Papers by Author: Ai Dang Shan

Abstract: To provide insight into the mechanical behavior and microstructural evolution of bulk nanograined (NG) Ni-based alloys during annealing, the Ni-based alloy sheets with grain size about 50 nm was produced through severe cold-rolling at room temperature, and then the cold rolled (CRed) Ni-based alloys were annealed at different states. The evolution of the nanostructure of the CRed Ni-based alloy during annealing and corresponding change in mechanical properties was investigated. The results showed that the CRed Ni-based alloy exhibited prominent enhancement in the yield strength (YS), ultimate tensile strength (UTS), which increased respectively from 253 MPa to 1455 MPa, 684 MPa to 1557 MPa. Further increase of the YS and UTS were obtained in the annealed-CRed Ni-based alloy with dual-phase. The YS and UTS of the NG dual-phase Ni-based alloy was respectively 2013 MPa and 2061MPa, which was annealed at 700 °C for 1h. In terms of the microstructural evolution, lower density of defects on the grain boundary were observed and the nanograins can be maintained about 100 nm even when annealed for 30 h at 700 °C, which suggests high thermal stability at this temperature. Both the high thermal stability and strength are due to the formation of the γ′ precipitates and slight grain growth of the NG matrix.

Abstract: Microstructures and properties of ultrafine grained (UFG) iron by different rolling processing are investigated. By applying the asymmetric rolling (ASR), the equiaxed UFG iron with the grain size of 0.9 μm has been obtained due to the shear strain induced by the mismatch of the two roll during ASR. With the same rolling reduction, the ASR processed iron shows higher yield strength than that of the symmetrically rolled one. When the ASR processed iron is further symmetrically rolled, the grain morphology maintains equiaxed and the grain size decreases to about 0.3 μm.

Abstract: The influence of ultrafine grains produced by severe plastic deformation technology on the weldability of Fe3Al and Al dissimilar materials was investigated. An ultrafine grain layer was produced on Fe3Al intermetallic compound by surface mechanical attrition treatment. Then the SMATed Fe3Al was diffusion bonded with 1060Al at 550°C for 90 min in the vacuum of 10-3 Pa. The microstructures of surface ultrafine grain layer and transition zone at the interface of SMATed-Fe3Al/Al joint were observed by scanning electron microscopy. The grain size of surface ultrafine grains was characterized by X-ray diffractometry. The elements distribution at the interface and the phase constituents of transition zone were measured by energy disperse spectroscopy. The results showed that a deformed layer about 10-20 μm wide and surface nanocrystallines about 35nm were produced after 15 min surface mechanical attrition. SMATed Fe3Al was well bonded with Al and 11-30 µm wide transition zone formed. The transition zone consisted of FeAl and FeAl3 phases. The surface nanocrystallines helped the atom diffusion and the formation of diffusion bonded joint with rough surface and lower pressure.

Abstract: The effects of asymmetric and symmetric rolling at room temperature on mechanical properties and microstructure of the commercial purity Ti were investigated by means of mechanical test, optical microscopy, X-ray diffraction and transition electron microscopy. The results show that through asymmetric and symmetric rolling processes the ultimate tensile strength is substantially increased from 450 MPa to 960 MPa. Microstructure observation illustrates this variation in mechanical property is caused by the grain refinement and work hardening.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: Asymmetric rolling has been considered as a possible way to obtain severe plastic deformation (SPD) since it will give an extra shear deformation to the processed materials during rolling. Previous researches have confirmed such a shear deformation. Very recently, the method of inserting-block is used to characterize the shear deformation through direct observation, but when the reduction is more than 70%, the lineation scratched on the side face of internal mark becomes vague and illegible. In order to directly observe the shear deformation of metallic material with large reduction, the internal mark method is employed in this research and asymmetric rolling was performed with pure aluminum and iron at room temperature. In severe plastic deformation, the shear deformation caused by asymmetric rolling was clearly observed and measured through employing internal mark method. Remarkable extra shear deformation during asymmetric rolling was confirmed. Very high equivalent strains were achieved when sheet samples were asymmetrically rolled to high reduction ratio. These strain values fall into the range of SPD.

Abstract: 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.