Zhuang Qi Hu

The Influence of Electric Current Pulses on the Microstructure of Magnesium Alloy AZ91D

The microstructure evolution of magnesium alloy AZ91D solidified with different electric current pulses and cooling rates was investigated and a new method, Low-voltage Electric Current Pulses (LVECP), to produce semi-slurry magnesium alloy was developed in this paper. The experimental results showed that the electric current pulses during solidification changed morphology of dendrites and the equiaxed, non-dendritic grains formed. The size of the primary a-Mg particles in semi-solid AZ91D alloy and the sphericity of the particles decreased with increase of discharging the voltage and treating time of LVECP. The increase of the cooling rate during the solidification of AZ91D alloy with LVECP promoted the formation of finer a-Mg particles, but the value of the sphericity of the particles rised. The formation of equiaxed, nondendritic structure by LVECP might be attributed to the electric current pulses increase the nucleation rate, restrained growth of the dendrites, and made dendrite arms remelted during the solidification of AZ91D alloy.

Effect of Temperature on Formation of Borides in TLP Joint of a Kind of Nickel-Base Single Crystal Superalloy

a kind of as-cast nickel-base single crystal superalloy was TLP bonded using Ni-Cr-B amorphous foil at different temperatures. Special attention is paid to the formation of boride in diffusion zone of TLP joints at different conditions. The chemical composition and microstructure of borides were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). At different bonding temperature, M3B2 precipitates appear distinct morphologies. At 1200°C, both blocky and plate-like borides formed owing to the diffusion of boron atoms into base metal and precipitation during the cooling process. At 1230°C or above, due to the diffusion of boron atoms the constitutional liquation of original γ/γ′ eutectics in the base metal occurs and borides formed when the system was cooled to room temperature. The analysis of TEM results reveals that M3B2 has a tetragonal structure and is rich in Mo, W, and Cr elements.

Role of Rhenium in Single Crystal Ni-Based Superalloys

The effect of rhenium on the microstructure and mechanical properties of single crystal superalloys with a nominal composition of Ni-3Cr-12Co-1Mo-6W-6Al-8Ta-0.1Hf-(0, 2, 4) Re has been studied. With the rise of rhenium content, the size of as-cast uf067uf0a2 particles becomes smaller. Rhenium addition elevates the incipient melting temperature and slows down the solid solution process. Even after 2000h prolonged aging both at 950°C and 1050°C, no topologically close-packed phase precipitation is found in the three alloys with Re content up to 4%. Re retards the uf067uf0a2 coarsening and is beneficial to improving the stress rupture life. The deformation mechanisms together with the dislocation configuration have been studied and discussed.

Surface Recrystallization and its Effect on Rupture Life of SRR99 Single Crystal Superalloy

The behaviour of surface recrystallization and its effect on rupture life of SRR99 single-crystal superalloy are studied. Results show that the recrystallization at lower temperature (such as 1240C) is of a cellular mode and the recrystallization rate is slower. At higher temperature (such as 1300°C), the recrystallized structure is consisted of grains. γ particles retard migration of recrystallizing grain boundary. The depth of recrystallized layer exhibits a linear relationship with shot peening intensity. At 950°C, when the nominal stress is lower than 320MPa, the rupture life is not affected seriously, but is reduced when the nominal stress is 400MPa. Deep local recrystallization leads to surprised reduction of rupture life due to the stress concentration resulted from recrystallized volume.

Mechanical Behaviour of a Single Crystal Superalloy DD32 - A Comparison with the Alloy SRR99

Stress rupture and tensile properties of a single crystal superalloy DD32 are investigated comparing with the alloy SRR99. It is shown that the alloy DD32 offers an improved creep temperature capability of more than 60°C at higher stresses. The g¢ precipitates in the stress ruptured samples were rafted to P-N type directional coarsening. The fracture mechanism of the stress ruptured samples was initiated from the micropores.

Wetting Behavior and Interface Characteristic of Ti32.8Zr30.2Ni5.3Cu9Be22.7/Ti6Al4V

The wetting behavior and the interaction at the liquid-solid interface are significant in preparation of metallic glass composite. In this paper, the wetting behavior and the interfacial interaction between Ti32.8Zr30.2Ni5.3Cu9Be22.7 (denoted as ZT3) bulk metallic glass (BMG) alloy melt and Ti6Al4V (denoted as TC4) substrate at different temperatures were investigated using sessile drop method. The results show that ZT3 alloy melt wetted the substrate well. With the increase of temperature, the droplet spread out rapidly and then came to equilibrium gradually. The equilibrium contact angle under 1273 K stabilized at about 15°. Dendrite phase generated at the interface during the wetting progress. The composition of the interface product transformed from Ti69.8Zr24.2Al2.2V1Cu2.8 to Ti73.6Zr15.3Al5V2.7Cu2.6Ni0.8 with the temperature rising from 973 K to 1273 K. Meanwhile, the dissolution of the TC4 substrate increased with increasing the temperature.

Isothermal Crystallization of Zr55Al10Ni5Cu30 Bulk Amorphous Alloy Near the Glass Transition Temperature

Isothermal crystallization of Zr55Al10Ni5Cu30 bulk amorphous alloy near the glass transition temperature has been investigated. The microstructures and thermal stability of the annealed amorphous alloy were examined by HRTEM, XRD and DSC. The amorphous phase in the Zr55Al10Ni5Cu30 bulk amorphous alloy crystallized at 420°C through the following processes of amorphous →amorphous with clusters + metastable phases→metastable phases.

Grain Refinement of GH4169G Alloy by the Combination of Heat Treatment and Cold Deformation

The precipitating behavior of δ phase in GH4169G alloy at 910°C was investigated and a novel process for achieving fine-grained GH4169G sheet with average grain size of ASTM13 was proposed. Globular δ phase precipitation precedes that of the plate-like δ phase, forming at grain boundary first and later in grain. After globular δ phase reaches a peak volume, it begins to transform into plate-like in grain boundary, and then the plate-like δ phase forms in grain in certain direction. The grain-fined process includes the following steps:(1) solution treated at a proper temperature, then air cooled; (2) soaked for δ phase precipitation; (3) cold rolled; (4) annealed for recrystallization. Microstructure analysis from optical microscopy (OM) and scanning electron microscopy (SEM) revealed that both plate-like and globular δ phase remarkably restrained the grain growth during annealing recrystallization. The optimum parameters for the grain refinement were determined finally.

Investigation on Denitrogenation Technology and Mechanism during VIM Refining Ni-Base Superalloy

The denitrogenation technology and mechanism of a new kind of Ni-base superalloy during vacuum induction melting (VIM) were studied. The nitrogen content could be reduced from 0.0005% to 0.0002% by this technology. The calculated interaction coefficient of Ti with N () is about -0.38, and the activity coefficient of N () is 0.714. The denitrogenation mechanism was discussed through the calculation of the reaction ratio constants and .

Effects of Phosphorus on Laves Phase Dissolution and Elemental Diffusion in GH706 Alloy

The effects of P on element segregation, Laves phase dissolution and elemental diffusion in GH706 alloy have been investigated. The results show that P promotes the segregation of Nb and Ti. Phosphorus was enriched in Laves phase, which enhanced the precipitation of blocky Laves phase, while constrained the formation of eutectic Laves phase. Moreover, more needle-like uf068uf020phase was formed around blocky Laves phase in high P-doped alloy. It also demonstrated that P slightly lowered the melting-point of Laves phase. During the homogenization process, P hindered the diffusion of Nb which blocks the diffusion of Nb and dissolution of Laves phase, and complicated the homogenization treatment.