Volume 113, Issue 8

A model is presented to derive both vacancy formation and migration characteristics from length change measurements upon modulated time-linear heating. The length variation with linear heating yields access to the equilibrium concentration of thermal vacancies. The modulation amplitude and the phase shift between modulated temperature and length change is determined by the ratio of equilibration rate and modulation frequency which yields access to the vacancy migration characteristics. The contribution from thermal lattice expansion is obtained from a reference measurement at high modulation frequencies. Compared to static isothermal equilibration measurements after temperature jumps, the processes are monitored under quasi-equilibrium conditions avoiding obstacles associated with fast temperature changes. Furthermore, in contrast to the static isothermal case where the equilibration rate is obtained from the time-exponential decay, its determination from the amplitude and phase shift of modulation offers higher precision. The method is suitable for materials with high thermal vacancy concentrations and low vacancy diffusivities, among which is the important class of intermetallic compounds with B2-structure.

Ceramic coatings were prepared on 6061 aluminum alloy in electrolytes containing La 2 O 3 particles using the micro-arc oxidation technique. The main work focuses on the microstructure, phase composition, elemental distribution and corrosion resistance of micro-arc oxidation coatings. The results showed that the addition of La 2 O 3 increased the oxidation voltage and promoted the formation rate. Scanning electron microscopy observations indicated that the quantity of discharge micropores decreased and the thickness increased, and thus, the microhardness increased. The X-ray diffraction results showed that the coatings mainly consisted of γ-Al 2 O 3 , α-Al 2 O 3 and SiO 2 . Electrochemical corrosion tests showed that the corrosion resistance of the coated sample with La 2 O 3 increased greatly because the addition of La 2 O 3 significantly reduced the micropores of the coating surface and made the coating thicker, smoother and harder. Hence, the addition of La 2 O 3 can optimize the structure of the micro-arc oxidation coating and improve its corrosion resistance.

Four groups of 5182, 5182-0.4 % Sc, 5182-0.1 % Sc-0.3 % Zr and 5182-0.3 % Sc-0.1 % Zr (wt.%) aluminium alloys were prepared. The microstructures of these alloys were examined using optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results showed that the addition of Sc leads to the effect of grain refinement, and adding both Sc and Zr makes this phenomenon even more obvious. The morphology and distribution of the second phase changed. The addition of Sc makes the tensile strength increase and the elongation decrease. Adding both Sc and Zr significantly improves the mechanical properties of the alloy. The tensile strength of 5182-0.3 % Sc-0.1 % Zr alloy reaches a maximum value of 223 MPa, and the Brinell hardness of the alloy reaches a maximum value of 88.7 HBW.

In this paper, the effect of composition and solution temperatures on microstructure, texture and mechanical anisotropy of Al–Zn–Mg–Cu–Zr–Mn alloys was investigated by means of optical microscopy, scanning electron microscopy, vickers hardness testing, tensile testing and electron backscatter diffraction analysis. The results indicate that the mechanical anisotropy can be reduced by increasing solution temperature and introducing iron-rich phases, in which equiaxed recrystallized grains and optimized texture distribution can be obtained, especially for the #1 alloy with the presence of iron-rich phases. Additionally, according to the evolution of microstructure and texture, the mechanism of recrystallization affected by solution temperature and iron-rich phases in the Al–Zn–Mg–Cu–Zr–Mn alloys was put forward. The relationship between mechanical anisotropy, microstructure and textures was also established in this paper.

In this research, the wear behavior and microstructural transformation of approximately single face-centered cubic phase AlCoCrFeNi high-entropy alloy at elevated temperatures (∼25–600 °C) were investigated in detail. The combined action of environmental temperature and friction force can induce significant and regular transformation of the preferred orientation of crystalline grains at the high-entropy alloy friction interface. Generally as the temperature rises its principal wear mechanism varies fairly regularly from abrasive wear to delamination wear, and then to adhesive wear. It is worth noting that at a test temperature of 100 °C the wear debris formed during friction was rolled repeatedly and then separated by delamination, which played a pivotal role in inhibiting wear. Furthermore, five specific wear mechanisms of face-centered cubic phase high-entropy alloy at elevated temperatures have been elucidated through this study.

Electron beam welding (EBW) with multiple beams is an advantageous technology to weld parts, as it contributes to decreasing welding distortions and improving mechanical properties. EBW with multiple electron beams was applied to welding simultaneously with auxiliary heating, in a rhombus scanning design, and the energy ratio was 1:1 for AF1410 steel welding. The welds of EBW with multiple beams were nail shaped with a width of 2.54 mm, which were more uniform than those of conventional EBW, and the transversal and longitudinal distortions were lower. The microstructures of specimens processed by EBW with multiple beams contained martensite and a small amount of austenite, and the martensite laths were smaller and more homogeneous than those of conventional EBW. The microhardness values were also more uniform than those of conventional EBW specimens. The tensile strengths of the joints were above 1754 MPa, equivalent to those of conventional EBW, and the percentage elongations were above 9.7 %, which were higher than those of conventional EBW. The microstructure homogeneity and refinement of EBW with multiple beams contributed to improving the mechanical performance.