Potential application of ultrasonic testing to characterize thermal aging embrittlement of duplex stainless steels was investigated. The materials to be tested were SUS329J1 and two simulated CF8M samples. The changes in acoustic properties such as attenuation and sound velocity were measured In addition to ultrasonic testing, micro bend tests, micro-Vickers hardness tests for ferrite phase and austenite phase were performed also. Microstructural variation due to thermal aging was observed by high resolution FEG-TEM and in-situ micro element analysis was performed by STEM-EDS. The modulated microstructure and composition modulation in ferrite phase due to spinodal decomposition were observed. This modulation was confirmed by Mossbauer measurements, which showed the increase in the hyperfine field with aging time. The hyperfine field which was dependent on spinodal decomposition and was determined by Mossbauer measurement showed the linear correlation with micro-Vickers hardness in process of thermal aging. And the linear relationship between the changes in sound velocity and the changes in micro-Vickers hardness was obtained also. Consequently, it is suggested that the thermal aging embrittlement can be evaluated by measuring sound velocity in material. The mechanism of thermal aging embrittlement has been interpreted tentatively in terms of the decrease in dislocation mobility.
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