Yicheng Feng

Fabrication and photodegradation properties of TiO2 nanotubes on porous Ti by anodization

Abstract Both Ti foil and porous Ti were anodized in 0.5%HF and in ethylene glycol electrolyte containing 0.5%NH 4 F (mass fraction) separately. The results show that TiO 2 nanotubes can be formed on Ti foil by both processes, whereas TiO 2 nanotubes can be formed on porous Ti only in the second process. The overhigh current density led to the failure of the formation nanotubes on porous Ti in 0.5%HF electrolyte. TiO 2 nanotubes were characterized by SEM and XRD. TiO 2 nanotubes on porous Ti were thinner than those on Ti foil. Anatase was formed when TiO 2 nanotubes were annealed at 400 °C and fully turned into rutile at 700 °C. To obtain good photodegradation, the optimal heat treatment temperature of TiO 2 nanotubes was 450 °C. The porosity of the substrates influenced photodegradation properties. TiO 2 nanotubes on porous Ti with 60% porosity had the best photodegradation.

Microstructure and mechanical properties of AM60B magnesium alloy prepared by cyclic extrusion compression

The cyclic extrusion compression (CEC) process was introduced into the AM60B magnesium alloy. The use of the CEC process was favorable for producing finer microstructures. The results show that the microstructure can be effectively refined with increasing the number of CEC passes. Once a critical minimum grain size was achieved, subsequent passes did not have any noticeable refining effect. As expected, the fine-grained alloy has excellent mechanical properties. The micro-hardness, yield strength, ultimate tensile strength and elongation to failure of two-pass CEC formed alloy are 72.2, 183.7 MPa, 286.3 MPa and 14.0%, but those of as-cast alloy are 62.3, 64 MPa, 201 MPa and 11%, respectively. However, there is not a clear improvement of mechanical properties with further increase in number of CEC passes in AM60B alloy. The micro-hardness, yield strength, ultimate tensile strength and elongation to failure of four-pass CEC formed alloy are 73.5, 196 MPa, 297 MPa and 16%, respectively.

Spheroidal microstructure formation and thixoforming of AM60B magnesium alloy prepared by SIMA process

The microstructure development in the semi-solid state of AM60B magnesium alloy prepared by strain induced melt activation (SIMA) was described. In the SIMA route, cyclic extrusion compression (CEC) and conventional compression were used to predeform as-cast AM60B, respectively. Partial remelting and thixoforming were also carried out within the semi-solid zone. The results showed that coarse-grained structure disappeared and a fine-grained structure resulted in the CEC formed alloy. However, coarse grains and recrystallized grains co-existed in the compression formed alloy. During partial remelting, the alloy treated by CEC exhibited ideal, fine microstructure, in which completely spheroidized grains contain little entrapped liquid. Polygonal grains were spheroidized to some extent but the previous irregular shape was still obvious in the compression formed alloy. The mechanical properties of the thixoformed AM60B produced by CEC plus partial remelting were better than those of the thixoformed alloy produced by compression plus partial remelting.

Grain coarsening in semi-solid state and tensile mechanical properties of thixoformed AZ91D-RE

Abstract For thixoforming to be possible, the microstructure of the starting material must be non-dendritic, which can be obtained by the strain induced melt activation (SIMA) route. Based on the SIMA route, as-cast AZ91D alloy with the addition of yttrium was deformed by cyclic closed-die forging (CCDF). Microstructure evolution of CCDF formed AZ91D-RE alloy during partial remelting were investigated. Furthermore, the mechanical properties of thixoformed AZ91D-RE magnesium alloy components were also studied. The results showed that prolonged holding time resulted in grain coarsening and the improvement in degree of spheroidization. The coarsening behaviour of solid grains in the semi-solid state obeyed Ostwald ripening mechanism. The coarsening rate constant of CCDF formed AZ91D-RE during partial remelting was 324 um3/s at 550 °C. The value of yield strength, ultimate tensile strength and elongation to fracture of four-pass CCDF formed AZ91D-RE magnesium alloy were 214.9, 290.5 MPa and 14%, respectively. Then the four-pass CCDF formed alloys were used for thixoforming. After holding at 550 °C for 5 min, the values of yield strength, ultimate tensile strength and elongation to fracture of thixoformed component were 189.6 MPa, 274.6 MPa and 12%, respectively. However, prolonged holding time led to remarkable decrease in mechanical properties of thixoformed components.

Microstructure in the semi-solid state and mechanical properties of AZ80 magnesium alloy reheated from the as-cast and extruded states

The multi-axial forging (MAF) process was introduced into the strain induced metal activation (SIMA) process to replace conventional forging. Microstructure evolution of MAF formed AZ80 magnesium alloy during partial remelting was investigated. Furthermore, the tensile mechanical properties for AZ80 magnesium alloy thixoextruded from the starting materials treated by MAF were determined. For comparison, as-cast AZ80 magnesium alloy was also thixoextruded. The results show that the SIMA route produced ideal, fine semi-solid microstructure, in which almost completely spheroidal primary solid grains had a little amount of entrapped liquid. The microstructure of the as-cast alloy in the semi-solid state is less spheroidized compared with the MAF alloy under the similar isothermal holding conditions. With prolonged holding time, the size of the solid grain increases and the degree of spheroidization is improved in the MAF formed alloys. However, the solid grain size of the as-cast alloys decreases initially, and then increases with further increasing temperature. The tensile mechanical properties for AZ80 magnesium alloy thixoextruded from the starting material produced by MAF are better than those of AZ80 magnesium alloy thixoextruded from the starting material produced by casting. The ultimate tensile strength, yield strength and elongation of the alloy thixoextruded from the starting material produced by MAF are 314 MPa, 238 MPa and 14%, respectively.