## 1 Introduction  
The selection and preparation of divertor materials to be implemented as plasma-facing materials has become one of the bottlenecks limiting the development of nuclear fusion. Plasma-facing components in nuclear fusion reactors can withstand harsh environments, such as steady thermal loads, transient thermal shock, plasma irradiation, and neutron irradiation [1]. The divertor is used to withstand the particles and heat flux coming from the core plasma, and at the same time, exhausts the helium ash generated in the nuclear fusion reaction process [2]. The advantages of tungsten and its alloys include having a high melting point, good thermal conductivity, good thermomechanical properties, a high sputtering threshold, low atomic activity, low deuterium and tritium retention, and resistance to plasma erosion [3]. In existing tokamak devices, such as EAST [4], WEST [5], JET [6], ASDEX-U [7], and KSTAR [8], tungsten and its alloys are widely used as plasma-facing materials, especially for divertor components. A tungsten divertor with a W/Cu block structure will also be used in ITER [9]. In addition, tungsten and its alloys are preferred as plasma-facing materials in other future fusion experimental reactors, such as DEMO [10] and CFETR [11]. However, during the process of plasma discharge, recovery and recrystallization will occur under a high heat flux and melting will happen under a higher heat load, which become the main constraints [12] that cause recrystallization brittleness [13], resulting in a decline in mechanical properties, a lower fracture toughness, and a higher ductile-to-brittle transition temperature [14; 15]. It seems that the recrystallization phenomenon and fatigue degradation mechanism caused by high temperatures and cyclic thermal loads increases the possibility of crack initiation and propagation [16]. Changes in the near-surface microstructure induced by transient heat loads, as well as the impact of combined plasma / heat loads, are likely to reduce the heat dissipation properties of tungsten, resulting in a reduction in the power-handling capability of the divertor target [17]. ITER has clear requirements for the microstructure and state of tungsten, which requires that the rolling direction of the rolled tungsten plate, that is, the grain elongation direction, should be parallel to the heat conduction direction. By limiting the grain direction and directing the crack propagation to occur along the grain elongation direction, the generation of cracks parallel to the surface can be reduced and the risk of plasma contamination caused by grain delamination and ejection can be minimized [18]. To clarify the service characteristics of tungsten under the relevant severe conditions, a series of experiments have been carried out on tungsten under the relevant heat loads or plasma irradiation. Through heat load or plasma irradiation experiments, [PERSON] [19] and [PERSON] [20] found vertical macrocracks in the middle of the thermal load surface. Long periods of servicing under high temperatures in the recovery and recrystallization process cannot be avoided. The recrystallization process will degrade the mechanical properties of materials and change the thermal shock resistance of tungsten [21], and thus, cracking is more likely to occur when suffering under repeated thermal cycling [22]. Although the macrocrack will not affect the heat dissipation capacity of the component, it still needs to be avoided as much as possible, considering its effects on the core plasma, such as the potential melting risk caused by the leading-edge effect [23]. [PERSON] [16] conducted a more detailed analysis of microstructures and damage evolution in monoblocks. Thermal stress introduced via a cyclic heat load on a monoblock resulted in surface deformation and surface roughness. The depth of the recrystallization area was examined by using EBSD and a hardness test, and the grain orientation and the initial texture were compared and analyzed.