Phosphorus Grain Boundary Segregation Under Moderate And High Applied Tensile Stress At High Temperature In A 2.25Cr1Mo Steel

The grain boundaries segregation of soluteatoms has an important influence on the mechanical behavior of engineering materials, and it is an interesting problem for metallurgists and material engineers. The segregation of soluteatomsat grain boundaries may be classified into equilibrium segregation and non-equilibrium segregation. The study on equilibrium grain boundaries segregation started long before, and its theory has approached almost perfect. However, there are much unknown about the non-equilibrium grain boundaries segregation, especially the non-equilibrium grain boundaries segregation under applied moderate and high stress.In the present work ,non-equilibrium grain boundaries segregation of phosphorus under moderate(200MPa) and high(350MPa) applied tensile stress at 520℃, were researched by Auger electron spectroscopy(AES). Phosphorus concentration on grain boundaries was caculated based on AES data. The result shows:While the materials were agedat 520℃under moderate(200MPa) and high(350MPa) applied tensile stress , non-equilibrium grain boundaries segregation of phosphorus obviously occurred and existed two segregation peaks.at the beginning of stress-ageing, phosphorus concentration on grain boundaries increases sharply with increasing stress-ageing time until reaching the first segregation peak, then it declines dramatically to a minimum quantity. After that, it grows gradually to the second segregation peak, andat last, it decreases slowly with increasing stress-ageing time and reaches a equilibrium concentration.When the applied tensile stress is 350MPa, the two segregation peaks appearat around 60min and 360min and the minimum quantity appearsat around 180min; when the applied tensile stress is 200MPa, the two peaks appearat around 15min and 600min and the minimum quantity appearsat around 60min. When the applied tensile stress is 350MPa,the grain boundaries segregation level is much higher. It is worthy mention that there exists grain boundaries dilution for the 200MPa stress-ageing.Compared with the phosphorus non-equilibrium grain boundaries segregation under applied low tensile stress(40MPa), which has only one segregation peak appearsat around 30min, phosphorus non-equilibrium grain boundaries segregation induced by moderate and high tensile stress are obviously different. Materials have no creep deformation under an applied low tensile stress of 40MPa, so the vacancy concentration in the matrix is only influenced by the applied tensile stress. However, there is apparent creep during ageingat 520℃under an applied tensile stress of 200Mpa and 350MPa. As a result, besides Xu's model, we should take diffusional creep mechanism into consideration, that is grain boundary diffusion of phosphorus from grain boundaries parallel to the tensile stress to those normal to it ; in addition, during the process of creep, there are work-hardening and recovery phenomenon, which both have influences on the dislocation density in the matrix ,and then it will influence the vacancy concentration in the matrix for the reason that vacancies mainly vanishat dislocations, eventually and consequently the phosphorus concentration on grain boundaries will be influenced. Therefore, phosphorus non-equilibrium grain boundaries segregation induced by moderate and high tensile stress is a result of the combination of the three mechanisms presented above.