Crystallography and interphase boundary of (MnS + VC) complex precipitate in austenite
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I. INTRODUCTION
INCLUSIONS and precipitates in austenite (fcc ) have been used as intragranular nucleation sites of proeutectoid ferrite for microstructure refinement in low-carbon structural steels to improve toughness when the deformation of austenite matrix before transformation is hardly applicable[1] such as the heat-affected zone formed in welding of structural steels. Among various kinds of inclusions and precipitates, a (MnS V(C,N)) complex precipitate is one of the most preferential nucleation sites of intragranular ferrite[2–6] and pearlite.[7] Those MnS particles are mostly incoherent to the surrounding matrix because the composition of steels and the processing routes employed result in irrational ORs between MnS and . The present authors recently studied[8] multiphase crystallography between ferrite, (MnS V(C,N)) complex precipitate and austenite in Fe-Mn-C alloys. VC holds a cube-cube OR, i.e., 001 //001VC, when it is formed directly within grains in Fe-12Mn-0.8C-0.3V. When VC precipitates nucleate on incoherent MnS particles dispersed in , there is no specific OR between the three phases. Yazawa et al.[9] found that after matrix containing fine VC precipitates is recrystallized, the morphology and crystallography of the VC change significantly, exhibiting no specific VC/ OR as well as no specific morphology. In those studies, clear planar facets were often observed on VC/ boundary even with irrational ORs, which deviate largely from the OR for ordinary precipitation. T. FURUHARA, Associate Professor, and T. MAKI, Professor, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Contact e-mail: [email protected]. kyoto-u.ac.jp T. KIMORI, formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University, is Engineer, Yawata Works, Nippon Steel Corporation, Fukuoka, 804-8501, Japan. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
Previously, interphase boundary structure of grain boundary bcc precipitate in fcc Ni-Cr was studied.[10] On both sides of the grain boundary, it was found that its interphase boundaries exhibit planar facets regardless of the presence of (near) a rational OR. Geometrical analysis with near coincidence site lattice (NCS) model[11] revealed that many of the facet planes correspond to the orientations with high NCS densities for either near-rational or irrational ORs. It was suggested in that periodic arrangement of NCS even in cases of low NCS density can provide some inhibition of boundary migration in recent studies on irrational boundary between massive phase and its matrix in Ti-Al.[12,13] However, syste
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