Metallic alloys containing multiple principal alloying elements have created a growing interest in exploring the property limits of metals and understanding the underlying physical mechanisms. ...Refractory high-entropy alloys have drawn particular attention due to their high melting points and excellent softening resistance, which are the two key requirements for high-temperature applications. Their compositional space is immense even after considering cost and recyclability restrictions, providing abundant design opportunities. However, refractory high-entropy alloys often exhibit apparent brittleness and oxidation susceptibility, which remain important challenges for their processing and application. Here, utilizing natural-mixing characteristics among refractory elements, we designed a Ti
V
Nb
Hf
refractory high-entropy alloy that exhibits >20% tensile ductility in the as-cast state, and physicochemical stability at high temperatures. Exploring the underlying deformation mechanisms across multiple length scales, we observe that a rare β'-phase plays an intriguing role in the mechanical response of this alloy. These results reveal the effectiveness of natural-mixing tendencies in expediting high-entropy alloy discovery.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Mo addition is widely used to retard ferrite transformation and increase hardenability of steels. It has been well recognized that Mo segregation at ferrite/austenite interface induces solute drag ...effect (SDE) and reduces migration rate of ferrite/austenite interface while retardation by Mo is less obvious in bainite transformation. However, element segregation, solute drag effects and its interface character dependence have not been clarified quantitatively. Therefore, in the present study, amount of Mo segregation, energy dissipation during interface migration and interface orientation relationship (OR) in ferrite and bainite transformations in Fe-0.4%C-0.5%Mo (mass%) alloy have been investigated quantitatively. Mo segregation at interfaces with non K-S OR is more significant than that at interface with near K-S OR. Amount of Mo segregation at interface with non K-S OR increases with increase in transformation time investigated while energy dissipation decreases. The amount of Mo segregation and energy dissipation measured at non K-S interface coincides well with SDE model using optimized segregation energy and interface thickness. On the other hand, energy dissipation at bainite/austenite interface is large in spite of negligibly small Mo segregation, which is inconsistent with the SDE model.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Theoretical understanding of the “quenching and partitioning” (Q&P) process allowed developing microstructures consisting of carbon-depleted martensite and retained austenite that deliver superior ...mechanical properties. Most of the models describing the Q&P process are limited to systems in which carbide precipitation in martensite and decomposition of austenite to bainite are totally suppressed. However, these reactions are often unavoidable, even in low-carbon steels containing a relatively high concentration of Si and Mn. This work investigates interactions between carbon partitioning, carbide precipitation and carbide-free bainite formation during the Q&P process of a 0.3C–1.6Si–3.5Mn (wt.%) steel with non-homogenous distribution of the alloying elements. It was found that prior to the partitioning step ɛ-carbide forms in martensite. The decomposition of this carbide is required for a full completion of the carbon partitioning from martensite to austenite. Slow kinetics of decomposition of ɛ-carbide retards the carbon partitioning process. Results show that a fraction of austenite becomes stable by carbon partitioning and does not decompose to bainite. In the specimens quenched to lower temperature, a higher fraction of austenite becomes stable and consequently a lower fraction of bainite is formed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Segregation of boron at random austenite grain boundaries with known misorientation angle in a boron-added low carbon steel quenched from austenite state at different temperatures was investigated ...quantitatively using three-dimensional atom probe. Boron segregation is reduced at low angle boundaries or low quenching temperatures. The latter result indicates that non-equilibrium boron segregation plays an important role and is enhanced at higher quenching temperature due to more excess vacancies and longer diffusion distance during quenching from higher temperature.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
A new method is developed for reconstruction of the local orientation of the parent austenite based on the orientation of lath martensite measured by electron backscattered diffraction. The local ...orientation of austenite was obtained by least squares fitting as the difference between the experimental data and the predicted martensite orientation was minimal, assuming the specific orientation relationship (OR) between martensite and the parent austenite. First, the average OR between austenite and lath martensite was precisely determined and it was shown that both close packed planes and directions between martensite and the parent austenite deviated by more than 1° in low carbon martensite. The quality of the reconstructed austenite orientation map depended strongly on the OR used for the calculation. When Kurdjumov–Sachs (K–S) or Nishiyama–Wasserman (N–W) ORs were used the austenite orientation was frequently mis-indexed as a twin orientation with respect to the true orientation because of the mirror symmetry of (011)α stacking in the K–S or N–W ORs. In contrast, the frequency of mis-indexing was significantly reduced by using the measured OR, where the close packed planes and directions were not parallel. The deformation structure in austenite was successfully reconstructed by applying the proposed method to ausformed martensite in low carbon steel.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The expanded austenite (γN) produced by low-temperature nitriding of austenitic stainless steels with a Cr content of 18–20 at% is conventionally regarded as a nitrogen-supersaturated fcc solid ...solution with Cr-N short-range ordering, while obvious clustering between Cr and N in γN was recently reported for a Fe-35Ni-10Cr (at%) alloy. To investigate the dependence of Cr-N cluster formation on Cr concentration in γN, a high-throughput approach is proposed in the present work where a diffusion couple of Fe-35Ni and Fe-35Ni-30Cr alloys was plasma-nitrided at 673 K for 30 h. Systematic nanostructure characterization of the γN conducted by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) revealed the variations in nanosized Cr-N clustering as modulated structures with different Cr content. Considering a simultaneous concentration fluctuation of Cr and N, computational thermodynamics of chemical driving force, strain energy, and modulation wavelength for coherent spinodal were performed, and the results were consistent with observed nanostructure evolution. The nanostructures of γN in austenitic stainless steels were also understood as further increments of Cr and N from a moderate composition lowered the driving force. The spinodal decomposition is also promoted by increasing Ni content in the alloys.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Expanded austenite (γN) formed by low temperature nitriding of austenitic stainless steel is a N-enriched fcc containing Cr-N short range ordering, but no direct observation of Cr-N clustering has ...been reported. In the present study, the nanostructure of an Fe-35Ni-10Cr (at%) alloy plasma-nitrided at 673 K was investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) technique in this study. Nanosized Cr-N clusters were directly observed in the γN accompanied with strong streaks in selected area diffraction, obvious modulated structure in TEM and Cr-N rich regions observed by 3DAP. Thermodynamic calculations suggested that Cr-N clusters were formed by spinodal decomposition due to a strong Cr-N attractive interaction. The hardness of the γN layer was much higher than that of Fe-N austenite steel, which indicates that the hardness is not only due to nitrogen solid-solution hardening but also to a synergetic effect of the coexistence of Cr and N.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The kinetics and element partitioning during austenite (γ) reversion from lath martensite in Fe-2Mn-1.5Si-0.3C (mass%) alloy have been investigated. Two different types of γ in terms of ...crystallography, with Kurdjumov-Sachs (K-S) and without K-S orientation relationships with respect to the surrounding tempered martensite matrix, are formed. A transition in kinetics and element partitioning is related to the differences in crystallography. At low temperature, the growth of both types of γ is accompanied with the partitioning of Mn and Si. Consequently, the growth rates and appearance of the K-S and non K-S γ are similar and the reverted γ structure consists of acicular γ dominantly. On the other hand, at high temperature, only the K-S γ grows with partitioning of alloying elements in contrast to the partitionless growth of the non K-S γ, which leads to the formation of coarse globular γ. DICTRA simulation reveals that the variations in driving force for reversion and the interface mobility induce the transition of element partitioning. At large driving force and interface mobility, partitionless growth is possible and consequently coarse globular γ is formed by the fast growth of the non K-S γ at high temperature. On the other hand, reduction of the driving force or interface mobility induces the transition from partitionless to partitioning growth, which results in the dominant formation of thin acicular γ at low temperature.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Addition of small amount of Nb can strongly retard the ferrite growth kinetics. The origin has been attributed to the solute drag effect (SDE) by Nb segregation at the migrating ferrite/austenite ...interface. However, due to limitation of characterization techniques, the relations between elemental segregation, SDE and interface velocity have not been quantitatively clarified yet. Meanwhile, the strong affinity between Nb and C atoms can induce carbide precipitation at the interface, namely interphase precipitation, which may influence the Nb segregation behavior and make the issue even more complicated. Therefore, in this study, the interface information including amount of Nb segregation, energy dissipation, NbC precipitates and interface velocity in Fe-0.08C-(0.035, 0.061)Nb (mass%) model alloys is quantitatively investigated. It reveals that the energy dissipation at the migrating ferrite/austenite interface decreases with longer holding time or higher transformation temperature. The Nb atoms prefer to segregate at the non K-S interface rather than the near K-S interface. Amount of Nb segregation at the non K-S interface increases with longer time, while raising bulk Nb content or lowering transformation temperature does not lead to a notable increment of segregation. The relations between Nb segregation, energy dissipation, and interface velocity can be well reproduced by the SDE model with optimized parameters (i.e., segregation energy, interface thickness and trans-interface diffusivity). Occurrence of NbC interphase precipitation affects the transformation kinetics indirectly by weakening the SDE via consumption of Nb solutes in ferrite. In contrast, their pinning effect plays a marginal role.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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