Relationship between crystallographic orientation at the boundaries and brittle crack propagation in Mn-Mo-Ni low alloy
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als I, W.O. Soboyejo, T.S. Srivatsan, and D. Davidson, eds., TMS, Warrendale, PA, 1993, pp. 27-37. 13. C. Mercer and W.O. Soboyejo: Acta Mater., 1997, vol. 45, pp. 961-71.
Relationship between Crystallographic Orientation at the Boundaries and Brittle Crack Propagation in Mn-Mo-Ni Low Alloy Steel MIN-CHUL KIM, YONG JUN OH, and JUN HWA HONG Grain boundaries in material can act as the barrier and/ or source of dislocation. Therefore, boundary properties between two adjacent grains with different crystallographic orientations are important factors in deciding the mechanical properties of materials. To clarify the effects of the grain boundary on deformation transfer, many workers have performed investigations using bicrystal or polycrystalline materials.[1–5] The concept of the geometric compatibility factor has been introduced in order to explain the relationship between the transfer of deformation mode and crystal orientation.[5–8] In the case of brittle crack propagation, the grain boundary also obstructs crack propagation, and the path of crack propagation can be determined by the orientation relationship between two adjacent grains. Brittle crack propagation, which occurs on a specific cleavage plane, is closely related to the arrangement of the possible cracking plane in two adjacent grains. Therefore, it is reasonable to investigate the characteristics of brittle crack propagation from the viewpoint of the crystallographic orientation relationship between possible cracking planes, and it is also possible to introduce the concept of geometric compatibility in analyzing crack propagation. In this work, the concept of geometric compatibility, which had been used for deformation transfer, is modified to explain brittle crack propagation, and the modified geometric compatibility factor is evaluated by Electron backscattered diffraction (EBSD) analysis on the crystallographic orientation of grains around a secondary crack. ASTM A508 cl. 3 forged low-alloy steel was investigated in this study. The chemical composition of the sample is shown in Table I, and the steel refining process and heat treatment condition are also presented under this table. A three-point bending test was performed at ⫺100 ⬚C, and a secondary crack below the fracture surface was analyzed in relation with the crystallographic orientation. The specimens used for the three-point bending test were the precracked Charpy V-notched specimen, and the fatigue crack grew to 50 pct of the specimen width at room temperature. The crystallographic orientation of the microstructure has been analyzed by the EBSD method using the Link OPAL system
MIN-CHUL KIM, Postdoctor, YONG JUN OH, Senior Researcher, and JUN HWA HONG, Director, are with the Nuclear Materials Technology Division, Korea Atomic Energy Research Institute, Yusong, Taejon 305600, Korea. Manuscript submitted April 21, 2000. VOLUME 32A, AUGUST 2001—2139
Table I. Chemical Composition of the ASTM A508 cl. 3 Steel Studied Chemical Composition (Wt Pct) C
Si
Mn
P
S
Ni
Cr
Mo
Al
Cu
V
0.17
0.25
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