The velocity of ultrasound in low-carbon steel deformed at the low yield limit

Abstract

The variation of a thin structure upon deformation of low-carbon steel at the yield limit is analyzed. The character of the interrelationship between the stage nature of a plastic flow of low-carbon steel and the velocity of ultrasound in it is established. It is shown that the velocity of ultrasound is a parameter for obtaining additional data on the development of plastic flow. The structural changes that exert an effect on the velocity of ultrasound in the deformation that corresponds to the yield site are studied.1. Formulation of the Problem. The heterogeneity of plastic flow and deformation localizations have recently been studied intensely. The known example of inhomogeneous flow is propagation of the Chernov-Liiders band upon loading of a material with a distinct yield limit. In this case, the object subjected to loading has a mobile narrow boundary, which divides two zones in which the material is in different states. The motion of the Chernov-Liiders band causes an increase in the volume of the plastically deformed region at constant stress; on the expansion diagram, the yield site corresponds to this increase. Zuev [1] interpreted the propagation of the Chernov-Liiders band as the development of one type of dissipative structures in the deformed material, the so-called autowave of switching [2]. Since the autowaves can generate only in an active medium whose properties change upon deformation [2], the data on the change of the material characteristics as the Chernov-Liiders band develops on the yield site are of great interest. This study is devoted to clarification of the indicated circumstances.2. Material and Techniques of Research. The studies were performed on flat samples of 09G2S low-carbon steel (0.09~ carbon, 2% manganese, and 1~ silicon) with a 50 • 10-mm working section; the specimens were cut from a hot-rolled sheet of thickness 3 mm. To remove the internal stresses, the samples were preannealed in vacuum at 950~ for 2 h. The average size of a grain after treatment was approximately 13 #m. The samples were stretched on an Instron-l185 test device with a velocity of 3.3 9 10 -3 mm/sec (d~/dt = 6.6.10 -5 sec-1). In these conditions, a "tooth" and a yield site were observed on the flow curve.The character of deformation localization was studied by the technique of two-exposure speckle interferometry. An ALMEX automated laser measuring complex developed for these purposes at the Institute of Strength and Materials Science Physics of the Siberian Division of the Russian Academy of Sciences allows one to obtain the field of displacement vectors, calculate the plastic-distortion tensor components, and construct their distributions over the entire surface of the sample at any stage of deformation.Additional information on the behavior of the deformed material at the stage of yield site was gained by simultaneous recording of small changes in the velocity of ultrasound (VU) in a sample, measured by an ISP-12 device [3] by the method of autocirculating the pulses of Rayleigh waves with a carrier freq...

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