The morphology and orientation of Mn5Si3 precipitates in an Mg-Sn-Mn-Si alloy

This paper reports a preliminary study of Mn₅Si₃ precipitates in an Mg-Sn-Mn-Si alloy. The transmission electron microscopy investigation reveals that the Mn₅Si₃ precipitates have a plate shape, approximately 50 nm in thickness and 200 nm in width. The orientation relationship (OR) between the precipitates and matrix is not unique, but all measured ORs obey the following relationship: g_{(1 0 − 1 0)Mn5Si3} // Δg₁ // Δg₂, where Δg₁ = g_{(2 − 1 − 1 0)Mn5Si3} − g_{(0 0 0 2)Mg}, Δg₂ = g_{(3 − 1 − 2 0)Mn5Si3} − g_{(0 0 0 2)Mg}. The precipitate morphology and their OR with the matrix are explained with a Δg parallelism rule, showing good agreement.     

钛表面激光熔覆原位制备Ti5Si3涂层结构及摩擦学性能

利用激光熔覆技术在钛表面预置硅粉原位制备了Ti5Si3涂层.用XRD、SEM和TEM分析了涂层的组成和组织结构.在UMT摩擦磨损试验机上对Ti5Si3涂层在不同载荷和不同滑动速度下的摩擦磨损性能进行了测试.实验结果表明:涂层的物相主要是Ti5Si3相和基材Ti相,涂层的显微结构为球状和块状晶,Ti5Si3涂层具有较高的显微硬度,涂层截面的平均显微硬度约为840 HV0.2,是钛基材的4.4倍;Ti5Si3涂层可显著提高钛基材的耐磨性能;Ti5Si3涂层的磨损机理为磨粒磨损和粘着磨损.     

Influence of Cu addition on the self-propagating high-temperature synthesis of Ti5Si3 in Cu–Ti–Si system

The self-propagating high-temperature synthesis (SHS) reactions can take place in Cu–Ti–Si systems with Cu additions of 10–50 wt.%, and the products only consist of Ti₅Si₃ and Cu phases, without any transient phase. In Ti–Si system, most of the Ti₅Si₃ grains synthesized exhibit the polygon-shaped coarse appearance with an obviously sintered morphology. When Cu content increases from 10 to 50 wt.%, however, the Ti₅Si₃ exhibits cobblestone-like shape with a relatively smooth surface, and its average size decreases significantly from 15 to 2 μm or less. The formation mechanism of Ti₅Si₃ in Cu–Ti–Si system is characterized by the solution, reaction and precipitation processes. Furthermore, the addition of Cu has a great influence on the volume change between green and reacted preforms. The volume change increases with Cu content increasing from 0 to 20 wt.%, and then decreases with the content further increasing from 20 to 50 wt.%. The addition of Cu to Ti–Si system significantly decreases the onset temperature of the reaction during differential scanning calorimetry process, which is even much lower than the α → β transition temperature of Ti (882 °C), suggesting that the reaction could be greatly facilitated by Cu addition. As a result, the role of Cu serves not only as a diluent but also as a reactant and participates in the self-propagating high-temperature synthesis reaction process.     

Preparation of Magnéli phases of Ti27O52 and Ti6O11 films by laser chemical vapor deposition

Magnéli phases of Ti₂₇O₅₂ and Ti₆O₁₁ films were prepared by laser chemical vapor deposition using Ti(dpm)₂(O-i-Pr)₂ as a precursor. Ti₆O₁₁ film was obtained at a laser power (P_L) of 200 W and a deposition temperature (T_dep) of 1270 K. Ti₂₇O₅₂ film was obtained at P_L = 150 to 200 W and T_dep = 1120 to 1250 K. Ti₆O₁₁ and Ti₂₇O₅₂ films had a faceted texture about 2 μm in grain size and a columnar cross section. The deposition rate of Ti₂₇O₅₂ and Ti₆O₁₁ films were 90 and 70 μm h^− 1, respectively.     

Explosion synthesis of Ti5Si3-Cu intermetallic compound

Synthesis of an intermetallic compound based on Ti₅Si₃ by an explosive compaction of elemental powders was studied with the detonation velocity as the main experimental variable. Prior to the explosion experiment, a computer simulation of the compaction process was conducted by using the DYNA program, and the result was utilized in designing the experiment. The relative density of the compacted compound increased with the detonation velocity. To enhance the density of the compound further, however, it was necessary to adjust other variables associated with the can, powder, and the backup tube. From an X-ray diffraction analysis of the explosion-compacted compound, it was confirmed that formation of the Ti₅Si₃ phase was complete. Although there is a room for further improvement of the density and the crack resistance of the compacted alloy, the present work verified that explosion synthesis is a potentially viable method to consolidate intermetallic compounds.     

Crystal structure of the orthorhombic U2-Al4Mg4Si4 precipitate in the Al–Mg–Si alloy system and its relation to the β′ and β″ phases

The atomic structure of a common precipitate in the Al–Mg–Si system has been determined. It is isotypic with TiNiSi (space group Pnma) and contains four units of MgAlSi in a unit cell of size a = 0.675 nm, b = 0.405 nm, c = 0.794 nm. EDS analyses support the composition. A model was based on the atomic structure of the β′ precipitate, electron diffraction and high-resolution transmission electron microscopy (HRTEM) images. A quantum mechanical refinement of the model removed discrepancies between simulated and experimental diffraction intensities. Finally, a multi-slice least square refinement confirmed the structure. The structural relation with β″ is investigated. A similar Mg–Si plane also existing in β″ and β′, can explain most coherency relations between the precipitate phases and with matrix.     

Study of transformation behavior in a Ti–4.4 Ta–1.9 Nb alloy

An alloy of composition Ti–4.4 wt.% Ta–1.9 wt.% Nb is being developed as a structural material for corrosion applications, as titanium and its alloys possess excellent corrosion resistance in many oxidizing media. The primary physical metallurgy database for the Ti–4.4 wt.% Ta–1.9 wt.% Nb alloy is being presented for the first time. Determination of the β transus, M_s temperature and classification of the alloy have been carried out, employing a variety of microscopy techniques, X-ray diffraction (XRD), micro-hardness and differential scanning calorimetry (DSC). The β transition temperature or β transus determined using different experimental techniques was found to agree very well with evaluations based on empirical calculations. Based on chemistry and observed room temperature microstructure, the alloy has been classified as an + β titanium alloy. The high temperature β transforms to either ′ or + β by a martensitic or Widmanstatten transformation. The mechanisms of transformation of β under different conditions and characteristics of different types of have been studied and discussed in this paper.     

Microstructure and strength of brazed joints of Ti3Al-base alloy with NiCrSiB

Brazing of Ti₃Al alloys with the filler metal NiCrSiB was carried out at 1273–1373 K for 60–1800 s. The relationship of brazing parameters and shear strength of the joints was discussed, and the optimum brazing parameters were obtained. When products are brazed, the optimum brazing parameters are as follows: brazing temperature is 1323–1373 K, brazing time is 250–300 s. The maximum shear strength of the joint is 240–250 MPa. Three kinds of reaction products were observed to have formed during the brazing of Ti₃Al alloys with the filler metal NiCrSiB, namely, TiAl₃ (TiB₂) intermetallic compounds formed close to the Ti₃Al alloy. TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds layer formed between TiAl₃ (TiB₂) intermetallic compounds and the filler metal and a Ni[s,s] solid solution formed in the middle of the joint. The interfacial structure of brazed Ti₃Al alloy joints with the filler metal NiCrSiB is Ti₃Al/TiAl₃ (TiB₂)/TiAl₃+AlNi₂Ti (TiB₂)/Ni[s,s] solid solution/TiAl₃+AlNi₂Ti (TiB₂)/TiAl₃ (TiB₂)/Ti₃Al, and this structure will not change with brazing time once it forms. The formation of over many intermetallic compounds TiAl₃+AlNi₂Ti (TiB₂) results in embrittlement of the joint and poor joint properties. The thickness of TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) in the brazed joints of Ti₃Al alloys with the filler metal NiCrSiB are 349 kJ/mol and 24.02 mm²/s, respectively, and the growth formula was y²=24.04exp(−41977.39/T)t. Careful control of the growth of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) can influence the final joint strength.     

The mechanism of order-induced intrinsic embrittlement in a stoichiometric Ni4Mo alloy by TEM and 3DAP characterization

To illustrate the mechanism of order-induced intrinsic embrittlement in a stoichiometric Ni₄Mo alloy, TEM and 3DAP were employed to investigate the phase separation during ordering in this paper. It showed that the atomic ordering initiated homogeneously, but some oriented ordered domains can grow preferentially later. Therefore, with atomic ordering, the average ordered domain size continues to increase, which improves the yield strength and ultimate strength due to increasing the critical shear stresses. However, except the growth of ordered phase, different phases with enriched molybdenum and depleted molybdenum were formed after ordering. The depleted molybdenum phase gradually reduces the Mo composition, which deteriorates the ultimate strength, and meantime the strength of grain boundary does not enhance, or even weakens. Hence, the atomic ordering induces embrittlement.     

Densification of In2O3:Sn multilayered films elaborated by the dip-coating sol-gel route

Indium Tin Oxide (ITO) thin films have been deposited by the Sol-Gel Dip-Coating technique, the starting solutions being prepared from chlorides. These multilayered films were crystallized by means of a classical heat treatment at temperatures ranging from 500 to 600 °C. Five stacked layers are necessary to obtain a global electrical resistivity value of 2.9×10⁻³ Ω cm, for 500 °C annealed film. The paper focuses on the study of the structure of such multilayered deposits, and on the densification process, using transmission electron microscopy, Rutherford Back-scattering Spectrometry and electrical resistivity measurements. This analysis reveals structural inhomogeneities and different crystallite growth processes as a function of annealing temperature and number of deposited layers.     

Formation of nanocrystallized and amorphous structures in the cavitation-eroded surface for Ti3Al–Nb alloy

Cavitation erosion of Ti₃Al–Nb alloy was investigated by using rotating disc equipment. It is showed that Ti–24Al–15Nb–1Mo alloy has excellent cavitation erosion resistance. The cavitation-eroded surface was observed by TEM and nanocrystallized and amorphous structures were found in some zones. On the basis of experimental results and analyses, the impacts of microjets can cause plastic deformation in the specimen surface layer. Because of the transient high temperature produced by the collapse of bubbles, some deformed zones are recrystallized and a nanocrystallized structure is formed. The nanocrystallized structure with lower melting point is then molten by transient high temperature and quickly solidified in water, which produces an amorphous structure.     

β to ω phase transformation due to aging in a Ti–Mo alloy deformed in impact compression

We investigated the roles of vacancies and their clusters introduced in a Ti–20mass% Mo alloy by high-speed compression in the formation of aged ω-phase crystals. Specimens were deformed by a static compression mode and a high-speed compression mode, and were then aged. The relationships between morphology of aged ω-phase crystals and deformation modes are discussed along with the roles of vacancies and their clusters in the nucleation and growth of aged ω-phase crystals. Aged ω-phase crystals were found to be smaller but of higher density in a high-speed deformation specimen. These results suggest that vacancies and their clusters easily become nucleation sites of aged ω-phase crystals. Several aged ω-phase crystals in a high-speed deformation specimen were of string-like shape. High-resolution electron microscopy confirmed that the string-like crystals have the ω-phase crystal structure. One of the roles of vacancies of and their clusters introduced by high-speed deformation is considered to be relief of compressive stress, which is predicted to arise in the course of transformation.     

Growth kinetics of Mo3Si layer in the Mo5Si3/Mo diffusion couple

The diffusion processes and growth kinetics of the Mo₃Si silicide layer occurring at annealing the Mo₅Si₃/Mo diffusion couple between 1180 and 1800 °C were studied by electrothermography. The experimental results are supplemented with calculations on the behaviour of the initial Mo₅Si₃/Mo diffusion couple on the basis of the reaction diffusion model describing the transformation of the Mo₅Si₃ layer into a Mo₃Si one. The values of the parabolic growth constant for Mo₃Si layer were determined and the silicon diffusion coefficient in the Mo₃Si phase was calculated: D = 0.165 exp[(− 247 ± 10) / RT], cm²/s, where the activation energy is expressed in terms of kJ/mol.     

Changes in the real structure and magnetoresistance of Co90Fe10/Cu and Co90Fe10/Cu85Ag10Au5 multilayers after annealing

Annealing of the (1.1 nm Co₉₀Fe₁₀/2.2 nm Cu)×20 and (1.1 nm Co₉₀Fe₁₀/2.2 nm Cu₈₅Ag₁₀Au₅)×20 multilayers at 235 °C improved their magnetoresistance as compared to the virgin samples. Annealing at higher temperatures resulted in degradation of the magnetoresistance effect. This observation raised the motivation of a detailed structural study using small-angle X-ray scattering, wide-angle X-ray diffraction, electron diffraction and transmission electron microscopy with the aim to link the structural changes in the system to the changes in the magnetoresistance. The structure studies have shown that the maximum of the magnetoresistance observed after annealing at 235 °C is related to the separation of Co₉₀Fe₁₀ and Cu, which are partly intermixed at interfaces after the deposition process. The decay of the GMR effect at higher annealing temperatures is caused by an increase of the interface roughness, which led in the Co₉₀Fe₁₀/Cu multilayers to occurrence of non-continuous interfaces and to short-circuiting of magnetic layers. In the Cu₈₅Ag₁₀Au₅ multilayers, the combination of small-angle X-ray scattering and wide-angle X-ray diffraction has shown that Cu₈₅Ag₁₀Au₅ did not form an alloy with the nominal composition: Only a part of Au and Ag was dissolved in the copper structure; the remainder of Ag and Au formed precipitates.     

Microstructure of Al2O3 nanocrystalline/cobalt-based alloy composite coatings by laser deposition

Composite coatings, made of nano-Al₂O₃/cobalt-based alloy, produced by a 5-kW CO₂ laser on Ni-based superalloy were investigated. The coatings were examined to reveal their microstructure using optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction instrument. The results showed that some equilibrium or non-equilibrium phases, such as γ-Co, Cr₂₃C₆, CoAl₂O₄, Al₂O₃ and ε-Co existed in the coatings. Fine and short dendritic microstructure and columnar to equiaxed transition occurred by adding nano-Al₂O₃ particle. With the increase of nano-materials (1%, mass fraction), fully equiaxed crystallization appeared. These were contributed to that nano-Al₂O₃ particles acted as new nucleation site and rapid solidification of the melted pool. The results also showed inhomogeneous dispersion of nano-Al₂O₃ that resulted in the formation of ε-Co phase in the coatings. The sub-microstructure of the clad was stacking fault. The mechanism of formation of equiaxed grains was also analyzed.     

A study on thermal emission of charges at Si3N4—GaAs interfaces after annealing in N2 and N2 + H2 mixtures

Si₃N₄—GaAs interfaces subjected to annealing in N₂ + H₂ mixture or pure N₂ atmosphere were investigated by a small-signal charge deep-level transient spectroscopy (Q-DLTS) method. The method measures the physical parameters of selective populations of the interface traps continuum. A dependence of the capture cross-section on activation energy was constructed for the continuum of interface states at the Si₃N₄—GaAs interface. The dependence shows an exponential character in the part of the gap ranging from 0.3 to 1.0 eV below the conduction band minimum. It was found that annealing in the temperature interval 400–450 °C reduces the zero-bias band bending by about 0.1 eV. At temperatures of 500 °C and more, degradation of the interface started; compared with annealing in pure N₂ ambient, annealing in an N₂ + H₂ mixture degraded the interface slightly more.     

烧结温度对(Ti5Si3+TiC+TiB)/Ti复合材料组织和力学性能的影响

为了获得性能优异的钛基复合材料和解决单一增强相对性能提升有限等问题,以Ti粉、SiC粉、TiB₂粉、C粉为原料,采用粉末冶金法,在不同烧结温度下原位自生制备了(Ti₅Si₃+TiC+TiB)/Ti复合材料。通过XRD、SEM、万能试验机等设备表征了复合材料的微观组织和力学性能。结果表明:随烧结温度的升高,复合材料的致密度提高,平均晶粒尺寸逐渐增大;烧结温度的升高使增强相数量增加的同时减少了较低烧结温度下的团聚现象。复合材料的洛氏硬度、屈服强度、抗拉强度随烧结温度的升高先增大后减小,断裂应变下降不显著。在1 300 ℃下,(Ti₅Si₃+TiC+TiB)/Ti具有最佳的综合力学性能,烧结态试样的抗压强度达到最高2 435 MPa,屈服强度1 649 MPa,洛氏硬度49.1HRC,断裂应变28.7%。分析可知,微米尺寸的TiC、TiB和亚微米尺寸的Ti₅Si₃增强相的协同作用在显著提高复合材料强度的同时也保持了一定的塑性。(Ti₅Si₃+TiC+TiB)/Ti复合材料的增强方式以细晶强化、弥散强化和载荷传递强化为主。     

Fabrication of low-resistivity and gold-colored TiN films by halide chemical vapor deposition with a low [NH3]/[TiCl4] flow ratio

This work describes the preparation of titanium nitride (TiN) films on Si (111) substrates by atmospheric pressure halide chemical vapor deposition (AP-HCVD). Various TiN films were obtained by exploiting TiCl₄ + NH₃ gas chemistry with flow ratios [NH₃]/[TiCl₄] from 0.2 to 1.4, and deposition temperatures (T_d) from 600 to 900 °C. When T_d = 800 °C gold-colored films with electrical resistivities of under 100 μΩ cm were formed at almost all of the investigated [NH₃]/[TiCl₄] flow ratios. In particular, a lowest resistivity of about 23.7 μΩ cm, which is quite close to that of bulk TiN, was achieved using an [NH₃]/[TiCl₄] flow ratio of 0.3. Atomic force microscopy indicated that the root mean square surface roughness of that film was only about 5.1 nm. Under the same [NH₃]/[TiCl₄] flow ratio as above, X-ray diffraction analyses revealed the presence of a cubic TiN phase with a preferred orientation of (200) for T_d ≤ 800 °C, while additional (111) and (220) orientations emerged when the film was deposited at 900 °C. In conclusion, a low resistivity (< 100 μΩ cm) TiN film can be formed by AP-HCVD with very low [NH₃]/[TiCl₄] flow ratios 0.3-1.4.     

Fluorescence line narrowing study of Cr ions in cordierite glass nucleating MgAl2O4 nanocrystals

The luminescence of Cr³⁺ ions in cordierite glass nucleating MgAl₂O₄ nanocrystallites has been investigated. The time resolved fluorescence line narrowing measurements and the temperature dependence of the homogeneous line width show that most Cr³⁺ ions are inside the nanocrystallites. Cr³⁺ ions occupy non-equivalent crystal sites, due to the Mg²⁺–Al³⁺ inversion effect. The values of the homogeneous line width compare well with those of previous studies in crystals. No surface effect has been observed due to the large size of the crystals.     

Raman study of stress effect on Ge nanocrystals embedded in Al2O3

Ge nanocrystals (NCs) embedded in Al₂O₃ were grown by RF-sputtering. X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy techniques were used to characterize the stresses on the NCs. While small NCs (< 10 nm) have been observed to be spherical and fully relaxed in the matrix, the larger ones (> 17 nm) demonstrated a compressive stress effect. This could be linked to the crystal structure of the adjacent Al₂O₃ matrix.