Takahisa Shobu

Uniaxial strain dependence of the critical current of DI-BSCCO tapes

In order to explain the effect of uniaxial strain on the critical current of DI-BSCCO-Bi2223 tapes, we employed a springboard sample holder that can smoothly and continuously apply both tensile and compressive strains to tape samples. Over a narrow tensile strain region, the critical current in the tapes decreased linearly with increasing strain and returned reversibly with decreasing strain. When compressive strain was applied, the critical current first increased and then reached a weak maximum. Thereafter, it decreased monotonically with further increases in compressive strain. At room temperature, the local strain exerted on BSCCO filaments was measured by means of a quantum beam diffraction technique. Over the whole tensile strain region up to 0.2% and the small compressive strain range, the local strain changed linearly with applied strain. When the compressive strain was applied beyond the relaxation strain, the local strain (measured by diffraction) versus the applied strain (measured using a strain gauge) deviated from linearity, which is characteristic of strain relaxation and the onset of BSCCO filament fracture. Thus, the strain at the maximum critical current corresponds to a crossover point in strain, above which the critical current decreased linearly and reversibly with increasing applied strain, and below which the critical current decreased due to the BSCCO filament fracture. In this paper, we clearly characterize the reversible range terminated by both compressive and tensile strains, in which filaments do not fracture. Our analysis of the compressive regime beyond the relaxation strain suggests that although BSCCO filament fracture is the primary factor that leads to a decrease in critical current, the critical current in those regions of filaments that are not fractured increases linearly and reversibly with decreasing applied strain at compressive strains well beyond the reversible region for the tape.

Local strain and its influence on mechanical–electromagnetic properties of twisted and untwisted ITER Nb3Sn strands

It is important to evaluate the local strain exerted on superconducting filaments in Nb3Sn strands, because it influences both superconducting and mechanical properties, in particular for the ITER (International Thermonuclear Experimental Reactor) project. The local strain in the twisted and untwisted Nb3Sn strands was directly measured at room temperature as well as at low temperatures by means of quantum beam techniques. The local strain consists of thermal strain and lattice strain. The latter changes as a function of external strain. The interrelation between the force-free strain and the intrinsic strain showing a maximum critical current was considered on the basis of the present experimental data as well as the recent theory. The thermal strains along both directions parallel and transverse to the strand axis were numerically evaluated. Their evaluated results could explain well the observed values, when To is the recovery temperature of pure Cu. The force-free strain along the axial direction is deduced to be distributed among grains with different crystal orientation with respect to the axial direction. It is suggested that this fact affects the definition of intrinsic strain.

Thermal strain exerted on superconductive filaments in practical Nb3Sn and Nb3Al strands

Practical superconductive (SC) wires such as the Nb3Sn and Nb3Al strands used in fusion reactors are typical composite materials consisting of brittle superconducting intermetallic compounds. Thermally induced strain is inevitably generated in the composite due to the different coefficients of thermal expansion and different moduli of elasticity among the constituent components. In order to evaluate quantitatively the thermal strain, local strain measurements during heating by means of quantum beams, and room temperature tensile tests were carried out. The stress versus strain curves of both Nb3Sn and Nb3Al strands showed a typical elasto–plastic behavior, which could be numerically evaluated on the basis of the rule of mixtures. The local strain exerted on SC filaments along the axial direction was compressive at room temperature and tensile at high temperatures, which is common for Nb3Sn and Nb3Al strands. Their temperature dependence was numerically evaluated by means of the iteration method. As a whole, it has been established that the temperature dependence of thermal strain can be reproduced well by the numerical calculation proposed here. It is pointed out that the thermal strain in SC filaments is affected by the creep phenomenon at high temperatures above a threshold temperature.

Microtwin Structure and Its Influence on the Mechanical Properties of REBCO Coated Conductors

The elastic properties of REBCO tape (i.e., REBa₂Cu₃ O_7-d, RE = <i>Y</i>, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strain <i>A</i>_hkl^l, which consists of thermal strain <i>A</i>_hkl^T and lattice strain <i>A</i>_hkl, was analyzed using a microscopic structure model, with two types of microtwin configuration along the tensile axis, i.e., with the [100] or [110] crystal axis oriented parallel to the longitudinal direction of the tape. Experimental results were: (a) slope <i>dA</i>_h00^l/<i>dA</i> is larger than <i>dA</i>_0k0^l/<i>dA</i> (where <i>A</i> is the external tensile strain), which is attributed to the elastic modulus <i>E</i>_a along the <i>a</i>-axis being smaller than <i>E</i>_b along the <i>b</i>-axis; (b) both <i>dA</i>_h00^l/<i>dA</i> and <i>dA</i>_0k0^l/<i>dA</i> are smaller than unity, but <i>dA</i>₁₁₀^l/<i>dA</i> is almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different as <i>A</i>_h00^T ≠ <i>A</i>_0k0^T due to the elastic behavior of the superconducting layer being modified because it is constrained by the substrate and the outer metallic layer; and (d) a large Poisson ratio is attributed to this constraint state. It is suggested that the microtwin structure is critical to understand and model this unusual stress/strain behavior.

Strain Dependence of Superconducting Properties for GdBCO Coated Conductor in High Field Under Tensile Load

The strain dependence of T_c and I_c for the GdBCO coated conductor was measured at 77 and 4.2 K in magnetic fields. T_c decreased with increasing applied tensile strain. For the I_c measurements, the magnetic field was applied in the direction parallel to the c-axis (perpendicular to the tape surface). I_c in fields was degraded with increasing strain, and the strain dependence of I_c increased with increasing magnetic field. The ratio of the I_c degradation at 4.2 K was smaller than that at 77 K. These results mean that the T_c degradation is one of the reasons for I_c degradation at 77 K at least. Further, in order to understand the strain effect at 4.2 K, the internal strain of the GdBCO coated conductor under tensile strain was measured using synchrotron radiation. It was found that the strain behavior for the a-axis is different from that for the b-axis. This result suggests that the strain dependence of I_c for GdBCO coated conductors under external tensile strain is related with the T_c degradation behavior of the crystal.

Internal Strain Behavior Exerted on YBCO Layer in the YBCO Coated Conductor

The stress/strain dependence of elastic property of the surround Cu stabilized YBCO coated conductor was precisely investigated by means of white X-ray and pulsed neutrons. Multiple diffraction peaks were observed along axial and lateral directions with the tape axis as a function of tensile load. The following unusual stress/strain behaviors exerted on the YBCO layer were made clear. Pairs of (020)/(200) and (040)/(400) were observed side by side in two rows and their diffraction intensity was almost constant with increasing tensile load. The ratio of diffraction strain to external strain became less than unity. The diffraction elastic constants estimated from (0k0) diffraction peaks were larger than from (h00) peaks. Their observations strongly suggest that the micro twin structure is key nanostructure to understand the microscopic elastic constant and strain obtained from the diffraction experiments.

Residual stress in EB-PVD thermal barrier coatings

A NiCoCrAlY bond coating was low-pressure plasma sprayed on a stainless steel sub- strate. Zirconia with 8 wt% yttria was deposited on the bond coating using an electron beam-physical vapor deposition (EB-PVD) method. The top coating had the preferred orientation with the h111i axis direction perpendicular to the coating plane. The distribution of the in-plane residual stress in the top coating was measured using laboratory Cr-K X-rays with a progressive layer removal method. The value of the in-plane stresses was determined by the sin2 method after the separation of the 133 and 331 peaks. The distribution of the out-of-plane strain in the top coating was measured using the strain scanning method with hard synchrotron X-rays. The out-of-plane strain was obtained from the 333 peak which had strong intensity due to the preferred orientation. The measured value of the in-plane stress in the top coating was a large compression, and showed a steep decrease near the in- terface between the top and the bond coatings. The distribution of the out-of-plane stress showed a compression, and its magnitude was smaller than that of the in-plane stress.

Pretransitional Phenomena at the First-Order Phase Transition in LaNbO4

X-ray diffraction experiments have been performed near the ferroelastic first-order phase transition temperature in lanthanum niobate LaNbO 4 , which transforms from a tetragonal phase to a monoclinic phase at T c =768 K with a discontinuous change of the lattice parameter Δβ=0.4° . With regard to the temperature dependence of the Bragg reflection near T c , such pretransitional phenomena as decreasing of the peak intensity and broadening of the profile were observed. Furthermore, in the high temperature phase, as the temperature was close to T c , the anisotropic diffuse scattering around the Bragg reflections extended to the directions where the Bragg reflections in the low temperature phase appeared was observed. These behaviors were contrasted with the well known precursor phenomena of the martensitic phase transformation in metals and alloys.

Residual Stresses in Austenitic Stainless Steel due to High Strain Rate

Austenitic stainless steel (SUS316L) was used as specimen material, and the plate specimens were deformed plastically with a wide range of strain rates (6.67×10-5~ 6.70×102/s). The residual micro-stress for each lattice plane was measured with hard synchrotron X-rays. The residual macro-stress due to tensile deformation depended on strain rate. The residual micro-stresses varied from tension to compression, depending on the diffraction elastic constant. The soft lattice plane had tensile residual stress, and the hard lattice plane had compressive residual stress. The higher the strain rate, the smaller the difference in residual micro-stresses. The residual micro-stresses of the surfaces peened with the laser-peening or water-jet-peening were examined. Both surfaces had exhibited large compressive residual stress. The residual micro-stress on the peened surfaces showed a tendency opposite to residual micro-stress due to tensile deformation.

Residual Microstress of Austenitic Stainless Steel Due to Tensile Deformation

Material of the specimen was austenitic stainless steel (SUS316L). The specimens were given tensile plastic strains from 0% to 55%. The Vickers hardness of the specimen corresponded to the plastic strain. The residual macrostress was measured by Mn-Kα radiations. The residual macrostress of the annealed specimen had a small compression and changed into a tension after ten- sile plastic deformation. The specimen with 1% plastic strain showed the maximum tensile residual stress. To examine the dependency of the residual stress on the lattice plane, the residual microstress for each lattice plane was measured by hard synchrotron X-rays. The residual microstress was related with Young’s modulus which was calculated by Kro¨ ner model. A new method, 2θ-cos2 χ method, was proposed to solve the problem of coarse grains and it was excellent in comparison with the sin2 ψ method.