Ultrahigh strength maraging-TRIP steels
•Télécharger en tant que PPTX, PDF•
3 j'aime•652 vues
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.
Recommandé
Contenu connexe
Similaire à Ultrahigh strength maraging-TRIP steels
Similaire à Ultrahigh strength maraging-TRIP steels (20)
Plus de Dierk Raabe
Plus de Dierk Raabe (20)
Ultrahigh strength maraging-TRIP steels
- 1. Düsseldorf, Germany WWW.MPIE.DE d.raabe@mpie.de 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
- 3. 2 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 (d.raabe@mpie.de)
- 4. 3 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 0 10 20 30 40 50 60 70 80 totalelongationtofracture[%] ultimate tensile strength [MPa] TRIP and complex phase martensitic Maraging-TRIP and advanced QP dual phase ferritic Motivation: Combine TRIP and maraging effects austenitic stainless advanced TWIP and TRIP Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141 Dierk Raabe (d.raabe@mpie.de)
- 5. 4 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 (d.raabe@mpie.de) Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
- 6. 5 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 martensite Retained austenite (TRIP) Controlled precipitation hardening What is maraging-TRIP ? Dierk Raabe (d.raabe@mpie.de)
- 7. 6 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°C Dierk Raabe (d.raabe@mpie.de)
- 8. Overview www.mpie.de 7 Dierk Raabe (d.raabe@mpie.de)
- 9. 0 5 10 15 20 25 0 300 600 900 1200 1500 1800 2100 2400 EngineeringStress(MPa) Engineering Strain (%) 0 5 10 15 20 25 0 300 600 900 1200 1500 1800 2100 2400 EngineeringStress(MPa) Engineering Strain (%) 8 0 5 10 15 20 25 0 300 600 900 1200 1500 1800 2100 2400 EngineeringStress(MPa) Engineering Strain (%) Maraging aged (450°C/48h) quenched maraging-TRIP, 12MnPH aged (450°C/48h) quenched Tensile tests (X3NiCoMoTi18-12-4) higher strength AND higher elongation Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141 Dierk Raabe (d.raabe@mpie.de)
- 10. 9 Tensile tests, maraging TRIP FCC BCC e=0% e=15% Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141 Dierk Raabe (d.raabe@mpie.de)
- 11. 10 Tensile tests, maraging TRIP Raabe, Ponge, Dmitrieva, Sander: Scripta Mater. 60 (2009) 1141
- 12. Overview www.mpie.de 11 Dierk Raabe (d.raabe@mpie.de)
- 13. 12 Microstructure hierarchy Dmitrieva et al., Acta Mater, in press 2010 Dierk Raabe (d.raabe@mpie.de)
- 14. Overview Calcagnotto et al. Mater. Sc. Engin. A 527 (2010) 2738 13 Dierk Raabe (d.raabe@mpie.de)
- 15. 14 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 °C Ni Mn Al Dierk Raabe (d.raabe@mpie.de)
- 16. Mn atoms Ni atoms Mn iso-concentration surfaces at 18 at.% APT results: Atomic map (12MnPH aged 450°C/48h) 70 million ions Laser mode (0.4nJ, 54K) Dmitrieva et al., Acta Mater, in press 2010 Martensite decorated by precipitations Austenite ? ? Dierk Raabe (d.raabe@mpie.de) 15
- 17. M A Mn layer 1 Mn layer 2 Mn layer2 Mn 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 zone nominal 12 at.% Mn APT results: chemical profiles Dmitrieva et al., Acta Mater, in press 2010 16 Dierk Raabe (d.raabe@mpie.de)
- 18. 17 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
- 19. Mean diffusion path of Mn in austenite (aging 450°C/48h) 2 nm M A Mn layer 1 Mn 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 AM PB migration Mn diffusion phase boundary aging New austenite (formed during aging) DICTRA AM original position phase boundary final position phase boundary APT results and simulation: DICTRA/ThermoCalc Dmitrieva et al., Acta Mater, in press 2010 18 Dierk Raabe (d.raabe@mpie.de)
- 20. Overview www.mpie.de 19 Dierk Raabe (d.raabe@mpie.de)
- 21. 20 20 2 nm 12 wt.% Mn maraging-aged (48 h, 450°C), TEM
- 22. 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 21
- 23. APT Characterization 10 nm 450°C/48h Iso-concentration surface at 14 at.% Ni 450°C/192h Ni Fe www.mpie.de Dierk Raabe (d.raabe@mpie.de) 22
- 24. 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 www.mpie.de Dierk Raabe (d.raabe@mpie.de) 23
- 25. Overview 24Raabe, Ponge, Dmitrieva, Sander: Adv. Eng. Mat. 11 (2009) 547 Dierk Raabe (d.raabe@mpie.de)
- 26. Conclusions 25 Mn atoms Ni atoms Mn iso-concentration surfaces at 18 at.% martensite with precipitates martensite with precipitates 70 million ions Laser mode (0.4nJ, 54K) martensite with precipitates austenit e Raabe, Ponge, Dmitrieva, Sander: Adv. Eng. Mat. 11 (2009) 547 Dierk Raabe (d.raabe@mpie.de)
Notes de l'éditeur
- Quantitative analysis of the chemical interfaces between austenite and martensite was performed using 1D concentration profiles computed over the region of interest (cylindrical units). We calculated the content of manganese averaged over the 0.5 nm thick cross sections of the cylinders (profile step size 0.5 nm). For both interfaces, strong increase of Mn content up to 26 at. % was observed. The content of the Mn on the austenitic side is about 12 at. %, whereas on the martensitic side a slight depletion of Mn down to 6 at. % can be observed. In order to avoid the contribution of the precipitates to the chemical profile within the martensitic area, we separately measured the 1D concentration profiles within the martensitic matrix after exclusion of the precipitates. These profiles are also plotted. In order to understand the reasons for the Mn accumulation on the phase boundary, we consider the measured Mn contents. Since the phase equilibrium concentration of Mn in the austenite is much higher than in the ferrite (martensite) as was calculated by using Thermo-Calc (26.7 vs. 3.3 at. %), we expect a redistribution of Mn atoms during aging: enrichment in the austenite and depletion in the martensite. However, the Mn content measured in the austenite remains the same as in the nominal alloy composition (about 12.2 at. %, see Table1). In the martensitic matrix, a slight Mn depletion down to 10.3 at. % was detected. The diffusion in the FCC lattice of austenite is widely suppressed. The martensitic matrix is depleted to 10.3 at. % which is mostly due to the enrichment of Mn in the precipitates. However, Mn content decreases continuously in the martensite toward the phase boundary and, just some nanometers before the Mn-rich layer starts, drops to about 5-6 at. %. The formation of such depletion zone indicates an enhanced diffusion behavior of Mn atoms from the martensite to the austenite. Due to the low diffusion in the austenite, Mn atoms accumulate in the phase boundary and built up a Mn-enriched layer. The Mn gradient obtained from the thermodynamic calculation using DICTRA provides nearly the same distribution of the Mn content on the austenite/martensite phase boundary (Fig. 4). Enrichment of Mn up to the content of 27 at. % is observed in the interface between austenite and ferrite. For the simulation of the diffusion of Mn in the martensite, we enhanced the mobility of the atoms given for ferrite by a factor of 45. (During time of annealing at given temperature in (α+γ) range, Mn moves from ferrite to austenite across interface surface between austenite and ferrite until equilibrium state of the chemical potentials of Mn in austenite and ferrite will be reached. The balance depends on temperature and time of annealing.)
- In order to understand the dynamics of the formation of the Mn-enriched layers on the phase boundaries, we consider the phase equilibrium contents of Mn. The averaged content of Mn measured for the Mn-enriched layers is about 26 at. %. This content corresponds to the phase equilibrium content of this element in the austenite which is 26.7 at. %. As known, local phase equilibrium can be easily reached on the grain/phase boundaries. Thus, right on the phase boundary, the equilibrium composition in austenite is reached, and a local phase transformation from martensite to austenite within the Mn-enriched layer can be expected. However, the With the growth of the Mn-enriched layer towards the martensitic grain, the material within the layer becomes austenitic and, thus, the phase boundary moves. The final thickness of the Mn-enriched layer is about 20 nm, thus, the phase boundary moved 20 nm during the aging treatment. The layer-to-austenite interface provides the information about the position of the original phase boundary between the retained austenite and the martensite before aging. Further diffusion of Mn into austenite during aging was suppressed just beyond the crystallographic BCC/FCC boundary. The martensite-to-layer interface, however, indicates the position of the final phase boundary when the Mn diffusion was stopped by the water quenching after the aging treatment. The Mn enriched area in-between these two layers therefore can be addressed as additional austenite formed during the aging. The growth of the austenite leads to an enhancement of austenite volume fraction during aging. This can be correlated to the growth of the existing austenite grains where the phase boundaries serve as nucleation seeds. We assumed an epitaxial formation of reverted austenite on the phase boundary of the retained austenite.