Grain Refinement by Cyclic Displacive Forward/ Reverse Transformation in Fe-High-Ni Alloys TADACHIKA CHIBA, SHIRAZI HASSAN, GORO MIYAMOTO, and TADASHI FURUHARA A novel method for grain refinement of martensite structures was proposed, in which transformation strain is accumulated by cyclic displacive forward and reverse transformations. This method can refine martensite structures in an Fe-18Ni alloy because a high density of austenite dislocations is introduced by a displacive reverse transformation in addition to an inheritance of dislocations in body-centered cubic martensite into austenite during cyclic transformation. The addition of a small amount of carbon accelerates structure refinement significantly, which results in the formation of ultra-fine-grained structures after ten cycles. DOI: 10.1007/s11661-017-4152-4 Ó The Minerals, Metals & Materials Society and ASM International 2017 I. INTRODUCTION 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 //[-  1  11] 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 gen- eral, two methods exist for martensite structure refine- ment: one is the refinement of austenite grain size that is usually achieved by austenite recrystallization or austen- ite reversion. A finer austenite grain size has a finer packet and block size. [5,6] Grange [7] proposed a cyclic 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 aus- forming. 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 trans- formation, and may accelerate martensite nucleation with its growth retardation. [8–10] An improvement in strength can be achieved without reducing the tough- ness. 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 lead to martensite structure refinement because of the accu- mulation of dislocations. A prior austenite grain struc- ture is recovered by reversion from the martensite structure in some cases, which is termed the austenite memory effect. [12–14] Displacive reversion may facilitate austenite memory [12] and be unfavorable in terms of austenite grain size refinement. Our research group has investigated displacive reverse transformation in Fe-18 mass pct Ni and Fe-23 mass pct Ni binary alloys by in situ observations. [15] Sharp surface relief during reversion was observed from the displacive reversion and because the martensite structure after quenching TADACHIKA CHIBA is with the Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa- ku, 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