The mechanical properties of materials strongly depend on their microstructure. Therefore, engineering the material's microstructure can lead to improving its mechanical properties. One method for enhancing the strength of metallic materials consists of refining the grain size down to the nanometer scale. Such nanostructured materials possess remarkable strength without using conventional metallurgical strengthening methods. However, this strength often comes at the expense of workhardening capacity, thus favoring flow localization and loss of ductility and toughness. The deformation behavior of nanostructured metallic materials has been extensively studied in the literature. However, little is known of their fracture behavior. In this study, the mechanical behavior of a nanostructured, nearly pure material is investigated in order to link processing conditions, microstructure, and fracture locus in stress space. With focus laid on BCC materials which can undergo a ductile-to-brittle transition, Interstitial- Free (IF) steel is chosen. The microstructure is refined using Severe Plastic Deformation (SPD) to achieve ultra-fine grain (UFG) materials with grain sizes in the range 100nm- 1 mu m. Equal Channel Angular Extrusion (ECAE) is used to obtain three types of UFG-IF steel microstructures by varying the extrusion rate and processing temperature. The deformation behavior is investigated for the three UFG materials using round smooth bars and is compared with the behavior of the as-received material. The damage behavior and the fracture mechanisms are studied using tensile round notched bars with varying notch radii. The findings indicate a remarkable combination of strength and notch ductility at room temperature, including for the material with the finest microstructure. They also point to the need for careful characterization of temperature effects before such materials can be considered in structural applications.

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.

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.

In this study, firstly, in order to achieve fine grains in solid solution strengthened AZ31 magnesium alloy by friction stir processing (FSP), various efforts have been made. It has found that with a newly designed cooling system, the microstructure of commercial AZ31 alloy can be refined dramatically by carefully controlling the FSP parameters. It is of scientific interest that nanometer grains have been observed in the resultant microstructure for the AZ alloy experienced by two-pass FSP. Besides, in order to modify the microstructure and mechanical properties, FSP is also applied to incorporate AZ31 Mg alloy with nano-ZrO2 particles, nano-SiO2 particles and different fractions of Al and Zn elements. The microstructure and mechanical properties of the modified alloy and composite samples are investigated and compared. By one-pass FSP coupled with rapid heat sink from liquid nitrogen cooling approach, the ultrafine grain size in AZ31 Mg alloy is successfully achieved. The grain boundaries are well defined and the mean grain size can be refined to 100~300 nm from the initial 75 £gm of commercial AZ31 Mg alloys sheets. The ultrafine grained structure can drastically increases the microhardness from the initial 50 up to 120 Hv, or an increment factor of 2.4 times. Furthermore, the nanometer grains can be even achieved by two passes FSP coupled with rapid heat sink. The resulting microstructure exhibits equiaxed grains ranging from 40 nm to 200 nm with an average grain size of less than 100 nm. The nanocrystalline grains can be characterized by the TEM observations and the diffraction rings in SAD patterns. The highest hardness point can reach ~150 Hv which is equal to triple of the AZ31 matrix, and the mean hardness also increases up to around 134 Hv. Bulk Mg-AZ31 based composites with 10~20 vol% of nano-ZrO2 particles and 5~10 vol% of nano-SiO2 particles are also successfully fabricated by FSP. The average grain size of the resultant composites could be effectively refined to 2~4 £gm, and it demonstrates much higher hardness values compared to commercial AZ31 billet. Moreover, for the Mg/ZrO2 composite fabricated by one pass and subsequent cooling pass FSP, the recrystallized grain size could be further refined to 0.4 £gm with the hardness value of 135 Hv. As for multi-element Mg base alloys fabricated by FSP, high fractions of Al and Zn elements can result in apparent grain refinement, this can be proved by the broadening of diffraction peaks. Multi-passes FSP can induce the appearance of intermetallic compounds, however, some of them are quasi-crystals with icosahedral point group symmetry. The average hardness of the resultant alloys reachs nearly 350 in Hv scale due to the generation of intermetallic compounds and grain refinement.

Kyoto University (京都大学) / 0048 / 新制・論文博士 / 博士(工学) / 乙第12650号 / 論工博第4078号 / 新制||工||1547(附属図書館) / 29728 / (主査)教授 辻 伸泰, 教授 落合 庄治郎, 教授 乾 晴行 / 学位規則第4条第2項該当

The microstructures of the near-surface layers on AA3104 aluminium alloys and their evolution through rolling and deep drawing processes have been investigated. The effect of the near-surface layers on the optical property of AA3104 aluminium alloy has also been assessed. It was revealed that two types of near-surface deformed layers, both with different microstructures different from the underlying bulk alloy, were generated on the surface of AA3104 aluminium alloy during rolling. Both of them are characterized by ultrafine, equiaxed grains, with diameters <100 nm for type A near-surface deformed layer and <200 nm for type B near-surface deformed layer. A high population density of nano-sized, oxygen-rich particles is present along grain boundaries within type A deformed layer. But type B deformed layer is free of oxygen-rich particles. Type A deformed layer was generated through two mechanisms, i.e. geometric dynamic recrystallization and mechanical alloying. Rolling introduced plastic strain in the surface/near-surface region of aluminium sheet was of sufficient magnitude to cause geometric dynamic recrystallization and thus microstructure refinement. In addition, the incorporation of oxides into the near-surface region was also involved in the formation of type A deformed layer. However, the formation of type B deformed layer was only attributed to severe strain induced geometric dynamic recrystallization. Type A deformed layer was mainly formed at the early stages of hot rolling. The subsequent rolling and deep drawing reduced the thickness of type A deformed layer by distributing it over a larger surface area. During cold rolling, type A deformed layer broke into patches with the extension of alloy surface. Type B deformed layer may be generated on the nascent surface if the strain is sufficiently severe to cause geometric dynamic recrystallization. For the hot rolled alloy sheets, the surface/near-surface region is mainly covered by type A deformed layer. However, for the alloys after cold rolling, only limited area is covered with type A deformed layer. The thicknesses of the near-surface deformed layers are not uniform across the surface of AA3104 aluminium alloy. The maximum thickness of type A deformed layer on transfer slab is approximately 4 µm, while that on re-roll gauge sheet is ~1 µm, and ~0.8 µm on the final gauge sheet, ~400 nm on formed cup and ~100 nm on formed can. Type A deformed layer is randomly distributed as patches on the cold rolled aluminium sheet. The reflectivity of oxygen-rich particles is lower compared with the reflectivity of aluminium. As a result, the type A deformed layer patches absorb more incident light than the area without type A deformed layer. Further, there are plenty of micro-scale mini-cracks present on type A deformed layer, their opening sizes are in the equivalent scale of the wavelength of visible light. The incident light may not able to be reflected out if they go into these mini-cracks. It is more prone to happen for short wavelength light since it is easier for them to go into the mini-cracks than long wavelength light. As a result, less short wavelength visible light is reflected from the type A deformed layer patches. Thus, such patches exhibit a yellow appearance while the surrounding area appears the original silver white aluminium appearance. The aluminium sheet with a high coverage of type A deformed layer exhibits a low total reflectance. Further, its total reflectance is with a significant “red shift”. Neither the macro-scale surface roughness nor the ultrafine grain size affects the total reflectance of aluminium alloys. The total reflectance of aluminium alloys is primarily dependent on the presence of type A deformed layer.

This thesis deals with the influence of grain refinement by ECAP on fatigue properties of magnesium alloy AZ 91. Tensile and fatigue tests were made on the as-cast state samples and samples after ECAP process. Metallographic analysis of the microstructure and fractographic analysis of the fracture surfaces was performed.

Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.