Hf-containing Alloys Research Articles

Synergistic enhancement the strength and ductility of crossover Al–Cu–Zn–Mg alloys via Zr(Sc or/and Hf) microalloying to create heterogeneous lamellar structure

A crossover Al–Cu–Zn–Mg alloy with a heterogeneous lamellar structure (HLS) was obtained by microalloying Zr(Sc or/and Hf) to form a multiscale second phase, and the microstructure and mechanical properties of the alloy were investigated. The Sc-containing alloys produce a large number of micron and submicron W phase particles and nano-sized Al3(Sc,Zr)/Al3(Sc,Hf,Zr) dispersions with core-shell structures. The Hf-containing alloys have a micron-sized second phase particles and nano-sized Al3(Hf,Zr) dispersions. The multiscale second phase formed by microalloying of Sc or/and Hf promotes recrystallization via particle-stimulated nucleation (PSN), whereas the nanoscale Al3M dispersions pinning sub-structures to inhibit grain growth, and these two mechanisms influence the recrystallization process to form a heterogeneous lamellar structure. The microalloying produces a large number of Al3M dispersions and the refinement grain by hindering recrystallization, which enhances the strength through precipitates strengthening and grain refinement strengthening. The Hf added alloy with typical HLS microstructure improve the ductility of the alloys due to the delamination toughening effect and the avoidance of the formation of coarse brittle phases.

Studies of the effect of Hf doping on the electrochemical performance of C15 Laves type metal hydride battery anode alloys

This paper focuses on the studies of the electrochemical performance of arc-melted Hf-modified Ti-Zr based AB2 Laves type metal hydride battery anode alloys with composition HfxZr24-xTi6.5V3.9Mn22.2Fe3.8Ni38La0.3Sn0.3 (x = 1–4). The phase composition and the morphology of the alloys were characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Metal-gas interactions and electrochemical performances were studied by building PCT diagrams and by electrochemical tests performed in a 9 N KOH electrolyte solution at ambient temperature when using the half-cells in a three-electrode assembly.All studied alloys were two-phase and contained the Laves phase intermetallics of C15 (major constituent) and C14 (minor constituent) types. An increase in Hf content caused a growth of the abundance of the C14 phase and was accompanied by a decrease in the electrochemical discharge capacities, exchange current densities, and H diffusion rates as compared to the Hf-free alloy. Among the Hf-containing alloys, an introduction of 2 wt% Hf into the alloy's composition resulted in a higher H diffusion rate, while the highest H storage capacity has been reached for the alloy with the lowest, 1 wt%, content of Hf. The surface of the alloys contained catalysing hydrogen exchange metallic nickel clusters with the highest enrichment observed for the 2 wt% Hf alloy. An influence of Hf on the hydrogen storage and electrochemical performance can be understood in terms of the modification of the intrinsic properties of the alloys following a substitution (Zr/Ti) → Hf as C15 type alloys show the fastest hydrogen diffusion rates while a gradually increasing content of the hexagonal C14 type in the Hf-containing alloys slows down the H diffusion process.

Comparing the role of Zr and Hf atoms on microstructure and mechanical properties optimization of Ti2AlN reinforced Ti48Al 0.5W composites

In order to further optimize the mechanical properties of TiAl composites, Ti2AlN/Ti48Al0.5W composites with 1 at% Zr/Hf addition were prepared by arc melting, which were termed as Base alloy, Zr-containing alloy and Hf-containing alloy. The grain size, element distribution, phase content and compressive properties were investigated and compared in this work. With 1 at% Zr/Hf addition, the lamellae colony size decreased from 33 µm to 25/23 µm, and Zr atoms almost dissolved into the single γ phase in the form of solid solution atoms, while Hf atoms distributed in both α2/γ lamellae and single γ phase, and part Hf atoms also existed in the form of B2 phase, which illustrated the higher solid solution of Zr atoms in TiAl composites comparing with Hf atoms. The γ phase content increased and the α2 phase content decreased, especially with 1 at% Zr addition, which acted as γ-phase stabilizing element. With 1 at% Hf addition, the compressive strength and strain increased from 2010 to 2221 MPa and from 23.9 % to 26.2 %, respectively, which was attribute to the solid solution strengthening of Hf atoms by substituting Ti atoms, refinement of α2/γ lamellae and increased γ phase content. Compared with 1 at% Hf addition, 1 at% Zr addition led to higher compressive strength and strain, which were 2344 MPa and 26.8 %, respectively, and this result was due to the higher concentration of Zr atoms in γ phase and higher content of γ phase. On account of density, cost and performance, Zr atoms are more suitable as an alloying element to further optimize the microstructure and mechanical properties of TiAl composites.

Influence of high melting elements on microstructure, tensile strength and creep resistance of the compositionally complex alloy Al10Co25Cr8Fe15Ni36Ti6

Due to its matrix/γ′ structure, the compositionally complex alloy (CCA) Al10Co25Cr8Fe15Ni36Ti6 has excellent properties that fulfill the requirements for a high-temperature material. This base alloy is alloyed with small amounts of high melting elements to a further improvement of its properties, which results in different shapes, fractions and sizes of the two phases γ′ and Heusler after various homogenization and annealing steps. By correlating this microstructure with time independent and dependent mechanical properties, conclusions can be drawn about the effects of the individual phases. The needle-shaped Heusler-phase leads to bad mechanical behavior if its phase fraction is too high. A fraction below 3 vol% is not critical in tensile tests, but it reduces the creep resistance compared to a purely two-phase matrix/γ′-alloy. Sharp-edged cubic γ′-particles and a coarse Heusler-phase without sharp edges in case of the base alloy with 0.5 at.% hafnium lead to the best tensile and creep properties in the high temperature range. At 750 °C, the Hf-containing alloy clearly outperforms two commercially used alloys in the targeted area of application when it comes to creep resistance.

Effect of Hf addition on solidification and hot isostatically pressed microstructures of the Ti-48Al-2Cr-2Nb (at%) intermetallic alloy

The microstructural evolutions during solidification and hot isostatic pressing (HIP) of Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated using both experimental observations and thermodynamic calculations. The primary solidification phase of all the studied alloys was β, albeit it was predicted by Thermo-Calc to be α for the alloys containing 4% and 6%Hf. In agreement with the Thermo-Calc calculations, the solidification path of the base 4822 alloy encountered a peritectic reaction. Although alloying by 2%Hf did not change the solidification path, further increase in Hf content caused an additional eutectic reaction following the peritectic one. The eutectic microstructure was composed of two orthorhombic phases with TiAl2 and Al3Hf2 compositions and this finding was the first evidence of such phases in a quinary alloy with Cr, Nb, and Hf. It was found that Hf acted as a weak β stabilizer as it showed a low partitioning tendency in β and the β single-phase field was not formed in the high-Hf alloys. A finer dendritic microstructure was characterized in the high-Hf alloys due to the Hf solubility limit in the 4822 alloy. Interestingly, α2 laths dissolved in the Hf-containing alloys after HIP treatment and instead rode-like precipitates of Al3Hf2 emerged at newly formed γ/γ interfaces. The precipitation mechanism of Al3Hf2 particles was discussed.

Alloying effect on solidification behaviour and grain refinement in Ti45Al2Nb2Ta0.8B and Ti45Al2Nb2Hf0.8B

The solidification process of Ti45Al2Nb2Ta0.8B (B4522Ta) and Ti45Al2Nb2Hf0.8B (B4522Hf) Bridgman samples were studied through directional solidification. Boron addition at 0.8 at. % was found to have changed the orientation relationship of peritectic α with β dendrites. The peritectic alpha is randomly oriented in both B4522Ta and B4522Hf, it was likely to be inoculated by boride precipitates at the β/liquid interfaces during peritectic transformation. The alloying effect of Ta and Hf on the peritectic reaction was similar. Borides precipitated earlier in Ta-containing alloy than that in Hf-containing alloy during solidification.

The formation mechanisms of HfO2 located in different positions of oxide scales on ni-al alloys

• Oxidation behavior of Ni-Al-Hf alloys with different phases is studied. • The distributions of HfO 2 are different in scales on Ni-Al-Hf alloys. • HfO 2 is distributed linearly within the scale on single β-phase Ni-49.9Al-0.1Hf. • For Ni-25Al-0.1Hf and Ni-33Al-0.1Hf, HfO 2 appears along the spinel/Al 2 O 3 interface. • Formation mechanisms of HfO 2 with various distributions are discussed. HfO 2 tends to grow on Hf-containing alloys during high-temperature oxidation and significantly affects the oxidation resistance. In this paper, cyclic oxidation behavior of single β-phase Ni-49.9Al, γ’-phase Ni-25Al and binary phase (γ’+β) Ni-33Al alloys doped with Hf was investigated at 1100 °C. HfO 2 was located at different positions of the oxide scale grown on the above alloys. The HfO 2 was distributed linearly within Al 2 O 3 scale in Ni-49.9Al-0.1Hf whilst HfO 2 primarily appeared along the spinel/Al 2 O 3 interface in Ni-25Al-0.1Hf and Ni-33Al-0.1Hf. In addition, the specific distribution of HfO 2 in Ni-33Al-0.1Hf was analyzed by HAADF-STEM and the corresponding formation mechanisms were proposed.

Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes.

Oxygen, one of the most abundant elements on Earth, often forms an undesired interstitial impurity or ceramic phase (such as an oxide particle) in metallic materials. Even when it adds strength, oxygen doping renders metals brittle1-3. Here we show that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials. Unlike traditional interstitial strengthening4,5, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs)6-10. The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength-ductility trade-off11. The oxygen complexes are ordered nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich atomic complexes whose formation is promoted by the existence of chemical short-range ordering among some of the substitutional matrix elements in the HEAs. Carbon has been reported to improve strength and ductility simultaneously in face-centred cubic HEAs12, by lowering the stacking fault energy and increasing the lattice friction stress. By contrast, the ordered interstitial complexes described here change the dislocation shear mode from planar slip to wavy slip, and promote double cross-slip and thus dislocation multiplication through the formation of Frank-Read sources (a mechanism explaining the generation of multiple dislocations) during deformation. This ordered interstitial complex-mediated strain-hardening mechanism should be particularly useful in Ti-, Zr- and Hf-containing alloys, in which interstitial elements are highly undesirable owing to their embrittlement effects, and in alloys where tuning the stacking fault energy and exploiting athermal transformations13 do not lead to property enhancement. These results provide insight into the role of interstitial solid solutions and associated ordering strengthening mechanisms in metallic materials.

Thermal behavior, structural relaxation and magnetic study of a new Hf-microalloyed Co-based glassy alloy with high thermal stability

In the present work, the influence of a minor Hf addition on the atomic structure, crystallization behavior, thermal stability and magnetic properties of a Co-based metallic glass was studied. Thermal analysis indicates that the thermal stability of new glassy ribbons microalloyed with 2.5 at.% Hf is notably enhanced through increasing the incubation time prior to devitrification and enlarging the width of the supercooled liquid region from 72 K to 96 K. Magnetic studies reveal that the new glass exhibits an excellent soft magnetic behavior, i.e., a very low coercivity of 0.26 A/m in the relaxed state, and a comparable saturation magnetization as the Hf-free ribbon. Structural relaxation of the Hf-containing alloy upon isothermal annealing below the glass transition temperature, Tg, was investigated by differential scanning calorimetry (DSC) and high-energy synchrotron X-ray diffraction (XRD). The evolution of the recovered enthalpy with annealing time can be expressed by the Kohlrausch-Williams-Watts (KWW) exponential function with a Kohlrausch exponent of 0.88, indicating a broad spectrum of relaxation times. Analysis of the reduced pair correlation functions, G(r), reveals volume shrinkage upon annealing caused by elimination of liquid-like sites including free-volume and anti-free-volume according to the shift in the positions of the G(r) maxima in the medium-range scale. The influence of structural relaxation on the variation of the Curie temperature and the coercivity of the new Hf-microalloyed glassy ribbon is discussed. A faster evolution of the Curie temperature with annealing time compared to the coercivity indicates a preferential dependence of the former on the chemical short-range order (SRO).

Microstructural characteristics of Nb–Si based ultrahigh temperature alloys with B and Hf additions

Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures.

Microstructure evolution at high temperature of chromium-rich iron-based alloys containing hafnium carbides

Abstract The high temperature behavior of three iron-based alloys rich in chromium and containing HfC carbides was studied in comparison with two ternary Fe – Cr – C alloys. All these alloys, contain 25 wt.% Cr, 0.25 or 0.50 wt.% C, and additionally 3.7 to 5.6 wt.% Hf for three of them. All alloys were produced in a foundry. They were exposed to air at 1 200 °C for 46 h. Their microstructures in the as-cast state and after exposure to high temperature were characterized using X-ray diffraction, electron microscopy and image analysis. The alloys were also subjected to indentation tests in their as-cast states and in their aged states. The obtained microstructures are composed of a dendritic bcc iron-based matrix and of HfC carbides. These carbides are of two types: script-like eutectic carbides mixed with matrix, and compact pre-eutectic carbides. Some chromium carbides were additionally obtained in two of the three Hf-containing alloys. These microstructures have roughly evolved during the high temperature exposure: variations of the surface fraction of the chromium carbides and increase in the HfC surface fractions. Consequently, the hardness was more (ternary alloys) or less (Hf-containing alloys) decreased. The morphologies of the carbides evolved: coalescence of the chromium carbides, coalescence and fragmentation of the HfC carbides. The behavior of these alloys in oxidation at high temperature is perfectible. It is to be improved to allow the use of such alloys at high temperature.

Improvement of strength and conductivity in Cu-alloys with the application of high pressure torsion and subsequent heat-treatments

Quenched and slowly cooled (annealed) Cu- 0.7 %Cr, Cu-0.9 %Hf, and Cu-0.7 %Cr-0.9 %Hf alloys were processed by high pressure torsion (HPT). The microstructures of the alloys were studied immediately after HPT and subsequent annealing. It has been shown that the microhardness and the thermal stability of the severely deformed microstructure increase, while the average grain size decreases in the order of Cu-0.7 %Cr, Cu-0.9 %Hf, and Cu-0.7 %Cr-0.9 %Hf alloys. The microhardness in all alloys is higher after quenching and HPT, than after annealing and HPT. The largest dislocation density is achieved by quenching and HPT in Hf-containing samples. Cu5Hf phase precipitations in Hf-containing alloys are more effective in retarding grain growth in comparison with Cr particles and lead to additional hardening during aging. It has been demonstrated that HPT-processing with subsequent heat-treatment might yield the combination of large hardness and high electrical conductivity in Cu alloys.

Alloys based on Cr–Cr2Ta containing Si

Dual phase alloys containing Cr-Laves phases have been considered as potential candidates for high temperature service. In this paper, the effect of Si additions on site occupancies in Cr–Cr2Ta alloys has been studied. Calculations using density functional theory predict that Si substitutes for the Cr atoms rather than Ta within the structure and that Si and Cr are relatively free to occupy 2a and 6h sites interchangeably. However, slight non-uniformity in the 6h Si–Cr minimum charge density and bond length leads to Si favouring 2a occupation within C14 (Cr2−xSix)Ta. These predictions were supported by studies using high-resolution synchrotron X-ray powder diffraction. The effect of quaternary additions on the microstructures and elevated temperature oxidation characteristics of Cr–Ta–Si–X alloys, where X = Ag, Ti, Hf, Mo, Al and Re have also been investigated. The microstructure of the alloys comprised primarily of a eutectic mixture of a Cr-rich solid solution and Cr2Ta-based Laves phase along with primary dendrites of C14 Cr2Ta. Upon isothermal exposure of the alloys for 100 h at 1000 °C, the Re-containing alloy was found to possess the best oxidation resistance, followed by the Hf-containing alloy. The alloys exhibited a significantly superior oxidation resistance at 800 °C owing to the formation of an adherent chromia layer. The oxidation behaviour of the alloys could be described using a combination of linear and parabolic growth laws.

Sulfur Segregation at Al2O3/γ-Νi + γ′-Ni3Al Interfaces: Effects of Pt, Cr and Hf Additions

The interfacial chemistry that developed as a result Al2O3-scale growth on γ-Νi + γ′-Ni3Al alloys at 1150 °C was studied using scanning Auger microscopy after the oxide layer was scratched to spall under ultra-high vacuum. The extent of scale spallation was used to evaluate semi-quantitatively the interfacial strength. The alloys investigated were primarily γ′ in structure, containing 22 at.% Al plus further additions of Pt, Cr and/or Hf. In the case of the binary γ + γ′ alloy, it was found that a sub-monolayer of sulfur segregated at the alloy/scale interface. Platinum reduced and hafnium eliminated sulfur segregation, but chromium enhanced it through Cr–S co-segregation, even on Pt- and Hf-containing alloys. Platinum also segregated slightly at the alloy/scale interface. The interface strength was a strong function of the sulfur content. Beyond the effect of eliminating S segregation, Pt and Hf both showed additional beneficial effects on alumina scale adhesion.

Early-Stage Oxidation Behavior of Pt-Modified γ′-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified γ′-Ni3Al-based alloys of composition (in at.%) Ni–22Al–30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al2O3 phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 °C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 °C; however, it was found that Hf delayed the metastable-to-α-Al2O3 phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al2O3 layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to α-Al2O3, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.

Development of Aluminium Alloys with Ultimate Recrystallisation Resistance

In the present work the precipitation behaviour and recrystallisation resistance of Alalloys containing Hf, Sc and Zr in different concentrations and combinations have been investigated. Special focus has been put on the Hf-containing alloys, as one of the objectives of this work was to find out if Hf can be used as a replacement for Sc. Additions of Sc, either alone or in combination with Zr, leads to the formation of coherent and homogeneously distributed dispersoids, which very efficiently inhibit recrystallisation. Despite these attractive properties, the high price of Sc has limited its use as an alloying element in aluminium. The present investigation has revealed that Hf cannot fully replace Sc, as only heterogeneous dispersoid distributions are obtained in the absence of Sc, i.e. in regions where the number density is low the alloys would still be prone to recrystallisation. However, as an extra addition to the already remarkably stable Sc+Zr-containing alloys, Hf can lead to further improvements and consequently open for the use of aluminium alloys at very high temperatures. Al3(Sc,Zr,Hf)-dispersoids were present at the largest f/r-ratios and also displayed lower coarsening rates than Al3(Sc,Zr)-dispersoids. Very promising results were obtained for an Al-Hf-Sc-Zr alloy, which maintained mainly an unrecrystallised structure after extrusion and large degrees of cold rolling.

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti, Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb ss/Nb 5Si 3 base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb 3Si silicide formed in the Hf-free alloys and transformed to Nb ss and αNb 5Si 3 during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb ss and Nb 5Si 3 were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb 5Si 3 only partially transformed to αNb 5Si 3, suggesting that Ti stabilises the βNb 5Si 3 to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb 5Si 3 (Mn 5Si 3-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb ss and was sensitive to the area fraction of Nb ss in the alloy.

Reaction and bonding of Hf and Zr containing alloys to alumina and silica

Abstract We have investigated the wetting and reaction behavior of two less common transition element additives to braze alloys to learn more about their reaction and bonding mechanisms. Alloys of Ag with different amounts of Hf or Zr were reacted in a controlled atmosphere furnace with sapphire, alumina of 99.6 and 96% purity, and fused silica. We determined contact angles during heating and examined cross sections after cooling using electron analytical techniques. Different interfacial microstructures were obtained when the active metal was varied in an otherwise constant system. The Hf-containing alloys reacted with polycrystalline Al2O3 and had a dispersion of Hf-containing particles near the interface. If there were a reaction layer with the Al2O3 it was below the resolution of the analysis. By contrast, Zr reaction products were more evident and appeared to have diffused away from the interface into the alloy. The microstructural observations are interpreted using available thermodynamic data and known phase relations.

Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form α-Al2O3 scales in a flow of pure O2 at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H2 −10% H2S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO2 particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.