To improve the mechanical properties of polycrystalline metallic materials, understanding the elementary processes involved in their deformation at the atomic level is crucial. In this study, firstly, we evaluate the transferability of the recently proposed alpha-Fe machine-learning interatomic potential (MLIP), constructed from mechanically generated training data based on crystal space groups, to the tensile deformation process of nanopolycrystals. The transferability was evaluated by comparing the physical properties and lattice defect formation energies, which are important in the deformation behavior of nanopolycrystals, with those obtained from density functional theory (DFT) and by comprehensively calculating extrapolation grades based on active learning methods for the local atomic environment in the nanopolycrystal during tensile deformation. These evaluations demonstrate the superior transferability of the MLIP to the tensile deformation of the nanopolycrystals. Furthermore, large-scale molecular dynamics calculations were performed using the MLIP and the most commonly used embedded atom method (EAM) potential to investigate the effect of grain size on the deformation behavior of alpha-Fe polycrystals and the effect of interatomic potentials on them. The uniaxial tensile deformation behavior of the nanopolycrystals obtained from EAM was qualitatively consistent with that obtained from MLIP. This result supports the results of many studies conducted using EAM and is an important conclusion considering the high computational cost of the MLIP. Furthermore, the construction method for the MLIP used in this study is applicable to other metals. Therefore, this study considerably contributes to the understanding and material design of various metallic materials through the construction of highly accurate MLIPs.
Medium carbon steel sheets of 2.6 mm thick were successfully welded by linear friction welding at various frequencies and applied pressures. The welding temperature decreased with the decreasing frequency, but unexpectedly with the increasing applied pressure. A medium carbon steel joint with no martensitic transformation could be obtained at a welding temperature below the A(1) point under the conditions of low frequency and high applied pressure. Moreover, the welding temperature evolution was found to be correlated with the behaviour of the flash expelling at the welding interface. It was revealed that the welding temperature can be determined as the temperature at which the ultimate tensile strength of the interface material decreased below the applied pressure.
Measurements of the local tetragonality in Fe-C martensite at microstructural length-scale through pattern matching of electron backscatter diffraction patterns (EBSPs) and careful calibration of detector geometry are presented. It is found that the local tetragonality varies within the complex microstructure by several per cent at largest and that the scatter in the axial ratio is increased at higher nominal carbon content. At some analysis points the local crystal structure can be regarded as lower symmetry than simple body centred tetragonal. A linear relation between the nominal carbon content and averaged local tetragonality measured by EBSD is also obtained, although the averaged axial ratio is slightly below that obtained from more classical X-ray diffraction measurements.
The large magnetic fields that hulk high-temperature superconductors (HTS) can trap, enable us to develop high specific power electric machines. However, very few bulk HTS machine prototypes have been developed worldwide. To explore the potential of bulk HTS machines, we developed a 30-kW low speed radial-flux synchronous machine comprised of a conventional stator with ferromagnetic teeth. The rotor has four-field poles. Each pole is a rectangular array of QMG melt-growth GdBa2Cu3O7-delta bulk superconductors. The poles are cooled by a 150 W capacity neon thermosyphon, which maintains an operating temperature of around 30 K. The drawback of the ferromagnetic teeth, however, is their saturation and core losses under high and variable magnetic flux. Apart from high flux above 1 T, the trapped field profile of bulk HTS poles exhibits a nonsinusoidal shape, the harmonic components of which increase core losses. Such core losses are normally divided into hysteresis losses and eddy current losses, both of which depend on the magnitude and frequency of the trapped field. Hysteresis loss is associated with domain walls movement, while eddy currents are circulating currents induced in the core. In this paper, core losses in a ferromagnetic stator of the 30-kW machine are determined while the poles are magnetized to 2 and 3 T. The results show that core losses increase with increasing magnetic flux and rotor speed. Core losses due to rotor pole fields at the operating speed of 190 r/min were 1.2% and 1.9% of the machine rated output under 2 and 3 T magnetization, respectively. Finally, the harmonic content of the machine output voltage was obtained.
Evolution of heterogeneous nano-structure in heavily cold-rolled SUS316LN stainless steels was investigated in detail. Transmission electron microscopic observations from the transverse direction (TD) of the 92% rolled specimen revealed the formation of a typical hetero-nano structure composed of ultra-fine lamellar grains embedded with deformation twin domains. The twin domains had prolate ellipsoidal shape elongated parallel to TD. Two types of twin domains with different crystallographical orientations to matri-ces could be identified, i.e. , i) <211> // rolling direction (RD) and <110> // TD or ii) <110> // RD and <211> // TD, although all the {111} twining planes of both twin domains were oriented nearly parallel to the rolling planes. The ultra-fine lamellar grains were elongated along <100> direction and nearly parallel to RD. Deformation twins with a few nano-meter spacing were also frequently observed to develop in the lamellar grains. Evolution sequence of the hetero-nano structure during cold rolling was also investigated. At an early stage of rolling, deformation twins were gradually formed in the whole grains. Then, the regions fragmented grains by twins were further subdivided by a numerous number of shear bands inclined at about 20–45 ° from the RD, resulting in the formation of “eye-shaped” twin domains surrounded by shear bands and their crystallographical rotation. Cold rolling up to 50% caused a considerable increase in strength and decrease in ductility. While the strength was raised more with increasing reduction up to 92%, both the strength and ductility eventually slightly decreased by further rolling.
