a b s t r a c tIn this study, microstructure and mechanical properties of an austenite NiTi alloy treated with laser shock peening (LSP) was investigated. It was found that the thickness of shock affected layer is about 250-300 μm. The surface hardness of the specimen is increased by approximately 10% after LSP. Laser induced shock introduces slightly residual compressive stress in the peened specimen. The superleastic stress-strain curves of the fully LSP processed NiTi material show no change in phase transition stress, about 100 MPa decrease in martensite yield stress, and a loss of maximum transition strain about 12% after LSP. The ultrahigh-strain-rate plastic deformation by LSP results in dislocation substructure and amorphization underneath the surface which are responsible for the hardness increase and superelastic strain loss.

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Laser shocking peening is a widely applied surface treatment technique that can effectively improve the fatigue properties of metal parts. We observe many micro-scale arc plastic steps on the surface of Zr47.9Ti0.3Ni3.1Cu39.3Al9.4 metallic glass subjected to the ultra-high pressure and strain rate induced by laser shock peening. The scanning electronic microscopy and atomic force microscopy show that the arc plastic step (APS) has an arc boundary, 50-300 nm step height, 5-50 m radius and no preferable direction. These APSs have the ability to accommodate plastic deformation in the same way as shear band. This may indicate a new mechanism to accommodate the plastic deformation in amorphous metallic glass under high pressure, ultra-high strain rates, and short duration. As one of the most promising structural materials, bulk metallic glasses (BMGs) have drawn much attention due to their unique properties such as high strength/weight ratio and large elastic deformation capability. PACS[1−3] Recently, in order to improve the performance in specific application areas, several surface modification methods have been applied on BMGs. For example, sand blasting [4] is used to improve the wettability of BMG for biomedical applications. Laser melting is utilized to induce residual stress [5] or to form nano-crystals [6] in a subsurface to improve the mechanical properties. Zhang et al. [7] successfully treated a BMG's surface with shot peening to generate a ∼80-µm-thick affected zone which contained many slight shear bands. It is speculated that these pre-existing shear bands are favored locations for resumed plastic flow, so that the plasticity is increased while fewer main shear bands are generated. Similar to but having more advantages over shot peening, laser shock peening (LSP) [8] has been widely used to improve wear resistance and fatigue performance in the metallic components of bulk materials. In the LSP process, a shock wave is generated and propagates into the target through the interaction of a pulsed high-intensity laser beam and absorption layer on the metallic target surface. If the amplitude of the shock wave exceeds the Hugoniot elastic limit (HEL) of the target material, plastic deformation occurs and residual compressive stresses are induced near the material surface, resulting in the enhancement of fatigue life. Since the beam size could be easily adjusted, a localized micro-scale peening technique [9] has been developed and applied on the metal films in the semiconductor industry. In addition, LSP is suitable in processing materials with complex structural geometry by manipulating laser beams and utilizing liquid confining media like water.[10] To our knowledge, LSP processing on BMGs is rarely reported. In this Letter, we report a novel micro-scale plastic deformation feature observed on the surface of Zr-BMG treated by LSP.A Zr 47.9 Ti 0.3 Ni 3.1 Cu 39.3 Al 9.4 BMG rod in diameter 5 mm was prepared by melting a mixture of elemental metals with purities ranging from 99.5% to 99.99% in an argon atmosph...

Summary This paper presents a new line commutated converter (LCC) and modular multilevel converter (MMC)‐based hybrid bipolar high‐voltage direct current (HVDC) transmission system. In the proposed circuit, each polar consists of one 12‐pulse LCC (12P‐LCC) and 1 MMC; meanwhile, the two 12P‐LCCs and the 2 MMCs locate diagonally. In each polar, the 12P‐LCC regulates DC voltage, while the MMC regulates the transmitted active and reactive power. Moreover, the MMC functions as an alternating current active power filter and a DC active power filter at the same time. Therefore, the 11th and 13th alternating current harmonics and the 12th DC current harmonic can be suppressed by the MMC without any other filters. It saves both footprint and cost of the system. In addition, the performance of this proposed system is almost the same as that of the MMC‐HVDC, while the footprint and cost are smaller than those of both the LCC‐HVDC and MMC‐HVDC systems. It means that the proposed system combines the benefits of both the MMC‐HVDC system and the LCC‐HVDC system. At last, the topology and control strategies of the proposed system are verified by the Power Systems Computed‐Aided Design/Electromagnetic Transients including DC simulations.