Eiichi Sukedai

β to ω phase transformation due to aging in a Ti-Mo alloy deformed in impact compression

Abstract We investigated the roles of vacancies and their clusters introduced in a Ti–20mass% Mo alloy by high-speed compression in the formation of aged ω-phase crystals. Specimens were deformed by a static compression mode and a high-speed compression mode, and were then aged. The relationships between morphology of aged ω-phase crystals and deformation modes are discussed along with the roles of vacancies and their clusters in the nucleation and growth of aged ω-phase crystals. Aged ω-phase crystals were found to be smaller but of higher density in a high-speed deformation specimen. These results suggest that vacancies and their clusters easily become nucleation sites of aged ω-phase crystals. Several aged ω-phase crystals in a high-speed deformation specimen were of string-like shape. High-resolution electron microscopy confirmed that the string-like crystals have the ω-phase crystal structure. One of the roles of vacancies of and their clusters introduced by high-speed deformation is considered to be relief of compressive stress, which is predicted to arise in the course of transformation.

Fine Structure of As-Quenched Omega-Phase of Ti-Mo Alloy Observed by HREM with Image Processing

The structure of the omega-phase which is formed by the quenching of Ti-14 wt%Mo alloy from 1223 K to 273 K was investigated at the atomic level using a high-resolution electron microscope (HREM). The HREM image were formed by using 15 diffracted waves including diffuse scattering due to the omega-phase. The focus of imaging was adjusted to aberration-free-focus (AFF) condition and the observed images were processed by a computer to reveal the structure of the omega-phase only. It was found that the omega-phase co-exists with an omega-like structure formed by vacancy clusters.

Annihilation Behaviors of Athermal omega-Phase Crystals Due to Electron Irradiation.

Annihilation behaviors of athermal omega-phase crystals formed by cooling at 131 K for 10.8 ks under four different electron irradiation conditions of acceleration voltages of 200 kV and 160 kV, and beam currents of approximately 20 pA/cm(2) and 5 pA/cm(2) were investigated using in situ dark field and HREM observation methods at 131 K. The effect of acceleration voltages on the lifetimes is recognized, i.e., in the case of approximately equal electron beam current, lifetimes at 200 kV become shorter than those at 160 kV. Also, lifetimes depend on the electron beam current at 200 kV, i.e., the higher the beam currents, the shorter the lifetimes become. However, no distinct dependence can be seen at 160 kV. Since annihilations of athermal omega-phase crystals begin after the electron irradiation for a certain period in each condition, which depends on acceleration voltages and beam currents, it is suggested that the annihilation behaviors have incubation periods.

Microstructure and tensile properties of a friction stir welded Al-Mg-Si alloy

Abstract The present paper describes correlations between microstructure and tensile properties of a friction stir (FS) welded Al–Mg–Si alloy (6061–T6 Al). Mechanical tests were performed to determine the hardness and tensile stress–strain properties for the base material and for the nugget (or stirred) zone in the FS weld. The microstructure in the nugget zone was characterized using a weak-beam technique, high-resolution electron microscopy, and scanning transmission electron microscopy. Reduction of flow stress in the nugget zone was caused by thermal softening of the base material during the friction stir welding (FSW) process. Fine needle-like precipitates, dislocation segments, and fine lattice defects were observed in the base material, while only isolated long dislocations were found in the nugget zone. These results suggest that dynamic recrystallization occurs during FSW. Deterioration of tensile properties in the nugget zone was caused by the annihilation of lattice defects formed during the thermal refining process. The strain-hardening rate of the nugget zone was higher than that of the base material, and this was caused by a simple dislocation multiplication due to the dynamic recrystallization of the base material.

Investigation of microstructure of mechanically alloyed TiMo particles using high-resolution electron microscope observations

Mechanically alloyed TiMo alloy particles were evaluated by high-resolution electron microscope observations. The particles were mechanically alloyed for 36, 72, 144 and 288 ks. Specimens for transmission electron microscopy were prepeared by electroplating the particles together with nickel on copper sheets and using an ion-milling machine. It was not difficult to distinguish MA particles in the specimens. MA particles consisted of small island crystalline parts, 20–50 nm diameter, and amorphous parts. Lattice fringes of island crystalline parts were measured and the materials produced by mechanical alloying were identified by their lattice spacings. After alloying for 36 ks, pure titanium and molybdenum still remained and a small amount of β-TiMo alloy was confirmed. After alloying for 72 ks, almost all crystalline parts were β-TiMo alloy. α-TiMo and TiFe were found after alloying for 144 ks. α-TiMo alloy seems to be transformed from β-TiMo alloy on alloying for 144 and 288 ks.