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Vol.60, No.3, 169 ~ 180, 2022
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| Title |
| Through-Thickness Microstructures and Yield Strength Enhancement for AZ31 Mg Sheets Treated by Ultrasonic Nanocrystal Surface Modification |
| 강명성 Myungsung Kang , 강주희 Joo-hee Kang , 박현욱 Hyeonuk Park , 김준형 Jun-hyung Kim , 편영식 Young-sik Pyoun , 백민재 Minjae Baek , 이동준 Dong Jun Lee , 이승진 Seung Jin Lee , 박상후 Sanghu Park , 김홍물 Hong Moule Kim , 박성혁 Sung Hyuk Park , 최윤석 Yoon Suk Choi |
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| Abstract |
| An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 μm from the surface) and twin-dominated (200~300 μm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of {1012} tensile twins (despite compressive strikes by the hemispherical tip), {1011}-{1012} double twins and {1011} compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress.
(Received 21 July 2021; Accepted 15 December 2021) |
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| Key Words |
| AZ31 Mg, ultrasonic nanocrystal surface modification (UNSM), through-thickness microstructures, effective depth, strength |
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