β-NiAl Phase Research Articles

Microstructure reconfiguration of duplex high entropy alloy/BCC ferrite bimetallic interface under electropulsing

Additive manufacturing of multi-materials is one of the effective pathways to enrich the performance diversity of metallic materials, and it is generally acknowledged that the bimetallic interface is the weakest position because of stress concentration and compositional mutation. Here it was found that the stress concentration along the BCC + FCC duplex Al-Co-Cr-Fe-Ni high entropy alloy/BCC ferrite bimetallic interface can be significantly released by electropulsing, and the compositional mutation region was also reasonably widened from 2 μm to 10 μm. Meanwhile, the neighboring grains rotated to form High Angle Grain Boundaries, which can reduce the Gibbs free energy of the system. Additionally, the nano-scale second phase including spherical BCC NiAlFe and rod-like B2 NiAl particles was generated at the interface. The BCC NiAlFe phase is coherent with the matrix and has numerous crystal defects inside the precipitates, whereas the B2 NiAl phase has a stable structure without stress fluctuation because of its greater maturation degree.

Studies on the growth mechanism of aluminide coating on K444 alloy surface by chemical vapor deposition

The studies of the growth mechanism of aluminide coatings by chemical vapor deposition (CVD) is an important prerequisite for optimizing the thermal protection properties of aluminide coatings and improving the CVD process parameters; however, the current research on the growth mechanism of CVD aluminide coatings on nickel-based alloys is relatively limited, and there is a lack of systematic studies in this area. In this work, aluminide coatings were prepared on the surface of nickel-based superalloys by the CVD method, and elemental diffusion and microstructure effects between coating and substrate at different deposition temperatures and times, as well as the growth kinetics and formation mechanism of CVD aluminide coatings were investigated. The results showed that the coating prepared on the surface of K444 nickel-based high-temperature alloy by CVD was an externally diffused aluminide coating with a bilayer structure: the outer layer of the coating was the β-NiAl phase. The interdiffusion layer was mainly composed of nickel-aluminum compounds, carbides, and topologically close-packed phase. The growth of the CVD aluminide coating was mainly controlled by the grain boundary diffusion mechanism.

New insights into the initial oxidation behavior of an ultra-corrosion resistant alumina-forming austenitic steel in supercritical carbon dioxide: The effects of precipitated intragranular intermetallic compounds

Effects of precipitated intragranular intermetallic compounds on the initial oxidation behavior of a new alumina-forming austenitic (AFA) steel and its base steel in supercritical carbon dioxide are investigated. Laves phase is the only precipitate in the base steel, which leads to the internal oxidation and destroys the integrity of oxide. B2-NiAl, Sigma and Laves phases are precipitated in AFA steel. B2-NiAl particle acts as Al-reservoir and Laves particle retards the formation of thick Cr-rich oxide. Sigma phase accelerates the growth of Cr/Si-rich oxides, which contributes to the formation of a thicker Cr/Si-rich oxide layer.

NaCl-induced hot corrosion behaviors of NiSiAlY coatings

In the marine environments, NiCrAlY coatings with high content of Cr would suffer much severer corrosion due to the effect of NaCl. Thus, NiSiAlY coatings with different Si content were proposed and deposited on Ni-based superalloys by multi-arc ion plating. The as-deposited coating was mainly composed of γ’-Ni3Al phase with a small amount of β-NiAl phase. NaCl-induced hot corrosion tests were performed on the coatings at 500°C, 600°C, and 700°C, respectively. Compared with Ni-based alloy substrates, the NiSiAlY coatings exhibited a good corrosion resistance to NaCl at elevated temperatures. However, with an excessive amount of Si, the NiSiAlY coating showed a degradation in the hot corrosion resistance. In this work, the corrosion mechanisms of the tested coatings were discussed. Moreover, the role of Si was also investigated.

The effect of Al and Ti on the microstructure, mechanical properties and oxidation resistance of γ′-Ni3(Al, Ti) strengthened austenitic stainless steels

Both Al and Ti are the key elements for the formation of γ′-Ni3(Al, Ti) phase and protective oxide scale in the γ′-Ni3(Al, Ti) precipitation strengthened austenitic stainless (AS) steels. To obtain good mechanical properties, especially at elevated temperatures, high volume fraction of γ′ phase is required, which is strongly dependent on the total amounts of Al and Ti. For a given concentration of (Al + Ti), changing Al or Ti addition also has significant effects on the microstructure, mechanical properties and oxidation resistance of alloys. In this study, three alloys are designed by moderately adjusting Al and Ti addition while keeping (Al + Ti) concentration constant (8.7 at.%). It is found that increasing Ti and decreasing Al addition lead to fine Laves precipitates and close spacing between them, which in turn refine grain size and enhance strength at temperature lower than 750 °C. But at about 800 °C, the strength of there alloys is almost the same. Increasing Al and decreasing Ti addition have no apparent influence on the γ′ phase but promote the formation of B2–NiAl phase, resulting in reduced ductility at elevated temperature. Besides, high Al addition facilitates the formation of continuous protective oxide layer (mainly alumina) and is beneficial for oxidation resistance at 750 °C for 500 h. As Ti replaces Al, the integrity of protective alumina layer is destroyed and the discontinuous Al-rich oxides are interlaced with Cr-rich and Ti-rich oxides.

Effect of high temperature exposure on microstructure, mechanical and tribological properties of cold sprayed NiCoCrAlTaY coatings

NiCoCrAlTaY coating is widely used in high temperature oxidation resistance, gas erosion protection and gas path sealing of hot-end parts of aeroengines and gas turbines. In this study, NiCoCrAlTaY coatings were prepared on cobalt-based superalloy substrates by high pressure cold spray technology. The microstructure, mechanical property and tribological behaviors of the coatings before and after thermal exposure at 1050 °C were systemically studied. The results showed that NiCoCrAlTaY coatings with very dense structure could be obtained by high-pressure cold spraying with nitrogen as carrier gas. The coating remained completely intimate with the substrate without any cracking and spalling after thermal exposure at 1050 °C for 500 h. Due to elemental diffusion at high temperatures, the coating formed a four-layer structure, including alumina film on the surface, β-NiAl phase depletion layer near the surface, intermediate layer containing β-NiAl phase and β-NiAl phase depletion layer near the substrate. With the prolongation of thermal exposure at 1050 °C, the thicknesses of β-NiAl phase depletion layers both near the surface and the substrate increased. The thickness and content of the intermediate β-NiAl phase layer decreased gradually during thermal exposure. Besides, the average hardness values of the coating rapidly decreased by about 30 % after thermal exposure at 1050 °C for 5 h. In particular, the hardness of the β-NiAl phase depletion layer near the surface substrate was lower than that of the intermediate area containing β-NiAl phase. Due to the microstructure and hardness evolution, the wear resistance of the coating decreased significantly after thermal exposure. The friction coefficient of the coating increased from 0.2 to 0.5, and the volume wear rate of the thermally exposed coating was about 4.5 times higher than that of the as-sprayed coating. The results of this study provide guidance for design and application of high-performance MCrAlY coatings.

Cyclic oxidation behavior of Pt modified NiCoCrAlYHfZr + AlNiY coating on a Ni-based single crystal superalloy

In this study, the NiCoCrAlY + AlNiY, NiCoCrAlYHfZr + AlNiY and Pt modified NiCoCrAlYHfZr + AlNiY coatings are prepared by combining arc ion plating (AIP), electroplating Pt and annealing treatment. All the coatings are composed of three different zones including an outer layer and an inner layer as well as an interdiffusion zone (IDZ), therein, the outer layer consists of β-NiAl phase with dispersed α-Cr phase and the inner layer is composed of NiCoCr-rich phase with dispersed β-NiAl phase. In addition, an Y-rich belt is formed between the outer layer and the inner layer. The cyclic oxidation behaviors of the three coatings at 1100 °C are investigated. Results indicate that Pt and Y-rich belt exhibit synergistic effect on preventing the interdiffusion between coating and substrate, thus a thinner secondary reaction zone (SRZ) forms in Pt modified NiCoCrAlYHfZr + AlNiY coating. The evolution processes of Y-rich belts in the three coatings are also discussed. Besides, a lower oxidation rate and enhanced oxide scale adherence are achieved in Pt modified NiCoCrAlYHfZr + AlNiY coating, the enhanced oxidation performance is mainly attributed to the co-segregation of Pt-Hf-Zr-Y at oxide scale grain boundaries and the synergistic effect of Pt-Hf-Zr-Y on suppressing interfacial voids growth.

Design of oxidation resistant and spallation resistant low-β/near-γ/γ′-MCrAlYs for thermally-sprayed TBCs

HVOF-MCrAlY + APS-YSZ TBCs, compared to its APS-MCrAlY + APS-YSZ counterparts, are increasingly attractive to gas turbine OEMs and coating developers in recent years; however, producing a TBC with oxidation resistant and spallation resistant single-layer HVOF-MCrAlY bond coating remains to be a challenge, due to cracking in the YSZ/TGO vicinity and along the TGO/MCrAlY interface region. This work studies the influences of alloying elements on the maximum amount of β-NiAl phase in the MCrAlYs, and proposes to develop a single-layer oxidation resistant and spallation resistant HVOF-MCrAlY with a low-β/near-γ/γ′ microstructure, to reduce the strain at the TGO/MCrAlY interface resulted from the B2 → L10 martensite transformation on cooling from elevated temperatures.

Grain refinement and strengthening of PH 13-8Mo martensitic stainless steel through TiC/TiB2 inoculation during wire arc additive manufacturing

This research investigates the role of TiC/TiB2 nano-inoculants’ addition to the PH 13-8Mo martensitic stainless steel structure during wire arc additive manufacturing (WAAM). The microstructural analysis revealed that the introduction of the ceramic-based inoculants promoted the transition of columnar-to-equiaxed grain structure, refined the primary δ-ferrite grains, slightly increased the retained austenite content, and provoked the precipitation of in-situ TiC/M3B2 phases. Additionally, mechanical properties investigations showed that the TiC/TiB2 inoculation abated the anisotropic ductility as compared to the non-inoculated sample, and also improved the microhardness and tensile strength. Moreover, post-printing solutionizing and aging treatments were found to further enhance the hardness and strength of both TiC- and TiB2-reinforced components owing to the precipitation of nano-sized β-NiAl phases, while the accumulation of M3B2 phases in the intergranular regions led to the embrittlement of the TiB2-inoculated sample. This work, for the first time, illustrates the great potential for the adoption of ceramic-based nanoparticles as reinforcing/grain refining agents during the WAAM process for the fabrication of reinforced-PH 13-8Mo stainless steel components with a homogenous microstructure, nearly isotropic behavior, and superior mechanical performance.

Effect of coating degradation on the hot corrosion behaviour of directionally solidified superalloy

The effect of coating degradation on the hot corrosion performance of coated alloys was investigated in Na2SO4/NaCl (75:25, wt./wt.) salt at 900 °C. Degraded coatings were obtained by thermal exposure before the hot corrosion test. The results indicate that when the β-NiAl phases within the coating were completely depleted, the hot corrosion process was delayed due to the Al-rich interdiffusion zone (IDZ) formed after the 900 °C thermal exposure to provide Al for the healing of the Al2O3 layer. Nevertheless, the Cr-rich IDZ formed after 1000 °C thermal exposure failed to repair the Al2O3 layer, causing the alloy to corrode aggressively even though Cr has been substituted for Al to participate in the corrosion process.

High-temperature oxidation and wear resistance of nano-ZrB2 reinforced CoNiCrAlY composite coating via a step-fashion mechanical alloying powder process

A nano-ZrB2 reinforced CoNiCrAlY composite coating with highly dense structures was prepared by HVOF via a step-fashion mechanical alloying (MA) powder process. The microstructure, mechanical properties, high-temperature oxidation and wear resistance were analyzed. Results show that a high-quality CoNiCrAlY-ZrB2 powder with expected powder size and homogenous dispersion of nano-ZrB2 can be obtained by the 35 h step-fashion MA. The addition of nanoscale ZrB2 greatly improves mechanical properties of CoNiCrAlY-ZrB2 coating, while the low content β-NiAl phase hinders its oxidation performance. Due to the formation of “upper enamel layer” during sliding process and the excellent hardness/fracture toughness, the wear resistance of CoNiCrAlY-ZrB2 coating (2.46 ×10−14 m3·N−1·m−1) at 1000 ℃ is much better than that of CoNiCrAlY coating (1.9 ×10−12 m3·N−1·m−1), showing that CoNiCrAlY-ZrB2 coating has a high potential for protecting hot parts.

Precipitation of TCP phases with R/P intergrowth structure during directional solidification in a Ru-containing nickel-based single crystal superalloy

The topologically close-packed (TCP) phases with R/P intergrowth structure formed at the final stage of directional solidification in a Ru-containing nickel-based single crystal (Ni-SX) superalloy were investigated. The nodular TCP phases were distributed in the interdendritic regions of the as-cast microstructure. Precipitation of the β-NiAl phases and the subsequent incomplete peritectic reaction (Liquid + β-NiAl → γ′ + β-NiAlresidual) resulted in the accumulation of the TCP-forming elements (Cr, Co, Mo, W, Ta, and Re) in the residual liquid around the β-NiAl phase, which provided a favorable nucleation condition, leading to the precipitation of TCP phase in the vicinity of the β-NiAl phases. In addition, the TCP phases were found to be the R/P intergrowth structure with an orientation relationship of [2,7,10]R//[001]P and (322¯)R//(200)P, (44¯2)R ∼3.2° from (11¯0)P. Further, the formation mechanism of these TCP phases with R/P intergrowth structure was systematically discussed in terms of crystal structure, interface characteristics, and chemical composition. The result can provide theoretical guidance and experimental data for the composition design and heat treatment process optimization of the novel Ru-containing high-generation Ni-SX superalloys.

Multi-scale modelling of the mechanical behaviour of a CoNiCrAlY bond coat alloy during small punch testing

The present work is undertaken to determine the ability of the small punch test to determine mechanical properties of CoNiCrAlY, a two phase bond-coat material found in thermal barrier coatings. It utilises a multi-scale modelling methodology in which 2-dimensional representative volume elements of CoNiCrAlY and finite element models of the small punch test are developed. Effective Young’s moduli and Poisson’s ratios are obtained from the representative volume elements and are compared with those obtained from analytical homogenization through the Voigt, Reuss and Voigt-Reuss-Hill schemes. The volume fraction of the β-Nial phase in CoNiCrAlY is varied and the force–displacement responses of representative volume elements are contrasted with those from small punch test finite element models, through comparison of ratios of discrete fréchet distances. The results of the study estimated Young’s modulus and Poisson’s ratio to be 178.56 ± 7.65GPa and 0.249 ± 0.037 respectively, and for an increasing β-Nial phase volume fraction, the Young’s modulus and Poisson’s ratio decrease. Lastly, the results suggest that the small punch test is insensitive to variations in material elastic behaviour because its results are more influenced by the yield properties of a material.

Analysis of solidification crack behavior for Amdry 997 coating on Inconel 713 LC superalloy by laser cladding process

In this research, solidification crack behavior in the laser cladding of Amdry 997 on Inconel 713 LC superalloy was investigated. This study aims to reveal the relation between laser parameters (laser scanning speed, pulse width, and laser frequency) and cracking phenomenon and finally, achieve a free-crack clad region. It is found that the dilution ratio has a substantial effect on the cracking phenomenon. Clad zones with large diluted area experience longer cracks due to intensive thermal stress applied in two dimensions, whereas the severity and length of cracks decrease by dropping the dilution ratio. Solidification crack is dominant in this study and mostly happens in the middle area of the clad zone, where thicker liquid films of β-NiAl phase and M(Mo, Ta)C carbides are formed between the coarser dendrites of γ. Laser scanning speed of 140 mm/min, pulse width of 7.5 ms, and laser frequency of 16 Hz result in the minimum dilution ratio (28%) and free-crack clad zone.

Cerium-alloyed ultra-high strength maraging steel with good ductility: Experiments, first-principles calculations and phase-field simulations

Improving ductility while achieving ultra-high strength has always been a challenge in developing of high performance maraging steels. To this end, the study of the influence of Ce on the mechanical performance of B2–NiAl precipitation strengthened maraging steel was performed via experimental characterization combined with first-principles calculations-based phase-field simulations and crystal plasticity simulations. The tensile test revealed that the Ce-bearing maraging steel possesses a remarkable combination of ultimate tensile strength of ∼2000 MPa and tensile ductility of ∼10%. The microstructures at various scales were characterized by EBSD, HRTEM and APT. It was found that Ce exhibits a notable effect on the precipitation behavior of B2–NiAl particles because the presence of Ce decreases the precipitation driving force of B2–NiAl phase. EBSD characterization revealed that Ce-bearing steel possesses a higher fraction of HAGBs which facilitates the improvement the ductility of the experimental steel. Reconstruction of parent austenite grains and analysis of martensite variants selection were performed, and the results indicated that the lowering of the Ms temperature of the steel caused by Ce leads to more length fractions of V1/V2 and V1/V5 inter-variant boundaries, which contribute to the formation of higher fraction of HAGBs.

High-temperature creep property deterioration of the alumina-forming austenitic steel: Effect of σ phase

The microstructural evolution of the alumina-forming austenitic (AFA) steel subjected to different cold rolling reductions during creep test at 973 K and 120 MPa was investigated by multiple characterization methods, and the high temperature creep property was also examined. It was found that the creep property of CR10 sample is not satisfactory although cold rolling truly improved it. The creep cavities were observed to nucleate at the interface between B2–NiAl phase and σ phase in the δ-ferrite. The serious coarsening of B2–NiAl phase and σ phase in δ-ferrite was considered as the main factors for the decline of creep property. In addition, the results also suggested that the shape of B2–NiAl phase changes from round to plate with increasing creep time, which resulted from the increase of elastic strain energy. Simultaneously, it was noteworthy that σ phase always grows adjacent to plate-shape B2–NiAl phase, which indicates the coarsening of σ phase is inseparable from that of B2–NiAl phase.

On the nucleation site and formation sequence of secondary phases during isothermal high-temperature aging of wire arc additively manufactured PH13–8Mo stainless steel

For the first time, the nucleation site and the formation sequence of secondary phases during isothermal high-temperature (600 °C) aging process of a wire arc additively manufactured PH13–8Mo martensitic stainless steel were investigated herein. The microstructural characterizations revealed that the aging process starts with the nucleation of homogenously distributed spherical nano-sized β-NiAl phase within the martensite laths, followed by the formation of ellipsoidal M23C6 carbides, which promoted the formation of reverted austenite within the martensite laths and also along the lath boundaries at intermediate aging times. Lastly, new M23C6 carbides were formed within the reverted austenite at the final stage of the aging process.

Solidification and Evolution of β-NiAl Phase in a Re-Containing Single Crystal Superalloy

Solidification and Evolution of β-NiAl Phase in a Re-Containing Single Crystal Superalloy

Oxidation and hot corrosion behaviors of Mo-doped NiMoAlY alloys at 750 °C

Oxidation and hot corrosion behaviors of NiCrAlY, NiAlY and Ni-xMo-Al-Y (x = 1, 3, 5 and 10 wt%) alloys have been investigated. The results of the hot oxidation tests indicated that doping Mo could promote the formation of dense α-Al2O3 scale. Among the Mo doped alloys, the Ni-5Mo-Al-Y showed the lowest final mass gain (0.300 mg/cm2). Moreover, by doping Mo, the internal corrosion of the alloy could be effectively prohibited during the hot corrosion test. However, the excessive high content of Mo results in the formation of a mixed phase of β-NiAl phase and γ/γ' phase, leading to a poor hot corrosion resistance.

Effect of Pt-doping on the oxidation behaviors of the γ'-Ni3Al and β-NiAl phases in the NiSiAlY alloy

The oxidation behaviors of the NiSiAlY+xPt (x = 0, 1, 5 and 10 wt%) alloys were investigated at 900 ℃, with the focus on the Pt effect, phase and oxidation stage. The doped Pt improved the oxidation resistance but caused different oxidation behaviors in β-NiAl and γ'-Ni3Al phases. During the initial oxidation process, the Pt-doped NiSiAlY alloys showed a faster increase in mass gain, which was mainly contributed by the β-NiAl phase. However, at the stable oxidation stage, the undoped NiSiAlY alloy had the highest oxide growth rate, it was dominated by the oxidation behavior of the γ'-Ni3Al phase.