The structure of ribbon borides in a Ti-44Al-4Nb-4Zr-1B alloy

The structure of ribbon borides in an as-cast Ti–44Al–4Nb–4Zr–1B (at.%) alloy has been studied using high-resolution transmission electron microscopy. The ribbon borides have a laminar structure comprised of thin faceted flakes of borides interspersed with layers of ordered B2 matrix. The dominant phase in the boride flakes is TiB with the B_f structure, containing a high density of planar faults on (010). Lattice images from these faults reveal that they correspond to nanoscale intergrowths with the C32 and D7_b structures. The B_f flakes also contain thin embedded layers with the B27 structure lying on {110}B_f. The orientation relationships between each of these phases have been identified. It is shown that these observations are consistent with the ribbon borides having formed as a sequence of eutectic phase mixtures in the liquid between primary β dendrites.     

不同SPS烧结温度Ti-45Al-5.5(Cr,Nb,B,Ta)合金的显微组织及力学性能(英文)

采用高能球磨和放电等离子烧结(SPS)技术,制备成分为Ti-45Al-5.5(Cr,Nb,B,Ta)的TiAl合金块体,随后对TiAl合金进行热处理。研究在不同SPS烧结温度下制备的TiAl合金经过热处理后的显微组织和力学性能。结果表明:高能球磨后的合金粉末形状不规则,粉末颗粒尺寸大约为几十微米。XRD分析表明,机械球磨后的粉末由TiAl和Ti3Al两相组成;烧结后的Ti-45Al-5.5(Cr,Nb,B,Ta)合金块体主要是TiAl相,以及少量的Ti3Al和TiB2相。当烧结温度为900°C和1000°C时,合金的显微组织为双相结构,并伴随有一些细小的等轴γ晶粒和细小的针状TiB2相。当烧结温度从900°C上升到1000°C时,Ti-45Al-5.5(Cr,Nb,B,Ta)合金的显微硬度变化不大,抗压强度从1812MPa提高到2275MPa,压缩率从22.66%增加到25.59%,合金的断裂方式为穿晶断裂。     

微量C，B对高铌TiAl合金显微组织与力学性能的影响

采用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)等设备，以及拉伸和蠕变试验系统研究微量间隙元素C，B对高铌TiAl合金显微组织与力学性能的影响。微量B元素对高铌TiAl合金没有明显的强化作用，但是微量B元素在合金中以条状或点状的TiB2存在，TiB2细化了高铌TiAl合金原始片层团晶粒，对改善高铌TiAl合金片层组织的室温塑性有利。加微量C元素的高铌TiAl合金在长时间的蠕变过程中，大量Ti3AlC沉淀相的析出提高了高铌TiAl合金全片层组织蠕变抗力。     

The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1–5 at.%) in TiAl alloys with different compositions, and in Ti₃Al with the compositions of Ti–26 at.%Al–(1–2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti–Al–V–Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti₃Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti₃Al. The experimental results are interpreted in terms of a Bragg–Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached.     

Enhanced mechanical properties of orthorhombic Ti<Subscript>2</Subscript>AlNb-based intermetallic alloy

Attempts were made to improve the mechanical properties of an orthorhombic Ti₂AlNb-based Ti−22Al−27Nb intermetallic alloy through microstructural and compositional modifications, and the dispersion of fine TiB particulates. A Ti−22Al−27Nb alloy with a prior B2 grain size ranging from 8 μm to 49 μm was successfully obtained using spherical α₂ particles as obstacles to grain growth. The finest grained material showed an excellent combination of room temperature tensile strength (around 1,000 MPa) and tensile ductility (more than 15%). Transition metal elements such as Mo, V and W for a portion of the Nb in the Ti−22Al−27Nb were substituted. The guideline for this compositional modification required that the beta phase stability in the modified alloy be equal to that of the Ti−22Al−27Nb. It was found that the substitution of 2% W for 7% Nb was quite effective in increasing tensile strength at temperatures above 923 K and reducing the steady state creep rate and primary creep strain. The Ti−22Al−27Nb alloy-based particulate composites reinforced with 6.5% TiB were successfully produced with the gas atomization P/M method. The dispersion of extremely small TiB was very uniform. Most of the mechanical properties of the composites, including the yield stress, tensile strength, Young's modulus, resistance to creep, and high cycle fatigue strength were greatly superior to those of the matrix alloy. This article is based on a presentation made in “The 16th Conference on Mechanical Behaviors of Materials” held at Oriental Hotel, Jeju, Korea. November 11, 2002, organized by The Korean Institute of Metals and Materials.     

Al—Ti—B晶粒细化合金中的有效形核相

研究了ＡｌＴｉＢ晶粒细化合金中各化合物相ＴｉＡｌ３、ＴｉＢ２和ＡｌＢ２对铝晶粒的细化作用。结果表明，ＴｉＡｌ３相是有效形核相，ＴｉＢ２和ＡｌＢ２相不能单独作为形核相，Ｂ对ＡｌＴｉＢ的细化作用有显著影响，但Ｂ及硼化物不能单独影响细化过程，而是富集在ＴｉＡｌ３相中对细化过程产生重要影响。     

First-principles study of site occupancy of dilute 3d, 4d and 5d transition metal solutes in L10 TiAl

Using a statistical-mechanical Wagner–Schottky model parametrized by first-principles density-functional (DFT-GGA) calculations on 32-atom supercells, we predict the lattice site occupancy of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition-metal elements in L1₀ TiAl intermetallic compound as a function of both alloy composition and temperature. The effects of local atomic relaxations, anisotropic lattice distortions, as well as magnetism on point defect energetics are fully taken into account. Our calculations show that, at all alloy compositions and temperatures, Zr and Hf consistently show a preference for the Ti sublattice, while Co, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt and Au consistently show a preference for the Al sublattice. In contrast, the site preference of V, Cr, Mn, Fe, Ni, Cu, Nb, Mo, Tc, Ta and W strongly depend on both alloy stoichiometry and temperature. Our calculated results compare favorably with the existing theoretical and experimental studies in the literature.     

The Systems Tantalum (Niobium)-Cobalt-Boron

Constitution of the ternary systems Nb-Co-B and Ta-Co-B was studied, employing optical and electron microscopy, x-ray powder, single crystal diffraction, electron probe microanalysis, DTA and Pirani melting point measurements. Ternary phase equilibria were determined within an isothermal section at 1100 °C. For the Co-rich part (≥50 at.% Co) of the system, a liquidus surface projection and a corresponding Schulz-Scheil reaction scheme were constructed in combination with data for primary crystallization from as-cast samples determined by SEM and EPMA measurements. The crystal structures of novel ternary compounds have been elucidated by x-ray powder and single crystal diffraction and were supported by TEM. {Nb,Ta}CoB with NbCoB-type exhibits a high temperature modification (ZrAlNi-type, a = 0.5953 nm, c = 0.3248 nm; a = 0.5926 nm, c = 0.3247 nm for Nb and Ta respectively), which was only present in as-cast alloys, but found to be stabilized by the addition of Fe to annealing temperatures of 1400 °C. Ta₃Co₄B₇ (a = 0.3189 nm, b = 1.8333 nm, c = 0.8881 nm) was proven to be isotypic with Nb₃Co₄B₇. The novel orthorhombic compounds {Nb,Ta}Co₂B₃ (TaCo₂B₃-type with space group Pnma; a = 0.53628 nm, b = 0.32408 nm, c = 1.24121 nm for TaCo₂B₃; a = 0.53713 nm, b = 0.32442 nm, c = 1.2415 nm for NbCo₂B₃) adopt unique structure types with branched boron zig-zag chains. {Nb,Ta}Co₂B were found to crystallize in a unique monoclinic structure type (space group P2 ₁/c; a = 0.9190 nm, b = 0.64263 nm, c = 0.63144 nm; β = 109.954°, for Nb) very close to an orthorhombic setting (Cmce, a = 0.63162 nm, b = 1.72810 nm, c = 0.64270 nm, for Nb). Substitution of Co by Ni stabilizes a smaller orthorhombic lattice with Re₃B-type structure (Cmcm) although no homologue compound in the Ni-system exists. The crystallographic relations among the structure types of Re₃B and pseudo-orthorhombic as well as monoclinic {Nb,Ta}Co₂B were defined in terms of a Bärnighausen scheme. DFT calculations revealed very close stabilities for the three competing structure types for {Nb,Ta}Co₂B. Detailed transmission electron microscopy (TEM) for Nb(Co,Fe)B, {Nb,Ta}Co₂B,  {Nb,Ta}(Co,Ni)₂B, and Ta₃Co₄B₇ confirmed lattice geometries and crystal symmetry. Vickers hardness was measured for {Nb,Ta}Co₂B, {Nb,Ta}(Co,Ni)₂B, {Nb,Ta}_2?x Co_21+x B₆ and {Nb,Ta}Co₂B₃ exhibiting the highest value of hardness of HV = 22.4 ± 1.1 GPa for TaCo₂B₃. Magnetic, specific heat and electrical resistivity measurements on the compounds TaCo₂B and Ta₂Co₂₁B₆ reveal paramagnetic and ferromagnetic metallic ground states, respectively.     

TiAl-B合金中TiB_2微观形态的主要存在方式

用XRD、SEM和TEM对原位自生法制备的Ti5 0Al x( % )B合金的金相组成和微观组织进行研究。结果表明 :合金中TiB2主要以片状、板片状、细棒状和块状形式存在 ;TiB2 微观形貌随着合金中B含量的变化而发生显著变化。当合金中B含量由w0 .4%增加到w1.4%时 ,TiB2 的微观形貌由片状依次演变为板片状、细棒状直至块状 ,而基体形貌没有发生明显的变化。根据TiB2 晶体形核与生长条件对TiB形貌演变过程进行了分析。     

Effects of heat shock at 800 °C on mechanical properties of γ-TiAl alloys

Effect of heat shock on the mechanical properties and the microstructure of TiAl alloys (Ti–47Al–2Cr–2Nb and Ti–45.3Al–2Cr–2Nb–0.1W–0.15B) with near fully lamellar structure were investigated. After heat shock process from room temperature to 800 °C for 500 cycles, the microstructure demonstrated that lamellar microstructure has been destructed by the presentation of some γ and α₂ block phases in lamellar structure, resulting in the ductility of as-polished Ti–47Al–2Cr–2Nb alloy decreased by about 70% and the ultimate tensile strength (UTS) reduced by about 25%, and the ductility of as-unpolished Ti–47Al–2Cr–2Nb alloy decreased by more than 70% and the ultimate tensile strength (UTS) was reduced by about 35%. The ductility of Ti–45.3Al–2Cr–2Nb–0.1W–0.15B alloy decreased by about 60% and the ultimate tensile strength (UTS) reduced by about 18% after heat shock, which was resulted from the appearance of small α₂ block phase at interfaces of lamellar colonies and microcracks at the interfaces of ribbon boride and lamellar structure.     

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys have been studied. In order to optimize ductility and strength of the orthorhombic alloys with the nominal compositions of Ti–22Al–23Nb–3Ta and Ti–22Al–20Nb–7Ta, various thermomechanical processing steps were implemented as part of the processing route. With a special heat treatment before rolling to obtain a fine and homogeneous rolled microstructure, the rolled microstructure resulted in a good combination of high tensile yield strength and good ductility of the alloys through available solution and age treatments. The duplex microstructure with equiaxed α₂/O particles and fine O phase laths in a B2 matrix, deforming in α₂+B2+O phase field and treating in O+B2 phase field, possesses the highest tensile properties. The R.T. yield strength and ductility of the Ti–22Al–20Nb–7Ta alloy are 1200 MPa, and 9.8% respectively. The yield strength and ductility values of 970 MPa and 14% were also maintained at elevated temperature (650°C).     

Effects of solid deformation and melt vibration on structure and refining performance of Al5Ti1B master alloy

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B和C对铸造TiAl基合金宏观和显微组织的影响

对比研究了B和C含量的变化对铸造TiAl基合金宏观和显微组织的影响。结果表明 ，B或C含量的增加均能细化TiAl基合金的晶粒，但二者的作用特点不同。B含量对宏观组织和晶粒尺寸的影响是渐进的，而C对其影响存在一个临界含量。显微观察表明，微量的B就能使合金析出TiB2，随着合金中B含量的增加，硼化物颗粒呈不同的形貌。对于含C合金，当C含量低于临界含量时，在光学尺度上观察不到碳化物相，而当C含量高于临界含量时，合金中生成大量的Ti2AlC碳化物颗粒。讨论了B和C细化TiAl基合金的机制。     

Fe- and Co-based bulk glassy alloys with ultrahigh strength of over 4000 MPa

Since the first synthesis of Fe-based bulk glassy alloys in Fe–(Al,Ga)–(P,C,B,Si) system in 1995, a number of Fe- and Co-based bulk glassy alloys have been developed up to date because their alloys are expected to exhibit high mechanical strength and good soft magnetic properties. The maximum diameter of Fe- and Co-based bulk glassy alloys exhibiting high fracture strength of over 4000 MPa is 5 mm for Fe–Co–B–Si–Nb system and 3 mm for Co–Fe–Ta–B–Mo–Si system. The addition of a small amount of Nb or Ta is essential for the increase in their glass-forming ability through the formation of network-like atomic configurations. The primary crystalline phase from supercooled liquid is a metastable complex FCC Fe₂₃B₆ or Co₂₃B₆ phase and hence the change to the local atomic configurations leading to the precipitation of the metastable Fe₂₃B₆ or Co₂₃B₆-type phase is thought to play an important role in the achievement of high glass-forming ability. The highest fracture strength reached as high as 4250 MPa for Fe–Co–B–Si–Nb alloy and 5545 MPa for Co–Fe–Ta–B–Mo alloy. The fracture strength has a good linear relation with Young's modulus, glass transition temperature or liquidus temperature. It is, therefore, concluded that the origin for the ultrahigh strength is attributed to the strong bonding nature among the constituent elements. Considering that Fe–Si–B amorphous alloy wires developed for several years between 1979 and 1983 have been used as high strength materials for the last two decades, the newly developed high-strength Fe- and Co-based bulk glassy alloys are expected to be used as a new type of ultrahigh strength material by utilizing the advantage points of much higher strength and three-dimensional material form.     

La2O3包埋渗硼TB2钛合金的腐蚀与磨损性能

为了改善TB2合金的表面性能,采用4％La2O3(质量分数)包埋渗硼法对TB2合金进行1100℃,20 h渗硼处理,研究TB2钛合金的渗硼层组成与厚度以及腐蚀与磨损性能.结果表明,La2O3在渗硼过程中促进硼化物层的生长,提高其连续性和致密性,TiB晶须长度从16.80增至21.84μm.这是因为La2O3能与B反应生...     

Experiment and modeling of TiB2/TiB boride layer of Ti−6Al−2Zr−1Mo−1V alloy

The influence of the boriding conditions on the boride layers was examined by boriding Ti−6Al−2Zr− 1Mo−1V alloy in the temperature range of 920−1120°C. The experimental results show that the boride layers were composed of a continuous thin outer layer of TiB₂ and a thick inner layer of TiB with whiskers or needle-like morphologies that extended into the substrate. Thick and compact boride layers were obtained when the boriding temperatures were 1000−1080 °C, and the treatment time exceeded 8 h. The boride layer depth increased with the boriding temperature and time, and the growth kinetics of the boride layers was characterized by a parabolic curve. The growth kinetics of the boride layers, including both TiB₂ and TiB layers, were predicted by establishing a diffusion model, which presented satisfactory consistency with the experimental data. As a result, the activation energies of boron in the TiB₂ and TiB layers were estimated to be 223.1 and 246.9 kJ/mol, respectively.     

Density of thermal vacancies in γ-Ti–Al–M,M=Si,Cr, Nb,Mo, Ta or W

Modifications to alloy chemistry are often used to tailor the intrinsic flow behavior of structural materials. Models of creep in intermetallic alloys must account for the influence of chemistry on the available intrinsic creep mechanisms. As in simple metals the presence of vacancies strongly influences bulk diffusion processes in these materials. Limiting the density of constitutional and thermal vacancies by alloying may produce materials with enhanced creep properties. The energy of intrinsic and substitutional point defects in L1₀ TiAl is calculated within a first principles, local density functional theory framework. Relaxed structures and energies for vacancies, antisites and solid solutions are calculated using a plane-wave-pseudopotential method. Calculated defect energies are used within a canonical ensemble formalism to estimate the point defect densities as a function of temperature and composition. The density of vacancies is found to be sensitive to the underlying stoichiometry of TiAl. The dependence of the vacancy concentration for solid solutions of Si, Cr, Nb, Mo, Ta and W is also predicted.     

A first principles study of the influence of alloying elements on TiAl: site preference

The electronic structure and binding energy of a number of TiAl-X alloy systems (X=V, Cr, Mn, Fe, Co, Ni, Y, Zr, Nb, Mo, Hf, Ta, W, Ga, Ge, In or Sb) were calculated using the discrete variational cluster method, based on the local density approximation of the density functional theory. The site preference of the ternary additions to TiAl was investigated by means of the binding energy data and the Bragg–Williams model. The results showed that Y, Zr, Nb, Mo and Sb preferentially occupy the Ti sublattice sites, Ga and In occupy the Al sublattice sites, while V, Mn, Cr, Co and Ge may occupy either site, depending on the Ti/Al ratio. Investigation of the total and local densities of states for representative elements showed that the substitution behaviour of ternary additions in TiAl is determined by the electronic structure of the systems. The present predictions of the site preference of alloying elements in TiAl show good agreement with the most recent experimental findings.     

Characteristics and wear performance of borided Ti6Al4V alloy

In this study, a 10 µm thick uniform boride layer, composed of TiB₂ and TiB phases, was formed on the surface of a Ti6Al4V alloy using a pack boriding technique. The hardness of the boride layer was over 2000 HV. Beneath the boride layer, a boron diffusion zone (BDZ) appeared with a thickness of about 50 µm. The microstructure of the BDZ was composed of randomly oriented TiB whiskers mixed with the structure of the base metal. In the BDZ, hardness decreased gradually towards the base metal owing to the reduction of the TiB volume fraction. The borided alloy exhibited excellent wear resistance along with a lower coefficient of friction against sapphire ball under both dry and smear lubricated sliding conditions when compared to the as-received state.     

Interfacial reaction studies of B4C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites

The interfacial reactions of B₄C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites were investigated by scanning electron microscope and transmission electron microscope. The detailed microstructures as well as the chemical composition throughout the reaction zone were identified. For SiC_f/B₄C/TiAl composite, the reaction zone from B₄C coating to TiAl matrix is composed of 4 layers, namely, a carbon-rich layer, a mixed layer of TiB₂ + amorphous carbon, a TiC layer and a mixed layer of TiB + Ti₂AlC. For SiC_f/C/TiAl composite, the reaction zone from C coating to TiAl matrix is composed of 3 layers, namely, a fine-grained TiC layer, a coarse-grained TiC layer and a thick Ti₂AlC layer. For both kinds of composites, the reaction mechanisms of the interfacial reactions were analyzed, and the corresponding reaction kinetics were calculated. The activation energies of interfacial reaction in SiC_f/B₄C/TiAl composite and SiC_f/B₄C/TiAl composite are 308.1 kJ/mol and 230.7 kJ/mol, respectively.