Laser ultrasonic method for determination of crystallographic orientation of large grain metals by spatially resolved acoustic spectroscopy (SRAS)

This thesis presents a crystallographic orientation determination technique which is based on a laser ultrasonic method: spatially resolved acoustic spectroscopy (SRAS). Surface acoustic waves (SAW) propagate on a solid surface with a phase velocity that is frequency independent, but which varies with the crystallographic orientation. By comparing the SRAS results with the calculated SAW velocities, the orientation of crystal can be determined. The SRAS technique allow the SAW velocity to be recorded in two slightly different approaches. According to the formula v=λf, the velocity v can be obtained by varying the k-vector (λ) or frequency f of the wave while the another multiplier is fixed. K-SRAS is implemented by firing a laser beam with a fixed intensity modulation frequency through a spatial light modulator (SLM); the SAW velocity is determined by varying the fringe spacing of the SLM image. F-SRAS uses a broadband (sharp pulse) laser, the beam passes through a chrome photomask with fixed fringe spacing, and the peak frequency is used to determine the SAW velocity. Scans are performed on single or multiple-grain titanium alloy, aluminium and nickel samples by both methods. The contrast of the velocity maps give adequate information of grain size and location. A SAW velocity model is developed according to the elastic constants and mass density of the material. The orientation of crystals can be determined by comparing the SRAS results and the SAW velocity model. The SAW velocities in different propagation directions are measured on nickel, aluminium and titanium α samples with known orientations, and agree well with the predicted velocities from the model. An overlap function is introduced as a search algorithm to link the SRAS results to the SAW velocity model. The results are compared with measurements taken using the Laue back-reflection technique; they gave very close crystallographic orientation with acceptable error within the industrial limit. At the end of the thesis, consideration is given to further research in the acoustic modelling and data processing algorithms that would improve the technique in the future