Non-equilibrium Grain-boundary Segregation Of Phosphorus Research Articles

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.

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.

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.

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.

Experimental study on the characteristic of grain-boundary segregation of phosphorus in Ni-base superalloy

In virtue of Auger electron spectroscopy, the levels of grain-boundary segregation of phosphorus in Ni-based superalloy after solution-treated at 1200 °C for 9.8 h and at 1020 °C for 68.5 h and then aged at 720 °C all for 2 h are measured. It is found that the levels increase with the increase in solution treatment temperature. The measurement result is analysed with the laws of equilibrium segregation and non-equilibrium segregation. Based on the analysis, the characteristic of non-equilibrium grain-boundary segregation of phosphorus in the superalloy is confirmed for the first time.

Fracture Behaviour and Temper Embrittlement Induced by NGS of Phosphorus

The non-equilibrium grain-boundary segregation (NGS) isotherms and its kinetics serve to provide a more complete understanding of inter-granular segregation behavior in relation to mechanical properties, not only for the engineering steels but also for a wide range of structural alloys. The NGS of phosphorus and temper embrittlement dynamics on the same heat treatment condition at the same isothermal holding time in two Cr-Mo steels, 12Cr1MoV and 2.25Cr1Mo, was experimentally studied. The fracture behaviour was also observed by tensile tests in situ in a scanning electron microscope (SEM). Results show that both the concentration of phosphorus atoms in grain boundaries and the degree of embrittlement reaches a maximum at the critical time. It can be satisfactorily elucidated by the temper embrittlement mechanism of NGS caused by cooling from solution temperature to isothermal holding temperature.

A Control Method of Grain-Boundary Brittleness by Step Cooling Technique

For most of the experiments and the practical problems about the non-equilibrium grain-boundary segregation (NGS), segregation will always occur during continuous cooling. The effective-time-method assumed that any continuous cooling curve for a sample can be replaced by a corresponding stepped curve. In this paper, the non-equilibrium grain-boundary segregation of phosphorus in step cooling process in an industrial steel12Cr1MoVis studied based on the effective-time-method and compared with that in isothermal holding process. Results show that the calculated result is in good accordance with the experimental observation. It provides a direct support for the effective-time-method of NGS. The step cooling process can speed-up the embrittlement of the steel. Based on the step cooling technique, a control method of grain-boundary brittleness is proposed.