Effect of Yttrium on High Temperature Oxidation Resistance of a Directionally Solidified Superalloy

The effect of rare earth element yttrium on the high temperature oxidation resistance of a directionally solidified Ni-base superalloy was studied with scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and X-ray diffraction(XRD) techniques. The results show that the oxidation resistance of the alloy is substantially improved by adding proper amount of yttrium.     

Mechanical Properties of Uncoated and Aluminide-Coated René 80

Nickel-base superalloys such as René 80 are widely used in manufacturing aircraft turbine blades. They are usually coated in order to increase their wear, oxidation, erosion, and hot corrosion properties against environmental degradation. In this article, the mechanical behavior (tensile and low-cycle fatigue (LCF)) of uncoated and aluminide-coated (CODEP-B) René 80 has been studied at 871 °C and 982 °C. Experimental results show that the tensile properties of coated specimens are relatively lower than those of uncoated ones in the same conditions, but application of coating increases the LCF life of René 80 at T = 871 °C, 982 °C, R = (ε _min/ε _max) = 0, strain rate of 2 × 10⁻³ s⁻¹, and Δε _t  = 0.8 pct. Scanning electron microscopy (SEM) studies of coated specimens at N = Nf show that the nucleation of cracks occurs merely in substrate, but cracks start from the surfaces in uncoated specimens. Transmission electron microscopy (TEM) investigations have been performed on fractured uncoated specimens to evaluate the microstructures at different temperatures. The misfit dislocation, pair dislocations, and cutting of γ′ were observed at T = 871 °C and 982 °C. The TEM studies also showed that at 982 °C stacking fault was observed in γ′ particles.     

Effect of Submerged Arc Softening Welding Flux Component on Temperature

Based on simplex algorithm of optimal design, the multicomponent mixture regression model was used to investigate physical properties of submerged arc welding flux. The effect of complex interaction of seven components in agglomerated flux on softening temperature was analyzed. The results indicate that the interaction of MgO-TiO2-CaCOa-AI20a increases the softening temperature of flux, but the additions of CaF2 and ZrO2 can decrease the softening temperature.     

Microstructure and Crystallization Evolution of Directionally Solidified Al–Cu–Si Alloys With the Assistance of a Static Magnetic Field

Metallurgical and Materials Transactions A - We report on the study of the evolution of microstructure and crystallization during directionally solidified Al–Cu–Si alloys with the...     

Microstructural Characterisations and High Temperature Oxidation Studies of Nanostructured Co–Al Coatings Deposited on Superalloy

The microstructural characterizations and high temperature oxidation of magnetron sputtered Co?CAl coatings on the superalloy substrate have been studied in the present work. FE-SEM/EDS and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. Thermo gravimetric technique was used to investigate the oxidation of the coatings, in air at 900°C. The growth kinetics of the oxide layers was predicted using weight change of the coated sample measured during oxidation. It was found that the oxidation rate of Co?CAl coated superalloy was lower than that uncoated superalloy due to the formation of continuous dense, adherent and protective oxide scale such as CoO, Al₂O₃, Cr₂O₃, CoCr₂O₄, and CoAl₂O₄ over the surface of the coatings exposed to air at high temperature oxidation, 900°C. The microstructural features and phases of the oxidised coatings were used to elucidate the mechanism of high temperature oxidation.     

Plasma Arc Welding of High Strength 0.3 % C–CrMoV (ESR) Steel

0.3 % C–CrMoV (ESR) steel has been proposed as an alternate cost effective material for fabrication of solid boosters for satellite launch vehicles. PAW studies on 7.8 mm thick 0.3 % C–CrMoV (ESR) steel have been conducted with filler wires of two different compositions. Welding parameters have been established for the steel. Welding has been carried out with stainless steel and copper back-up bars and effect of the same has been evaluated. Weldment has been characterized through optical microscopy, microhardness and mechanical properties evaluation. It is found that filler wire chemistry (low carbon content) does not significantly affect the properties of weldments subjected to post weld hardening and tempering treatment. It is attributed to low dilution in thicker plate welding and diffusion of carbon from base metal side during hardening treatment of the weldment. Use of different back- up bars also show similar results, except for process difficulties noted while using stainless steel. Weld efficiency >85 % has been observed with weldments having microhardness in the range of 480–520 VHN.     

Application of Computational Thermodynamics to the Design of a Co-Ni-Based γ′-Strengthened Superalloy

A currently available commercial Calphad thermodynamic database was utilized to investigate its applicability to alloy design in the new class of Co-Ni-based γ′-strengthened high-temperature alloys. A simple primary design criterion was chosen: maximize the γ′ solvus temperature in the six-component Co-Ni-Al-Ti-W-Ta system while ensuring no formation of secondary, potentially deleterious phases. Secondary design considerations included the effects of alloying elements on equilibrium γ′ volume fraction and on solidus and liquidus temperatures. The identified composition, Co-30Ni-9Al-3Ti-7W-2Ta-0.1B (expressed in mole percent), representing a conservative estimate of the maximum allowable concentrations of alloying additions Al, Ti, W, and Ta, was subsequently produced and characterized. The experimentally measured γ′ solvus temperature of the new alloy was 1491 ± 3 K (1218 ± 3 °C), about 35 K (35 °C) above any previously reported two-phase γ−γ′ Co-(Ni)-based alloy. No secondary phases were observed in the alloy after annealing at temperatures between 1173 K and 1473 K (900 °C and 1200 °C). Additional alloy compositions with experimentally measured γ′ solvus temperatures in excess of 1533 K (1260 °C) were also identified employing the same basic approach. The efficacy of currently available thermodynamic databases in their application to Co-based γ′-strengthened superalloy development is discussed, including expanding design efforts to include additional alloying elements, as well as specific areas for improvement of future databases.

     

Effect of Magnesium on Inclusion Formation in TiKilled Steels and Microstructural Evolution in Welding Induced CoarseGrained Heat Affected Zone

 Effects of Mg on the chemical component and size distribution of Ti bearing inclusions favored grain refinement of the welding induced coarse grained heat affected zone (CGHAZ), with enhanced impact toughness in Ti killed steels, which were examined based on experimental observations and thermodynamic calculations. The results indicated that the chemical constituents of the inclusions gradually varied from the Ti O+Ti Mg O compound oxide to the Ti Mg O+MgO compound oxide and the single phase MgO, as the Mg content increased from 0002 3% to 0006%. A trace addition of Mg (approximately 0002%) led to the refinement of Ti bearing inclusions by creating the Ti Mg O compound oxide and provided favorable size distribution of the inclusions for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ with enhanced impact toughness. Otherwise, a high content of Mg (approximately 0006%) produced a single phase MgO, which was impotent to nucleate an acicular ferrite, and a microstructure comprised of a ferrite side plate and a grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.     

Influence of CeO_2 on the Microstructure and Wear Resistance of M_(80)S_(20) Thermal Spray and Thermal Spray Welding Coatings

The influence of rare earth oxide CeO_2 on microstructure and wear resistance of M_(80)S_(20) thermal sprayand thermal spray welding coatings is studied using optical microscope,scanning electron microscope,X-rayenergy dispersion spectroscope,X-ray diffractometer and wear testing machine.The results show that the addi-tion of 8% CeO_2 can improve the microstructure,microhardness and wear resistance of coatings significantly.     

Tempering-Induced Microstructural Changes in the Weld Heat-Affected Zone of 9 to 12 Pct Cr Steels and Their Influence on Sliding Wear

Increasing amount of tribological applications is working under alternating high/low temperature conditions where the material is subjected to temperature fatigue mechanisms such as creep, softening due to annealing, and at the same time must withstand mechanical wear due to sliding contact with pairing bodies. Steam turbine valves, gate valves, valve heads, stems, seats and bushings, and contacting surfaces of the carrier elements are some examples of such applications. The purpose of the present study is to evaluate the potential of X20 and P91 steels as materials for applications operating under combined effect of mechanical wear and alternating high/low temperature conditions. It was focused on how the microstructural changes occurring in the weld zone affect the wear properties of the selected materials. Generally, with longer tempering time and higher tempering temperature, the number of carbide precipitates decreased, while their relative spacing increased. Before tempering, the morphology of the steel matrix (grain size, microstructure homogeneity) governed the wear resistance of both steels, while after tempering wear response was determined by the combination of the number and the size of carbide particles. After tempering, in X20 steel larger number of stable M₂₃C₆ carbides was observed as compared with P91 steel, resulting in lower wear rates. It was observed that for both steels, a similar combination of number density and size distribution of carbide particles provided the highest wear resistance.     

Effect of Ice Sheet on Stochastic Response of Offshore Wind Turbine–Seawater–Soil Interaction Systems Subjected to Random Seismic Excitation

To investigate the effect of the presence of surrounding ice sheet on the stochastic response of offshore wind turbines subjected to random seismic excitation, this research uses a three-dimensional, numerical, finite-element model that includes viscous boundaries. The model of the ice–seawater–offshore wind turbine–soil interaction system uses the Lagrangian fluid (displacement-based) and solid-quadrilateral-isoparametric finite elements. Random seismic excitation from a filtered white-noise model and applied to each support point of the three-dimensional finite-element model of the coupled interaction system provides the experimental environment. A parametric study examines the effects of both the presence of and variation in mechanical and geometric properties of the surrounding ice sheet on the stochastic response of offshore wind turbines. The investigation also includes the effect of the wind turbine’s wall thickness in relation to the ice sheet on the stochastic seismic response of the coupled interaction system.     

Aging Effects on Heat Treatment Response and Mechanical Properties of Al-(1 to 13 pct)Si-Mg Cast Alloys

The effects of solution treatment time and Si content and morphology on microstructures and mechanical properties of heat-treated Al-Si-Mg cast alloys were investigated systematically. Five alloys, with Si levels ranging from 1 to 13 pct, were tested in as-cast, T4, and T61 conditions. The eutectic Si was both unmodified and Sr-modified. Results show that the microstructures are affected significantly by alloy composition, eutectic Si morphology, and solution treatment time. Si content has significant effects on ultimate tensile strength (UTS), yield strength (YS), and elongation as well as a strong influence on solution treatment response. In T61 treatment with different solutionizing times, UTS and YS reach their maximum values in ~1 hour of solutionizing followed by a decrease, then a slight increase, and finally, a plateau close to the maximum level. Elongation of alloys with a high Si content, 7 pct and 13 pct, increases rapidly at solutionizing times of 1 to 2 hours then varies in a wide range, showing improvements in the 4 to 10 hours range. The data indicate that a solution treatment time of ~1 hour is sufficient to achieve maximum strength. The changes in mechanical properties were correlated to changes in microstructure evolution—Mg-Si precipitation, Si particle fragmentation, and microstructure homogenization. Empirical models uniquely relating Si content to UTS and YS are given for T61 heat-treated alloys.     

Dissimilar Impact Welding of 6111-T4, 5052-H32 Aluminum Alloys to 22MnB5, DP980 Steels and the Structure–Property Relationship of a Strongly Bonded Interface

Metallurgical and Materials Transactions A - The ability to weld high-strength aluminum to high-strength steel is highly desired for vehicle lightweighting but difficult to attain by conventional...