12%Cr合金凝固过程夹杂物析出的实验研究与热力学分析

铁基12%Cr合金常用来制造先进发电机转子,这要求材料具有良好的性能和细小的夹杂物,合金中的大尺寸夹杂物会导致材料性能恶化。该研究采用了缓冷实验后淬火的工艺,从而将12%Cr合金熔体冷却与凝固过程中析出的夹杂物保留下来,并分别采用扫描电镜和能谱,对夹杂物的形貌及化学成分进行了观察和测定。实验结果表明,除各种形状的氧化物夹杂外,在试样中也发现了氮化钛夹杂,但在熔炼的过程中却并没有钛元素的加入。对凝固过程夹杂物的析出进行热力学计算,结果表明氮化钛夹杂析出与凝固阶段的末期。即便是痕量来自原料中的钛元素也可以导致氮化钛的析出。这些氮化钛在凝固时能够生长至大尺寸并损害材料性能。与此同时,氧化铝夹杂在液态熔体中即可生成。依据计算结果,对钛和氧的含量控制给出建议,这将对12%Cr合金生产过程起到帮助。通过研究钛和铝之间的浓度竞争,本文也讨论了实验过程中三氧化二钛夹杂析出的可能性。     

Oxidation resistance and thermal stability of Ti(C,N) and Ti(C,N,O) coatings deposited by chemical vapor deposition

Ti_0.51C_0.24N_0.25 and Ti_0.51C_0.26N_0.21O_0.02 coatings were deposited by medium temperature chemical vapor deposition, their oxidation resistance and thermal stability at 600 °C were compared. Ti(C,N,O) coating is oxidized faster than Ti(C,N) coating. The oxidation resistance of Ti(C,N,O) coating is impaired not only by the existence of more pores, fissures and cracks in the oxide layer, but also by lower interfacial adhesion. Higher tensile stress has a negative impact on the oxidation resistance of Ti(C,N,O) coating. It promotes the formation and development of more defects and cracks during cyclic oxidation, it is also harmful to the interfacial adhesion. Although the oxidation resistance of Ti(C,N,O) coating is inferior, the thermal stability is improved. The incorporation of oxygen slows down the phase transformation of Ti(C,N,O) at 600 °C. During the annealing, the hardness of Ti(C,N,O) coating is always higher than that of Ti(C,N).     

Experimental investigation on tool wear characteristics of PVD and CVD coatings during face milling of Ti6242S and Ti-555 titanium alloys

Ti6242S and Ti-555, as two typical titanium alloys, are often used to manufacture high-temperature aeroengine parts and landing gear components, respectively. They have different chemical composition and microstructure, which make them have different mechanical properties, and also affect their machinability. In this paper, face milling experiments were carried out to evaluate the wear performance by using CVD-Ti(C, N) + Al₂O₃ + TiN, PVD-(Ti, Al)N + TiN coated and uncoated tools. The results show that Ti-555 has the worse machinability than that of Ti6242S. When milling Ti6242S, all tools suffered adhesive wear and diffusion wear; the wear of Ti(C, N) + Al₂O₃ + TiN coated tool was more serious than that of other tools due to the blunt cutting edge; (Ti, Al)N + TiN coated tool suffered micro chipping and coating peeling with the minimal wear loss. When milling Ti-555, uncoated tool suffered serious chipping, abrasive wear and adhesive wear; Ti(C, N) + Al₂O₃ + TiN coated tool suffered serious chipping and coating peeling with short tool life; (Ti, Al)N + TiN coated tool suffered coating peeling, adhesive wear and diffusion wear. Overall, (Ti, Al)N + TiN coated tools have the longest tool life and are preferred for face milling of Ti6242S and Ti-555 titanium alloys.     

Oxidation Resistance of Ti5Si3 and Ti5Si3Zx at 1000°C (Z = C,N, or O)

The oxidation behavior of Ti₅Si_3+y (y=0 or 0.2) and Ti₅Si₃Z_x (Z=C, N or O, x=0.25 or 0.5) was studied at 1000°C in air or argon–oxygen mixtures for up to 500 h. Ti₅Si₃ has poor oxidation resistance in air because of the formation of an oxide scale rich in rutile and subscale formation of TiN, TiSi, TiSi₂ and Si. In contrast, Ti₅Si_3.2 has excellent oxidation resistance because of the formation of a silica scale. Samples with interstitial oxygen or nitrogen show only slight improvements in the early stages of oxidation, compared to Ti₅Si₃, which is in stark contrast to previous research. However, samples with interstitial carbon displayed excellent oxidation resistance at 1000°C, consistent with previous research.     

Dry sliding wear behavior of PVD TiN,Ti55Al45N,and Ti35Al65N coatings at temperatures up to 600 °C

Three PVD nitride coatings (TiN, Ti₅₅Al₄₅N, and Ti₃₅Al₆₅N) with different Al content were deposited on the cemented carbides by cathode arc-evaporation technique. Microstructural and fundamental properties of these nitride coatings were examined. The friction and wear behavior of these coatings were evaluated at temperatures up to 600 °C. The wear surface features of the test samples were examined by scanning electron microscopy. Results showed that the friction coefficient of these nitride coatings is different depending on the temperature. The friction coefficient of TiN coating increased with the increase of test temperature; while the friction coefficient of Ti₅₅Al₄₅N and Ti₃₅Al₆₅N coatings with the addition of Al decreased with the increase of test temperature. The Ti₅₅Al₄₅N and Ti₃₅Al₆₅N coatings exhibited higher wear resistance over the one without Al (TiN coating). The wear resistance of these nitride coatings at high temperature wear tests is significantly dependent on their tribological oxidation behavior. The Ti₅₅Al₄₅N and Ti₃₅Al₆₅N coatings with the addition of Al exhibited improved wear resistance as compared to the TiN coating, which was attributed to that their tribo-chemically formed Al₂O₃ exhibited better tribological properties than the TiO₂ of the latter.     

Effect of reacted compounds in Al2O3+Ti investment mold on alpha-case formation for Ti casting

This paper proposes a newly developed alpha-case controlled mold material for Ti castings. An Al₂O₃ mold containing alpha-case reaction compounds, titanium oxide (TiO₂, Ti₂O and Ti₆O) and titanium silicide (Ti₅Si₃) was manufactured via a reaction between Al₂O₃ and Ti powder under different firing conditions in air and a vacuum. In comparison with the Al₂O₃ and Al₂O₃+Ti mold fired in a vacuum, the micro-Vickers hardness and nano indentation profiles of Al₂O₃+Ti indicated that the alpha-case thickness was significantly reduced from 350 μm to ～45 μm. The alpha-case formation in the Al₂O₃+Ti mold was reduced due to the presence of TiO₂, which formed the TiO intermediate phase that acted as a diffusion barrier. In addition, Ti₅Si₃ was effective in minimizing Si contamination at the casting surface due to the reaction between Ti and the colloidal SiO₂ binder. Therefore, alpha-case reaction compounds, such as TiO₂ and Ti₅Si₃ in Al₂O₃, can effectively reduce alpha-case formation at the casting surface.     

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. TiC, TiN and TiC+TiN powder mixtures containing 50% CaF₂ flux were deposited on the surface of a Ti–6Al–4V alloy substrate, and irradiated by high-energy electron beam to form 1-mm-thick, defect-free surface composite layers. The surface composite layers contained a large amount (over 30 vol.%) of precipitates such as TiC, TiN, (Ti_xAl_1−x)N and Ti(C_xN_1−x) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification including the formation of hard precipitates and hardened matrices in the surface composite layers improved hardness and high-temperature hardness two to four times greater than that of the substrate. In particular, the surface composite fabricated with TiN powders had the highest hardness because of the highest volume fraction of TiN and (Ti_xAl_1−x)N distributed in the hardened N-rich acicular α-Ti matrix. These findings suggest that the (TiC,TiN)/Ti–6Al–4V surface composites can be used for structural materials requiring excellent thermal resistance.     

Cu + TiB2 composite filler for brazing Al2O3 and Ti-6Al-4V alloy

Al₂O₃ and Ti-6Al-4V alloy were brazed using Cu + TiB₂ composite filler, which manufactured by mechanical milling of Cu and TiB₂ powders. Typical interface microstructure of joint was Al₂O₃/Ti₄(Cu,Al)₂O/Ti₂Cu + Ti₃Al + Ti₂(Cu,Al)/Ti₂(Cu,Al) + AlCu₂Ti/Ti₂Cu + AlCu₂Ti + Ti₃Al + Ti₂(Cu,Al) + TiB/Ti(s.s) + Ti₂Cu/Ti-6Al-4V alloy. Based on temperature- and time-dependent compositional change, the formation of intermetallics in joint was basically divided into four stages: formation of interfacial Ti₄(Cu,Al)₂O in Al₂O₃ side, formation of Ti₂Cu, Ti₃Al, TiB, Ti₂Cu, and AlCu₂Ti in layers II and IV, formation of Ti₂(Cu,Al) and AlCu₂Ti in layer III, formation of Ti + Ti₂Cu hypereutectoid organization adjacent to Ti-6Al-4V alloy. TiB in situ synthesized in joint not only acted as low thermal expansion coefficient reinforcement to improve the mechanical properties at room temperature, but also as skeleton ceramic of joint to increase high temperature mechanical properties of Al₂O₃/Ti-6Al-4V alloy joint increasing. When the joint containing 30 vol.% TiB brazed at 930 °C and 10 min of holding time, the maximum room temperature shear strength of joint was 96.76 MPa, and the high temperature shear strength of joint was 115.16 MPa at 800 °C.     

Comparative studies on corrosion and tribological performance of multilayer hard coatings grown on WC-Co hardmetals

Multilayer TiN/TiCN/TiCN/TiC/TiN and TiN/TiCN/TiCN/TiC/Al₂O₃ hard coatings with total thicknesses of 15.7 μm and 9.3 μm were deposited on WC-10Co substrates using a chemical vapor deposition system. Evaluation of surface, cross-section morphologies, chemical composition and phases of coatings were analyzed by field emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) analyses respectively. Corrosion properties were evaluated in 3.5 wt% NaCl medium using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Tribological properties of fabricated multilayer hard coatings were evaluated using pin-on-disk tests. Results show that active dissolution of the WC-10Co occurred while the coated samples showed more anodic slopes as well as lower corrosion current densities. The corrosion current densities of 3.3 × 10⁻⁷ A/cm² and 7.5 × 10⁻⁸ A/cm² were obtained for the TiN/TiCN/TiCN/TiC/TiN and TiN/TiCN/TiCN/TiC/Al₂O₃ coated specimens which are much lower than 4 × 10⁻⁶ A/cm² of substrate. EIS analysis confirmed the results of potentiodynamic polarization curves. Delamination of the TiN coating and formation of titanium oxide compounds on the surface of the TiN/TiCN/TiCN/TiC/TiN coating revealed that oxidative wear mechanism is dominant for this sample, while adhesive wear mechanism was dominant for the TiN/TiCN/TiCN/TiC/Al₂O₃ coated sample.     

Structure and mechanical properties of arc evaporated Ti-Al-O-N thin films

The structure, mechanical properties, and machining performance of arc evaporated Ti-Al-O-N coatings have been investigated for an Al_0.66Ti_0.34 target composition and O₂/(O₂+N₂) gas flow-ratio varied between 0 to 24%. The coating structure was analysed using SEM, EDX, XRD, XPS, TEM, and STEM. Mechanical properties were analysed using nanoindentation and the deformation behaviour was analysed by probing the nanoindentation craters. The coatings performances in cutting tests were evaluated in a turning application in low carbon steel (DIN Ck45). It is shown that the addition of oxygen into the arc deposition process leads to the formation of a dual layer structure. It consists of an initial cubic NaCl-structure solid solution phase formed closest to the substrate, containing up to 35 at.% oxygen (O/O+N), followed by steady-state growth of a nanocomposite compound layer comprised of Al₂O₃, AlN, TiN, and Ti(O,N). The addition of oxygen increases the ductility of the coatings, which improves the performances in cutting tests. At high levels of oxygen, (> 13 at.%), however, the performance is dramatically reduced as a result of increased crater wear.     

Al_2O_3-TiC复合陶瓷与Q235钢扩散连接界面组织结构

采用Ti/Cu/Ti复合中间层通过液相扩散连接技术实现了Al2O3-TiC复合陶瓷与Q235低碳钢的扩散连接.采用扫描电镜、电子探针及X射线衍射等测试手段对Al2O3-TiC/Q235扩散连接接头的显微组织、断口形貌及相组成进行了分析.结果表明,Al2O3-TiC/Q235界面结合紧密,没有显微孔洞、裂纹及未连接区域;Al2O3-TiC/Q235界面附近有各种各样的新相生成,如TiO,Ti3Al,Cu2Ti4O及Cu3Ti3O,所生成的TiO相及复杂结构氧化物Cu3Ti3O和Cu2Ti4O都具有金属特性,对于促进Al2O3-TiC/Q235的可靠连接起到重要作用;接头抗剪强度达143MPa,断口表现为脆性断裂特征,Al2O3-TiC/Q235接头断在界面附近的Al2O3-TiC内.     

Theoretical calculations and experimental measurements of the structure of Ti5Si3 with interstitial additions

The equilibrium structural parameters, enthalpies of formation and partial densities of state for Ti₅Si₃ and Ti₅Si₃Z_0.5 (Z= B, C, N or O) were calculated based on first-principle techniques. Enthalpy of formation calculations suggest that of the known structures for transition metal (TM) silicide compounds containing TM₅Si₃ (D8₈, D8_l and D8_m) the D8₈ structure is the most stable form of Ti₅Si₃, and the stability of the structure increases as Z atoms are added. The theoretically determined structural trends as a function of interstitial element, Z, agreed well with experimentally determined values. Both indicate bonding between Ti and Z atoms based on contraction of Ti–Z separations. The calculated partial densities of state suggest that p(Si)–d(Ti) and d(Ti)-d(Ti) interactions are responsible for most of the bonding in pure Ti₅Si₃, which agrees with previous studies. As Z atoms are added, p(Z)–d(Ti) interactions become significant at the expense of weakening some of the d(Ti)–d(Ti) interactions.     

PTLPB of Si3N4 to FA-129 using nickel as a core interlayer

Partial transient liquid phase bonding was applied to a silicon nitride/iron aluminide alloy (FA-129) joint. This joint was composed of two independent joining systems. The joint configuration was Si₃N₄/75Cu–25Ti/Cu/Ni/Al/FA-129. The results demonstrate that homogenization has no effect on the kinetic of the reaction layer formation between the ceramic and the nickel core due to a previous dissolution reaction. The reaction layer composition evolves from a TiN–Ti₅Si₃ layer system after reaction during the first soaking time, to a single TiN layer after the complete cycle. The dissolution of the Ti₅Si₃ layer in the Ni core becomes the driving factor influencing the properties of the joint. The composition of the copper–nickel alloy interlayer changes with homogenization treatment, passing from a maximum of 70–20 wt% Cu, depending on the conditions. NiAl and Ni₃Al intermetallics were observed at the interface between the nickel and the FA-129 and their thicknesses varies with the homogenization conditions. The strength values obtained were within the range of 80 MPa, with the ceramic reaction layer being the region where all failures occurred.     

Nanostructure of wetting triple line in a Ag–Cu–Ti/Si3N4 reactive system

Nanometer scale structures around wetting triple lines were studied in a Ag–Cu–Ti/Si₃N₄ reactive system. Changes in the contact angle and radius of the molten metal on the substrate as a function of time were also measured in the system as macroscopic wetting behaviors. The macroscopic wetting behaviors showed two wetting stages and double layered reaction products consisting of upper Ti₅Si₃ and lower TiN layers were observed in both first and second stages. The reaction product always lay in front of the triple line defined as a triple junction of Ag–Cu–Ti alloy/Ti₅Si₃/atmosphere. At the front of the reaction product, a dominant phase changed from TiN in the first stage to Ti₅Si₃ in the second stage. It is considered that the structural change is one of the reasons why the macroscopic wetting behavior changed, and that the structural change was caused by a decrease of Ti activity as the reactive wetting progressed.     

Effect of micro-blasting on the tribological properties of TiN/MT-TiCN/Al2O3/TiCNO coatings deposited by CVD

The effect of micro-blasting on the tribological properties of TiN/MT-TiCN/Al₂O₃/TiCNO coatings was studied. The multilayer coatings were deposited on cemented carbides by chemical vapor deposition. The microstructure, mechanical and tribological properties were investigated using X-ray diffraction, scanning electron microscopy (SEM), nano-mechanical testing system, scratch tester and reciprocating tribometer. The results show that micro-blasting significantly reduces the surface roughness and converts the residual tensile stress of Ti(C,N,O) top-layer and Al₂O₃ layer into compressive stress. Affected by the residual compressive stress, the hardness and adhesion strength are increased. More importantly, the friction coefficient is decreased attributed to the decreased surface roughness and improved hardness. Also, the wear resistance of micro-blasted TiN/MT-TiCN/Al₂O₃/TiCNO is superior due to higher hardness of Ti(C,N,O) top-layer, Al₂O₃ layer and adhesion strength of coatings. Especially for the total sliding time of 2 h, the wear volume and wear rate of micro-blasted coatings are 69.4% of as-deposited coatings, because micro-blasting helps to increase the adhesion strength and micro-cracking resistance, which play important roles in the improvement of wear resistance. Micro-blasting has a positive effect on the friction and wear properties of TiN/MT-TiCN/Al₂O₃/TiCNO multilayer coatings since the adverse impact of top-layer thinning is offset.     

Microstructure and properties of CVD coated Ti(C,N)-based cermets with varying WC additions

In this paper, multilayer coatings of TiN/TiCN/Al₂O₃/TiN are deposited on the Ti(C, N)-based cermets containing WC, and the effect of WC on the growth and adhesion strength as well as the mechanical properties of the coating are investigated. The multilayer coatings deposited by chemical vapor deposition (CVD) are uniform and dense. TiN coating exhibits a dense fine-grained structures and the Ti (C,N) on TiN coating shows dense columnar structure. The α-Al₂O₃ layer deposited on transition coating presents coarse grains with limited voids. The grain size of the columnar crystals deposited on the substrates gradually decreases with WC addition. The Al₂O₃ layer shows a preferred growth orientation of (104) plane. For TiN/TiCN phase, a change in orientation from (111) to (200) is observed. Generally, the (200) preferred orientation enhances and (111) preferred orientation diminishes with increasing WC addition. Strong adhesion of the CVD coating is obtained due to a sufficient amount of chemical elements, especially tungsten, diffusing from the substrate to the interfacial layer. Scratch tests show that the adhesion strength of TiN/TiCN/Al₂O₃/TiN films gradually increases firstly, and then decreases. With the addition of WC, the hardness, elastic modulus and plasticity index increase at the beginning, and then decrease. The change in nanohardness and elastic modulus is related to the grain size, elemental diffusion, and preferred orientation of the coating.     

High-Temperature Oxidation of TiN-Ti5Si3 Composites Prepared by Polymer Pyrolysis

Composites consisting of a TiN matrix containing about 32 vol.% Ti₅Si₃ were synthesized via polymer pyrolysis, and oxidized in the temperature range of 700 to 900°C for up to 120 h in air. Oxidation occurred mainly in the TiN matrix, because Ti₅Si₃ was resistant to oxidation. The oxidation of the TiN matrix was preceded by the outward diffusion of Ti ions to form an outer TiO₂ layer, and the inward diffusion of oxygen ions to form an inner TiO₂ layer, together with nitrogen evolution from the TiN matrix.     

Combustion synthesis and development of Ti-O-C aluminium composites

Aluminium matrix composite reinforced with Ti compounds was successfully fabricated by SHS combustion synthesis and squeeze casting course. Prepared samples from mixture containing Ti, C and Al₂O₃ fibres were heated in microwave reactor to ignite synthesis and produce porous preform for subsequent infiltrating with liquid metal. Studies showed that synthesizing temperature has been remarkably increased by applying higher magnetron power and addition of graphite. Synthesis of specimens prepared from preliminary ball milled Ti and C powders proceeded at the highest propagation wave velocity. Microwave heating of metal Ti powder in the stream of CO₂ resulted in formation of corrugated precipitates composed of titanium oxide with carbon inclusions TiO(C) and Ti₂O₃. The produced preforms were impregnated by squeeze casting with the aluminium alloy AlSi7Mg. Proper interface with slight reduction of Ti oxide between the reinforcement and the matrix was developed. Subsequently, the samples were annealed at 500 and 1000 °C. Annealing at the lower temperature induced creation of Ti₃O₂(C) and Al₂O₃. This process was continued at 1000 °C, and additionally some Ti(Al_0,8Si_0,2)₃ pellets appeared in the matrix. With prolonged annealing, oxygen was completely removed from Ti compound and oval grains of Ti(C) were created, enveloped with Al₂O₃. In the matrix, larger and numerous Ti₃AlSi₅ pellets were formed. Hardness examination showed that the best strengthening effect was achieved after annealing at 1000 °C.     

液料等离子热喷法制备Fe3+/TiO2纳米粉末

通过对液料等离子热喷前驱物添加掺杂成分实现了液料等离子热喷TiO2纳米粉末的掺杂改性,并利用TEM,XRD及XPS对其进行表征.结果表明,采用液料等离子热喷法可以制备Fe3 掺杂TiO2纳米粉末,所制备粉末形貌基本呈球形或近球形,粒径分布为10～35 nm,掺杂量小于2.0%时粉末为锐钛矿及金红石相混晶,Fd3 掺杂促进锐钛矿向金红石相的转变,掺杂量为10.0%时析出了Fe2Ti3O9相.Fe3 掺杂不会引起TiO2粒径的大范围波动.粉末中含有O,Ti,Fe和C等元素,Fe元素在TiO2中仍为 3价.     

The mechanical milling of Al/TiO2 composite powders

The processing of Al/TiO₂ composite powders produced by high-energy mechanical milling leads to production of a range of valuable, titanium-based materials. They include Ti(Al,O)/Al₂O₃ and Ti_xAl_y(O)/Al₂O₃ composite powders, bulk composites and Ti₃Al/TiAl alloy powders, and corresponding bulk materials. The strength of the Ti(Al,O)/Al₂O₃ and Ti_xAl_y(O)/Al₂O₃ composites is moderate, but their high-temperature oxidation resistance is exceptionally high, making the titanium-based composite powders favorable feedstock materials for protective coatings. The hardness of the Ti(Al,O)/Al₂O₃ and Ti₃Al(O)/Al₂O₃ composites is also very high (10–16 GPa). For more information, contact D.L. Zhang, University of Waikato, Waikato Centre for Advanced Materials, Department of Materials and Process Engineering, Private Bag 3105, Hamilton, New Zealand; 011-64-7-838-4783; fax 011-64-7-838-4835; e-mail d.zhang@waikato.ac.nz.