Mechanical Behavior on Micro-compression Test in Ultra-low Carbon Steel Produced by High Pressure Torsion
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Mechanical Behavior on Micro-compression Test in Ultra-low Carbon Steel Produced by High Pressure Torsion. Takashi Nagoshi1, Akinobu Shibata2, Masato Sone3 and Yoshikazu Todaka4 Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 2 Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan 3 Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8503, Japan 4 Toyohashi University of Technology, Toyohashi 441-8580, Japan 1
ABSTRACT Mechanical behavior of ultra-fine grained (UFG) steel fabricated by high pressure torsion (HPT) was investigated by micro-sized compression test of a micro-sized pillar with uniform dimensions (non-tapered, non-filleted) carefully fabricated by FIB milling. After HPT process, grains were elongated to shear direction and its grain size was decreased down to 300 nm in diameter with increasing strain amount. Compression test confirmed that the uniform elongations of ultra-fine grained materials were lower than 3% and do not depend on the grain size. INTRODUCTION Ultra-fine grained (UFG) materials have been attractive research topics in recent years in terms of their extremely high strength. However, elongation of UFG materials is limited compared to coarse grain materials and it decreased with decreasing grain size. Yu et al. confirmed when the grain size reaches sub-micron scale, failure mode is only necking without uniform deformation in tensile test of UFG aluminum [1]. Uniaxial compression testing is required to obtain substantial plastic deformation, and it allows us to evaluate work hardening behavior which presumably corresponds to poor ductility in UFG materials. UFG structure can be introduced by severe plastic deformation (SPD) process such as high pressure torsion (HPT), equal-channel angular pressing (ECAP) and accumulative rollbonding (ARB) [2]. The HPT process can apply extremely high strain by only single pass. The samples subjected to HPT have inhomogeneous distribution of microstructure from center to edge of the samples [3], because induced strain is different from center to edge of the samples. This inhomogeneity combined with micro-sized specimen allows us to fabricate specimens with various grain sizes. In this study, we proposed a novel micro-compression test with micro-sized pillar to investigate the mechanical properties of UFG materials. It is well known that the size effects often arise when the specimen dimension is in the scale of micro meter. According to the systematic study of Uchic and Dimiduk [4], the size effects are absent in single crystal Ni super alloy with sample size of 10 Pm. In order to eliminate the occurrence of size effect, we use the sample with dimensions above 20 Pm. On the sample preparation, conventional fabrication method with ion beam from pillar direction is not feasible in terms of pillar dimension (taper)
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and damage to the fixed end [4]. On the other hand, our proposed new fabrication metho
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