Gradient Microstructure Induced by Surface Mechanical Attrition Treatment (SMAT) in Magnesium Studied Using Positron Annihilation Spectroscopy and Complementary Methods

Abstract

Surface mechanical attrition treatment (SMAT) was used to generate a gradient microstructure in commercial grade magnesium. Positron annihilation lifetime spectroscopy and variable energy positron beam measurements, as well as microhardness tests, electron backscatter diffraction, X-ray diffraction, and electrochemical corrosion tests, were used to investigate the created subsurface microstructure and its properties. It was found that SMAT causes an increase in dislocation density and grain refinement which results in increased hardness of the subsurface zone. The mean positron lifetime values indicate trapping of positrons in vacancies associated with dislocations and dislocation jogs. The increase of the SMAT duration and the vibration amplitude influences the depth profile of the mean positron lifetime, which reflects the defect concentration profile. Electrochemical measurements revealed that the structure induced by SMAT increases the susceptibility of magnesium to anodic oxidation, leading to the enhanced formation of hydroxide coverage at the surface and, as a consequence, to the decrease in corrosion current. No significant effect of the treatment on the residual stress was found.

Temperature studies using positron lifetime spectroscopy of the subsurface zone in pure titanium exposed to a dry sliding test

“…However, changes go much deeper than the work hardened zone and define the boundaries of the so-called subsurface zone (SZ). 4 The generation of this zone accompanies each surface treatment, for example: surface machining, blasting, and so on, 5 but the frictional process is the most spectacular because in this case processes in this zone are responsible for the wear of the sliding materials.…”

Positron Annihilation Spectroscopy Complex for Structural Defect Analysis in Metal–Hydrogen Systems

“…The second component with a lifetime of τ A = 232 ± 1 ps is significantly lower than the values typical for monovacancies (253 ps [27,30], 292 ps [26]), and dislocations (244 ± 4 ps [28,31,32]). This component seems to be related to deformation defects (dislocations associated with twin boundaries [32]), which are typical for milled powders. In addition, two long-lived components are distinguished in all spectra: τ B = 392 ± 2 ps and τ C = 2.4 ± 0.02 ns.…”

“…The component with the shortest lifetime τ F = 226 ± 1 ps corresponds to the positrons’ annihilation in a defect-free lattice of magnesium [ 26 , 27 , 28 , 29 ]. The second component with a lifetime of τ A = 232 ± 1 ps is significantly lower than the values typical for monovacancies (253 ps [ 27 , 30 ], 292 ps [ 26 ]), and dislocations (244 ± 4 ps [ 28 , 31 , 32 ]). This component seems to be related to deformation defects (dislocations associated with twin boundaries [ 32 ]), which are typical for milled powders.…”

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment in Grade 2 Titanium Studied Using Positron Annihilation Spectroscopy and Complementary Methods

“…The increase in the sonotrode amplitude leads to larger and deeper pits which are visible for the SMAT-2 sample with twice the sonotrode amplitude. In the case of magnesium SMATed with the same parameters as in the present studies, the differences in the surface roughness and morphology between the samples such treated were smaller [ 45 ].…”