Effect of Irradiation with Low-Energy He2+ Ions on Degradation of Structural, Strength and Heat-Conducting Properties of BeO Ceramics

The paper is devoted to the study of radiation-induced damage kinetics in beryllium oxide ceramics under irradiation with low-energy helium ions with fluences of 1015–1018 ion/cm2. It was revealed that at irradiation fluences above 1017 ion/cm2, a decrease in radiation-induced damage formation and accumulation rate is observed, which indicates the saturation effect. At the same time, the main mechanisms of structural changes caused by irradiation at these fluences are amorphization processes and dislocation density increase, while at fluences of 1015–1016 ion/cm2, the main mechanisms of structural changes are due to the reorientation of crystallites and a change in texture, with a small contribution of crystal lattice distorting factors. It was discovered that the radiation-induced damage accumulation as well as an implanted helium concentration increase leads to the surface layer destruction, which is expressed in the ceramic surface hardness and wear resistance deterioration. It was determined that with irradiation fluences of 1015–1016 ion/cm2, the decrease in thermal conductivity is minimal and is within the measurement error, while an increase in the irradiation fluence above 1017 ion/cm2 leads to an increase in heat losses by more than 10%.

Experimental Study on the Relationship between Dislocation Density and Internal Stress in Micro Electroforming Layer

In the micro electroforming process, the existence of electroforming layer defects caused by macro internal stress seriously limits the application and development of the micro electroforming technology. Currently, some studies have shown that ultrasonic can reduce the internal stress. But the formation process of the internal stress and the mechanism of ultrasonic stress relief in micro electroforming layer are still unclear now. In this paper, the relationship between dislocation density and internal stress under ultrasonic was studied. The results show that the ultrasonic can make the dislocation density increase and the compressive stress decrease. When the ultrasonic power is 200W, the dislocation density and the compressive stress culminate 3.8×10-15m-2 and-144.4MPa, respectively. The ultrasonic can excite the movement of dislocation proliferation, pile-up and opening, which leads to a micro plastic deformation in the crystal, and thereby releases the internal stress.

Effect of Lath Width and Dislocation Density on Resistance to Irradiation of Warmly Deformed SCRAM Steel

The martensitic lath width (0.83 ± 0.45μm ∼ 0.48 ± 0.14 μm) and dislocation density (1.3 ± 0.3 × 1015 m−2 ∼ 6.4 ± 1.6 ×1015 m−2) change of Super-clean Reduced Activation Martensitic (SCRAM) steel caused by warm deformation on Gleeble-3500 thermo-simulation machine have been examined. The irradiation-induced helium bubbles and hardening were observed in all the specimens after helium implantation to 1e + 17/cm2 at 723 K. The helium bubbles became smaller and more numerous while the distribution was more homogeneous when the lath width decrease and dislocation density increase. The nano-indentation hardness indicated that the sample, the martensitic lath width is 0.83 ± 0.45μm and the dislocation density is 1.3 ± 0.3 × 1015 m−2, exhibited the maximum nano-indentation variation (ΔH) and the ΔH decreased with the lath width decreasing and dislocation density increasing. The hardening occurred in all helium implanted samples can mainly be ascribed to helium bubbles.

Tensile Deformation Behaviors of Ultra-Fine Subgrained Aluminum

Tensile deformation behaviors up to peak stress of the ultra-fine subgrained aluminum (2024Al) with the subgrain sizes of about 250 nm and low dislocation density inside were investigated. The results show that the ultra-fine subgrained aluminum exhibited high strain hardening and large uniform plastic strain (19.3 %), but little post-deformation hardness/dislocation density increases (99.6 HV vs. 100.3 HV and 0.79×1014 m-2 vs. 1.03×1014 m-2, respectively). The theoretical calculation based on Taylor equation demonstrated that the dislocation density increase during the tensile deformation up to peak stress was very enormous (1.64×1015 m-2). These results not only implied that the dislocations involved in the tensile deformation were in large quantities but most of them disappeared upon the unloading of the tensile deformation, but also demonstrated that high strain hardening capacity is not a sufficient factor for ultra-fine subgrained metals to store deformation dislocations leading to post-deformation hardness increases.

The Analysis of SPD Paradox by Computer Modeling Technique

The paper has viewed the manifestation of the paradox of severe plastic deformation (SPD), caused by the occurrence of preexisting deformation twins in ultrafine-grained Cu, which has been obtained by the combination of the SPD method, accomplished by an equal-channel angular pressing with the conventional methods of deformation-thermal treatment. The high strength of the obtained samples has proved to be conditioned by the occurrence of the high density of the coherent twin boundaries, serving as effective obstacles on the way of slipping dislocations. Moreover, the occurrence of the twins creates favorable conditions for the dislocation density increase both in the grains with the twins and in the grains without them. As a result the sample hardens, contributing additionally into its strength. Simultaneously it manifests high ductility. By doing so the deformation behavior of the sample is mainly conditioned by the grain boundaries of grains free from the twins. The results were obtained on the basis of the dislocation-based model which develops models of Y. Estrin and L. Tóth, M. Zehetbauer, and L. Remy.

Mechanical Properties of AS21 Magnesium Alloy Based Composites

The high temperature behaviour of composites with the AS21 magnesium alloy matrix, reinforced by short Saffil fibres was investigated in the temperature interval from room temperature to 300 °C. The yield stress and the maximum stress decrease with increasing temperature. Two types of specimens were investigated – one with fibres plane oriented parallel to the stress axis and the other with perpendicular fibres plane orientation. Light and scanning electron microscopy were used for study of the microstructure of composites. Possible hardening and softening mechanisms are discussed. The shear stress at fibre/matrix interface was of greatest importance in this regard, though the contribution resulting from the dislocation density increase was also significant.