关键词： amorphous silicon   amorphous silicon germanium alloys   mean free path   photoelectronic excitations   optical properties   spectroscopic ellipsometry  

摘要： Hydrogenated amorphous silicon germanium alloy (a-Si1-xGex: H) films are prepared by plasma enhanced chemical vapor deposition (PECVD) and characterized by in situ real time spectroscopic ellipsometry (RTSE). From complex dielectric function spectra extracted, the broadening width energy (G) of the primary absorption feature centered near 3.7 eV is quantified using the Cody-Lorentz oscillator model. Mean free path length of photoelectronic excitations is calculated from Gamma based on reasonable estimates for speed of electron-hole photoexcitations. A model is applied with excited state lifetime assumed to be limited by scattering from network disorder and provides a relative measure of short-range order. The simple model applied is an extension of that widely used to characterize broadening of optical transitions in polycrystalline semiconductors. Decreases in mean free path of up to similar to 30% occur with germanium. Relative increases in mean free path with increased hydrogen during PECVD a-Si:H, a-Si0.73Ge0.27:H, and a-Si0.60Ge0.40:H occur from similar to 5% to similar to 8% and with hydrogen plasma treatment of a-Si:H by similar to 6%. Mean free path changes are tracked with thickness to provide short range order evolution and the effect of the underlying material. Mean free path of photoexcitations in electronic quality a-Si1-xGex: H is estimated at similar to 3.5 angstrom, on the same order as interatomic spacing.

关键词： Amorphous alloy   Brazing   Titanium alloy   Microstructure   Mechanical property  

摘要： The Ti_(51.44)Zr_(16.26)Cu_(30)Sn_(2.3)(at%)amorphous filler alloy in a ribbon form was prepared by melt *** Ti-6 Al-4 V alloy was brazed in vacuum furnace by using the Ti_(51.44)Zr_(16.26)Cu_(30)Sn_(2.3)amorphous filler *** effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of the Ti-6 Al-4 V brazed joints were *** brazed joints are composed ofα-Ti,β-Ti,and(Ti,Zr)_(2)Cu *** shear strength of the joints increases with brazing temperature increasing from 1153 to 1193 K and holding time from 10 to 15 min,while decreases with brazing temperature and time further *** temperature brazing results in the formation of microcracks and voids in the seams,which are harmful for the shear strength of the *** the joint brazed at 1193 K for 15 min,a fineα+βWidmansta¨tten structure beneficial to the mechanical property is formed,and the joint exhibits relatively high joint strength of 180 MPa.

关键词： K4169 superalloy   Amorphous filler   Brazed joint   Microstructure   Mechanical property  

摘要： Three nickel-based amorphous filler materials with composition of [B-Ni9-xCrx]BSiNi (x = 0,1,2) were designed to braze K4169 superalloy according to cluster-plus-glue-atom model. The microstructure of K4169 nickel-based alloy brazed joints was examined, and the shear strength of the joint was also measured via pressure-shear tests. The brazed seam consists of gamma-Ni solid solution, Nb6Ni16Si7 and CrB phases of joint brazed with [B-Ni7Cr2]BSiNi filler. More Nb6Ni16Si7 phase formed in the brazed seam with more Cr addition in filler material, because Cr promoted the diffusion of Nb atoms from base material to brazed seam. Diffusion zone was composed of needle-like Cr-Mo rich borides and blocky MC carbides (NbC, TiC). The amount of Cr-Mo rich borides went up with the enhancement of Cr content in filler material. The shear strength of joint reached 689 MPa when brazed with [B-Ni-9]BSiNi filler, followed by 562 MPa with [B-Ni8Cr1]BSiNi filler and 450 MPa with [B-Ni7Cr2]BSiNi filler. In addition, Cr-Mo rich borides and continuous precipitation (Nb6Ni16Si7 and CrB) exhibited low resistance to crack, resulting in the degradation of joint strength.

关键词： Magnetic properties   Magnetocaloric effect   Refrigeration capacity   Rare earths  

摘要： Amorphous Fe78-xRExSi4 Nb5 B12Cu1(RE=Gd,Dy) ribbons with different RE contents were prepared by melt spinning to investigate the effect of heavy rare earth(Gd,Dy) substitution on the hyperfine structure,magnetic properties and magnetocaloric *** Curie temperature of RE substituted alloys,hyperfine field and magnetic moments of Fe atoms initially increase up to 1 at% RE content and then decrease monotonously for increasing RE content up to 10 at%.The dependence of magnetic entropy change(-△SM) and refrigeration capacity(RC) of the alloys on RE contents displays the same *** RCAREA values of the alloys substituted with 1 at% Gd and Dy are similar to those of recently reported Fe-based metallic glasses with enhanced RC values compared with those of Gd5 Ge1.9Si2 ***-△SM and RC values,negligible coercive force and hysteresis commonly make these Fe78-xREx-Si4 Nb5 B12Cu1 amorphous alloys as low-cost candidates for high-temperature magnetic refrigeration.

关键词： Rapid solidification   Amorphous alloys   Differential scanning calorimetry   X-ray diffraction   Positron annihilation lifetime spectroscopy  

摘要： The paper presents the calorimetric and structural analysis of the rapidly solidified GdxZr10Fe58-xCo10B15Mo5W2 (where: x = 0, 1, 2, 3, 4, 5) alloy ribbons in as-cast state. The base alloys were obtained by the arc-melting of the high purity constituent elements under a low pressure of Ar protective atmosphere. Subsequently, the ribbon samples were produced by the single-roll melt spinning technique under the low Ar pressure at the linear velocity of the copper roll surface of 32 m/s. X-ray diffraction (XRD) studies have shown amorphous structure of all the alloy. The microstructure of the samples was examined by scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) measurements were performed at various heating rates. These allowed to determine the thermal stability of the amorphous phase. The Kissinger, Ozawa and Augis-Bennett plots were constructed to determine the activation energies E-a for crystallization. The positron annihilation lifetime spectroscopy (PALS) was used to study the structural changes as well as a degree of disorder in the atomic structure caused by doping the base alloy with Gd. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Titanium matrix composites   Brazing   Microstructure  

摘要： (TiB + Y2O3)/titanium matrix composites (TMCs) were brazed with CuTiZrNi amorphous filler alloy. The joint showed integral interfaces without any imperfections. The joint brazed at 900 degrees C can be divided into 2 zones. Microstructure of zone I was demonstrated as (Ti,Zr)(2)(Cu,Ni) + alpha-Ti + TiB and zone II was a continuous TiCu reaction layer. Higher temperature accelerated the elemental diffusion between the joint and substrates, thickened the brazing seam and TiCu layer diminished when brazed at 980 degrees C. The amount and size of (Ti,Zr)(2)(Cu,Ni) phase decreased while the amount of TiB and Y2O3 phase increased with increasing brazing temperature. When brazing temperature rose higher than 980 degrees C, the joint was occupied by reticulation-distributed (Ti,Zr)(2)(Cu,Ni) + alpha-Ti eutectic structure. At higher brazing temperatures, TiB was greatly refined as the decomposition of the original TiB enhanced and more reconstructed TiB precipitated during cooling. The joint shear strength increased greatly with increasing brazing temperatures and reached the maximum of 646.0 MPa when brazed at 1020 degrees C. Further increasing the brazing temperature to 1060 degrees C, the joint strength did not change obviously. The fracture locations changed when brazed at different temperatures. The joints at different parameters showed cleavage fracture or quasi-cleavage fracture.

关键词： gamma-TiAl-based alloy   Ni-based superalloy   Amorphous filler   Low temperature   Shear strength  

摘要： The gamma-TiAl-based alloy and Ni-based superalloy (GH536) were successfully brazed utilizing a low-melting-point amorphous Ti35Zr25Be30Co10 filler below 800 degrees C. Before brazing, the wetting ability of the filler was investigated and it was found that the amorphous filler exhibited a good wetting ability with a wetting angle less than 30 degrees. A sound joint with no cracks and holes was obtained after brazing and dendritic phases formed in the wetting interfacial regions and across the brazed joint. The structural evolution of the wetted interfaces and brazed joints were studied by x-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS). The results show that temperature played an important role in the formation of the dendritic phases. Coarse grains were presented and dendritic phases decreased when the joint was brazed at a higher temperature, which seriously deteriorate the shear strength of the joint. The shear tests showed that the maximum shear strength of the joint was 300 MPa when the brazing temperature and holding time were 770 degrees C and 20 min.

关键词： Vacuum brazing   Ti-Cu-based amorphous filler metal   Interfacial microstructure   Mechanical property  

摘要： The effect of brazing parameters on the interfacial microstructure and mechanical properties of Ti6Al4V titanium alloy/316 L stainless steel brazed joint was investigated. The joint presented sectionalized interfacial microstructure by forming four reaction zones. The diffusion of Cu and Fe into Ti6Al4V titanium alloy substrate caused beta-Ti transformation and its dissolution. Raising the brazing temperature promoted the dissolved blocky beta-Ti to migrate to 316 L stainless steel side. Compared to the brazing temperature, the brazing time had relatively less impact on the microstructure of joint. The diffusion of Ti into steel substrate led to the formation of the transition zone which contained three different reaction layers-Fe2Ti, tau + alpha-(Fe, Cr), and gamma-(Fe, Ni) + sigma. With the increase of brazing temperature, the transition zone gradually thickened. The optimized joint shear strength was 65 MPa obtained at 960 degrees C/5 min. Contraction difference between two base metals generated stress concentration at Ti-Cu-Fe/Fe2Ti interface, which was a liquid/solid interface upon solidification. During shear test, cracks initiated at the Ti-Cu-Fe/Fe2Ti interface, and then mainly propagated within the reaction layers of Fe2Ti and tau + alpha-(Fe, Cr).

关键词： Electrical resistance spot brazing   Ni-based amorphous alloys   Behavior of stainless steel sheets brazed joints  

摘要： In this study the behavior of brazing joints with nickel-silicon-based amorphous alloys ribbons was analyzed in terms of mechanical strength and corrosion resistance. Two Ni78Si8B15 and Ni68Fe3Cr7Si8B14 metallic glasses ribbons designed and fabricated in our laboratory have been studied as brazing foils. The base material of the joint is the 316L austenitic stainless steel. Note that the two tested chemical compositions of the ribbons are recommended for brazing stainless steels. A comparison between brazed joints and welded steel by electrical resistance spot has also been conducted. As results, Ni78Si8B15 amorphous alloy presents a very good castability, wettability and flowability. However the addition of chromium and iron to form Ni68Fe3Cr7Si8B14 ignificantly increases the melting temperature, the corrosion resistance and the mechanical strength. Finally we found that the brazed joint with Ni68Fe3Cr7Si8B14 metallic glasses ribbon has the same yield strength than that of the welded joint, but it presents a better corrosion resistance.

关键词： Divertor   First wall   Tungsten   RAFM steels   Brazing   Amorphous filler metal  

摘要： The effective joining of tungsten (W) and reduced activation ferritic-martensitic (RAFM) steels is crucial to fabrication of the divertors and first wall of future fusion reactors. In the present work, a low-activation Fe-based amorphous alloy of Fe67.8Cr11.5Si2.1B18.6 (at. %) is designed as the filler metal for brazing W and RAFM steels. Crack-free joint has been achieved with the amorphous alloy filler and a vanadium interlayer metal by short-time vacuum brazing at 1270 degrees C. Layer structures composed of intermetallic phases and single solid-solution phases, respectively, are alternately formed in the W/RAFM steel joint, exhibiting alternating hard and soft mechanical characteristic. Long-distance diffusion of W atoms to the steel substrate is blocked by the formation of FeW2B2 and Fe3B phases, and Si and B elements are confined within the main bonding seam of the joints. Microstructure recovery for the steel is realized following the standard heat treatment procedures. The present results suggest a promising way of making strong and tough W/RAFM joints with low-activation Fe-based amorphous alloys.