Nucleation Of α-Al2O3 Research Articles

Excellent oxidation resistance of rapidly crystallizing Cr2AlC coatings at high temperatures: Effect of microstructure

Two kinds of Cr2AlC coatings with different microstructures were prepared by direct crystallization during oxidation at 1000 ℃ and conventional crystallization at 700 ℃ for 1 h for amorphous Cr-Al-C coatings prepared by magnetron sputtering, respectively. The effect of microstructure on the oxidation behavior of the coatings at 1000 ℃ was investigated. The grain boundary density and dislocation density of the Cr2AlC coatings crystallized directly during oxidation were much higher, which provided much more sites for heterogeneous nucleation of α-Al2O3 and fast diffusion paths for Al. Therefore, the coating formed complete pure α-Al2O3 scales earlier with better antioxidant properties. More importantly, the present work demonstrates feasibility and advantages of directly applying amorphous Cr-Al-C coatings at high temperatures without additional crystallization treatments, which simplifies preparation process of Cr2AlC coatings, facilitates their industrial applications, and provides a new method for rapid preparation of MAX-phase coatings.

Accelerating formation of α-Al2O3 scale on a nanograined Ni-Al alloy by self-generated NiO top layer

Alfa (α)-Al2O3 is widely used to resist oxidation at high temperature. However, presence of some transient phases can reduce the oxidation resistance. Several strategies have been proposed to solve this issue. Here, we found that nucleation of α-Al2O3 can be accelerated by a NiO layer on the surface of nanostructured Ni-7 wt%Al alloy. The oxidation rate constant is an order of magnitude lower than that of β-NiAl after 1000 °C oxidation. The enhanced growth compressive stress in scale is supposed to facilitate the α nucleation. This provides a novel strategy to develop advanced engineering alloys with high temperature applications.

Influence of the nucleation surface on the growth of epitaxial Al2O3 thermal CVD films deposited on cemented carbides

This work aims at understanding the nucleation and growth of alumina films grown on Ti(C,N)-based layers using an industrial-scale CVD system. Firstly, Al2O3 layer was deposited on Ti(C,N)-based layers without any nucleation treatment, pure α-Al2O3 is obtained, whereas no orientation relationship at the Ti(C,N)/α-Al2O3 layers interface can be observed. Secondly, Al2O3 layer was deposited on the bonding layer consisting of rutile TiO2, which is obtained by oxidizing the uppermost part of Ti(C,N)-based layers. Herein, α-Al2O3 single-phased layer is obtained, and the epitaxial growth of α-Al2O3 on rutile is observed. The orientation relationships can be found as:(1¯20)α-Al2O3//(101¯)rutile, (003)α-Al2O3//(010)rutile and [210]α-Al2O3//[101]rutile. Finally, Al2O3 layer was deposited on the bonding layer, produced from a TiCl4-H2-N2-CH4-CO-AlCl3 gas mixture. Regarding this intermediate layer, despite additions of CO and AlCl3, no evidence for oxide phases, e.g. TiO2 and Al2O3 can be found, while Ti(C,N) needle-shaped grains develop. In this case, κ-Al2O3 is epitaxially grown on Ti(C,N), with the orientation relationships found as:.(01¯3)κ-Al2O3//(2¯20)Ti(C,N), (100)κ-Al2O3//(1¯1¯1)Ti(C,N) and [031]κ-Al2O3//[112]Ti(C,N). Since processing parameters for the alumina depositions were always the same, it is revealed that the nucleation of α-Al2O3 and κ-Al2O3 can be accurately controlled with deposition of specific bonding layers.

Effects of Minor Alloying Elements on Alumina Transformation During the Transient Oxidation of β-NiAl

The transient stage of alumina scale formation on alumina-forming alloys is often accompanied by the formation of metastable Al2O3 phases that may affect oxidation kinetics and scale microstructure. To clarify the role of dopants on the transient stage of NiAl oxidation, we characterized Al2O3 scales on a series of oxidized model β-NiAl samples that were un-doped or doped with additions of Ti, Y, Si, Ti + Y, and Si + Y. Our results confirm that different dopant elements (Ti, Si, and Y) have varied effects on the length of transient oxidation of alumina-forming alloys. Ti and Si accelerated the θ- to α-Al2O3 transformation via two different mechanisms. Ti promoted α-Al2O3 nucleation, while Si delayed nucleation but accelerated the lateral growth of the α-Al2O3 patches. In contrast, Y had little effect on alumina transformation, possibly due to the low Y concentration used in the current study.

Pt effect on early stage oxidation behaviour of Pt-diffused γ-Ni/γ’-Ni3Al coatings

The early stage oxidation behaviours of γ/γ’-based NiAl coatings with different Pt additions are investigated. It is found that Pt addition can slow down the θ-Al2O3 to α-Al2O3 transformation. High resolution phase mapping by scanning diffraction analysis shows that α-Al2O3 nucleation in the θ-Al2O3 scale is inhomogeneous along the coating/scale interface. Spatially resolved PLPS results show a clear correlation between the θ-Al2O3 to α-Al2O3 transition and the γ or γ’ microstructure in the underlying alloy: where Pt stabilises the γ’ structure, the suppression of θ-Al2O3 to α-Al2O3 transition is observed. The slower θ-Al2O3 to α-Al2O3 transition rate due to Pt addition leads to a lower compressive stress of the stable oxide scale, which contributes to the long term stability of the scale as a result of Pt addition.

Wear resistance of FeCoCrNiMnAlx high-entropy alloy coatings at high temperature

The FeCoCrNiMnAlx high-entropy alloy (HEA) coatings were produced on 4Cr5MoSiV steel by laser cladding, which can improve the high temperature oxidation resistance and wear resistance of the substrate steel in its service environment. The effects of Al addition on the microstructure and properties of the coatings were systematically investigated. The addition of Al element promoted the transition of FCC to FCC + BCC (B2) phases in the cladding layer and also refined the grains. Thereby, with the Al addition from 0 to 0.75, the micro-hardness and room-temperature wear weight loss of the HEA coatings increased and decreased, respectively, i.e. from 224.4 HV0.5 and to 344.2 HV0.5 and from 6.0 mg to 1.1 mg, compared with those of the base metal (227.1 HV0.5 and 10.5 mg). In the high-temperature oxidation process, fine and dense α-Al2O3 was preferentially generated because of its lowest absolute Gibbs free energy compared to the other oxides. The generation of α-Fe2O3 and Cr2O3 also promoted the heterogeneous nucleation of α-Al2O3. The high-temperature oxidation weight gain and wear weight loss of the FeCoCrNiMnAl0.75 cladding layer was approximately 58% and 17% of those of the base metal, respectively, and the wear mechanism was mainly abrasive wear.

Structural variation in improving thermal stability of transition alumina with La dopant

Structural variation in alumina directly determines the thermal stability of transition alumina with La dopant at high temperature. In this work, the thermal stability and alumina structure from well-crystallized ultrafine boehmite doped with various La-bearing compounds were studied. La2O3, LaAlO3, and LaAl11O18 could not efficiently inhibit transition alumina changing into α-Al2O3, α-Al2O3 seed benefited α-Al2O3 formation in the presence of La dopant. La(NO3)3 doping into boehmite effectively improved the thermal stability of θ-Al2O3 above 1200 °C. The nano-diameter active spots with low melting point led to the formation of α-Al2O3 with a smooth edge and sintering neck from the blank boehmite, whereas La dopant contributed to rhombus θ-Al2O3 with considerable pores, jagged edge, and wedge at 1200 °C for 2 h. Moreover, the Al and La contents fluctuated asynchronously during calcination, and the atomic ratio of Al/O decreased with the addition of La dopant. These results are attributed to La migration, assembling in active spots and increasing their melting point. Furthermore, relative to nearly 100% AlO6 coordination in α-Al2O3, La dopant significantly increased the AlO4 content above 900 °C. Therefore, La dopant increased the melting point of the active spots, hindered AlO6 formation, and inhibited α-Al2O3 nucleation on the alumina surface, thereby improved the thermal stability of the transition alumina.

Control of continuous α-Al2O3 fibers by self-seeding and SiO2-Sol doping

Abstract The effects of self-seeding and SiO2-sol doping on the sol–gel preparation of α-Al2O3 fibers are investigated. Self-seeding induces heterogeneous nucleation of α-Al2O3 from the precursor gels at a low temperature of 800 °C. Using transmission electron microscopy and X-ray diffraction analysis, the primary crystallite size (dp) of α-Al2O3 in the seeded fibers is determined as ∼30 nm, which is much smaller than the size (∼50 nm) in the unseeded fibers. Moreover, self-seeding increases the γ-to-α transformation kinetics and lowers the transformation temperature by 175 °C. Heating at 1500 °C could sinter the self-seeded α-Al2O3 fibers nearly to full density with a grain size of ∼1 μm. By doping with SiO2 sols, this final grain size can be decreased with suppression of the grain boundary migration, at the expense of increased phase transition temperature. Therefore, dense α-Al2O3 fibers of submicron grains could be achieved by the combination of self-seeding and SiO2-sol doping.

Coupling effect of chromia doping and vacuum on the phase transition of alumina prepared by co-precipitation process

Synthesis of thermodynamically stable corundum structure alumina under lower temperature is of great significance in practical applications, exploration on new technique and the involved mechanism therefore becomes increasingly essential. In this work, the coupling effect and mechanism of chromia doping and vacuum condition on the phase transition of Al2O3 prepared via co-precipitation process were investigated and explored. It was found that chromia doping by itself did not promote the nucleation of α-Al2O3, or in some sense inhibited the transformation process. Cr doped γ-Al2O3 powders treated in a vacuum condition (10−5Pa) however showed a gradual transition to α-Al2O3 even at low temperature of 600°C, demonstrating that surface nucleation sites introduced by a coupled effect of vacuum and Cr2O3 doping contributed to the dramatic reduction (200–400°C) of the phase transformation temperature. Based on the comprehensive characterizations and conventional diffusional nucleation mechanism, it is concluded that the surface nucleation sites for phase change to α-Al2O3 increased significantly during vacuum calcination with the combination of template effects of Cr2O3. This finding may provide a new way in fabrication and understanding of stable alumina formation under relatively moderate temperature condition.

The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe–Al alloys

The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe–Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks.

First stage of reaction of molten Al with MgO substrate

The Al/MgO couple was produced in vacuum (~5×10−4Pa) by contact heating from RT up to 1000°C and holding at that temperature for 1h of a small 4×4×4mm aluminium (5N) sample placed on the [100] MgO single crystal substrate. TEM observations backed with electron diffraction analysis indicated that the interaction between liquid aluminium and MgO starts from a redox reaction producing a continuous layer of MgAl2O4 spinel on the substrate surface. Its growth is controlled by solid state out-diffusion of magnesium and oxygen towards the surface being in contact with liquid metal. The thickening of spinel layer is accompanied by its cracking and infiltration with aluminium. The above process enables local dissolution of the MgO substrate and formation in it of a thin region of interpenetrating metallic channels walled with spinel. The removal of dissolved magnesium through open aluminium channels towards the drop and to vacuum locally produces areas of aluminium enriched with dissolved oxygen, which results in the nucleation of α-Al2O3 at spinel clad walls. The growth of α-Al2O3 is controlled only by the dissolution rate of MgO by aluminium, liquid state diffusion of Mg to drop/vacuum and oxygen to the front of the of α-Al2O3 crystallites growing into MgO substrate.

Deuterium permeation of Al2O3/Cr2O3 composite film on 316L stainless steel

In this work, an Al2O3/Cr2O3 composite film on 316L stainless steel was prepared by metal-organic chemical vapor deposition (MOCVD). The template effect of α-Cr2O3 interlayer on α-Al2O3 growth in the Al2O3 layer is examined. It is found that α-Cr2O3 is formed in the composite film, however no crystallized alumina is detected. The composite film consists of amorphous Al2O3 and α-Cr2O3. Thus, the α-Cr2O3 template does not stimulate the nucleation of α-Al2O3. Furthermore, the deuterium permeation behavior of the composite film is examined. It is found that the deuterium permeation suppression property of the Al2O3/Cr2O3 composite film is not simply the addition of the deuterium permeation suppression properties of amorphous Al2O3 and α-Cr2O3 single films. The composite film exhibits much lower permeability than the single films of Al2O3 and Cr2O3 with similar thickness. The permeability reduction factors (PRF) were increased by the Al2O3/Cr2O3 composite film to 230–544 at temperatures of 823–973 K, while the PRF values of the Al2O3 film and the Cr2O3 film are 95–247 and 24–117 at temperatures of 823–973 K. Therefore, the Al2O3/Cr2O3 composite film can significantly enhance the deuterium permeation suppression property.

Kinetic Study of the Competitive Growth Between θ-Al2O3 and α-Al2O3 During the Early Stages of Oxidation of β-(Ni,Pt)Al Bond Coat Systems: Effects of Low Oxygen Partial Pressure and Temperature

An oxidation study of β-(Ni,Pt)Al commercial bond coat systems was carried by means of TGA analysis during isothermal treatments at temperatures from 1273 K to 1423 K (1000 °C to 1150 °C). The effect of oxygen partial pressure on their oxidation kinetics was studied and complemented by photo-stimulated luminescence spectroscopy and SEM. Pre-oxidation treatments performed on as-aluminized samples at 10−5 atm O2 did not accelerate the θ-Al2O3 → α-Al2O3 transformation, even after 5 hours of oxidation relative to samples oxidized in 0.21 atm O2, with the exception of the sample treated at 1273 K (1000 °C), where apparently α-Al2O3 nucleation started earlier for the sample treated at a low pO2.

Observation on the Interface of α-Al2O3/Cr2O3: Prepared by oxidation of Al45Cr7

To explore the effects of Cr2O3 on the nucleation of α-Al2O3 and the nucleation interface of α-Al2O3/Cr2O3, the oxidation of Al45Cr7 powder at 650–720°C for 9–500h in air was carried out in this paper. The morphologies and structures of oxidized powder were investigated by XRD, SEM and TEM. The results showed that Cr2O3 formed at the outset of the oxidation, and acted as the nucleus of α-Al2O3 immediately. Then the oxidation of Al at the interface of Cr2O3/Al45Cr7 occurred, resulting the direct nucleation of α-Al2O3 on (001) facet of Cr2O3. The interface of α-Al2O3/Cr2O3 was found to be (001)α-Al2O3/(001)Cr2O3, with an angle about 5 degree between them. Besides, the effects of temperature on the nucleation of α-Al2O3 were also studied.

Characterization and hot corrosion performance of LVPS and HVOF-CoNiCrAlYSi coatings

The CoNiCrAlYSi coatings were produced by using low vacuum plasma spray (LVPS) and high velocity oxy-fuel (HVOF) techniques on the Inconel-738. Hot corrosion behavior and microstructure characterization was investigated by exposing the sample to a molten film of Na2SO4-20٪ wt NaVO3 at 880 °C for up to 560 h. The hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. The result of weight change measurements showed better hot corrosion resistance for HVOF-CoNiCrAlYSi coatings. This was attributed to the α-Al2O3 nucleation during the HVOF coating process and replacement of non-protective oxide due to a fluxing mechanism. It was also observed that the non-protective and porous oxides such as (Co,Ni)Al2O4 and (Co,Ni)Cr2O4 were formed on the both types of coatings due to large β-depleted zone after long exposure time of hot corrosion testing.

Impact of transition oxides on growth stresses and texture of alumina scales formed during oxidation of iron aluminides

The internal stresses and crystallographic texture in α-Al2O3 scales grown on iron aluminides at 1100 °C were determined in situ using synchrotron X-ray diffraction. In the first hour of oxidation, α-Al2O3 was formed by direct nucleation and by conversion from transition oxides (either θ-Al2O3 or a mixed Fe–Al oxide). A sharp texture develops connected with the direct nucleation of α-Al2O3, in contrast to the weaker texture observed in α-Al2O3 originated by previous transformations, which also yielded tensile stresses in early oxidation stages.

Rapid Formation of α-Al2O3 Scale on an Fe–Al Alloy by Pure-Metal Coatings at 900 °C

Rapid formation of an α-Al2O3 scale on Fe–50 at.%Al by pure metal thin coatings of Ni, Al, Ti, Cr or Fe was investigated, and the effects of those elements on Al2O3-scale evolution were assessed. The oxidation behavior of samples with and without coatings could be divided into two groups: the samples with/without Ni and Al, and those with Ti, Cr and Fe. The mass gains of samples coated with Al and Ni were almost the same as that of non-coated Fe–50 at.%Al alloy. The mass gains of samples coated with Ti, Cr, and Fe were much lower than that of the Fe–50 at.%Al alloy. A stable α-Al2O3 scale was found to develop from the beginning of oxidation on the samples coated with Ti, Cr and Fe. However metastable θ-Al2O3 remained after long-time oxidation of non-coated and Ni- and Al-coated samples. The direct α-Al2O3 scale formation on the samples with Cr or Fe coatings was speculated to be due to sympathetic nucleation of α-Al2O3 on the surface of Al-supersaturated Fe2O3 for Fe-coated sample, and composition changes from (Cr,Al)2O3 to (Al,Cr)2O3 for the Cr-coated sample. Initial formation of an oxide having a corundum structure was inferred to provide a nucleation site for precipitation of α-Al2O3 without prior formation of a metastable Al2O3 scale.

Elaboration of Nickel Aluminide Diffusion Coatings: Application to Oxidation Resistance

Aluminide diffusion coatings are widely used in aeronautic domain. Nickel based superalloys present good mechanical properties and creep resistance at high temperature but their corrosion resistance is somewhat poor. In order to enhance their lifetime at high temperature, aluminide coatings can be applied on alloy surface. Aluminium present on material surface can form, at high temperature, alumina scale which is a protective oxide scale. In the present study, nickel aluminide coatings were carried out by aluminisation in a single step pack cementation process on nickel based superalloys 600 and AM1. Ni-Al intermetallic was obtained in each case. Oxidation tests were carried out at 900°C in air under atmospheric pressure. At this temperature, transient alumina as γ-Al2O3 and θ-Al2O3 appear on surface and are less protective than the stable α- Al2O3. Reactive elements can enhance α-Al2O3 nucleation and change diffusion mechanism during oxidation leading to a better adherence of alumina scale. In the present work, yttria was introduced by Metal-Organic Chemical Vapour Deposition (MOCVD) prior to the aluminisation. Yttria doped and undoped sample behaviours were compared under oxidising atmospheres.

Effects of preoxidation on the nucleation and growth behavior of chemically vapor-deposited α-Al2O3 on a single-crystal Ni-based superalloy

A chemical vapor deposition (CVD) procedure was developed for preparing a high-quality α-Al2O3 coating layer on the surface of a single-crystal Ni-based superalloy using AlCl3, CO2 and H2 as precursors. A critical part of this procedure was a short-time preoxidation step (1 min) with CO2 and H2 in the CVD chamber, prior to introducing the AlCl3 precursor. Without this preoxidation step, extensive whisker formation was observed on the alloy surface. Characterization results showed that the preoxidation step resulted in the formation of a continuous oxide layer (∼50 nm) on the alloy surface. The outer part of this layer (∼20 nm) appeared to contain mixed oxides, whereas the inner part (∼30 nm) mainly consisted of α-Al2O3 grains with θ-Al2O3 as a minor phase. We observed that the nucleation of α-Al2O3 in the preoxidized layer was promoted by (1) rapid heating (10 seconds) of the alloy surface to the temperature region where α-Al2O3 was expected to nucleate; (2) the low oxygen pressure environment of the preoxidation step, which kept the rate of oxidation low; and (3) contamination of the reactor chamber with HfCl4. The preoxidized layer served as an effective diffusion barrier for mitigating the interaction with some of the alloying elements such as Co and Cr with the CVD precursors and eliminating whisker formation on the alloy surface.

The formation of α-Al2O3 from θ-Al2O3: The relevance of a “critical size” and: Diffusional nucleation or “synchro-shear”?

The coarsening of θ-Al2O3 crystals to a 'critical size' is often interpreted as the first step in the shear nucleation of α-Al2O3. The existence of this so-called critical size has also been used to explain the observation that α-Al2O3 “nuclei” are generally twice as large as the crystals in the θ-Al2O3 matrix. This paper discusses the important issues in the nucleation of α-Al2O3 from θ-Al2O3. A few key experiments are also presented to clarify the nucleation process. It is concluded that a critical θ-Al2O3 crystal size is not a prerequisite for α-Al2O3 nucleation, but is primarily a result of the incubation time required to produce α-Al2O3 nuclei by diffusional nucleation. It is proposed that the large observed α-Al2O3 crystal size also does not result from a shear nucleation event in a 'critical size' θ-Al2O3 crystal, but is due to the intrinsically low α-Al2O3 nucleation density, together with rapid growth of α-Al2O3 after nucleation.w