Chonghe Li

Electrochemical extraction of Ti5Si3 silicide from multicomponent Ti/Si-containing metal oxide compounds in molten salt

Ti5Si3 silicide has been extracted directly from complex multicomponent Ti/Si-containing metal oxide compounds by electro-deoxidation in molten calcium chloride using an inert solid oxide oxygen-ion-conducting membrane (SOM) based anode. Studies on the microstructure evolution and electrochemical extraction mechanism show that the formation of Ti5Si3 and the removal of impurity elements happened simultaneously during the electro-deoxidation process. It is found that the electro-deoxidation generated Ti5Si3 micro-particles typically possess a smooth surface, which could contribute to create a continuous anti-oxidation surface layer with excellent high-temperature oxidation resistance property. Consideration is also given to the parameters of electrolysis and the electrochemical characteristics including chemical and/or electrochemical reactions during the electro-deoxidation process, and then a relevant kinetic model is proposed.

Phase transformation and reduction kinetics during the hydrogen reduction of ilmenite concentrate

The reduction of ilmenite concentrate by hydrogen gas was investigated in the temperature range of 500 to 1200°C. The microstructure and phase transition of the reduction products were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM). It was found that the weight loss and iron metallization rate increased with the increase of reduction temperature and reaction time. The iron metallization rate could reach 87.5% when the sample was reduced at 1150°C for 80 min. The final phase constituents mainly consist of Fe, M3O5 solid solution phase (M=Mg, Ti, and Fe), and few titanium oxide. Microstructure analysis shows that the surfaces of the reduction products have many holes and cracks and the reactions take place from the exterior of the grain to its interior. The kinetics of reduction indicates that the rate-controlling step is diffusion process control with the activation energy of 89 kJ·mol−1.

Preparation of single crystal of TiNi alloy and its shape memory performance

The unidirectional solidification equipment based on Bridgman method with high temperature gradient was designed, and the single crystal of Ti-50.0at%Ni alloy was successfully fabricated by this equipment as well as a selective growing zigzag-shaped crystallizer and a steady growth container that were made of electro graphite. The microstructure of single crystal sample was studied by means of Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS); the orientation of single crystal was measured by X-ray technology; the phase transformation points were determined by Differential Scanning Calorimetry (DSC). It is resulted that, the single crystal of TiNi shape memory alloy (SMA) can be prepared with a set of suitable process parameters; the microstructure of the single crystal obtained in this study is dendritic, there is Ti2Ni intermetallic between the dendrites, the angle between the orientation of single crystal and [111] plane is about 15 degree; the shape memory performances are improved obviously and the maximum recoverable strain reaches 10%.

Effect of CaO Additive on the Interfacial Reaction Between the BaZrO 3 Refractory and Titanium Enrichment Melt

The BaZrO3 and CaO-doped BaZrO3 ceramics were fabricated at 1750 ℃ for melting the Ti2Ni alloys, respectively. Employing XRD, OM and SEM, the influence of CaO on the composition of BaZrO3 and the interaction between the BaZrO3 ceramic and titanium alloy was investigated. The results showed that the CaO doped BaZrO3 was consisted of CaO and Ba1−xCaxZrO3. The grain on the surface of BaZrO3 ceramic was loose after melting, and the thermodynamic analysis indicated that the dissolution-erosion was responsible for the interaction mechanism. An amount of BaZrO3 refractory was attached to the Ti2Ni alloy bottom after the alloy was cooled. However, no refractory was found on the bottom of Ti2Ni alloy, which was cooled down on the CaO doped BaZrO3 ceramic. CaO additive can effectively reduce the interaction between the BaZrO3 refractory and the titanium melt, indicating that it was a very promising refractory for preparing the titanium alloy.

The Unidirectional Solidification of Ti-46Al-8Nb Alloy with BaZrO 3 Coated Al 2 O 3 Mould

The TiAl alloy with nominal composition of Ti-46Al-8Nb (at.%) is directionally solidified(DS) in a BaZrO3 coated Al2O3 mould with the certain withdrawing rates (120 μm/s), and the metal-BaZrO3 interface, microstructure and chemical composition of the alloy are evaluated. The result demonstrates that after the directional solidification a fully lamellar γ/α2 microstructure without BaZrO3 particles contamination is observed, and the BaZrO3 protective coating exhibits an effective barrier capability to avoid the direct contact between the mould base material and the TiA1 melt. The interface reaction between the mould and TiAl melt was analyzed by scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDS). The results showed that a certain thickness interface reaction layer is formed at the metal-mould interface with this withdrawing rate. And the reason may be the BaZrO3 coating is found to suffer some erosion and be slightly dissolved by the molten TiAl.

BaZrO3 refractory applied to the directional solidification of TiAl alloys

Recently, much attention has been paid to the refractory used for the directional solidification process of TiAl intermetallics, the Y2O3 crucible/Y2O3 coated moulds seem to be the suitable candidate. However, the use of Y2O3 is limited by its low thermal shock resistance and high cost. In this work, a novel BaZrO3 refractory was introduced to the directional solidification of TiAl intermetallics. The melt of this alloy contained in BaZrO3 crucibles were heated for 30 minutes at 1600, 1650, 1700 °C, respectively, then cooled within the crucible for the investigation of the interface between the melt and the refractory. The scanning electron microscopy (SEM) was used to evaluate the surface topography and microstructure of the samples, and the energy dispersive spectroscopy (EDS) was used to analyse the chemical composition of the samples. The results indicated that no interfacial interaction layer and no obvious element diffusion were observed between the crucible and the metal, which may imply that the BaZrO3 is a promising candidate of refractory for the directional solidification of TiAl alloys. This work can provide a basis for the further study of directional solidification of TiAl alloys by using BaZrO3 shell mould.