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

100%

Čížek J., Procházka I., Smola B., Stulíková I., Kužel R., Matěj Z., Cherkaska V., Islamgaliev R., Kulyasova O.

Acta Physica Polonica A

2005

vol. 107

issue 5

738-744

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

81%

Čížek J., Procházka I., Kužel R., Matĕj Z., Cherkaska V., Cieslar M., Smola B., Stulíková I., Brauer G., Anwand W., Islamgaliev R., Kulyasova O.

Acta Physica Polonica A

2005

vol. 107

issue 5

745-752

Recent investigations of ultra fine-grained metals (Cu, Fe, Ni) performed within a Prague-Rossendorf-Ufa collaboration will be reviewed. The specimens were prepared by severe plastic deformation: the high-pressure torsion and equal channel angular pressing. Positron annihilation spectroscopy was used as the main method including (i) the conventional lifetime and the Doppler broadening measurements with ^{22}Na and (ii) the slow-positron implantation spectroscopy with the Doppler broadening measurement. Other methods were also involved: transmission electron microscopy, X-ray diffraction, and microhardness. First, the mean grain size was determined and defects were identified in the as-deformed materials. Defects concentration and spatial distribution were studied in detail. Dislocations situated in distorted regions along grain boundaries, and a few-vacancy clusters distributed homogeneously inside dislocations-free grains, were observed in the ultra fine-grained Cu, Fe, and Ni. Subsequently, the thermal evolution of the ultra fine-grained structures during isochronal annealing was studied.

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2 Acta Physica Polonica A

2 2005

2 Cherkaska V.

2 Islamgaliev R.

2 Kulyasova O.

2 Kužel R.

2 Procházka I.

2 Smola B.

2 Stulíková I.

2 Čížek J.

1 Anwand W.

1 Brauer G.

1 Cieslar M.

1 Matĕj Z.

1 Matěj Z.

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

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