Tomo Ogura

Quantitative characterization of precipitate free zones in Al -Zn -Mg( -Ag) alloys by microchemical analysis and nanoindentation measurement

Abstract To correlate quantitatively the mechanical properties of precipitate free zones (PFZ) with the corresponding microstructural and compositional characteristics, TEM observation, EDX analysis and nanoindentation measurement have been performed in the vicinity of grain boundaries in Al–4.9 mass%Zn–1.8 mass%Mg (–0.28 mass%Ag) alloys. The remarkable decreases in hardness and solute concentrations were observed towards grain boundaries even in the regions just outside PFZ. With increasing aging time, it is firstly revealed that the hardness inside PFZ monotonously decreases although the hardness inside grains increases in the earlier stage of aging. Three distinct regions of “PFZ”, “Transition-area” and “Grain-region” were therefore proposed to explain the origins of such age-hardening behavior observed in this work. In the Ag-added alloy, on the other hand, the hardness could be maintained up to closer regions to grain boundaries at the same level as that inside grains.

A novel metal-to-metal bonding process through in-situ formation of Ag nanoparticles using Ag2O microparticles

The metal-to-metal bonding has been successfully achieved via the bonding process using Ag metallo-organic nanoparticles at a bonding temperature of around 300-, which can be alternative to the current microsoldering in electronics assembly using high-temperature solders. However, further reduction of bonding temperature and/or bonding pressure is needed. In the present research, a novel bonding process through in-situ formation of Ag nanoparticles instead of the filler material of the Ag metallo-organic nanoparticles has been developed. The Ag nanoparticles can form by the reduction of Ag2O particles. In this study, the Ag2O particles were mixed with triethylene glycol as a reducing agent to form a paste for bonding. The Au coated cylindrical specimens were bonded using the paste. The Ag nanoparticles formed at around 130 to 160 through the reduction process of Ag2O particles with triethylene glycol. The Ag nanoparticles were immediately sintered each other due to a great surface energy per volume. A transmission electron microscope observation revealed that the sintered Ag metallurgically bonded to the Au substrate at around 160 and a dense Ag layer formed after further heating. The tensile strength of the joint bonded at 250 under a bonding pressure of 5MPa was around 60MPa

Mechanical properties and microstructures of resistance spot welded DP980 steel joints using pulsed current pattern

Abstract Dual phase 980 MPa grade (DP980) steel sheets were resistance spot welded using a pulsed current, and the effects of the pulsed current on the strength properties of the joints were investigated. The pulsed current improved the mechanical properties of the joints in cross tensile tests. In situ observations during tear tests revealed that the ductility of the nugget was improved and that the propagation of cracks into the nugget was inhibited when the pulsed current was used. Microstructural observations and electron probe microanalysis (EPMA) of the nugget showed that the segregation of phosphorus at the nugget was reduced in the joint welded using the pulsed current, suggesting that the pulsed current improved the ductility of the nugget by altering their microstructures.

Microscale evaluation of mechanical properties of friction stir welded A6061 aluminium alloy/304 stainless steel dissimilar lap joint

Abstract Microscale evaluation of the mechanical properties of a friction stir welded A6061/SUS 304 grooved lap joint was performed using a microtensile test and transmission electron microscopy. The microtensile test revealed that ∼62% of the area along which the rotating tool passed the specimen was regarded as the bonded region and that the joint was fractured at the A6061 matrix owing to the formation of very thin interfacial reaction layers. Equiaxed aluminium grains were observed at the interface of the specimen after it was fractured, indicating that the interface deformed only slightly during the microtensile test. It should be noted that although the maximum tensile strength of the joint was approximately the same as that of the base alloy, the proof stress of the joint decreased with the dissolution of the β″ phase in the A6061 aluminium alloy.

Dispersoid Formation and Recrystallization Behavior in an Al-Mg-Si-Mn Alloy

The nucleation and precipitation of Mn-containing dispersoids in an Al-Mg-Si-Mn alloy (6082) have been studied by optical microscopy, EPMA (electron probe microanalysis) and TEM (transmission electron microscopy). The in∞uence of Mn-containing dispersoids on the recrystallization behavior was also investigated. The size and distribution of dispersoids were strongly afiected by both the homogenization process and the alloying element distribution formed in the direct chill cast procedure. The Mn-containing dispersoids were observed to nucleate preferentially on the fl 0 -Mg2Si phase and to be aligned along the direction of the matrix. After cold deformation, the morphology of dispersoids greatly in∞uences the recrystallization and grain growth behavior in the annealing process.

Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

Abstract The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.

Effects of Solvents in the Polyethylene Glycol Serieson the Bonding of Copper Joints Using Ag2O Paste

The effects of reducing solvents on the bonding process using silver oxide paste in a copper joint were investigated. Three solvent types were tested: diethylene glycol (DEG), triethylene glycol (TEG), and polyethylene glycol (PEG). The strength of the joints was assessed by fracturing, which occurred at the interface of the copper oxide layer and the copper substrate in DEG and TEG samples and at the bonded interface in the PEG sample. Analysis of the samples revealed that, in the DEG and TEG samples, the copper substrate was oxidized during the bonding process, which compromised the shear strength of the joints. In contrast, the PEG sample exhibited nonuniform sintering of the silver layer while retaining good shear strength. It was found that the combination of DEG and PEG produced optimum shear strength in the copper joint, as PEG suppressed the growth of copper oxide and DEG promoted the formation of a dense sintered silver layer. The bonding strength achieved was higher than that of the gold-to-gold joint made using standard Pb-5Sn solder.

Formation of High-Density Dislocations and Hardening in Femtosecond-Laser-Shocked Silicon

High-density dislocations are formed in silicon by a femtosecond-laser-driven shock wave. The silicon is hardened to around 6.4 times harder than the matrix material as measured by micro-indentation hardness test using nanoindentation. Because the depth, size, and inhomogeneity of the area where the high-density dislocations exist are the same as those of the hardened area, we conclude that the hardening is caused by the high density of dislocations. We suggest that the femtosecond laser shock compression plays an important role in creating high-energy-density states in materials.

Quantitative Correlation between Strength, Ductility and Precipitate Microstructures with PFZ in Al-Zn-Mg(-Ag, Cu) Alloys

The quantitative correlation between strength, ductility and precipitate microstructures in the vicinity of grain boundaries with precipitate free zones (PFZ) was evaluated for Al-Zn-Mg(-Ag, Cu) alloys using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and tensile test. In the Al-Zn-Mg ternary and Cu-added alloys aged at 433K, larger widths of PFZ were observed by TEM and resulted in lower elongations to fracture, independent of the size of grain boundary precipitates. On the other hand, the elongation of the Ag-added alloy was higher, if compared at the same levels of proof stress, due to the much smaller width of PFZ. This strongly suggests that PFZ is harmful to fracture of the investigated alloys. From a 3DAP analysis, furthermore, it was revealed that Ag and Cu atoms are incorporated in the nanoclusters from the initial stage of aging. In this work, the elongation was well correlated to the width of PFZ, size of grain boundary precipitates and the level of proof stress, enabling to predict ductility of the alloys from known microstructural factors.

Evaluation of Interfacial Bonding Utilizing Ag2O-Derived Silver Nanoparticles Using TEM Observation and Molecular Dynamics Simulation

The interfacial bonding utilizing Ag2O-derived silver nanoparticles was evaluated using TEM observation and molecular dynamics simulation. The TEM observation reveals that the crystal orientation of the sintered silver corresponded to that of the gold substrate. This is considered that the epitaxial layer of silver was formed through in-situ formation of silver nanoparticles from Ag2O paste, and oriented in the direction of the gold crystal. MD simulation successfully recreated the sintering behavior of silver nanoparticles and the gold substrate. The simulation results clearly showed the epitaxial layers of silver atoms were formed on the substrate. The existence of the closed pore indicates the acceleration of the sintering between nanoparticles and the gold substrate to minimize the total sum of surface energy and grain boundary energy.