Zhiyong Gao

Magnetic properties and structural phase transformations of NiMnGa alloys

The magnetic properties and structural phase transformations of Heusler alloys Ni/sub 2+x/Mn/sub 1-x/Ga and Ni/sub 2-x/Mn/sub 1+x/2/Ga/sub 1+x/2/ were investigated. It was found that the martensitic transformation temperature increases and the Curie temperature decreases with decreasing Mn concentration in the Ni/sub 2+x/Mn/sub 1-x/Ga system, but the magnetic properties decrease faster than those do in the Ni/sub 2-x/Mn/sub 1+x/2/Ga/sub 1+x/2/ system. This suggests that the influence of the conduction electron concentration and the distance between Mn atoms on the Curie temperature is stronger than the magnetic dilution effect. The martensitic transformation temperature shows a peak value at x=0.06 in the Ni/sub 2-x/Mn/sub 1+x/2/Ga/sub 1+x/2/ system. A large stress-free magnetic-field-induced strain (MFIS) of 1.8% has been obtained at room temperature in a single crystal sample with the composition of the first system.

Microstructure and Mechanical Properties of Carboxylated Carbon Nanotubes/Poly(L-lactic acid) Composite

Poly(L-lactic acid) (PLLA)/MWNTs composites were prepared by mixing solubilized PLLA with solutions of MWNTs treated by four kind of acids. Fourier transform infrared (FT-IR) spectra revealed that carboxyl groups were grafted to the surface of MWNTs. The water solubility showed that the MWNTs treated by HNO 3/H2O2 and HNO3/H2SO 4 could suspend in the air at room temperature for more than 100 days. Thermogravimetric analysis (TGA) showed that MWNTs treated by HNO3/H 2O2 and HNO3/H2SO4 obtained relatively high COOH content. Mechanical properties of composites showed that the Youngs modulus of the carboxylated MWNTs/PLLA composites increased compared to the pure PLLA. Scanning electron microscopy (SEM) images of fracture morphology confirmed that the dispersion of carboxylated MWNTs was more homogeneous than the pristine MWNTs in polymer matrix. MWNTs treated by HNO3/H 2O2 could get more COOH group and less damage.Poly(L-lactic acid) (PLLA)/MWNTs composites were prepared by mixing solubilized PLLA with solutions of MWNTs treated by four kind of acids. Fourier transform infrared (FT-IR) spectra revealed that carboxyl groups were grafted to the surface of MWNTs. The water solubility showed that the MWNTs treated by HNO 3/H2O2 and HNO3/H2SO 4 could suspend in the air at room temperature for more than 100 days. Thermogravimetric analysis (TGA) showed that MWNTs treated by HNO3/H 2O2 and HNO3/H2SO4 obtained relatively high COOH content. Mechanical properties of composites showed that the Youngs modulus of the carboxylated MWNTs/PLLA composites increased compared to the pure PLLA. Scanning electron microscopy (SEM) images of fracture morphology confirmed that the dispersion of carboxylated MWNTs was more homogeneous than the pristine MWNTs in polymer matrix. MWNTs treated by HNO3/H 2O2 could get more COOH group and less damage.

INVESTIGATION ON MARTENSITIC TRANSFORMATION AND MECHANICAL PROPERTIES OF THE Ni50Mn37-xSn13Fex (x=0, 2, 5, 10) ALLOYS

The microstructures, martensitic transformation behavior and mechanical properties of Fe-doped Ni-Mn-Sn ferromagnetic shape memory alloys are investigated. The influence of composition on the precipitation of second phase and the improvement of ductility are also explained in this paper. The results show that the phase transformations of Ni-Mn-Sn-Fe alloys are still one-step thermo-elastic transformation. The transformation temperatures increase with the increase of Fe content, and the hysteresis decreases after being doped Fe element, this is mainly due to the difference of atom size and the precipitation of second phase. After homogeneously annealing at 1123K for 12h, the Ni50Mn35Sn13Fe2 alloy becomes single phase due to the solid solubility of Fe. The breaking strength and maximum compressive strain increase with the increase of doped Fe content due to the solid solution strengthening and second phase strengthening. The fracture of the Ni-Mn-Sn alloy is intergranular crack, and becomes transgranular crack after doping Fe element. The ductility is improved by this means, which is in favour of the application of the Ni-Mn-Sn alloys.

Microstructural evolution in an Ni-Mn-Ga alloy during compression

Abstract Microstructural aspects and interfacial characteristics of deformed martensite variants in a compressed Ni55Mn20Ga25 (at.-%) alloy have been investigated using transmission electron microscopy. It is shown that the undeformed specimen exhibits a well accommodated morphology and a straight and well defined interfacial boundary. After deformation, reorientation of martensite variants occurs at the expense of unfavourable martensite variants, and the interfacial boundary changes from sharp and straight to curved and irregular. Meanwhile, lattice defects are formed inside the variants even in slightly compressed samples. With increasing deformation, high density lattice defects are generated and rearranged inside the variants as well as in the interfacial boundaries between variants.

FIRST PRINCIPLE STUDY ON THE EFFECT OF Fe CONTENT ON THE PHASE STABILITY OF THE Ni-Mn-Ga ALLOY

The effect of Fe elements on the total energy of Ni2MnGa ferromagnetic shape memory alloy was investigated. It is shown that Fe addition has little effect on the majority-spin density of state (DOS), but changes minority-spin DOS significantly. Substituting Fe for Mn decreases the total DOS of the parent phase, leading to the increase in parent phase stability. When Fe content is higher than 12.5at%, the increasing of martensitic transformation is related to the precipitation of γ phase.