Non-equilibrium Grain Boundary Segregation Research Articles

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

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.

Mechanism Study on Hot Ductility of 2.25Cr1Mo Alloy Based on Non-Equilibrium Grain-Boundary Segregation

Mechanism Study on Hot Ductility of 2.25Cr1Mo Alloy Based on Non-Equilibrium Grain-Boundary Segregation

Nonequilibrium grain-boundary segregation mechanism of hot ductility loss for austenitic and ferritic stainless steels

Abstract

Investigation of non-equilibrium grain boundary segregation on CGHAZ in reactor pressure vessel steel

In order to explore the reasons for coarse grained embrittlement, post-weld heat treatment (PWHT) with different times was conducted on the coarse grained heat affected zone (CGHAZ) of A533B steel. The results reveal that the impact toughness of the CGHAZ at room temperature decreases with the prolongation of PWHT time from 0.5 to 4 h. As the PWHT time increases to 4 h, the fracture feature of the CGHAZ presents intergranular cleavage ruptures, and the impact toughness exhibits the minimum value. After PWHT for 4 h, the area of the intergranular cleavage fractures decreases, and the impact toughness gradually increases. Further research reveals that there is phosphorus grain boundary segregation in the CGHAZ. The theory of non-equilibrium grain boundary segregation (NGS) is proposed to explain the phenomenon. It is found that the PWHT time with minimum impact toughness is consistent with critical time of phosphorus NGS.

Mechanism of hot ductility loss in C–Mn steels based on nonequilibrium grain boundary segregation of impurities

Abstract

Identical mechanism of isochronal and isothermal embrittlement in Ni(Bi) alloy: Thermo-induced non-equilibrium grain-boundary segregation of Bi

Isochronal and isothermal plasticity after thermal pre-treatments are obtained by tensile tests to characterize the embrittling behaviors of Ni(Bi) alloy. Both isochronal and isothermal plasticity show evident minima. Fractography observed by scanning electron microscopy displays intergranular fracture for samples of low plasticity. The microstructure is found to be free of precipitates within grains and at grain boundaries by focused ion beam and transmission electron microscopy. Atom probe analysis indicates a strong tendency of Bi segregation to grain boundaries. By these results, the existing interpretations are discussed to be inadequate and both embrittlement are confirmed to be identical in mechanism, i.e. thermo-induced non-equilibrium grain-boundary segregation of Bi.

Non-equilibrium grain-boundary segregation of Bi in Cu bicrystals

The observations of grain-boundary segregation of Bi in Cu bicrystals were analyzed. According to equilibrium grain boundary segregation (EGS) model and non-equilibrium grain-boundary segregation (NGS) model, respectively, the segregation kinetics of isothermal annealing at 500 °C and that of isochronal annealing for 24 h of Bi in Cu bicrystals were investigated. By qualitative analysis and quantitative analysis, it is concluded that the grain-boundary segregation of Bi agrees well with the theory of NGS. Based on the kinetics model of NGS, some parameters that are useful to predicting and controlling the Bi-induced embrittlement in Cu alloys are calculated as follows: the diffusion coefficient of Bi-vacancy complexes Dc=7.8×10−5exp[–1.46/(kT)]; the apparent diffusion coefficient of Bi atoms DiA=7.66×10at+bexp[–1.76/(kT)], where a=8.45×10−8 and b=–13.37.

Non-equilibrium Grain-boundary Segregation of Phosphorus in an Fe-17Cr Alloy

In order to confirm the segregation characteristic of phosphorus in an Fe-17Cr alloy at grain boundary, the phosphorus segregation was obtained by Auger electron spectroscopy. The results show that a maximum phosphorus segregation appeared at 450 °C for all specimens aged for 30 min at 350, 400, 450, 500, and 600 °C after being solution-treated at 1000 °C for 1 h. These results were analyzed by the characteristics of segregation peak temperature of non-equilibrium segregation.

Precipitation sequence of η phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy during artificial aging

The precipitation sequence of η(MgZn2) phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 °C for various times from 5 min to 6 h. High resolution transmission electron microscopy (HRTEM) observations and energy dispersive X-ray spectroscopy (EDX) analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution (SSS)→vacancy-rich clusters (VRC)→GP II zones→η′→η. Based on the theory of non-equilibrium grain boundary segregation (NGS) and non-equilibrium grain boundary co-segregation (NGCS), the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP II zones, η′ phase as well as η phase.

Grain Boundary Fracture Behavior and Segregation of P under Low Tensile Stresses in Steel 2.25Cr1Mo and 12Cr1MoV

Experimental investigation about the grain boundary fracture behavior and segregation behavior of phosphorus under low tensile stresses in steel 2.25Cr1Mo and 12Cr1MoV was carried out in this paper. AES (Auger electron spectroscopy) experiments and dynamic analyses on the non-equilibrium grain-boundary segregation (NGS) of phosphorus and the SEM photos of grain boundary fracture in Auger specimens of both steels were obtained. The variation of phosphorus segregation level in grain boundary at different aging time was studied. Results show that the non-equilibrium segregation of phosphorus occurred at the grain boundaries in the two steels while subjected to a tensile stress of 30MPa and held at 500°C. The corresponding critical time was about 1 hour for steel 2.25Cr1Mo and 3.5hour for steel 12Cr1MoV respectively. SEM photos of grain boundary fracture in Auger specimens of the test steels show that the grain boundary fracture rate increased with increasing concentration of phosphorus, and that the fracture toughness of steel 12Cr1MoV is much lower than the one of steel 2.25Cr1Mo.

Effect of non-equilibrium grain-boundary segregation of phosphorus on the intermediate-temperature ductility minimum of an Fe–17Cr alloy

The ductility of an Fe–17Cr alloy was examined experimentally in the range of 350–700°C. The results showed that the ductility of the alloy, evaluated by the reduction in area, had a minimum at an intermediate temperature of approximately 450°C. Auger electron spectroscopy demonstrated that the ductility minimum corresponded to a maximum in grain boundary segregation of phosphorus, which could be elucidated by the theory of non-equilibrium grain-boundary segregation. Thus, the intermediate temperature ductility minimum for the Fe–17Cr alloy could be explained on the basis of non-equilibrium grain-boundary segregation of phosphorus.

Unified mechanism of intergranular embrittlement based on non-equilibrium grain boundary segregation

The review is aimed at presenting a unified approach in understanding the mechanism of non-equilibrium grain boundary segregation, which can satisfactorily describe the three types of intergranular embrittlement, namely, reverse temper embrittlement of steels, intergranular corrosion embrittlement of stainless steels and intermediate temperature embrittlement of metals and alloys. The review starts with a broad perspective of non-equilibrium grain boundary segregation, including thermally induced non-equilibrium grain boundary segregation and stress induced non-equilibrium grain-boundary segregation. Next, it focuses on the recent progress made in the non-equilibrium grain boundary segregation, including (1) critical time, (2) segregation peak temperature, (3) segregation peak temperature movement for thermally induced and stress induced non-equilibrium grain boundary segregation, and (4) the effect of temperature difference on thermally-induced non-equilibrium grain boundary segregation. Next, the attention is focused on the grain boundary coverage of elements and intergranular embrittlement phenomena. Three types of intergranular embrittlement is analysed in terms of (1) the ductility healing effect induced by the critical time, (2) embrittlement peak or ductility trough induced by the segregation peak temperature, (3) embrittlement peak or ductility trough movement induced by the segregation peak temperature movement and (4) widening and deepening of ductility trough induced by differences in temperature. These experimental phenomena concerning the three types of intergranular embrittlement are consistent with the models of thermally induced and stress induced non-equilibrium grain boundary segregations of impurities, instead of precipitation or equilibrium grain boundary segregation. Towards the end, we visit the subject of grain boundary segregation and associated embrittlement process from the viewpoint of fracture resistance and briefly discuss different perspectives that are of practical significance.

Non-equilibrium grain-boundary segregation of Bi in binary Ni(Bi) alloy

The minimum ductility of Ni(Bi) alloy caused by isothermal annealing at 750 °C or 650 °C is determined through tensile tests at room temperature. Tensile samples with minimum ductility display intergranular facets. Strong grain-boundary segregation of Bi in nanocrystalline Ni–Bi layer is observed by atom probe tomography. The minimum ductility, intergranular facets and grain-boundary segregation of Bi confirm the non-equilibrium grain-boundary segregation of Bi in Ni(Bi) alloy for the first time.

Grain-Boundary Segregation of Phosphorus and Inter-Granular Fracture Behavior under Low Tensile Stresses

The present work is an effort to provide experimental results focusing on segregation behavior of phosphorus at grain boundary and the intergranular fracture behavio under low tensile stresses. AES (Auger electron spectroscopy) experiments and dynamic analyses on the non-equilibrium grain-boundary segregation (NGS) of phosphorus and the SEM photos of intergranular fracture in Auger specimens in 12Cr1MoV steel were carried out in this paper. The variation of phosphorus segregation level in grain boundary under different low tensile stresses and at different temperature were obtained. Results show that NGS of phosphorus occurred in the experimental steel while subjected to low tensile stresses. Maximum values of phosphorus segregation level were obtained at the critical times. SEM photos of intergranular fracture in Auger specimens of the test steel show that the intergranular fracture rate increased with increasing concentration of phosphorus. The intergranular fracture behavior is accordant with the segregation behavior of phosphorus at grain boundary.

Study on the Diffusion Coefficient in NGS under Low Tensile Stress

In this paper, the diffusion coefficient of vacancy-solute complex and diffusion coefficient of solute atoms, P, in non-equilibrium grain-boundary segregation under low tensile stress in steel 12Cr1MoV are calculated based on the model developed by Xu. Both the simulation and experimental kinetics curves are achieved. Results show that the calculated result with the kinetic equations perfectly fits with the experimental observations for phosphorus in steel 12Cr1MoV under low tensile stress. In the segregation process, the rate of phosphorus segregation is high due to a large diffusivity of the complex. In the de-segregation process, the level of phosphorus segregation decreases with increasing stress aging time. But the rate of phosphorus desegregation from grain boundaries to centre is slower compared with that in segregation for the diffusion coefficient of phosphorus atoms is far lower than the complexes.

A unified model of grain-boundary segregation kinetics

A unified model of grain-boundary segregation kinetics is established, which can simulate the equilibrium and non-equilibrium grain-boundary segregation kinetics during isothermal ageing. The results of model fittings/predictions are in a good consistence with those of the experiments. Values of several important parameters, such as vacancy formation energy, vacancy migration energy, and complex migration energy derived by model fittings, show a reasonably good agreement with those in the references. Relationship between the equilibrium and non-equilibrium grain-boundary segregation mechanisms is discussed in detail. The segregation peak/depletion trough observed in the non-equilibrium segregation kinetics may be ascribed to the vacancy flux. The equilibrium segregation kinetics is considered as a special case in non-equilibrium segregation kinetics with the zero vacancy flux. Both the equilibrium and non-equilibrium segregation kinetics are believed to be dominated by vacancy-solute complex flux. The proposed unified model can be a significant progress in grain-boundary segregation theory, with applications in guiding the design of materials.

Intermediate temperature embrittlement in high-purity Ni and binary Ni(Bi) alloy

The tensile ductility of high-purity Ni and binary Ni(Bi) alloy were studied in the temperature range of 400–850 °C. Ni(Bi) alloy shows evident intermediate temperature embrittlement with a minimum ductility between 700 and 750 °C, while high-purity Ni does not. It is concluded that this embrittlement must be an impurity effect. On the basis of quantitative estimates, the embrittlement is interpreted as being caused by thermo-induced non-equilibrium grain-boundary segregation of Bi.

Graphical representation for isothermal kinetics of non-equilibrium grain-boundary segregation

Abstract A model of non-equilibrium grain-boundary segregation of solute is expressed with graphs for the isothermal aging at various temperatures after quenching from a solution temperature. It is found from the graphical representations that when the samples are aged for a certain time at various temperatures there is a maximum value in the segregation concentration at a certain temperature. The aging time is equal or close to the critical time of non-equilibrium segregation corresponding to this temperature. This finding is experimentally confirmed with sulfur in an Ni-base superalloy with the aid of Auger electron spectroscopy. As an application of the new finding, a mechanism for intermediate temperature embrittlement of alloys is suggested and identified experimentally with tension tests of the superalloy.

Non-equilibrium grain-boundary segregation of phosphorus in an Ni–Cr steel

In order to confirm the segregation characteristic of phosphorus in Ni–Cr steel at grain boundary, a grain boundary segregation kinetic curve of phosphorus is obtained using Auger electron spectroscopy. The results show that a peak concentration of phosphorus appears at about 60 min during isothermal ageing at 600 °C after quenching from 1000 °C. The boundary concentration of phosphorus decreases with decreasing (increasing) ageing time, when the ageing time is shorter (longer) than 60 min. These results are analyzed by the model of non-equilibrium grain boundary segregation, indicating that this peak is corresponding to the critical time for phosphorus non-equilibrium grain boundary segregation.