The Influence of Hard Coatings on Fatigue Properties of Pure Titanium by a Novel Testing Method.

This study investigates the impact of hard coatings on the fatigue properties of pure titanium. A specialized fatigue test which ensured machine equivalence was conducted to compare the fatigue behavior of coated and uncoated metals. The findings reveal that the application of coatings adversely affects the fatigue properties of pure titanium due to stress concentration from the coating, which accelerates fatigue crack propagation within the substrate material. Notably, zigzag fatigue cracks at the interface between the coating and substrate and multiple micro-cracks initiated within the coating are found.

Texture-Induced Corrosion Resistance of Dissimilar AA7204/AA6082 Friction Stir Welded Joints.

The quasi in situ EBSD test was applied to study the effect of grain orientation on corrosion behaviors of the thermomechanically affected zone I (TMAZ I) of dissimilar AA6082/AA7204 friction stir welding (FSW) joints in this work. The results show that the structure with grain orientation close to the brass texture ({110}<112>) has excellent corrosion resistance, which contributes to the better corrosion performance of the TMAZ I of the 7204-AS joint than the 7204-RS joint. Furthermore, the brass texture around by S texture ({213}<364>) in the TMAZ I of the 7204-AS joint is slightly corroded, and the orientation of the remaining structure is closer to the ({110}<112>) than before, which indicates that the corrosion, like deformation, is carried out alongside the {110} plane for the structure with grain orientation near {110}<112>. Those findings could provide new insight into the designed FSW joints and improve the corrosion resistance of the wrought aluminum alloy.

Dynamic Recrystallization Critical Conditions and a Physically-Based Constitutive Model of Al-4.8Mg Alloy Under Hot Working Conditions.

The microstructure evolution and the mechanical behavior of Al-4.8Mg alloy were investigated by means of isothermal compression tests at various temperatures (280-520 °C) and strain rates (0.01-10 s-1). The results shown that there are three main mechanisms of dynamic softening of samples within the range of selected process parameters: dynamic recovery, dynamic recovery + dynamic recrystallization, and dynamic recrystallization, and the equiaxed dynamic recrystallization grain tends to be formed under higher temperature and higher strain rate. In order to accurately describe the dynamic recrystallization condition of Al-4.8Mg alloy under a wide range of hot deformation parameters, an improved dynamic recrystallization critical conditions model is proposed based on deformation activation energy and work-hardening rate. Additionally, a two-stage physically constitutive model considering the influence of work hardening-dynamic recovery and dynamic recrystallization is established. Comparisons between the predicted and experimental data indicate that the proposed model can adequately predict the flow stress in the range of selected process parameters with the average absolute relative error of 4.02%.

Effect of Zn and Mg Content on Crashworthiness of Al-Zn-Mg Alloy Thin-Walled Square Extrusions.

The effects of Zn and Mg content in thin-walled square extrusions of Al-Zn-Mg alloys on its crashworthiness were investigated, and the correlation between the crushing properties, mechanical properties, and microstructures of the profiles were investigated. The results showed that the strength and the compression properties were gradually increased with a decrease in the Zn/Mg ratios (from 12.48 to 4.57). When the Zn/Mg ratio is lower (less than 6.29), an increase in the Mg content simultaneously improves the alloy strength and the compression properties. An increase in Zn content (from 5.07 to 6.77) can improve the strength of the alloy however, it does not affect the compression properties. However, the higher Zn contents (6.77%) would lead to cracking in advance during the compressing, which reduces the compression energy absorption capacities of the product. Therefore, in order to obtain higher strength and excellent compression properties, the Zn/Mg ratio should be reduced. For the upper limit, the Zn content should not be too high (less than 6.77), as this may lead to early cracking and failure. For the lower limit, the Mg content should be higher (more than 0.91) to make sure that the alloy has excellent compression properties and higher strength.

Mechanism of Precipitate Microstructure Affecting Fatigue Behavior of 7020 Aluminum Alloy.

The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 107 cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m1/2.

Effect of the Zn/Mg Ratio on Microstructures, Mechanical Properties and Corrosion Performances of Al-Zn-Mg Alloys.

The effect of the Zn/Mg ratio on microstructures, mechanical properties and corrosion performances of Al-Zn-Mg alloys was studied. Microstructures were characterized using the optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Tensile tests, intergranular corrosion (IGC) and stress corrosion cracking (SCC) tests were conducted to study the properties. Microstructures results indicated that with the decrease of the Zn/Mg ratio, the recrystallization proportion and the fraction of second phase decreased, while the size of η' (MgZn2) phases in grain interior also significantly decreased. The number density of η' phases in grain interior increased and grain boundary precipitates developed discontinuous distribution with the decrease of the Zn/Mg ratio. These microstructures contributed to the significant improvement of the strength and corrosion resistance. The tensile strength and yield strength increased by 34.1% and 47.4%, respectively, with the Zn/Mg ratio decreased from 11.4 to 6.1. Calculating results indicated that the enhancement of strength mainly contributed from the solid-solution strengthening, grain-boundary strengthening and precipitation strengthening. The intergranular corrosion degree was greatly relieved and the stress corrosion sensitivity index decreased from 0.031 to 0.007 with the Zn/Mg ratio decreased from 11.4 to 6.1.

Enhancing the Corrosion Resistance of Al-Cu-Li Alloys through Regulating Precipitation.

The influences of aging treatments on microstructures and the corrosion properties of an Al-Cu-Li alloy were investigated through an immersion test in intergranular corrosion (IGC) solutions, a potentiodynamic polarization test, and electrochemical impedance spectra (EIS), combined with scanning and transmission electron microscopy. The results demonstrated that the Al-Cu-Li alloy displayed outstanding comprehensive mechanical properties and IGC resistance after treating with pre-strain deformation and a double aging process (PDA). Both the PDA and pre-strain followed by creep aging (PCA) treatments significantly increased the number densities of T1 and θ' precipitates in the grain interior. The increase in precipitates in the grain interior greatly reduced the Cu-rich precipitates on the grain boundaries and inhibited the formation of a precipitate-free zone (PFZ). The electrochemical characteristics of the Al-Cu-Li alloy were influenced by the precipitates in the grain interior and grain boundaries. The studied alloy gained high IGC resistance due to the refinement of its microstructure, and the main corrosion mode was intra-granular pitting corrosion; thus, the corrosion diffusion rate was slowed down.

Influence of Minor Zn Addition on Precipitation Behavior and Intergranular Corrosion Properties of Al-Mg-Si Alloy.

The effect of 0.2 wt.% Zn addition on microstructure, age hardening and intergranular corrosion (IGC) properties of Al-Mg-Si alloy were investigated by scanning electron microscope, transmission electron microscope, hardness testing, and electrochemistry testing. The results showed that the addition of Zn can accelerate the transformation of GP zones into ß″, and make the intragranular precipitates become smaller and with higher density. This is beneficial to the precipitation strengthening of the alloy, leading to obtaining higher hardness and enhancing the age hardening response. The peak hardness of the alloy with the addition of Zn is 125.8 HV which means increasing the hardness by 12.7 HV, compared with the alloy without Zn. However, the addition of Zn makes the precipitate-free zone (PFZ) of the alloy wider, and coarsens the grain boundary precipitates slightly, which result in the reduction of IGC resistance of Al-Mg-Si alloy. The maximum corrosion depth of the Zn-containing alloy is 121.3 µm in the peak age condition, which is 35.7 µm deeper than the alloy without Zn. The result of the potentiodynamic polarization curves also demonstrated the increase of IGC sensitivity. The corrosion current density of the alloy with added Zn is 0.595 µA/cm2 in the peak age condition, while that for the alloy without Zn is 0.199 µA/cm2.

Effect of Travel Speed on Microstructure and Mechanical Properties of FSW Joints for Al-Zn-Mg Alloy.

The microstructures and mechanical properties of friction stir welded (FSW) Al-Zn-Mg alloy plate under different travel speeds were investigated. Both the average grain sizes (AGSs) of the shoulder affected zone (SAZ), nugget zone (NZ), and the widths of thermo-mechanically affected zone (TMAZ) decreased with the increase of travel speed. Moreover, the AGSs of NZ are always about 60% of that of SAZ at different travel speeds. The fractions of high-angle grain boundaries (HAGBs) in the FSW joints reduce with the distance away from the stir zone (SZ). Furthermore, the initial η' strengthening precipitates in NZ and TMAZ dissolve and GP zones form during subsequent natural aging, so that the hardness is similar in the two zones. The precipitate evolution in the heat-affected zone (HAZ) at hardness minima are affected by travel speeds, which induce the hardness minima and ultimate tensile strength (UTS) of FSW joints and increase with the increase of travel speed, and a fracture tends to occur at hardness minima location of HAZ during tensile testing.

Energy Level Evolution and Oxygen Exposure of Fullerene/Black Phosphorus Interface.

The heteroepitaxial growth of fullerene (C60) on single-crystal black phosphorus (BP) has been studied using low-energy electron diffraction, X-ray and ultraviolet photoelectron spectroscopy, and density functional theory simulation. The occupied orbital features from C60 observed in the photoelectron spectra for C60/BP interface are slightly broadened at higher coverages of C60 and exhibit no direct evidence of hybridization, demonstrating that the C60/BP interaction is physisorption. Oxygen exposure of interface leads to obvious oxidation of BP in which C60 bridges the large electron-transfer barrier from BP to oxygen and plays an important role for the production of O2- and oxidation of BP. Our findings suggest that C60 does not form an ideal protection layer as the other n-type semiconductors. With the assistance of density functional theory calculations, the oxidized phosphorus at the interface prevents further charge transfer from BP to C60.

A Study on Dissimilar Friction Stir Welded between the Al⁻Li⁻Cu and the Al⁻Zn⁻Mg⁻Cu Alloys.

In this paper, the microstructure of friction stir welded between the Al⁻Li⁻Cu alloy and the Al⁻Zn⁻Mg⁻Cu alloy was studied using optical microscope(OM), electronic backscattered diffraction (EBSD), SEM, and TEM. The hardness profile revealed that the range of heat affect zone for the Al⁻Zn⁻Mg⁻Cu alloy was slightly wider than the Al⁻Li⁻Cu alloy when they suffered from the thermal transient. Additionally, the characterization of precipitates inside the different zones was obviously different, which corresponded to the microhardness distribution profile. At the periphery of the kissing line, the Al⁻Li⁻Cu and the Al⁻Zn⁻Mg⁻Cu alloys mutually diffused during the weld process. The magnesium element in the Al⁻Zn⁻Mg⁻Cu alloy diffused into the Al⁻Li⁻Cu alloy. But the copper and zinc had no change because of the low diffusion coefficient in aluminum. The heat affected zone of the Al⁻Zn⁻Mg⁻Cu alloy showed a higher corrosion susceptibility immersed into the corrosion environment, and the Al⁻Li⁻Cu alloy did not show severe corrosion.