Cryogenic (NDT) non-destructive testing method is based on the effect of the microcrystalline structure on the transmission of ultrasound. This method can be used to non-destructively evaluate the kinetics and completion of the time-temperature-transformation between structural phases. The basis for ultrasonic testing is based on the difference between the Young’s moduli of the two phases at any given transformation temperature. Since the velocity of longitudinal sound waves is a function of the square root of the elastic constants, an increase in velocity is expected during a typical transformation. Hence, the velocity should be somewhere between the two extremes with a mixture of the phases having a velocity in between. Fortunately, voids do not affect velocity since the sound waves will pass only through the solid material. In addition to velocity changes, sound attenuation is also highly structure sensitive which may also give insight into the mechanism of the transformation.

More specifically, the velocity of the longitudinal sound wave (Vl) relates to the elastic constants as described by Hooke’s law. The elastic constant relates to the linear stress and strain which are fundamentally functions of the thermodynamic state of the material and the interatomic binding forces in the microcrystalline lattice structure. The longitudinal velocity is related to the Youngs’s modulous (E), density (P), and Poisson’s ratio (o) by 

In order to utilize this to non-destructively assess the transformation within an alloy, sound waves are assessed for transit time through a structure. This time (T) will be related to longitudinal velocity (V)  and path length (L) by

Since changes in length will be small during the phase transformation, changes in transit time will reflect changes in the properties of the product and the progress of transformation. Hence the use of ultrasonic transit time measurements can allow the non-destructive assessment of the materials.

By performing real-time ultrasound testing procedures using electromagnetic acoustic transmission (EMAT) or conventional ultrasonic (UT) while performing cryogenic NDT testing, any changes in a materials velocity and amplitude can be documented. The time and temperature is recorded upon any change in velocity or amplitude and phase transformation detection of the start of transition and completion of transition can be documented. This cryogenic (NDT) testing process becomes quite valuable in establishing quality control procedures for materials manufacturers and as an incoming materials inspection process.