Al-Mg solid solution alloys of various grain sizes were prepared by friction stir processing (FSP). The mechanical properties and micro-structure evolution were studied. The results show that the mechanical properties including tensile strength and ductility are improved by increasing Mg weight fraction. The homogeneous deformation is enhanced by fined slip bands within the grains. On the other hand, Dynamic strain aging or serrated flow stress has been wildly investigated in Al-Mg alloys. Effects of strain rate and magnesium content on dynamic strain aging are also discussed.

In this study, ultrafine-grained commercial purity AA1050 aluminum was produced by equal channel angular extrusion (ECAE).Annealing at 250¢J was able to give a grain size of 0.59£gm. Specimens were compressed along different ECAE axis under a strain rate of 7.1¡Ñ10-4 s-1at room temperature. Compression tests were also performed under 5¡Ñ10-5 s-1, 7.1¡Ñ10-4 s-1 ,and 10-1 s-1 strain rates at 100¢J,150¢J ,and 175¢J. Surface morphology of specimens was observed by optical and scanning electron microscopes to study the generation of shear bands. Texture within shear bands was analyzed by electron backscattered diffraction (EBSD). The present research found that, different compression direction has little effect on the generation of shear bands. Increasing compression temperature and decreasing strain rates have the effect of decreasing the degree of strain localization of shear bands. Shear band deformation is compatible with the uniform deformation occurred outside shear bands. Texture change within shear bands is rotated about an axis perpendicular to the specimen surface, and strengthens the texture component.

In the study, the effect of current density and rotation speed of grinding disk on characters of Ni-Diamond composite coating are investigated. Experimental results show that current density and film thickness are almost linearly depend. When the current density is increased, the film thickness is increased, too. And it can cover diamond particles much more efficiently. The rotation speed of grinding disk is 20 rpm, the average deposition rate is approx. 2£gm/min in 5 ASD. When reduce the current density to 2.5ASD, the average deposition rate reduce to approx. 1.08£gm/min. The current density is 5 ASD, the covered area of diamond particle in Ni-Diamond composite coating is 60% when the rotation speed of grinding disk is 0rpm. Increasing the rotation speed up to 100 rpm, the covered area of diamond particle in Ni-Diamond composite coating is down to 24% because diamond particle can`t stay in the same position in a long period. Secondary, we use composite electroplating on grinder in process to grind CVD diamond films, the effect of current density and loads on grinding characters of CVD diamond films by using the composite electroplating on grinder in process are investigated. The load is 4.2 kg, the surface roughness Ra is about 0.2 £gm when composite coating grind CVD diamond with no electroplating. But the current density is up to 2.5 ASD, Ra can down to 0.12£gm. The load is increasing to 6.3 kg, the Ra of CVD diamond films is about 0.16£gm.

It has been shown that alloys with submicron-grained structure can be produced by severe plastic deformation (SPD). However, our understanding about the characteristics of mechanical behaviors of these materials is still limited. According to the literature, many alloys exhibit quite different mechanical properties as the grain size decreasing to submicrometer range. In this study, commercial purity aluminum (AA1050) of grain size ranging from 0.35 to ~ 45 mm was obtained by the proper combination of equal-channel angular extrusion (ECAE) and annealing treatment. The influences of grain size, testing temperature and boundary character on the mechanical properties were studied in this work. Generally speaking, the materials of grain sizes below 1mm have quite different mechanical properties than those of coarser grain sizes. In tensile tests, they exhibited yield drop immediately followed by work softening at RT, while they showed Lüders extension followed by work hardening at 77K. In addition, their yield strength at RT was about 20% higher in compression than in tension. The submicron-grained aluminum has much higher strength but lower tensile ductility than large grained aluminum at room temperature, while it exhibits both high strength and good ductility at 77K. This finding suggests that the poor tensile ductility of submicron-grained alloys at room temperature may be improved by reducing the dynamic recovery rate. The Hall-Petch slope in the submicrometer grain size range showed positive deviation from that extended from coarser grains at both room temperature and 77K. This might be arisen from the phenomenon of inhomogeneous yielding as grain size below 1 mm. In addition, the grain boundary character distribution was found to have influence on the tensile properties of matrials of submicrometer grain sizes. As the grain size increases to the range between 1 mm and 4 mm, the tensile deformation at RT proceeds by the propagation of Lüders band initially, and followed by strain hardening. For materials of grain sizes greater than 4 mm, a normal strain hardening behavior of coarse-grained aluminum resumes.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16502号 / 工博第3495号 / 新制||工||1529(附属図書館) / 29159 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 乾 晴行 / 学位規則第4条第1項該当

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In the current study, we have investigated the cyclic loading behavior of conventional as well as novel alloy system exhibiting fine and ultrafine-grained structure. While in case of conventional alloy systems (here aluminum alloy AA5024), the effect of three different grain sizes was investigated. Improvement in fatigue properties was observed with decreasing grain size. The unique microstructure produced via Friction stir processing was responsible for the improved fatigue response. Additionally, microstructures consisting of a high fraction of special boundaries within the fine and ultrafine-grained regime were also subjected to cyclic loading. The hierarchical features introduced in the eutectic high entropy alloy deflected the persistent slip bands, responsible for fatigue cracking, thus resulted in delayed crack initiation and improved fatigue life. The selective nature of fatigue was learnt in the fine grain Al0.5CoCrFeNi, where the introduction of hierarchical features did not result in improved fatigue properties. The weak links in the microstructure, while not affecting the tensile properties, got exposed during cyclic loading. Further study on the medium entropy alloy revealed the inherent reason for the improved fatigue properties. The medium entropy alloys utilized the benefit of UFG single-phase FCC matrix. The UFG matrix showed signs of transformation of FCC phase into the HCP phase during fatigue deformation and hence exhibited improved work-hardening. Alongside atomic scale transformation, stacking faults and nano-twins can also be attributed for obtained cyclic properties.

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