Scaling up of High-Pressure Sliding (HPS) for Grain Refinement and Superplasticity
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TRODUCTION
GRAIN refinement is an important requirement for high strength and/or enhanced ductility in metallic materials. For the former, the strength increases with a decrease in the grain size through the Hall–Petch relation.[1,2] Extra strengthening may be expected if the grain size is reduced to the submicrometer or to the nanometer levels and the fine-grained structure is free of dislocations.[3] For the latter, the ductility is well enhanced as an advent of superplasticity, although the deformation should be carried out at elevated temperature.[4] According to the constitutive equation for the deformation, the superplasticity may appear at higher strain rates when the grain size is reduced to the submicrometer ranges.[4–10]
YOICHI TAKIZAWA, Deputy Director, is with the Technology Department, Nagano Forging Co., Ltd, Nagano 381-0003, Japan, and Ph.D. Student with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan, and also with WPI, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), KyushuUniversity. Contact e-mail: [email protected] TAKAHIROMASUDA, KAZUSHIGE FUJIMITSU, TAKAHIRO KAJITA, and KYOHEI WATANABE, Graduate Students, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University. MANABU YUMOTO, Chief, and YOSHIHARU OTAGIRI, Director, are with the Technology Department, Nagano Forging Co., Ltd. ZENJI HORITA, Professor, is with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, and also with WPI, International Institute for CarbonNeutral Energy Research (WPI-I2CNER), Kyushu University. Manuscript submitted January 31, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
It is well known that the grain refinement to the submicrometer and/or nanometer range is achieved using the process of severe plastic deformation (SPD).[11] Although several processes are available for the SPD,[12,13] the process under high pressure, known as high-pressure torsion (HPT),[14–16] is effective for the grain refinement in hard and less ductile materials even such as intermetallics,[17–22] semiconductors,[23–26] and ceramics.[27,28] The sample for the HPT processing is used in a form of disk[14] or ring[29–31] and it may be extended to ribbons or wires if they are processed in a continuous way using continuous HPT (CHPT).[32,33] However, the direct application to sheet or rod samples should be feasible when sliding under high pressure, called high-pressure sliding (HPS), is used as schematically illustrated in Figure 1.[34] This process was developed such that two samples of sheets[34] or rods[35] were placed between the anvils and plunger. The plunger was then pushed for shearing while applying a load on the samples between the anvils. In this study, the HPS facility is scaled up so that it is possible to increase the sample size and apply to high-strength materials such as a Ni-based superalloy and a Ti-based alloy. It is then demonstrated that the s
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