Structural mechanism of scale for 12Cr–W–Mo–Co steels with enhanced oxidation resistance

The phase constituent, morphologies, layer structures of the scale of 12Cr–W–Mo–Co heat resistant steel (HRS) formed in dry air and air with 10% vapour were systematically investigated. The interface between the scale and ferritic/martensitic matrix of this HRS was also studied. For the scale formed in air, single particle- and sheet-shaped oxide layer, which are composed of (Fe, Co, Cr)₂O₃, were formed. The scale combines with steel matrix via coherent or semi-coherent structure. For the scale formed in air with 10% vapour, the oxides take the shape of particulate. Layering phenomenon has been observed, i.e. the external layer is composed of (Fe, Co)₂O₃/(Fe, Co)₃O₄, the internal layer with spinel (Fe, Co, Cr)₃O₄ and the transition layer with Cr rich and Cr poor regions. The interface between the transition layer and the matrix is tight and steady, but the region linking the internal scale and transition layer is shaky and brittle. Both the oxidation processes in air and air with 10% vapour are considered to be controlled by diffusion mechanism.     

Effect of Al on High-Temperature Oxidation of Cr–W Alloys

The effect of Al on the high temperature oxidation behavior of Cr–10 wt.%W alloy was investigated using a cyclical oxidation test at 1,000 °C in dry air. First, Al was added into the Cr–W alloy as an alloying element up to 8 weight percent. Although alloying with Al reduced the spalling, it did not eliminate it. Secondly, Al was applied to the surface using an aluminizing process. Forming an Al–Cr layer on the Cr–W alloy reduced oxidation rate significantly and eliminated spalling completely.     

The Oxidation Behaviour of Alloys Based on the Ni–Co–Al–Ti–Cr System

The isothermal oxidation behaviour of a series of quinary Ni–Co–Al–Ti–Cr alloys were studied at 800 °C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-h exposure, as did the alloys richest in Ni and Al for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni and Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni and Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the elemental partitioning behaviour and the increased mole fraction of γ′ precipitates forming in the alloy.     

High temperature deformation of Ti–Al–Nb–Cr–Mo alloy with ultrafine microstructure

Ti–Al–Nb-based alloys have shown promise for high temperature applications, however limited research has been conducted in γ(TiAl) + σ(Nb₂Al) alloys that are aimed towards increasing strength and operating temperatures through microstructural control. Alloys with less than 30% σ-phase have been investigated, exploring relationships between microstructure and deformation mechanisms. An alloy with composition Ti–45Al–14Nb–5Cr–1Mo has been produced, characterized, and tested at high temperature under compression at strain rates ranging from 10⁻³ to 10⁻⁵ s⁻¹. The flow behavior of this alloy shows strain rate and temperature dependence with constant strain rate sensitivity at each temperature in the testing regime. Microstructural analysis reveals the γ-phase is primarily responsible for the alloy's ability to accommodate deformation to large strains under compression.     

Interfacial reaction between Co–Cr–Mo alloy and liquid Al

The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)₂Al₉ layer close to alloy matrix and “(Cr,Mo)₇Al₄₅ + Al” layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer.     

Solidification structures of Ti–Al–Cr alloys

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)₂ Laves phase. The L1₂ τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.     

Structural stability of the Laves phase Cr2Ta in a two-phase Cr–Cr2Ta alloy

Detailed microstructural analysis of a two-phase alloy of composition Cr–9.8 at.% Ta and lying in the Cr–Cr₂Ta region of the Cr–Ta binary system confirmed that the existing phase diagram is inaccurate; in the cast and annealed condition (1273 K/24 h), blocky primary Cr₂Ta precipitates were observed although the phase diagram indicates the eutectic composition to be ∼13 at.% Ta. The eutectic structure is composed of Cr solid solution and the Laves phase Cr₂Ta; the morphology is primarily lamellar although the rod morphology was occasionally observed. The Laves phase eutectic microconstituent exhibits the C14 (2H) hexagonal structure with a low stacking fault (basal faults) density and an average composition corresponding to 28.5 at.% Ta. After a prolonged high-temperature anneal (1573 K/168 h), the morphology breaks down to form discrete particles of Cr₂Ta; the C14, C36 and C15 structures were all recognized in this annealed condition, often more than one form being present in a single precipitate. The C15 structure was not twinned but contained some stacking faults on the {111} planes. Composition measurements confirmed that these structural transformations were accompanied by composition changes, the precipitates becoming more Ta-rich as they transitioned from the C14 via the C36 to the C15 phase. These observations are coupled with the results from earlier studies to present a discussion on factors that influence the stability and C14/C36/C15 transformation kinetics.     

A solidification model for prediction of castability in the precipitation-strengthened nickel-based superalloys

The Scheil equation was used to model the solidification path, microsegregation of alloying elements in the interdendritic regions, solidification temperature ranges, and to predict the formation of secondary structures and the castability behavior of as-cast superalloys. 4 experimental alloys with pre-specified γ-Ti,Nb,Al,Mo composition containing different Nb, Ti and Al contents were designed using vacuum induction melting furnace. The produced as-cast superalloys were characterized using optical and scanning electron microscopy equipped with energy dispersive X-ray spectrometer and TG–DSC analysis. The experiments showed logic conformity to the modeling results. The model and experiment confirmed the highest segregation behavior for Ti and Nb. All the experimental superalloys indicated the remarkable tendency to form secondary eutectic structures at the last stages of solidification. Superalloy with chemical composition of γ-3.5%Mo,1.8%Al,4%Ti,2.9%Nb showed the shorter solidification temperature range and the best castability.     

The isothermal-oxidation behavior of several nickel-base single-crystal superalloys with and without coatings

Several commercial single-crystal superalloys (CMSX-2, CMSX-3, CMSX-4, CMSX-6, SRR 99) and some laboratory versions of one of them (CMSX-4) with various Y-additions were investigated concerning their oxidation resistance in air at temperatures between 800 and 1200°C. The investigations also included two materials (CMSX-6, SRR 99) with an RT-22 coating. Weight change was recorded for times of up to 1000 hr (in some cases up to 1600 hr). Oxidized coatings and substrates were characterized by metallography, SEM, and microprobe analysis. Most of the alloys showed good oxidation resistance up to 1000°C, while there was complete spalling during cooling after oxidation at 1150°C and 1200°C for the uncoated and Y-free alloys. Coated alloys were superior, however the best behavior was shown by a laboratory version of CMSX-4 containing between 10 and 60 ppm Y. Interdiffusion at 1000°C is tolerable for the coated alloys, but there was extremely rapid degradation of the coating by interdiffusion at 1200°C.     

Phenomenological modeling of the effect of specimen thickness on the creep response of Ni-based superalloy single crystals

Isothermal creep tests on single-crystal Ni-based superalloy sheet specimens show a thickness-dependent creep response that is known as the thickness debit effect. A size-dependent creep response at similar length scales has also been observed in a wide variety of other materials. We focus on Ni-based single-crystal superalloys and present a phenomenological nonlinear parallel spring model for uniaxial creep with springs representing the bulk and possible surface damage layers. The nonlinear spring constitutive relations model both material creep and evolving damage. The number of springs and the spring creep and damage parameters are based, as much as possible, on recent experimental observations of the thickness debit effect under two creep test conditions: a low-temperature, high-stress condition, 760 °C/758 MPa, and a high-temperature, low-stress condition, 982 °C/248 MPa. The bulk damage mechanisms accounted for are the nucleation of cleavage-like cracks from pre-existing voids and, at the higher temperature, void nucleation. The surface damage mechanisms modeled at the higher temperature are an oxidation layer, a γ′-precipitate-free layer and a γ′-precipitate-reduced layer. Model results for the creep response and for the thickness debit effect are in close quantitative agreement with the experimental results. In addition, the model predicts qualitative features of the failure process that are in good agreement with experimental observations. The simplicity of the model also allows parameter studies to be undertaken to explore the relative roles of bulk and surface damage as well as the relative roles of cleavage-like cracking and void nucleation in the bulk.     

固溶处理参数对一种单晶高温合金微观偏析的影响

采用液态金属冷却法,对一种含4wt.％Re的单晶高温合金进行高温度梯度定向凝固试验,并通过改变固溶处理温度和时间对其进行多步固溶处理.结果表明:高温度梯度定向凝固可以显著地细化枝晶,降低Re、W、Ta等难熔元素的枝晶偏析;溶质的特征扩散距离也随枝晶的细化而减小,经多步固溶处理后,合金各组元的残余偏析程度显著降低;此外,适当提高固溶热处理温度,可以起到减轻枝晶偏析的作用.     

Development of two rhenium- containing superalloys for single- crystal blade and directionally solidified vane applications in advanced turbine engines

A team approach involving several turbine engine companies using the concepts of simultaneous engi-neering has been used to successfully develop CMSX-4 ® alloy for turbine blade applications. CMSX-4 al-loy is a second-generation, single-crystal cast nickel-base superalloy containing 3% Re and approximately 70% volume fraction of γ. The high level of balanced properties determined by labora-tory evaluation has been confirmed during field testing of the Solarγ Mars T-14000 industrial gas turbine with CMSX-4 single-crystal (SX) blades in both the coated and bare condition. A similar collaborative ap-proach has resulted in the successful development of CM 186 LCγ alloy for complex, directionally solidi-fied (DS) columnar grain vane segments. CM 186 LC alloy is a second-generation DS columnar grain cast nickel-base superalloy containing 3% Re and approximately 65% volume fraction of γ. Excellent com-ponent producibility and quality is demonstrated. Turbine engine testing is scheduled to commence by the end of 1993.     

Processing and microstructure of investment casting turbine blade NITAC in-situ composites

Directional solidification was used to produce turbine blades by the Bridgman method. NITAC alloys with various carbon contents were investigated; the optimum range was found to be 0.40 to 0.48%. Within this range, except for the blade locking piece edges, the blade structure consisted predominantly of aligned eutectics. The in- situ eutectics were aligned tantalum fibers embedded in a γ- phase matrix. The blades were produced using an alloy displacement rate of 1.86 x 10 ^{- 6} m/s. Measurements of fiber spacings along the blade height indicated that the rate of displacement of the solidification front exhibited some variations. These variations were closely associated with dimensional changes in the turbine blade cross sections.     

Ni-65wt%Nb alloy by aluminothermic reduction process

Ni–Nb alloys have been used as master-alloy in the production of Ni-based superalloys containing niobium. In the case of INCONEL 718, the most used superalloy, the niobium content is in the range 5.0–5.5 wt%. The aim of this work is to present results related to the production of Ni-65wt%Nb alloy by aluminothermic reduction process. Two different reactant mixtures were used: Nb₂O₅ + Ni + Al and Nb₂O₅ + NiO + Al. For each type of mixture the Al excess was varied from 0% to 15% over the stoichiometric amount. All the alloys were produced in a bulk form with Nb and Ni composition according to specifications. The best results came from the experiments involving mixtures of Nb₂O₅ + NiO + Al, where an overall metallic yield of 86% was obtained. The alloys produced from Nb₂O₅ + Ni + Al mixtures showed low metallic yields and high oxygen contents associated to the presence of a niobium oxide-phase in the microstructure.