Stability of thermal-induced phase transformations in the severely deformed equiatomic Ni–Ti alloys

Nickel–Titanium (Ni–Ti) alloys of two different near-equiatomic chemical compositions (Ni-rich and Ti-rich) are subjected to severe plastic deformation by means of high pressure torsion (HPT) by higher rotation speed and larger total number of rotations. Further, the as-received and severely deformed specimens are subjected to heat treatments at 300 and 350 °C. Phase transformations of the specimens under different conditions are analyzed by employing differential scanning calorimetry and by X-ray diffraction. The results obtained show that in Ti-rich Ni–Ti alloy the sequence of phase transformations is found to be stable against heat treatments and independent of previous HPT process. Also, in Ni-rich Ni–Ti alloy, when it is subjected to HPT, the sequence of phase transformations found to remain unaltered. However, with or without HPT, after the heat treatments at 300 and 350 °C, the sequence of the phase transformation is found to be affected.     

Low temperature oxidation of copper alloys—AEM and AFM characterization

Oxidation kinetics and oxide structures of three polycrystalline copper grades at different temperatures were studied by analytical transmission electron microscopy (AEM) and atomic force microscopy (AFM). The copper samples were oxidized in air at 200 and 350 °C for 1–1,100 min. AEM and AFM studies indicated that alloying and increase of temperature, accelerated oxidation. At 200 °C local oxidation was observed in the unalloyed copper samples while a uniform oxide layer formed on the alloyed coppers. At 350 °C a uniform oxide layer formed on all copper samples. The oxide structure was nanocrystalline cubic Cu₂O after all oxidation treatments at 200 °C and after 5 min oxidation at 350 °C. After 25 and 100 min oxidation at 350 °C the crystal size of copper oxide had grown and the oxide structure was monoclinic CuO.     

Microstructural characterization of surface layers formed on a low alloy steel during erosion – oxidation in a fluidized bed

Low alloy steel specimens were subjected to erosion-oxidation in a simulated fluidized bed environment in the range 100–600° C. Transmission electron microscopy was carried out on the surfaces which experienced particle impacts, with specimens prepared by backthinning. At low temperatures (≤ 200° C) a dense, protective layer of bed particle fragments formed on the steel surface due to fragmentation of particle asperities and their subsequent comminution to a very fine size (< 10 nm). At intermediate temperatures thin oxide films developed on the wear scar surfaces; these were predominantly magnetite with a fine grain size. There was rapid material loss with particle impacts removing the oxide and some metal below, but the thin oxide rapidly regrew due to the fine grain size, absence of a haematite layer and the mechanical damage during particle impact. At high temperatures the oxide became sufficiently thick to be mechanically protective. Erosion occurred within the fine grained surface haematite layer, while there was grain growth in the lower magnetite layer. This revised version was published online in November 2006 with corrections to the Cover Date.     

Formation of a silicate layer between lead oxide and a silicon-wafer surface during heat treatment

PbO thin films were deposited on a silicon substrate by plasma-enhanced chemical vapour deposition (PECVD) using Pb(C₂H₅)₄ and oxygen at 250°C. The interdiffusion reaction phenomena between the PbO thin film and the silicon substrate during heat treatments were investigated in a horizontal furnace in the temperature range between 350 and 650°C under a nitrogen ambient for 1 h. The PECVD PbO film deposited on the silicon substrate at 250°C, was amorphous and contained carbon-related contaminants which could almost be removed by heat treatment at 350°C. The PbO on the silicon substrate initially participated in the interdiffusion reaction in the temperature range between 400 and 450°C. This produced a silicate layer containing lead components. The lead content in the film varied with the depth of film and heat-treatment temperature. Metallic lead was observed as a cluster in the specimen heated at 550°C. This cluster was produced by the agglomeration of metallic lead originating from PbO decomposition. The oxygen source for silicate formation was not ambient oxygen coming from the decomposition of Pb–O bonding. The metallic lead clusters dissolved as weakly bound metallic lead or as an unbound nanosized metal particle in the silicate layer at 650°C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of annealing on propagation in ion-implanted contiguous-disk bubble devices

Bubble propagation margins are found to be affected by heat treatment in ion-implanted contiguous-disk devices fabricated on liquid-phase epitaxial (LPE) grown double-layer garnet films which support 1-μm bubbles. When an optical reflector is deposited directly on the driving layer interfacial diffusion takes place, and this raises the coercivity of the implanted layer, which in turn causes a severe degradation of propagation margins. In samples fabricated with optical reflectors isolated from the driving layer, the adverse effects of annealing on propagation margins are stronger in close-packed minor loops due to interaction of long-range charged walls as compared with isolated loops and isolated disks, and they cease to propagate bubbles after annealing in the temperature range of 350-400°C, whereas the loss of margins in the isolated loops and disks are typically less than 30 percent after 600-650°C annealing treatments. Stress gradients caused by the discontinuity in the overlay patterns contribute significantly to the increase of threshold drive field after anealing. The rapid degradation of propagation margins found after annealing in the temperature range of 350-400°C in all samples is caused by reordering of the damaged lattice.     

Preparation of CdTe films by electrodeposition

Single-phase CdTe thin films have been prepared by depositing sequentially a layer of tellurium and a layer of cadmium on a molybdenum substrate followed by a short thermal treatment. Deposition of tellurium films was done in an aqueous solution containing TeO₂ at a current density of ≈ 1 mA/cm². An aqueous solution containing cadmium sulfate was used for cadmium deposition with a current density of ≈1 mA/cm². Solution temperature was ≈ 95°C for tellurium film deposition and was 50°C for cadmium deposition. It was found that after a heat treatment at ≈ 370°C for 10 min the deposited Te/Cd layers were converted to CdTe thin films with a cubic structure. Compositional uniformity of the films was also investigated by electron probe microanalysis.     

Texture effect on serrated flow in 2090 Al‐Li alloy

Tensile specimens of 1×6×25 mm in gauge dimension were cut from the surface and centre of 12.7 mm thick 2090 Al‐Li alloy plate, which were solution treated at 550 °C for 30 min, peak‐aged at 190 °C for 18 h, or reversion‐treated at 275 °C for 2 min. The flow stress of the centre layer was higher than that of the surface layer, regardless of the heat treatments. The textures of the surface and centre layers were approximated by the {001}<110> and {011}<211> orientations, respectively. The solution‐treated specimens gave rise to extensive serrations in their flow curves at a strain rate of 2×10^‐4 s^‐1. The serration amplitude was drastically reduced after the specimens were peak‐aged or reversion‐treated. However, for the peak‐aged alloy, the surface‐layer specimens underwent complex, serrated flows, whereas the flow curve of the centre‐layer specimen was almost devoid of serration. The serration, especially fine‐type serration in the peak‐aged and reversion‐treated specimens tended to disappear with increasing strain rate. The tensile behavior was explained in terms of the texture and strain rate.     

Hardness, elastic modulus and structure of indium nitride thin films on AlN-nucleated sapphire substrates

The structural properties of epitaxial indium nitride thin films were characterized and related to the mechanical properties as measured by nanoindentation. The seven epitaxial InN films examined were deposited over the temperature range 200–400°C by reactive magnetron sputtering on (0 0.1) sapphire substrates buffered with a thin AlN layer. The hardness, surface texture and crystal quality of the InN films were functions of the deposition temperature, with the maximum hardness (11.2 GPa), the smoothest surface, the minimum c-lattice constant (0.5708 nm), and the minimum (0 0.4) X-ray rocking curve width (0.6°) all occurring at the deposition temperature of 350°C. The crystal quality and the hardness of the InN films degraded at both higher and lower temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of heat treatments on apatite-forming ability of NaOH- and HCl-treated titanium metal

Titanium (Ti) metal was soaked in HCl solution after NaOH treatment and then subjected to heat treatments at different temperatures. Their apatite-forming abilities in a simulated body fluid (SBF) were discussed in terms of their surface structures and properties. The nanometer scale roughness formed on Ti metal after NaOH treatment remained after the HCl treatment and a subsequent heat treatment below 700°C. Hydrogen titanate was formed on Ti metal from an HCl treatment after NaOH treatment, and this was converted into titanium oxide of anatase and rutile phases by a subsequent heat treatment above 500°C. The scratch resistance of the surface layer increased with the formation of the titanium oxide after a heat treatment up to 700°C, and then decreased with increasing temperature. The Ti metal with a titanium oxide layer formed on its surface showed a high apatite-forming ability in SBF when the heat treatment temperature was in the range 500–700°C. The high apatite-forming ability was attributed to the positive surface charge in an SBF. These positive surface charges were ascribed to the presence of chloride ions, which were adsorbed on the surfaces and dissociated in the SBF to give an acid environment.     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Relationship between rupture stress and microstructure parameters of plastically deformed heat-resistant steel

The paper addresses the influence of plastic prestraining in tension and combined tension at 150 and 350°C on the rupture stress in 15Kh2MFA steel upon heat treatment that simulates irradiation embrittlement of materials in a WWER-440 type reactor towards the end of its lifetime. The dependence of rupture stress on the dislocation density in the material upon plastic prestraining is studied. It is found out that as the plastic prestraining in tension and combined tension grows, the dislocation density within small-angle boundaries increases and thus results in a larger rupture stress. An increase in rupture stress in the case of prestraining at 350°C is more intensive than that at 150°C. We analyze the microcrack nucleation and growth micromechanisms in 15Kh2MFA steel during tensile plastic deformation and discuss the effect of the material substructure on the microcrack arrest. __________ Translated from Problemy Prochnosti, No. 4, pp. 19–30, July–August, 2007.     

Apatite-forming ability of Ti–15Zr–4Nb–4Ta alloy induced by calcium solution treatment

Ti–15Zr–4Nb–4Ta alloy free from cytotoxic elements shows high mechanical strength and high corrosion resistance. However, simple NaOH and heat treatments cannot induce its ability to form apatite in the body environment. In the present study, this alloy was found to exhibit high apatite-forming ability when it was treated with NaOH and CaCl₂ solutions, and then subjected to heat and hot water treatments to form calcium titanate, rutile, and anatase on its surface. Its high apatite-forming ability was maintained even in 95% relative humidity at 80°C after 1 week. The surface layer of the treated alloy had scratch resistance high enough for handling hard surgical devices. Thus, the treated alloy is believed to be useful for orthopedic and dental implants.     

Investigation of the drying temperature dependence of the orientation in sol–gel processed PZT thin films

The crystal orientations of lead zirconate titanate (PZT) thin films have been investigated by using various drying temperatures in the sol–gel process. The films were dried at different temperatures between 310 and 350°C for pyrolysis and then were heat treated at 650°C using rapid thermal annealing (RTA). TG/DTA and FTIR spectroscopy were used to detect the remnants of organic materials in the thin films prior to the final heat treatment. In order to examine the relationship between the film orientation and the remaining organic materials for the prior and final heat treatment, the films were fabricated with different coating cycles and dried for different holding times and then annealed at 650°C. The preferred orientations were investigated using X-ray diffraction. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of sintering temperature and particle size on the translucency of zirconium dioxide dental ceramic

The aim of this study was to evaluate the effects of sintering temperature and particle size on the translucency of yttrium stabilized tetragonal zirconia polycrystals (Y-TZP) dental ceramic. Eighty disc-shaped and cylindrical specimens were fabricated from zirconia powers of particle size 40 and 90 nm. These specimens were sintered densely at the final sintering temperature 1350, 1400, 1450 and 1500°C, respectively. The visible light transmittance, sintered density and microstructure of the sintered block were examined. The results showed that the sintered densities and transmittances increased with the temperature from 1,350 to 1,500°C. Y-TZP could gain nearly full density and about 17–18% transmittance at the final sintering temperature of 1,450–1,500°C. The 40-nm powders had higher sintered density and transmittance than the 90-nm. The translucency of Y-TZP dental ceramic could be improved by controlling the final sintering temperature and primary particle size.     

On the development of two characteristically different crystal morphology in SiO2–MgO–Al2O3–K2O–B2O3–F glass-ceramic system

The present work demonstrates how crystals with two different characteristic morphologies can be formed in SiO₂–MgO–Al₂O₃–K₂O–B₂O₃–F glass-ceramic system by adopting two sets of heat treatment experiments. In our study, single stage heat treatment experiments were performed at 1,000°C for varying holding time of 8–24 h with 4 h time interval and as a function of temperature in the range of 1,000–1,120°C with 40°C temperature interval. The constant heating rate of 10°C/min was employed for both sets of experiments. The microstructural changes were investigated using Fourier transformed infrared spectroscopy (FT-IR), SEM-EDS and XRD. For temperature variation batches, the microstructure is characterized by interlocked, randomly oriented mica plates (‘house-of-cards’ morphology). An important and new observation of complex crystal morphology is made in the samples heat treated at 1,000°C for varying holding times. Such morphology appears to be the results of composite spherulitic-dendritic like growth of mica rods radiating from a central nucleus. The possible mechanism for such characteristic crystal growth morphology is discussed with reference to a nucleation-growth kinetics based model. The activation energy for crystal nucleation and Avrami index are computed to be 388 kJ/mol and 1.3 respectively, assuming Johnson–Mehl–Avrami model of crystallization. Another important result is that a maximum of around 70% of spherulitic-dendritic like crystal morphology can be obtained after heat treatment at 1,000°C for 24 h, while a lower amount (~58%) of interlocked plate like mica crystals is formed after heat treatment at 1,040°C for 4 h.     

Low-temperature crystallization of oriented ZnO film using seed layers prepared by sol–gel method

Transparent zinc oxide (ZnO) films were coated on seed layers prepared by the sol–gel method by chemical solution deposition method. Firstly, seed layers were prepared from zinc acetate and monoethanolamine, 2-methoxyethanol by the sol–gel method on a silicon substrate or a slide glass. Next, the substrate coated with a seed layer was immersed in zinc nitride solution with hexamethylenetetramine, and ZnO films were obtained. The transmittance of the ZnO films depended on the morphology and crystallinity of the seed layers. When the seed layer were dried on a hot plate, the seed layer had flat surface and transparent ZnO film could be obtained on the seed layers dried at temperatures above 200 °C. When the seed layer was prepared from zinc acetate dihydrate dried in a petri dish, the seed layer were smooth without cracks and the transparent ZnO films were obtained at temperature below 100 °C.     

Replacement of conventionally complex processing for low alloy steel 20Cr2Ni4A by rare earth carburising at lower temperature

Abstract

This paper deals with the effects of rare earth (RE) addition on the microstructure and properties of 20Cr2Ni4A steel carburised at 860°C. Compared with conventional 920°C carburising and other complex treatments, finer and more dispersed carbide precipitates formed in the surface layers. A superfine martensitic structure and a less residual austenite were obtained by direct quenching after RE 860°C carburising. The microstructure exhibited a higher microhardness, a wear resistant surface layer, and it also led to an increase in impact toughness. In addition, the heat treatment period for this steel was remarkably reduced by the replacement of conventional complex treatments by RE 860°C carburising.     

In vitro bioactivity of sol–gel-derived hydroxyapatite particulate nanofiber modified titanium

A chemically-etched titanium surface was modified by electrospinning a sol–gel-derived hydroxyapatite (HAp) that was subjected to calcination within the temperature range of 200–1400°C in the normative atmospheric condition. After heat treatment, crystal structures of the filmed titanium oxide and HAp on the titanium’s surface were identified using wide-angle X-ray diffraction. A highly porous layer of HAp was found to have formed on the oxidized titanium surfaces. The surfaces of three different samples; (1) electrospun HAp, (2) HAp calcined at 600°C, and (3) HAp calcined at 800°C, were investigated for their ability to foster promotion, proliferation, and differentiation of human osteoblasts (HOB) (in the 9th passage) in vitro up to 6 days. Among the three samples, cells cultured on the HAp calcined at 800°C titanium surfaces displayed the best results with regard to adhesion, growth, and proliferation of HOB. This novel method for fabrication of titanium substrates would provide a promising improvement for titanium-based medical devices over the current standards, which lack such substrates. These titanium substrates explicitly provide enhanced HOB proliferation in terms of both desired surface properties and their produced bulk quantity.     

Improvement of crystal quality and UV transparence of dielectric Ga2O3 thin films via thermal annealing in N2 atmosphere

Ga₂O₃ thin films were deposited on sapphire (0001) substrates by low-pressure metal organic chemical vapor deposition. The influence of annealing in N₂ atmosphere at the temperature in the range of 800–1,000 °C was investigated by X-ray diffraction and optical transmittance spectra. With an increase of annealing temperature from 800 to 950 °C, the transformation from the initial amorphous film to polycrystalline β-Ga₂O₃ thin film was observed, and the transmittance was also improved remarkably. The optical band gap energy of the sample annealed at 950 °C was evaluated as ~5 eV. Whereas, after an annealing at 1,000 °C, the crystal quality became worse and the transmittance degraded. The mechanism of annealing in N₂ atmosphere was discussed in view of phase transition.     

Untersuchungen zum Pulveraluminieren von niedriglegiertem Stahl (13 CrMo 44)

Investigations of Pack Aluminizing of Low-Alloyed Steel (0,13% C, 1% Cr, 0,5% Mo) A diffusion layer after pack-aluminizing of low alloyed steel can be described by the Al-concentration in the surface of the sample, the layer thickness and the surface structure. It is reported, how these characteristic values depend on the composition of the powder mixtures, coating temperature and coating time. After 16 weeks heat treatment at 608 °C and 707 °C, pack-aluminized samples didn't show an increase of layer thickness or decrease of Al-concentration in the sample surface. After the heat treatments a new phase occurs in the layer, which has the composition Fe₃Al.