Lan Ting Zhang

Growth of Single Crystals and Low Temperature Oxidation Behaviors of MoSi2 and NbSi2

In this paper, single crystals, around 8mm in diameter, of MoSi2 and NbSi2 have been grown by optical heating floating zone method. X-ray analysis confirmed that the as-grown ingots were single phase and single crystalline material. Oxidation behavior of the poly-crystalline and single crystalline MoSi2 and NbSi2 were characterized by measuring their weight changes as a function of exposure time. For arc-melted poly-crystalline samples, MoSi2 and NbSi2 fully turned into white powders after 160 and 3hrs exposure at 773K and 1023K respectively, which is known as the “pesting” phenomenon. As a comparison, no pesting was found in the dense spark plasma sintered (SPS) poly-crystalline samples and single crystals. The weight change of single crystals during exposure is found to be much lower than that of the SPS sample, indicating grain boundary plays an important role in the low temperature oxidation behavior of these two silicides.

Microstructure and Properties of Directionally Solidified NbSi2/ Nb5Si3 Composites

NbSi2 is an attractive material for high temperature applications due to its high melting point, low density and good oxidation resistance. The high-temperature strength of NbSi2 is expected to be further improved by incorporation with Nb5Si3, which performs a high creep resistance and strength at high temperature due to its complex crystal structure. In this paper, directionally solidified NbSi2/ Nb5Si3 in-situ composites have been prepared using an optical floating zone method. Scanning Electron Microscopes (SEM) and X-ray diffraction (XRD) have been used to investigate the phase constitution and microstructure. The orientation relationship between Nb5Si3 and NbSi2 is investigated by transmission electron microscopy (TEM). High-temperature properties of alloys are tested by compression at the strain rate of 1×10-4/s at 1673K and 1773K. It was found that high temperature strength and phase constitution of directionally solidified alloys depended on the addition of Mo.

Growth of Crystals and Formation of C40/C11b Lamellar Structure in (Mo0.85Nb0.15)Si2

In this paper, high-quality single crystals of (Mo0.85Nb0.15)Si2, around 8 mm in diameter and 90 mm in length, have been grown by optical heating floating zone method. Effects of chemical composition and growth rate on forming C40 structured single crystal were studied. Aligned C40/C11b lamellar structured can be formed in the as-grown crystals after post annealing at temperatures between 1473 and 1873 K. Chemical composition as well as annealing temperature are found to be two important factors to form C11b lamellae in the C40 matrix. Fully C40/C11b lamellar structure was formed after annealing at 1873 K in the present work. The aligned C40 and C11b lamellae follow a crystallographic orientation relationship of (0001)C40//(110)C11b. Dislocations were observed in some coarse C11b lamellae but never in C40 lamellae of the duplex structure. This is probably due to accumulation of misfit strain during formation of C40/C11b lamellae.

Grain Boundary Modification by Si3N4 Additions and the Effect on Corrosion Behaviors in Two Sintered Nd-Fe-B Magnets

Two sintered magnets Nd15Dy1.2Fe77Al0.8B6 and Nd22Fe71B7 were modified by intergranular additions of Si3N4. The remanence as well as sintering density of the two magnets increased slightly with appropriate amount of Si3N4 additives. Meanwhile, there was an obvious increase in coercivity of the Nd-rich Nd22Fe71B7 magnet after 0.3 wt. % Si3N4 was added to magnets. Besides the effects on magnetic properties, an improved corrosion resistance was observed. Compared with the native magnets without any additions, corrosion potential of the magnets with Si3N4 additives is more positive and the current density in the anodic branch of the polarization curve is reduced. Microstructure observation reveals that Si3N4 additives have been incorporated into the intergranular phases in the magnets. Si is found to enrich in the Nd-rich intergranular phase with low oxygen content. With the introduction of Si3N4 additives, more intergranular phase with high oxygen content is formed, which may contribute to improved corrosion resistance. In addition, addition of Si3N4 refines the grain size of Nd22Fe71B7.

Study on Microstructure and Properties of Low-Mo Fire-Resistant Steel

The number of steel-frame buildings is increasing as a result of development of social economy. However, the fire-resistant property of steel-frame buildings is much weaker than that of brick-frame buildings and RC-frame buildings. In response to such demand, fire-resistant steel has been developed since the last two decades. Mo is one of the most effective strengthening elements for the high-temperature strength of steels. With the increase of the Mo content in steels, there is a dramatic increase in costs which is unacceptable for the cost-sensitive construction area. Therefore, a fire-resistant steel with a low Mo content is highly desired. Two fire-resistant steels with low Mo content (<0.3%) were designed in this paper. The interdependence of microstructure, properties and temperature was studied and analyzed in hot-rolled testing steels. The results show that the low-Mo fire-resistant steels have improved elevated temperature strength (The yield strength of Nb content steel is 240MPa, which can met the requirements of grade Q345 fire-resistant steel), low yield ratio (<0.6) and good welding performance (Ceq≈0.40). The bainite and fine grains are mainly beneficial to the elevated temperature UTS. Nb is an alloying element effective in increasing elevated temperature strength (especially for YS) as well as Mo.

Effect of W on Deformation Resistance of Co-Bearing 12Cr Ferrite Heat-Resistant Steel for USC Steam Turbines

12Cr heat resistant steels with different concentration of Co and W, while Mo equivalent (Mo+1/2W) was fixed at 1.6, were prepared by arc-melting and hot rolling. Mechanical properties were evaluated by tensile tests conducted with the strain rate 2×10-5S-1 at 575oC, 600oC and 625oC instead of time-consuming creep tests. The results show that when Co content is fixed, the steel with 1.5 wt% W is found having the best deformation resistance which is strong work hardening and slow strain softening. Apparent activation energy of the steel with 3.1 wt% Co and 1.5 wt% W is in the range of 370~413 kJ/mol, higher than those of the other steels in our study, which are close to the self-diffusion activation energy of iron (239 kJ/mol). Therefore, the steel with 3.1 % Co and 1.5% W is suggested as a potential candidate material for 625oC~650oC class USC steam turbines.

Oxidation Behaviours of MoSi2 Single Crystal at the Temperatures of 773K and 1473K

MoSi2 is a potential high temperature structural materials and shows excellent oxidation resistance at high temperatures. To explore its oxidation behaviour and mechanism, MoSi2 single crystals were prepared and investigated in the present work. The experimental results showed that different from its polycrystalline, MoSi2 single crystal showed good oxidation resistance at both 773 and 1473K. Near parabolic law of oxidation kinetics was followed for all the investigated surfaces of MoSi2 single crystal. The close-packed (110) surface of MoSi2 single crystal showed the best oxidation resistance at both temperatures. After exposure at 773K, the molybdenum oxide formed on the (110) surface of MoSi2 single crystal was found to be Mo4O11 instead of MoO3 formed on the other surfaces. Morphology observation showed a columnar growth of Mo4O11 on the (110) surface. No difference was found on the oxide formed for different surfaces of MoSi2 single crystal at 1473K.