Papers by Author: Shin Takeuchi

Abstract: In order to investigate flip-flop transitions of shear transformation zones (STZs) which are believed to be the elementary deformation sites in metallic glasses, internal friction measurements have been conducted for a commercial bulk metallic glass of a Zr-based alloy (Johnson alloy). Since the STZ is an asymmetrical two-level system, it is necessary to measure the internal friction at a high amplitude or under a bias stress condition to detect the transitions. Measurements have been made at temperatures between 130 K and 573 K at frequencies between 0.1 Hz to 10 Hz. The results showed that quite a broad peak appears between 200 K and 500 K in high amplitude internal friction measurements. The broad peak, observed for the first time in metallic glass, is interpreted to be due to flip-flop transitions of STZs having a broad spectrum, 0.5∼1.2 eV, of the activation enthalpy.

Abstract: Peierls stresses P of a variety of pure crystals, bcc metals, NaCl type crystals, elemental and compound tetrahedrally coordinated crystals, intermetallic compounds and ceramic crystals, have been estimated from the critical resolved shear stress (c) vs. temperature curves. For high P crystals where CRSS data are available only at high temperatures, P has been estimated from the critical temperature T0 at which steep temperature dependence of c vanishes: T0 is related to the kink-pair formation energy which is a function of P, material parameters and dislocation character controlling the deformation. The estimated p/G values are semi-log plotted against h/b value, where G is the shear modulus, h the slip plane spacing and b the Burgers vector. Two facts should be noted. First, P/G values for a group of crystals with the same crystal structure are within a range of a factor of 10. Second, most of the data points lie in between the classical Peierls-Nabarro relation and the Huntington’s modified relation. These facts indicates that Peierls stress is primarily determined by the crystal structure.

Abstract: In a hydrogen-doped metallic glass, there appear low-temperature and high-temperature internal friction peaks respectively associated with a point-defect relaxation and the crystallization. The high-temperature-side slope of low-temperature peak and also the low-temperature-side slope of high-temperature peak enhance the background internal friction near the room temperature. A hydrogen-doped Mg-base metallic glass was proposed as a high-damping material to be used near and somewhat above the room temperature. Stability of the high damping was also checked.

Abstract: Effects of hydrogen doping on the internal friction (IF) of ternary Ti50Ni50-xCux (x=15, 20, 25)
shape memory alloys, prepared by rolling and annealing laminating Ti and Ni-Cu alloy sheets, have been measured with a damping mechanical analyzer in a forced bending oscillation mode at temperatures from 173K to 423K at three frequencies, 0.1, 1 and 5Hz. The effects of hydrogen doping on IF are common to the three alloys: a hydrogen IF peak appears at around 260K; the IF peak value (tanφ) increases rapidly with increasing hydrogen concentration up to tanφ=0.03 at 0.5at% and then decreases; the peak temperature also increases rapidly and then gradually decreases. From the frequency dependence of the peak temperature, the activation energy E and the pre-exponential factor ω0 have been analyzed to be E=0.6-0.7eV and ω0=1013-14s-1. The origin of the hydrogen IF is interpreted to be the Snoek-Koester effect due to interaction of twin boundary dislocations and segregated hydrogen atoms. Effects of hydrogen on mechanical properties of the alloys have also been studied.