Compositional effects on the high temperature ductility of 1 Cr-1.25 Mo-0.25 V Steel
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I.
INTRODUCTION
THE brittle
intergranular failure mode observed in most metals at elevated temperatures is caused by nucleation, growth, and coalescence of cavities on grain boundaries. The extent of grain boundary cavitation, and therefore the degree to which the ductility is reduced at any given temperature, is determined by both the bulk properties of the alloy, e. g., composition and hardness and by the properties of grain boundaries, e. g., grain boundary composition and the amount and distribution of additional phases in the grain boundaries. A large literature has developed in recent years on the subject of grain boundary cavity nucleation and growth at elevated temperatures; see the reviews by Perry, Svensson and Dunlop, 2,3 Ashby et al,4 and Dyson. 5 Also, the effects of grain boundary segregation on the creep ductility of ferritic steels have been recently reviewed by Pope and Wilkinson. 6 The nucleation of cavities depends on the cohesion between grain boundary particles and the matrix 7-1~ and on the grain boundary self-diffusion rate which can control the rate of stress relaxation around grain boundary particles. 9'~~ Both of these processes can be altered by changes in grain boundary composition. Cavity growth is generally thought to be caused by the movement of vacancies from the grain boundary plane to the cavity surfaces, coupled with bulk plastic deformation, such as power-law creep. (The contribution of bulk plastic deformation becomes less as the temperature increases and the applied stress decreases.) Therefore, changes in grain boundary composition are expected to change the cavity growth rate by altering the grain boundary self-diffusion coefficient. Also, changes in the free surface composition of the cavities are expected to change the cavity growth rate by altering the free surface energy and free surface self-diffusion coefficient. In particular, when the free surface self-diffusion coefficient is lower than the grain boundary self-diffusion
coefficient, the cavity growth rate is controlled by free surface self-diffusion. 12 Results obtained recently in this laboratory on a 2.25 Cr1 Mo steel, ~~ which showed that P can improve the creep ductility, were interpreted using the assumption that P, when segregated to the grain boundaries, retards the cavity growth rate by reducing the grain boundary self-diffusion coefficient. It was also reported in Reference I0 that the material containing no deliberately added impurities (Mn is also considered to be an impurity in this context) was remarkably brittle. This was interpreted as being the result of a combination of the above-mentioned P effect on cavity growth and the effect of unscavanged residual S on the cavity nucleation rate. The existence of this S effect was demonstrated by the fact that material doped only with Mn showed substantially higher ductility than the undoped material. Parallel studies on these same materials 13'14 showed that the most creep ductile materials are the most temper embrittled and vice versa. This indicates that t
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