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Deformation and crystallization behaviors of metallic glass powders during warm rolling
김휘준 Graduate School, Yonsei University 2010 국내박사
Bulk metallic glass alloys exhibit several superior properties associated with the atomic structure. These unique properties that can be rarely found in crystalline materials are attractive for the practical applications as new classes of structural as well as functional materials. Recently, bulk metallic glass alloys with high glass forming ability were developed mainly in the Zr-, Ti-, Ni-, Cu- and Mg-based alloy systems. However, due to the requirement of high cooling rate for the formation of amorphous phase from a liquid state, practical applications for the near net shape fabrication are restricted. The fabrication of bulk metallic glass alloy by consolidation of metallic glass powders provides an advantage since it enables the production of complex shapes without limitations in sample shape and dimension. Since the consolidation of metallic glass powders is possible using the significant viscous flow of the supercooled liquid, the metallic glass alloy should have a wide supercooled liquid region, ΔTx = Tx - Tg (where Tg is glass transition temperature and Tx is crystallization temperature). Many studies have previously been performed on the consolidation of metallic glass powders by various techniques including warm extrusion, hot pressing and equal channel angular extrusion. However, no results have been reported on the consolidation of metallic glass powders by warm rolling, which is a useful method since it is possible to produce plate-type bulk metallic glass alloys. In the present study, the behaviors of consolidation, viscous flow deformation and crystallization in rolled BMGs sheet during warm rolling were investigated in order to fabricate thin gauge BMGs sheet with fully amorphous phase using warm rolling of the metallic glass Cu54Ni6Zr22Ti18 powder. The effect of rolling parameters on the crystallization and crack formation was studied. Furthermore, the finite element method (FEM) technique was used to shear strain states in rolled BMGs sheet during warm rolling of MG powder, leading to understanding the effect of both friction element and geometric element on the shear strain states and crack formation. Since deformation of rolled sheet occurs at high strain rate of over 10 s-1, and monolithic BMG sample is fractured at that strain rate under uniaxial compression test, no reports have been performed on the flow stress-strain curve of monolithic BMG in the supercooled liquid region at high strain rate of over 10 s-1 . In the present study, using the rule of mixture and flow stress curves obtained from both BMG/crystalline composite and crystalline Ni, the flow stress-strain curves of monolithic BMG were able to be calculated at high strain rate of 10 s?1 in the supercooled liquid region. The FEM simulated effective strain was compared with the effective strain measured from microstructure of rolled BMG plate in order to verify FEM results of BMG warm rolling process. The experimentally measured effective strain agreed with that simulated using FEM at the surface layer of rolled BMG plate (s=0.9). There was good agreement between the simulated and measured effective strain under thickness reduction of 30 % and 50%, demonstrating the ability of the FEM analysis to evaluate the strain states of rolled BMG plate during warm rolling process. The cause of crack formation during warm rolling of BMGs can be classified into two types; crystallization resulting from temperature increase during warm rolling, inhomogeneous shear strain states due to temperature gradient and displacement rate gradient in the rolled BMG sheet. In order to prevent crystallization resulting from work heat, reduction ratio in one pass should be controlled, resulting in remaining the temperature of rolled plate under Tx. The thin gauge Cu?Ni?Zr?Ti BMG strips with thickness of 1mm were produced by multi-pass warm rolling of the MG powders. Besides, homogeneous shear strain states should be controlled by roll temperature and can thickness. The roll temperature of under 420 K increased the temperature gradient between surface layer and center layer of rolled BMG plate, leading to the formation of crack during warm rolling. The finite element method simulation disclosed that sandwich rolling with thick outer sheets minimized the shear strains leading to a strain state close to the plane strain in the rolled BMG sample. In the present study, a thick BMG plate with nominal composition of Zr41.2Ti13.8Cu21.5Ni10Be22.5 was first produced by vacuum centrifugal casting. In order to obtain homogeneous deformation of BMG during rolling, the as-cast BMG plate was sandwiched between the outer copper sheets and then jointly warm-rolled in the supercooled liquid region. It was found that as-cast BMGs plate as well as the plate rolled from MG powders was able to be rolled into thin gauge sheet by means of the sandwich warm rolling. The effect of the outer copper sheet on the evolution of strain states in the sandwiched samples was studied by finite element method (FEM) simulations. From the future understanding and application of deformation behavior in the supercooled liquid region, it can be possible to fabricate intermediate-shaped materials of BMGs such as plate, sheet, rod, and tube, which can result in extending the application of BMGs in the field of various industries. Thus, the study on BMGs has an important scientific value for the development of future material.
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