistance

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 .

Introduction

• Diffusion in ternary and multicomponent Al-

Co-Cr-Ni-Ti alloys

• Review of thermodynamic and kinetic data

• Lifetime estimation of MCrAlY coatings

Objectives

Alloy Preparation

• Mixture of high-purity metals:

Al, Co, Cr, Ni, Ti.

• Uniaxially pressed at 400 MPa.

• Furnace melt at Tliq+200 K in Ar.

• Homogenisation 3h, 1100 ºC in

Ar.

Diffusion couples

• Al-Co-Cr /

+ /

• Al-Co-Ni / + ’

• Al-Co-Cr-Ni + /

• Al-Co-Cr-Ni-Ti + /

+ / +Ni3Ti

• Diffusion annealing: 1100 ºC,

24-72 h

• Diffusion profiles: EDAX

Materials and experimental procedure

• 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

Thermodynamic description

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 / +

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.)

Co-7.7Al-32.0Cr

f =0.59 (meas.)

f =0.51 (calc.)

Calculations from the binaries

(no ternary parameters)

Good agreement for +

region

GT29: a commercial MCrAlY

coating: Co-6Al-29Cr-[0.5Y]

• 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

– Al-Ni

– Al-Ti

– Co-Ni

– Cr-Ni

– Ni-Ti

– Al-Cr-Ni

– Al-Ni-Ti

Kinetic description

Non assessed sub-systems:• Al-Co• Co-Cr• Co-Ti• Cr-Ti• Other ternary sub-systems

Diffusion only in -fcc phase

• Atomic mobilities:

• Redlich-Kister polynomials:

• Accepted approximations:

Kinetic description (fcc phase)

RT

Q

RTRT

Q

RT

MM iii

i

*0

exp1

exp

)ln( 0*

iii MRTQQ

p pj k

k

jp

pj

i

k

jp

j

j

iji xxAxxQxQ )(*

Ni

Al

Ti

Al QQ

Cr

Ni

Cr

Co QQ

Co

Co

Ti

Co QQ

Ni

Cr

Ti

Cr QQ

Ni

Ti

Ti

Ti

Co

Ti

Al

Ti QQQQ

Cr

Al

Cr

Ti QQ

Ni

Ni

Ti

Ni QQ

Fe

Fe

Co

Al 5QQ

Diffusion in Al-Co-Cr ( / couples)

C1: Co-4.2Al / Co-8.9Cr

1100 ºC, 72 hC2: Co-4.0Al / Co-14.1Cr

1100 ºC, 72 h

Diffusion in Al-Co-Cr ( + / couples)

C3: Co-8.2Al / Co-11.1Cr

1100 ºC, 72 h

C4: Co-9.1Al / Co-17.0Cr

1100 ºC, 72 h

C5:Co-10.0Al / Co-30.0Cr

1100 ºC, 72 h

Diffusion in Al-Co-Cr ( + / couples)

Regression of phase

Original interface

C4: Co-9.1Al / Co-17.0Cr

+ <

Diffusion in Al-Co-Ni ( / + ’ couples)

C6: Ni-5Al-30Co / Ni-10Al-23.3Co

1100 ºC, 48 h

> + ’

Original interface

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

+ <

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

C11: Co-7.7Al-32Cr

/ Ni-5Al-38Co

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

C11: Co-7.7Al-32Cr

/ Ni-5Al-38Co

Regression of phase

Diffusion in Al-Co-Cr-Ni-Ti ( + / )

Original interface

C12: Co-6.7Al-29.4Cr / Ni-20Cr-6Ti

1100 °C, 72 h

+ < + Ni3Ti]

C13: Co-6.5Al-28Cr / Ni-5.5Cr-8.9Ti-1.4Al

1100 °C, 72 h

+ < + ’]

Diffusion in Al-Co-Cr-Ni-Ti ( + / )

C12: Co-6.7Al-29.4Cr

/ Ni-20Cr-6Ti

1100 ºC, 72 h

+ < + Ni3Ti]

C13: Co-6.5Al-28Cr /

Ni-5.5Cr-8.9Ti-1.4Al

1100 ºC, 72 h

+ < + ’]

• 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

Lifetime estimation of GT coatings

Lifetime estimation of GT coatings

GT29

CMSX-4

100/200 m

4 mm

Depletion of -phase

800

850

900

950

1000

1050

1100

1150

1.E+05 1.E+06 1.E+07 1.E+08 1.E+09

time (s)

Te

mp

era

ture

(ºC

)

100 m

200 m

1 month 1 year 10 years

• 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

Conclusions

Future CALPHAD conferences