Designing, Processing and Properties of Advanced Engineering Materials

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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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Authors: In Bae Kim, Yong Su Park, Kyung Hyun Kim, In Gon Kim
Abstract: Effects of Si and Cr additions on the fatigue properties of Al-Zn-Mg-Cu cast alloy were investigated by low and high cycle fatigue tests. It was found that in the low cycle fatigue test, fatigue life of base alloy showed the maximum value of 3,075 cycles, whereas in Si and Cr containing alloys, it was 2,993 and 1,413 cycles, respectively. The same trend was obtained in high cycle fatigue test, i.e., the fatigue strength in base alloy showed the highest value of 104MPa and decreased to 100MPa for Cr containing alloy and 81MPa for Si containing alloy. The fatigue ratio was about 0.20 for all three alloys. The tensile strength of base alloy also showed the maximum value of 513MPa, and decreased with the addition of Si and Cr to 400 and 500MPa, respectively. Metallographic observation revealed that the fatigue crack initiated at the surface and propagated along the grain boundary.
617
Authors: Kyung Mok Cho, Il Dong Choi, Ik Min Park
Abstract: Compositionally graded Al-SiCp composites were fabricated using pressureless infiltration process. Microstructure was examined and thermal properties were characterized for Al-SiCp composites. Al-SiCp composites with fairly uniform distribution and compositional gradient of SiC reinforcement in the Al matrix though the thickness direction was successfully fabricated. The thermal conductivity of Al-SiCp composites was measured at room temperature, 200°C and 400°C using laser flash method. Thermal conductivity of Al-SiCp composites increases non-linearly with decreasing the volume fraction of SiC. Cyclic thermal shock fatigue tests were performed by immersing Al-SiCp functionally graded materials(FGM) into water from the various heating temperatures of 400°C , 300°C and 200°C , repeatedly. After cyclic thermal shock fatigue tests, micro-hardness was measured and formation of cracks was investigated.e fati
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Authors: Yong Suk Kim, T.O. Lee, Dong Hyuk Shin
Abstract: The ARB process has been carried out up to seven cycles on a commercial purity 1100 aluminum alloy to obtain ultra-fine grains with the average grain size of 500 nm. Microstructural evolution of the ARB processed aluminum alloy was examined by a transmission electron microscopy as a function of accumulated total strain. Mechanical properties including hardness, tensile property, and sliding wear characteristics of the severely deformed Al alloy were also investigated. Grain boundaries of the ARB processed alloy were diffusive and poorly defined after the initial ARB cycles, however they changed to well-defined high angle boundaries with the increase of the accumulated strain. Though hardness and strength of the ARB processed alloy were enhanced significantly, wear resistance of the processed alloy hardly increased. The mechanical properties were discussed in connection with the microstructure.
625
Authors: M. Tagami, A. Sugafuji, Y. Yabumoto, Ken-Ichi Ohguchi, A. Muto
Abstract: The stirring technique was used to produce Al-6mass%Sn-3mass%Si-1mass%Cu alloy with 5~8wt% graphite additions. The friction coefficient and wear loss of the graphite-dispersed Al-Sn-Si alloys were measured against commercial stainless steel (SUS304) in the boundary lubrication. Although the graphite particles added up to 8mass% do not improved the friction coefficient of as-cast alloys at the lower applied load, an applied load for the Al-Sn-Si alloy with 5~8wt% of graphite additions reaches 0.60MPa,whereas that for the non-added base alloy is only 0.45MPa.
629
Authors: Yoshihito Oda, Junji Sugishita, Hatsuhiko Usami, Tomokazu Hattori
Abstract: The tactile wear ("tezure" in Japanese) is an abrasion phenomenon caused by the contact of humans hand with the surfaces of materials over a long period of time. Though this phenomenon has been the focus of various articles, an extensive study with regard to the wear characteristics is of profound importance. To date, we have several remarkable examples such as the statue of Pindola Bharadvaja (Buddhist) and the St. Peter statue (Christian). Followers of the respective religions who are deeply attached and rooted have been touching the statues as part of their rituals for many generations over centuries. In this study, an attempt is done to verify the friction and wear characteristics of the human finger with various soft materials and woods.
633
Authors: Y.B. Huang, Bong Sun You, Byung Ho Choi, Won Wook Park, Il Sup Chung, S.K. Suh
Abstract: The liquid oxidation behavior of Mg-Ca base alloys containing Be has been presented in this paper. The ignition temperature test and microbalance measurement indicated that the oxide film formed at elevated temperature was protective, resulting in the improved oxidation resistance and ignition-proof properties. A Ca-rich zone was found at the very near surface of oxide layer. With Be addition the oxide layer became dense and compact, which was impermeable for the rapid diffusion of oxygen and magnesium through the oxide layer. It was concluded that the formation of BeO in the oxide layer suppressed the continuous growth of the oxide layer.
637
Authors: Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: The effects of antimony addition on the microstructures and creep behavior of AZ31 have been studied. Constant load creep tests were carried out at 200°C and an initial stress of 50MPa for AZ31 alloys containing antimony up to 0.84% by weight. Results show that Small additions of antimony to AZ31 effectively decreased the creep extension and steady state creep rates. The steady state creep rate of AZ31 was reduced 2.5 times by the addition of 0.84% of antimony by weight. The main reason for the higher creep resistance is due to the presence of high volume fraction (~20%) of second phases including Mg3Sb2, which effectively hindered the movement of dislocations during the elevated temperature creep.
641
Authors: Si Young Chang, Sang Woong Lee, Jin Chun Kim, Young Seok Kim, Dong Hyuk Shin
Abstract: The commercial AZ31 and AZ61 Mg alloys were subjected to equal channel angular pressing (ECAP) after hot rolling at 673 K. The hot-rolled AZ31 alloy could be ECA pressed at 493 K. The 4 ECA pressed AZ31 alloy revealed the microstructure of dynamically recrystallized grains with a grain size in range of 1 to 10μm. Despite the dynamic recrystallization during ECAP at higher temperatures ( > 1/2 Tm), the yield stress and tensile strength of AZ31 and AZ61 alloys drastically increased after 1 pressing. The yield stress gradually decreased with increasing the number of pressings, which contrasts with the behavior of the ECA pressed Al and Fe alloys, while the tensile strength increased slightly. In particular, the alloys showed nearly 3 times higher elongation than as-annealed one after 4 ECAPs, without sacrificing the tensile strength. These tensile deformation characteristics were explained based on the observation of the deformed microstructure in the vicinit of fracture surface.
645
Authors: In Bae Kim, Jin Young Hong, Bong Gil Hong, Kyung Hyun Kim
Abstract: The precipitation behavior of Mg-Y-Nd-Zr alloy was investigated by using transmission electron microscopy. The precipitation sequence of Mg-Y-Nd-Zr alloy was confirmed as follows ; Supersaturated solid solution → β''→ β'→ β.The β'' phase was identified Mg3Nd having DO19 structure with the lattice constant of a=2aMg=0.64nm and c=cMg=0.52nm. The β' precipitate was identified as Mg12NdY having base centered orthorhombic structure with the lattice constants a, b and c of 0.64, 2.23 and 0.52 nm, respectively. The β phase was confirmed as equilibrium phase having the fcc structure with the lattice constant of a=2.22 nm.
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