Olya B. Kulyasova

Microstructure and Thermal Stability of Ultra Fine Grained Mg-Based Alloys Prepared by High Pressure Torsion

In the present work we studied microstructure of ultra fine grained (UFG) pure Mg and UFG Mg-based alloys. The initial coarse grained samples were deformed by high pressure torsion (HPT) using pressure of 6 GPa. Such deformation leads to formation of UFG structure in the samples. The severe plastic deformation results in creation of high number of lattice defects. Therefore, we used positron annihilation spectroscopy (PAS) for defect characterizations. PAS represents a well developed non-destructive technique with high sensitivity to open volume defects like vacancies, vacancy clusters, dislocations etc. In the present work we combined PAS with TEM and XRD to obtain complete information about microstructure of the UFG samples studied. We have found that microstructure of HPT-deformed Mg contains two kinds of regions: (a) ”deformed” regions with UFG structure (grain size 100-200 nm) and high number of randomly distributed dislocations, and (b) ”recrystallized” regions with low dislocation density and grain size of few microns. It indicates some kind of dynamic recovery of microstructure already during HPT processing. On the other hand, homogenous UFG structure with grain size around 100 nm and high density of homogeneously distributed dislocations was formed in HPT-deformed Mg-9.33 wt.%Gd alloy. After characterization of the as-deformed microstructure the samples were subsequently isochronally annealed and the development of microstructure with increasing temperature and recovery of defects were investigated.

Structure and Fatigue Properties of the Mg Alloy AM60 Processed by ECAP

This paper reports on the microstructures and fatigue properties of ultrafine-grained (UFG) AM60 magnesium alloy processed by equal channel angular pressing (ECAP) at various temperatures. After ECAP processing, samples exhibited an increase in fatigue endurance limit, which correlates well with a decrease in grain size. In case of lowest ECAP temperature, the mean grain size is as small as 1 2m which leads to an increase in fatigue endurance limit by 70 % in comparison to coarse-grained alloy. The temperature of ECAP not only governs the grain size and misorientation angles of grain boundaries but also the volume fraction of precipitates, thus affecting the probability of twinning and grain growth after fatigue treatment.

The Influence of the ECAP Temperature on Microstructure and Mechanical Properties of a Magnesium Alloy

The influence of the equal channel angular pressing (ECAP) temperature (150-350oC) on microstructure of the AM60 magnesium alloy has been investigated using transmission electron microscopy. It was demonstrated that application of various ECAP regimes leads to significant difference in a grain size and volume fraction of precipitates in investigated material. Thermal stability of precipitates and correlation between microstructure and tensile strength are discussed.

Positron annihilation studies of microstructure of ultra fine grained metals prepared by severe plastic deformation

In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al2O3 nanocomposite (GlidCop) is compared.

Microstructure Development and Ductility of Ultra-Fine Grained Mg-Gd Alloy Prepared by High Pressure Torsion

Microstructure of ultra fine grained (UFG) Mg-Gd alloy prepared by high-pressure torsion (HPT) was investigated in the present work. Lattice defects introduced by HPT were characterized at first. Subsequently thermal stability of UFG structure and its development with annealing temperature were studied and correlated with changes of hardness and ductility. Precipitation effects in the alloy with UFG structure were compared with those in a conventional coarse-grained alloy. Defect studies were performed by positron annihilation spectroscopy (PAS), which represents well established non-destructive technique with a high sensitivity to open volume lattice defects like vacancies, dislocations, misfit defects etc. PAS investigations were combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). Changes of mechanical properties were monitored by Vicker’s microhardness (HV) and deformation tensile tests. It was found that HPT deformed Mg-Gd alloy exhibits UFG structure with mean grain size of 100 nm and a dense network of dislocations distributed uniformly throughout the whole sample. Although recovery of dislocations takes place at relatively low temperatures, it is not accompanied by grain growth and the mean grain size remains around 100 nm up to 300oC. Tensile tests performed at elevated temperatures to examine ductility showed that HPT-deformed alloy exhibits a superplastic behavior at 400oC. Moreover, it was found that the precipitation sequence in HPT-deformed alloy differs from that in conventional coarse-grained material.

Microstructure development and precipitation effects in ultra fine grained Mg-3Tb-2Nd alloy prepared by high pressure torsion

Mg-Tb-Nd ternary alloy represents a novel hardenable Mg-based alloy with enhanced strength and favorable creep properties. In the present work we studied microstructure of ultra fine grained (UFG) Mg-Tb-Nd alloy prepared by high pressure torsion (HPT). Lattice defects introduced into the specimen by the severe plastic deformation play a key role in physical properties of UFG specimens. It is known that positron lifetime (PL) spectroscopy is highly sensitive to open volume defects (like vacancies, dislocations, etc.). Therefore, PL spectroscopy is an ideal tool for defect characterizations in the HPT deformed specimens. In the present work we combined PL studies with transmission electron microscopy and microhardness measurements. After detailed characterization of the as-deformed structure, the specimens were step-by-step isochronally annealed and we investigated the development of microstructure with increasing temperature.

Microstructure and Mechanical Properties of Ultrafine-Grained Mg-Zn-Ca Alloy

This paper studies the structure and mechanical properties of the Mg-1%Zn-xCa system subjected to high-pressure torsion (HPT) treatment. It was found that the chemical composition had a notable effect on the processes of grain refinement in the alloy. As is shown, HPT of Mg-1%Zn-0.005%Ca resulted in the formation of grains with a mean size of 250 nm, while HPT of the alloy with an increased content of Са up to 0.2% led to the formation of a nanostructure with a mean grain size of 90 nm. It is demonstrated that high microhardness is typical of all HPT-processed samples. The formation of fine Mg2Ca particles was established to increase the heat resistance of the alloy.

The Enhanced Kinetics of Precipitation Effects in Ultra Fine Grained Mg Alloys Prepared by High Pressure Torsion

Precipitation effects in ultra fine grained (UFG) lightweight Mg-based alloys were studied in the present work by means of positron lifetime spectroscopy, transmission electron microscopy, and microhardness. The UFG samples with grain size around 100 nm were fabricated by high pressure torsion (HPT). The UFG structure contains a significant volume fraction of grain boundaries and exhibits a high number of lattice defects (mainly dislocations) introduced by severe plastic deformation during the HPT processing. A high dislocation density and volume fraction of grain boundaries enhance the long range diffusion of solute elements. Moreover, dislocations and grain boundaries act as nucleation centers for precipitates. As a consequence, the precipitation effects are facilitated in the UFG alloys compared to the conventional coarse-grained samples. This phenomenon was examined in this work by comparison of the precipitation sequence in Mg alloys with UFG structure and solution treated coarse-grained alloys.

Defect in Ultra-Fine Grained Mg-Based Alloys Deformed by High-Pressure Torsion

Applications of Mg-based alloys at elevated temperatures are limited by the low melting point of Mg. This difficulty can be overcome by an addition of rare earth elements. A number of novel promising Mg-based hardenable alloys with high creep resistance at elevated temperatures have been developed, e.g. Mg-Gd, Mg-Mn-Sc etc. Despite the favorable strength and thermal stability, a disadvantage of these alloys consists in a low ductility, which is not sufficient for industrial applications. Grain refinement is known as a way how to improve ductility. It has been demonstrated that an extreme grain size reduction can be achieved by methods based on severe plastic deformation (SPD). In the present work we used high pressure torsion (HPT), which is the most efficient in grain size reduction among the SPD-based techniques, for preparation of selected Mg-based alloys with ultra fine grained (UFG) structure. Microstructure investigations and defect studies of HPT deformed UFG Mg-based alloys are presented in this paper. The extraordinary properties of UFG materials are closely related with defects (grain boundaries, dislocations) introduced by HPT. Positron lifetime (PL) spectroscopy [1] is a well-established non-destructive technique with high sensitivity to open volume defects. It enables identification of the defect types present in the material studied and determination of defect densities. Thus, PL spectroscopy represents an ideal tool for defect studies of UFG materials. In the present work PL spectroscopy was combined with X-ray diffraction (XRD), microhardness measurements, and direct observations of microstructure by TEM.