Volume 109, Issue 9

This study aims to understand the impact of various lubricant additives on the friction- and wear behavior of aluminum alloys against steel. Employing the aluminum alloy Al 6061 and the steel AISI E 52100 as representatives, we studied the effects of the main additives typically found in fully formulated lubricants individually by formulating model lubricants consisting of base oil and just one additive at a time. The additives we studied include sulfurized olefin, complex alkyl phosphate amine, alkyl phosphite, overbased calcium sulphonate, zinc dithiophosphate (ZDP), and calcium sulfonate detergents. For each model lubricant, as well as for the base oil and the fully formulated oil, we performed ball-on-disc experiments with aluminum alloy balls on steel discs. We obtained the friction coefficient, determined the wear volume of the aluminum alloy, and analyzed the composition of the wear surface. We found that all additives except calcium sulfonate detergents promote the formation of aluminum oxide. Among all individually tested additives, zinc dithiophosphate provides the best wear resistance, while sulfurized olefin provides the lowest friction. The lowest friction combined with the second lowest wear rate was actually observed for a fully formulated oil (containing an optimized mixture of several additives).

Grain boundary serration is an effective way to increase the high temperature resistance of superalloys and steels. The popular Alloy 625 Ni-based superalloy was until now believed not to form serrated grain boundaries based on previous considerations of serrability criteria. Following the recent strain-induced serration mechanism, a special heat treatment involving continuous slow cooling between the solution and aging temperature was designed. As a result, significant serration was observed for the first time for Alloy 625 promoted by slow cooling. Grain boundary M 23 C 6 carbides were systematically detected from either degenerescence of solidification M C carbides or heterogeneous nucleation. Upon aging, serration amplitude increased and precipitation of the δ phase proliferated.

Microstructural stability is of great importance for the application of rhenium-containing nickel-based single-crystal superalloys. The microstructures of single-crystal superalloy DD6, especially the precipitation characteristics before and after thermal exposure, were characterized using scanning electron microscopy, transmission electron microscopy, and related techniques. Acicular M 6 C carbides, rod-like M 6 C carbides, and granular μ phases have been observed in the aged alloys, and the orientation relationships of these precipitates have been identified. During the early stage of M 6 C precipitation, the carbide maintains a cube-on-cube orientation relationship with the matrix, whereas further growth of the M 6 C carbides leads to a twin-related orientation relationship. For the granular μ phases containing a high concentration of Re, no special relationship has been observed. The results of the study will be beneficial for the understanding and improvement of nickel-based single-crystal superalloys.

This paper is intended to examine the dynamic evolution of the metastable structure and nano precipitation of AA7055 under thermal deformation. Results indicate the second-phase particles produced will break up and spheroidize at low temperature. When the strain reached 0.4, precipitated η (MgZn 2 ) was detected to occur and gradually coarsen. Under high temperature and high strain rate, not many second phase particles were left in the alloy and the particles were nearly spherical. Coarse rod-like T (Al 2 Mg 3 Zn 3 ) particles appeared during thermal deformation, which would gradually coarsen and tend toward uniform orientation. There were also long rod-like S (Al 2 CuMg) particles and some nanoscale coarse particles. During thermal deformation, dislocations quickly proliferated, entangling into dislocation cells, and then formed subgrains through slipping and climbing. When the strain reached its maximum, large-angle grain boundaries were detected below 450°C, suggesting that dynamic recrystallizion had taken place. Below 300°C, however, only dynamic recovery took place. As the strain increased, the dislocation density reduced. Subgrains developed quite completely. The intergranular misorientation was modest. The subgrains were 0.2∼ 0.6 μm in size with quite straight boundaries, but these subgrains were not stable enough. The boundary angle also displayed a tendency of developing toward the 120°. According to the diffraction pattern of zonal axis [⥘11] Al , the orientation relationship of the η′ and η (MgZn 2 ) particles to the aluminum matrix was (0001) η //(111) Al . Under high temperature (450°C), when the strain was 0.4, subgrains with relatively clear boundaries were observed. Under low temperature, at the same strain, subgrains in the alloy were still entangling dislocaion cell grains with high intragranular and boundary dislocation densities. A few subgrain boundaries were becoming clear. The subgrain boundaries were heavily curved. Quite a lot of the subgrain boundaries were still unclear. When the strain was 0.6, under high temperature, the subgrain boundaries began to transform toward 120° stable state. Subgrains began to grow. Under low temperature, in the same state, many dynamically recrystallized grains formed in the alloy structure. The grains were small in size with clear boundaries. Some of the subgrains were still in the nucleation stage of recrystallization nuclei. When the strain was 0.8, under deformation temperature 450°C, the dislocation density in the structure reduced significantly. Equiaxial or sub-equiaxial grains more than 0.5 μm in size were observed. Under low temperature, subgrains were fairly completely developed, though the boundaries were still unstable and tended to transform toward the 120° stable state, but the recrystallized grains were fine and sized 0.2∼0.6 μm. The dislocation density in the structure reduced. Yet low-density dislocation walls not having evolved into subgrain boundaries were still observed on the boundaries of a few grains.

Microstructural evolution and compressive properties of an extruded Mg-2Dy-0.5Zn (at.%) alloy sheet at 350°C were investigated. As the compressive strain increased, the volume fraction of dynamic recrystallization increased, the fine lamellar 14H long period stacking ordered phase precipitated in the dynamic recrystallization grain, and the Mg 12 ZnDy phase with an 18R long period stacking ordered structure gradually bent. These secondary phases not only acted as nucleation sites to promote dynamic recrystallization but also restrained grain growth by inhibiting dislocation movement and grain boundary sliding. The compressive yield strength, ultimate compressive strength, and compressive strain of the alloy sheet were 161 MPa, 212 MPa, and 12.4% at 350°C, respectively. The high compressive strengths were mainly attributed to grain refinement, kink band strengthening of the 18R long period stacking ordered phase and precipitation strengthening of the fine lamellar 14H long period stacking ordered phase in the dynamic recrystallization grain.

In this paper, residual stress distribution is investigated by a novel X-ray cosα diffraction technique in a nanostructured austenitic stainless steel after hydrostatic extrusion processes. Hydrostatic extrusion performed at 20°C and with a total true strain of 2.3 leads to the creation of a nanostructure consisting of nanotwins and shear bands. The results reveal that the greatest compressive residual stresses of −1 GPa are found 3 mm from the surface of the nanostructured austenitic stainless steel. These compressive residual stresses restrict crack growth into the material, thereby preventing catastrophic failure.

Manganese was added to face-centered-cubic (fcc) high-entropy CoCrCuFeNi alloy to investigate the effects of Mn content on the microstructures, phase selection, and properties of CoCrCuFeMn x Ni. CoCrCuFeMn x Ni showed typical dendrite and interdendritic structures, and nanoprecipitates including Cu-rich cubic and Cr-rich acicular ones, were obtained in the interdendritic region. Calculated phase-selection-related parameters indicated that the formation of fcc phase was favored. Although two fcc structures were detected by X-ray diffraction, the interdendritic region showed body-centered-cubic, Cr-rich nanoprecipitates when x < 0.5. It is worth noting that the CoCrCuFeMn 2.0 Ni interdendrites showed an amorphous phase. CoCrCuFeMn x Ni showed considerable ductility and increasing compressive strengths with increasing Mn content.

By means of the CALPHAD (CAlcultion of PHAse Diagram) technique, the Hf–Pt system was critically assessed. Based on the experimental data, the four solution phases (liquid, fcc, bcc and hcp) were described with the substitutional solution model. The intermetallic compounds Hf 3 Pt 4 and αHfPt were treated as the formula (Hf,Pt) m (Hf,Pt) n by a two-sublattice model. Based on the solid solution range, the intermetallic compounds HfPt 4 and Hf 2 Pt were treated as the formula (Hf,Pt) 1 (Pt) 3 and (Hf) 2 (Hf,Pt) 1 , respectively. The intermetallic compound Hf 2 Pt 3 was treated as a stoichiometric compound. The formulas (Hf,Pt) 0.5 (Hf,Pt) 0.5 · (Va) 3 and (Hf,Pt) 0.25 (Hf,Pt) 0.75 (Va) 0.5 were applied to describe the compounds βHfPt with CsCl-type structure (B2) and HfPt 3 with Ni 3 Ti-type structure (D0 24 ) to cope with the order-disorder transition from bcc-A2 to bcc-B2 and hcp-A3 to hcp-D0 24 . A set of self-consistent thermodynamic parameters of the Hf–Pt system was obtained.

NiCrAlY-Mo-20Ag composite coating was fabricated by atmospheric plasma spraying. The influence of heat treatment on the mechanical properties, high temperature oxidation resistance properties and tribological properties of the coating were investigated. After 500°C heat treatment, the microstructure of the coating becomes more homogeneous and the amount of precipitated Y 2 O 3 phase clearly increases. The micohardness and adhesive strength of the composite coating are effectively improved after heat treatment. The coating heat-treated at 500°C shows the highest micohardness and adhesive strength of 433.3 HV and 42.9 MPa as well as excellent lubricating properties over a wide range of temperature. Furthermore, the coating presents excellent oxidation resistance properties at 700°C and 800°C.

Porous Ni–Cr–Al alloy samples were fabricated by pressureless sintering of Ni, Cr, and Al elemental powders. The phase transformation, swelling behavior, pore structure parameters, microstructure, and pore formation mechanisms of porous Ni–Cr–Al alloys were systematically investigated. Results show that obvious swelling behavior occurs during sintering. The open porosity of the Ni–Cr–Al alloy increases as the sintering temperature increases below 920°C, followed by a relatively declining value for phase transformation beyond 920°C. The porous Ni–Cr–Al alloy samples exhibit an open porosity of 35.02% when sintered at 1200°C for 3 h. The main pore formation mechanisms are the Kirkendall effect and the development of interparticle pores.

An ionic liquid derived Co 3 O 4 /carbon nano-onions composite has been prepared by carbothermal reduction followed by oxidation. The introduction of carbon nano-onions improves the conductivity and structural stability of Co 3 O 4 electrode material. Electrochemical measurements indicate that the redox reversibility is significantly improved. The Co 3 O 4 /carbon nano-onions composite shows a large specific capacitance of 402.35 F g –1 at a current density of 0.5 A g –1 . After 9000 cycles, the specific capacitance retention remained 76% at 1 A g –1 . The as-prepared Co 3 O 4 /carbon nano-onions composite delivers superior capacitive performance with good rate capability, large specific capacitance, and excellent cyclic performance, showing great application potential for high-performance electrochemical supercapacitors.

SnO/reduced graphene oxide photocatalysts were prepared using ultrasonic synthesis with SnCl 2 · 2H 2 O, NH 4 HCO 3 , and GO powders. Photocatalytic properties of samples were estimated by the degradation of methyl orange (MO). An SnO/graphene oxide composite containing small amounts of SnO 2 was obtained. Numerous nano SnO/SnO 2 particles were loaded uniformly on the RGO thin flakes. The composites possessed excellent photocatalytic activity, which may degrade 99% of MO under visible light irradiation within 50 min.