Crystallization Of Amorphous Phase Research Articles

Magnetron-Deposited FeTiB Films: From Structural Metastability to the Specific Magnetic State

Results of XRD and TEM studies of a metastable phase state in Fe73Ti5B19O3 and Fe55Ti16B27O2 films, which is formed upon magnetron deposition under preset conditions, and of the evolution of the state in the course of subsequent annealing at 500 °C for 1, 5, and 9 h and experimental data on the magnetic microstructure and magnetic properties are reported. The annealed films were found to be characterized by a nanocrystalline structure, which is represented by two crystalline phases, namely, the ferromagnetic solid solution αFe(Ti), and nonferromagnetic boride FenB. The Ti content in the films determines the grain size of the αFe(Ti) phase, whereas the content of B localized within the grain boundaries determines the ratio of the volume fractions of amorphous and nanocrystalline phases in the structure. In contrast to the ferromagnetic Fe73Ti5B19O3 films, the Fe55Ti16B27O2 films are superparamagnets both in the deposited state and after annealing at 500 °C for 1 and 5 h because of the higher volume fraction of the amorphous phase in the structure. The 9 h annealing of the Fe55Ti16B27O2 films transfers them into the ferromagnets owing to the development of the amorphous phase crystallization, increase in the content of nanocrystalline ferromagnetic phase αFe(Ti) grains, and realization of exchange interaction between them.

Tropomyosin induces the synthesis of magnesian calcite in sea urchin spines

Calcium carbonate is present in many biominerals, including in the exoskeletons of crustaceans and shells of mollusks. High Mg-containing calcium carbonate was synthesized by high temperatures, high pressures or high molecular organic matter. For example, biogenic high Mg-containing calcite is synthesized under strictly controlled Mg concentration at ambient temperature and pressure. The spines of sea urchins consist of calcite, which contain a high percentage of magnesium. In this study, we investigated the factors that increase the magnesium content in calcite from the spines of the sea urchin, Heliocidaris crassispina. X-ray diffraction and inductively coupled plasma mass spectrometry analyses showed that sea urchin spines contain about 4.8% Mg. The organic matrix extracted from the H. crassispina spines induced the crystallization of amorphous phase and synthesis of magnesium-containing calcite, while amorphous was synthesized without SUE (sea urchin extract). In addition, aragonite was synthesized by SUE treated with protease-K. HC tropomyosin was specifically incorporated into Mg precipitates. Recombinant HC-tropomyosin induced calcite contained 0.1–2.5% Mg synthesis. Western blotting of sea urchin spine extracts confirmed that HC tropomyosin was present in the purple sea urchin spines at a protein weight ratio of 1.5%. These results show that HC tropomyosin is one factor that increases the magnesium concentration in the calcite of H. crassispina spines.

Explanation and prediction of the selective crystallization of boscalid by mid-frequency Raman difference spectroscopy analysis

The selective crystallization of amorphous phase, monohydrate and solutions of boscalid (BOS) were studied by experiment and mid-frequency Raman difference spectroscopy. The possible routes to obtain BOS solids with good purities were proposed.

Metal-Induced Crystallization in Metal Oxides.

ConspectusThe properties of a material depend upon its physical characteristics, one of these being its crystalline state. Next generation solid-state technologies will integrate crystalline oxides into thermal sensitive processes and composite materials. Crystallization of amorphous phases of metal oxides in the solid state typically requires substantial energy input to induce the amorphous to crystalline phase transformation. In the case of silica, the transformation to α-quartz in a furnace occurs above 1300 °C and that of titania, above 400 °C. These calcination processes are costly in energy but also often degrade complex material architectures or compositions.Thus, low temperature crystallization techniques are required that preserve macro- and mesostructures and complex elemental composition (e.g., organic-, metal-, and semiconductor-metal oxide hybrids/composites). Some solution-based techniques exist to directly fabricate crystalline metal oxides. However, these are not always compatible with the specificities of the system or industrial constraints. A postsynthetic, solid-state approach that reduces crystallization temperature in metal oxides is metal-induced crystallization (MIC).MIC is the introduction of catalytic amounts of a cation, which can be an s-block, p-block, or d-block cation, that migrates through the solid metal oxide lattice. The cation is thought to temporarily break metal oxide bonds, allowing [MOx] polyhedra to rotate and reform bonds with neighboring [MOx] groups in a lower energy crystalline phase. Depending on the system, the cation can favor or defavor the formation of a particular crystalline phase, providing a means to tune the purity and crystalline phase ratios. An advantage of MIC is that, although the crystallization occurs in the solid state, the crystallization process can be accomplished for particle suspensions in liquid media. In this case, the energy required to induce the crystallization can come from, for example, a microwave or an ultrasound bath. The crystallization of particles in suspension avoids aggregation from particle–particle sintering. In the case of thin films, the energy for crystallization typically comes from a laser or calcination.MIC is only recently being used as a low temperature metal oxide crystallization technique, despite being widely used in the semiconductor industry. Here, the mechanism and previous studies in MIC are presented for titania, silica, and other oxides. The beauty of this technique is that it is extremely easy to employ: cations can be incorporated into the system postsynthetically and then are often expelled from the lattice upon phase conversion. We expect MIC to enrich materials for photochromic, optoelectronic, catalyst, biological, and other applications.

Wear behavior at high temperatures of ZrO2 - Al2O3 plasma sprayed coatings and an electro-melted AZS refractory

Two ZrO 2 -Al 2 O 3 atmospheric plasma coatings were applied to a pressed and sintered aluminosilicate refractory brick, which were compared to an electro-melted alumina-zirconia-silica (AZS) refractory typically used in the glass industry for its high temperature wear resistance. The samples were subjected to sliding contact using a ball-on-disk test at 25, 500, 750, and 1000 °C and then, physical, crystallographic, and mechanical characterizations were performed before and after the wear tests in order to monitor the wear behavior. Both ZrO 2 -Al 2 O 3 coatings exhibited better wear performance than the electro-melted AZS refractory at all temperatures evaluated, owing to their high hardness and fracture toughness , which were mainly related to the increase of α-Al 2 O 3 and t-ZrO 2 phases from γ-Al 2 O 3 and the crystallization of amorphous phases respectively. In the same way, both coatings also showed the same wear mechanisms at all temperatures evaluated. At 25 °C, wear by ductile deformation was identified, while at 500 and 750 °C, wear by brittle deformation was produced, and at 1000 °C, the transition of wear from brittle to ductile deformation was evident. For the electro-melted AZS refractory, until 750 °C, wear by brittle deformation was developed, while at 1000 °C, wear by both, brittle and ductile deformation was detected. • The wear resistance at 1000 °C of two different ZrO 2 -Al 2 O 3 coatings was evaluated. • The quantity of α-Al 2 O 3 and t-ZrO 2 increased at 1000 °C in both coatings. • Both ZrO 2 -Al 2 O 3 coatings showed similar wear resistance at 1000 °C. • Both coatings showed better wear resistance at 1000 °C than the reference refractory. • Both coatings can protect the refractory used as substrate at 1000 °C.

Achieving fast hydrogenation by hydrogen-induced phase separation in Mg-based amorphous alloys

Mg-based amorphous alloys are one of the potential hydrogen storage materials. The challenges which are blocking application of such alloys involve unfavorable crystallization of amorphous phase in hydrogenation and dehydrogenation and sluggish low-temperature de/hydrogenation kinetics. To solve those challenges, the Mg60RExNi30−xCu10 (RE=La and Ce; x = 5, 7.5, 10 and 15) amorphous alloys were prepared by melt spinning and the structural transformation of amorphous phase and its correlation to hydrogenation kinetics has been investigated. It is observed that all the amorphous powders are able to absorb more than 3.0 wt%-H at 130 °C under 4.5 MPa-H2, and the hydrogen absorption rate of the Mg60RE10Ni20Cu10 amorphous alloys is much faster than other alloys by forming a dual amorphous phase structure during hydrogenation. XRD characterization of Mg60Ce10Ni20Cu10 hydrides proved that such dual amorphous phase structure is resulted by hydrogen-induced amorphous phase separation. According to EDX results, the dual amorphous phase structure consists of Ce-rich and Ni-Cu-rich domains with a size of about 5 nm which gradually form by element aggregation during hydrogen absorption. Meanwhile, it is also confirmed that the element aggregation is the precursor of hydrogen-induced crystallization as crystallization is observed in the Ni–Cu-rich domain. It is verified that the hydrogen-induced amorphous phase separation of the Mg-based amorphous alloys is tunable and can be used for obtaining superior hydrogenation performance. The hydrogen-induced structural evolution revealed in this work might inspire the development of new Mg-based hydrogen storage amorphous alloys.

Comparison for Grain Growth Dynamics of Severely Deformed Austenite and Martensite NiTi Shape Memory Alloys after Complete Crystallization of Amorphous Phase

Severe plastic deformation (SPD) based on local canning compression is imposed on Ni-rich Ni50.7Ti49.3 (at.%) shape memory alloy (SMA) with B2 austenite phase and equiatomic Ni50Ti50 (at.%) SMA with B19′ martensite phase in order to induce the amorphization of the two alloys. SPD-processed Ni50.7Ti49.3 and Ni50Ti50 samples are annealed at 450°C for various times in order to comparatively investigate grain growth dynamics of severely deformed austenite and martensite NiTi SMAs after complete crystallization of amorphous phase. The grain growth exponent of B2 austenite Ni50.7Ti49.3 SMA is 2.78, whereas the grain growth exponent of B19′ martensite Ni50Ti50 SMA is 3.18. The results show that grain growth dynamics of amorphous NiTi samples is sensitive to the chemical composition. The grain growth of equiatomic B19′ martensite Ni50Ti50 sample is inhibited to a greater extent than that of Ni-rich B2 austenite Ni50.7Ti49.3 one. The mechanisms for grain growth of NiTi SMAs deal with formation of nano-clusters due to crystallization of amorphous phase, recovery of retained nanocrystalline grains containing dislocations under thermal driving force, growth of retained nanocrystalline grains as well as new nanocrystalline grains and incorporation of nano-clusters or smaller nanocrystalline grains into larger nanocrystalline grains. The grains of Ni50.7Ti49.3 sample absorb nano-clusters more readily than that Ni50Ti50 one during grain growth. Investigation on grain growth dynamics of severely deformed NiTi SMAs lays the foundation for fabricating nanocrystalline NiTi SMAs with tailored grain size.

Effect of microstructure evolution and crystal structure on thermal properties for plasma-sprayed RE2SiO5 (RE = Gd, Y, Er) environmental barrier coatings

In this work, the microstructure evolution, thermal expansion, thermal conductivity, and thermal shock resistance properties of the plasma-sprayed X1-Gd2SiO5, X2-Y2SiO5, and X2-Er2SiO5 coatings were evaluated and compared by experimental measurement and theoretical exploration. Results showed that significant microstructure evolution such as crystallization of amorphous phase, grain growth, and defects reduction was observed in the RE2SiO5 coatings after thermal aging at 1400 °C. The X1-Gd2SiO5 coating exhibited higher CTE values than the X2-Y2SiO5 and X2-Er2SiO5 coatings, which was related to their crystal structure. The thermal conductivity of thermal-aged RE2SiO5 coating was much higher than that of the as-sprayed RE2SiO5 coating, and thermal conductivity was determined not only by crystal structure but also mainly by the microstructure of the coatings. The X2-Y2SiO5 and X2-Er2SiO5 coatings with lower thermal mismatch stresses presented much better thermal shock resistance than the X1-Gd2SiO5 coating.

Formation of Metastable Phases during Crystallization of Amorphous Iron-Based Alloys

The structure of metastable Fe3B boride and its change during phase transformations of alloys of the Fe–B system from amorphous to equilibrium crystalline state have been investigated by X-ray diffraction analysis and transmission electron microscopy. It is shown that metastable boride undergoes the Fe3B ( $$I\bar {4}$$ ) → Fe3B ( $$P{{4}_{2}}{\text{/}}n$$ ) → Fe3B (P) structural transformations and retains its lattice parameters during amorphous-phase crystallization. The thermal stability of each lattice depends on the B concentration in the alloy.

Degradation of the plasma sprayed GdMgAl11O19 thermal barrier coating resistant to calcium-magnesium-aluminum-silicate attack at 1350 °C

Degradation of plasma sprayed GdMA and YSZ coatings attacked by CMAS was comparatively investigated at 1350 °C for 1 h, 12 h and 48 h in air. Results show that GdMA coating exhibited much better CMAS attack resistance than the YSZ coating. Presences of Gd, Mg and Al elements facilitated formations of the Ca2Gd8(SiO4)6O2 apatite, MgAl2O4 spinel and CaAl2Si2O8 anorthite crystals effectively arresting the molten CMAS glass penetration. YSZ coating underwent a typical dissolution-precipitation degradation mode. Precipitated fine ZrO2 grains leading to significantly increased grain boundaries provided pathways for further penetration of the melted CMAS causing a more and more serious degradation to YSZ coating along thickness direction as duration extended. GdMA coating with amorphous phase provided enough Al3+ dissolved in melted CMAS leading to fast formation of CaAl2Si2O8 crystals first. Local enrichment of Mg2+ concentration from the starting CMAS and as well as GdMA dissolution contributed to formations of the MgAl2O4 crystals. Ca2Gd8(SiO4)6O2 apatite crystals growth was Gd3+ concentration and local CMAS loading controlled, they effectively healed the vertical microcracks induced by crystallization of amorphous phase. CaAl2Si2O8 anorthite mainly as the top layer, MgAl2O4 spinel mainly as an relative thin intermediate layer followed by a third layer mainly consisting of Ca2Gd8(SiO4)6O2 apatite and integrity GdMA phase generally contributed to an entire degradation layer of the CMAS-degraded GdMA coating.

Enhanced mechanical properties of 3D printed Zr-based BMG composite reinforced with Ta precipitates

3D printing technique based on selective laser melting (SLM) provides a new approach to fabricate complex bulk metallic glass (BMG) components. However, the structural relaxation and partial crystallization of the pre-formed amorphous phase in heat affect zones (HAZs) caused by repeated laser scanning deteriorate the mechanical properties, i.e., reducing plasticity and fracture toughness. Here, we attempted the preparation of Zr-based metallic glass composites reinforced with precipitation of Ta particles by SLM technique. The results show that, in a wide range of energy densities, the 3D printed BMG composites exhibit high amorphous fraction (>90%) and good mechanical properties with fracture strength of 1.9 GPa, plastic strain of around 2.15% and fracture toughness (Kq) of 60 MPa m1/2, and the plastic strain is the highest values achieved in 3D printed BMGs so far. The suppression of crystallization of amorphous phase in HAZs and the precipitation of Ta particles in amorphous matrix can account for the enhancement of plasticity and fracture toughness while maintaining high strength. This work provides a hint for the preparation of toughening BMG composites by using 3D printing technique.

Deposition of amorphous hardening coatings by electrospark treatment in a crystalline granule mixture

The article focuses on the preparation of amorphous coatings on the Steel 1035 surface by electric spark treat the coating composition control by changing the granule mixture composition was studied. EDS analysis showed that the coatings obtained contain W, Mo, Co and Ni in different ratios. The weight of granules having different compositions decreased by 11–16 wt.% in 6 hours of treatment as a result of electric erosion. The mass transfer coefficient varied from 33 to 54 %. X-ray diffraction analysis showed the predominance of the amorphous phase in the composition of layers deposited. Annealing of the coatings at 1150 °C led to amorphous phase crystallization into M23(C,B)6 type borocarbide and α-Fe. The coatings had an increased microhardness of 10–15 GPa, and their wear resistance under dry sliding wear conditions at 10 and 50 N loads was 3,3 and 1,6 times higher, respectively, than in Steel 1035. The highest values at both loads were shown by samples without nickel, while samples without tungsten featured the lowest values. The coatings had a friction coefficient within 0,27–0,31 that is lower than for Steel 1035 by 13–30 %. Wear resistance of the coatings under dry abrasive wear conditions at the 25 N load was 3 to 5 times higher as compared to uncoated Steel 1035. Samples without nickel demonstrated the best performance, while samples without cobalt had the worst indicators. Thus, it was shown that tungsten and cobalt increase wear resistance of iron-based amorphous alloys, while nickel and molybdenum tend to worsen their tribotechnical behavior.

Phase Formation and Magnetic Properties of Melt Spun and Annealed Nd-Fe-B Based Alloys with Ga Additions

The structural transformations and magnetic property changes of the Nd16.2FebalCo9.9Ga0.5B7.5 (SG1, SG2) and Nd15.0FebalGa2.0B7.3 (SG3) nanocomposite alloys obtained by melt spinning in the as-quenched state and after annealing at a temperature range of 560–650 °C for 30 min were studied. The methods used were X-ray diffraction analysis, magnetic property measurements, TEM studies, X-ray fluorescence analysis and Mössbauer spectroscopy. Amorphous phase and crystalline phase Nd2Fe14B (P42/mnm) were observed in the alloy after melt spinning. The content of the amorphous phase ranged from 20% to 50% and depended on the cooling rate. Annealing of the alloys resulted in amorphous phase crystallization into Nd2Fe14B and led to the increased coercivity of the alloys up to 1840 kA/m (23.1 kOe) at 600 °C annealing for 30 min. The alloy with the maximum coercivity had a grain size of the Nd2Fe14B phase ≈50–70 nm with an Nd-rich phase between grains.

Hydrogenation and crystallization of amorphous phase: A new mechanism for the electrochemical capacity and its decay in milled Mg[sbnd]Ni alloys

MgNi-based alloys have been regarded as very promising anode material for nickel-metal hydride battery owing to their electrochemcial capacity much higher than that of AB5 alloys. However, MgNi alloy anodes generally suffer very serious capacity decay in cycling, which is previously ascribed to the corrosion of Mg in alkali electrolyte in the past decades. To further reveal the capacity fading mechanism of MgNi-based alloys, the electrochemcial hydrogen storage properties of Mg2Ni and amorphous Mg50Ni50 alloys and their microstructural evolution during cycling were comparatively studied. It has been firstly demonstrated that it is the amorphous MgNi phase instead of Mg2Ni phase contributes to electrochemical discharge capacity, because the Mg2NiH4 hardly released hydrogen under electrochemcial conditions. Then, it has been found that the charge input level has significant effect on the cyclic performance of amorphous Mg50Ni50 alloy anode. At full charge of 500 mAh g−1, the amorphous MgNi phase easily crystallized to the nanocrystalline Mg2NiH4, which leaded to the capacity decay immediately. Comparatively, with the low charge input of 100 mAh g−1, the discharge capacity of amorphous Mg50Ni50 alloy remained almost unchanged for 425 cycles. The excellent cyclability is attributed to the less hydrogenation-induced crystallization at low charge input. This work clearly demonstrates that the hydrogenation-induced crystallization is a dominating reason for the electrochemical capacity decay in the milled amorphous MgNi anode. It provides a new approach to improve the electrochemcial properties of MgNi alloy electrode by inhibiting the hydrogenation-induced crystallization.

Effect of Plane Strain Compression and Subsequent Recrystallization Annealing on Microstructures and Phase Transformation of NiTiFe Shape Memory Alloy

The effect of plane strain compression and subsequent recrystallization annealing on microstructures and phase transformation of NiTiFe shape memory alloy (SMA) is investigated. Inhomogeneous plastic deformation at various deformation degrees occurs in NiTiFe SMA during plane strain compression. Nanocrystalline phase and amorphous phase increase as the deformation degree increases. B2 austenite, B19′ martensite, nanocrystalline and amorphous phases coexist in the NiTiFe samples subjected to large plastic strain. The static recrystallization mechanisms depend on the microstructures of the deformed NiTiFe samples. The static recrystallization mechanisms deal with nucleation and growth of the recrystallized grains, growth of nanocrystalline phase and crystallization of amorphous phase. Grain size, subgrain boundaries, geometrically necessary dislocation density and Schmid factor are captured on the basis of electron backscattered diffraction data. The process of recrystallization annealing cannot eliminate the deformation texture completely. The slip direction [110] is the most favorable slip direction in the recrystallized NiTiFe sample. Plane strain compression along with subsequent recrystallization annealing changes the phase transformation path of as-rolled NiTiFe SMA, and it results in the decreasing martensite transformation start temperature. The three annealed NiTiFe samples exhibit the similar phase transformation behavior since complete recrystallization annealing leads to the similar microstructures.

Sliding wear of as-deposited and heat-treated nanocrystalline nickel-tungsten alloy coatings

The aim of the present study is to understand the sliding wear response of pulse electrodeposited nanocrystalline nickel (Ni) – tungsten (W) alloy coatings in both as-deposited and heat-treated conditions. Ni-W alloy coatings with a wide range of W content (0% to 25 at%) were deposited using pulse electrodeposition and subsequently annealed at 700 °C up to 1 h. The tribological behaviour of Ni and Ni-W coatings (both as-deposited and heat-treated) was studied using a unidirectional pin-on-disc configuration at two different normal loads 3 kg and 5 kg respectively. All tests were performed in atmospheric air, at room temperature and at a constant sliding speed of 0.67 m s−1 using WC-Co disc as countersurface. Wear test results indicated that Ni-W alloy coatings exhibit greater wear resistance when compared to pure Ni coatings at both the loads. The wear mechanism in pure Ni coatings was attributed to severe abrasive wear along with brittle fracture whereas Ni-W alloy coatings exhibited mild ductile wear characterized by micro-ploughing. In the case of as-deposited Ni-W alloy coatings, the addition of W up to 17 at% increased the wear resistance due to improvement in hardness, however, further increase in W content resulted in amorphization and reduced the hardness with corresponding decrement in wear resistance. Heat treatment of coatings resulted in grain growth, crystallization of amorphous phase and precipitation of nanoprecipitates (beyond 12 at% W). Presence of nanoprecipitates and crystallization of amorphous phase resulted in significant improvement in wear resistance of the heat-treated coatings compared to their as-deposited counterparts. Addition of W to Ni did not vary the coefficient of friction (CoF) significantly and was found to be independent of W content, load and hardness of the coatings. The effect of load and hardness of the coatings on wear rate did not follow the wear relationship proposed by Archard.

Crystalline – Amorphous AA4048 – Al60Nb40 composites processed by mechanical alloying and powder compaction

Commercial crystalline AA4048 powders and mechanically alloyed amorphous Al60Nb40 (at.%) powders were used for fabrication of crystalline-amorphous composites containing 10, 20 and 30 vol% of amorphous phase. High pressure high temperature technique was used for powders compaction. The applied pressure was 7.7 GPa and temperature was in the range 600–1000 °C. The powders and bulk samples were characterized by structural investigations (X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, X-ray fluorescence spectroscopy), hardness and microhardness tests and measurements of density. The obtained sinters revealed relative density above 98.7%. The amorphous component was observed as agglomerates or as single particles surrounding the grains of crystalline AA4048 phase. The crystallization of amorphous phase was not observed. Simultaneously, up to 2.5 wt% of Fe was detected as impurity from milling media. Significant increase of hardness was observed, from 226 to 288HB, resulting from the presence of amorphous component which prevent from cracks propagation during deformation.

About thermostability of biocompatible Ti–Zr–Ag–Pd–Sn amorphous alloys

Ti-based amorphous alloys produced by ultra-rapid melt cooling represent an excellent option as biomaterials because of their mechanical properties and corrosion resistance. However, complete elimination of toxic elements is affecting the glass-forming ability and amorphous structure could be obtained only for thin ribbons or powders that are subsequently processed by powder metallurgy. Amorphous ribbons of special Ti42Zr40Ta3Si15 alloy, which is completely free of any toxic element, were produced by melt spinning, and the thermostability of resulting material was investigated in order to estimate its ability for further heat processing. Isochronal differential scanning calorimetry (DSC) was used to determine transformation points such as glass transition temperature T g or crystallization temperature T x. The activation energy for crystallization of amorphous phase was calculated based on Kissinger method, using heating rates ranging between 5 and 20 °C min−1. Amorphous structure of resulting ribbon was evidenced by means of X-rays diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). It was determined that amorphous Ti42Zr40Ta3Si15 alloy has a high activation energy for crystallization, similar to other Ti-based amorphous alloys, which provides good thermal stability for subsequent processing, especially by means of powder metallurgy techniques.

Effect of heat treatment on microstructure and property of plasma-sprayed lanthanum hexaaluminate coating

Amorphous phase is commonly found in LaMgAl11O19 (LMA) coating prepared by atmospheric plasma spraying. In order to improve their performance, the as-sprayed LMA thermal barrier coatings on a nickel-based DZ125 substrate were heat-treated at 900, 1000 and 1100 °C for 5–20 h, respectively. The effect of heat treatment on the microstructure, porosity, phase composition, and crystallization behavior of the LMA coatings was investigated using scanning electron microscopy, X-ray diffraction, differential scanning calorimetry analysis. Samples before and after heat treatment for 20 h were thermally cycled at 1127 °C. Results indicated that the annealing of the as-sprayed LMA coating prior to thermal cycling is preferable to promote the crystallization of amorphous phase. Variations in microstructure, phase composition and thermal cycling lifetime of LMA coatings have been correlated to the heat treatment. Heat treatment at 900 °C improved the lifetime of LMA coatings, whereas their thermal cycling performance did not significantly improved when the aging temperature increased to 1000 or 1100 °C. The corresponding mechanism had been explored by tracing the microstructure and thermo-physical properties evolutions during thermal annealing process.

Nanograined Al2O3-ZrO2 (Y2O3) ceramics through pressureless sintering of powders with different degrees of crystallinity

In this work, Al2O3-ZrO2 (Y2O3) nanoceramics were prepared via pressureless sintering of powders with different degrees of crystallinity. It was demonstrated that, the crystallization of amorphous phase in starting powders highly enhanced the densification during sintering. When the crystal content was 25vol%, the powders exhibited better sinterability than other powders, and they could be sintered into bulk nanoceramics with relative densities higher than 98% at 1400°C and 1500°C. The superior sinterability of C25 powders was due to the crystallization of amorphous phase promoted by the as-existed nanocrystalline powders as nucleation seeds. The obtained nanoceramic exhibited homogenously distributed ZrO2 and Al2O3 particles with grain size of about 450nm, and displayed excellent mechanical property with Vickers hardness of 15.3GPa, fracture toughness of 5.2MPam1/2, and flexure strength of 1190MPa. This work may provide new guidance for optimization in preparing bulk nanoceramics by managing the crystallinity of starting powders.