Designing, Processing and Properties of Advanced Engineering Materials

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Authors: Young Sang Na, Young Mok Rhyim, J.Y. Lee, Jae Ho Lee
Abstract: In order to quantitatively analyze the critical strain for the initiation of dynamic recrystallization in Ni-Fe-based Alloy 718, a series of uniaxial compression tests was conducted in the temperature range 927°C - 1066°C and the strain rate range 5 x 10-4 s-1 - 5 s-1 with varying initial grain size. The critical strains were graphically determined based on one parameter approach and microscopically confirmed. The effect of γ'' (matrix-hardening phase) and δ (grain boundary phase)on the critical strain was simply discussed. The constitutive model for the critical strain of Alloy 718 was constructed using the experimental data obtained from the higher strain rate and the temperature range between 940°C and 1040°C.
Authors: Yong Nam Kwon, Young Seon Lee, J.H. Lee
Abstract: Thermomechanical behavior of Al-Mg-Si alloys has been studied to investigate the effect of microstructural features such as pre-existing substructure and distribution of particles on the deformation characteristics. The controlled compression tests have been carried out to get the basic information on how the alloy responds to temperature, strain amount and strain rate. Microstructural features after forging have been discussed in terms of the thermomechanical response of Al-Mg-Si alloys. As already well mentioned, we have found that the high temperature deformation of Al-Mg-Si brought the recovered structure. However, the abnormally large grains seems to occur in a certain condition as a result of deformation assisted grain growth, which means that hot forging of Al-Mg-Si alloys could lead to the undesirable microstructures and the consequent mechanical properties such as fatigue strength.
Authors: In Bae Kim, Kwang Nyeon Kim, Kyung Hyun Kim, In Gon Kim
Abstract: Effects of Si and Cr additions and aging treatment on the stress corrosion cracking(SCC) resistance of Al-Zn-Mg-Cu cast alloys were investigated by C-ring test and electrical conductivity measurement. The SCC resistance of Al-Zn-Mg-Cu cast alloys decreased in the order of Cr containing alloy, Si containing alloy and base alloy. The lowest electrical conductivity of Cr containing alloy exhibited the best SCC resistance and this is probably due to an improved corrosion resistance by forming a passive film on the surface. The over-aged alloys showed the better SCC resistance. This is due to the increased distance between coarse particles at grain boundary. The fracture mode of the alloy was confirmed as intergranular type and showed brittle fracture surface. The SCC mechanism of Al-Zn-Mg-Cu cast alloys was found to be the anodic dissolution model.
Authors: Si Young Chang, Sang Ho Seo, Seong Hee Lee, Chang Seog Kang, Sung Kil Hong, Dong Hyuk Shin
Abstract: The nano-structure in the commercial 5083 Al alloy was introduced by the equal channel angular pressing technique. The nano-sized grains of ~300 nm were obtained after 8 ECAPs at 373 K and 473 K. It was also obvious that much improvement in strength was obtained at lower pressing temperature, 373 K, than 473 K. In particular, there were mainly two different tensile characteristics; one was that the strength in the alloy ECA pressed at 373 K was much higher than that of as-annealed alloy but the elongation to failure was significantly smaller, while, in case of pressing at 473 K, the improvement in strength was found without sacrificing much of the elongation. The other was that the work hardening with increasing the amount of deformation was found in the alloy ECA pressed at 473 K. These tensile deformation characteristics were analyzed based on the observations of microstructure by TEM and fracture surface by FE-SEM.
Authors: Jun Hyun Han, Dong Ik Kim, Kwang Koo Jee, Kyu Hwan Oh
Abstract: Orientation rotation behaviors taking place in aluminum single crystal and polycrystal were studied during uniaxial tensile deformation at room temperature. Specimens were elongated in an in-situ deformation stage equipped on a high resolution electron backscattered diffraction (HREBSD)system. A series of crystallographic evolution and deformation behavior of concerned areas in the specimens during deformation was directly analyzed using HR-EBSD.
Authors: Yong Suk Kim, J.S. Ha, Woo Jin Kim
Abstract: Dry-sliding-wear behavior of ultra-fine grained 6061 Al alloy and AZ61 Mg alloy was investigated. The accumulative roll bonding (ARB) and the equal channel angular pressing (ECAP) processes were employed to obtain refined microstructures in the Al and Mg alloys, respectively. Pin-on-disk wear tests of the processed alloys were carried out with various applied load against a 304 stainless steel counterpart. In spite of the increased hardness and strength, wear resistance of the ultra-fine grained 6061 Al alloy was lower than that of the coarse-grained starting alloy. The strength and wear resistance of the ECAP processed AZ61 Mg alloy did not change appreciably despite the refined microstructure. Recrystallization was found to occur during the ECAP process of the Mg alloy. Worn surfaces and cross-sections of the wear-tested specimens were examined to investigate the wear mechanism of the ultra-fine grained Al and Mg alloys.
Authors: Won Bae Lee, Hyung Sun Jang, Yun Mo Yeon, Seung Boo Jung
Abstract: The hardness distribution related to the precipitates behaviors as friction stir welded and PWHT (post weld heat treated) 6061 Al alloy have been investigated. Frictional heat and plastic flow during friction stir welding created a fine, eqiuaxed and elongated microstructure near the weld zone due to dynamic recovery and recrystallization. A softened region which had been formed near the weld zone couldn't be avoidable due to the dissolution and coarsening of the strengthening precipitates. PWHT (SHT+ Aging) homogeneously recovered the hardness distribution over that of the base metal without softening region, resulted from non-homogeneously distributed hardness only aging treated. 36ks aging followed by SHT gave a higher hardness overall weld than that of the base metal due to a higher density of the spherical shaped precipitate.;
Authors: Dong Woo Suh, Sang Yong Lee, Jun Yun Kang, Kyu Hwan Oh
Abstract: Rapid grain growth and artificial aging characteristics during heat treatment is investigated for hot extruded Al-Zn-Mg-Cu-(Sc) alloys. Two Al-0.1wt%Sc alloys with different alloying element content are hot extruded to make T-shape bars at 380°C, and then the bars are solution treated for 2 hours at 480oC followed by artificial aging for 24 hours at 120°C. Microstructural evolution of the hot extruded bar is analyzed with optical microscope and electron back scattered diffraction (EBSD) mapping. Two kind of extruded bar shows different grain growth behavior at surface region and different artificial aging characteristics. The interaction between the precipitates and the grain growth during the heat treatment is thought to be responsible for the different grain growth behavior.
Authors: Woo Jin Park, Tae Kwon Ha, Sang Ho Ahn, Young Won Chang
Abstract: The spray forming process was used to produce Al-25Si alloy in this study. The microstructure of the hypereutectic Al-25Si alloy appeared to consist of Al matrix and equiaxed Si particles with average diameter of 5~7 µm. To deduce the extrusion condition, a series of compression tests has been conducted at temperatures ranging from RT to 500oC. The strain rate sensitivity parameter (m) of this alloy has been found to be very low ( ≤0.1) below 300oC and reached maximum value of about 0.13 at 500°C. The extrusion has been successfully conducted at the temperatures of 300°C and above with the ratio of area reduction of 28 and 40. The processing map for hot working has also been constructed.
Authors: Seong Hee Lee, Chung Hyo Lee, Si Young Chang
Abstract: Aluminum based metal matrix composite (MMC) was processed by accumulative roll bonding (ARB) for ultra grain refinement and high strengthening. The ARB process up to 4 cycles was performed for the composite with 5vol.%SiC at ambient temperature under unlubricated conditions. The ARB of unreinforced aluminum powder compact was also performed for comparison. The tensile strength of the composite increased with the number of ARB cycles, and reached a maximum of 375MPa at the 3rd cycle, which is 1.8 times higher than that of the initial material. An increment of the strength per cycle was much larger in the composite than that in the unreinforced 6061 aluminum powder compact. The elongation of the composite decreased gradually with the number of ARB cycles, became almost zero after 4 cycles. TEM observation revealed that the composites fabricated by 1 to 3 cycles showed a dislocation cell structure, but after 4 cycles it showed an ultra-fine grained structure with mean grain size below 500nm. The ultra-fine grains developed at lower cycles in the composite than in the unreinforced one.

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