Gradient layer of ultrafine equiaxed grains produced by ultrasonic energy accelerated dynamic recrystallization

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2020-09, Vol.795, p.139958, Article 139958
• Main Authors: Wang, Yan-li, Zhu, You-li, Cai, Zhi-hai
• Format: Article
• Language: English
• Subjects: Aluminum base alloys; CAD; Computer aided design; Diameters; Dynamic recrystallization; Equiaxed structure; Finite element method; Grain refinement; Indentation; Irradiation; Metal sheets; Multiplication; Plastic deformation; Stress waves; Ultrafine grain; Ultrafines; Ultrasonic energy
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2020.139958

Single site indentation experiments with ultrasonic and quasi-static indentations were performed on aluminum alloy sheet to study the effect of induced ultrasonic energy on microstructure evolution during indentation. Electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) techniques were employed to investigate the dislocation structures and grain refinement mechanisms of the material underneath the indents. Process simulation about the indentation under ultrasonic irradiation via the ANSYS LS-Dyna finite element code was conducted to analyze the stress wave and spin rate in the material for further explanation of the mechanisms. Results showed that, after 10 s of ultrasonic indentation, a 300 μm deep gradient ultrafine-grained layer was produced underneath the ultrasonic indent, where the top surface exhibited an equiaxed ultrafine dislocation-free grains layer of 60 μm in thickness with grain diameter ranged 200–300 nm. The hardness of the gradient ultrafine equiaxed grain layer was increased substantially. It is supposed that the ultrasonic irradiation effectively enhanced the dislocation multiplication, movement and vibration, which accelerated sub-grain rotations and strongly facilitated the dynamic recrystallization. •Ultrasonic energy favored dislocation multiplication, vibration and annihilation and promoted dynamic recrystallization.•Both rotational and grain boundary nucleation mechanisms for the dynamic recrystallization occurred, while sub-grain rotation dominated.•Topsurface stress amplitude and spin rate were ~1000 MPa and ~10000/s, respectively.•A 300 μm thick gradient ultrafine-grained layer is produced in a 10-s ultrasonic indention.•The equiaxed ultrafine-grained layer of 60 μm in thickness is expected to have good ductility and thermal stability.