We extend a thermal-elastic stress model by the finite element method to evaluate anisotropic three-dimensional thermal stress in AlN bulk crystals grown on on-axis 2H-AlN and 6H-SiC seeds. The distribution of stresses in the growing AlN crystals at various crystal thicknesses is simulated based on the developed model. The simulation results show that a high von Mises stress layer with strong fluctuations at the 6H-SiC/2H-AlN interface is observed for the heteroepitaxial growth, and a critical crystal thickness of 1 mm for the stress relaxation is required to avoid cracking. On the contrary, a smooth evolution of the von Mises stress is observed for the homoepitaxial growth on the 2H-AlN seed. The maximum total resolved shear stress inside the crystal when using SiC seeds is slightly higher than that of using AlN seeds at the initial growth stage, while this phenomenon reverses after the crystal thickness exceeds approximately 3 mm. Whether using AlN or SiC seeds, the magnitude of the total resolved shear stress increases steadily, and the difference between the homoepitaxial and heteroepitaxial growth is quite small during the whole bulk AlN growth by the physical vapor transport process.

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The low-temperature plasticity of aluminum nitride (AlN) is determined by the interaction between edge and screw dislocations. However, the motion of screw dislocations and their glide mechanisms have not been evaluated. In this study, the motion of a ⟨0001⟩{11̅ 00} screw dislocation in a single crystal of AlN is explored by molecular dynamics simulations using LAMMPS software with the Stillinger−Weber (SW) potential. Four modes of thermally activated motion are observed under different conditions of temperature and stress: double kinks, Shockley partials, self-pinning, and debris and dislocation loops. The mobilities of a ⟨0001⟩{11̅ 00} screw dislocation and a 1/3⟨112̅ 0⟩{11̅ 00} edge dislocation are compared under various conditions. Our results show that the mobilities of the ⟨0001⟩{11̅ 00} screw and 1/3⟨112̅ 0⟩{11̅ 00} edge dislocations are quite low at T < 600 K. The ⟨0001⟩{11̅ 00} screw dislocation moves faster at 900 < T < 1500 K and seems less dependent on the temperature than does the 1/3⟨112̅ 0⟩{11̅ 00} edge dislocation at 1200 < T < 2200 K. However, the opposite phenomenon is observed at higher temperatures. The mobility of the ⟨0001⟩{11̅ 00} screw dislocation is slightly lower than that of the 1/3⟨112̅ 0⟩{11̅ 00} edge dislocation at T > 1800 K, although the mobility difference can reach several orders of magnitude at 900 < T < 1200 K due to different Peierls barriers.

The average cyclic load of heavy-haul railway trains is generally larger than that of a conventional mixed passenger and freight railway. This load leads to more severe fatigue damage to structures, including the concrete in a tunnel invert. This study focuses on the fatigue damage of a tunnel invert under a cyclic load of 33 tonnes. The damage classifications for the tunnel inverts are given based on field investigations. With large-scale in-situ tests on the Zhang-Tang Heavy-Haul Railway Tunnel, the pressure–time distributions for the additional dynamic stresses on the surface of the track-bed for various classes of the surrounding rock are proposed. They were subsequently validated against numerical simulation using the ANSYS Workbench module. Fatigue damage of the tunnel invert is demonstrated using both numerical and monitoring methods. It has been observed that the damage to the tunnel invert becomes severe and extensive if the quality of the surrounding rock degrades. Damage zones develop first at the top of the invert and then expand to a deeper position, depending on the rock grade.

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