Advanced Materials Research Vol. 409

Abstract: Rheo-extrusion of hypereutectic Al-14.8%Si-4.5%Cu-1.1%Mg (in mass%) alloy which is considerably difficult to hot-extrude was tried using semi-solid slurry with fine solid granules and almost finely dispersed primary Si particles, made by a newly developed rotor process. Trials of rheo-extrusion at the constant extrusion ratio of 36 using this semi-solid slurry were carried out at the different conditions of the extrusion temperature and the ram speeds ranging from 520 to 560oC and from 5 to 20mm/s, respectively. Under the all rheo-extrusion conditions, this semi-solid slurry could easily be rheo-extruded to the round bars of 6mm diameter with sound surface at much lower extrusion force than that of hot-extrusion at very low extrusion ratio. In addition, the characteristic age-hardening behavior which is thought to be essential for the rheo-extrusion process has been found out, namely the peak hardness values of aged bars rheo-extruded at 520 and 540oC were considerably higher, 30HV at maximum, than that of a gravity casting.

Abstract: It is known that Al-Mg-Ge alloy shows the similar precipitation sequence to Al-Mg-Si alloy, and its equilibrium phase is β-Mg₂Ge according to its phase diagram. In this study, the precipitation sequence of Al-Mg-Ge alloys containing different contents of Mg₂Ge has been investigated by hardness test, TEM and HRTEM observation to understand the effect of Mg₂Ge contents on age-hardening behavior of the alloys. The hardness of as-quenched and peak-aged samples have been improved by increasing Mg₂Ge contents. The precipitates in the peak-aged samples have been classified into some metastable phases, such as the β’-phase and parallelogram-type precipitate by HRTEM observation. The relative frequency of these precipitates in the is has been changed with Mg₂Ge contents.

Abstract: In the present work, b’ phase in alloys Al -1.0 mass% Mg₂Si -0.5 mass% Ag (Ag-addition) and Al -1.0 mass% Mg₂Si (base) was investigated by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) to understand the effect of Ag addition. The b’ phase is rod-shape with the longest direction parallel to <001>_Al. HRTEM images and SAED patterns obtained along the direction were similar for the b’ phase in both alloys. The unit cell of b’-phase in Ag-addition alloy is hexagonal with the same c-axis dimension as the Ag-free b’, but shorter a-axis. Ag was found in the composition of the rod-shaped precipitates in Ag-addition alloy by energy dispersive X-ray spectroscopy (EDS). In addition, the distribution of Ag was investigated by energy filtered mapping and high annular angular dark field scanning transmission electron microscopy (HAADF-STEM). The Ag-containing atomic column was observed in every b’ unit cell, and the unit cell symmetry is slightly changed as compared with the Ag-free b’. The Ag-containing b’ rods have complicated domain structures. The interfaces of these particles are enriched with Ag atoms that occupy the lattice positions in the Al matrix. The occupancy of the Ag-containing atomic columns seem to vary both inside particles, as well as at the interfaces.

Abstract: In the present study, the substructure within grains of different orientations was investigated by means of EBSD technique. A 70% cold rolled commercial AA8079L alloy was annealed at two temperatures, 280°C and 320°C, for short times from 2 to 60 seconds. The subgrain size within the deformed grains of different orientations was measured. The results revealed no pronounced difference in subgrain size right after cold deformation. During further annealing at both investigated temperatures, Cube grains showed the highest recovery rate. It is concluded that the Cube oriented nuclei require an incubation time which is, obviously, much smaller than for grains of other orientations.

Abstract: The Zn-Al eutectoid alloy has been well known as a typical superplastic metallic material with small grain sizes. In the present study, controlling conditions in hot rolling such as temperature, total number of passes, reduction in a pass, etc. have been investigated in a ZnAl eutectoid alloy to obtain finegrained microstructures. Homogenized ingot specimens were hotrolled by a total rolling reduction of 90% with different pass schedule. Microstructural observation on the final sheet showed that nearly equiaxed finegrained microstructure with grain size of about 2µm was formed when the ingot was waterquenched after homogenization.

Abstract: It has been known that Cu-and Ag-addition Al-1.0mass%Mg₂Si alloys (Al-Mg-Si-Cu alloy and Al-Mg-Si-Ag alloy) have higher hardness and elongation than those of Al-1.0mass%Mg₂Si alloy. In this study, the aging behaviour of Al-Mg-Si-Cu alloy, Al-Mg-Si-Ag alloy and (Cu+Ag)-addition Al-1.0 mass% Mg₂Si alloy (Al –Mg –Si-Cu-Ag alloy) has been investigated by hardness test and TEM observation. The Al-Mg-Si-Cu-Ag alloy has the fastest age-hardening rate in the early aging period and the finest microstructure at the peak hardness among three alloys. Therefore the microstructure of the precipitate in Al–Mg–Si-Cu-Ag alloy has been investigated by HRTEM observation to understand the effect of Cu and Ag addition on aging precipitation.

Abstract: Investigation on the behavior of hydrogen is needed to spread the use of hydrogen fuel cell vehicles. Hydrogen microprint technique (HMPT) has been known as an effective method to investigate the hydrogen behavior by visualizing the microscopic location of hydrogen in relation to the microstructure. In the present study, the behavior of electrolitically charged hydrogen in 6061 and 7075 aluminum alloys with T6-temper has been investigated by means of HMPT. Both in the two alloys, hydrogen was detected on constituent particles and in the matrix. Total amount of detected hydrogen was markedly larger in 7075 than in 6061, although the distribution in depth direction far narrower and the fraction of hydrogen detected in the matrix with respect to that on the constituent was larger. These fact was presumed to be caused by the difference in the fine precipitates between the two alloy formed during final aging treatment.

Abstract: The transition metals such as chromium and manganese are usually added to 6000 series Al-Mg-Si alloys to control recrystallization and grain size and thus the properties of alloys. In Cr/Mn-addition alloys, Cr or Mn will expense Si to form the dispersoids as AlMnSi or AlCrSi and tend to decrease its aging effect. The aim of this work is to investigate the effect of transition metals (TMs) addition on the hardness and the microstructural features of Al-Mg-Si alloys. Al-Mg-Si alloys, which can be remarked as the quasi-binary alloys of Al-Mg₂Si, are prepared with Cr or Mn addition by laboratory casting. Some other transition metals, such as Co and Ni, are also added to Al-Mg-Si alloys. The grain size of four alloys decreases with TMs addition, which consequently increases the as-quenched hardness of the alloys comparing with that of the Al-Mg₂Si alloy without TMs addition. The difference between Cr/Mn-addition alloy and Co/Ni-addition alloy is that the dispersoids are formed in Co/Ni-addition alloy without expensing Si. Therefore, there is little effect on the aging effect of Si in Co/Ni-addition alloy. Keywords: transition metals, hardness, microstructural, Al-Mg₂Si, dispersoids.

Abstract: Hydrogen embrittlement has been known to occur by invading hydrogen (environmental hydrogen) in some metallic materials under several certain conditions, although the cause has not been clarified yet. To clarify the cause, investigations on the behavior of environmental hydrogen in metallic materials are needed. It has been reported that environmental hydrogen invades 7075 aluminum alloy through the second-phase particles (Al₇Cu₂Fe). However it has not been clarified yet whether environmental hydrogen invades aluminum through the interface between the matrix and second-phase or through the bulk of second-phase particles. In this study, tritium autoradiography technique, TARG, has been applied to an aluminum alloy containing a single kind of second-phase particles (Al₇Cu₂Fe), to elucidate the invasion behavior of hydrogen from two different environments: tritiated water and tritium gas. In TARG, hydrogen (tritium) atoms that stay in the vicinity of the surface can be detected as silver particles. Silver particles (hydrogen atoms) were primarily detected on the interface between the matrix and second-phase. Thus most of the invading hydrogen is concluded to be trapped by the interface.

Abstract: Silk fibroin (SF) is a protein fiber spun by Bombyx mori silkworm. SF fibers are about 10-25 μm wide in diameter and a single cocoon may provide over 1000 m of SF fibers. SF can present several conformations regarding protein secondary structure which ultimately define the structural properties of SF-based materials. For this reason, a rigorous control on its processing conditions shall be performed. It is known that SF has excellent properties to be used in biomaterials field, controlled release and scaffolds for tissue engineering. In addition, SF can be processed in several forms, such as films, fibers, hydrogels or microparticles. This work seeks to provide an overview on SF processing conditions, regarding the preparation of SF membranes (dense and porous), hydrogels and biocomposites, focusing on biomaterials application.