Controllable rectification on the irreversible strain limit of 2G HTS coated conductors

The second generation high-temperature superconducting coated conductors (CCs) have excellent electrical and mechanical properties， and are extensively used in superconducting devices such as fault current limiters, magnets and motors. During the operation of these superconducting devices, superconducting CCs inevitably bear the combination of electromagnetic force and thermal mismatch stress, resulting in straining of YBCO layer along the tape length. It is well known that the strains of superconducting CCs cause degradation of critical current (Ic). Generally, the irreversible strain limit ( ) is used to characterize the phenomenon that Ic of superconducting CCs degrades with axial strain. When the axial strain of superconducting CCs is less than , Ic can be reversibly recovered by over 99% after being unloaded. Therefore, is a key parameter for the design and application of superconducting CC devices. For this reason, to carry out a practical engineering method for improving of superconducting CCs has become a challenge and aroused interests among researchers. This study is based on the idea of precompression. A 316LN stainless steel tape was pretensioned at 77K to improve its elastic strain limit. Then, two superconducting CCs were soldered onto both surfaces of pretensioned stainless steel tape respectively. As a result, of the superconducting CCs can be controlled manually with different precompressions. Taking YBa2Cu3O7-δ (YBCO) CCs produced by SuperPower Inc. as an example, the measurement results show that the of the YBCO CCs increased from 0.39% to 0.73%. Meanwhile, the thickness of the sample did not increase more than once.

A crystal plasticity model for metal matrix composites considering thermal mismatch stress induced dislocations and twins

Originated at heterogeneous interfaces with distinct coefficient of thermal expansion (CTE), thermal mismatch stress is one of the critical influential factors to mechanical properties of metal matrix composites (MMCs). This stress is normally accommodated plastically by various defects, for example, high-density dislocations and twins in Al near heterogeneous interfaces in SiC/Al composites. Basic knowledge on the influence of defect characteristics is important but difficult to extrapolate from experimental results. However, existed theoretical models more focus on the influence of dislocation density, but less focus on defects variety, volume and distribution. In this paper, we propose a physics-based crystal plasticity model that has the capability of dealing with thermal mismatch stress induced dislocations and twins (denoted as TMDT model). The proposed TMDT model that is implemented in the Visco-Plastic Self-Consistent (VPSC) method considers defect heterogeneous distribution (gradient range), defect type (dislocations vs. twins) and defect volume fraction (twin spacing vs. twin volume). We demonstrate the validity and the capability of the VPSC-TMDT model in SiC/Al composites with thermal mismatch induced dislocations or twins. Furthermore, this model predicts the ultra-high strength of Graphene/Copper composites with high-density nanoscale twins, which is in turn the future aim for such nanocomposites.

Internal Factors Affecting the Surface Rumpling of a β-NiAlHf Coating

β-NiAl coatings on a superalloy substrate will inevitably result in severe rumpling at elevated temperatures; however, the associated rumpling mechanisms are not completely understood. The scale rumpling behavior of a β-NiAlHf coating deposited by electron beam physical vapor deposition (EB-PVD) on single crystal superalloy IC21 was investigated in this work. Some internal factors, including the mismatch in the coefficient of thermal expansion and the stress induced by the growth of oxide scale and the phase transformation, were taken into consideration. The thermal mismatch stress between the coating and substrate was the main internal factor responsible for rumpling behavior during thermal cyclic loads, while the phase degradation from β-NiAl to γ’-Ni3Al in the coating played a dominant role during static thermal loads.

Super-elastic ferroelectric single-crystal membrane with continuous electric dipole rotation

Ferroelectrics are usually inflexible oxides that undergo brittle deformation. We synthesized freestanding single-crystalline ferroelectric barium titanate (BaTiO3) membranes with a damage-free lifting-off process. Our BaTiO3 membranes can undergo a ~180° folding during an in situ bending test, demonstrating a super-elasticity and ultraflexibility. We found that the origin of the super-elasticity was from the dynamic evolution of ferroelectric nanodomains. High stresses modulate the energy landscape markedly and allow the dipoles to rotate continuously between the a and c nanodomains. A continuous transition zone is formed to accommodate the variant strain and avoid high mismatch stress that usually causes fracture. The phenomenon should be possible in other ferroelectrics systems through domain engineering. The ultraflexible epitaxial ferroelectric membranes could enable many applications such as flexible sensors, memories, and electronic skins.