CRACK-IMPURITY INTERACTIONS AND THEIR ROLE IN THE EMBRITTLEMENT OF FEALLOY CRYSTALS CHARGED WITH LIGHT-ELEMENTS

The effect of mobile impurity doping on fracture behaviour has been in vestigated using plasma charging of light elements for Fe-2.5wt.%Si al loy crystals with particular attention to the role of crack-impurity e lastic interactions. Fe-Si crystals are markedly embrittled by plasma charging of helium as well as hydrogen at around room temperature, thi s being accompanied by slow crack growth. Neon charging contributes li ttle to the embrittlement, but argon charging does not contribute. The crystals are also embrittled by nitrogen charging in the tests at 450 K and exhibit slow crack growth during the tests. Elastic analyses in dicate that crack-impurity interactions are induced not only by the ap plied K-1 field but also by the stress modification due to ambient imp urities in the presence of a crack. The interactions serve effectively to concentrate mobile impurities ahead of a crack tip, leading to the increase in the local stress intensity k(1). The effect of interstiti al impurities on crack extension is discussed in connection with the m odification of stress states due to impurities around a crack tip.