Defects in Ultra-Fine Grained Mg and Mg-Based Alloys Prepared by High Pressure Torsion Studied by Positron Annihilation

Abstract

Despite the favourable strength and thermal stability, a disadvantage of the Mg-based alloys consists in a low ductility. Recently it has been demonstrated that ultra fine grained metals with grain size around 100 nm can be produced by high pressure torsion. A number of ultra fine grained metals exhibit favourable mechanical properties consisting in a combination of a very high strength and a significant ductility. For this reason, it is highly interesting to examine microstructure and physical properties of ultra fine grained Mg-based light alloys. Following this purpose, microstructure investigations and defect studies of ultra fine grained pure Mg and ultra fine grained Mg-10%Gd alloy prepared by high pressure torsion were performed in the present work using positron annihilation spectroscopy combined with X-ray diffraction, TEM observations, and microhardness measurements. Positrons are trapped at dislocations in Mg and Mg-10%Gd alloy deformed by high pressure torsion. A number of dislocations increases with the radial distance r from the centre to the margin of the sample. No microvoids (small vacancy clusters) were detected. Mg-10%Gd alloy deformed by high pressure torsion exhibits a homogeneous ultra fine grained structure with a grain size around 100 nm and high dislocations density. On the other hand, pure Mg deformed by high pressure torsion exhibits a binomial type of structure which consists of "deformed regions" with ultra fine grained structure and a high dislocation density and dislocation-free "recovered regions" with large grains. It indicates a dynamic recovery of microstructure during high pressure torsion processing.

Ultra Fine-Grained Metals Prepared by Severe Plastic Deformation: A Positron Annihilation Study

“…As it was discussed in Ref. [4], the uniform spatial distribution of dislocations seems to be present predominantly in HPT-deformed metals with the hcp structure as a consequence of a lower number of slip systems compared to the fcc and bcc metals.…”

“…Hencefore, one concludes that τ 1 -component in the UFG materials of Table arises from positrons trapped at dislocations situated in the distorted regions along GBs. However, this cannot be regarded as a general conclusion because, as reported at this Conference [4], a uniform distribution of dislocations was found in the HPT-processed UFG Mg-10%Gd alloy. As it was discussed in Ref.…”

“…The Mg-based UFG metallic systems have been studied within the above collaboration, too. These results are presented elsewhere at this Conference [4].…”

“…Vacancies are mobile at RT in the metals under study [15] so that they can diffuse to sinks at GBs, decorate dislocations present in the sample and a portion of vacancies can form small vacancy clusters. The latter process obviously becomes more likely inside the dislocation-free grain interiors as it appeared to be suppressed in HPT-deformed Mg-10%Gd alloy with homogeneously distributed dislocations [4]. The average size of vacancy clusters, i.e.…”

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

“…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.…”