Fully austenitic Fe-28Mn-10Al-1.0C steel with high stacking fault energy exhibited exceptionally high uniform elongations (85 to 100 pct) and total elongations (100 to 110 pct) at room temperature. The origin of such exceptional room-temperature ductility was rationalized in terms of strain accommodation mechanisms of reduction of glide plane spacing in Taylor lattice (TL) formation at low strains and TL rotation forming domain boundaries (DBs) and microbands (MBs) at high strains.Transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) associated with austenite are attractive for enhancing the mechanical properties of steels. [1] Phase transformation of austenite to martensite in TRIP and mechanical twin formation in TWIP depend on the austenite stability and stacking fault energy (SFE). TRIP and TWIP dominantly occur when SFE of austenite is <20 mJ/m 2 and 20 to 50 mJ/m 2 , respectively. [2,3] Meanwhile, by investigating deformation of a fully austenitic steel having high SFE (~90 mJ/m 2 ), the present authors [4] recently suggested a new plasticity called ''microband induced plasticity (MBIP)'' in which microbands (MBs) formed by planar glide accommodate a large amount of strain. The MBIP steel exhibited exceptionally high uniform elongation of~80 pct and total elongation of~100 pct at room temperature, which cannot be attained in any class of existing steels. The present study is aimed at rationalizing the origin of such extraordinary high elongation of the fully austenitic steel with the high SFE by correlating the microstructural evolution during deformation to its strain hardening behavior in detail in light of MBIP.The steel having composition of 0.98 C, 28.2Mn, 9.95Al, 0.003P, 0.0038S, and the balance Fe in wt pct was supplied in the form of 12-mm-thick hot-rolled plates (Kwangyang Mill, POSCO, Kwangyang, Korea). The plates were fully recrystallized (1000°C for 1 hour), cold rolled (65 pct reduction), solution treated (1000°C to 1200°C for 1 hour), and water quenched. Roomtemperature tensile tests were performed on the samples (the gage dimension of 25.4 mm 9 6 mm 9 2 mm) taken from quenched plates using a universal testing machine (Instron* model 4484) at the initial strain rate of 10 À3 s À1 . Some tests were interrupted at predetermined strains to observe the microstructures developed at the different strain levels. Microstructures were optically examined by etching the mechanically polished samples with 2 pct Nital. Phase identification was performed by using X-ray diffraction (XRD, Rigaku**, D/max 2500H). Submicrostructures of deformed samples were observed using a transmission electron microscope (TEM, JEOL 3011) operated at 300 kV. Figure 1 shows the optical micrographs of the steel solution treated at 1000°C to 1200°C and the corresponding XRD profiles. The XRD results revealed that the steel was fully austenitic due to the high Mn content. The austenite grain size was~63 ± 7 lm,~114 ± 14 lm, and~350 ± 52 lm for 1000°C, 1100°C, and 1200°C, respectively, and most grains cont...

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Abstract. The shear formability and the metal jet formability are important for the kinetic energy penetrator and the chemical energy penetrator, respectively. The shear formability of ultrafine grained (UFG) steel was examined, mainly focusing on the effects of the grain shape on the shear characteristics. For this purpose, UFG 4130 steel having the different UFG structures, the lamellar UFG and the equiaxed UFG, was prepared by equal channel angular pressing (ECAP). The lamellar UFG steel exhibited more sharper and localized shear band formation than the equiaxed UFG steel. This is because a lamellar UFG structure was unfavourable against grain rotation which is a main mechanism of the band propagation in UFG materials. Meanwhile, the metal jet formability of UFG OFHC Cu also processed by ECAP was compared to that of coarse grained (CG) one by means of dynamic tensile extrusion (DTE) tests. CG OFHC Cu exhibited the higher DTE ductility, i.e. better metal jet stability, than UFG OFHC Cu. The initial high strength and the lack of strain hardenability of UFG OFHC Cu were harmful to the metal jet formability. IntroductionUltrafine grained (UFG) materials usually exhibit higher strength due to the Hall-Petch strengthening but lower ductility due to shear localization than coarse grained (CG) counterparts at room temperature [1]. Meanwhile, some UFG materials show high strain rate superplasticity (HSRS) at high temperatures [2]. The shear localization is beneficial for the self-sharpening of the kinetic energy penetrator. HSRS is possible to operate on the metal jet formation of the metal liner in the chemical energy penetrator [3]. Accordingly, UFG materials are promising as the high performance penetrator materials. In this study, the shear localization behaviour of UFG steel was examined focusing on the effects of the grain shape. Besides, the metal jet formability of UFG Cu was compared to that of CG counterpart by means of the dynamic tensile extrusion test. Experimental4130 steel and OFHC Cu were selected as the model materials to examine the shear localization and the metal jet formability, respectively. Both were subjected to equal channel angular pressing (ECAP) in order to fabricate a UFG structure. Tempered martensitic 4130 steel was subject to 4 passes of ECAP with routes A and B c at 400• C. Routes A and B c with 4 passes resulted in a lamellar UFG structure and an equiaxed UFG structure, respectively [4]. Well-annealed OFHC Cu was subjected to 16 passes of ECAP with route C at room temperature. Route C of 16 passes also introduced an equiaxed UFG structure [4].In order to examine the shear localization behavior, tensile tests with the initial strain rate of 10 −3 s −1 were carried out on both lamellar and equiaxed UFG 4130 steels. a e-mail: ktpark@hanbat.ac.krThe shear localization during tensile test was monitored by a high speed camera.Dynamic tensile extrusion (DTE) [5] was employed to examine the metal jet formability of CG and UFG OFHC Cu. DTE tests were carried out by launching the sphere sample ...