Jiansheng Wu

Multicomponent mechanical alloying of Fe-Cu-Nb-Si-B

Abstract The mechanical alloying (MA) process of an elemental powder mixture of Fe 73.5 CuNb 3 Si 13.5 B 9 was investigated in this paper. X-ray and electron diffraction results showed the MA product was an α solid solution with BCC structure. Transmission electron microscopy (TEM) dark field image revealed this α solid solution had a nanocrystalline structure and the grain size was about 10 nm. The results of Mossbauer analysis indicated that a part of the powders was superparamagnetic owing to the extremely fine size obtained by ball milling. The Fe atoms occupy four kinds of ferromagnetic local environments. The values of the hyperfine fields of three of the corresponding subspectra decreased with milling time because of the increasing alloying level. The fourth subspectrum could be attributed to the contribution of iron atoms in nanograin boundaries.

The effect of Co addition on the fracture strength of NdFeB sintered magnets

Abstract The bending strength and fracture toughness of the ternary and Co-containing NdFeB sintered magnets have been studied. The microstructure and the fracture behavior were examined utilizing micro-analysis. It is shown that the bend fracture of the NdFeB magnets is mainly intergranular. The Co addition with proper content increases the bending strength of NdFeB sintered magnets by about 80% and this is mainly due to the strengthening of Nd-rich grain boundary phase.

Electronic structure and transport properties of doped CoSi single crystal

The lattice parameter and electronic structures of CoSi and CoSi1−xYx (Y=Al and P, x=0.03125 and 0.125) were calculated using the full-potential linear augmented plane wave approach based on density functional theory. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semimetal and its density of states is very small at the Fermi level. Doping with Y on the Si site can tune the Fermi level and the effective masses as well as the density of the hole carriers or electron ones in the electronic structure of CoSi, which is a valuable way to modulate the transport properties of this compound. Based on the calculated electronic structures and our experimental results on CoSi, the intrinsic relations between the electronic structures and transport properties of doped and undoped CoSi are discussed in detail.

Gas nitriding behavior of TiAl based alloys in an ammonia atmosphere

Abstract Gas nitridation of TiAl based alloys in ammonia is investigated, together with microstructural and mechanical properties of the nitrided alloys. A mechanism to explain the gas-nitriding behavior of the alloys is proposed.

Dysprosium Nitride-Modified Sintered Nd–Fe–B Magnets with Increased Coercivity and Resistivity

The magnetic properties, electrical resistivity and related microstructures of (Nd,Pr)14.5Dy1.2FebalAl0.8B6 magnets modified by DyN addition have been investigated. The coercivity of the magnets increases almost linearly as a function of DyN content without a considerable reduction in remanence. Similarly, the resistivity in the transversal orientation increases from 1.6 to about 2.0 µΩ m. N dissolves in the grain boundary Nd-rich phase, leading to the formation of an N-containing amorphous phase. This amorphous phase is found to enhance the electrical resistivity. Dy tends to diffuse into the matrix phase, enriched on the matrix side of the interface between the grain boundary phase and the matrix. The significant improvement of the coercivity is ascribed to the increased anisotropy of R2Fe14B by partial substitution of Dy for Nd.

Tensile behavior of Ti50.6Pd30Ni19.4 alloy under different tensile conditions

Ti50.6Pd30Ni19.4 high-temperature shape memory alloy (SMA) has been prepared to investigate the mechanical properties under various tensile conditions. It has been found that shape memory effect (SME) exists even at temperature higher than austenite finish temperature (Af), the mechanism is the reverse transformation of the stress-induced martensite at a higher temperature. The stress–strain curve exhibits a cycle when the alloy was subjected to loading–unloading cycling at different temperature. But the shape and characteristic of the hysteresis cycle are related to the loading history. Complete linear pseudoelasticity is obtained when the specimens are tested at temperature near Af, while elastic hysteresis for the specimens of a stable single phase.

Oxidation behavior of TiNi-Pd shape memory alloys

Abstract The oxidation behavior of TiNi, TiNiPd and the Ce-containing TiNiPd shape memory alloys are investigated using thermogravimetric analysis, X-ray analysis, scanning electron microscopy and electron probe microanalysis. It has been found that the oxidation scale of the alloys exhibits different sub-layers. Generally, the outmost layer is mainly composed of rutile TiO2. The TiNiO3 phase exists for all the alloys, and the Ti4Pd2O phase for TiNiPd alloys as well. These oxides act as important media during the ion exchanging process during the oxidation and cause different oxidation resistance. The addition of Ce in the TiNiPd alloy effectively impedes the Ti ions outward diffusion, causes a wide distribution of Ni and Pd over the scale depth, and obviously improves the oxidation resistance.

Dislocation dissociations and fault energies in Ni3Al alloys doped with palladium

Abstract Dislocation structures in polycrystalline Ni 3 A1 alloy doped with palladium deformed at room temperature have been investigated by transmission electron microscopy. The structure consists mainly of dislocations dissociated into a/2 super partials bounding an anti-phase boundary (APB). Dislocations dissociated into a/3 super Shockley partials bounding a superlattice intrinsic stacking fault (SISF) were often observed. The majority of the SISFs are truncated loops, i.e. the partials bounding the SISF are of similar Burgers vector. These faulted loops are generated from APB-coupled dislocations, according to a mechanism for formation of SISFs proposed by Suzuki et al. (Suzuki K, Ichihara M, Takeuchi S. Acta Metall 1978;26:183) and recently modified by Chiba et al. (Chiba A, Hanada S. Philos Mag 1994;69:751). The APB energies for {111} and {100} slip planes are measured to be 144±20 and 102±11 mJ/m 2 , respectively, and the SISF energy has been estimated to be 12 mJ/m 2 in this alloy. It is concluded that the dislocation structure in Ni 74.5 Pd 2 Al 23.5 alloy deformed at room temperature is similar to that in binary Ni 3 Al and the difference in fault energies between these two alloys is small. Thus, it seems unlikely that the enhancement of ductility of Ni 74.5 Pd 2 Al 23.5 results from only such a small decrease of the ordering energy of the alloy. SISF bounding dislocations also have no apparent influence on the ductilization of Ni 74.5 Pd 2 Al 23.5 alloy.

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.

Oxidation kinetics of the nitrided TiAl-based alloys

Abstract The oxidation kinetics of the nitrided TiAl-based alloys have been investigated at 800–1000 °C in hot air. It is concluded that nitridation is detrimental to the oxidation resistance of TiAl-based alloys under the present conditions. The nitrided alloys exhibited increased oxidizing rate with the prolongation of nitridation time at 800 °C. However, alloys nitrided at 940 °C for 50 h displayed a sign of better oxidation resistance than the other nitrided alloys at more severe oxidizing conditions. The parabolic rate law was considered as the basis of the data processing and interpretation of the mass gain vs. time data. As a comparison with it, attempts were made to fit the data with the power law. The oxidation kinetic parameters k n , k p and n were measured and the trends were discussed.