Fast noncontact imaging of material micro structure using local surface acoustic wave velocity mapping

The make-up of the material microstructure of multi-grained materials such as titanium alloys and aluminum is of great interest to many in industries such as aerospace. The ability to map the material microstructure - in effect to image the grains - quickly and in a nondestructive manner would be useful from both a process control perspective and in the area of nondestructive evaluation. There are several techniques in the field that are capable of imaging grain structure, including simple etching, orientation imaging microscopy and scanning acoustic microscopy; all have their strengths and weaknesses. We present a new ultrasonic technique that can directly and quantitatively image the local surface acoustic wave (SAW) velocity over the surface of a material. Material microstructure can be determined if the phase velocity of the grains varies with grain orientation. The acoustic waves are generated and detected by lasers and as well as being noncontact, the technique is relatively fast, can cope with large samples, and is totally nondestructive. The new technique involves varying the spatial parameters of the excitation pattern in real time to maximize the generation efficiency of the surface acoustic waves at the phase velocity of the material, in the region of excitation. By repeating this over the sample surface, a surface wave phase velocity map can be produced. As well as describing the velocity mapping technique in detail, several example results of industrially-relevant materials acquired using our new instrument are presented. The limit to the quantitative lateral resolution of the instrument is discussed, and how this relates to the qualitative lateral resolution. Results indicating the practical limit of the accuracy of velocity measurements are also presented. We demonstrate imaging on several samples (40mm2) and different materials. This shows that the the instrument can reveal the underlying microstructure and image areas of anomalous grain structure that may be significant for the performance of the material. The instrument currently works on smooth samples but can be extended to work on rough surfaces and components with unprepared surfaces and consequently this has high industrial relevance.