Feathery Microstructure Research Articles

Pr doped La2Zr2O7 TBCs by EB-PVD: Thermal property, morphology and degradation mechanism

Pr doped La2Zr2O7 (LPZ) TBCs have been deposited by electron beam physical vapor deposition. This work concentrates on the thermophysical properties and microstructure evolution of LPZ/YSZ duplex TBCs before and after thermal cyclic so as to probe into the merit and the application feasibility. The thermal expansion, thermal conductivity and phase structure have been measured and analyzed in comparison to other classical TBCs materials. Typical feathery microstructure and intra-columnar gaps did not provide desired thermal cyclic counts for LPZ/YSZ TBCs. Two principal categories of degradation mechanism limited the cyclic lifetime. The first principal degradation mode is the pulverization of LPZ after thermal cyclic due to the Pr rare earth elements modification in lanthanum Zirconates. Pr instability created the pulverization degradation mode instead of spalling. The second degradation mode is the cracks extension paralleled to interface inside TGO due to the brittle spinel generated by excessive oxidation. This study of thermal properties and degradation mechanism might offer thought and inspiration for other advanced TBCs.

LaNdZrO thermal barrier coatings by electron beam physical vapor deposition: Morphology, thermal property and failure mechanism

Thermal barrier coatings (TBCs) have been widely studied over the last decade. Lanthanum zirconate (LaZrO) is known for high melting point and good thermal stability in aircraft engine environment. The thermal property and failure mechanism of these advanced TBCs still remain a challenge. This study focuses on phase structure, thermal expansion coefficient, thermal conductivity, morphology, composition, thermal durability, and failure behaviors in the LaNdZrO/YSZ/NiCoCrAlYHf TBCs. The LaNdZrO and YSZ ceramic coatings have been deposited by EB-PVD, and NiCoCrAlYHf metallic coating has been deposited by AIP-PVD. The as-deposited TBCs exhibit good thermal cycling life and thermal shock life at 1100 °C due to the improvement of TEC and the formation of feathery microstructure. In the thermal durability test, the locally broken region of the YSZ/TGO interface, YSZ coating and further the LaNdZrO coating might facilitate propagation of the instability. The element diffusion and cracks evolution might be two critical factors for the failure mechanism of TBCs. This study would further explain the failure of other advanced TBCs.

Phase transformation pathway and microstructural refinement by feathery transformation of Ru-containing γ-TiAl alloy

The phase transformation sequence and transition temperatures of Ti–48Al–4Nb–2Cr-0.3Ru alloy are studied by using in-situ synchrotron radiation high energy X-ray diffraction, which was confirmed as α → α+γ → α/α2+β/B2+γ. The α transus and β/B2 solvus temperatures are located at 1390 °C and 1300 °C, respectively. Significantly, the acceleration of Ru element on the precipitation of metastable massive and feathery microstructures was found for the first time. Based on that, a multi-step heat treatment consisting of homogenization treatment, cyclic air-cooling heat treatment, γ equiaxed treatment and lamella reprecipitation treatment was designed to optimize the Ti–48Al–4Nb–2Cr-0.3Ru alloy. The coarse microstructure after homogenization was refined by feathery microstructure during cyclic air-cooling heat treatment. Finally, a fine duplex microstructure with γ grain and lamellar colony sized approximately 25 μm and 40 μm, respectively, was achieved. First-principles calculation revealed that the interfacial energies can be decreased when Ru atom occupies the Al sublattice position of true twin and Σ5 coincidence site lattice boundaries, which promote the occurrence of metastable transformations. Comparing with the initial microstructure, the tensile properties of the duplex microstructure were highly improved both at room temperature and 800 °C.

Thermal property and failure behavior of LaSmZrO thermal barrier coatings by EB-PVD

SummaryLa2Zr2O7 coatings are promising candidates to substitute YSZ coatings in advanced gas turbine engines. In this work, Sm-doped La2Zr2O7 coatings were deposited by physical vapor deposition. This work focuses on the crystal structure, thermal conductivity, thermal expansion coefficient, morphology, composition, and thermal durability of LaSmZrO coatings. The LaSmZrO ceramics exhibit low thermal conductivity (1.69 W/mK at 800°C) and high thermal expansion coefficient (9.72∗10−6 K−1 at 1500°C) compared with La2Zr2O7. The LaSmZrO/YSZ coatings with feathery microstructure show relatively good thermal durability (8183 cycles or 856 h) under high temperature. The broken regions are observed at the ceramic coating/bond coating interface. The failure behaviors are relevant with crack evolution and thermally grown oxide growth. This work might guide the investigation of advanced coatings under high temperature.

Thermal property and failure behaviors of Gd doped LaZrCeO coatings with feathery microstructure

LaZrCeO coatings are promising candidates to substitute Y2O3-stabilized ZrO2 in advanced gas turbine engines. In this study, Gd doped LaZrCeO coatings were deposited by electron beam physical vapor deposition. This study focuses on the phase, microstructure, thermal property, and thermal durability of (La1-xGdx)2(Zr0.7Ce0.3)2O7 coatings. The as-deposited coatings show relatively good thermal shock life and thermal cycling life. The broken regions are observed on the interface of thermal barrier coatings. The failure behaviors are relevant with crack evolution and thermally grown oxide growth. This study might guide the investigation of advanced coatings under high temperature.

Effects of Er stabilization on thermal property and failure behavior of Gd2Zr2O7 thermal barrier coatings

This work focuses on the effects of Er stabilization on thermal property and failure behavior. The GdErZrO/YSZ TBCs exhibit a defect fluorite structure with high thermal expansion coefficient and low thermal conductivity. The as-deposited TBCs show a high thermal life and a good microstructure stability at 1100 °C due to the Er stabilization and the feathery microstructure. After thermal cycling test, the broken of thermally grown oxide is generally attributed to the Al2O3 growth and cracks evolution. Furthermore, the element diffusion might be critical factor for cracks extension leading to the spallation and delamination failure.

Thermal shock life and failure behaviors of La2Zr2O7/YSZ, La2Ce2O7/YSZ and Gd2Zr2O7/YSZ DCL TBCs by EB-PVD

Double layer ceramic (DCL) thermal barrier coatings (TBCs) have attracted extensive attention due to their high thermal stability under high temperature. The thermal shock life and failure behaviors of DCL TBCs still remain a challenge. This work focuses on crystal structure, thermal expansion coefficients, thermal conductivity, microstructure and failure behaviors of the La2Zr2O7/YSZ (LZ/YSZ), La2Ce2O7/YSZ (LC/YSZ) and Gd2Zr2O7/YSZ (GZ/YSZ) DCL TBCs. The advanced ceramic material exhibits low thermal diffusivity and thermal conductivity. Furthermore, the advanced DCL TBCs exhibit good thermal shock life at 1100 °C due to the feathery microstructure with inter-columnar gaps and inter-columnar pores. After thermal shock test, the broken regions occur on the cracks area within ceramic coatings, TGO and interface leading to the TBCs instabilities. The TBCs interface instabilities play a key role on the failure behaviors of DCL TBCs.

Effective removal of lead(II) migrated from kitchen faucets using multi-slice feathery γ-alumina

In this study, γ-Al2O3 was synthesized using a hydrothermal method and its morphology and chemical composition was characterized by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and X-ray diffractometer (XRD). The TEM images, EDX and XRD spectrum highlight good crystallization and multi-slice feathery microstructure, involving relatively pure crystal sintering. The Pb(II) concentration analysed by inductively coupled plasma mass spectrometry (ICP-MS) after centrifuging is linked to the γ-Al2O3 adsorption, and a removal method for Pb(II) from water passing through kitchen faucets is proposed. The Pb(II) value below 0.06 mg g−1 relative to the γ-Al2O3 value reveals 94% adsorption. Further assessment of the alumina separation effect for faucets shows Pb(II) precipitation from the downstream of the valve spool is below 0.16 μg L−1. About 0.12 g adsorbent is mixed with 1.0 L tap water to produce a suspension for effective soaking. The proposed method produced satisfactory results, providing a theoretical basis for authorities to exploit on heavy metal removal from kitchen faucets.

LZC/YSZ double layer coatings: EB-PVD, microstructure and thermal cycling life

Low thermal conductivity ceramic materials have triggered tremendous attention in gas turbine engines coatings. It still remains a challenge for investigating the thermal cycling life, diffusion and failure of LaZrCeO (LZC). A LZC/YSZ double layer coatings (DCL) TBCs have been prepared by vacuum arc ion-plating (ARC) and electron beam-physical vapor deposition (EB-PVD). The phase structures of LZC/YSZ TBCs are composed of La2Zr2O7 and La2Ce2O7. The as-deposited LZC/YSZ shows columnar and feathery microstructures. The as-deposited coating showed high thermal cycling and shock lifetime. After thermal cycles, the DCL coating shows a good thermal stability. The double layer structure and feathery microstructure appear to be relevant with the high lifetime of TBCs.

LZC/YSZ DCL TBCs by EB-PVD: Microstructure, low thermal conductivity and high thermal cycling life

Thermal barrier coatings (TBCs) with low thermal conductivity have triggered tremendous attention due to their promising application in the gas turbine engines. Albeit recent studies have investigated double ceramic layers (DCL) with pyrochlore (A2B2O7) phase, it still remains a big challenge for controlling element content and investigating the relationship between the complex hierarchical architectures and their thermal performances. Here we describe a series of DCL La2O3-ZrO2-CeO2 (LZC)/Y2O3-stabilized ZrO2 (YSZ) coating under different current of electron beam by electron beam-physical vapor deposition (EB-PVD). The formation of hierarchical architecture with feathery microstructure and intra-columnar have been investigated in detail. The DCL coatings achieve a high thermal cycling life and relatively low thermal conductivity at controlling current of electron beam from 1.0 A to 1.3 A. This work may open new opportunities to rationally design other promising TBCs.

Feathery Microstructure Formation of Ti48Al2Cr2Nb Alloy by Rapidly Quenched Solidification

AbstractFeathery microstructure is observed in the rapidly solidified sample of Ti48Al2Cr2Nb (at%) alloy. The results show that the microstructure changes significantly from the surface to bottom of the sample, that is, cellular → dendrite → dendrite cell → feathery γ phase (γf). This phenomenon is attributed to the variation of cooling rates in local regions. The cooling rates of sample by rapidly quenched solidification are calculated by ANSYS software. A rapid solidification at about 3.67 × 103 K s−1 is obtained at the bottom of sample, causing the formation of γf. High dislocation density is detected in γf by transmission electron microscopy (TEM), attributed to the releases of the stresses during rapid solidification. The formation of γf is considered to be solid‐state phase transformation because of the strong chilling. Rapidly quenched solidification can provide high driving force for the nucleation of γf during rapid solidification.

3D Analysis of Porosity in a Ceramic Coating Using X-ray Microscopy

Suspension plasma spraying (SPS) is a new, innovative plasma spray technique using a feedstock consisting of fine powder particles suspended in a liquid. Using SPS, ceramic coatings with columnar microstructures have been produced which are used as topcoats in thermal barrier coatings. The microstructure contains a wide pore size range consisting of inter-columnar spacings, micro-pores and nano-pores. Hence, determination of total porosity and pore size distribution is a challenge. Here, x-ray microscopy (XRM) has been applied for describing the complex pore space of the coatings because of its capability to image the (local) porosity within the coating in 3D at a resolution down to 50 nm. The possibility to quantitatively segment the analyzed volume allows analysis of both open and closed porosity. For an yttria-stabilized zirconia coating with feathery microstructure, both open and closed porosity were determined and it could be revealed that 11% of the pore volumes (1.4% of the total volume) are closed pores. The analyzed volume was reconstructed to illustrate the distribution of open and closed pores in 3D. Moreover, pore widths and pore volumes were determined. The results on the complex pore space obtained by XRM are discussed in connection with other porosimetry techniques.

A mixture of massive and feathery microstructures of Ti48Al2Cr2Nb alloy by high undercooled solidification

A mixture of massive and feathery microstructures was observed in Ti48Al2Cr2Nb alloy subjected to the undercooled solidification rather than the heat treatments in most cases. Double recalescence events and primary β solidification confirmed that massive γ phase did not directly nucleate from the undercooled melt but formed during the solid-state transformations. It is believed that small white areas (aluminium-poor) along lamellar grain boundaries may be closely related to the formation of massive γ phase and feathery γ phase. High dislocation density and stacking faults were detected in massive γ phase by transmission electron microscopy. The high energy of defects and undercooling in the solid state phase transformation can provide sufficiently high driving force for the nucleation of massive γ phase.

Influence of alumina content on the nucleation crystallization and microstructure of barium fluorphlogopite glass-ceramics based on 8SiO2·y Al2O3·4MgO·2MgF2·BaO Part II Microstructure, microhardness and machinability

The influence of alumina content, heat treatment time and temperature on the microstructure, hardness and theoretical machinability of barium fluorphlogopite glass-ceramics were investigated. The glass-ceramics were based on glasses of the general composition 8SiO2-yAl2O3-3.75MgO-2.25MgF2-BaO. Glasses with high alumina contents with y > 2.0 were found to give fine feathery microstructures that did not coarsen readily to give the classic house of cards microstructure associated with machinability. The glass with the lowest alumina content (y = 1.5) that gave a house of cards microstructure was therefore investigated in detail. The Vickers hardness decreased slightly on formation of the barium fluorphlogopite phase, but decreased significantly on forming an interconnected house of cards microstructure. Machinability was highly dependent on the formation of an interconnected microstructure.

Microstructural Control for Superplasticity Simply by Heat Treatment without Thermomechanical Processing in a Ti-46Al-3.5Cr Alloy.

Microstructural evolution only by heat treatment has been studied for a Ti-46at%Al-3.5at%Cr alloy, in order to obtain a microstructure which causes superplasticity at high temperatures. By changing the cooling rate from 1 613 K in the α-Ti single-phase region, three kinds of microstructures were identified. Namely, lamellar microstructure appeared by furnace cooling, feathery microstructure took place by air-cooling and massive microstructure prevailed by oil quenching. During subsequent annealing at 1 473 K in the β+γ two-phase region, the feathery microstructure turns to fine microdual structure with the equiaxed y grains with 13 μm in grain size and the β precipitates formed along the y grain boundaries. In a tensile test at 1 473 K with a strain rate of 3.2×10 -4 s -1 , this β/γ microdual structure exhibits remarkable superplastic deformation with an elongation of 450%, which is the same with that obtained by the β/γ microdual structure prepared by a thermomechanical processing. On the other hand, the lamellar microstructure and the massive microstructure are not transformed to the β/γ microdual structure with the equiaxed y grains, resulting in low elongations of 30% and 110% at 1 473 K, respectively.

Formation of α phase in the massive and feathery γ-TiAl alloys during aging in the single α field

Precipitation of {alpha} phase in massive and feathery microstructures was studied during aging of a Ti-48 pct Al-2 pct W-0.5 pct Si alloy in the single {alpha} field. It was found that the {alpha} phase mainly precipitates along the {gamma}-plate interfaces as laths in the feathery structure, while it nucleates at various sites in the massive structure in the form of idiomorphs and especially of plates. The {gamma}{sub m} {yields} {alpha} reaction proceeds by the growth of pre-existing {alpha} precipitates and chiefly by the development of new {alpha} plates. The {alpha} plates are likely to originate from the splitting of unit dislocations into Shockley partials and to grow by the diffusional ledge mechanism, which shows both diffusional and shear character. During aging, the stacking faults (SFs) in the massive {gamma} domains evolve into SF-shaped {alpha} precipitates through a transition {gamma}{prime} phase.

Study of the polymorphism and the crystallization kinetics of tripalmitin: A microscopic approach

The morphology and kinetics of crystallization of tripalmitin have been examined in detail by optical microscopy. The α‐crystallization process is characterized by a fast heterogeneous nucleation and spherulitic growth, even at low undercooling, resulting in intense birefringence and smooth spherulitic entities. Four different β’‐microstructures have been found—grainy, fibrous, feathery and lamellar. Around 47°C, a clear change from a grainy to a fibrous β’‐microstructure is observed. This transition seems to take place without a drastic change in nucleation or in crystal growth. At 50°C, both nucleation and crystal growth exhibit a clear discontinuity, indicating interference from β‐crystallization. Around 52°C, the β’‐form changes again from a fibrous to a more feathery microstructure; the transition is accompanied by a distinct decrease in crystal growth rate. The lamellar β’‐structure exhibits the highest stability and can be obtained onlyvia an accelerated nucleation at low temperature, followed by further growth at elevated temperature near the melting temperature of the β’‐form. Determination of the β‐form on the basis of its microstructure is not always precise, because the microstructure strongly depends on whether the β‐crystals are obtained from a transformation of α or β’, or whether β‐crystallization occurs directly from the melt. Clear confirmation of the polymorphic nature of the solid state can be obtained from melting point determination.

Laser Processing of Cast Iron for Enhanced Erosion Resistance

The surfaces of nodular and gray cast iron specimens have been modified by CO2 laser processing for enhanced hardness and erosion resistance. Control of the near-surface microstructure was achieved primarily by controlling resolidification of the laser melted layer through variations in laser beam/target interaction time and beam power density. Typical interaction times and power densities used in this study were 5 msec and 500 kW/cm2. Analysis of the laser melted surface showed a dramatic increase in hardness and a greatly refined microstructure. Depending on the processing parameters, two basic kinds of microstructure can be produced in the laser hardened layer—a feathery microstructure with a very high hardness (up to 1245 HV) and a dendritic microstructure with a metastable, fully austenitic matrix and a lower hardness (600 to 800 HV). Erosion testing was done in a rotating paddle device using slurries of SiO2 or SiC in water. Weight loss and crater profile measurements were used to evaluate the erosion characteristics of the various microstructures. Both ductile and gray cast iron showed marked improvement in erosion resistance after laser processing.