STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY I. Microstructure Evolution in the Transition Region

对Ti--43Al--3Si (原子分数, %) 合金在3---100 μm/s 的生长速度下进行了系统的定向凝固实验. 研究了生长速度对固/液界面形态及初始过渡区组织演化规律的影响. 合金在3---60 μm/s 的生长速度范围内均以胞晶形态生长, 胞晶间距随着生长速度的增大而减小; 当生长速度达到90 μm/s 时, 开始出现枝晶生长. 在定向凝固初始启动阶段, 存在清晰的热过渡区, 热过渡区内Ti₅Si₃ 相分布及过渡区组织与定向凝固区组织的关联性对于籽晶材料的引晶效果有重要影响. 生长速度在10 μm/s 以内时, 热过渡区内Ti₅Si₃ 相分布连续, 且热过渡区组织与定向凝固区组织的关联性好, 有利于该合金的引晶.     

Mechanical milling of Ti–Ni–Si filler metal for brazing TiAl intermetallics

The Ti–Ni–Si filler metal was manufactured by mechanical milling of TiH₂, Ni and Si powder mixture. The microstructure of the filler metal and TiAl brazed joint was analyzed by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The effect of milling time on the brazing powder was investigated. It was found that NiSi phase formed when the milling time exceeded 120 min. The typical microstructure of the TiAl brazed joint using Ti–Ni–Si filler metal was TiAl/Ti₃Al/TiAlNi₂/Ti₃Al + Ti₅Si₃/TiAlNi₂/Ti₃Al/TiAl. The effect of Si on the microstructure was investigated and the result suggested that Si addition resulted in the aggregation of Ti and formation of Ti₃Al phase in the middle of joint. The optimal parameters were brazing temperature of 1140 °C and holding time of 30 min. The fracture was brittle and propagated between the TiAlNi₂ layer and Ti₃Al + Ti₅Si₃ layer.     

Microstructure,high-temperature deformability and oxidation resistance of a Ti5Si3-containing multiphase MoSiBTiC alloy

A Ti₅Si₃-containing multiphase MoSiBTiC alloy with a composition of 38Mo–30Ti–17Si–10C–5B (at.%) was designed and produced by arc-melting. The alloy was composed of five phases—Mo solid solution (Mo_ss), Mo₃Si, Mo₅SiB₂ (T₂), Ti₅Si₃ and TiC—and consistently has good thermal stability at least up to 1700 °C. The density of the alloy was ∼7.0 g/cm³, which is considerably smaller than that of Ni-base superalloys. Microstructure was carefully examined and microstructural anisotropy was confirmed. The anisotropy was considered to be generated by thermal gradient during the solidification process. Microcracking was remarkable across the primary Ti₅Si₃ phase, which was caused by thermal expansion anisotropy of the Ti₅Si₃ phase. High-temperature deformability was examined by high-temperature compression tests at 1500 °C. Two kinds of loading axes were chosen for the compression tests with respect to the microstructural anisotropy. The alloy exhibited a peak stress of 450–550 MPa, followed by good deformability at the testing temperature. Microstructure refinement and reduction in microcrack density were observed after hot working. Oxidation tests were conducted on the alloy at 1100 °C and 1300 °C for 24 h. The oxidation curves demonstrated that rapid mass loss finished within several minutes. After that, the mass loss began to slow down and then the specimens' mass decreased almost linearly with increasing testing time. Cross-section observation indicated that oxygen propagated through Mo_ss, whereas T₂ and Ti₅Si₃ phases acted as barriers against oxygen attack during the tests. In addition, it was found that the alloy gained better oxidation resistance after high-temperature deformation, suggesting a positive effect of phase refinement on its high-temperature oxidation resistance.     

Joining of Cf/SiC Composite and TC4 Using Ag-Al-Ti Active Brazing Alloy

Carbon fiber reinforced SiC (C_f/SiC) composite was successfully joined to TC4 with Ag-Al-Ti alloy powder by brazing. Microstructures of the brazed joints were investigated by scanning electron microscope, energy dispersive spectrometer, and x-ray diffraction. The mechanical properties of the brazed joints were measured by mechanical testing machine. The results showed that the brazed joint mainly consists of TiC, Ti₃SiC₂, Ti₅Si₃, Ag, TiAl, and Ti₃Al reaction products. TiC + Ti₃SiC₂/Ti₅Si₃ + TiAl reaction layers are formed near C_f/SiC composite while TiAl/Ti₃Al/Ti + Ti₃Al reaction layers are formed near TC4. The thickness of reaction layers of the brazed joint increases with the increased brazing temperature or holding time. The maximum room temperature and 500 °C shear strengths of the joints brazed at brazing temperature 930 °C for holding time 20 min are 84 and 40 MPa, respectively.     

Phase evolution and dendrite growth in laser cladding of aluminium on zirconium

Aluminium was laser clad on a pure zirconium substrate using the blown powder method. The microstructure across the laser-clad coating was studied. Starting from the bottom to the top surface of the coating, a series of phase evolutions had occurred: (Zr) → (Zr) + AlZr₂ + AlZr₃ → Al₄Zr₅ + Al₃Zr₂ → Al₃Zr₂ + AlZr₂ → Al₂Zr → Al₂Zr + Al₃Zr. This resulted in an epitaxial columnar crystal growth at the re-melt substrate boundary, a band of backward growth Al₃Zr₂ dendrites towards the lower half of the coating, and a two-phase eutectic dendritic growth of Al₂Zr + Al₃Zr towards the top of the coating. The evolution of the various phases and microstructures is discussed in conjunction with the Al-Zr phase diagram, the criteria for planar interface instability, and the theory of eutectic growth under rapid solidification conditions (the TMK model).     

Phase equilibria in the Ti-rich corner of the Ti-Si-Ga system

Phase relations in the ternary Ti-Si-Ga system have been established experimentally by means of a study of alloy samples in the as-cast condition and annealed at 1350 °C. The alloys were prepared by arc melting. The investigation was carried out using physico chemical methods of analyses (metallography, X-ray powder diffraction, differential thermal analysis, and electron probe microanalysis over a limited composition range with samples containing less than 38 at.% Ga and more than 62 at.% Ti. Liquidus and solidus surface projections, the isothermal section at 1350 °C, and the isopleth at 68 at.% Ti are presented. Three surfaces of primary crystallization of phases have been established: extended ones for Ti₅(Si,Ga)₃ and β (Ti-base solid solution) and a narrow one of Ti₂Ga. The monovariant curves separating these are due to the eutectic reactions L↔β+Ti₅(Si,Ga)₃ and L↔β+Ti₂Ga and to the L+Ti₅(Si,Ga)₃↔Ti₂Ga peritectic reaction. The three-phase region (β+Ti₅(Si,Ga)₃+Ti₂Ga) results from the four-phase eutectic reaction L↔β+Ti₅(Si,Ga)₃+Ti₂Ga. The composition of the ternary eutectic point E and the compositions of the coexisting solid phases have been determined. The solubilities of Si in the gallides, and of Ga in Ti₅Si₃ and of both the elements in Ti are given.     

Phase equilibria in the Ti-rich corner of the Ti-Si-Ga system

Phase relations in the ternary Ti-Si-Ga system have been established experimentally by means of a study of alloy samples in the as-cast condition and annealed at 1350 °C. The alloys were prepared by arc melting. The investigation was carried out using physico chemical methods of analyses (metallography, X-ray powder diffraction, differential thermal analysis, and electron probe microanalysis over a limited composition range with samples containing less than 38 at.% Ga and more than 62 at.% Ti. Liquidus and solidus surface projections, the isothermal section at 1350 °C, and the isopleth at 68 at.% Ti are presented. Three surfaces of primary crystallization of phases have been established: extended ones for Ti₅(Si,Ga)₃ and β (Ti-base solid solution) and a narrow one of Ti₂Ga. The monovariant curves separating these are due to the eutectic reactions L↔β+Ti₅(Si,Ga)₃ and L↔β+Ti₂Ga and to the L+Ti₅(Si,Ga)₃↔Ti₂Ga peritectic reaction. The three-phase region (β+Ti₅(Si,Ga)₃+Ti₂Ga) results from the four-phase eutectic reaction L↔β+Ti₅(Si,Ga)₃+Ti₂Ga. The composition of the ternary eutectic point E and the compositions of the coexisting solid phases have been determined. The solubilities of Si in the gallides, and of Ga in Ti₅Si₃ and of both the elements in Ti are given.     

Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

Role of nitrogen on the oxidative stability of Ti5Si3 based alloys at elevated temperature

Ti₅Si₃ has been extensively studied as a candidate material for high temperature application due to its high melting point (2130 °C), low density (∼4.3 g/cm³) and excellent oxidation resistance in oxygen above 1000 °C. However, stoichiometric Ti₅Si₃ alloy experiences accelerated oxidation during exposure in air above 1000 °C. It was proposed that nitrogen was responsible for the increased oxidation in air. In the present study, the isothermal reaction kinetics of Ti₅Si₃ in nitrogen at 1000 °C was investigated. Compared to a slow parabolic oxidation rate in oxygen, a faster linear reaction rate was observed when Ti₅Si₃ is exposed to nitrogen. Further studies on the oxidation behavior for changing nitrogen/oxygen atmospheres showed that Ti₅Si₃ is stable for exposure up to 400 h at 1000 °C when the gas contained 50% N₂. Breakaway oxidation occurs after short exposures when the gas contained at least 75% N₂, and the reaction rate increased as the concentration of N₂ increased. Furthermore, time to breakaway oxidation decreases with the increasing nitrogen partial pressure. Extensive analysis of the oxidation products with SEM and XRD revealed that the formation and fast growth of a nitride-containing subscale interferes with the establishment of the continuous protective silica scale and contributes to the breakaway oxidation.     

STUDY OF MICROSTRUCTURES IN γ/γ'--αMo DIRECTIONAL EUTECTOID ALLOY INDUCED BY LASER RAPID MELTING SOLIDIFICATION

对γ/γ'--αMo定向共晶合金进行激光快凝实验. 结果表明, 激光快凝后,合金组织细化, 偏析减少, 硬度显著提高. 在激光脉冲平行于定向共晶生长方向时,熔凝区重新生长出更细密的定向共晶组织; 激光脉冲垂直于定向共晶生长方向时, 熔凝区由更致密的胞状晶和枝晶组成. 在熔凝区的枝晶组织中, 枝晶臂由γ固溶体及从该固溶体析出的细小γ'相组成; 枝晶间由含有γ'的γ固溶体和富Mo亚稳相的共晶组成, 亚稳相包括[αMo_(ordered)], Ni₃Mo和[Ni₃Mo_(ordered)].     

CRYSTALLIZATION PROCESS AND NON–ISOTHERMAL CRYSTALLIZATION KINETICS OF MELT–SPUN Nd–Fe–B AMORPHOUS THICK RIBBONS

利用熔体快淬法在12 m/s的辊速下制备了Nd₆Fe₇₂B₂₂和Nd₆Fe₆₈Ti₄B₁₇C₅非晶厚带. 通过DSC和XRD, 并借助Kempen模型和 Kissinger方程, 研究了合金的非晶晶化过程及非等温晶化动力学. 结果表明, 两种合金厚带具有不同的晶化过程以及晶化动力学机制. Nd₆Fe₇₂B₂₂合金的晶化过程分为三步完成: 非晶
相 (AP)→ Nd₂Fe₂₃B₃→Nd₂Fe₁₄B+ α--Fe +Fe₃B→Nd₂Fe₁₄B+α--Fe+Fe₃B+NdFe₄B₄, 而Nd₆Fe₆₈Ti₄B₁₇C₅ 合金一步完成晶化: AP→Nd₂(Fe, Ti)₁₄(B, C)+α--Fe + Fe₃B. 与Nd₆Fe₇₂B₂₂合金由界面控制的多晶型晶化不同, Nd₆Fe₆₈Ti₄B₁₇C₅合金以扩散控制的共晶型晶化为主.     

700 °C Isothermal Section of the Al-Ti-Si Ternary Phase Diagram

The 700 °C isothermal section of the Al-Ti-Si ternary phase diagram has been determined experimentally by means of scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray powder diffraction. Fourteen three-phase regions have been determined experimentally in the isothermal section at 700 °C. The ternary phases τ₁ (I4₁/amd, Zr₃Al₄Si₅-type) and τ₂ (Cmcm, ZrSi₂-type) are confirmed in the system at 700 °C. The compositions of τ₁ and τ₂ are found as Al_6.2-9.3Ti_32.4-34.0Si_57.5-60.9 and Al_10.0-11.6Ti_34.2-34.5Si_53.9-55.6, respectively. The τ₃ and Ti₃Al₅ phases are not found in the section. The Ti-rich corner at 700 °C shows the presence of three three-phase equilibriums, i.e., (TiAl + Ti₃Al + Ti₅Si₃), (α-Ti + Ti₃Si + Ti₅Si₃) and (α-Ti + Ti₃Al + Ti₅Si₃). The maximum solubility of Al in Ti₅Si₃, Ti₃Si and α-Ti is 6.0, 1.5 and 13.9 at.% at 700 °C, respectively. The maximum solubility of Si in L-Al, TiAl₃, TiAl₂, TiAl, Ti₃Al and α-Ti is 24.1, 13.6, 1.5, 0.8, 2.3 and 2.3 at.%, respectively.     

Microstructure and properties of Ti–Co–Si ternary intermetallic alloys

Ti–Co–Si ternary intermetallic alloys with Ti₅Si₃ as the main reinforcing phase and intermetallic TiCo as the toughening matrix were fabricated by the laser-melting deposition (LMD) process. Microstructure of the intermetallic alloys was characterized by OM, SEM, XRD and EDS. High-temperature oxidation resistance of the alloys was evaluated by isothermal oxidation at 1173 K and metallic dry-sliding wear property was evaluated at room temperature. The effect of reinforcing phase Ti₅Si₃ content on hardness, oxidation and wear resistance of the alloys was investigated. Results indicate that microstructure of the alloys transforms from hypoeutectic to hypereutectic, while hardness and oxidation resistance increases with the increasing Ti₅Si₃ content. The alloys have good oxidation resistance at 1173 K and the oxidation kinetic curves are approximately parabolic. Wear resistance of the alloys is insensitive to the microstructure and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr under dry-sliding wear test conditions. The excellent wear resistance of alloys is attributed to the effective reinforcement of Ti₅Si₃ and the excellent toughness of the intermetallic TiCo.     

Optimization of the Ti3SiC2 MAX phase synthesis

The synthesis of Ti₃SiC₂ MAX phase by self-propagating high-temperature synthesis (SHS) and pressureless argon shielding synthesis has been investigated following different pathways pertaining to the reactant systems Ti/Si/C, Ti/SiC/C and Ti/TiC/Si. Silicon in excess ranging from 10 to 50 mol% was employed to obtain powders mainly constituted by Ti₃SiC₂.Optimizing the excess of silicon and the pressing technique, the resultant powders with Ti₃SiC₂ content near to 100% were obtained. Result was consequent to the use of pressureless argon shielding synthesis obtained with 30 mol% of silicon excess in the examined different systems. The Ti₃SiC₂ was also obtained by SHS, but with lower proportion (88% and 86% from 3Ti + 1.2SiC + 0.8C and 3Ti + 1.3Si + 2C respectively). These results driving from XRD patterns were confirmed by FESEM observations and the EDAX analyses.     

Rapid self-sustaining consolidation of titanium silicide (Ti5Si3) via transient liquid phase reaction induced by an electric discharge

The fabrication of Ti₅Si₃ in the form of a solid product directly from an elemental 37.5 at.% Si and 62.5 at.% Ti powder mixture was carried out by two different powder metallurgy routes. The first was by uniaxial pressing of the reactant powder mixture with sequent vacuum-sintering, and the second was by electric discharge sintering (EDS) of reactant powder mixture. The pressing process combined with vacuum-sintering produced a porous compact with multi phases of titanium silicide such as Ti₅Si₃, Ti₅Si₄, TiSi₂, and TiSi, including elemental Ti, which indicated an incomplete phase transformation into Ti₅Si₃. On the other hand, the EDS induced the phase transformation mostly into Ti₅Si₃ with a small amount of Ti₅Si₄ in <180 μsec, which had a sequent consolidation into a solid compact with an average crystallite size of 30.4 nm and a lattice parameter of a = 7.42 Å and c = 4.91 Å. The significantly higher hardness value of the EDS compacts can be the result of the high density and the fine microstructure stemming from the homogeneous dissolution of the elements and the constrained grain growth. The formation of Ti₅Si₃ solid compact from the stoichiometric Ti and Si powder mixture by EDS can be dominated by the solid to liquid phase transformation mechanism.     

Experimental phase diagram of the Ti-Si-Sn ternary system at 473 K

The phase diagram of the ternary system Ti-Si-Sn system at 473 K was investigated by means of powder X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscope (SEM) with energy dispersive analysis (EDX). The isothermal section consists of 14 single phase regions, 26 binary phase regions and 13 ternary phase regions. The 10 binary compounds, namely Ti₃Si, Ti₅Si₃, Ti₅Si₄, TiSi, TiSi₂, Ti₃Sn, Ti₂Sn, Ti₅Sn₃, Ti₆Sn₅, Ti₂Sn₃, have been confirmed at 473 K. Moreover, a ternary phase with the crystal structure of tetragonal W₅Si₃ structure type and I4/mcm space group is confirmed in the Ti-Si-Sn system. The combined results of both EDX and XRD show that the composition of this ternary phase is 60.25-61.03 at.% Ti, 15.01-21.77 at.% Si and balance Sn.     

The microstructure evolution of directionally solidified Nb-22Ti-14Si-4Cr-2Al-2Hf alloy during heat treatment

To obtain a homogenous and optimizing microstructure, non-equilibrium directionally solidified (DS) samples at high withdrawal rate were heat treated (HT). The heat treatments were carried out at 1500 °C for 3–100 h. The DS and HT samples consisted of Nb_SS, α-Nb₅Si₃ and γ-Nb₅Si₃, and the DS morphology altered significantly after heat treatment. The well developed Nb_SS dendrites were compromised and the size decreased from more than 100 μm to less than 60 μm when the heat treated time was 6 h. On the other hand, the Nb_SS in eutectic interconnected and meanwhile connected with the dendrites, and the frequency of small size got lower with the heat treated time increasing. When heat treated for 100 h, the size of Nb_SS was comparatively uniform and concentrate in the region of 5–20 μm with a value of 63% for Ostwald ripening. The dendric and eutectic morphologies exited in the DS sample disappeared, turning into a net-work structure. The components of primary Nbss dendrites and Nbss in the intercellular regions were homogenized after heat treated for 3 h. However the enrichment of Ti, Cr and Hf in γ-Nb₅Si₃ compared to α-Nb₅Si₃ could not be eliminated by homogenizing treatment even for 100 h.     

Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the αNb₅Si₃ and βNb₅Si₃ is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb–24Ti–18Si–5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 °C) alloys were the niobium solid solution, (Nb,Ti)_ss, the niobium 5–3 silicides, αNb₅Si₃ and βNb₅Si₃, and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of βNb₅Si₃, while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the βNb₅Si₃ phase transformed to αNb₅Si₃ according to the reaction βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss. The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 °C in the two Cr-rich alloys. Ta and Cr retard the βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti)_ss, αNb₅Si₃ and the C14 silicide Laves phase, in the Nb–24Ti–18Si–6Ta–8Cr–4Al alloy.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.