Phase Field Study of the Diffusional Paths in Pearlite-Austenite Transformation
LI Sai1, YANG Zenan2, ZHANG Chi1, YANG Zhigang1()
1.School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 2.Science and Technology on Advanced High Temperature Structural Materials Laboratory, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China
Cite this article:
LI Sai, YANG Zenan, ZHANG Chi, YANG Zhigang. Phase Field Study of the Diffusional Paths in Pearlite-Austenite Transformation. Acta Metall Sin, 2023, 59(10): 1376-1388.
In the development of advanced steel, accurate and detailed knowledge about the kinetics of phase transformations and microstructure formation is critical. The critical issue in pearlite-austenite transformation is the consideration of diffusional paths of the alloy element. Simulation has been an available method to study the diffusion of alloy elements and the migration rate of the phase boundary in the complex morphological evolution of austenite growth. The isothermal pearlite-austenite transformations at 720 and 740oC in Fe-0.6%C-2%Mn (mass fraction) alloy were studied by phase-field methods based on MICRESS. At different temperatures, the effects of diffusional paths on the austenite transformation were discussed. To achieve a semiquantitative verification of the simulated results, the migration rates of the austenite/pearlite boundary at 720 and 740oC were estimated from the experimental kinetics curves by fitting the JMA equation. By measuring the Mn profile in austenite, the modes of the austenization at 720 and 740oC can be verified as partitioned local equilibrium (PLE) and non-partitioned local equilibrium (NPLE) modes. The heterogeneous distribution of Mn in austenite at 740oC can be observed with STEM-EDS. However, the homogenous distribution of Mn can be found near the pearlite/austenite boundary in austenite at 720oC. The cases considering the γ, α, and interface-diffusional paths were simulated by phase-field methods to compare with the migration rates of the austenite/pearlite boundary. Because carbon is an interstitial element in steel and has an interstitial diffusional mechanism, it can be speculated reasonably that the diffusion of C mainly proceeded in austenite and ferrite through the considerations of the atom-size of carbon and the experimental results. Phase-field methods were used to study Mn diffusion in the lamellar pearlite-austenite transformation. With the analysis of the experimental estimations, the interface-diffusional path was observed as the dominant path for the Mn diffusion. It is because the Mn atoms have greater diffusivity in interfaces than in γ or α-diffusional paths. Furthermore, the diffusional activation energy is closely related to the diffusivity of Mn at the interface. Moreover, compared with the γ-diffusional path, the diffusional flux of Mn in ferrite is much larger than that in austenite. Thus, it can be concluded that the contribution of the α-diffusional path to the migration rate of the pearlite/austenite boundary is larger than that of the γ-diffusional path. As a result, considering the α-diffusional path in the thermodynamics analysis under NPLE mode makes more sense. However, ignoring the interface- and α-diffusional path, which is different from the traditional cognition in PLE mode, will result in a magnitude error for the thermodynamics analysis.
Table 1 The conditions for the different simulations
Physical quantity
Item
Value
Unit
Interface energy
γ/α, γ/θ, α/θ
1.5 × 10-5
J·cm-2
Interface mobility
γ/α, γ/θ
1.5
cm4·J-1·s-1
α/θ
1
cm4·J-1·s-1
Frequency factor
C in γ
0.1996
cm2·s-1
C in α
0.419
cm2·s-1
C in interface
0.1996
cm2·s-1
Mn in γ
0.1501
cm2·s-1
Mn in α
6.7119 × 103
cm2·s-1
Mn in interface
0.1501
cm2·s-1
Activity energy Q
C in γ
1.390 × 105
J·mol-1
C in α
1.086 × 105
J·mol-1
C in interface
0.97 × 105
J·mol-1
Mn in γ
2.602 × 105
J·mol-1
Mn in α
3.062 × 105
J·mol-1
Mn in interface
1.55 × 105
J·mol-1
Thickness of the diffusional interface
0.5
nm
Table 2 Materials data used in the phase-field calculations[26,36]
Fig.1 High temperature confocal laser scanning microscopy images of the microstructure of the isothermal austenite at 720oC (a) and 740oC (b) (AGB—austenite grain boundary)
Fig.2 Kinetics curves of the austenization at 720 and 740oC from the initial pearlite (t—time)
Fig.3 SEM image of the sample held at 720oC for 60 s (a), TEM image of the region in the box in Fig.3a (b), and Mn profiles along line A (c) and line B (d) in Fig.3b
Fig.4 SEM image of the sample held at 740oC for 20 s (a), TEM image of the region in the box in Fig.4a (b), and Mn profile along the line A in Fig.4b (c) (RA—retained austenite)
Fig.5 Microstructural evolutions of the pearlite-austenite transformation at 740oC with the γ + α + interface diffusional path, the spacing layers of the initial pearlite are 150 nm (a-d) and 75 nm (e-h)
Fig.6 Microstructural evolutions of the pearlite-austenite transformation at 720oC with the 150 nm-spacing layer of the initial pearlite, the diffusional paths are γ + α + interface (a-c), γ + α (d-f), and γ (g-i), respectively
Fig.7 C (a, c) and Mn (b, d) distributions in the austenite held at 740oC for 0.004 s with the spacing layers of 150 nm (a, b) and 75 nm (c, d)
Fig.8 C (a) and Mn (b) distributions in the austenite held at 720oC for 3.81 s
Fig.9 Migration rates of the austenite/pearlite boundary under different simulated conditions (Inset is the enlargement of the green dash box region. ExpC and ExpMn represent the experimental value of NPLE and PLE mode, respectively. The meanings of A-I are shown in Table 1)
Fig.10 Distributions of the alloy element beside the austenite/pearlite boundary with the γ + α diffusional path (a-c) the simulated morphology (a) and C profiles in austenite (b) and ferrite (c) in group B (d-f) the simulated morphology (d) and C profiles in austenite (e) and ferrite (f) in group E (g-i) the simulated morphology (g) and Mn profiles in austenite (h) and ferrite (i) in group H
Fig.11 Distributions of the fluxes of C in austenite/ferrite held at 740oC (a) and Mn in austenite/ferrite held at 720oC (b) along the austenite/pearlite boundary
Fig.12 Variation of the migration rate of P/γ boundary with the activity energy of interfacial diffusion (Qint), where the circle point is the calculated value and the diamond point is the experimental estimated value
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