The internal structure of a component will influence how sound propagates through it. Most metal specimens are comprised of small single crystals called “grains”. Depending on the metal used and its processing history, the average grain size can range from a few microns to many centimeters. Some metal specimens, such as Nickel alloys used in jet engine components have one primary type of grain (i.e., grains of “one phase”). Other metal specimens contain more than one phase. For example jet-engine titanium alloys often contain a mixture of grains having cubic and hexagonal symmetries, respectively.
The speed of sound through a given grain depends on the direction of sound propagation with respect to the crystalline symmetry axes. When a sound wave encounters a boundary between two grains, the change in sonic properties causes some of the incident sound energy to be reflected. This leads to detectable ultrasonic signals known as “grain noise”. On the one hand, such noise can hamper an inspection for a small or subtle defect. On the other hand it can also be used as a tool for material characterization, e.g., for making grain size estimates. The microstructure of a metal is also responsible for a phenomenon known as ultrasonic attenuation. Attenuation describes the loss of energy (due to scattering and absorption) of a sound pulse as it propagates through a material.