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DC short-circuit current calculation is necessary to design and protect modular multilevel converter-based (MMC-based) multi-terminal DC (MTDC) grids. In this paper, a general and analytical short-circuit current calculation method is proposed for bipolar MTDC grids. For pole-to-pole (P2P) faults, the superposed fault equivalent network of the MTDC grid is first developed in the frequency-domain. Then, a decoupling method based on the network inductance ratio is proposed, which simplifies the high-order network into the superposition of several independent two-order RLC circuits. As a result, the short-circuit current can be expressed as the sum of the currents of these RLC circuits. For P2G faults, the phase-domain DC line impedance considering the coupling effects is determined based on its modal-domain parameters, so that the P2G fault current can be calculated using the method of the P2P fault. The proposed methods effectiveness is verified through simulations and experiments, and the calculation error is classified and analyzed. Compared with the existing methods, the proposed method is more accessible, more accurate, and available for hand-solution.

An ultrahigh damping capacity with internal friction close to 0.35 was detected over a large temperature range of 75 °C (from -40 to 35 °C) in Ti49.2Ni50.8 shape memory wires by adjusting the B2→R phase transformation through a low-temperature aging treatment. Microstructural analysis showed that the excellent damping performance is attributed to high-density nanodomains in the R phase, which are induced by short-range Ni segregation. These results are expected to further expand capacity for designing damping microstructure.

We discovered Elinvar effect in a severely cold worked Ti-50.8(at%)Ni thin belt that showed a modulus variation of less than 0.54GPa(∼1%) over a wide temperature range (−50 to 100 °C). Microstructural analysis showed an unraveled amorphous matrix dotted with nanosized crystallites, which demonstrated that the softening of the nanodomains in the retained nanosized crystallites and hardening of the amorphous matrix during cooling over wide temperature range can trigger the Elinvar effect.

A class of stochastic Runge–Kutta–Nyström (SRKN) methods for the strong approximation of second-order stochastic differential equations (SDEs) are proposed. The conditions for strong convergence global order 1.0 are given. The symplectic conditions for a given SRKN method to solve second-order stochastic Hamiltonian systems with multiplicative noise are derived. Meanwhile, this paper also proves that the stochastic symplectic Runge–Kutta–Nyström (SSRKN) methods conserve the quadratic invariants of underlying SDEs. Some low-stage SSRKN methods with strong global order 1.0 are obtained by using the order and symplectic conditions. Then the methods are applied to three numerical experiments to verify our theoretical analysis and show the efficiency of the SSRKN methods over long-time simulation.

We report a Ni50.8Ti49.2 alloy that exhibits an ultrahigh damping capacity with an internal friction (IF) value of ~ 0.35 (tan δ) at low frequencies in a wide temperature range. This alloy undergoes low-temperature aging (LTA) after insufficient annealing from a severely deformed state (~90% cold work). Via phase transformation (PT) analyses and microstructure characterization, we demonstrate that this high broad IF plateau originates from nanodomains and the stable coexistence of the B19’- and R-phases over a wide temperature range. The abundant phase boundaries (PBs) and movable interfaces between nanodomains contribute significantly to this high broad IF plateau. Further analyses validate the critical role of LTA in inducing nanodomains, i.e., resulting in localized Ni segregation. The abnormal appearance of needle-like B19’- martensites near room temperature is attributed to the defects that remain after insufficient annealing, after which LTA promotes localized depletion or enrichment of Ni. In addition, the results of this study indicate that high density nanodomains are generated from the two steps of insufficient annealing and LTA in the severely deformed Ni50.8Ti49.2 alloy. The idea of a high-damping microstructure design based on “domain engineering” is realized.

We report a Ni50.8Ti49.2 alloy that exhibits an ultrahigh damping capacity with an internal friction value of ~ 0.35 (tan δ) at low frequencies in a wide temperature range. This alloy undergoes low-temperature aging after insufficient annealing from a severely deformed state (~90% cold work). Via phase transformation analyses and microstructure characterization, we demonstrate that this high broad internal friction plateau originates from nanodomains and the stable coexistence of the B19’-phase and R-phase over a wide temperature range. The abundant phase boundaries and movable interfaces between nanodomains contribute significantly to this high broad internal friction plateau. Further analyses validate the critical role of low-temperature aging in inducing nanodomains, i.e., resulting in localized Ni segregation. The abnormal appearance of needle-like B19’ martensites near room temperature is attributed to the defects that remain after insufficient annealing, after which low-temperature aging promotes localized depletion or enrichment of Ni. In addition, the results of this study indicate that high density nanodomains are generated from the two steps of insufficient annealing and low-temperature aging in the severely deformed Ni50.8Ti49.2 alloy. The idea of a high-damping microstructure design based on “domain engineering” is realized.

The B19-phase transition and the related tensile properties of Ti50Ni30Cu20 shape memory alloys were investigated after hydrogen doping. The results revealed that the presence of hydrogen suppressed the B2–B19 transition by decreasing transformation enthalpies. Aging at room temperature lowered martensitic transformation temperature, which might have been associated with the variation of hydrogen contents and states. Ti50Ni30Cu20 shape memory alloys manifested high thermal cycling stability after the aging treatment. In addition, the increase in the hydrogen content was accompanied by an increment in detwinning stress of B19 phase, whereas the elongation of Ti50Ni30Cu20 alloys decreased. Based on our analysis, these effects were caused by the combined activities of different hydrogen states, including those of hydrides and solid-solution hydrogen.