과냉 액상 영역에서 Ti계 벌크 비정질 합금의 변형 거동과 성형성 및 그에 관한 유한 요소 분석

Abstract

Bulk metallic glasses (BMGs) exhibit unique physical properties such as high strength and hardness, excellent corrosion resistance and improved wear resistance, which they have generated growing attention in the past decade. They have, however, disadvantage of poor ductility at room temperature due to rapid propagation of primary shear band. Bulk metallic glass composites (BMGCs) have, on the other hand, been developed in order to overcome the disadvantage by formation of the secondary phases through the in-situ and ex-situ methods. Deformation behavior of BMG and BMGC alloys is, in general, inhomogeneous at room temperature developing localized narrow shear bands, even though the ductility of BMGCs becomes larger than that of monolithic BMGs. In contrast with deformation behavior at room temperature, the high temperature deformation behavior was observed to become homogeneous along the whole specimen within supercooled liquid region (SLR). Furthermore, they shows have a good processability in a supercooled liquid state enabling near-net-shape fabrication through a conventional mold casting followed by warm deformation. In this study, attempts have been made in the above regard to investigate the deformation behavior of Ti43.3Zr21.7Ni7.5Be27.5 (at. %) BMG and develop an optimum process condition for a warm forming. Three different types of true stress-strain curves were identified under uniaxial compression tests within SLR depending on the strain rates and test temperatures. The first type did show a large viscous flow without stress overshoot, i.e. a Newtonian viscous flow, typically observed at high temperature and low strain rate region. The second type, a steady-state flow after stress overshoot, was observed at intermediate temperature and strain rate region. Finally the third type, a brittle fracture caused by shear localization, was generated at low temperature and high strain rate region. As a result, an experimental deformation map could be constructed including the boundaries among three different deformation behaviors.The basic processing maps based on dynamic materials model (DMM) have been constructed to establish feasible forming conditions. This alloy shows high efficiency of forming at the intermediate temperature range of SLR, where the input energy was used most effectively for mechanical deformation. Crystallization during warm forming of this BMG alloy due to poor thermal stability couldn’t, however, estimated through the processing map and then conditions for sound forming was very limited though high efficiency region of forming has widely distributed on the map. Therefore, the processing map has been needed to modify by addition of the empirical crystallization onset time region and then optimum processing condition could be estimated from the modified map. In order to analyze the high temperature deformation behavior and warm formability which are important properties for reliable applications of advanced materials, the free volume model based constitutive equation of BMG alloy has been combined with the finite element method (FEM) and applied to the temperature and strain rate dependent deformation behavior at the temperatures within SLR. Validity of the current approach was assessed through the reproduction of compression data for the Ti43.3Zr21.7Ni7.5Be27.5 (at. %) BMG alloy by FEM. The FEM simulations combined with the free volume constitutive model well reproduced the deformation behavior, such as Newtonian viscous flow without stress overshoot and non-Newtonian viscous flow with stress overshoot in SLR under slow and fast strain rate conditions, respectively. In particular, warm formability of this BMG alloy within SLR was successfully estimated using FEM analysis. This constitutive model could be also combined with Iso-Work model to apply the high temperature deformation behavior of Zr76.11Ti4.20Cu4.51Ni3.16Be1.49Nb10.53 (at. %) BMGC alloy and then confirmed to well reproduce the deformation behavior. Especially, high temperature deformation behaviors of each phase such as metallic glass matrix and dendrite phase which are impossible to observe through experimental compression tests were able to be estimated through the model.