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In this study, we investigate a new simple scheme using a planar undulator (PU) together with a properly dispersed electron beam (
beam) with a large energy spread (
) to enhance the free-electron laser (FEL) gain. For a dispersed
beam in a PU, the resonant condition is satisfied for the center electrons, while the frequency detuning increases for the off-center electrons, inhibiting the growth of the radiation. The PU can act as a filter for selecting the electrons near the beam center to achieve the radiation. Although only the center electrons contribute, the radiation can be enhanced significantly owing to the high-peak current of the beam. Theoretical analysis and simulation results indicate that this method can be used for the improvement of the radiation performance, which has great significance for short-wavelength FEL applications.
Gas nitridation of TiAl based alloys in an ammonia atmosphere was carried out in the present work. The nitride layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The evaluation of the surface hardness and wear resistance was performed to compare with those of the non-nitrided alloys. It is concluded that the nitride layers are composed of Ti2AlN as the inward-growing layer and TiN as the outward-growing layer. The nitridation temperature and time were two major factors influencing the thickness of the nitride layers of the alloys. The high temperature nitridation raised the surface hardness and the wear resistance of the TiAl based alloys markedly. The tribological behaviors of the nitrided alloys were also discussed.
Nb addition to Ti3Al has been found to be effective to improve its oxidation resistance. Recently, the beneficial effect of nitrogen in the oxidation behaviour of Nb contained T3AI-based alloys has been noticed. In this paper, the oxidation behaviour of five alloys with different Nb content from 0 to 20 at. % exposed in room air at different temperatures (from 600 to 950,C) has been examined with the focus on the TiN formation in the oxide scale. No TiN is detected in the scales of binary Ti3Al under all temperatures tested. On the contrary, in alloys containing Nb, TiN is identified in the scales irrespective of the oxidation temperature. The amount of TiN formation differs with differentNb content. EPMA analysis of the cross section of the scales is performed and discussion is made accordingly.
Development of α 2-Ti3A1 based dual phase alloys have shown some promising potentials in property improvement by introducing Ti5Si3 sulicide phase into the matrix via Si alloying. However, the presence of coarse network of Ti5Si3phase formed by eutectic reaction in the as-cast state also embrittles the alloy. Both hot rolling and powder metallurgy are considered to be the possible ways to refine the Ti5Si3 phase in the matrix. Two Ti-Al-Si-Nb alloys whose Si contents are 2 and 5 at. % respectively were arc melted into ingots and then hot rolled to sheets in this investigation. Optical metallographic examination correlates the microstructures of the as-cast and as-rolled alloys with the different rolling amounts, showing that the coarse silicide network is broken into small particles after hot rolling. Mechanical property testing from room temperature to 8000°C indicates that the strength and plastic elongation of the hot-rolled alloys are much higher than those of the as-cast ones. The data obtained in this investigation are comparable with those obtained in the P/M processed specimens. Fracture surfaces of the alloys are also examined.
Titanium silicide Ti5Si3 whose melting temperature is 2130°C bears the potential for very hightemperature application. This paper reports our results on the alloying behaviour of Nb or Cr addition to this compound. Total energy calculation shows that the substitution of Ti by Nb or Cr atoms in Ti5Si3 crystal stiffens the bonding between the atoms. In experiment, two means of alloying are considered: stoichiometric and off-stoichiometric alloying. Stoichiometric alloying in Ti5Si3 results in compounds consisting of single Ti5Si3 phase while off-stoichiometric alloying yields hypereutectic microstructure with Ti5Si3 being the primary phase. The Ti5Si3 phase in both cases dissolves certain amount of Nb or Cr alloying element and its composition agrees with the stoichiometric composition of (Ti,Nb) 5Si3 or (Ti,Cr) 5Si3 The moduli of the stoichiometric alloys increase with the increase of alloying element addition, indicating an enhancement in Ti5Si3crystal.
A disadvantage to the application of Ti3Al is its poor high temperature oxidation resistance. It is found that the element Nb or(and) Si can greatly reduce the oxidation rate of Ti3A1. A hot stage in situ X-ray diffractometer was used to determine the formation sequence of the oxide layers. At 800°C, TiO2 as well as Al2O3 was detected on the surface of binary Ti3Al at the beginning of the oxidation process. The addition of 5 at % Si to Ti3Al alloy did not favor the formation of an A12O3 layer. On the contrary, it inhibited the onset of Al2O3 to nearly 20 hours from the start of the oxidation process at the temperature of 800°C. The Nb addition also did not promote the formation of Al2O3. TiO2 formed first on the surface of Ti3Al-11 at % Nb alloy while TiN and TiAl formed consequently. Al2O3 was further delayed to 20 hours from the beginning of the oxidation process. When the Nb addition increased to 15 at %, however, TiN and TiAl were not found. It is suggested that the favorable effect of Nb and Si to the oxidation resistance of Ti3Al alloy is not due to their promotion of A12O3 layer, but probably due to some other mechanisms, such as the formation of TiN which serves as a diffusion barrier and decreases porosity in the TiO2 layer.
A sufficient Si addition to TiAl matrix has led to TiAl + Ti5Si3 dual phase alloys, showing coupled-growth microstructure. Compression tests at R.T. as well as high temperatures indicated that the yield stress increased with increasing Ti5Si3 volume fraction, and decreased at higher temperature. The reinforcement from Ti5Si3 phase was obvious, while high Si and Al contents resulted in low ductility. The fracture surfaces were quasi-cleavage. Further research should concern with the adjustment of the shape and amount of the second phase.
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