Tensile cracking during thermal cycling of alumina films formed by high-temperature oxidation

Plastic relaxation of the stresses, produced by high-temperature oxidation and thermal contraction mismatch on cooling, decreases residual compression in the oxide. For this reason, slow cooling from the oxidation temperature is expected to have a beneficial effect on spalling resistance of protective alumina films. However, there exists a possibility of creating tensile stress in the oxide upon subsequent heating. With increasing stress relaxation on cooling, the magnitude of the tensile stress also increases which may lead to cracking of the oxide at high temperature. Experimental evidence shows an adverse effect of stress relaxation on the mechanical integrity of alumina films formed on Fe–Cr–Al heat-resistant alloys. High cooling and heating rates between oxidation cycles are required to prevent tensile failure of the oxide.     

Cyclic Oxidation Behaviors of TiAl–Nb–Si-Based Alloys

For this study, several TiAl–Nb–Si-based alloys were designed for a ductility improvement, whereby the high-temperature strength and oxidation resistance were not sacrificed. The environmental properties under the cyclic oxidation behaviors of the TiAl alloys were evaluated at 900 °C for up to 360 cycles. The compositions of the as-cast alloys determined their microstructures, and the cyclic oxidation behavior of the selected alloy was relatively comparable to that of a commercial TiAl alloy that is currently used in automotive engines.     

The reactive-element effect in the high-temperature oxidation of Fe-23Cr-5Al commercial alloys

The effect of yttrium and hafnium additions on high-temperature oxidation resistance of modified Fe-23Cr-5Al commercial, heat-resistant alloys was studied. The oxidation resistance was evaluated by the use of accelerated-life tests and isothermal oxidation for wires and flat samples, respectively. A solgel method of modification of the surface properties of ready-made heating elements was examined. The effect of several sol-gel coatings on the alloy oxidation behavior in the accelerated-life test was studied.     

Phase transformations in some TiAl-based alloys

Samples of a range of TiAl-based alloys have been cooled directly to room temperature at rates between 0.1 and 500 °C s⁻¹ in order to define the transformation behaviour during continuous cooling (CCT). In addition other samples have been cooled rapidly to predetermined temperatures where they have been held for times up to 18,000 s before cooling rapidly to room temperature in order to determine their time-temperature-transformation (TTT) behaviour. It has been found that the massive transformation occurs at the highest cooling rates used (500 °C s⁻¹) in all the alloys studied apart from Ti–44Al–4Nb–4Zr–0.2Si–1B. In this alloy the high-temperature beta phase partially transformed during rapid cooling to lenticular alpha which, together with the remaining beta, was retained at room temperature. The effects of holding at selected temperatures were as anticipated from the CCT curves and the equilibrium diagrams. In all cases the room temperature tensile properties were improved for the finest microstructures—i.e. for the fastest cooling rates used, although with alloys with B addition (i.e. grain-refined alloys) the effect of cooling rate was less important. The changes in microstructure and changes in the tensile properties and hardness of samples which have been tempered after quenching have also been determined. Appropriate tempering of samples which had been cooled at a rate which caused them to transform massively gives rise to fine microstructures of intimately mixed equilibrium phases. In the case of Ti–48Al–2Nb–2Cr this leads to a mixture of convoluted alpha and gamma grains of about 50 μm (even although it contains no B and is therefore not grain-refined) and to a plastic elongation of 1.3% which is significantly better than the 0.5% found in coarse-grained air-cooled or furnace-cooled samples of this alloy.     

The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

Shape-Memory Wires

In wire made from high-temperature shape-memory alloys, partial substitution of palladium for nickel in NiTi increases the martensite to austenite transformation temperature, while minor boron additions increase the tensile ductility and formability. By heating deformed wires, transformation forces have been measured as a function of temperature. Further, the influence of variations in titanium level about stoichiometry on shape-memory behavior and tensile properties has also been determined.     

Phase transformations and structural changes in the course of the high-temperature superplastic deformation of aluminum alloys

Phase and structural transformations, which develop under conditions characteristic of high-temperature superplasticity in matrix multicomponent aluminum alloys, have been investigated. It has been shown that the appearance of a metastable liquid phase is in principle possible in the structure of such alloys in the process of the establishment of the equilibrium state during their heating to the temperature of tests and in the course of superplastic deformation. The characteristic properties of the structural changes that occur in the course of the superplastic deformation of such alloys have been studied. The basic conditions that lead to the origin and the development of fibrous species in the course of the high-temperature superplastic deformation have been determined. The determining role of magnesium in the process of origin and development of such fibrous formations is shown.     

Kinetics of high temperature oxidation of chromium rich HfC reinforced cobalt based alloys

Mostly known to improve the high temperature oxidation resistance of superalloys, hafnium may also form carbides. Several per cents of Hf allow developing a dense carbide network to mechanically strengthen alloys. Here, the high temperature oxidation behaviour of three HfC containing cobalt alloys was characterised at all steps of a thermogravimetry test: heating, isothermal stage and cooling, compared with two Co–Cr–C model alloys. The five alloys were heated in synthetic air, maintained at 1200°C during 50 h and then cooled. The mass gains were plotted versus time or according to (m×dm/dt?=?Kp–m×Kv) to specify the isothermal kinetic constants, or versus temperature to determine how oxidation acts during heating and oxide spallation occurs during cooling. Compared to the ternary alloys, the oxidation of the HfC reinforced alloys starts earlier but leads to lower mass gains during heating, the isothermal oxidation is faster and oxide spallation occurs later.     

Mechanical properties of Al–Si–Cu alloys produced by the continuous casting process with heating process

The mechanical properties of aluminium alloys produced by the continuous cast process and heating process (heat-cast-sample) were investigated, where the aluminium alloys are heated continuously to high temperatures for 1 h immediately following heated mould continuous casting (HMC) and sand gravity casting (SGC). The material strength and ductility of the aluminium alloys were irregularly altered depending on the heating temperature. The mechanical properties decreased when the heating temperature increased to 400 °C and were then recovered when the temperature increased to 520 °C. However, these properties decreased again when heated to more than 540 °C. The mechanical properties of the HMC-heat-cast-sample showed overall higher than those for the SGC-sample. In addition to high tensile strength, high ductility was obtained for the HMC-520 °C samples compared with those for the as-cast-sample. Such changes were found to be directly attributable to the different severity of precipitate; moreover the crystal orientation was unchanged even after the heating process.     

α2-Ti-25Al-xNb合金力学性质的第一原理计算

采用第一原理赝势平面波方法计算了D0_19结构的α_2-Ti-25Al-xNb(x=O—12,原子分数,%)晶体的弹性模量(B, G和E)和抗拉强度(σ_b),并利用Cauchy压力(c_(12)-C_(44))与G/B比值表征和评判了不同浓度Nb合金化时α_2一Ti-25Al- xNb合金的韧脆化倾向.结果表明:在x=2—12时,α_2-Ti-25Al-xNb晶体的抗拉强度(σ_b)与σ_2相合金的弹性模量(B, E和G)随x增加而增大;在x=0—6时,α_2-Ti-25Al-xNb合金脆性有一定改善,且x值越大韧化效果越好;但在x=7—9时,相对于α_2-Ti_3Al,合金脆性不但没有得到弱化,反而随x增加而加剧;随后,当x进一步增大时,合金脆性又随x增加再次得到改善,至x=12时,α_2-Ti-25Al-xNb合金的韧化效果最好.通过电子态密度(DOS)和投影电子态密度(PDOS)等电子结构的分析,初步解释了Nb的这种强化与韧化作用.     

Friction welding of pipes of a zirconium alloy with 2.5% niobium

Investigations were carried out into the ductility properties of a number of titanium alloys in heating from 750 to 1050°C and ATs5K5 and AMts aluminium alloys. The extreme temperature ranges with increased ductility are determined. The conditions of diffusion welding of T-joints in alloys are determined to produce joints without visible defects with the simultaneous formation of a smooth transition (fillet) in the fin–web section. Examples of a number of welded joints are discussed.     

Observations on the role of oxide scales in high-temperature erosion-corrosion of alloys

The results are presented of exposure to a controlled high-temperature erosive gas stream of a series of alloys, which were selected to represent the range of microstructures and mechanical properties available in commercial high-temperature alloys. Analysis of the kinetic and morphological data suggested that the high-temperature oxidation behavior of a given alloy plays a very important role in determining its erosion-corrosion behavior under the conditions studied. In terms of relative behavior, alloys which are weak but ductile at temperature, and which form tenacious oxide scales, exhibited the highest resistance to high-temperature erosion-corrosion. Simple models were developed to describe the expected interaction between high-temperature oxidation and erosion.     

Surface oxide cracking associated with oxidation-induced grain boundary sliding in the underlying alloy

When thin sections of FeCrAlY alloys having initially flat surfaces are oxidized at high temperature, the surface becomes uneven as some metal grains on the surface displace above and below the macroscopic surface. On subsequent cooling, the oxide formed by high-temperature oxidation cracks and spalls at the edges of the uplifted grains. Microstructural observations indicate that the grain uplift is associated with grain sliding and it proposed that the sliding at the oxidation temperature occurs in response to the evolving compressive growth stress in the oxide. The observed spalling patterns are consistent with a fracture mechanics model in which incipient flaws at the oxide-alloy interface, in the vicinity of the step edges, propagate in the residual stress field in the oxide.     

Effect of sulphur on hot ductility and heat affected zone microfissuring in Inconel 718 welds

Abstract

The influence of sulphur on microfissuring has been studied by characterising the hot ductility and heat affected zone (HAZ) microfissuring in welded wrought Inconel 718. A series of Inconel 718 base alloys, with sulphur concentrations in the range 7–110 wt-ppm and with the lowest possible concentrations of carbon, boron, and phosphorus were used in the study. The hot ductility of the alloys was measured using a Gleeble 1500 system, and their weldability was evaluated by bead on plate electron beam welding. The effect of sulphur on microfissuring in the weld HAZ was compared with that of boron in boron containing, but sulphur free, alloys. Sulphur reduced the weldability of the material, but its influence was very moderate in comparison with that of boron. The rate of increase of the brittle temperature range hot ductility parameter with increasing sulphur concentration was significantly smaller compared with that with increasing boron concentration. This may be because sulphur and boron seem to influence grain boundary liquation in the HAZ differently during welding of these alloys. Sulphur influences the weld HAZ microfissuring mainly by reducing the solidus temperature of liquid films during the cooling part of the welding cycle. Boron, however, not only reduces the solidus temperature, but also promotes grain boundary liquation during the heating part, and is therefore more effective than sulphur in promoting HAZ microfissuring.     

Dependence on the Chromium Content of the High-Temperature Oxidation Behavior of Ta-Rich Nickel-Based Cast Alloys

The high-temperature stability of primary tantalum carbides is a problem of importance for chromium-rich cast alloys, based on cobalt or nickel. The focus of this study was nickel-based alloys, as these alloys are particularly sensitive to a lack of TaC in the as-cast state and by dissolution due to high-temperature exposure. In this work, a possible way for promoting the formation of many TaC precipitates by changing from the usual 30 wt% chromium content was investigated. Five alloys with Cr content varying from 10 to 50 wt% were prepared and then subjected to microstructure characterization and to oxidation tests. In contrast with what was expected, decreasing the Cr content in comparison with the Ni–30Cr–0.4C–6Ta reference alloy did not succeed in obtaining more TaC precipitates, but instead had the opposite effect. Concerning the high-temperature oxidation behavior at 1127 and 1237 °C, loss of resistance was observed only for a Cr content at the lower level of 10 wt%. It was noticed that a subscale CrTaO₄ developed during oxidation and seemed to promote oxide spallation during cooling.     

A New High-Temperature,Oxidation-Resistant Ti-Based Material

Ti₃Al(O)–Al₂O₃ composites were fabricated in situ using a mechanical-alloying and reaction-sintering technique. Their oxidation behaviors were studied in air at 700–900°C. The oxidation rates were much lower than those of Ti₃Al. The behaviors of isothermal and cyclic oxidation were very similar. The oxide scales that formed exhibited excellent spallation resistance under all testing conditions. No scale cracking or spallation could be observed, even along the edges or corners of the samples, implying that growth and thermal stresses generated during heating and cooling periods had been effectively released. The mechanisms of the decrease in oxidation rate and the improvement on spallation resistance are discussed based on microstructure studies. This composite has advantages of light-weight, simple fabrication from inexpensive materials, and superior high-temperature oxidation resistance; it may provide opportunities to be used in some high-temperature structural applications.     

Effects of cooling rate and stabilization annealing on fatigue behavior of β-processed Ti-6Al-4V alloys

The effects of stabilization annealing and cooling rate on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-processed Ti64 alloys were examined. After β-process heating above β transus, two different cooling rates of air cooling (β-annealing) and water quenching (β-quenching) were utilized. Selected specimens were then underwent stabilization annealing. The tensile tests, HCF and FCP tests on conducted on the β-processed Ti64 specimens with and without stabilization annealing. No notable microstructural and mechanical changes with stabilization annealing was observed for the β-annealed Ti64 alloys. However, significant effect of stabilization annealing was found on the FCP behavior of β-quenched Ti64 alloys, which appeared to be related to the built-up of residual stress after quenching. The mechanical behavior of β-processed Ti64 alloys with and with stabilization annealing was discussed based on the micrographic examination, including crack growth path and crack nucleation site, and fractographic analysis.     

A Review of Superplasticity and Related Phenomena

Unusual ductility effects can be obtained by mechanical deformation during a phase change or allotropie transformation. The current knowledge on superplasticity—the enhanced ductility associated with structural changes under applied stress—is reviewed. Of particular interest is the recent Soviet work on this phenomenon. The high-temperature creep ductility observed in alloys tested at temperatures coinciding with the solubility limit of the phase diagram and the extra ductility found in certain steels at relatively low temperatures are also discussed.     

Laser processing of titanium aluminides

Using a Nd-YAG laser, laser processing of a series of Ti-Al alloys including pure Ti and Ti-Al intermetallic compounds has been studied. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and optical microscopy were used to determine the surface morphological, chemical, and compositional characteristics of the laser-processed samples. Analysis of results showed that cracks along grain boundaries caused by rapid heating and cooling of laser processing were the dominant characteristics of the surface morphologies of the laser-processed samples. The Al content in the Ti-Al alloys played a very important role in crack initiation and/or development. The more Al content in the samples, the more severe the cracks that developed after laser processing under the same conditions. The experiments were conducted at ambient conditions, resulting in surface oxidation layers being observed on the processed samples. The XPS results indicated that the oxidation layer consisted of adsorbed O₂, Al₂O₃, TiO₂, and TiO. In addition, Al enrichment was found in the oxide film of TiAl as well as in the oxidation layers formed on the surfaces of TiAl and Ti₃Al intermetallics that were processed by the laser; this differs from the reported results for traditional oxidation of TiAl at elevated temperature.     

Effect of Tempering on the Oxidation Resistance of a Work Roll Grade High-Chromium White Cast Iron

The high-temperature oxidation resistance of as-cast and heat-treated specimens of a high-chromium white cast iron for hot mill work rolls was investigated by gravimetric means under isothermal oxidation conditions in dry air at 650 °C. Results from the experiments revealed that the chromium content of the matrix and the carbide area fraction of the samples significantly affect the extent of oxidation. Even so, the oxidation phenomenon could always be described by the parabolic rate law. The surface morphology and cross-section of the samples were studied by means of X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analyses, confirming that the oxidation behaviour of the alloys is sensitive to tempering heat treatment schedule. The impact of heat-treatment on the oxidation resistance of the work roll alloys is discussed in terms of their microstructural modification.