NEW MODEL OF GRAIN-BOUNDARY FAILURE IN TEMPER EMBRITTLED STEEL

A new model of grain boundary failure for intergranular fracture modes is developed. After segregation during heat treatment the embrittle active group B (IV, V, VI) metalloids (I), which have a ns2px electron configuration, are supposed to form a layer with dominating covalent bond character along the grain boundaries. It is suggested that embrittlement is due to: 1. (1) direction depending p-orbitals of the rigid metalloid bonds; 2. (2) metal-metal atom (4s)-bond disruption in the neighboured metal (M) layers caused by the formation of strong I(3p)-M(4s) electron pairing; 3. (3) mutual I-I attraction resulting in a reduction of M-I bond strength. The model is based on the experimentally proved equivalence of free surface and bulk interface chemical reactions. Results are therefore extracted from quantitative valence bond theory calculations of sulfur chemisorption on a Ni(100) model free surface as developed in theoretical works of catalysis. Quantitatively a two-dimensional 53° model grain boundary is discussed in order to demonstrate structural similarity. © 1979.