It is generally known that the yield stresses of 304 series steels at cryogenic temperatures are remarkably low compared to those of other more stable stainless steels such as 316L. In addition, load drops in the serration at cryogenic temperatures are larger for 304 series steels than in the latter steels. Martensitic transformation induced during deformation is thought to contribute to such behavior, but the details have not been clarified. Therefore, the effects of the transformation on the deformation behavior of 304L and 316L steels at cryogenic temperatures were examined using computer simulation. The simulation was carried out based on procedures previously developed by Shibata et al. As for the effects of the transformation, strain, heat and strengthening generated by the transformation were considered. Results showed that heat generated by the transformation increases the load drops in the serration. The low-yield stress was revealed by the transformation strain induced by an applied elastic stress. But Ogata et al. reported that X-ray diffraction detected no martensitic transformation at ambient temperature in 304L steel deformed at 4K by a very small plastic strain. On the other hand, Nyilas et al. reported that the transformation induced by a very small strain at cryogenic temperatures reversed to austenitic phase during heating to ambient temperature. Hence, it is expected that, in 304L steel, the martensitic transformation to ε induced by an elastic stress suppresses the yield stress at cryogenic temperature and the ε reverses to austenitic phase in the process of heating to ambient temperature.
We have developed high critical current, Ic, and long GdBa2Cu3O7-X (GdBCO) coated conductors using ion-beam assisted deposition (IBAD) / multi-plume and multi-turn pulsed laser deposition (MPMT-PLD) method. We have successfully fabricated 32 and 60.7 m long GdBCO coated conductors with high Ic of 205 and 183 A at 77 K and 0 T, respectively. Moreover, the Ic of the 32 m long GdBCO coated conductor exceeded 20 A at all angles for the applied magnetic field of 3 T. This Ic value at 3 T was 2.5 times higher than that of the 200 A class YBa2Cu3O7-X(YBCO) coated conductor. The production rate of the GdBCO layer was 7.5 to 10 m/h, which is about 2.6 times faster than that for a YBCO layer using the MPMT-PLD method and 10 times faster than that for a YBCO layer using the conventional PLD, single-plume and single-turn PLD, method. In addition, the material yield of the GdBCO layers using the MPMT-PLD method was about 26 to 28%. The GdBCO coated conductors with a high Ic in the magnetic field at 77 K, and capable of being produced quickly and with a high material yield are promising for practical applications.
We have reported high-Jc Sm1+xBa2-xCu3Oy (SmBCO) films prepared by a low-temperature growth (LTG) technique. Despite the low-substrate temperature, the LTG-SmBCO films showed complete c-axis orientation and cube-on-cube texture. In addition, the LTG-SmBCO films showed a Jc of 0.28 MA/cm2 (77 K, B//c, B=5 T), because these included high-dislocation densities and nanosize low-Tc particles. In this study, we discuss the mechanism of c-axis orientation and crystal growth for high-Jc LTG-SmBCO films. We newly suggest a model for “Nucleated growth on c-axis oriented Seed layer (NCS)" to understand the c-axis orientation mechanism for LTG-SmBCO films. Based on the NCS model, when the LTG-SmBCO films nucleated on a seed layer have two-dimensional (2D) nuclei with a 1 unit SmBCO step cell height, the interfacial energy between the nucleus and the seed layer and the critical Gibbs free energy of the nucleus are lower than those of PLD-SmBCO films. Therefore, the LTG-SmBCO films show high crystal quality and a wide processing window for biaxial orientation. We suggest that the LTG technique is effective for not only increasing Jc in magnetic fields, but also improving the processing window and crystal quality.