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고영건 ( Young Gun Ko ),김우겸 ( Woo Gyeom Kim ),박경태 ( Kyung Tae Park ),이종수 ( Chong Soo Lee ),신동혁 ( Dong Hyuk Shin ) 대한금속재료학회 ( 구 대한금속학회 ) 2006 대한금속·재료학회지 Vol.44 No.5
Strain hardening behavior of ultrafine grained (UFG) dual-phase (DP) steels via equal-channel angular pressing and subsequent intercritical annealing followed by water quenching was investigated at ambient temperature. The formation of UFG martensite islands of ~1㎛ was not confined to the prior pearlite colonies, but uniformly distributed throughout UFG ferrite matrix. Their room temperature tensile properties were examined and compared to those of coarse grained DP steels. Although uniform and total elongations were not degraded, the strengths of UFG-DP steels were much higher than that of coarse grained DP steels. In contrast to conventional UFG steels fabricated by equal-channel angular pressings, UFG-DP steels exhibited significant strain hardening rate, which was attributed to uniform distribution of island-typed martensite as well as grain refinement of each constituent phase throughout the microstructure. Such strain hardening behavior was discussed in relation to modified C-J analysis based on Swift relationship.
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ECAP으로 제조된 초미세립 5083 Al 합금의 동적 변형거동
김양곤,황병철,이성학,김우겸,신동혁 대한금속재료학회 2005 대한금속·재료학회지 Vol.43 No.1
Dynamic deformation behavior of ultra-fine-grained aluminum alloys fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four aluminum alloy using a torsional Kolsky bar, and then the test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear band formation. The ECAP'ed specimens consisted of a considerable amount of second phase particles, which were refined and had an equiaxed shape as the ECAP pass number increased. The dynamic torsional test results indicated that maximum shear stress increased, while fracture shear strain remained constant, with increasing ECAP pass number. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that a number of voids initiated mainly at second phase particle/matrix interfaces, and that the number of voids increased with increasing pass number. Adiabatic shear bands of 100-150μm in width were formed in the as-extruded and 1-pass specimens having coarser second phase particles, while they were hardly formed in the 4-pass and 8-pass specimens having finer second phase particles. The possibility of the adiabatic shear band formation was explained by concepts of absorbed deformation energy and void initiation. (Received October 29, 2004)
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