Materials For High-temperature Applications Research Articles

Effect of co-substitution of Mn and Al on thermoelectric properties of chromium disilicide

CrSi2 was earlier reported to be an interesting thermoelectric material for high temperature applications because of its high oxidation resistance and good mechanical properties. In order to enhance its figure of merit, Mn at Cr site and Al at Si site were substituted into CrSi2. Our results indicate that Cr1−xMnxSi2−xAlx solid solutions exhibit significantly lower thermal conductivity and a higher figure of merit than CrSi2.

The Mechanical Behavior of a 25Cr Super Duplex Stainless Steel at Elevated Temperature

Super duplex stainless steel (SDSS) is a candidate material for production tubing in oil and gas wells and subsea pipelines used to transport corrosive hydrocarbon fluids. The suitability of this material for high temperature applications is examined in this article. The uniaxial tensile properties are determined for a 25Cr SDSS over a range of temperature relevant to high pressure-high temperature oil and gas wells. It is shown that there is a significant effect of temperature on the uniaxial tensile properties. Elevated temperature was shown to reduce the Young’s modulus and increase the strain hardening index; temperature effects on these two parameters are usually neglected in the design of subsea pipelines and oil well tubulars, and this could lead to wrong predictions of the collapse pressure. The manufacturing process of the super duplex tubular did not lead to significant anisotropy in the hardness and the ultimate tensile and uniaxial yield strengths.

Thermoelectric Properties of Manganese Monosilicide Synthesized by Mechanical Alloying Process

Manganese silicide is one of the potential thermoelectric materials for high temperature application. As a fundamental investigation on these materials group, manganese monosilicides were synthesized by the mechanical alloying of stoichiometric elemental powder compositions, and the as-milled powders were consolidate by vacuum hot pressing. Phase transformation and microstructure evolution during mechanical alloying and hot consolidation were investigated using XRD and SEM. Near single phase of monosilicides were successfully produced in this process. Thermoelectric properties as a function of temperature were evaluated in terms of Seebeek coefficient, electrical resistivity, thermal conductivity and the figure of merit for the hot pressed specimens. Mechanically alloyed manganese monosilicide, MnSi, appeared to have a potential as a thermoelectric material in this study.

Electrochemical characterisation of novel γ/γ′-strengthened Co-base superalloys

Electrochemical behaviour of a new γ/γ′-strengthened Co-base superalloy with the composition Co–9Al–9W–0.12B (in at%) was studied in comparison with pure Co at room temperature in 1M Na2SO4 (pH 5.9) and 0.1M NaOH (pH 12.8) aqueous solutions. Pure Co and the Co-base superalloy show passive behaviour in both solutions. Impedance spectra measured at the corrosion potential indicate comparable impedance values for the alloy and pure Co in neutral solution, but in alkaline solution the alloy shows a lower corrosion rate. Polarisation curves indicate a limited range of passivity for both materials in neutral solution. In alkaline solution, the alloy and pure Co show similar (albeit not identical) primary and secondary ranges of passivity, as previously described in the literature for pure Co. Furthermore, lower passive current densities are observed for the superalloy in the primary range of passivity. As the superalloy studied in this work is a promising material for high temperature applications, we moreover compared the corrosion behaviour of oxidised alloy surfaces with the bare alloy surface. For this, isothermal oxidation of the γ/γ′-strengthened Co-base superalloy was carried out in air at 800°C and 900°C for 24h and 500h. Compared to unoxidised Co–Al–W–B alloys and pure Co samples, the high temperature oxide scales provide orders of magnitude higher protection with good barrier properties in a potential range between −1 and +2V (vs. Ag/AgCl). In general, longer oxidation times and a higher oxidation temperature result in the formation of more protective oxide layers. The protective effect of the high temperature oxide layers is discussed in view of their thickness and chemical composition.

Room temperature synthesis of high temperature stable lanthanum phosphate–yttria nano composite

A facile aqueous sol–gel route involving precipitation–peptization mechanism followed by electrostatic stabilization is used for synthesizing nanocrystalline composite containing lanthanum phosphate and yttria. Lanthanum phosphate (80wt%)–yttria (20wt%) nano composite (LaPO4–20%Y2O3), has an average particle size of ∼70nm after heat treatment of precursor at 600°C. TG–DTA analysis reveals that stable phase of the composite is formed on heating the precursor at 600°C. The TEM images of the composite show rod shape morphology of LaPO4 in which yttria is acquiring near spherical shape. Phase identification of the composite as well as the phase stability up to 1300°C was carried out using X-ray diffraction technique. With the phases being stable at higher temperatures, the composite synthesized should be a potential material for high temperature applications like thermal barrier coatings and metal melting applications.

Studies on SiC, DLC and TiO2 thin films as piezoresistive sensor materials for high temperature application

The use of thin films as sensing elements for microsensor applications has been shown very attractive due to their low-cost fabrication, potential for integration with standard CMOS technologies and possibility of deposition on different substrate types. In particular, piezoresistive sensors based on thin films have been commonly developed because can be easily implemented using microfabrication processes and present the best relation between sensitivity and system complexity, which showing great advantages in term of device integration. In our previous works (Fraga et al. 2010, 2011a), we studied undoped and nitrogen-doped PECVD a-SiC thin films as alternative materials to replace the silicon piezoresistors in strain and pressure sensors for harsh environments. Here, we focused our attention on the piezoresistive properties of sputtered silicon carbide (SiC), diamond-like carbon (DLC) and titanium dioxide (TiO2) thin films. These materials were evaluated in terms of sensitivity or gauge factor and of the influence of the temperature on this sensitivity, allowing a preliminary analysis of the applicability of these thin films in high temperature piezoresistive sensors.

Relationship Between Mechanical Properties of Lead-Free Solders and Their Heat Treatment Parameters

The research was undertaken to establish mechanical properties of as-cast and heat-treated Sn-Zn-based alloys of binary and ternary systems as candidates for lead (Pb)-free solder materials for high-temperature applications. The heat treatment of as-cast alloys was made under different combinations of processing parameters (168 h/50 °C, 42 h/80 °C, and 24 h/110 °C). The systematic study of structure-property relationships in Sn-Zn, Sn-Zn-Ag, and Sn-Zn-Cu alloys containing the same amount of Zn (4.5, 9, 13.5 wt.%) and 1 wt.% of either Ag or Cu was conducted to identify the effects of chemical composition and heat treatment processing parameters on the alloy microstructure and mechanical behavior. Structural characterization was made using optical microscopy and scanning electron microscopy techniques coupled with EDS analysis. Mechanical properties (initial Young’s modulus E, ultimate tensile strength UTS, elastic limit R 0.05, yield point R 0.2, elongation A 5, and necking Z) were determined by means of static tensile tests. All the examined Sn-Zn-based alloys have attractive combination of mechanical characteristics, especially tensile strength, having values higher than that of common leaded solders and their substitutes of Pb-free SAC family. The results obtained demonstrate that the Sn-Zn-based alloys present competitive Pb-free solder candidates for high-temperature applications.

Computational Approach to Modelling Fracture Behaviour of Polypropylene/Talc Composites

Fracture represents one of the major problems associated with the selection and use of engineering materials for high temperature applications. The fracture toughness is of special relevance on the design of components. In this work, the fracture behavior of Polypropylene/Talc composites was studied. From the results of the tensile and flexural tests conducted on the composite, scatter diagrams were made using Microsoft Excel to evaluate and show the effect of the addition of the talc filler as it affects the tensile strength, percentage elongation at break, flexural strength and modulus. In order to give additional analysis, the talc filler content effect was presented mathematically to further describe explicitly the various equations associated with each scatter diagram earlier developed using Microsoft Excel. The mathematical expression developed shows the actual talc filler content on the fracture mechanical properties of the sample composite.

Broaching Performance of Superalloy GH4169 Based on FEM

The nickel-based superalloy GH4169 is an important material for high temperature applications in the aerospace industry. However, due to its poor machinability, GH4169 is hard to be cut and generates saw-tooth chips during high speed machining, which could significantly affect the dynamic cutting force, cutting temperature fluctuation, tool life, and the surface integrity of the parts. In this paper, the saw-tooth chip formation mechanism of superalloy GH4169 was investigated by the elasto-viscoplastic finite element method (FEM). Using the finite element software of ABAQUS/Explicit, the deformation of the part during high speed machining was simulated. The effective plastic strain, the temperature field, the stress distribution, and the cutting force were analyzed to determine the influence of the cutting parameters on the saw-tooth chip formation. The study on broaching performance has great effect on selecting suitable machining parameters and improving tool life.

The cyclic softening and evolution of microstructures for Mg–10Gd–2.0Y–0.46Zr alloy under low cycle fatigue at 573 K

As a candidate material for high temperature applications, the Mg–10Gd–2.0Y–0.46Zr alloy may undergo the alternate loading during the service lifetime. Under the total strain-controlled triangular waveform loading, the strength and the evolution of microstructures for the alloy at 573K have been explored. The results show that the alloy presents cyclic softening characteristics at diverse total strain amplitudes. The grain boundary sliding has been detected near the cracks on the surface of the specimen, suggesting that the grain boundary sliding plays an important role in coordination deformation at 573K. Microcracks initiate mostly either at grain boundaries for lower strain amplitudes or at persistent slip bands for higher strain amplitudes, and then propagate along grain boundaries. Due to twinning shear and the dislocations slip, the precipitates in the deformation twins disintegrate to form a small equilibrium phase. The dislocation cells, small angle grain boundaries, as well as dynamic recrystallization are observed in the precipitation-free zone.

Tool wear and surface quality in milling of a gamma-TiAl intermetallic

Advanced structural materials for high-temperature applications are often required in aerospace and automotive fields. Gamma titanium aluminides, intermetallic alloys that contain less than 60 wt.% of Ti, around 30–35 wt.% of aluminum, and other alloy elements, can be used as an alternative to more traditional materials for thermally and mechanically stressed components in aerospace and automotive engines, since they show an attractive combination of favorable strength-to-weight ratio, refractoriness, oxidation resistance, high elastic modulus, and strength retention at elevated temperatures, together with good creep resistance properties. Unfortunately such properties, along with high hardness and brittleness at room temperature, surface damage, and short and unpredictable tool life, undermine their machinability, so that gamma-TiAl are regarded as difficult to cut materials. A deeper knowledge of their machinability is therefore still required. In this context the paper presents the results of an experimental campaign aimed at investigating the machinability of a gamma titanium aluminide, of aeronautic interest, fabricated via electron beam melting and then thermally treated. Milling experiments have been conducted with varying cutting speed, feed, and lubrication conditions (dry, wet, and minimum quantity lubrication). The results are presented in terms of correlation between cutting parameters and lubrication condition with tool wear, surface hardness and roughness, and chip morphology. Tool life, surface roughness, and chirp morphology showed dependence on the cutting parameters. Lubrication conditions were observed to heavily affect tool wear, and minimum quantity lubrication was shown to be by far the method that allows to extend tool life.

Thermoelectric Properties of Half-Heusler ZrNiSn1-xSbxSynthesized by Mechanical Alloying Process and Vacuum Hot Pressing

Half-heusler phase ZrNiSn is one of the potential thermoelectric materials for high temperature application. In an attempt to investigate the effect of Sb doping on thermoelectric properties, half-heusler phase () was synthesized by mechanical alloying of stoichiometric elemental powder compositions, and consolidated by vacuum hot pressing. Phase transformations during mechanical alloying and hot consolidation were investigated using XRD. Sb doped ZrNiSn was successfully produced in all doping ranges by vacuum hot pressing using as-milled powders without subsequent annealing. Thermoelectric properties as functions of temperature and Sb contents were evaluated for the hot pressed specimens. Sb doping up to x

Thermal shock resistance of brittle ceramic materials with embedded elliptical cracks

An analytical model is established for the prediction of the thermal shock resistance of a ceramic plate with an embedded elliptical crack with ideal thermal insulation. The thermal stress intensity factors at the crack fronts are formulated by fitting the thermal stress to a polynomial form in an un-cracked plate. The thermal shock resistance of sapphire ceramic is evaluated as an example. In addition, the numerical solution of thermal shock resistance suitable for practical engineering applications is provided. The model developed applies to any brittle material in high temperature applications.

Atomistic configurational effects on piezoelectric properties of La3Ta0.5Ga5.5O14 and a new piezoelectric crystal design

Single crystalline langatate (LTG, La3Ta0.5Ga5.5O14) has been widely used as a sensor material in high temperature applications because of its thermally stable piezoelectric properties. In this research, to elucidate the relationship between piezoelectric tensors and local ionic configurations, first-principles calculations based on density functional perturbation theory (DFPT) were performed on various local ionic structures. The results indicate that two independent relaxed-ion piezoelectric coefficients, e11 and e14, increased with increases in La(3e)–O and Ta(1a)–O distances or decreases in Ga(3f,2d)–O distances. Thus, to obtain high piezoelectric constants in this crystal, ions larger than La3+ should be incorporated at 3e sites to open the distance between 3e ions and oxygen ions, and ions smaller than Ga3+ should be introduced at 2d and 3f sites to reduce the distance between Ga and O ions. Finally, from this design rule, a new crystal, BTAS (Ba3TaAl3Si2O14), which belongs to the same P321 group, is proposed. The calculated relaxed-ion piezoelectric coefficient e11 of BTAS was 17.7% higher than the coefficient of a LTG crystal. This significant increase confirms BTAS as a useful new piezo-material, especially in applications where there is also a need to reduce the use of more expensive elements.

Technological and Environmental Aspects in Milling of γ-TiAl

Gamma-titanium aluminides present an attractive combination of low density, high melting temperature, good oxidation and burn resistance, high modulus and strength retention at elevated temperatures, and good creep properties. For this reason they are promising candidates as advanced structural materials for high temperature applications. However due to their high strength, high hardness and brittleness at room temperature, g-TiAl alloys are regarded as difficult-to-cut materials. Many efforts have been done during the last 20 years to introduce TiAl-based alloys into the aerospace and automotive market as engineering components. But for the application and diffusion of this new heat-resistant light-weight material for high performance components, a deeper knowledge of their machinability is still required. In this context, and given the increasing attention to the environmental aspects related to the manufacturing processes, the present research work is aimed at investigating two key aspects: the influence of cutting parameters and cooling conditions, on tool wear, surface finishing and power consumption in milling operations of a g-TiAl alloy.

Production of nanoporous superalloy membranes by load-free coarsening of γ′-precipitates

Nickel-based superalloys are predominantly used as structural materials in high-temperature applications due to their exceptional high-temperature strength based on precipitation hardening by the coherent γ′-phase. In modern superalloys, the γ′-phase fraction can amount to 75%, at which γ′-precipitates align as cubes parallel to the 〈0 0 1〉 directions of the crystal lattice. At high temperatures and under a mechanical load, e.g. during service in gas turbines, the γ′-cubes coalesce to γ′-rafts, generating an interpenetrating microstructure of γ and γ′. By extracting one phase of this interpenetrated network, nanoporous superalloy membranes containing channel-like interconnected pores are produced. So far, this can only be achieved by applying simultaneously thermal and mechanical loads during tensile creep deformation. Here, a new production process is presented. Due to internal stresses, load-free aging of single-crystalline superalloys, e.g. CMSX-4, also generates an interpenetrating microstructure of γ and γ′. This can be utilized to manufacture nanoporous superalloy membranes in the absence of an external mechanical load. The advantage of this process is its simplicity and the potential to fabricate larger membranes than possible by the costly tensile creep deformation process currently used.

Diffusion of chromium and manganese in γ-TiAl

The γ-TiAl intermetallic compound with suitable alloying additions has shown considerable promise as a material for high-temperature applications. Diffusion studies in this alloy system are useful in assessment of their creep behaviour and structural stability in service conditions. Tracer diffusion coefficients of 51Cr and 54Mn in a γ-TiAl intermetallic compound containing 54.1 at. % aluminium were determined in the temperature range from 1095 to 1470 K. The temperature dependence of both the diffusing species follows a linear Arrhenius behaviour and can be expressed as D Cr = 4.4 × 10−3exp(−350 kJ mol−1/RT) m2 s−1 and D Mn = 1.2 × 10−3 ×exp(−326 kJ mol−1/RT) m2 s−1. The data are analysed on the basis of empirical correlations between the diffusion and melting parameters applicable for conventional mono-vacancy diffusion mechanism in metals. It is concluded that impurity diffusion in γ-TiAl occurs through the migration of thermal vacancies via nearest-neighbour or next-nearest neighbour jumps.

Thermoelectric Properties of Er-doped InGaN Alloys for High Temperature Applications

ABSTRACTThermoelectric (TE) properties of erbium-silicon co-doped InxGa1-xN alloys (InxGa1-xN: Er + Si, 0≤x≤0.14), grown by metal organic chemical vapor deposition, have been investigated. It was found that doping of InGaN alloys with Er atoms of concentration, N[Er] larger than 5x1019 cm-3, has substantially reduced the thermal conductivity, κ, in low In content InGaN alloys. It was observed that κ decreases as N[Er] increases in Si co-doped In0.10Ga0.90N alloys. A room temperature ZT value of ~0.05 was obtained in In0.14Ga0.86N: Er + Si, which is much higher than that obtained in un-doped InGaN with similar In content. Since low In content InGaN is stable at high temperatures, these Er+Si co-doped InGaN alloys could be promising TE materials for high temperature applications.

Improvement of Dielectric Performance of a Prototype AlN High Temperature Chip-level Package

Aluminum nitride (AlN) has been proposed as a packaging substrate material for reliable high temperature electronics operating in a wide temperature range. However, it was discovered in a recent study that the dielectric properties of some commercial polycrystalline AlN materials change quite significantly with temperature at high temperatures. These material properties resulted in undesired large and temperature-dependent parasitic parameters for a prototype chip-level package based on an AlN substrate with the yttrium oxide dopant. This paper reports a method using a coating layer of a commercial thick-film glass on the AlN substrate surface to significantly reduce both the parasitic capacitances and parasitic conductances between neighboring inputs/outputs (I/Os) of a prototype AlN chip-level package. The parasitic parameters of 8-I/Os low power chip-level packages with the insulating glass coating were characterized at frequencies from 120 Hz to 1 MHz between room temperature and 500°C. These results were compared with the parameters of AlN packages without the glass coating. The results indicate that the parasitic capacitances and conductances between I/Os of the improved prototype AlN packages are significantly reduced and stable at high temperatures. The method using a glass coating provides a feasible way to mitigate the temperature dependence of dielectric properties of AlN and further utilize AlN as a reliable packaging substrate material for high temperature applications.

Thermodynamic analysis and characterization of Bi–Cu–Sn alloys as advanced lead-free solder materials for high temperature application

Due to the frequent use of Cu as a substrate material in electronics, it is important to understand the interactions between the solders and the substrates, based on the knowledge of the phase equilibria, thermodynamics and other characteristics of the Bi–Cu–Sn system. The results of thermodynamic analysis and characterization of Bi–Cu–Sn alloys as advanced lead-free solder materials for high temperature application are presented in this paper. The research includes analytical investigations done by thermodynamic predicting according to general solution model in temperature range 1,073–1,273 K and experimental investigations performed by differential thermal analysis, differential scanning calorimetry, optical microscopy, hardness and electroconductivity measurements for the sections with molar ratio Bi:Cu = 1:1, 1:3 and 3:1.