Interdiffusion Zone Research Articles

Synergistic effects of Pt and Y addition in (Ni, Pt)CrAlY bond coat on oxide spallation resistance and growth of interdiffusion zone between bond coat and Ni-based single crystal superalloy

This study shows the beneficial effect of Pt addition to the (β+γ)-NiCrAlY coating. Adding Y in NiCrAl increases oxide growth kinetics but improves spallation resistance. Pt with Y does not change the oxidation rate compared to only adding Y but significantly reduces spallation when cooled to room temperature. Continuous layers of Al2O3 and Cr2O3, along with other oxides such as YAlO3, NiCr2O4, and NiAl2O4, are observed. Moreover, Pt addition reduces the thickness of the interdiffusion zone where Al goes through an uphill diffusion profile, minimizing Al loss and significantly decreasing the growth of deleterious TCP phases.

Si modified aluminide coatings on a Ni-base superalloy prepared using Al-Si plasma irradiation: Growth mechanism and oxidation behavior

The growth mechanism and high temperature oxidation behavior of Si-modified aluminide coatings prepared on a Ni-base single crystal superalloy DD98M using vacuum plasma irradiation were investigated. The growth of the coatings under plasma irradiation was much faster than thermal diffusion-controlled process. The coatings comprised an aluminide outer zone, where Si existed as solid-solute, and an inter-diffusion zone where silicide particles precipitated in aluminide matrix. The crystalline structures of the aluminides were found dependent on the irradiation power. β-NiAl was formed at higher irradiation power while δ-Ni2Al3 was formed at lower irradiation power. The β-NiAl coatings had higher oxidation resistance than the δ-Ni2Al3 ones usually, with an exception of one β coating which was prepared at the highest substrate bias which underwent severe internal oxidation. The presence of numerous voids in the oxide scale of Ni2Al3 may be related to the large number of Kirkendall voids present in the as-prepared coating.

Phase evolutions and the growth of Kirkendall voids in the V–Zn system

The V-Zn phase diagram became available after 1990. The existence of the VZn16 phase has been questioned mainly. A solid–state diffusion experiment is conducted to reinvestigate the stability of phases. The interdiffusion zone is examined using scanning electron microscopy and energy dispersive spectroscopy. Our analysis confirms that VZn16 is one of the stable phases. Additionally, the growth of the VZn9 product phase is observed in the V/Zn diffusion couple, indicating that this newly discovered phase is also stable. Furthermore, the formation of the Kirkendall voids is observed in the VZn3 phase, which can affect the physico–mechanical reliability of any component during load-bearing application. Moreover, this study's findings could also be beneficial for the ongoing efforts of designing promising Zn–based biomaterials.

Phase-field modeling of interdiffusion between dissimilar Fe-Cr-Ni alloys during non-isothermal hot isostatic pressing

Powder metallurgy hot isostatic pressing (PM-HIP) has emerged as a promising alternative to welding for joining dissimilar metals. During HIP, interfacial bonding is mediated by solid state diffusion. The interdiffusion zone across the interface depends on processing conditions, calling for the need for accurate numerical tools capable of simulating interdiffusion and possible phase transformation in order to optimize processing parameters. Here, a phase-field (PF) model based on CALPHAD-based free energy functionals is developed to simulate the interdiffusion and phase evolution between dissimilar Fe–Cr–Ni based steels undergoing HIP and is demonstrated using the interface between 316L and SA508 steels. To overcome the numerical challenges caused by the singular magnetic and entropy terms in the CALPHAD free energy models in the Fe–Cr-Ni system, polynomial functions are fitted with temperature dependent coefficients represented by Fourier series to accurately describe the phase stability of both fcc and bcc phases in the composition and temperature space. This enables simulations of non-isothermal HIP cycles. Diffusivity data from commercial software and literature are taken to parameterize the kinetic parameters. A discrete nucleation model is incorporated for possible phase transformation. The modified thermodynamic models are validated against previous experiments at 923 K and 1273 K. The interdiffusion kinetics are benchmarked against new HIP experiments joining powder and bulk 316L to bulk SA508 with three different HIP cycles. The good agreement between simulations and experiments on both phase stability and interdiffusion indicate that the model is suitable for simulating interdiffusion between Fe–Cr–Ni alloys during HIP cycles. It is also found that using powder and bulk 316L gives similar interdiffusion profiles at elevated temperature when a dense interface forms during HIP.

Long-term oxidation resistance of CrAl(Si)N coatings deposited on TiAl-based alloy at 950 °C

CrAl(Si)N coatings were deposited on Ti-48Al-2Cr-2Nb alloy by arc ion plating. Long-term cyclic oxidation behaviors of the CrAl(Si)N-coated alloy were investigated at 950 °C. The oxidation resistance of the Cr0.4Al0.5Si0.1N coating was significantly better than that of the Cr0.5Al0.5N coating. The addition of Si to the CrAlN coating promoted the formation of an almost continuous inner Al2O3 layer in the oxide scale. The continuous and dense Cr2O3/Al2O3 oxide layer acted as a diffusion barrier and retarded the inward diffusion of oxygen and outward diffusion of the coating elements. However, interdiffusion between the CrAl(Si)N coatings and the alloy was serious and a thick interdiffusion zone formed.

Effect of coating-substrate microstructure on nucleation and growth of surface microcracks in a PtAl-coated Ni-based SX superalloy with sheet specimens during HCF at 900 °C

The fatigue crack initiation process accounts for the main portion of the high-cycle fatigue (HCF) life. Although coated superalloys are well-known to be prone to surface microcracks, the influence of the coating-substrate microstructure on the damage mechanism during HCF of the coated single crystal (SX) superalloys remains unclear. In the current study, HCF failure and interrupted tests were conducted at 900 °C under a low applied maximum stress on a PtAl-coated third-generation SX superalloy using sheet specimens. The HCF crack initiation was investigated by analyzing the evolution of surface microcracks and coating-substrate microstructure. The results show that the surface microcracks nucleation and growth are controlled by the initial microstructure of the coating-substrate region and the oxidation process during the HCF crack initiation stage. The rapid nucleation and growth of the surface microcracks are promoted by grain boundaries (GBs) in the coating and the interdiffusion zone (IDZ). Comparatively, the surface microcracks are blunted by oxidation in the SX substrate due to the loss of GBs, leading to a slower microcrack growth. However, the surface microcracks can gradually grow further into the substrate by repeating the processes as follows: oxidation at the crack tip, formation of γ′-depleted region due to oxidation, recrystallization at the γ′-depleted region and cracking along the GBs in the recrystallized γ′-depleted region. Subsequently, the surface microcracks, which reach the critical size, propagate along the slip bands during the progressive propagation stage. This study will contribute to improve the HCF durability of the PtAl-coated SX superalloy.

Protective silicide coating fabricated on ductile VNbTa refractory complex concentrated alloy: Microstructure, high-temperature oxidation and wear behaviors

VNbTa refractory complex concentrated alloy (RCCA) possesses broad application prospects in a wide temperature window owing to its remarkable strength-ductility synergy and super rolling formability, whereas poor oxidation resistance will give rise to excessive wear above 800 ℃. In this contribution, we aim to fabricate a protective silicide coating on VNbTa RCCA for the purposes of enhancing high-temperature oxidation as well as wear resistance. Microstructural characterizations revealed that a uniform and intact coating was obtained under the optimized processing condition of pack cementation. The coating is mainly composed of a single-phase (VNbTa)Si2, along with a thin M5Si3-type interdiffusion zone. Compared with bare alloy, the coated specimens exhibit remarkable oxidation resistance at 1000 ℃ due to the preferential formation of protective SiO2 scale. Meanwhile, the coated specimens maintain desirable wear resistance from 800 ℃ to 1000 ℃. The results of this study would offer a valuable reference to expedite the future engineering applications of RCCAs.

Characterization of microstructure and oxidation behavior of Al modified MoSi2 coating on Nb-Si based alloy

The aim of this work is to investigate the effects of Al on the microstructure formation and oxidation behavior of MoSi2 coating on Nb-Si based alloy by slurry sintering. The results indicate that the addition of a small amount of Al has little effect on the formation of MoSi2 coating. The phase constituent and structure of the inter-diffusion zone (IDZ) at the interface change significantly with the increase of Al content. The Al2O3 formed during the coating preparation process can hinder the diffusion of Si and Fe, thus suppressing formation of Mo5Si3 and Nb4Fe3Si5 layers in IDZ of the coatings with higher Al, but instead forming Fe-rich (Ti,Nb)5Si4 layer and (Nb,X)5Si3 layer. The higher amount of Al2O3 will increase the sintering resistance between the Mo and Si, resulting in a high porosity of the MoSi2 coating. By comparison, the 0.1Al coating exhibits the best oxidation resistance at 1250 °C. The preferential oxidation of Al promotes the formation of SiO2, which is conducive to the rapid filling of pores in the lower part of MoSi2 coating, forming a dense mixed layer of MoSi2+Mo5Si3+SiO2. However, adding a higher content of Al reduces the oxidation resistance of the coating.

Investigation on the interdiffusion mechanism between single-crystal superalloy and FeCrNiAl-based medium entropy alloys

High and medium entropy alloys (HEAs, MEAs) have emerged as promising candidates for bond-coat materials in the thermal barrier coating systems due to their exceptional oxidation resistance. However, understanding the interdiffusion behavior between these alloys and single-crystal superalloys (SXs) substrates requires further investigation. In this study, diffusion couples were created between SXs and two types of MEAs, namely FeCrNiAl and FeCrNi0.4Al0.4, and the interdiffusion behavior was simulated using the commercial DICTRA code. The results reveal significant interdiffusion at 1100 °C in the couples, attributed to the substantial elemental differences between SXs and MEAs. This led to the formation of the interdiffusion zone (IDZ) and the secondary reaction zone (SRZ). Within the SRZ, phase transformation of SXs occurred, resulting in the precipitation of topologically close-packed (TCP) phases. Three types of TCP phases (σ, μ, and P) were observed. Along the {11¯02} plane of the μ phase, twins and stacking faults have formed. The glide of P phase contributed to the bending of bar-like precipitates. Notably, the amorphization of the σ phase was observed for the first time, suggesting an intermediate state during the phase transformation. Despite elemental differences between the two MEAs, the growth mechanism of IDZ and SRZ, as well as the precipitation of TCP phases, exhibited similarities. This underscores the importance of Cr and Ni diffusion alongside Al diffusion. The pronounced interdiffusion behavior raises concerns about the suitability of 3-d transition metal MEAs as bond-coat materials due to the lack of slow diffusion effect.

Equivalence study of the resin-dentine interface of internal tunnel restorations when using an enamel infiltrant resin with ethanol-wet dentine bonding

This preregistered ex vivo investigation examined the dentinal hybrid layer formation of a resinous infiltrant (Icon), with reference to both thickness (HLT) and homogeneity when combined with modified tunnel preparation (occlusal cavity only) and internal/external caries infiltration. The adhesives Syntac and Scotchbond MP were used as controls (Groups 1 and 3) or in combination with Icon (Groups 2 and 4). A split-tooth design using healthy third molars from 20 donors resulted in 20 prepared dentine cavities per experimental group. The cavity surfaces (n = 80) were etched (37% H3PO4), rinsed, and air-dried. Rewetting with ethanol was followed by application of the respective primers. After labeling with fluorescent dyes, either Syntac Adhesive/Heliobond or Scotchbond MP Adhesive was used alone or supplemented with Icon. HLT, as evaluated by scanning electron microscopy, did not significantly differ (P > 0.05), and confocal laser scanning microscopy revealed homogeneously mixed/polymerized resin-dentine interdiffusion zones in all groups. Icon can be successfully integrated into an ethanol-wet dentine bonding strategy, and will result in compact and homogeneous hybrid layers of comparable thickness considered equivalent to the non-Icon controls, thus allowing for preservation of the tooth’s marginal ridge and interdental space in the case of internal/external infiltration of proximal caries.

Effect of PtAl coating on the HCF property of a coated third-generation single crystal superalloy with sheet specimens at 900 ℃

High-cycle fatigue (HCF) is one of the primary failure modes for turbine blades in aero-engines. Therefore, comprehending the effect of coating on the HCF property of single crystal (SX) superalloys is vital for the safe service of turbine blades. In order to mimic the degradation of the fatigue property for the turbine blades during service, this study investigated the effect of PtAl coating on the HCF property of a third-generation Ni-based SX superalloy using sheet specimens at 900 ℃ and different maximum stresses (σmax). The results indicated that both coating and σmax affected the fatigue properties and crack initiation process of SX superalloy. A transition in fatigue crack initiation site from the internal micropores to the surface cracks for both uncoated and coated alloys has been observed as the σmax decreased. However, the coating significantly increased the σmax required for the transition of the crack initiation site. Meanwhile, the HCF property of the SX superalloy was reduced with the deposition of PtAl coating at lower σmax, and the debit in HCF life was more pronounced with decreasing the σmax. The coating facilitated the rapid nucleation of surface cracks and promoted the transition of the fatigue crack initiation site under higher σmax. On the other hand, since the HCF life is controlled by the crack growth rate, grain boundaries in the interdiffusion zone (IDZ) and secondary reaction zone (SRZ) accelerated the growth of the surface crack under lower σmax, resulting in a shorter crack initiation stage and lower fatigue life compared to the uncoated alloy. This study will be helpful in accurately predicting the HCF life of the coated Ni-based SX superalloys.

Microstructural development and oxidation resistance of MSi2-type multicomponent silicide coatings on Nb alloys by spark plasma sintering

MSi2-type multicomponent silicide coatings were prepared on Nb alloys by spark plasma sintering using mixed powders of Mo, Ta, W, Zr, Ti and Si. The coatings were composed of thick outer layers and thin inner layers (interdiffusion zone). A two-phase structure consisting of C11b (Mo,Ta,W,Ti)Si2 and C40 (Ti,Ta,Zr)Si2 was observed in the outer layers of all the coatings sintered for holding times ranging from 15 s to 5 min, but the distribution of these elements in each phase gradually became uniform with increasing holding time. The coatings demonstrated excellent protection during oxidation at 1300 °C for 100 h, and a continuous dense oxide scale mainly composed of ZrSiO4 and SiO2 was generated. The alternation of heating and cooling during oxidation caused cracks to appear at the interface between the outer layer and inner layer due to a mismatch of thermal expansion coefficients. However, crack extension was observed only along the inner layer toward the interior due to the multicomponent structure of the outer layer of the coating. The microstructural development and oxidation behavior of the coatings were discussed.

High-throughput pseudo-binary diffusion couple approach for alloy design in cobalt-based superalloys

The paper describes a high-throughput method for alloy design in in Co-based superalloy. The approach is an extension of the diffusion couple method used for the determination of diffusion coefficients. We present the use of such a method in low-density cobalt-based superalloys Co-30Ni-10Al-5Mo-2Ta-2Ti-xCr (x = 0–14 at. %) exploring the effect of Cr content on the microstructural evolution, mechanical, and oxidation properties. The composition profile is developed at relatively high temperature with Co-30Ni-10Al-5Mo-2Ta-2Ti and Co-30Ni-10Al-5Mo-2Ta-2Ti-14Cr as end members in which only the solid solution γ phase grows in the interdiffusion zone. This is subsequently subjected to heat treatment for the evolution of g and γ՛ phases. The composition-dependent microstructure and respective mechanical properties were probed using FE-SEM and nano-indentation respectively. The following composition-dependent properties were successfully probed from the high-throughput diffusion couple: 1) The morphological transition of γ՛ precipitates, 2) The solubility limit of Cr in the alloy for TCP formation, 3) Hardness and average elastic modulus, and 4) The nature and topology of oxidized layers as a function of chromium concentration. Therefore, this method facilitates the optimization of multiple properties in a single experiment, identifying the optimal composition range typically between 5 and 8 at. % of Cr in the present alloy.

Chromium diffusion coatings for improving the oxidation behavior of refractory metals at intermediate temperatures

Due to their very high melting points and high mechanical strength at elevated temperatures, refractory metals are promising structural materials for high temperature applications. However, their application is strongly restricted by their insufficient oxidation resistance. In this fundamental study, four different refractory metals (Mo, Nb, Ta, and W) were chromized by pack cementation to investigate the influence of chromium diffusion coatings on their oxidation resistance at intermediate temperatures. Thermodynamic calculations were used to find the most suitable conditions for the pack cementations. The formation of a homogeneous single-phase bcc Cr layer with a thickness below 10 μm on the sample surface was observed on all four refractory metals. In the case of Nb and Ta, intermetallic Cr2X-Laves phases (X = Nb, Ta) formed underneath the bcc Cr layer while an interdiffusion zone formed on Mo. The concentration profiles of the deposited Cr layers were used to determine the diffusion coefficients graphically via Boltzmann-Matano analysis. The Cr-coated samples were analyzed via thermogravimetric analysis at 700 °C and 900 °C for up to 100 h in dry synthetic air. Optical and electron microscopy, EPMA, EBSD and XRD were used to investigate the chromium diffusion layers and the oxide scales.

Diffusion-controlled growth and microstructural evolution between Pt and Pd containing B2-NiAl bondcoats and Ni-based single crystal superalloy

Pt-modified NiAl bond coats are used to extend the lifetime of blades in gas turbine engine applications. However, they suffer from the growth of deleterious precipitates within the interdiffusion zone. Partial substitution of Pt by Pd is advantageous in reducing the interdiffusion zone thickness between the bond coat and the superalloy, providing comparable oxidation properties, and critically reducing the propensity to form brittle (Pt,Ni)Al2 precipitates. In this study, we utilize the pseudo-binary diffusion couple method to estimate the interdiffusion coefficients of Ni and Al in Pd, Pd-Pt and Pt-modified nickel aluminides. Additionally, the main and cross-interdiffusion coefficients are estimated in the ternary diffusion couples. The estimated diffusion coefficients in the β-NiAl phase reflect on the thickness of the β phase in the superalloy bond coat interdiffusion zone. Pd reduces the pseudo-binary interdiffusion coefficients of Ni–Al and also decreases the main and cross-ternary interdiffusion coefficients. This correlates with the reduction of the interdiffusion zone thickness by Pd. The diffusion process is strongly assisted by point defects in this phase. Ab initio-informed defect calculations are done to explain diffusion retardation in the presence of Pd (compared to Pt) with decreased defect concentrations. Furthermore, through in-depth microstructure characterization performed by electron probe micro analyser, transmission electron microscopy, and atom probe tomography, the presence of σ, R and μ phases as TCP precipitates in the interdiffusion zone are identified. The results give insights into the coatings’ diffusional properties that influence the service life of the product.

Preparation and characterization of ReCr co-doped β-NiAl coating on a Ni-based single crystal superalloy

To prepare ReCr co-doped β-NiAl coating, an innovative method that electroplating NiRe combined with arc ion plating (AIP) Cr, AIP Al and followed by diffusion treatment was proposed. ReCr co-doped β-NiAl coating contained an outer β-NiAl layer with fine dispersive ReCr-rich precipitates, an inner β-NiAl layer with fine dispersive W-rich precipitates and bottom interdiffusion zone (IDZ). And ReCr-rich phases were mainly distributed in the interior of coating grain. The TEM results proved that ReCr-rich precipitates were α-Cr phase as main phase, wherein some Re solubilized in α-Cr phase and it did not change the phase structure of α-Cr.

Effect of TiAlN diffusion barrier at MoSi2-SiO2 composite coating/Mo alloy interface

In order to reduce the interdiffusion at interface of the silicide coating/Mo alloy and improve the service life of the coatings. MoSi2-SiO2 composite coatings with TiAlN diffusion barrier were prepared on Mo alloy by magnetron sputtering and followed by hot press sintering in the present study. Oxidation and interdiffusion behaviors of the samples were investigated at 1200 °C in static air. The results show that MoSi2-SiO2 composite coatings have excellent high temperature oxidation resistance, and the inward diffusion of Si is greatly inhibited with the addition of TiAlN diffusion barrier. After oxidation at 1200 °C for 100 h, the mass change of the MoSi2-SiO2 composite coatings is only -0.059 mg/cm2 to -0.022 mg/cm2, the thickness of the interdiffusion zone is just only 3.4 μm that is one order of magnitude smaller than TiAlN-free coating. The TiAlN diffusion barrier can effectively inhibit the interdiffusion of elements between the MoSi2-SiO2 composite coatings and the Mo alloy substrates.

Grain size effect on the phase growth in CoNi/Sn sandwich diffusion couples

Grain size effect during interdiffusion is investigated by designing a sandwich couple of CG-CoNi/Sn/UFG-CoNi; (CG coarse grained (>200 µm), UFG: ultrafine grained (∼240 nm)). Phase growth is examined at temperatures 175 & 200 ℃ for various times. CG CoNi and UFG CoNi have been fabricated by Vacuum arc melting and spark plasma sintering of mechanically alloyed CoNi powders, respectively. At 175 ℃, (CoNi)Sn3 intermetallic phase is detected at CG CoNi/Sn interface and (CoNi)Sn2 phase at UFG CoNi/Sn interface. At 200℃, formation of (CoNi)Sn2 and (CoNi)Sn3 phases is observed at CG interface and only (CoNi)Sn2 at the UFG interface. Phase compositions have been confirmed by electron probe microanalysis while the corresponding crystal structure with micro-x-ray diffraction. Phase selection in the interdiffusion zone (IDZ) of CG CoNi/Sn is governed primarily by kinetic factors, while thermodynamic driving forces, assessed using CALPHAD approach, also contribute to phase formation in the IDZ of UFG CoNi/Sn. Time-dependent measurements at 175 & 200℃ for UFG CoNi/Sn reveal an accelerated growth of phases when compared with the CG CoNi/Sn. The enhanced kinetics and preferential formation of the (CoNi)Sn2 phase at the UFG CoNi/Sn interface can be ascribed to the higher proportion of grain boundaries (GBs), leads to an increase in overall diffusion flux. The sandwich diffusion couple approach ensured that all the comparisons between CG and UFG interfaces are conducted under similar conditions yielding dependable correlations. The formation of intermetallic phases and role of GBs are studied at a finer detail using correlative microscopy approach.

On the oxidation and interdiffusion behavior of an AlCoCrFeNiY high-entropy alloy bond coat on a directionally solidified Ni-based superalloy

The oxidation and interdiffusion behavior of a novel AlCoCrFeNiY bond coat deposited on a directionally solidified Ni-based superalloy were systematically studied at 1050, 1100 and 1150 °C, and compared with a conventional NiCoCrAlY coating deposited on the same substrate. The AlCoCrFeNiY bond coat exhibits lower oxide growth rates due to its large columnar grains and low Al activity at the oxide scale/bond coat interface. Meanwhile, AlCoCrFeNiY has higher resistance to oxide spallation than NiCoCrAlY, which is attributed to the formation of a clean and defect-free metal/oxide interface. Significant interdiffusion occurs across the AlCoCrFeNiY/superalloy substrate interface. Our experimental evidence and thermodynamic modelling suggest that Fe accelerates interdiffusion and destabilizes the γ’ phase, thereby causing the formation of a thick and γ’-depleted interdiffusion zone. In addition, the AlCoCrFeNiY bond coat undergoes more Al depletion and subsequent β to γ transformation compared with NiCoCrAlY. Based on the findings in this work, a novel AlCoCrFeNiY/NiCoCrAlY double-layer bond coat was designed, tested and validated to achieve optimal balance between oxidation and interdiffusion.

Very high cycle fatigue rupture mode at high temperatures of Ni-based superalloys coated with a slurry aluminide

The failure mode in the very high cycle fatigue (VHCF) regime of four different nickel-based single-crystal (SX) superalloys coated with a slurry aluminide has been investigated at high temperature (1,000 °C) and under fully-reversed conditions (Rε = -1). The 3rd generation superalloy CMSX-4 Plus presented a lifetime one to two order(s) of magnitude higher than AM1, CMSX-4 and Rene N5 for the same applied alternating stress (180 MPa). The crack initiation zones were distinct. CMSX-4 Plus samples failed from the interdiffusion zone (IDZ) while AM1, CMSX-4 and Rene N5 samples failed from casting pores close to the IDZ. The interaction between the casting pore, the IDZ and the rough zone is believed to control the premature crack initiation. A new crack initiation mode in VHCF for coated specimens is proposed in this article and the competition between crack initiation from a casting pore or from the surface is considered and discussed.