thermodynamic database me-fe1.3 release notes

Thermodynamic database ME-Fe1.3 RELEASE NOTES

Database Manager: Erwin Povoden-Karadeniz [email protected]

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ME-Fe1.3 database contains edited and newly assessed thermodynamic data for steel alloys. It is validated for various thermokinetic simulations in various steel systems (e.g micro-alloyed steels, tool steels, 9-12% Cr steels, austenitic stainless steels, PH maraging steels), as well as solidification modeling (Scheil-Gulliver calculation). More sophisticated applications of the database include the description of coupled nitride/sulfide precipitates in micro-alloyed steels, formation of carbon clusters in tempered martensite or precipitation of g’ (gamma prime) phase in Ti-alloyed irradiated austenitic steels. Composition range used for database validation

Element Max. content

[wt.%] Element

Max. content

[wt.%] Element

Max. content

[wt.%]

Al 10 Mn 25 S 0.5

B 1 Mo 5 Si 5

C 2 N 1 Ti 0.5

Co 3 Nb 1 V 0.5

Cr 25 Ni 26 W 3

Cu 3 O 0.5 Y 0.5

Hf 0.5 P 0.1

La 0.5 Pd 4

Major updates compared to the previous version (ME-Fe1.2) include:

• Improved descriptions of fcc, bcc and liquid phases for TRIP steel applications.

• Assessment of Cu-solubilities in B2-NiAl and Eta-Ni3Ti phases for multi-component

maraging steel grades

• Improved modeling of G-phase in stainless steels • Optimized phase stabilities in Mo-containing 9 to 12% Cr-steels • Assessment of Ta-solubility in MX carbo-nitride and Z-phase



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• Improved sigma-phase description

Improved descriptions of fcc, bcc and liquid phases for TRIP steel applications

Precise knowledge of the peritectic composition and temperature and the austenite and ferrite stability limits are relevant for the processing of TRIP steels. The new ME-Fe database version contains an improved descriptions of fcc, bcc and liquid phases in the system Fe-Si-Mn-Al-C. With this improvement, phase stabilities from low-Si and -Mn to high-Si / high-Mn (validated up to 3wt.% Mn and 3wt.% Si) steel are correctly described.

Comparison of experimental phase boundaries and calculation with ME-Fe1.3 in Fe bal.-2Si-2Mn-C (left side).

Comparison of experimental phase boundaries with different calculations, using ME-FE1.3, Thermocalc TCFE8 and “Iron Alloy Database” (Miettinen2019) in Fe bal.-2Si-2Mn-C (right side)

The reproduction of both tested materials with ME-Fe1.3 is excellent. In fact, in particular the peritectic reaction as well as liquidus line can also be reproduced appropriately by Miettinen 2019 (IAD), however their model for matrix phases lack physicality due to the simplification of carbon distributed among the substitutional crystallographic sublattice.



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Assessment of Cu-solubilities in B2-NiAl and Eta-Ni3Ti phases for multi-component maraging steel grades

Cu-solubilities in B2-NiAl, gamma-prime Ni3Al-based and eta Ni3Ti-based solid solutions are described by assessment completion of the ternary systems Al-Cu-Ni and Ni-Cu-Ti. Moreover, the Al-Ni-Ti system description has been completed in ME-Fe1.3. This allows for simulations of competitive precipitate evolution in maraging steel grades with varying alloying in the system Fe-Ni-Cr-Al-Ti-Cu with ME-Fe1.3.

Comparison of experimental Al-Ni-Ti phase diagram (Huneau1999, left side) and computed Al-Ni-Ti (ME-Fe1.3, right side), at 1173K.

Comparison of previous Al-Cu-Ni assessment (Wang2019, left side) and computed Al-Cu-Ni (ME-Fe1.3, right side), at 1273K showcase.



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In fact, the full solid solution B2-bcc Cu3Al, as proposed by Wang2019 is not supported by thermodynamics of forming compounds. Huge non-idealities by excess mixing energies of bcc (e.g.: first order Al,Cu,Ni interaction -518491.506+213.890T) were required by Wang2019 to obtain the phase diagram, together with stabilized B2-AlCu compound energy compared to first-principles analysis. With ME-Fe1.3, a description with less fitting is preferred, resulting in the phase diagram above. Moreover, this approach leads to best reproduction of B2-thermodynamics in the Al-Cu-Ni extension:

Comparison of B2-thermodynamics, reported in previous Al-Cu-Ni assessment (Wang2019, left side) and computed data with ME-Fe1.3 and calorimetric data (Hu2007). The overall reproduction of experimental data is best with ME-Fe1.3.

With the implementation of assessed subsystems in ME-Fe, extensions to maraging steel grades are realized. As a model example, resulting phase stabilities of a complex alloy Fe bal. –12Cr–10Ni–1Al-0.5Ti-1.5Cu-1Mo are presented in the following equilibrium phase fraction plot computed with ME-Fe1.3.



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Predicted thermodynamic equilibrium phase stabilities in maraging steel.

In addition to Cu-fcc (“FCC_A1#01), Cr-bcc (“BCC_A2#01), B2-phase (“BCC_B2”) and Ni3Al-based gamma-prime, intermetallic Fe-Cr-Mo-Ni Laves-phase solid solution is relevant. Low-temperature Ni-Fe fcc-phase with almost 50:50 (at.%) composition is stable. The Cu-solubility in gamma-prime is negligible. Instead, Cu has some preference to dissolve in B2-ordered NiAl, even though indeed the mayor part of Cu forms 95% pure fcc-Cu (with some minor solubilities of Ni, Al and Mo). Eta-Ni3Ti is not stable in the computed steel. Only at 1.5 wt.% Ti, Eta-Ni3Ti appears at relatively low temperature around 400°C.



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Predicted thermodynamic equilibrium phase stabilities in maraging steel with increased Ti-alloying (1.5 wt.% Ti).High-T “B2” is actually almost completely disordered, i.e. delta-ferrite.

Cu-solubility in different intermetallic Al-Ni-Ti phases in maraging steel with increased wt.% Ti=1.5 as function of temperature. Cu-solubility is preferred most in B2 and insignificant in gamma-prime.



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Improved modeling of G-phase in stainless steels

Ni16Nb6Si7-based G-phase (G_PHASE) stabilities in ternary subsystems have been revised, leading to improvement of its computed stability and composition in multicomponent stainless steel grades. In this framework, a recent long-term aging (20 years) study (Abbasi2019) documents the importance of local Si-enrichment for G-phase stabilization at 700°C aging, but destabilization at 900°C. Typically, Si is locally increased to approx.. 1 wt.% relative to 0.8 wt.% alloyed to the studied steel grade with the composition Fe bal.-0.41C-25Cr-34.8Ni-0.8Si-0.7Mn-0.8Nb-0.1Mo-0.12Ti-0.07V-0.04N (wt.%).

Computed phase stabilities in Fe bal.-0.41C-25Cr-34.8Ni-1Si-0.7Mn-0.8Nb-0.1Mo-0.12Ti-0.07V-0.04N (wt.%).



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With ME-Fe1.3 we obtain G-phase up to 800°C for 1wt.% S, which reproduces the observation. The computed G-phase dissolution for 0.8 wt.% Si is at 750°C. This agrees well with the reported Si-effect on G-phase stability.

at.% exp. (Abbasi2019) ME-Fe1.3 ME-FE1.2 Cr 4.3 3.64 0.48 Ni 52.6 49.2 55.2 Si 16.1 24.1 24.1 Nb 19.7 15.3 18.3 Fe 7.3 6 1.9 Mo not measured 1.65 --- Ti --- 7.50E-05 9.00E-04

The computed composition of G-phase is also in good agreement with experimental results. Compared to previous database versions, ME-Fe1.3 predicts Cr- and Fe-solubility in G-phase closer to the experimental findings.

Optimized phase stabilities in Mo-containing 9 to 12% Cr-steels

All available databases up to now revealed a problem of too high dissolution of Mo in M23C6, which indirectly destabilizes Fe2Mo-base Laves-phase (LAVES_PHASE) resulting in a too low solvus temperature. This problem is solved in ME-Fe1.3.

Assessment of Ta-solubility in MX carbo-nitride and Z-phase

The modeled sublattice description of Ta-containing Z-phase (ZET) in ME-Fe1.3 reads (Cr,Fe)(Cr,Nb,Mo,Ta,V)(N,Va), allowing for reproduction of reported nonstoichiometry of Z-phase Cr1+xTa1-xN (Ettmayer1971), with x being around 0.2. Relative phase relations between MX and Z-phase obey the criterion of metastable MX carbo-nitride, in agreement with previous work (Danielsen2012)



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Improved sigma-phase description

In previous ME-Fe database versions, delicate setting of mayor constituents in sigma-phase (SIGMA) was required in order to prevent the formation of an artificial miscibility gap in high-alloyed stainless steel grades in a wide temperature range. This problem has been solved in ME-Fe1.3 by improved sigma-phase description. As a consequence, also the model applicability in ferritic / superferritic stainless steels is improved considerably: Previous ME-Fe versions predict a dissolution temperature of sigma-phase in 27Cr-4Mo-2Ni SFSS (superferritic stainless steel) of 927°C. However, sigma-phase was observed experimentally after aging at 950°C (Qu2012). This discrepancy between experimental data and computation with ME-Fe has been decreased with ME-Fe3. We now obtain a dissolution temperature of sigma-phase in 27Cr-4Mo-2Ni of at 965°C.

Literature:

1) J. Miettinen, V.-V. Visuri, T. Fabritius, Thermodynamic description of the Fe-Al-Mn-Si-C system, Acta Universitatis Ouluensis, C Technica 704, 2019.

2) B. Huneau, P. Rogl, K. Zeng, R. Schmied-Fetzer, M. Bohn, J. Bauer, The ternary system Al-Ni-Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation. Intermetallics 7 (1999) 1337.

3) W. Wang, H.-L. Chen, H. Larsson, H. Mao, Thermodynamic constitution of the Al-Cu-Ni system modeled by Calphad and ab initio methodology for designing high entropy alloys, Calphad 65 (2019) 346.

4) R. Hu, H.-N. Su, P. Nash, Enthalpies of formation and lattice parameters of B2 phases in Al-Ni-X systems, Pure Appl. Chem. 79 (2007) 1653.

5) M. Abbasi, I. Park, Y. Ro, Y. Ji, R. Ayer, J.-H. Shim, G-phase formation in twenty-years aged heat-resistant cast austenitic steel reformer tube, Mater. Charact. 148 (2019) 297.

6) P. Ettmayer, The crystal structure of the complex nitrides NbCrN and Ta1-xCr1+xN, Monatsh. Chem. 102 (1971) 858.

7) H. Danielsen, J. Hald, M. Somers, Atomic resolution imaging of precipitate transformation from cubic TaN to tetragonal CrTaN, Scripta Mater. 66 (2012) 261.



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8) H.P. Qu, Y.P. Lang, H.T. Chen, F. Rong, X.F. Kang, C.Q. Yang, H.B. Qin, The effect of precipitation on microstructure, mechanic properties and corrosion resistance of two UNS S44660 ferritic stainless steels, Mater. Sci. Eng. A, A 534 (2012) 436.

thermodynamic database me-fe1.3 release notes

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