Grain Refinement by Cyclic Displacive Forward/Reverse Transformation in Fe-High-Ni Alloys
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LATH martensite is used in high-strength steels as a base microstructure in industrial structural materials. Lath martensite is formed with a high dislocation density and holds near the Kurdjumov–Sachs (K–S) orientation relationship (OR) ((111)fcc//(011)body-centered cubic (bcc), [101]fcc//[-111]bcc) with respect to the parent austenite phase.[1] Because it is close to the K–S OR, 24 variants can be formed in a single austenite grain, which results in the formation of a characteristic hierarchical structure in lath martensite. An austenite grain is divided into packets, each of which is composed of laths with a nearly parallel habit plane, and a packet is further divided into blocks that consist of laths with nearly the same orientation. Because most packet and block boundaries are of a high angle and impede crack propagation and slip deformation,[2–4] a refinement of the block or packet structure is an effective way to improve strength without reducing toughness. In general, two methods exist for martensite structure refinement: one is the refinement of austenite grain size that is usually achieved by austenite recrystallization or austenite reversion. A finer austenite grain size has a finer packet and block size.[5,6] Grange[7] proposed a cyclic
TADACHIKA CHIBA is with the Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showaku, Nagoya, Aichi, 466-8555 Japan. Contact e-mail: chiba.tadachika@ nitech.ac.jp SHIRAZI HASSAN is with the School of Metallurgy and Materials Engineering, University of Tehran, 14395-731 Tehran, Iran. GORO MIYAMOTO and TADASHI FURUHARA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. Manuscript submitted November 10, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation process in which austenite grain growth is retarded by short and rapid heating followed by quenching. A finer martensite structure can be obtained, which leads to further grain refinement of reverted austenite in a subsequent cyclic process. Grange applied cyclic transformation to low-alloyed steels in which austenite was formed by diffusional transformation and reported that austenite grains are refined after a few cycles. The other method is martensite transformation from work-hardened austenite, which is termed ausforming. Martensite that is transformed from work-hardened austenite grains is refined because of a high density of dislocations in austenite that are introduced by deformation prior to martensitic transformation, and may accelerate martensite nucleation with its growth retardation.[8–10] An improvement in strength can be achieved without reducing the toughness. Carbon addition enhances the strengthening effect by ausforming.[11] If displacive reversion occurs during heating, an introduction of dislocations into austenite should be accompanied by reversion, and martensite dislocation will be inherited to austenite. Therefore, a cyclic displacive forward and reverse transformation will
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