Subsequent Vacuum Heat Treatment Research Articles

Bulk Nanostructured Silicide Thermoelectric Materials by Reversible Hydrogen Absorption-Desorption.

The production of bulk nanostructured silicide thermoelectric materials by a reversible hydrogen absorption-desorption process is demonstrated. Here, high-pressure reactive milling under 100bar hydrogen is used to decompose the Ca2Si phase into CaH2 and Si. Subsequent vacuum heat treatment results in hydrogen desorption and recombination of the constituents into the original phase. By changing the heat treatment temperature, recombination into Ca2Si or Ca5Si3 can be achieved. Most importantly, the advanced synthesis process enables drastic and simple microstructure refinement by more than two orders of magnitude, from a grain size of around 50µm in the initial ingot to 100-200nm after the hydrogen absorption-desorption process. Fine precipitates with sizes ranging from 10-50nm are forming coherently inside the grains. Thus, the route is promising and can be used for reducing thermal conductivity due to phonon scattering from grain boundaries as well as through nanostructuring with second-phase precipitates. Moreover, the process is environmentally friendly since hydrogen is reversibly absorbed, desorbed, and can be fully recovered.

Tungsten Heavy Alloys with Two phase Matrix

WNiCo alloys subjected to a two-stage or cyclic heat treatment develop a unique microstructure wherein apart from tungsten grains and matrix phase, fine tungsten precipitates are distributed in the matrix. This is unlike conventional heavy alloys such as WNiFe and WNiFeCo where the matrix is single phase without any secondary microstructural features. The purpose of developing a two-phase matrix is to realise superior mechanical properties compared to conventional alloys, especially strength with comparable or superior elongation and impact toughness. This advantage has rendered WNiCo alloys (with two-phase matrix) suitable candidates for advanced kinetic energy penetrators. The present study focusses on processing 92W-5Ni-3Co alloy using cyclic heat treatment and optimisation of parameters involved in cyclic heat treatment as well as subsequent vacuum heat treatment. Any refinement in processing parameters will help in improving the mechanical properties given the fact that processing parameters, microstructural features and mechanical properties are strongly interdependent in the case of tungsten heavy alloys.

Evaluation of vacuum heat-treated α-C films for surface protection of metal bipolar plates used in polymer electrolyte membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) assembled with metal bipolar plates (BPPs) is a promising power source for new energy vehicles. However, metal BPPs have serious corrosion issues and the surface electrical performance degrades in the harsh PEMFC environment. Amorphous carbon (α-C) films exhibit improved properties with both high corrosion resistance and electrical conductivity. Subsequent vacuum heat treatment of the as-coated α-C films can change their phase composition and film structure, modifying their performance. This study prepared α-C films with a titanium interlayer on a SS316L substrate by DC balance magnetron sputtering and then subjected them to vacuum heat treatment at different temperatures (400–700 °C). These treated α-C films are systemically analyzed in terms of surface and cross-sectional morphologies, sp bond hybridization, interfacial electrical conductivity, and corrosion resistance. The results indicate that the conversion of sp3 to sp2 and the compact density of α-C films are greatly enhanced with an increase in temperature, greatly improving the corrosion resistance and surface conductivity of the films. These promising results lead to a potential direction for the post-coating treatment of α-C films on metal BPPs in PEMFCs.

Effect of grit blasting and subsequent heat treatment on stress rupture property of a Ni-based single-crystal superalloy SGX3

In the present study, the effect of grit blasting and subsequent heat treatment on the stress rupture properties of a third-generation nickel-based single-crystal superalloy SGX3 sheet was studied. It was found that the stress rupture life of alloy SGX3 sheet at 980 °C/250 MPa was reduced by about 60% by only vacuum heat treatment at 1100 °C for 200 h and further reduced by 20% and 70% respectively with grit blasting of 0.3 MPa/1 min and 0.5 MPa/2 min before heat treatment. The microstructure analysis results indicated that the degradation of stress rupture life of alloy SGX3 sheet by vacuum heat treatment was mainly attributed to the variation of γ/γ′ microstructure, i.e., the decrease in γ′ volume fraction and the coarsening of γ′ precipitates. Furthermore, such degradation by grit blasting and subsequent vacuum heat treatment should be attributed to the formation of cellular recrystallization with different thicknesses at the surface of alloy SGX3 sheet, which not only acts as the vulnerable site for cracks to initiate and propagate but also reduces the effective loading area.

Tougher TiAl alloy via integration of hot isostatic pressing and heat treatment

A special processing method integrating the hot isostatic pressing and heat treatment was developed to produce TiAl alloy. As compared with the traditional two-step method, which consists of separate hot isostatic pressing and subsequent heat treatment in vacuum, the integrated approach will introduce an isostatic pressure throughout the whole process from the consolidation of powder to the heat treatment of coupon. Two types of microstructure, i.e., lamellar and duplex, have been generated by integrated and separate methods. Tensile test at room temperature indicates that the yield strength and tensile elongation of lamellar microstructure generated by integrated method are 650±30MPa and 2.1±0.2% respectively, which are improved simultaneously as compared with 550±28MPa and 0.6±0.1% for sample generated by separate approach. Moreover, such simultaneous enhancement of yield strength and ductility is also observed in duplex microstructure, where the yield strength and tensile elongation increase from 420±18MPa and 2.4±0.4% in samples with separate method, to 540±25MPa and 3±0.3% in those generated by integrated approach. Additionally, the microstructural examination also revealed the influence of microstructure to the mechanical performance. The results show that the simultaneous improvement of yield strength and tensile elongation is mainly attributed to the suppression of cracking, which is prone to happen during heat treatment without atmospheric pressure. Using the integrated method, the isostatic pressure could sustain the equilibrium pores during heat treatment, and provides an exterior force to balance the potential internal stresses due to phase transformation.

Physicochemical Characteristics of Nitrogen Sorption on High-Porous Powders of Graphene-Like Boron Nitride

The porous structure of graphene-like boron nitride t-BNg powders is qualified. The material is homogenous and micro-mesoporous, with pores being distributed in the range 3.53–4.12 nm corresponding to 55% volume and 60% specific surface area of mesopores. The specific surface area of micropores calculated with the Dubinin–Kadlets method (t-method) is 28.3 m2/g and the specific surface area of mesopores calculated with the Barrett–Joyner–Halenda method is 141 m2/g. Powders of graphene-like t-BNg synthesized from carbamide contain impurities of polycondensed flat particles of boron oxonitride. Impurities as a refined sublimate (white powder) are separated out of the washed and dried t-BNg powder by subsequent vacuum heat treatment at 540 K and pressure to ≤1.0 Pa. The sublimate is identified as a heterocyclic six-membered dimer with molecular formula H(OH)[(BON)2](OH)H.

Impact of processing conditions and feedstock characteristics on thermally sprayed MCrAlY bondcoat properties

One of the options to manufacture MCrAlY bondcoats (M=Co, Ni) for thermal barrier coating systems is High Velocity Oxy-Fuel spraying (HVOF). In this work, particle diagnostics were applied to investigate the impact of processing conditions and feedstock characteristics on the relevant bondcoat properties. The results showed that compromises must be made on the oxygen/fuel ratio, spray distance, and particle size distribution to strike a balance between low oxidation and dense microstructures.These limitations initiated the development of the High Velocity Atmospheric Plasma Spray process (HV-APS) as a further alternative process. In this work, HV-APS process parameters were developed for a three cathode torch in combination with a 5mm diameter high speed nozzle. A one-dimensional calculation of the expansion through this nozzle to atmospheric pressure yielded supersonic conditions with a Mach number of 1.84. The calculated plasma temperatures at the nozzle exit and in the expanded jet are 8400K and slightly above 5200K, respectively, which is low compared to conventional APS processes.A very fine powder with a median particle size of 18μm was identified to be most suitable. Although the spray conditions were relatively cold, reasonable deposition efficiencies up to 61% and rather dense coatings were achieved using this feedstock. The as-sprayed porosity was ≈2% which was reduced by the subsequent vacuum heat treatment to <1%. The oxygen content determined by chemical analysis for a sample sprayed at a spray distance of 100mm was 0.41±0.04wt%.Moreover, reference samples were manufactured by Low Pressure Plasma Spraying (LPPS). The oxidation behavior was compared in isothermal and cyclic oxidation tests. The oxidation rates of the HV-APS coatings were found to be significantly lower than those of LPPS coatings. The thermally grown oxide scale showed less yttrium incorporation and better adherence in case of HV-APS. The latter is suggested to be related to a unique new distribution of Y-rich nano-sized oxide precipitates. The cyclic oxidation test confirmed the better oxidation resistance of the HV-APS coatings.

Diffusion mechanisms and microstructure development in pack aluminizing of Ni-based alloys

Despite the large industrial use of pack aluminizing processes for the protection of parts (e.g. cooling channels) of gas turbine blades, systematic studies relating the formation mechanisms and the chemical composition of pack aluminized layers to important process parameters, including the nature of the base alloy and the temperature, are scarce. In this study, 4 different alloys (pure Ni, Ni–20Cr, Inconel 738 and directionally solidified CM247LC) were subjected to a pack aluminizing process at three different temperatures (950°C, 1000°C, 1040°C), using a pack mix containing a fluorine-based activator; the results were compared to those obtained with a chemical vapor deposition (CVD) aluminizing process performed at 1040°C with the same fluorine-based gaseous precursor.The microstructural characterization, performed by SEM+quantitative EDX analysis, XRD and nanoindentation testing, shows that, during the heating stage of the pack aluminizing process, Al is transported to the sample surface at temperatures too low to allow significant simultaneous diffusion of Ni; therefore, a δ-Ni2Al3 outer layer is formed by inward Al diffusion below the alloy surface, and its growth then continues during the isothermal stage as well. As a result, the chosen isothermal treatment temperature does not affect growth mechanisms, although it modifies the overall thickness of the aluminized layer. δ-Ni2Al3 is converted to β-NiAl after a subsequent vacuum heat treatment at 1120°C. In a CVD process, where gaseous precursor are introduced only after attaining the isothermal treatment stage, Al and Ni diffuse simultaneously from the very beginning of the aluminizing process and β-NiAl is directly developed.Less mobile species (heavy atoms, such as W) in the alloy composition hinder all diffusion phenomena, both during pack aluminizing and during subsequent vacuum heat treatment: after aluminizing, precipitates are developed within the δ-Ni2Al3 outer layer and, after vacuum heat treatment, the resulting β-NiAl layer exhibits a compositional gradient.

Effect of Grinding in Hydrogen and Vacuum Treatment on the Phase Composition of SmCo5 Alloy

It is shown that the phase composition of commercial ferromagnetic KS37 alloy depends on the conditions of its grinding in hydrogen and subsequent heat treatment in vacuum. Treatment parameters (hydrogen saturation pressure and grinding of the alloy, mill rotation frequency, grinding time, vacuum treatment temperature) that allow a wide variation in the phase composition of the alloy and in the quantitative ratio of the phases have been established. Depending on the above-mentioned conditions, several phase compositions involving SmCo5, Sm2Co17, Sm2Co7, and Sm5Co19 can be obtained.

A Novel Method for the Preparation of Nano-Sized Nickle Powders

Nano-sized nickel powders were prepared through a wet chemical reduction, of NiCl2 by sodium in liquid ammonia at -45 °C, and a subsequent heat-treatment in vacuum at 300 °C. The prepared product was systematically characterized by X-ray diffraction (XRD), scan electron microscopy (SEM), transmission electron microscopy (TEM), and BET specific surface area measurement. The results show that the product was composed of nano-sized nickel particles, with average particle diameter of about 20 nm, and specific surface area of about 30 m2g-1. The possible formation mechanism of the nano-sized nickel powder was also discussed briefly.

LuNi5Sn: A first representative of RNi5Sn stannides with CeCu5Au structure

Abstract Single crystals of LuNi 5 Sn were synthesized from the pure metals by arc-melting with subsequent heat treatment in vacuum. The crystal structure of the LuNi 5 Sn compound was determined and refined from single-crystal X-ray diffraction. It belongs to CaCu 5 family of intermetallics and is the first stannide with CeCu 5 Au structure type in R–Ni–Sn ternary systems (R is rare-earth element).

The corrosion–wear response of Cr–Ti coatings

Titanium alloys display problematic behaviour in environments where wear and corrosion take place either simultaneously or sequentially. While various surface treatments have been proposed for the alleviation of corrosion and wear of Ti-6Al-4V, there is still scope for new and novel approaches. In the work reported here, three coating compositions: Cr–13 at.%Ti, Cr–33 at.%Ti and Cr–48 at.%Ti, were synthesised by unbalanced magnetron sputter deposition and applied to Ti-6Al-4V substrates. In the as deposited state the coatings were single phased, while subsequent vacuum heat treatment at 800 °C resulted in the formation of the Laves phase (α-TiCr 2). Coated Ti-6Al-4V test pieces were subjected to reciprocation sliding corrosion–wear tests in 0.9% NaCl solution at 37 °C. All the coated variants tested showed a reduction in corrosion–wear compared to uncoated Ti-6Al-4V. The most effective coatings were those that contained the Laves phase (α-TiCr 2). It is proposed that the passive film formed on the Laves phase had greater mechanical integrity than any others formed during corrosion–wear testing and this phenomenon was a key factor in providing the observed enhanced surface durability.

Amorphous-to-quasicrystalline transformations in the Ti–Zr–Ni and Ti–Hf–Ni systems

We have synthesized Ti–Zr–Ni- and Ti–Hf–Ni-based quasicrystals by the mechanical alloying of elementary powders with an average particle size of 35 μm in a high-energy planetary ball mill followed by a vacuum heat treatment. The X-ray diffraction (XRD) spectra indicate that the mixture was to a large extent amorphous after mechanically alloying for more than 40 h. These largely amorphous precursors, after various times of mechanical alloying, were subsequently analysed by means of differential scanning calorimetry (DSC) in order to obtain the kinetic parameters of crystallization, such as the temperature and the activation energy. DSC was used in combination with XRD to identify the structural changes related to the peaks in the DSC scans. The morphologies and compositions were analysed using scanning electron microscopy with an energy-dispersive spectrometer. We observed agglomerates ranging in size from 5 to 50 μm; these agglomerates consisted of smaller particles with sub-micrometer diameters. The composition of the agglomerates was not homogenous, since small areas (∼1 μm) of Zr-rich secondary phase were observed inside them. Using a vibrating-sample magnetometer (VSM) we found that the saturation magnetization decreases with mechanical treatment, and is additionally decreased with the time and temperature of a subsequent vacuum heat treatment. The saturation magnetization ( M s) decreases fairly linearly, from the initial mixture of crystalline elements, which is ferromagnetic due to nickel, through to a largely amorphous phase, and then to the quasicrystalline phase, where the M s value is the lowest. As a result, measuring the magnetic properties allows us to probe the structural transformations in the material.

Effects of Fe-ion irradiation and annealing on the optical absorption in sapphire

We have studied the optical absorption in sapphire after Fe-ion irradiation and subsequent vacuum heat treatment. The absorption parameters and the nature of the transitions between localized states and allowed bands have been shown to be influenced by substitutional defects, intrinsic radiation-induced defects, and their complexes. We have evaluated the contributions of individual substitutional defects, their clusters, and complexes of substitutional defects with native vacancies and the effect of the iron oxides forming during annealing on the absorption in sapphire. The nature of the substitutional defect clusters and impurity-native defect complexes has been analyzed in detail.

Highly coercive SmCo5 magnets prepared by a modified hydrogenation-disproportionation-desorption-recombination process

The hydrogenation-disproportionation-desorption-recombination (HDDR) process was applied to SmCo5 using extreme conditions, namely high hydrogen pressures and reactive milling under hydrogen. Investigations on the hydrogen absorption behavior of SmCo5 by differential scanning calorimetry under hydrogen pressures between 1 and 7 MPa showed absorption events due to an interstitial absorption at about 100 °C and a disproportionation reaction at about 600 °C. X-ray diffraction showed the disproportionation of SmCo5 into Sm hydride and fcc-Co. A favorable effect of high hydrogen pressures on the disproportionation reaction was observed which can be explained by a decrease of the free enthalpy of the samarium hydride for increasing hydrogen pressures. Reactively milled SmCo5 showed also the products of the disproportionation reaction. The recombination to the original SmCo5 phase on hydrogen desorption in a subsequent heat treatment in vacuum was successful for both methods. However, Sm2O3, Sm2Co17, and Sm2Co7 were detected as minor phases. Maximum coercivities μ0JHC of 2.1 and 4.7 T were achieved for high pressure and reactively milled HDDR powders, respectively. The high coercivities originate from the high anisotropy field of the SmCo5 phase in combination with the grain refinement due to the HDDR treatment.

Effect of Ni–Al intermetallic sublayers on tribological properties of crystallised Ni–P coatings on aluminium substrates

The effect of Ni–Al intermetallic sublayers on the tribological properties of Ni–P coated aluminium substrates has been investigated. For this purpose an Ni–P coating was directly deposited on to an 1100 aluminium substrate and Al–Ni diffusion sublayers were produced by subsequent vacuum heat treatment. The diffusion layers were characterised by optical and scanning electron microscopy (SEM), X-ray diffrac tion, microanalysis, and tribological testing. The results show that after annealing for 10 h at 520°C, three layers are formed with a total thickness of 3–4 times that of the initial Ni layer thickness. The layers grown during diffusion annealing are the intermediate phases of the Ni–Al system where their formation is controlled by process kinetics. The outermost layer was identified as a crystallised Ni–P residue along with two distinct layers of Ni2 Al3 and NiAl3 intermetallic compounds formed beneath it. The results of wear testing indicate much improved wear resistance after diffusion treatment, believed to result from a smooth hardness gradient and the formation of intermetallic phases at the interface, which acts as a firm sub strate beneath the hard crystallised Ni–P surface layer. From SEM studies of the wear debris and the worn specimens it can be concluded that delamination is the predominant wear mechanism of the surface layer.

Microstructural changes in Nd&lt;SUB&gt;2&lt;/SUB&gt;Fe&lt;SUB&gt;14&lt;/SUB&gt;B permanent magnet alloys during HDDR phenomena

Microstructural changes in Nd-Fe-B alloys during the hydrogenation, disproportionation, desorption, and recombination (HDDR) phenomena were studied by XRD and TEM. These phenomena are caused by a heat treatment in hydrogen (H-treatment), in which the disproportionation occurs, and a subsequent heat treatment in vacuum (V-treatment), in which the recombination reaction occurs. The disproportionated mixture is formed of submicron sized colonies, which consist of NdH 2 rods embedded in an α-Fe matrix, at the first stage of the disproportionation reaction. Some relationship in crystallographic orientation was observed among the NdH 2 , α-Fe, and the undecomposed Nd 2 Fe 14 B phases. H-treatment at higher temperatures causes the grain growth of the disproportionated phases, but the relationship in crystallographic orientation remains between the NdH 2 and α-Fe phases. The recombined Nd 2 Fe 14 B grains in the alloys V-treated at temperatures higher than 800°C, exhibit the same texture as the original, untreated alloys, and this grain alignment appears to begin as the recombination reaction occurs. The alloys V-treated at temperatures lower than 800°C show isotropic characteristics.

Field emission from ion-milled diamond films on Si

Diamond grains were grown on Si substrates by plasma-enhanced chemical vapor deposition. Ar ion milling was applied to the diamond/Si structures. It has been found that sharp diamond cones can be formed by ion milling if diamonds are in the form of isolated grains. It has also been found that the field emission current from diamond/Si samples is drastically increased by Ar ion milling and subsequent heat treatment in vacuum.

Phase Changes and Magnetic Properties of Sm2Fe17Nx Alloys Heat-Treated in Hydrogen

This paper describes the use of hydrogen in the processing of Sm 2 Fe 17 N x powders with good coercivity. The hydrogen absorption and desorption behavior of the compound Sm 2 Fe 17 was studied. Green compacts of Sm 2 Fe 17 powders were heat-treated in hydrogen at 600 to 1000°C, and the latter decomposed into ?-Fe and samarium hydrides. Samples underwent dehydrogenation by subsequent heat treatment in vacuum, resulting in the reformation of the Sm 2 Fe 17 phase.

The effect of silicon and zirconium additions on the microstructure of vacuum-plasma-sprayed CoNiCrAlY coatings

Abstract To improve the oxide scale adherence to high temperature alloys, active elements such as yttrium are added in amounts of 0.5–1 wt.%. Recently, silicon and zirconium have proved to be successful as alternative active elements in CoNiCrAlY and NiCrAlY alloys. To understand the mechanisms behind this improvement and to compare various alloy systems, it is useful to investigate the effect of active elements on the microstructure. In this study, the effect of silicon and zirconium additions on the microstructure of the CoNiCrAlY system was investigated. The CoNiCrAlSiZrY coatings were prepared by vacuum plasma spraying in a full-scale production unit and subsequent vacuum heat treatment (at 1120 °C for 2 h and at 845 °C for 24 h). The microstructural features were revealed by scanning and transmission electron microscopy. The chemical composition of the secondary phases was determined by energy-dispersive X-ray analysis. In the as-deposited condition, the matrix structure of the CoNiCrAlSiZrY coatings is single-phase γ (f.c.c.). As a consequence of the silicon and zirconium additions the ternary silicide G phase is frequently present as a dispersion in the coating microstructure. It is proposed that the presence of silicides explains the observed improvements of oxide scale adherence. Other secondary phases are Cr23C6, Cr7C3 and Y2O3. Furthermore, both the secondary and the matrix phase increase in grain size with increasing substrate temperature and with heat treatment. During heat treatment the matrix structure transforms into a three-phase γ (f.c.c.) + γ′-Ni3Al (ordered f.c.c.) + α-Cr (b.c.c.) structure.