Temperature dependence of dynamic Young's modulus and internal friction in LPPS NiCrAlY

The piezoelectric ultrasonic composite oscillator technique (PUCOT), operating near 80 kHz, was used to measure the temperature dependence, in the range 23–1000 °C, of dynamic Young's modulus,E, and internal friction,Q ^–1 in three compositions of low-pressure plasma-sprayed NiCrAlY: Ni-15.6Cr-5.2Al-0.20Y (16-5), Ni-17.2Cr-11.6Al-0.98Y (17–12), and Ni-33Cr-6.2 Al-0.95 Y (33–6). Ambient temperature (23 °C) dynamic Young's moduli for the three alloys were 205.0, 199.8, and 231.0 GPa, respectively. In each case, dE/dT was found to be — 0.06 GPa °C^–1 over temperature ranges 23–800, 23–400 and 600–900, and 23–700 °C, respectively. Internal friction was essentially independent of temperature to about 600 °C (700 °C for the 16–5 alloy), at which point a temperature dependence of the formQ ^–1 =A exp (C/RT) was observed. The constantA for the three alloys was determined to be 62.7, 555, and 2.01 × 10⁶, respectively. The constantC for the three alloys was determined to be 82.8, 111, and 170 kJ/mol^–1, respectively. While the physical mechanism is not fully understood, both the pre-exponential constantA and the activation energyC correlate with durability in thermal barrier coatings (TBCs) wherein these alloys are used as bond coats.     

Formation of nano-sized Y2O3 dispersoids in mechanically alloyed Ni-(Cr, Y2O3, Y) alloys during heat-treatments

In the present work, the evolution of nanoparticles during annealing and hot-consolidation in mechanically alloyed Ni-22Cr-1.5Y, Ni-22Cr-1.5Y2O3 and Ni-3% Y2O3 was examined. The high-energy ball-milling of elemental powders resulted in the complete dissolution of the constituent Cr, Y, or Y2O3, forming a Ni-based solid solution. During the subsequent annealing, however, oxide particles precipitated from the solid solution. In the case of mechanically alloyed Ni-22Cr-1.5Y2O3, over-grown Cr2O3 precipitated at a temperature as low as above approximately 500 degrees C and ternary YCrO3 particles precipitated at 1100 degrees C. In the case of mechanically alloyed Ni-22Cr-1.5Y, on the other hand, the binary Y2O3 phase precipitated at 1100 degrees C during spark plasma sintering. The presence of Cr in the alloy composition facilitated the formation of Cr2O3 or YCrO3, and the precipitated oxides were highly prone to grain growth during hot-consolidation, sometimes reaching several micrometers. In Cr-exempt Ni-3%Y or Ni-3% Y2O3, however, the growth of nanodispersoids was restrained even at temperatures as high as 1000 degrees C and the resulting dispersoid was only nano-sized Y2O3.     

Microstructure evolution during mechanical working of three RS aluminium alloys: Al-4Cr-1Fe,Al-5Cr-2Zr and Al-6.43Cr-1.67Zr

Three Al-Cr alloys containing additions of Zr and Fe have been fabricated via cold compaction and hot extrusion. The decomposition of the powder microstructure and the subsequent coarsening during thermomechanical treatment have been studied. Detailed electron microscopy investigations were performed at different locations of partially extruded billets at 450 °C. The microstructure of the dead metal zone reflects the effect the induction heating exerts on the as-atomized powder microstructure. In the low Cr alloys, Al-4Cr-1 Fe and Al-5Cr-2Zr, decomposition of the rapidly solidified microstructure commences at the rich intercellular network, whereas the microstructure of the Al-6.43Cr-1.67Zr alloy remains relatively unaffected. Within the deformation zone the precipitation kinetics are affected by the shearing and the temperature rise. The cells and the powder particles are aligned along the extrusion direction. Precipitation is taking place within the primary segregation-free areas, observed in Al-4Cr-1 Fe and Al-5Cr-2Zr alloys, whereas in the Al-6.43Cr-1.67Zr alloy, decomposition of the powder microstructure starts at the Cr-rich intermetallic particles. The as-extruded microstructure is fibrous and heterogeneous. The heterogeneity of the as-extruded microstructure is a result of the microstructural variation observed within different size powder particles and within individual ones.     

Mechanical properties of precipitation-strengthened Ni-Al-Cr alloy based on an NiAl intermetallic compound

Mechanical properties of a ternary alloy Ni-30.3 at. % Al-6.6 at. % Cr have been studied in the temperature range 25 to 1100° C. The material was heat-treated to produce a stable dispersion of incoherent rod-shaped Ni₃Al precipitates, 1m in diameter and 20m long. The tensile properties were found to be temperature dependent. Below 750° C the material had high strength, low ductility and low strain-rate sensitivity, whilst above 750° C the strength fell, ductility increased and the material became strain-rate sensitive. The room temperature fracture toughness of the single-phase material was 6 MN m^–3/2 and increased to 50 MN m^–3/2 in the two-phase material. This can be attributed to the effect of Ni₃Al on crack nucleation and propagation.     

Oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet fabricated by Electron Beam Physical Vapor Deposition at 800 °C

Isothermal oxidation of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet fabricated by Electron Beam Physical Vapor Deposition (EB-PVD) at 800 °C in air was investigated. The oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet followed a parabolic power law at initial oxidation stage, and cubic power law for long oxidation terms. The fine oxide grains could be formed on microcrystalline superalloy surface. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with cubic power law was derived. The excellent results in agreement with the experimental data were obtained.     

Structural peculiarities of cermets design based on titanium carbide

The influence of chemical composition on the ductile brittle transition (DBT) temperature, microstructure and mechanical properties of cermets based on titanium carbide (chemical composition of cermets TiC _x , (x=0.55; 0.65; 0.75 and 1.0) — nickel-based superalloy (Russian Trade Mark GS6U containing (wt%) Ni-10W-10Co-9Cr-5.5Al-2.5Ti-2Mo-0.15C) was studied. It was shown that high strength in cermets can be achieved if the structure of the frames is retained and the DBT temperature of the refractory phase is controlled. By changing the microstructure and its homogeneity, the chemical composition of the frames, the porosity value and the binding material, it is possible to synthesize new cermets for certain temperature regions. The optimum operating temperature of such materials depends on the DBT temperature and the temperature of the transition to a superplastic state of refractory phases.     

掺杂纳米CeO2对ZrO2-Y2O3热障涂层隔热性能的影响

以纳米ZrO₂-8 wt%Y₂O₃(YSZ)和在纳米ZrO_2-8wt%Y₂O₃中分别掺杂25wt%和50wt%纳米CeO₂团聚处理后作为隔热层材料,NiCrAlY(Ni-25Cr-5Al-0.5Y,wt %)作为粘结层材料,用等离子喷涂(APS)方法在GH30高温合金表面制备三种材料体系的热障涂层(TBC) 。通过扫描电镜(SEM)和X射线衍射仪(XRD)对掺杂了25wt%纳米CeO₂涂层的微观组织结构进行分析研究,测定了三种材料涂层在室温和 300、500、700℃时的热导率,并在相同边界条件下测试了它们的隔热性能。结果表明：掺杂纳米CeO₂涂层组成相为稳定的t相(t-ZrO₂、t-Zr_0.82Y_0.18O_1.91、t-Zr_0.82Ce_0.18O₂)和c相(c-CeO₂) , 涂层中存在闭合的孔隙和微裂纹;掺杂纳米CeO₂能够降低涂层的热导率,并且隔热性能随CeO₂含量的增加而提高。对于 400μm厚的CeO₂/ZrO₂-Y₂O₃涂层(CYZ , 掺杂25wt%CeO₂)对基体产生的温降比纳米YSZ涂层提高了10.7%,当CeO₂的含量从25wt%提高到50wt%时,隔热性能也提高了7.1%。     

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.     

Comparison of densification and distortion behaviors of W-Ni-Cu and W-Ni-Fe heavy alloys in liquid-phase sintering

This paper aims to correlate the densification and distortion behaviors of liquid-phase sintered 80W-14Ni-6Cu and 80W-14Ni-6Fe heavy alloys with the melting characteristics of the Ni-Cu and Ni-Fe matrices. Differential thermal analysis (DTA) of die-pressed compacts reveals that the melting range of the Ni-Cu matrix is extended from 1235°C to 1453°C by the in situ alloying between elemental Cu and Ni powders, whereas the melting of the Ni-Fe matrix is limited to a narrow temperature range between 1464°C and 1480°C. Dilatometry and furnace sintering tests show that densification due to liquid-phase sintering of 80W-14Ni-6Cu starts at 1287°C and proceeds at a low rate to 1450°C, where full densification without distortion is achieved. In contrast, densification due to liquid-phase sintering of 80W-14Ni-6Fe occurs at a very high rate above 1475°C, and full density can be obtained at 1500°C. For both alloy compacts, distortion is induced by prolonging the sintering time or elevating the sintering temperature after full densification. Crack-like voids develop in the 80W-14Ni-6Cu compact to accommodate the gravity-induced distortion, while spherical pores are dominantly formed in the 80W-14Ni-6Fe compact as a result of water vapor entrapment.     

Coatings for protecting nickel-base TaC eutectics against oxidation

A class of very high strength eutectic superalloys consisting of TaC fibers in a γ- Ni/γ'-Ni₃Al matrix is being critically evaluated as a potential turbine blade material. However, the strongest alloys in this system lack the inherent oxidation resistance needed for extended use at 1100°C and above. The present study describes a program aimed at improving oxidation resistance and coating-substrate compatibility for a substrate of a γ/γ'-TaC which has been designated NiTaC- 13.A preliminary study considered seven different electron beam vapor-deposited coating systems of the MCrAlY type (M≡Ni, Co or Fe). The most promising coating was a NiCrAlY composition which showed excellent oxidation resistance; however, during high temperature exposure of all of these coated materials, a region with complete absence of carbide fibers developed in the substrate at the coating-substrate interface.The mechanism of carbide denudation was investigated using samples coated with Ni-20Cr-10Al-lY. It was concluded that a primary cause of fiber denudation was diffusion and dissolution of C in the C-free coating rather than carbon diffusion through the coating with resulting oxidation at the coating-atmosphere interface.NiCrAlY coatings containing C were deposited by several techniques to determine whether C additions would be useful in improving coating-substrate compatibility. The most promising coating was a duplex coating; the first layer of Ni-20Cr-5Al-0.1Y-0.1C was deposited by flame spraying and this was subsequently overcoated with Al. This coating withstood 2000 h of cyclic oxidation with little loss of coating thickness and absence of fiber denudation.Sequential metallographic examinations were made through the 2000 h of testing to show the development of the coating-substrate interactions. Microstructural and phase changes are described and are compared with macrostructural changes and the change in sample weight indicative of oxidation and spallation.     

Tensile microstrain properties of vapour quenched and rolled Al-alloys

Al-alloy deposits produced by quenching from the vapour phase were rolled into sheet and tensile stress-strain curves were obtained in the microstrain region (<10⁻⁴ strain). Two binary alloys (Al-5% Cr and Al-11% Cr) and two ternary alloys (Al-8% Cr-1.2% Fe and Al-6% Cr-1.0% Fe) were tested (compositions in wt%). The corresponding tensile strengths were 375 and 590 MPa for the binary alloys, and 679 and 662 MPa for the ternary alloys. The microyield stresses (the stress to produce a residual plastic strain of 2×10⁻⁶) for the vapour quenched alloys were less than 26 MPa compared with 100 to 260 MPa for commercial aluminium alloys. When the vapour quenched alloys were retested immediately after prestrain, the values were much greater (166 to 254 MPa), but after resting at room temperature for 7 to 106 days low values were again obtained. The low microyield stresses were attributed to the high density of dislocations present in the rolled sheet and the behaviour observed may be characteristic of high strength dispersion hardened Al-alloys produced by rapid solidification and rolling.     

Ni—Al—Hf—Cr—W系合金中相的金相显示

针对Ｎｉ－Ａｌ－Ｈｆ－Ｃｒ－Ｗ复合金，对体系中会出现的各种相进行研究，根据它们的化学特性，寻找单相显示的不同方法，为合金主相的鉴别、分布以及相的定量分析提供条件。     

Influence of the microstructure on the plastic behaviour of duplex stainless steels

Microstructure change of α (ferrite) + γ (austenite) two-phase structure in duplex stainless steels deformed by hot torsion tests is briefly analyzed. Two types of stainless steels containing different volume fractions of ferrite and austenite were torsion deformed at temperatures ranging from 900 to 1250 °C. Steel A (25.5Cr-4.9Ni-1.6Mo) contained Cr_eq/Ni_eq = 4.8 and steel B (22.2Cr-5.6Ni-3Mo) contained Cr_eq/Ni_eq = 3.5 bring about different microstructures and flow stress behaviour. The results show that the shape of the flow stress curves depends on the material and on deformation conditions. Four different flow curve shapes were observed. At high temperatures, steel A displayed a plastic behaviour typical of ferritic stainless steels. As the deformation temperature decreased, the flow curves presented peak stresses at low-temperature deformation. When the austenite particles are distributed coarsely in the matrix (steel B), the plastic flow curve displays a stress peak separating extensive regions of hardening and softening. When both phases have the same volume fractions, the microstructure is characterized by percolation of the two phases in the samples, and the plastic flow curve takes on a very distinctive shape in hot torsion tests. The role of the microstructure present during deformation on the shape of the flow stress curves is analyzed.     

The high temperature impact-sliding wear and oxidation resistance of several cobalt-based oxide- containing detonation gun coatings

The wear resistance of several detonation gun coatings under conditions of combined thermal shock and impact-sliding wear at elevated temperatures was evaluated. The conditions of testing were chosen to help rank materials for use in the turbine section of gas turbine engines, on high temperature steel mill equipment and in other severe industrial environments. The coatings studied had nominal compositions of (Co-25Cr-10Ta-7Al-0.5Y-2C)-10Al₂O₃ (LCO-17A), (Co-30Cr- 10Ta-0.5Y-2C)-10Al₂O₃ (LCO-19A) and (Co-28Cr-19W-5Ni-1V-0.85C)- 3Cr₂O₃ (L-103A). These were compared with Co-28Cr-19W-5Ni-1V-0.85C (CM 64) weld deposits.The tests comprised self-mating wear of the materials on a 6.35 mm pin with a coated end 12.7 mm in radius against a flat pad. In each mechanical cycle, the pin struck the pad, slid across it for a distance of 6.35 mm under a load of 5 kgf, reversed its sliding direction for the same distance and lifted off the pad; this occurred at a frequency of 530 cycles min^-1. The simultaneous thermal cycle consisted of heating for 50 s with a propane-H₂-O₂ throat-burning torch, followed by cooling with a forced air jet to near ambient temperature for 50 s. Maximum temperatures were 800, 900 or 1000°C. Each material was tested for about 300 thermal cycles.The results indicated that the coatings were superior to CM 64 at all three temperatures. On the basis of surface and metallographic observation of the scars, it appears that the formation of oxide scales had an important bearing on the wear rates. These results combined with oxidation and fretting wear test results suggest that LCO-17A is the material of choice at the higher temperatures but does not replenish its oxide film rapidly enough at low temperatures. L-103A is superior at the lower temperatures but has too high an oxidation rate above 900°C. LCO-19A is probably the best choice at intermediate temperatures.     

Oxidation Behaviour of Sputtered Ni-3Cr-20Al Alloy

1. IntroductionNi-base superalloys are the important materialsused at high temperature and greatly rely on the presence of Cr and/or Al to form a protective scale ofCrook or Allot. Therefore, the investigation of oxidation of the simple Ni-Cr-Al ternary alloys is beneficial to the understanding of oxidation patterns of complex Ni-base superalloys. Some works have been conducted on the Ni-Cr-Al alloys with normal grain sizeand the results indicated that different oxides couldbe formed accor…     

Fabrication,properties and structure of a high temperature light alloy composite

Paniculate alumina reinforced Al-4Cr-1 Fe alloys were fabricated from rapidly solidified aluminium alloy powder and commercially purchased alumina powder by traditional powder metallurgical techniques involving powder mixing and cold compaction followed by hot extrusion. The tensile tests at ambient temperature indicated a considerable improvement in the mechanical strength at the expense of ductility and modulus. Poor values of modulus were explained by the presence of porosity in the composites. The high temperature mechanical properties of the matrix, tested at 350 °C after prolonged exposure to the test temperature under static air conditions, were intrinsically poor. Additions of the filler material, alumina particles, up to a weight fraction of 15% did not improve the high temperature performance of the matrix substantially. Possible causes for this are discussed and alternatives proposed.     

An investigation of the fatigue and fracture behavior of multicomponent Nb-11Al-41Ti-1.5Mo-1.5Crintermetallic

This paper presents the results of a recent study of the fracture and fatigue crack growth behavior of a newly developed multicomponent niobium aluminide intermetallic Nb-11Al-41Ti-1.5Mo-1.5Cr alloy (compositions quoted in at % unless stated otherwise). The alloy is shown to have attractive combinations of room-temperature tensile ductility (approx. 11%) and fracture toughness (approx. scds 83 MPa ) in the as-forged condition. However, the tensile properties and fracture toughness are degraded somewhat by direct aging at 750 °C for 25 h. The direct aged Nb-11Al-41Ti-1.5Mo-1.5Cr intermetallic is also shown to have comparable fatigue crack growth resistance to pure Nb, IN 718 and mill annealed Ti-6Al-4V at room- and elevated-temperature. Fatigue and fracture mechanisms are elucidated prior to a discussion of the implications of the current results for potential high temperature structural applications.     

Enhanced grain refinement and mechanical properties in a severely plastic deformed Ni-30Cr alloy

The present study was carried out to evaluate the microstructures and mechanical properties of severely deformed Ni-30Cr alloys. Cross-roll rolling (CRR) was introduced as a severe plastic deformation (SPD) process. Ni-30Cr alloy sheets were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the recrystallized microstructure. Electron back-scattered diffraction (EBSD) was introduced to analyze grain boundary character distributions (GBCDs). The application of CRR to the Ni-30Cr alloy effectively enhanced grain refinement through heat treatment; consequently, the average grain size was significantly refined from 33 μm in the initial material to 0.6 μm. This grain refinement directly improved the mechanical properties, in which yield and tensile strengths significantly increased relative to those of the initial material. We systematically discuss the grain refinement and accompanying improvement in mechanical properties in terms of the effective strain imposed by CRR relative to conventional rolling (CR).     

On the influence of alloying elements on the bainite reaction in low alloy steels during continuous cooling

The CCT diagrams of a class of Fe-(0.1–0.6)C-(0.4–2.0)Si-(0.4–2.0)Mn-(0.5–2.0)Cr-(0.0–0.8)Mo steels are predicted by an artificial neural network (ANN) model. The model indicates that an increase in carbon concentration (C wt%) gives rise to a decrease in bainite start (BS) temperatures. The rate of decrease depends also on cooling rate. Additions of Si, Mn, Cr and Mo all decrease the bainite start temperature. The dependencies for different alloying elements vary: 32, 100–120, 100–130, and 70–150°C per wt% of Si, Mn, Cr, and Mo, respectively. Mn shifts the whole bainite transformation region to the far right-hand side of the CCT diagram, while C, Cr, and Mo have considerable, and Si has minor effects on the incubation period of bainite. Mn and Cr significantly decrease the MS temperature, while Si only has a minor influence. When Mo < 0.5 wt% it has a minor influence, whilst when Mo > 0.5 wt%, it increases MS temperature. Quasi-isochronal and quasi-isothermal methods have been used to analyze the influence of the proportion of Mo and C upon the BS and incubation period. Attempts, for qualitative explanations using the shear and diffusion mechanism, as well as a certain amount of thermodynamic analysis, have been made to interpret the influence of alloying elements on the nucleation of the bainite reaction. The results support that bainite reaction takes place utilizing a diffusion-controlled mechanism.     

Effect of yttrium on elastic strains and fracture of Cr2O3 scales on Ni-30Cr alloys

It has long been known that the addition of so-called reactive elements, such as Y, to alloys forming chromia scales improves the protectiveness of those scales by reducing the rate of scale growth and by reducing the tendency of the scales to spall during thermal cycling. It has been suggested that reduced spallation might arise from better scale adherence or from improved resistance to fracture.

In this study Ni-30Cr and Ni-30Cr-0.5Y alloy samples were oxidized at 1000°C in pure O₂ for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. Scale fracture events, which are detected by acoustic emission, were collected throughout the experiment. An in situ X-ray diffraction technique was used to measure the elastic strains in the oxide scales at the isothermal scale growth temperature and at several temperatures during cooling.

These measurements found higher elastic strains in the Cr₂O₃ scales, during both isothermal oxidation and cooling, but for fewer cracking events, on the yttrium-containing alloy than on Ni-30Cr. We infer that the addition of yttrium increased the adherence of the scale to the substrate, since the scale was able to withstand higher elastic stresses.