James B. Spicer
Dept. of Mater. Sci. and Eng., Johns Hopkins Univ., Rm. 102 Maryland Hall, Baltimore, MD 21218
Laser ultrasonics as a remote, noncontacting ultrasonic testing method has been used extensively for materials characterization. However, the unique character of the laser source has not been exploited fully. This work on steel at elevated temperatures has made use of relationships which exist for the thermoelastic laser source in monitoring microstructural changes in steel. Ultrasonic monitoring of microstructural evolution in solids is performed by making measurements of ultrasonic velocity and attenuation. At elevated temperatures, both the ultrasonic velocity and the attenuation are affected independently of the material microstructure. Thermal expansion of the sample complicates the determination of velocity from ultrasonic data since an independent measurement of sample length is needed. Contacting transducers allow only intermittent determination of velocity and attenuation since continuous contact might adversely affect either the transducer or the material. In this study, the laser-ultrasonic source characteristics are used to address these issues related to the testing of materials at elevated temperatures. Results obtained for the testing of austenitic and martensitic stainless steels to temperatures in excess of 1273 K indicate that characteristic microstructural changes may be detected by analysis of the laser ultrasonic data. This analysis permits real-time, continuous monitoring of microstructural evolution.