Non-equilibrium Segregation Research Articles

Inhibition of the second phase precipitation and improvement of intergranular corrosion resistance by boron segregation at the grain boundary of S31254 superaustenitic stainless steel

The inhibition of Mo segregation and phase precipitation is vital for improving the hot workability and corrosion resistance of superaustentic stainless steels (SASS). The boron non-equilibrium segregation of S31254 SASS was implemented through solid solution, air cooling, and diffusion at low-temperature treatment (SADT). The precipitation process and intergranular corrosion (IGC) of S31254 SASS with various boron distributions were researched at a sensitive temperature. The second phases were observed and identified by SEM and TEM. IGC susceptibility was evaluated by double-loop potentiodynamic reactivation (DL-EPR) measurements. The SADT treatment promoted more segregation of B at the grain boundary, leading to lower amounts of grain boundary precipitation before aging for 6 h. The decrease of σ phases in B-regulated samples enhances the IGC resistance, compared with the samples without B addition specimens.

Lattice Boltzmann modeling of convective heat and solute transfer in additive manufacturing of multi-component alloys

Additive manufacturing (AM) is a remarkable breakthrough technology, allowing for the direct fabrication of three-dimensional components through the layer-by-layer stacking of materials. A novel free Surface lattice Boltzmann (LB) model is developed to simulate the heat and solute transfer in AM of multi-component alloys. The behavior liquid phase is described by using the free surface LB model, and the phase transitions between solid and liquid are modeled by using the LB-enthalpy method. A LB equation is directly constructed, which accounts for solute transfer in a certain multi-component alloys system. The thermodynamic information used in the calculation of phase transitions are determined by an extensive thermodynamic database. The model is validated via several benchmark examples of fluid flow and heat transfer. The characteristics of convective heat and solute transfer within various AM are investigated. Finally, the non-equilibrium convective heat transfer, phase transitions, and macroscopic segregation are discussed within the AM melt pools. The melt pool expands and convective heat transfer is enhanced by the Marangoni effect. Thus there is a consistent decrease in solute segregation. The solute segregation is more severe near the surface of the deposit layer. Additionally, the thermal convection experiences cyclic intensification attributed to the successive impact of multiple droplets in wire feeding and melting AM. This continuous impact serves to diminish the solute segregation. The results underscore the significant potential and advantages of LB method in accurately simulating the AM, which provides valuable insights for understanding the underlying mechanism of heat and solute transfer in materials and manufacturing.

Influence of boron segregation on the hot ductility of T23 steel CGHAZ

Boron segregates in a non-equilibrium segregation mode at lath, packet, block boundaries as well as prior austenite grain boundaries (PAGBs) in 2.25Cr1.6WVNb steel coarse grain heat-affected zone (CGHAZ) during the thermal cycle, and tends to segregate only at PAGBs in the same mode during the high-temperature tensile test. Boron segregation along PAGB increases GB plasticity but not inter-granular strength. This is because the boron segregation at PAGBs occupies the vacancies and other defect positions, which inhibits the nucleation and growth of micro-voids, and reduces plasticity consumption. As a result, the hot ductility of CGHAZ is enhanced.

Effect of boron addition on the microstructure and cryogenic mechanical properties of N50 stainless steel after aging treatment

The effect of boron on the microstructure and cryogenic mechanical properties in N50 austenitic stainless steel after aging treatment was revealed by a comparative investigation of 0B and 25B (25 ppm) N50 steels. The results showed there is a large amount of irregular block and rod-like Cr2N precipitated along grain boundaries with intra-granular precipitation of lamellar Cr2N in 0B steel; nevertheless, the nucleation and growth of Cr2N were significantly suppressed in 25B steel. It is related to the retarded diffusion of Cr, Mo, and V atoms caused by the pre-occupied B atoms during the precipitation of nitride. Theoretical calculations verify that the high B segregation at grain boundaries is caused by both equilibrium and non-equilibrium segregation. The weakened precipitation of nitride and the enhanced cohesion of grain boundaries by segregated B synergistically suppress the initiation and extension of intergranular cracks. The impact toughness of 25B–N50 steel at both 77 K and 4.2 K can reach 200 J, which is about 2.5 times higher than that of 0B–N50 steel.

Boron segregation at austenite grain boundaries: An equilibrium phenomenon

High-resolution secondary ion mass spectrometry imaging, atom probe tomography and transmission electron microscopy have been employed to investigate boron segregation at austenite grain boundaries (γGB) in a high-strength steel. Boron content in solid solution, interfacial excess at γGB and precipitate number density were quantified after heat treatments specifically designed to promote non-equilibrium segregation of boron. The samples were first soaked at 1100°C, followed by rapid cooling to different isothermal holding temperatures, and then quenched to room temperature. We found that boron distribution does not evolve after holding at 460°C, even after bainitic transformation, suggesting a low mobility of boron at this temperature. In contrast, after holding at high temperatures (780°C and 820°C), boron distribution evolves remarkably: boron content in solid solution close to the interface and boron interfacial excess decrease similarly with the increase of holding time, while their ratio remains almost constant; at the same time, transient precipitation of M23(B,C)6 particles occurs at the grain boundaries. The experimental results on interfacial excesses at γGB and the solute profiles in the vicinity of the grain boundary agree with the results of a numerical model of coupled diffusion and equilibrium segregation phenomena. These results unambiguously exclude the hypothesis that non-equilibrium segregation controls the kinetics of boron segregation and confirm that boron segregation is only controlled by the equilibrium segregation mechanism.

Improvement of strength and ductility synergy and the elimination of the yield point phenomenon of ultrafine-grained Mg–Al–Zn–Ag alloy sheet

Strength and ductility synergy in ultrafine-grained AZQ310 alloy sheet was successfully improved at the expense of grain boundary segregation via multi-pass rolling. Before rolling, the as-extruded alloy has high yield strength (TYS, 382 MPa) but poor elongation (4.1%) mainly due to the ultra-fine grains (414 nm) and the co-segregation of Al/Zn atoms at grain boundaries. More importantly, this non-equilibrium segregation dies away during the rolling process, and the sheet exhibits a tensile yield strength of 370 MPa, an ultimate strength of 422 MPa, and an elongation of 9.8% along the rolling direction after 5th pass rolling. The results show that the high strength-ductility synergy is mainly attributed to the ultra-fine grains and residual dislocations. Furthermore, it is surprising that the yield point phenomenon (YPP) prominently in as-extruded sample is weakened and finally eliminated in as-rolled sheets. Microstructure evolution in the room temperature tensile test manifests that the underlying reason for the YPP is the stress-induced grain growth and grain boundary segregation, which in turn expounds the difficulty in the refining of ultrafine-grained Mg alloys by traditional processing methods.

“Non-equilibrium” grain boundaries in additively manufactured CoCrFeMnNi high-entropy alloy: Enhanced diffusion and strong segregation

Grain boundary diffusion in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy is systematically investigated at 500 K under the so-called C-type kinetic conditions when bulk diffusion is completely frozen. In the as-manufactured state, general (random) grain boundaries are found to be characterized by orders-of-magnitude enhanced diffusivities and a non-equilibrium segregation of (dominantly) Mn atoms. These features are explained in terms of a non-equilibrium state of grain boundaries after rapid solidification. The grain boundary diffusion rates are found to be almost independent on the scanning/building strategy used for the specimen’s manufacturing, despite pronounced microstructure differences. Grain boundary migration during diffusion annealing turned out to preserve the non-equilibrium state of the interfaces due to continuous consumption of the processing-induced defects by moving boundaries. Whereas the kinetic “non-equilibrium” state of the interfaces relaxes after annealing at 773 K, the non-equilibrium segregation is retained, being further accompanied by a nano-scale phase decomposition at the grain boundaries. The generality of the findings for additively manufactured materials is discussed.

Premature failure induced by non-equilibrium grain-boundary tantalum segregation in air-plasma sprayed ZrO2−YO1.5−TaO2.5 thermal barrier coatings

Premature failure induced by non-equilibrium grain-boundary tantalum segregation in air-plasma sprayed ZrO2−YO1.5−TaO2.5 thermal barrier coatings

Grain boundary segregation and toughness of friction-stir-welded high-phosphorus weathering steel

Friction stir welding (FSW) of a high-P (0.3 wt%) weathering steel was performed at temperatures above A3 and below A1. Three-dimensional atom probe tomography analysis revealed that the grain boundaries in the stir zone formed below A1 (which had a considerably finer grain size) had higher P and lower C concentrations than those of the grain boundaries in the stir zone formed above A3. The effect of grain size on non-equilibrium grain boundary segregation during cooling after FSW was analyzed using the diffusion equation, assuming grain boundaries acted as sink sites for solutes. The fast diffusion of C is postulated to result in an almost equilibrium segregated state, such that its segregation decreases with decreasing grain size. In contrast, P diffuses only an extremely small distance, presumably leading to the grain-size-independent segregation of P. Therefore, the experimental result that the grain boundary concentration of P in the stir zone formed below A1 was larger than that in the stir zone formed above A3 is assumed to stem from the site competition effect between P and C and/or the enhanced P diffusion to grain boundaries due to dynamic recrystallization. Moreover, the segregation heat treatment was performed after the FSW to investigate the effect of the grain boundary segregation of P on the toughness. An investigation into the correlation between grain boundary segregation and toughness revealed that when the grain boundary concentration is less than approximately 7 at%, the effect of P segregation on intergranular fracture is relatively small and the toughness can be effectively improved by grain refinement.

Microstructure and Elements Concentration of Inconel 713LC during Laser Powder Bed Fusion through a Modified Cellular Automaton Model

In this study, a hybrid finite element (FE) and cellular automaton (CA) model is developed to explore crystallization behavior and alloying of Inconel713LC during Laser powder bed fusion. A cellular automaton model is considering the surface nucleation, equiaxed bulk nucleation, and grain growth kinetics. In addition, the equation for solute diffusion is coupled with a cellular automaton model to simulate the IN713LC elements segregation. During the phase change, the non-equilibrium segregation model is applied to insert the effect of ultra-fast solidification happening during LPBF. It is found that, during LPBF processing of IN713LC, the micro segregation of Nb, Ti, and C is accrued at the grain boundaries. It is further shown that the micro segregation intensity depends on the solidification speed, which is determined in turn by the laser heat input. In particular, a lower laser heat input increases the solidification speed and results in a more uniform solid phase, thereby reducing the risk of crack formation. Finally, using a comparison between simulation results and experimental observation, it was shown that the proposed model successfully predicts the bulk element concentration of IN713LC after laser melting.

Tensile Properties and Fracture Mechanism of Strain Rate Brittleness on Quenched Cold‐Rolled Carbon Steel

For metal materials, the corresponding mechanical indicators obtained by tensile experiments at different strain rates are different. Herein, after the quenching and cold‐rolling process, a deformed lath martensite structure is obtained in low‐carbon steel. A scanning electron microscope (SEM), transmission electron microscope (TEM), and 3D atom probe (3DAP) analyses are used in this research. The results show that measurement uncertainty exists and when the strain rate is 0.0021 s−1 at room temperature, the elongation is at its minimum value, which gets into an extreme point. C atoms segregate into the martensite boundary when the strain rate brittleness occurs from 3DAP analysis. This phenomenon indicates that the supersaturated C atoms in the steel undergo nonequilibrium segregation at the lath boundary during the elastic phase of stretching, which results in strain rate brittleness.

Flash oxidation tests to evidence the segregation of boron at the grain boundaries of superalloy Inconel 718

In this study very short oxidation tests, called flash oxidation tests, were carried out at high temperature on samples of superalloy Inconel 718 containing some boron. Following these tests, a specific oxidation pattern was evidenced: spots were observed on the oxide film, these spots being discontinuously distributed along the grain boundaries. Whether outside or inside the spots, the oxide film exhibited the same three oxide layers which were likely rutile oxide (Cr,Nb,Ti)O2, chromia oxide Cr2O3 and spinel oxide (Ni,Fe)(Cr,Fe)2O4. However, inside the spots the oxide film was characterized by thicker oxide layers, a slightly different chemical composition of the spinel layer, and vitreous oxides at the top. Moreover, all the intergranular (Nb,Mo)2CrB2 borides of the microstructure were involved in this specific oxidation pattern, thus leading to the suspicion of high boron concentrations within the spots. FEG-EPMA analyses confirmed that, during the oxidation process, boron atoms diffused from the matrix to the oxide film, especially within the spots. Actually, the spots revealed areas where boron was highly concentrated just before oxidation. Such areas originated from the non-equilibrium segregation of boron near grain boundaries, which is a classical phenomenon when cooling and/or reheating a boron-containing alloy. Thus, the spots of the oxide film highlighted the grain boundary segments where boron segregated the most, confirming that the segregation of boron was not uniform within the grain boundary network.

Emergence of active topological glass through directed chain dynamics and nonequilibrium phase segregation

This work shows how glassy behavior emerges from the active, directed polymer dynamics that is triggered by the non-equilibrium phase separation.

Simulations of Non-Equilibrium and Equilibrium Segregation in Nickel-Based Superalloy Using Modified Scheil-Gulliver and Phase-Field Methods

Solute segregation significantly affects material properties and is an essential issue in the additive manufacturing (AM) process. In the present study, we have investigated (i) non-equilibrium segregation in solidification, and (ii) equilibrium segregation at grain boundary in Ni-based Hastelloy-X (HX) superalloy using the modified Scheil-Gulliver model (i.e., Scheil-Gulliver model with back diffusion) and a phase-field model. We have found that the concentrations of all solute elements on grain boundary differ from those in face-centered cubic (FCC) phase matrix even in equilibrium state. In the non-equilibrium segregation, the segregations of Mo, Cr, and Mn and the depletion of Fe become more remarkable than the equilibrium segregation. Moreover, we have investigated the segregation in HX-based alloys with different Fe concentrations to propose a guide for tailoring the chemical composition of HX via the control of the segregation behaviors. The equilibrium-segregation simulation revealed that the Cr segregation in the grain boundary phase increased with the increase of Fe concentration. This result suggests that by controlling the Fe concentration, the Cr concentrations on grain boundaries can be controlled without changing directly the Cr concentrations. This finding opens new way of controlling materials properties which are dominated by the nature of grain boundaries such as corrosion resistance, crack sensitivity, high temperature strength.

Non-Equilibrium Solidification and Segregation of a MnFeCoNiCu Alloy Investigated Through In-Situ Synchrotron Diffraction

Non-Equilibrium Solidification and Segregation of a MnFeCoNiCu Alloy Investigated Through In-Situ Synchrotron Diffraction

On solute depletion zones along grain boundaries during segregation

We propose a model for predicting the depletion zone arising next to grain boundaries during non-equilibrium segregation. The model directly links to distribution of segregation energies as provided by atomistic simulations. We expose the theoretical framework based on the thermodynamic extremal principle and propose an efficient algorithm to solve the underlying equations. The example of the W-25at%Re is discussed to illustrate the main features of the model. Multi-component segregation kinetics is discussed for segregation of B, C, and N in Mo to illustrate site-competition scenarios. Comparison with earlier results obtained without depletion illustrates the importance of this effects. Finally the depletion zones along GBs are investigated for many different compositions to asses for which material composition and heat treatments they may be observed experimentally. We find that extended depletion zones arise for very small solute concentrations.

Effect of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC joint

Influence of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC superalloy was investigated systematically. The microstructure of as-cast base metal (BM) mainly consisted of “ogdoadically” diced cube γ′ particles, fanlike γ-γ′ eutectics, MC carbides, Cr–Mo borides and Ni–Zr intermetallics. Grain-boundary liquation cracking was observed in the HAZ of as-cast joint, resulting from the segregation of elemental boron. Pre-weld heat treatment had a significant influence on the size, volume fraction and distribution of γ′ particles. It also decreased the non-equilibrium segregation of boron during cooling, which inhibited the formation of grain-boundary liquid film and liquation cracking. The precipitation of MC carbides and nanoscaled γ′ particles in the fusion zone (FZ) improved the micro-hardness of the FZ. The micro-hardness of the BM and the HAZ were influenced by the volume fraction and the size of γ′ particles. All the joints fractured in the BM at RM and 900 °C. The as-cast and water quenched (WQ) joints failed with a quasi-cleavage fracture mode at RM and 900 °C while the tensile fracture of furnace cooled (FC) joint showed a ductile rupture at RM and 900 °C.

Influence of cyclic non-isothermal heat treatment on microstructure, mechanical property and corrosion behavior of Al–Zn–Mg alloy

The influence of cyclic non-isothermal heat treatment times on microstructure, mechanical properties and corrosion behavior of Al–Zn–Mg alloy is investigated. The results show that Mg segregation at grain boundary becomes severer with the increase of non-isothermal heat treatment times, and the tensile strength and elongation of the alloy decrease gradually. The increase of Mg segregation at grain boundary can be interpreted by non-equilibrium segregation theory. The size and quantity of MgZn2 precipitation decrease after one time of non-isothermal heat treatment, but increase significantly after two and three times of non-isothermal heat treatment. The change of MgZn2 precipitation results in slight improvement of corrosion resistance of Al–Zn–Mg alloy after one time of non-isothermal heat treatment, and serious deterioration of corrosion resistance of Al–Zn–Mg alloy after two and three times of non-isothermal heat treatment.

Study of Nb non-equilibrium segregation at prior austenite grain boundary in welding using atom probe tomography and modeling

The non-equilibrium segregation of Nb at prior austenite grain boundary (PAGB) induced by welding is experimentally investigated by means of atom probe tomography (APT). The Nb concentration at PAGB appeared to be 0.115 at.% for experimental steel containing 0.06 at.% Nb at the simulated welding thermal cycle for 10 s from 1320 to 950 °C. The critical time tc for achieving maximum segregation is calculated to be 565 s for the experimental steel. For the first time, the diffusion coefficient of Nb in desegregation process is calculated for the temperature of Ti = 950 °C using non-equilibrium segregation model and APT experimental result and is confirmed to be 9.23 × 10−11 m2/s. Based on it, the kinetics of Nb non-equilibrium segregation at the PAGBs in welding process is outlined, and the segregation process and desegregation process are discussed. The Nb segregation at PAGB is also predicted over the t8/5 range of 3–600 s. The critical t8/5, at which the highest Nb concentration of 0.144 at.% at PAGB is predicted to be achieved, is ~ 29 s.

Effect of cooling rate on phosphorus segregation behavior and the corresponding precipitation of γ″ and γ′ phases in IN718 alloy

Effect of segregation behaviors of P at different cooling rates on the precipitation of γ″ and γ′ phases and the corresponding strength are investigated. The precipitation of γ″ and γ′ phases during cooling is sensitive to P concentration. With increasing the concentration of P, the amount of γ″ and γ′ particles increases after air cooling. With decreasing the cooling rate, the accelerating effect of P on the precipitation of γ″ and γ′ phases decreased first and then increased, which demonstrates the concentration of P dissolved in the grain interior decreases first and then increases. The different effects of P on γ″ and γ′ phases with different cooling rates were analyzed by the kinetic characteristic of nonequilibrium grain-boundary segregation. The characteristic of nonequilibrium grain-boundary segregation of P in superalloy is further confirmed, and the phenomenon caused by critical cooling rate is captured.