Diffusion bonding behaviour of β-γTiAl alloys containing high niobium with Ti interlayer by spark plasma sintering

High niobium β-γ TiAl alloy(HNBG) was diffusion bonded using spark plasma sintering with pure Ti as interlayer. The joint microstructural evolution, growth kinetics and mechanical properties were investigated. The joint included three diffusion zones. The β/B2 phase formed in the Zone Ⅰ, α_(2)phase in the Zone Ⅱ, and β-Ti and α-Ti phases in the Zone Ⅲ. The thickness of β/B2 phase, the average grain size of α_(2)phase and the amount of β-Ti phase increased with the increase of bonding temperature or bonding time. The growth activation energies of β/B2 and α_(2)phases were 582 and 253 kJ/mol, respectively. The joint acquired at 1000 °C, 10 min and 10 MPa showed the maximum shear strength of 308 MPa. Fracture mainly occurred along the interfaces between Zone Ⅰ and HNBG alloy, and between Zone I and Zone Ⅱ. Fracture mechanism of the joint was characterized by brittleness rupture along the phase boundary.

Microstructures of PM Ti-45Al-10Nb alloy fabricated by reactive sintering

Powder metallurgy Ti-45Al-10 Nb alloys was successfully fabricated by reactive sintering of milled powders at 1350 °C for 2 h in vacuum. The microstructure, in particular, constituent phases, grain size, precipitates of the product was investigated in detail. The results indicate that a double phase γ＋α2 microstructure containing Ti2 Al C precipitate is obtained under the sintering condition, and the alloy exhibits a fine and homogenous microstructure compared with traditional Ti Al base. In fine grain structure, the average grain sizes for the γ and α2 phases are（2.3±0.05） and（5.6±0.05） μm, respectively. Additionally, numerous dislocations are observed in the alloy, and the dislocation density in γ phase is significantly higher than that in α2 phase. The formation mechanisms of the dislocation in the present alloy were also discussed in detail.

Effect of heat treatment and thermomechanical processing on microstructure and tensile property of Ti-44Al-8Nb-0.2W-0.2B-0.5Y alloy

High Nb-TiAl (Ti-44Al-8Nb-0.2W-0.2B-0.5Y,at.%) ingot was fabricated by vacuum arc remelting (VAR).The as-cast ingot was hot-isostatic pressed (HIP) and homogenizing annealing processed.The influence of heat treatment temperature and thermomechanical processing on the microstructure and tensile property of the alloy was investigated by X-ray diffractometry (XRD),scanning electron microscopy (SEM) and tensile tests.It was found that the high Nb-TiAl alloy after HIP and annealing was mainly composed of coarse α2/γ lamellae,β/B2 phase and γ phase and the solidification path of this alloy was:L→L+β→β→α+β→α→α+β+γ→α2+β+γ.The water quenching results showed that the alloy was in α single phase region at 1,340 °C.After heating at 1,340 °C for 30 min followed by furnace cooling,the alloy showed a full lamellar microstructure and its ultimate tensile strength was about 538 MPa,with an elongation of 0.3% at room temperature.Free-crack forged pancakes with fine-grained fully lamellar structure (FFLS) were obtained with an initial deformation temperature of 1,340 °C and the ultimate tensile strength of forged alloy was about 820 MPa,with an elongation of 0.9% at room temperature,which was much higher than that of alloy after HIP and annealing because of microstructural refinement.

First-Principles GGA+U Study of Intermediate-Band Characters from Zn_(1-x)M_xO(M=3d Transition-Metal) Alloys Suitable for High Efficiency Solar Cell

The electronic structure characters are calculated for the Zn1-∞MxO alloys with some Zn atoms in ZnO substituted by 3d transition-metal atoms （M）, in order to find out which of these alloys could provide an intermediate band material used for fabricating high efficiency solar cell. Especially, among of these alloys, the electronic structure character and optical performance of Zn1-xCr∞ 0 alloys clearly show an intermediate band filled partially and isolated from the VB and the CB in energy band structure of ZnO host, and the intermediate band characters can be preserved with increasing Cr concentrations no more than 8.33% in Zn1-xCrxO alloys, at the same time, the ratio 0.52 of Eg^FC to EVE in Zn1-xCrxO, （x = 4.16%） alloy is closest to the optimal ratio of 0,57. Besides, compared to the ZnO, the optical absorption does indicate a great improved absorption below the calculated band gap of the ZnO and an enhancement of the optical absorption in the whole solar spectral energy range.