Effect of Solute Segregation on Fracture Toughness in a Ni-Cr Steel
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Q U E N C H E D
and tempered alloy steels are well known to exhibit susceptibility to intergranular embrittlement after prolonged exposure to temperatures ranging from 400 to 650 ~ ] Ample evidence from intergranular fracture surface analyses by Auger electron spectroscopy (AES) has led to the assumption that intergranular segregation of Groups IVA to VIA metalloid elements reduces cohesion across grain boundaries. A fundamental study of intergranular embrittlement was recently performed2,3 by measuring the strength of grain boundaries tr*, (which varies directly with the cohesive energy 3' and plastic work ~/p)4.5 as a function of the quantity and type of segregated solute. For engineering purposes, however, the plain strain fracture toughness K~ is used to examine the propensity for crack extension in solids. The physical significance of Ktc is based on a Griffith-type energy balance criterion in elastic materials. On the macroscopic scale, the energy balance criterion is assumed to be applicable to elastic-plastic materials when brittle fracture is associated with plasticity on a scale which is negligible compared with the precrack size.6 Although this assumption is not valid on the microscopic scale, Ktc is nevertheless expected to be related to inherent material properties (~/and 7p) of brittle fracture in some way since it characterizes the critical local stress field in which a fracture micromechanism is operative ahead of the precrack. Attempts to interpret the variations of Kt~ with microstructural variables have primarily been made in the study of transgranular fracture. 7,8 In such cases it is generally not possible to vary 7, and as a result one cannot study the dependence of Kit on 7. However, in the case of intergranular brittle fracture one can vary ~, by varying the concentration or kind of segregated impurity which causes the intergranular weakness. J U N K A M E D A is Research Associate, D e p a r t m e n t of Materials Science a n d Engineering, University of Pennsylvania, Philadelphia, PA 19104. Manuscript submitted January 13, 1981. METALLURGICAL TRANSACTIONS A
Hence, the present work was undertaken to gain a fundamental understanding of the variation of Ktc with solute segregation. For this purpose, a correlation between the macroscopic Klc and the microscopic kzc (which is estimated from the measurement of o*) 2,3 is experimentally determined, and a model of stressgradient-control of brittle fracture is proposed. EXPERIMENTAL PROCEDURE Vacuum-melted heats of Ni-Cr steel individually doped with Sb, Sn, and P, and having the chemical compositions listed in Table I, were austenitized at 1025 ~ for 1 h and tempered at 600 ~ for 1 h to obtain an average pr~.or austenite grain size of 120/tm and a hardness of Rc 30. They were then aged at 480 ~ for various lengths of time to cause various amounts of segregation along grain boundaries. Some Sb-doped specimens were homogenized at 1250 ~ for 24 h and then isothermally transformed (to refine the coarsened austenite grain structure resulting from the
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