Li Chonghe

Investigation on Microstructure and Memory Property of NiTi Single Crystal Shape Memory Alloys

Abstract Single crystals of NiTi (with 50.0at% Ni) were prepared in a self-designed unidirectional solidification furnace with a graphite mold shell connected with a crystal selector. Both macrostructure and microstructure of the alloy were observed by OM and SEM with EDS, the grain orientation of single crystal was determined by XRD and pole figures, and shape memory effect was evaluated by an instrument developed by ourselves. The results show that dendritic features were presented in both cross section and longitudinal section; the direction of dendrites growth were close to solidified direction; there was no obvious grain boundary observed; the angle between normal direction of cross section and [111] orientation was 15°. Compared with normal NiTi polycrystals, the maximal recoverable strain and memory fatigue property of the alloy were improved because the bad effect of grain boundary on the martensite phase transformation was eliminated, and also [111] orientation was a preferred orientation of NiTi alloy which can provide larger recoverable strain.

Hydration Resistance of Y2O3 Doped CaO and Its Application to Melting Titanium Alloys

Various amount Y2O3(1–8 mol%) doped CaO powder was synthesized by the solid state reaction method, the pellete and crucible were fabricated by the cold isostatic pressing and were sintered at 1750°C for 4h. The microstructural characterization was revealed by X-ray diffraction(XRD) and scanning electron microscopy(SEM).The XRD results showed that when Y2O3 doped 2 mol%, metastable CaY2O4 phase existed in CaO grain boundary, when Y2O3 doped 3 mol%–8 mol%, in addition to the above structure, Y2O3 phase also be found in CaO grain boundary. Hydration experiment results showed when Y2O3 doped 0 mol%–4 mol%, CaO had excellent hydration resistance performance, Y2O3 doped 2 mol% had the best hydration resistance, its weight addition stored after 7 weeks (49 days) was only about 0.2 wt%. Melting experiment results showed that it was no reaction between crucible and alloy layer. Oxygen, calcium, titanium, nickle and yttrium element not diffusion between the CaO crucible and TiNi alloy, it was no oxygen content increase after melting.

Interface Reaction Between Y 2 O 3 Doped BaZrO 3 and TiNi Melt

Using the solid reaction method, the Y2O3 4mol%-doped BaZrO3 powder was prepared by BaCO3, ZrO2 and Y2O3 as raw materials, the effect of sintering temperature on the property and microstructure of Y2O3-doped BaZrO3 pellets was investigated, the crucible made of this Y2O3-doped BaZrO3 was utilized to melting the TiNi alloy by the vacuum induction method. The phase constitution of doped powder and the interface reaction between the crucible and the melt were studied by XRD and SEM. The results showed that this Y2O3-doped BaZrO3 consists of two phase (BaZrO3 and Ba2YZrO6), the pellets sintered at 1750°C for 6h reached the maximum relative density of 98.0%. Besides, there was no obvious interaction layer and element diffusion between the crucible and the melt, which may imply that the Y2O3-doped BaZrO3 refractory is a promising candidate for melting titanium alloys.

Thermodynamic Reassessment of BaO-YO1.5 System

Based on the available experimental information, the thermodynamic reassessment of BaO-YO1.5 system was carried out. The liquid phases were modeled with substitutional solid solution model, the compounds BaYO2O4 and Ba3Y4O9 were treated as stoichiometric phases, the BaOss (halite BaO solid solution), (α-Y2O3 and β-Y2O3 were treated as pure compounds. The existing discrepancies were summarized, and a new assessment was carried out based on the formation enthalpy of two compounds (BaY2O4 and Ba3Y4O9). The thermodynamic parameters of all phases were optimized by Calphad method, a self consistent set of the optimized Gibbs energy parameters was derived, which can be safely used to extrapolate into the multicomponent system. Compared with the experimental data and the results in this work as well as the results reported previously, it is demonstrated that the present thermodynamic assessment is consistent with the most of experimental data.