Nickel-based superalloys have already been extensively used in aviation manufacturing, particularly in the production of jet engine components such as turbine blades, airframe parts, and nuclear power plant components. The mechanical properties of these alloys make them highly desirable for these applications. To ensure the successful application of single crystal nickel-based superalloys, it is crucial to have a comprehensive understanding of their anisotropic properties. This includes knowledge of the elastic coefficients, thermal expansivity, and thermal conductivity. For these purposes, acoustic wave velocity is often employed as a primary quantity to access the parameters of these alloys since it is influenced by the module of elasticity and density. Any variation in material properties like porosity, residual stress, or even coating thickness in the case of surface waves can lead to changes in acoustic wave velocity. Monitoring the acoustic wave velocity can provide valuable information about the ongoing processes and their effects on material properties. Measuring the changes in acoustic wave velocity makes it possible to assess and track the progress of these processes. This information is important in evaluating the quality and integrity of manufactured components, as it helps in identifying deviations or abnormalities that may affect the final product.

Nondestructive testing (NDT) has offered a powerful approach for safety critical material inspections such as those in aerospace and nuclear industries by minimizing the risk of failure, thereby reducing costs and maximizing safety. Laser ultrasound technology (LUT) uses a pulsed laser source to locally heat the sample, and acoustic waves are then generated due to thermal elastic processes. A second laser is used to probe the generated acoustic wave. As thermal elastic constants are highly related to samples' density, stress, as well as crystallographic structures, one can then access the macroscopic or microscopic information of the sample and thus find the defects at both scales. As a consequence, LUT is receiving growing attention thanks to its great potential in the evaluation of defects, crystallographic orientation, and residual stress. Moreover, as LUT uses lasers to excite and detect the signals, the entire process is contactless, and it thus shows great advantages when inspecting samples with complex geometric structures and brings great convenience when used in environments with elevated temperatures or toxicity. In addition, LUT can inspect small specimens with high spatial revolution by using lenticular systems that allow the pump and probe lasers to focus on the surface of the sample. In recent years, laser ultrasound systems combined with wavefront modulation techniques have shown the capability of generating and detecting surface acoustic wave (SAW) with a specific frequency, which makes the mathematical modeling much simpler during data processing and in turn leads to an easier approach to access critical material properties such as crystallographic structures.

We discuss a new LUT that utilizes wavefront control to investigate the relationship between SAW velocity and crystal orientation in nickel-based superalloys. Nickel-based superalloys are widely used in aerospace turbofan blades due to their thermal resistance. Understanding the mechanical anisotropy of these materials is crucial for ensuring the mechanical performance and flight safety of turbofan blades. The technology combines numerical simulation and experiments to accurately measure the propagation velocity of SAWs and analyze the material's mechanical properties. A laser ultrasonic finite element numerical simulation model based on wavefront control is proposed. The simulation results reveal that the anisotropy ratios of SAW velocity in single crystal nickel-based superalloys in the (100) and (001) planes are 0.073 and 0.18, respectively (Fig. 4 & Fig. 6). To further investigate the relationship between crystal orientation and acoustic velocity in nickel-based superalloys, a laser ultrasonic microscopy system is employed (Fig. 7). This system enables the scanning and imaging of the surface acoustic velocity in both single crystal and polycrystalline nickel-based superalloys, facilitating the visualization analysis of surface defects and grain distribution (Fig. 10).

The simulation and experiment results indicate that the SAW velocity is sensitive to the orientation of the crystalline axis, which proves the capability of laser ultrasound systems combined with wavefront modulation techniques in the field of crystalline orientation determination and defect detection in an NDT manner