On grain refinement of a γ-TiAl alloy using cryo-milling followed by spark plasma sintering

A fully dense fine-grained γ-TiAl based alloy was fabricated by cryo-milling pre-alloyed powders followed by spark plasma sintering. The consolidation was performed at 1050 or 1200 °C. The effect of cryo-milling on the densification kinetics and final microstructure was studied. Results indicate that cryo-milled (8 h) powder is fully densified at a temperature nearly 125 °C lower than that of un-milled powder. The microstructure of the alloy fabricated at different temperatures consisted of fine grains of γ-TiAl and α₂ (Ti₃Al) phases in different volume fractions. The average grain size of cryo-milled powders compacted at 1050 and 1200 °C were determined as 0.6 and 0.9 μm, respectively. It is inferred that the reduction in crystallite and particle size induced by cryo-milling is the likely reason for such enhancement in densification and grain refinement.     

Molecular-dynamics simulation of the synthesis of intermetallic Ti–Al

In the present paper, results of a molecular-dynamics simulation study of the synthesis of intermetallic Ti–Al in the regime of thermal explosion at constant volume are reported. The structure of the polycrystalline intermetallic compound formed upon cooling was studied, and the dependence of crystallite sizes on the cooling rate was examined.     

Tribological properties of titanium aluminides coatings produced on pure Ti by laser surface alloying

Titanium aluminides coatings were in-situ synthesized on a pure Ti substrate with a preplaced Al powder layer by laser surface alloying. The friction and wear properties of the titanium aluminides coatings at different normal loads and sliding speeds were investigated. It was found that the hardness of the titanium aluminides coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating. Friction and wear tests revealed that, at a given sliding speed of 0.10 m/s, the wear volume of pure Ti and the titanium aluminum coatings all increased with increasing normal load. At a given normal load of 2 N, for pure Ti, its wear volume increased with increasing sliding speed; for the titanium aluminides coatings, the wear volume of Ti₃Al coating and TiAl coating first increased and then decreased, while the wear volume of TiAl₃ coating first decreased and then increased with increasing sliding speed. In addition, the friction coefficients of pure Ti and the titanium aluminides coating decreased drastically with increasing sliding speed. Under the same dry sliding test conditions, the wear resistance of the titanium aluminium coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating.     

Influence of transformation temperature on microtexture formation associated with α precipitation at β grain boundaries in a β metastable titanium alloy

The influence of transformation temperature on microtexture development associated with α precipitation at β/β grain boundaries (GB) in the near-β Ti17 alloy was studied using electron backscatter diffraction and considering isothermal treatments. For the alloy studied and the temperature range considered, decreasing the transformation temperature decreased the local microtexture strength within each prior β grain because of a larger number of α_WGB colonies (standing for α Widmanstätten GB) formed per β grain, each colony increasing by one the number of α orientations inside each prior β grain. This larger number of α_WGB colonies was a consequence of faster formation along β/β GB of their precursors, the allotriomorphic α_GB grains (standing for α-GB) at lower transformation temperatures, as evidenced by detailed examination of the first stages of α_GB formation. α_GB crystallographic orientations frequently followed a variant selection (VS) criterion based on the alignment of (0 1 1)β//(0 0 0 1)α_GB//(0 1 1)β. From a statistically relevant number of observations, VS was found to be more frequent at a lower transformation duration and a lower temperature, but the effect was not significant enough to influence the final α microtexture, considered at the scale of one prior β grain. α_GB grains that followed the VS criterion emitted two α_WGB colonies on either side of the β/β GB more frequently than those with no particular orientation.     

Tribological Behavior of In Situ TiC/Graphene/Graphite/Ti6Al4V Matrix Composite Through Laser Cladding

The TiC/graphene/graphite/Ti6Al4V composite coating was prepared by laser cladding.The microstructure and tribological behavior of the coating were studied.The in situ reaction between graphene and Ti occurred,and feathery TiC was formed.The feathery TiC was homogeneously distributed between α'acicular martensites which was refined with the addition of graphene.Some graphene was transformed into a11otrope graphite under the laser irradiation.The TiC hard particles and the self-lubrication of graphene/graphite improved the wear resistance of composite coating.The wear rate and friction coefficient of TiC/graphene/graphite/Ti6A14V composite coating decreased with the increase in sliding speed,a mechanical mixing layer (MML) was formed on the wear surface of the composite coating under the frictional heat,which protected the substrate and reduced the contact.Because of the self-lubricating properties of graphene/graphite,interlayer sliding occurred easily,which also effectively reduced friction.The wear rate of TiC/graphene/graphite/Ti6A14V composite coating increased with the increase in load,but the friction coefficient decreased.The plastic deformation of subsurface layer was more serious under high load,and a stable self-lubricating MML with a protective effect was formed between the wear interfaces,which reduced the friction coefficient.With the increase in load,the wear mechanism changed from abrasive and oxidation wear to delamination,fatigue and oxidation wear.     

Sulphidation/oxidation behaviour of TiAlCr and Al2Au coated Ti45Al8Nb alloy at 750 °C

In this paper, we present the sulphidation/oxidation behaviour of a Ti45Al8Nb (at%) alloy coated with different protective surface films. Two intermetallic coatings are considered; TiAlCr and Al₂Au deposited by physical vapour deposition. The coated alloy was subjected to a H₂/H₂S/H₂O yielding pS₂ - 10⁻¹ Pa and pO₂ - 10⁻¹⁸ Pa potentials at 750 °C for up to 1000 h. The corrosion kinetics were determined by means of discontinuous gravimetry and the as-received and exposed samples were characterised using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The materials showed the development of a multilayered structure. In the case of the TiAlCr coated Ti45Al8Nb - base alloy, Al₂O₃, TiO₂ and Cr₂S₃ developed. For the Al₂Au coated Ti45Al8Nb samples an Al₂O₃ scale containing TiO₂ nodules was observed at the surface.     

Physical erosion of yttria crucibles in Ti–54Al alloy casting process

Y₂O₃ crucibles with different porosities were manufactured to investigate the physical erosion during the casting of Ti–54Al (at%) alloys. The effects of multiple superheating times on the chemical composition of the casting, the introduction of inclusions into the alloy and on the metal–crucible interface were studied. A large number of ceramic inclusions were introduced by the physical erosion of the crucible walls in contact with the molten alloy. The mechanisms by which the physical erosion was reduced are discussed in relation to the crucible wall permeability coefficient A′. By reducing the porosity content of the crucible, which led to a decrease of A′, the net increments of oxygen of the remelted Ti–Al as-cast samples were limited to a minimum level at 1873 K.     

硼对Ti47Al合金固态相变的影响

以Ti47Al二元合金为研究对象,探讨了硼添加及冷却速度对合金α→γ+α2相变的影响规律。结果表明,添加0.5at%B显著细化了Ti47Al合金的片层团组织,Ti47Al合金中块状组织主要在原始α晶界上形核,还有少量的在晶内形核;添加硼可以促进块状转变的发生,这是由于硼化物有效细化了原始α晶粒,还与固溶的硼元素有关。添加硼可以提高片层组织形成的临界冷速,抑制羽毛状组织和魏氏组织的形成。     

Reaction behavior and pore formation mechanism of TiAl–Nb porous alloys prepared by elemental powder metallurgy

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α₂-TiAl₃/γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.     

Partial liquidus projection and vertical sections in the system Al–Fe–Si–Ti

The quaternary system Al–Fe–Si–Ti was characterized in the Fe-rich part for thermal reactions, which were studied by differential thermal analysis (DTA). As-cast alloys were investigated in order to gain additional information about primary crystallization fields. Three sections through the liquidus projection were constructed at 50, 60 and 70 at.% Fe, considering the experimental data and those from literature. In addition, five selected vertical sections are presented. The microstructures of selected as-cast alloys in the primary crystallization fields of the Laves phase Fe₂Ti, the A2/B2 phase, FeSi and τ2 (FeSiTi) are discussed.     

Thermal cycling of Ti46Al8Nb1B

The effects of thermal cycling on the mechanical properties and microstructure of a TiAl-based alloy (Ti46Al8Nb1B) have been investigated for samples with and without an imposed external stress. The temperature range (300–800 °C) and stress (0 or 300 MPa) were chosen to reflect likely industrial usage. Unstressed samples showed no significant effect of thermal cycling. Thermal cycling with external stress is found to reduce the yield stress, ductility and UTS and to increase the amount of primary creep. The microstructural changes include dislocation and twin generation, ₂/γ ledge formation, dissolution of ₂ lamellae and microcrack formation. The changes in mechanical properties are interpreted in terms of the changes in microstructure.     

Multi-step heat treatment design for nano-scale lamellar structures of a cast Ti-45Al-8.5Nb-(W,B, Y) alloy

A multi-step heat treatment is proposed to optimize the lamellar structure of as-cast high Nb containing TiAl alloys. The β-segregation is removed effectively after the first two step heat treatment, a short-time holding within β single phase field then annealing in α+β phase field. During the third step heat treatment, a subsequent low-temperature aging, new γ lamellae precipitate by the means of isolated nucleation or twin-related nucleation. After the proposed multi-step heat treatment, the alloy presents nano-scaled lamellar structure with the colony size about 140 μm.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Lamellar orientation control of Ti–47Al–0.5W–0.5Si by directional solidification using β seeding technique

In this paper, we report a β seeding technique for the lamellar orientation controlling in TiAl alloy, which is a novelty and effective method for aligning the lamellar orientation of Ti–47Al–0.5W–0.5Si with primary α phase. The shorter composition transition zone and simpler process procedure can improve the deficiency of α seeding technique. The proper temperature gradient and normal growth rate are necessary for aligning the lamellar orientation in TiAl alloy with primary α phase using β seeding technique.     

Precipitation behaviors in a quenched high Nb-containing TiAl alloy during annealing

The precipitation of γ phase and heterogeneous nucleation of ω_o phase within β_o phase areas are common phenomena in TiAl alloys. However, detailed explanation on the corresponding phase transformation mechanisms is still lacking. In this study, the precipitation behaviors of γ and ω_o phases in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy are investigated. The results show that large γ grains form after quenching whereas small γ particles can directly nucleate within the remaining β_o phase during annealing. Semi-coherent interfaces are observed between γ and β_o phases and the average distance between dislocations is evaluated. The heterogeneous nucleation of ω_o phase at the lamellar colony boundary is imaged by HRTEM. Edge-to-edge method is used to calculate the orientation relationship between γ and ω_o phases. The γ phase grows up faster than ω_o phase within the β_o phase areas during annealing at 800 °C.     

Selective electron beam melting of Ti–48Al–2Nb–2Cr: Microstructure and aluminium loss

In the present paper a broad parameter window for processing Ti–48Al–2Cr–2Nb by selective electron beam melting is investigated. Data for the aluminium loss during the process in relation to used beam parameters and process strategies for its minimisation are presented. Al losses as low as 0.5 at% were achieved. Using different beam parameters a wide range of microstructures from lamellar to massively transformed γ could be realized. This finding in principle gives the opportunity to locally adjust the microstructure of net or near-net-shape parts from TiAl during the process.     

Fatigue response of a grain refined TiAl alloy Ti-44Al-5Nb-1W-1B with varied surface quality and thermal exposure history

Fatigue specimens with four types of designed surface (EDM plane-sited, EDM notched, shot peened, electropolished) were assessed under three exposure conditions (no exposure, block exposure, individual exposure-oxidation at 700 °C for 10000 h) to quantify the effects of surface roughness, stress concentration, oxidation and inner microstructural embrittlement on fatigue behaviour of a grain refined TiAl alloy Ti-44Al-5Nb-1W-1B. With the yield strength of 568 MPa, fatigue is found to occur under a loading condition of σ_max<σ_0.1. Local plastic deformation is difficult to occur. The alloy becomes sensitive to surface damages but not to V-notch because the small surface area sampling the highest stress significantly reduces the EDM impact. Electropolishing rather than shot peening is found to be more effective in improving fatigue strength for the high strength alloy. When subjected to block exposure, both annealing effect (beneficial) and microstructural embrittlement (detrimental) occurred on all the surfaces, and the latter was dominant in governing fatigue behaviour except for EDM surfaces. After individual exposure-oxidation, fatigue performance deteriorated significantly for the shot peened and moderately for the electropolished but not for EDM surfaces. The mechanism for specific fatigue behaviour is discussed individually based upon whether or not the beneficial effects outweigh the detrimental effects.     

Producing fully-lamellar microstructure for wrought beta-gamma TiAl alloys without single α-phase field

Fine-grained fully-lamellar (FL) microstructure is desired for TiAl components to serve as compressor/turbine blades and turbocharger turbine wheels. This study deals with the process and phase transformation to produce FL microstructure for Mo stabilized beta-gamma TiAl alloys without single α-phase field. Unlike the α + γ two-phased TiAl or beta-gamma TiAl with single α-phase field, the wrought multi-phase TiAl–4/6Nb–2Mo–B/Y alloys exhibit special annealing process to obtain FL microstructure. Short-term annealing at temperatures slightly above β-transus is recommended to produce the desired FL microstructure. The related mechanism is to guarantee the sufficient diffusion homogenization of β stabilizers during single β-phase annealing, and further avoid α decomposition by α → γ + β when cooling through α + β + γ phase field. The colony boundary β phase contributes to fine-grained nearly FL microstructure, by retarding the coarsening of the α phase grains.     

Ti6Al4V表面磁控溅射NiCoCrAlY涂层的组织及性能

为了提高钛合金的高温抗氧化能力,采用非平衡磁控溅射技术在Ti6Al4V钛合金的表面沉积了NiCoCrAlY涂层,通过SEM,EDS和XRD分析了涂层氧化前后的物相组成和组织形貌。结果表明:长时间沉积后,受基材原始表面状态和涂层择优生长作用的影响,涂层表面有大量微米级的细小颗粒和少量直径约10μm的圆丘形突起;NiCoCrAlY涂层成分与靶材相近。NiCoCrAlY涂层明显提高了钛合金的抗氧化性能,使其氧化增重速率大大降低。经高温氧化后,涂层与基底的界面处Ti,Ni等元素发生互扩散形成过渡层,随氧化时间延长,过渡层逐渐增厚;并且受高温作用,涂层表面化合物相晶态特征有增强的趋势。