The present work is aimed at characterizing the strain hardening behavior of AISI 316L austenitic stainless steel using ultrasonic velocity measurements. For this purpose, microstructural studies and ultrasonic velocity measurements were carried out on the samples deformed to different levels of strain at room temperature. Strikingly, the ultrasonic velocity-strain plot of the alloy exhibited a three-stage behavior that was similar to the strain hardening rate-strain response of the alloy. At strains lower than about 0.06 (stage A), a falling regime of velocity was observed that was related to the increase of dislocations density. This stage was followed by a stage of a nearly constant velocity (stage B). The initiation of this stage was concurrent with the onset of deformation twinning in the microstructure. Beyond a strain of about 0.2, the second falling regime of velocity (stage C) was developed. The occurrence of this stage was attributed to the difficulty of new twins formation with an increasing strain.In recent years, ultrasonic velocity measurement has been used increasingly as a nondestructive technique to study various characteristics and processes of materials. Several investigations have been performed to correlate velocity measurements with the microstructural features, [1][2][3] physical properties, [4] mechanical properties, [5][6][7] heat treatments, [8][9][10] grain size, [11] and the amount of deformation. [12][13][14] However, the correlation between strain hardening behavior and ultrasonic velocity, especially in low stacking fault energy (SFE) alloys, has not received attention so far.The strain hardening technique is widely used for strengthening the metallic components. From a technological viewpoint, an accurate characterization of strain hardening behavior of a material is necessary to develop models with good predictive capabilities concerning the force-displacement relationship in deformation processes. In addition, it provides clear indications of the beginning or collapse of many of the microstructural phenomena occurring during deformation. A transmission electron microscope (TEM) and an optical microscope (OM) usually are applied to characterize the strain hardening behavior of materials; however, sample preparation for these techniques is difficult and time consuming. Also, in a recent study, it has been shown that the deformation mechanism of SFE alloys cannot be characterized solely by OM. [15] Strain hardening behavior of low SFE face-centeredcubic (fcc) alloys (such as Hadfield steels, austenitic stainless steels, and multiple Co-Ni superalloys) have been studied extensively. [16][17][18][19] It is known that the strain hardening rate (h)-true strain (e) plots of these alloys exhibit four distinct regimes of hardening. In stage A, the strain hardening rate decreases continuously with deformation. This stage is followed by a second stage of nearly constant hardening rate, stage B. The second falling regime of strain hardening is observed in stage C. At large strains, this stage...
