Papers by Author: Xin Qing Zhao

Abstract: The microstructure stability and tensile properties of Ti-15-3 titanium alloy were investigated by aging the alloy at different conditions. The results showed that when the temperature was below 250°C, ω phase precipitates and the tensile strength increased with increasing aging time. Although there were a little of ω phase after aging at 200C for 1000h, Ti-15-3 alloy still presented good strength and ductility. When the aging temperature was 250°C~400°C, ω precipitation or β’ phase separation occurred from β matrix, and finally transformed to α precipitates. The tensile strength increased firstly and then decreased with increasing aging time. When aged at above 400°C, α phase precipitated fast from the alloy, and the strength increased gradually.

Abstract: To get bioorganic surface with improving biological properties, NiTi shape memory alloy was bonded poly (ethylene glycol) (PEG) by sequentially piranha solution treating, silanizing, and then γ-ray irradiation induced grafting. Piranha solution treating gave hydroxylated surfaces for the benefit of next silanization. The trichlorovinylsilane (TCVS) was performed as a bridge to covalent bond treated NiTi substrates and PEG by γ-irradiation. X-ray photoelectron spectroscopy (XPS) was used to indicate that PEG was bonded on silanized NiTi surface by irradiation. Osteoblast culture of 1 day and methyl-thiazol-tetrazolium (MTT) assay showed that PEG bonded on NiTi surface enhanced cell proliferation and cell amount increased significantly with increasing the concentration of bonded PEG.

Abstract: The phase transformation and mechanical behaviors of cold-rolled NiTi shape memory alloys ultra-thin sheet with 100μm in thickness are investigated. The transformation behaviors of the NiTi sheet are found to be remarkably influenced by heat treatment using electric resistance vs. temperature measurements. The martensitic transformation temperature reduced by annealing at 400°C or 600°C, and R-phase transformation appears when annealing at 400°C. The martensitic reorientation occurs when the NiTi sheets annealed at 400°C is deformed at room temperature and the maximum shape memory strain is 3.5% at 100% recovery ratio. For the NiTi sheets annealed at 600°C, a superelastic strain of 5% and a transformation stress about 500MPa are achieved.

Abstract: The effect of Mo on the oxidation behavior of TiNiAl at 1073K has been investigated. It is found that 1at.% Mo addition can increase the diffusion of Al in the alloys and promote the formation of dense and continuous Al-rich oxide layer. Therefore the oxygen diffusion can be effectively impeded and the oxidation behavior of TiNiAl is improved. The observation of the cross-sectional oxidation layer showed that beneath the top oxide scale a Mo-rich oxide layer formed. Because the oxide of Mo is volatile at high temperature, voids formed in the oxide scales during the oxidation process. 3at.% Mo addition could cause cracks between the oxide scale and the substrate, resulting in poor adhesion of the oxide scale to the substrate.

Abstract: A series of NiTiAl based alloys with different amount of Nb addition were prepared and the effects of Nb on both the mechanical properties and microstructure evolution were investigated. The addition of Nb can remarkably enhance the high temperature yield strength of these alloys. The highest yield strength of the alloys with 10 at% Nb reaches 1237MPa at 873K and 875MPa at 973K, respectively. The Ti2Ni(Al,Nb) precipitates and the solution strengthening effect might be responsible for the high yield stress achieved at high temperature.

Abstract: A first principle study was performed to discuss the high temperature oxidation mechanism of NiTi alloys with the special emphasis on the effect of Nb on the oxidation behavior. The calculation results suggest that the Nb atom prefers the Ti site in Ni(Ti,Nb). The addition of Nb will not only reduces the electron density of Ti-d and Ni-d states near the Fermi energy level but the their electron contributions to the p-orbital of Ti. In addition, the Nb atom increases the formation energy of the Ti defect, which will decrease the diffusion of Ti atoms. All these Nb-induced effects account for the improvement of high temperature oxidation resistance, which agrees well with the experimental results.

Abstract: Nanostructured layers were fabricated on the surface of 0.4C-1.0Cr and 1.0C-1.5Cr low alloy steels by using an ultrasonic particulate peening (USPP) technique. The microstructures and mechanical properties of the nanocrystallized layers were characterized by means of transmission electron microscopy, and nano-indentation test. Results showed that the average grain size in the surface nanocrystallized layer of 0.4C-1.0Cr and 1.0C-1.5Cr low alloy steel was about 5nm and 10nm, respectively. The nano-indentation hardnesses of the surface nanocrystallized layer were enhanced significantly and reach upwards of 8.0 GPa and 12.5 GPa, respectively.