Abstract
Measuring the grain structure of aerospace materials is very important to understand their mechanical properties and in-service performance. Spatially resolved acoustic spectroscopy is an acoustic technique utilizing surface acoustic waves to map the grain structure of a material. When combined with measurements in multiple acoustic propagation directions, the grain orientation can be obtained by fitting the velocity surface to a model. The new instrument presented here can take thousands of acoustic velocity measurements per second. The spatial and velocity resolution can be adjusted by simple modification to the system; this is discussed in detail by comparison of theoretical expectations with experimental data.
Highlights
Imaging and measuring the microstructure of aerospace materials is becoming more important as the materials are pushed ever closer to their working limits
Small changes to the grain structure or orientation can have a large effect on the performance of the material, for example, small changes to the primary and secondary angles of a single crystal turbine blade [1] can lead to different stress responses for the same loading conditions, potentially impacting the fatigue life of the blade [2]
The orientation of grains in a material is especially important for certain types of alloys that are susceptible to dwell fatigue [6, 7], of which one contributing factor is the presence of rogue grain combinations in the loading direction leading to premature failure of the material [8, 9]
Summary
Imaging and measuring the microstructure of aerospace materials is becoming more important as the materials are pushed ever closer to their working limits. Understanding the evolution of the change in grain structure is important for understanding the strength of these bonds [11], and how ultrasonic bulk waves interact with the weld microstructure in non-destructive evaluation (NDE) applications Additive manufacturing techniques, such as direct laser deposition [12] and arc weld deposition [13], are being increasingly used to manufacture aerospace components. We have developed an instrument around an acoustic technique that can perform similar grain orientation measurements, on samples of any size and varying degrees of surface finish, albeit with a lower spatial resolution (tens of microns) than obtained by electron microscopy techniques. The instrument and its capabilities will be described in more detail
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