On the mechanism of intergranular embrittlement by phosphorus in transformer steel
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METALLURGICAL TRANSACTIONS A
segregation since the dimensions associated with this phenomena are orders of magnitude smaller than those reported for nonequilibrium segregation. There are at least two models presently suggested for explaining intergranular failure due to equilibrium intergranular segregation. One mechanism is based on Gibb's adsorption and the Griffith fracture criterion. This model will be referred to as the Gibb-Griffith (G-G) model. 13q6 According to this model an excess of impurity (F/b) concentrates in the grain boundary, thereby reducing the intergranular fracture energy (~ff) associated with Griffith type failure. The minimum value of ~,~ is:
r~ > ( 2 ~ - ~,0
[11
where ~,~bis the equilibrium surface energy per unit area associated with grain boundary fracture surfaces and ~'b is the energy of the unstressed grain boundary. Jos_~'_ and Stein, I~ for example, calculated a large potential reduction of 27~b - 7b in copper due to intergranular segregation of bismuth. This result was primarily due to the much stronger effect that bismuth is believed to have on reducing 3,~ gb than g0- Thermodynamic calculations by Mason Iv have also shown that the right hand side of Eq. [1] can generally be reduced by segregation. A second mechanism to explain intergranular embrittlement by equilibrium intergranular segregation has been proposed by Seah? 8.~9This model proposes that 2"Ysgb -- ~b is unchanged by I'~b and instead the cohesive strength between atoms in the grain boundary is reduced. The reduced cohesive strength is proposed to be a function of impurity atom size. Solute atoms larger than the solvent cause reduced strength while solute atoms smaller than the solvent cause increased strength. It was suggested 19that phosphorus in iron resulted in intergranular embrittlement due to solid solution hardening of the boundary, a mechanism which is assumed to be similar to that proposed for certain types of nonequilibrium segregation.~l,~2 This was proposed because phosphorus and iron atoms are similar in size and hence should not result in a strongly embrittling system based on size effect considerations. The Seah model has provoked debate on several
ISSN 0360-2133/81/0713-1339500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, JULY 1981--1339
points. This debate centers on Seah's conclusion that segregation does not appreciably change the low temperature fracture energy of a grain boundary. In particular, his usO 8 of 1/2F~b as a general fracture surface adsorption level in calculating equilibrium surface energy has been critically reviewed. ~7The form of his atomic interaction energy and his use of a maximum-force failure criterion have also been debated, z~ Seah's work did demonstrate the need to be more rigorous in defining equilibrium or nonequilibrium fracture surface energies as pointed out by Rice. z~ It is recognized that large atom embrittlement by the G-G model or the Seah model would be difficult to differentiate by fracture tests. However, embrit
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