Intermetallic Alloys Based on Orthorhombic Titanium Aluminide

Orthorhombic titanium aluminides represent the youngest class of alloys emerging out of the group of titanium aluminides. These new materials are based on the ordered orthorhombic phase Ti₂AlNb, which was discovered for the first time in the late 1980s as a constituent in a Ti₃Al‐base alloy. In the 1990s primarily simple ternary Ti–Al–Nb orthorhombic alloys were investigated in countries such as the US, UK, India, France, Japan, and Germany. The drive was mainly provided by jet engine manufacturers and related research labs looking for a damage‐tolerant, high‐temperature, light‐weight material. This follows the aim of further extending the use of lower density titanium‐base materials in temperature regimes, where heavy nickel‐base superalloys are the only alternative today. The present understanding of microstructure–property relationships for orthorhombic titanium aluminides reveals an attractive combination of low and high temperature loading capabilities. These involve high room‐temperature ductility and good formability, high specific elevated temperature tensile and fatigue strength, reasonable room‐temperature fracture toughness and crack growth behavior, good creep, oxidation, and ignition resistance combined with a low thermal expansion coefficient. This article reviews the aspects of composition–microstructure–property relationships in comparison to near‐α titanium, TiAl, and nickel‐base alloys. Special emphasis is also placed on the environmental degradation of the mechanical properties.     

Application of high temperature DSC technique to nickel based superalloys

Heat capacity is a critical input parameter in mathematical models of solidification and casting. It appears in its own right, and is required for the extraction of thermal conductivity from thermal diffusivity data. It also provides a measure of the latent heat associated with changes of phase, and the precipitation or dissolution of precipitates. Calorimetry is a well-established technique and is used to solve a wide range of materials problems such as studying precipitation or phase changes in alloy systems, and the kinetics of these phase transformations. The quality and range of data that can be obtained will be demonstrated by applying it to nickel base superalloys.This paper discusses important aspects of the measurement including control of the atmosphere, suitable reference materials for temperature and enthalpy calibration, size of the sample and the rate of heating and cooling. The optimised DSC technique was used to investigate the properties of two nickel based superalloys (CMSX4 and IN738LC), comparing the results with previous values and determining sources of error in the method.     

L1<Subscript>2</Subscript>-phase cutting during high temperature and low stress creep of a Re-containing Ni-base single crystal superalloy

In the present study, we investigate dislocation processes in a Ni-base single crystal superalloy (LEK94) after high temperature and low stress creep. Specimens were creep deformed at 240 MPa and 980 °C to a strain of 20%. We use diffraction contrast transmission electron microscopy (TEM) to show that two γ-channel dislocations with different Burgers vectors combine and form a super dislocation that shears the γ′-particle. This type of cutting event has now been observed for three single crystal superalloys with different alloy chemistry (CMSX-4, CMSX-6 and LEK94 in the present work) and we therefore conclude that it represents a generic elementary dislocation process which governs high temperature (around 1000 °C) and low stress (around 240 MPa) creep. The present paper provides microstructural evidence for this type of cutting processes and discusses the results in the light of previous work published in the literature.     

微量元素对镍基高温合金微观组织与力学性能的影响

在高温环境中镍基高温合金具有良好的高温强度、抗氧化性能、抗腐蚀性能和抗疲劳性能,被广泛应用于航空航天等领域。镍基高温合金优异的综合性能与其微观组织紧密相关。综述了微量元素B, C, Y, Ce, Hf, Re, Ru, P对镍基高温合金微观组织及其力学性能的影响。针对不同的镍基高温合金,对微量元素的不同作用进行讨论分析。镍基高温合金微观组织及其力学性能与微量元素的含量及其分布有关。添加于镍基高温合金中的微量元素分布在合金基体或者其析出相中,通过偏聚于晶界处或者元素偏析等方式,改变合金的微观组织,从而影响其力学性能。     

High rate directional solidification and its application in single crystal superalloys

In the present paper there are two parts contributing to the discussion of high rate directional solidification and its application. The first part aims to characterize the high rate directional solidification of various kinds of alloys. It was found that the relevant cooling rate of the high rate directional solidification is defined to be within 1–10³ K/s (solidification rate is 10^-4–10^-1 m/s as G_L= 100 K/cm) and that it is located in the region between the near-equilibrium slow growth rate and the rapid solidification rate beyond the equilibrium condition, whilst at the same time there occurs a series of turning effects of interface stability and morphologies. With the increase in the growth velocity the interface with the plane front evolves to cells and dendrites at the stage of near-equilibrium and with a further increase in growth rate they transformed reversibly from dendrites to cell structure and then to the absolute stability of a planar interface. The change of solute segregation reveals the same from a lowsegregation, then increased and finally reduced again. An explanation based on effective constitutional supercooling about the evolution of interface morphologies with respect to the changes of growth rate is proposed.

The second part is devoted to introducing experimental results for single crystal superalloys using the rate directional solidiication principle. It is shown that the single crystal superalloys CMSX–2 and NASAIR 100 exhibit significant improvement in microstructure segregation and mechanical properties at high temperature both in the as-cast and after-heat-treatment conditions with the high rate directional solidification technique.     

Fatigue crack initiation and propagation in electron beam vapor-deposited coatings for gas turbine superalloys

Thermal fatigue crack initiation and propagation in NiCoCrAlY-coated nickel base superalloys have been evaluated. At relatively low strain ranges, the cycles required to initiate coating cracking are a significant fraction of the total fatigue life of a component. Crack initiation occurs preferentially at pit and flake defects in the coating. The strain-temperature cycle shape has a significant effect on coating crack initiation; cycle I, which peaks tensile strains at the minimum temperature, is the most severe from a coatings standpoint. Cycle I thermal fatigue crack initiation and propagation in NiCoCrAlY coatings are dependent on the thermal expansion mismatch with the substrate. The rate of coating crack propagation is also dependent on coating thickness.     

Glass transition kinetics and optical band gap in Se85−xSb15Snx (x = 10, 11, 12.5, and 13) chalcogenide glasses

Se_85−xSb₁₅Sn_x (10 ≤ x ≤ 13) chalcogenide glasses were prepared by melt quenching technique. The glass transition temperature T_g of the samples was recorded at different heating rates using differential scanning calorimeter DSC. From the heating rate dependence of T_g, the activation energy for thermal relaxation E_t was calculated using Moynihan model and Kissinger equation. It is found that T_g increases with Sn content due to enhancement of both the degree of cross-linking parameter D_cl and the mean bond energy of the average cross-linking per atom 〈E_cl〉. The observed increase in D_cl and 〈E_cl〉 is attributed to the formation of SnSe_4/2 structural units of energies higher than that of Se–Se and Se–Sb bond energies. The decreasing trend of E_t with the addition of Sn is an indication of improving thermal stability as it is also evident from the values of the temperature difference T_c − T_g. Correlation of T_g values with the physical parameters of the studied glasses (for instance, the average coordination number 〈Z〉, the average heat of atomization (H_s), the overall mean bond energy 〈E〉, and the optical band gap (E_g)) reveals that T_g increases linearly with 〈Z〉, H_s, and 〈E〉 but the behavior with E_g is opposite.     

低膨胀高温合金抗氧化性能预测的研究

通过测定六种不同成分的低膨胀高温合金氧化增重曲线,提出合金元素作用系数,利用分析得到的合金元素作用系数对实验合金的氧化增重进行预测计算,其结果与实验结果吻合得很好。利用合金元素作用系数的概念和本研究所提出的方法可对低膨胀高温合金的氧化增重提供预测计算,借此可减少研究低膨胀高温合金氧化特性的试验工作量,为今后低膨胀高温合金的发展打下了基础。     

Physicochemical properties of Nd2-x-yCexSry,CuO4 solid solutions in the range 293–1023 K

The electrical properties and thermal expansion of Nd_2-x-yCe_xSr_yCuO₄ solid solutions were studied in the temperature range 293–1023 K. The temperature dependences of conductivity, dielectric permittivity, and tan δ show anomalies at 488 K for Nd₂Cu0₄ and 400 K for the solid solutions. These anomalies are accompanied by sharp changes in the linear thermal expansion coefficient     

A modified normal strain ratio fatigue life model based on the hybrid approach of critical plane and crystallographic slip theory

Based on the method combining the critical plane with crystallographic slip theory, an anisotropic low cycle fatigue life model is proposed to reflect the effects of orientation dependence and damage factors on fatigue life. According to this method, the crystallographic slip plane is adopted as the critical plane by searching for 30 potential slip systems. In addition, considering the effects of normal strain and strain ratio on fatigue failure, the normal strain ratio is introduced into model and regression model is obtained by fitting method. The proposed model is verified by estimating the low cycle fatigue lives of single crystal nickel–based superalloys PWA1480, CMSX‐2 and DD3 for different loading conditions. The results show that the proposed model is applicable for more complicated loading situations and give a higher prediction accuracy compared to Sun's model.     

A dip process for thermal barrier coating on superalloys

A new cerium oxide thermal barrier coating for superalloys has been investigated. The coating is applied by hot dipping superalloy substrates into a molten bath of low melting point Ce-Ni alloy. Annealing and selective oxidation of cerium at low oxygen activities produce a duplex coating of a continuous outer layer of cerium oxide (CeO₂) and an inner composite CeO₂-substrate layer.The effects of coating composition and processing variables on the microstructure and integrity of the coating on nickel- and cobalt-base alloys were studied in detail. Isothermal and cyclic oxidation tests showed that coating spallation could occur and was associated with the oxidation of nickel and cobalt incorporated in the coating. A model for the formation of the CeO₂ barrier and the subscale region is proposed that explains the presence of unwanted nickel and cobalt.     

Surface damages and tool wear mode in end milling of hastelloy‐C276 under dry and wet conditions

Hastelloy‐C276 is a nickel based superalloy that is widely used in chemical, petro‐chemical, environmental and nuclear industries due to its outstanding performance in a wide range of corrosive mediums. The superior properties of nickel based superalloys impair their machinability which increases the difficulty in obtaining a good surface finish. Because most of the components' failures are initiated from surface defects, several researchers have been concerned about surface integrity in machining aerospace superalloys particularly Inconel‐718. Due to the lack of studies done on machining corrosion‐resistant superalloys, this study aims to investigate surface damages and tool wear modes in milling Hastelloy‐C276 under dry and wet conditions. The absence of cooling and lubricating actions in dry machining resulted in the formation of craters, severe plastic deformation, voids, debris re‐deposition and materials drag. The breakage of the nucleated carbide phases resulted in the formation of nucleated cavities on the machined surface in both wet and dry machining. Adhesive tool wear was less in dry machining due to the formation of oxide layers on tool faces which suppressed the formation of built‐up edges due to the weak adhesion properties of oxide compounds which resulted in less surface roughness at v_c = 50 m/min. On the other hand, the higher temperature and friction in dry machining resulted in severer tool coating delamination.     

Untersuchung zur Stabilität von Gefügen einkristalliner Nickelbasis‐Superlegierungen

Investigation of Microstructural Stability of Single Crystal Nickel Based Super Alloys The present study investigates the kinetics of the widening of γ‐channels in the single crystal superalloy CMSX‐4 during high temperature and low stress creep. Shear creep specimens subjected to high resolution shear creep testing were investigated using scanning and transmission electron microscopy (SEM and TEM) in combination with quantitative image analysis. {011} 〈011¯〉‐shear creep tests were performed at a shear stress τ = 50 MPa and a temperature of 1080°C. For the macroscopic crystallographic shear system {011} 〈011¯〉 it was found that γ′‐rafts form in an angle of 45 ° to the macroscopic shear direction. The kinetics of γ‐channel widening can be described by a parabolic rate law. Therefore it is concluded that the diffusion of atoms through the γ‐channels of γ/γ′‐microstructures is the rate controlling process for rafting.     

Silicide formation with nickel and platinum double layers on silicon

Diffusion effects and silicide formation in double layers of electron-gun-evaporated thin films of nickel and platinum on 〈100〉 and 〈111〉 silicon substrates were studied by megaelectronvolt backscattering spectrometry, transmission electron microscopy and glancing angle X-ray diffraction as a function of heat treatment (200–900 °C) for both sequences of thin films. It was found for the Si/Ni/Pt(Si/Pt/Ni) system that Ni₂Si(Pt₂Si) starts growing first. When all the nickel (platinum) has been consumed by this compound growth, platinum (nickel) diffuses through the Ni₂Si(Pt₂Si) layer and accumulates at the SiNi₂Si(SiPt₂Si) interface. This platinum (nickel) diffusion seems to be a grain boundary diffusion.For 〈100〉 Si/Ni/Pt samples with thin platinum layers it has been shown that platinum acts as a marker for the moving species in the transition from Ni₂Si to NiSi. For thick platinum layers it was observed that similar processes occur, leading to essentially a four-layered silicide where the layers are alternately rich in nickel and rich in platinum (450 °C, 20 min). In the silicide for the 〈100〉 Si/Pt/Ni system the distribution of nickel and platinum is approximately the reverse of the asdeposited distribution (about 450 °C, 20 min). In the further evolution of the profiles the elemental distribution becomes smooth and flat for both sequences of the layers (750 °C, 20 min). We suggest the existence of a ternary of the type SiNi_1?xPt_x.     

弹性气凝胶的制备及其力学、热学性能研究

以甲基三甲氧基硅烷(MTMS)为硅源、水为溶剂、醋酸和氨水作为酸碱催化剂, 采用酸碱两步法和酒精超临界干燥制备了透明块体气凝胶。用扫描电镜、比表面积与孔径分析仪、动态力学分析仪和hotdisk热分析仪等表征了气凝胶的微观形貌、孔结构、力学、热学等性能。制备的气凝胶具有很好的弹性性能, 压缩60%后可回复到原长的78%, 经热处理后反弹到原长的94%。气凝胶同时具有较好的保温隔热性能, 常温热导率仅为0.028 W/(m?K)。其接触角达154°, 表现出较好的疏水性能。气凝胶的耐热温度为440℃, 高于此温度将导致甲基氧化分解。气凝胶优良的保温隔热性能和力学性能使其在保温隔热领域中具有广泛的应用前景。     

Structure and Properties of Boron-Doped LiNbO<Subscript>3</Subscript> Single Crystals

We have studied the macro-and microstructure and properties of LiNbO₃〈B〉 crystals in comparison with LiNbO₃〈Zn〉 and stoichiometric and congruent lithium niobate crystals. The optical characteristics of the LiNbO₃〈B〉 crystals have been shown to approach those of the stoichiometric LiNbO₃ and LiNbO₃〈Zn〉 crystals.     

Structure and Hardness of Ceramics Produced through High-Temperature Nitridation of Titanium Foil

We have studied the variation in the phase composition, elemental composition, and microstructure of rolled titanium samples during thermal annealing in a nitrogen atmosphere at 1300, 1500, 1700, and 2000°C. The results demonstrate that the nitridation process can yield compact titanium nitride-based ceramics or TiN/TiN _x /α-Ti〈N〉 heterostructures. X-ray diffraction data for the near-surface region of the samples before and after nitridation and those for cross-sectional fracture surfaces of heterostructures are used to infer orientation relationships between the TiN phase and α-Ti〈N〉 solid solution. Scanning electron microscopy results for cross-sectional fracture surfaces indicate that complete nitridation of the samples leads to the formation of a three-layer microstructure.     

喷射成形镍基高温合金的研究进展

喷射成形是一种新型快速凝固技术,采用这项技术制备的镍基合金,具有细小,球状的晶粒和均一的组织,从而提高其性能,综述了喷射成形镍基高温合金的发展概况及组织,性能特点。     

Au/Ni bilayers on n-type silicon: Metallurgical and electrical behaviour

Diffusion effects during the formation of silicides in the Ni-Au-Si system were investigated by means of ⁴He⁺ MeV Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar⁺ ion sputtering and X-ray diffraction as a function of the heat treatment temperature (280–350°C) and time (10–1000 min). Schottky barrier heights were used to identify the type of metal present at the silicon surface. Au/Ni/Si and Ni/Au/Si structures were prepared by electron gun deposition of thin gold and nickel films onto n-type Si〈111〉 single crystals. After thermal treatment only Ni₂Si and NiSi compounds were observed and their formation follows the phase order confirmed by previous investigations on the Ni/Si system, with a growth controlled by a lattice diffusion process. In the Ni/Au/Si〈111〉 structure the diffusion of the silicon through the gold film was detected during the formation of nickel silicide and the kinetics of growth of Ni₂Si and NiSi were similar to those studied in the Ni/Si〈100〉 system. A diffusion of gold towards the Si-NiSi interface was observed during the growth of NiSi in the Au/Ni/Si〈111〉 structure. The Schottky barrier height measurements confirm these findings.