Tetsu Ichitsubo

Rafting mechanism for Ni-base superalloy under external stress: elastic or elastic–plastic phenomena?

Abstract Rafting mechanism in Ni-base single-crystal superalloys has been discussed with the total mechanical energy calculated for typical microstructures. We found that the actual rafting phenomena cannot be explained within the coherent elastic regime. The present calculations reveal that (i) only the transverse rafted structure with laminates normal to the stress direction can be realized, regardless of tensile or compressive stresses, and (ii) the lattice misfit is not relevant to the choice of the rafted structures. However, when the eigenstrain of the spherical (dilatational) symmetry changes into that of the tetragonal symmetry with misfit dislocations on the γ/γ′ interfaces, the signs of lattice misfit and external stress govern the choice of the transverse or longitudinal rafts. It is concluded that the rafting belongs to an elastic–plastic phenomenon.

Anisotropic elastic constants of lotus-type porous copper: measurements and micromechanics modeling

Abstract We studied the elastic constants of a lotus-type porous copper, regarding it as a composite material showing hexagonal elastic symmetry with the c-axis along the longitudinal direction of the pores. We used the combination of resonance ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine the elastic constants of the composite. The resulting Young’s modulus E∥ decreases linearly and c33 does slowly with porosity, while E⊥ and c11 drop rapidly and then slowly. Micromechanics calculations considering the elastic anisotropy of the copper matrix can reproduce the measured anisotropic elastic constants. This indicates that the elastic properties of various types of porous metals can be predicted and designed with the present approach using micromechanics modeling.

Elastic and anelastic behavior of Zr55Al10Ni5Cu30 bulk metallic glass around the glass transition temperature under ultrasonic excitation

Elastic and anelastic properties of Zr55Al10Ni5Cu30 bulk glass have been investigated at high temperatures with the electromagnetic acoustic resonance technique within the frequency range of 300–1500 kHz. The elastic constants of ascast sample decrease monotonically up to the glass transition temperature Tg and jump up just above Tg, but such jumps disappear in the cooling and subsequent heating processes. This indicates that an irreversible structural stabilization occurs at Tg under ultrasound vibration. As for ultrasonic attenuation, prominent peaks appear at Tg in the first heating process. These attenuation peaks can be attributed to the atom movements in the initial glassy state. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Influence of the elastic strain on the band structure of ellipsoidal SiGe coherently embedded in the Si matrix

We report on a theoretical investigation of the elastic strain in an ellipsoidal SiGe inclusion coherently embedded in Si and its influence on the band structure of SiGe. The strain was calculated as a function of the Ge fraction in SiGe and the aspect ratio of the ellipsoid, and utilized to derive the shift of the band edge. When the principal axis of the ellipsoid was chosen to be parallel to [001], the band structure of SiGe was predicted to be Si like regardless of the aspect ratio. The band gap of strained SiGe was also calculated, and the deviation of the aspect ratio from unity was found to be effective to decrease the band gap due to the breaking of the crystal symmetry. These results suggest the importance of controlling strain, shape, and local Ge fraction in multicrystalline SiGe, which we propose as a promising material for solar cell applications.

Elastic anisotropy of rafted Ni-base superalloy at high temperatures

Abstract The elastic constants of the rafted Ni-base single-crystal superalloy, TMS-26, at high temperatures (~1000 °C) have been studied with acoustic resonance methods. The rafted superalloy can be regarded as an elastic body of tetragonal symmetry. The elastic anisotropy factors, c 11 / c 33 and E 1 0 0 /E 0 0 1 , increase with temperature up to 1.010–1.025. Although a micromechanics model can reproduce this trend, the calculated anisotropy (~1.001) is much smaller than the measurements, which can be related with the internal strain due to the lattice misfit. The elastic anisotropy of E 0 0 1 1 0 0 =E 0 1 0 contributes to the transverse rafting under external stress.

Elastic property of aged duplex stainless steel

We have measured the elastic constants of duplex stainless steel, JIS-SCS14A (CF8M), aged isothermally at 400 C up to 10,000 h, using resonant ultrasound spectroscopy method. The elastic constant c11 (¼ k þ 2l) increases monotonically with the aging time, but c44 (¼ l) remains virtually unchanged. Using a micromechanics model, we have deduced the elastic constants of a-phase on the assumption that those of c-phase are unchanged with the aging. The increase of the a-phase elastic constants reflects the spinodal decomposition of a-phase. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Elastic-stiffness coefficients of a silicon carbide fibre at elevated temperatures: Acoustic spectroscopy and micromechanics modelling

We have determined all five independent elastic-stiffness coefficients Cij of a silicon carbide fibre with transverse isotropy from room temperature up to 873 K. Firstly, we measured the Cij of a titanium-alloy-matrix composite reinforced unidirectionally with the fibres and the matrix alloy alone. Electromagnetic acoustic resonance detected the free-vibration resonance frequencies of the specimens to determine their Cij . Secondly, we applied a micromechanics calculation to deduce the fibre Cij from the measured composite and matrix Cij . The resulting fibre Cij shows strong anisotropy in the temperature derivatives of the Cij ; the temperature derivatives for the fibre-axis-direction Cij are much smaller than the others.

Elastic constant measurement of Ni-base superalloy with the RUS and mode selective EMAR methods

This paper reports the elastic constants of the Ni-base single crystal superalloy (TMS-26) with a rafted (lamellar) structure having tetragonal symmetry. The elastic constants have been measured at room temperature with the resonance ultrasound spectroscopy method and the mode-selective electromagnetic acoustic resonance method. The value of the elastic constant C33 (250.4 GPa) is almost equal to that of c11 (252.5 GPa), which indicates that the rafted structure virtually has the elastic anisotropy of cubic system.

Evolution of Internal Stress Field in Ni-Base Superalloy through Creep Deformation

Internal elastic strain, and its change accompanied with the raft formation during creep deformation in the Ni-based single crystal superalloy (TMS-26) have been investigated by X-ray diffractometry. The elastic strain caused by the lattice misfit between g and g phases has markedly been changed by creep deformation especially in the directions perpendicular to the [001] tensile axis. The change in the elastic strain can be explained by the effect of creep dislocations stacked at g/g interfaces. The evolution of the elastic stress field estimated from the elastic strain has explained well the transition from primary creep stage to the second one.

Elastic constants predicted from sintered porous MgB2 via micromechanics modeling

Elastic constants of MgB2 from acoustic measurements using sintered porous samples are much different from the values by ab initio calculations. This paper considers the discrepancy in terms of the pore shape with micromechanics modeling. The model calculations showed that ab initio calculations are consistent with the values corrected for the oblate ellipsoidal pores from the measured elastic constants, and the extremely small values can be measured with sintered samples.