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Segregation and partitioning phenomena at phase boundaries of complex steels are important for their microstructural, mechanical, and kinetic properties. We present nanoscopic atom probe tomography results across martensite/austenite phase boundaries in a precipitation-hardened maraging TRIP steel after aging at 450°C for 48 hours (12.2 at.% Mn, 1.9 at.% Ni, 0.6 at.% Mo, 1.2 at.% Ti, 0.1 at.% Si, 0.3 at.% Al, 0.05 at.% C). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched ~2.1 and 1.2 times, respectively, relative to the average matrix content. In contrast, Ti is depleted ~6.9 times relative to the average content, Al ~6.6 times, Mo ~2.0 times, and Fe ~1.2 times. The strong accumulation of Mn at the interfaces is of particular interest as it strongly affects the transformation equilibrium and kinetics in steels. We observe up to 27 at. % Mn in a 20 nm thick layer at the martensite/austenite phase boundary. This can be explained by a large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn-flux towards austenite. The low diffusivity in austenite does not allow accommodation of this flux within the matrix. Consequently, the phase boundary moves towards martensite with a Mn-composition given by the local equilibrium condition. This interpretation relies on diffusion calculations performed with the method DICTRA. A mixed-mode approach involving finite interface mobility was also applied to refine the agreement with the experiments. In order to achieve a good agreement the diffusivity in martensite had to be increased compared to ferrite. This can be attributed to a high defect density.
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Düsseldorf, [email protected]
MS&T‘10 Conference 18. Oct. 2010 Houston, USA
D. Raabe, D. Ponge, O. Dmitrieva, J. Millán, P. Choi, G. Inden
Ultrahigh strength maraging-TRIP steels
Motivation and alloy design
Mechanical properties
Microstructure
Mn partitioning and simulations
Nano-precipitates
Conclusions
Overview
2www.mpie.de
Dierk Raabe ([email protected])
3
Motivation: Combine TRIP and maraging effects
Mn is among the most important alloying elements for the design of advanced high strength steels
It affects the stabilization of the austenite, the stacking fault energy, and the transformation kinetics
Mn has very low diffusion rates in the austenite and a high segregation or respectively partitioning tendency at interfaces
This context makes Mn a very interesting candidate for an atomic-scale study of compositional changes across austenite/martensite interfaces.
Dierk Raabe ([email protected])
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tota
l elo
ngatio
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ultimate tensile strength [MPa]
TRIP and complex phaseTRIP and complex phase
martensiticmartensitic
Maraging-TRIPand advanced QPMaraging-TRIPand advanced QP
dual phasedual phase
ferriticferritic
Motivation: Combine TRIP and maraging effects
steels with very good formabilitysteels with very good formability steels with extreme strength and acceptable formabilitysteels with extreme strength and acceptable formability
austenitic stainlessaustenitic stainless
advanced TWIP and TRIP
advanced TWIP and TRIP
Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
Dierk Raabe ([email protected])
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Motivation: Combine TRIP and maraging effects
The material studied here is a precipitation-hardened alloy that is referred to as maraging TRIP steel
It combines the TRIP mechanism with the maraging effect (maraging: martensite aging)
The TRIP effect exploits the deformation-stimulated transformation of metastable austenite into martensite and the resulting plasticity required to accommodate the transformation misfit
The maraging effect uses the hardening of the heavily strained martensite through the formation of nano-sized intermetallic precipitates during aging heat treatment
The maraging TRIP steels used in this work reveal the surprising property that both strength and total elongation increase upon aging reaching an ultimate tensile strength of nearly 1.3 GPa at an elongation above 20%
Dierk Raabe ([email protected])
Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
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Fe-Mn based maraging TRIP steel development
TRIP: deformation-stimulated transformation of instable austenite into martensite and accommodation plasticity (e.g. Mn, Ni, low C)
Maraging effect: hardening of heavily strained martensite via nano-sized (intermetallic) precipitates (Ni, Al, Ti, Mo)
(see also conventional Maraging steels)
* TRIP: transformation-induced plasticity* Maraging: martensite aging
Raabe, Ponge, Dmitrieva, Sander: Adv. Eng. Mat. 11 (2009) 547
Quenched austenite: ductile low carbon martensiteRetained austenite (TRIP)Controlled precipitation hardening
What is maraging-TRIP ?
Dierk Raabe ([email protected])
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Low carbon: ductile martensite
Steel C Ni Co Mo Ti Al Mn Fe
Maraging 0.01 18 12 4 1.6 0.15 0.05 Balance
09MnPH 0.01 2 - 1 1.0 0.15 9 Balance
12MnPH 0.01 2 - 1 1.0 0.15 12 Balance
15MnPH 0.01 2 - 1 1.0 0.15 15 Balance
Precipitation Hardenable
Mn (+Ni): austenite (TRIP)
Compositions in mass%
PH
PH
PH
D. Raabe et al. Scripta Materialia 60 (2009) 1141
Martensite aging after quenching at 450°CDierk Raabe ([email protected])
Overview
www.mpie.de 8
Motivation and alloy design
Mechanical properties
Microstructure
Mn partitioning and simulations
Nano-precipitates
Conclusions
Dierk Raabe ([email protected])
0 5 10 15 20 250
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Eng
inee
ring
Str
ess
(MP
a)
Engineering Strain (%)
0 5 10 15 20 250
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ess
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0 5 10 15 20 250
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inee
ring
Str
ess
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a)
Engineering Strain (%)
Maragingaged
(450°C/48h)
quenched
maraging-TRIP, 12MnPH
aged (450°C/48h)
quenched
Tensile tests
(X3NiCoMoTi18-12-4)
higher strengthAND
higher elongation
Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
Dierk Raabe ([email protected])
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Tensile tests, maraging TRIP
FCCBCCFCCBCC
e=0%e=0% e=15%e=15%
Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
Dierk Raabe ([email protected])
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Tensile tests, maraging TRIP
Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
Overview
www.mpie.de 12
Motivation and alloy design
Mechanical properties
Microstructure
Mn partitioning and simulations
Nano-precipitates
Conclusions
Dierk Raabe ([email protected])
13
Microstructure hierarchy
Dmitrieva et al., Acta Mater, in press 2010
Dierk Raabe ([email protected])
Overview
Calcagnotto et al. Mater. Sc. Engin. A 527 (2010) 2738 14
Motivation and alloy design
Mechanical properties
Microstructure
Mn partitioning and simulations
Nano-precipitates
Conclusions
Dierk Raabe ([email protected])
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R. Kainuma, M. Ise, K. Ishikawa, I. Ohnuma, and K. Ishida, Phase Equilibria and Stability of the B2 Phase in the Ni-Mn-Al and Co-Mn-Al Systems, J. Alloys Compd., 1998, 269, p 173-180
Ni-Mn-Al isothermal section at 850 °CNi-Mn-Al isothermal section at 850 °C
Ni Mn
Al
Dierk Raabe ([email protected])
Mn atomsNi atomsMn iso-concentration surfaces at 18 at.%
APT results: Atomic map (12MnPH aged 450°C/48h)
70 million ionsLaser mode (0.4nJ, 54K)
Dmitrieva et al., Acta Mater, in press 2010
Martensite decorated by precipitations
Austenite
?
?
Dierk Raabe ([email protected])
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M A
Mn layer 1Mn layer 2
Mn layer2Mn layer 1
Mn iso-concentration surfaces at 18 at.%
Thermo-Calc
Phase equilibrium Mn-contents:
27 at. % Mn in austenite (A)
3 at. % Mn in ferrite (martensite) (M)
1D profile: step size 0.5 nm
M A M
depletion zonenominal 12 at.% Mn
APT results: chemical profiles
Dmitrieva et al., Acta Mater, in press 2010 17Dierk Raabe ([email protected])
18
precipitates in a`
no precipitates in
12MnPH after aging (48h 450°C)
nmDtxDiff 302
nmxDiff 2
Raabe, Ponge, Dmitrieva, Sander: Adv. Eng. Mat. 11 (2009) 547
Mean diffusion path of Mn in austenite
(aging 450°C/48h) 2 nm
M A
Mn layer 1Mn layer 2
nominal 12 at.%
Thermo-Calc
Phase equilibrium Mn content:
27 at. % in austenite
3 at. % in ferrite (martensite)
10 nm
Ti, Si, Mo
Mn-rich layer
AMPB migration
Mn diffusion
phase boundary
aging
Newaustenite
(formed during aging)
DICTRA
AM
original positionphase boundary
final positionphase boundary
APT results and simulation: DICTRA/ThermoCalc
Dmitrieva et al., Acta Mater, in press 2010 19Dierk Raabe ([email protected])
Overview
www.mpie.de 20
Motivation and alloy design
Mechanical properties
Microstructure
Mn partitioning and simulations
Nano-precipitates
Conclusions
Dierk Raabe ([email protected])
2121
2 nm
12 wt.% Mn maraging-aged (48 h, 450°C), TEM
APT Characterization
Iso-concentration surface at 14 at.% Ni
450°C/0.5h
10 nm
Ni
Fe
450°C/6h
10 nm
Ni
Fe
www.mpie.de 22
APT Characterization
10 nm
450°C/48h
Iso-concentration surface at 14 at.% Ni
450°C/192h
Ni
Fe
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Dierk Raabe ([email protected])
23
48h 192h
0.5 hours 6 hours 48 hours 192 hours
Volume fraction 0.06% 0.8% 1.5% 4.3%
Number density of particles (m-3) 4.8x1022 7.8x1023 3.6x1024 1.9x1024
Mean diameter (nm) 2.7 ± 0.9 2.5 ± 0.7 4.7 ± 0.7 6.1 ± 2.2
6hAging time: 0.5h
APT Characterization
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Dierk Raabe ([email protected])
24
Overview
25
Motivation and alloy design
Mechanical properties
Microstructure
Mn partitioning and simulations
Nano-precipitates
Conclusions
Raabe, Ponge, Dmitrieva, Sander: Adv. Eng. Mat. 11 (2009) 547
Dierk Raabe ([email protected])
Conclusions
Maraging-TRIP as a new GPa steel design approach
Unexpected simultaneous increase in strength and elongation
Mn partitioning, predicted by ThermoCalc/DICTRA
Austenite stability predicted using ab initio methods
Heusler phase nano-precipitates
Next steps: lean composition, alloy variants, higher strength, partitioning, nano-precipitates
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Mn atomsNi atoms
Mn iso-concentration surfaces at 18 at.%
martensite with
precipitates
martensite with precipitates
70 million ionsLaser mode (0.4nJ, 54K)
martensite with
precipitates
austenite
Raabe, Ponge, Dmitrieva, Sander: Adv. Eng. Mat. 11 (2009) 547
Dierk Raabe ([email protected])
