PHASE AND MORPHOLOGY OF PbNb2O6 OBTAINED BY MOLTEN SALT SYNTHESIS

The morphologic development of lead metanlobate powder particles in KC1 is examined with special emphases on the heating- conditions and starting compositions. Potassium ions are substituted in the Pb-x K_2xNb₂0₆(PKN) With the Sthorhoobic modifications.The.morophology of particles changed during PKN formation and particle growth. During the formation process of PKN, equiaxed and rod-shaped particles were obtained both above and below the melting temperature of KC1. The rod-shaped particles have a columnar structure but decrease in number with Increasing potassium content in the starting mixtures. During the growth process, equiaxed particles grew to simple rod-shaped particles without a columnar structure. Potassium in the starting oxide was found to promote the formation of simple rod-shaped particles. PKN powder composed of only simple rod-shaped particles is obtainable.     

Effect of Yttria content and alumina addition on the formation of a textured microstructure during the surface nitridation of Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP)

The condition for the production of a textured microstructure was investigated in the process of nitridation of yttria-stabilized zirconia. The nitrided surface layer was composed of either columnar or equiaxed cubic grains. The amount of the cubic phase in the matrix, which was determined by yttria content and the sintering temperature, was the principal parameter that affected the morphology of the nitrided layer. With the increase in the cubic phase in the matrix, the chances of finding columnar grains decreased. The driving force of growth, which was provided by the free energy difference determined by the phase and grain size, decreased with the increase in the cubic phase in the matrix, and eventually led to the suppression of the growth of columnar grains. The cubic zirconia in the matrix also played the role of a transient pinning center due to its chemistry and lattice parameter that were dissimilar to those of the columnar grain. Such role of the cubic phase could be mimicked by the introduction of Al₂O₃ particles as an intended pinning center. With the introduction of the pinning center, the growth rate of the texture was suppressed and equiaxed grains appeared at the front of the texture. Therefore, by preparing the matrix with the utmost tetragonal phase, successful texturing of zirconia during nitridation could be achieved.     

Reaction products and their solidification process of the plasma sprayed Fe2O3–Al composite powders

Fe₂O₃–Al composite powders were deposited onto steel substrate by plasma spraying. The reaction products of the Fe₂O₃–Al composite powders in the plasma flame and their solidification process were investigated. The results showed that the reaction products of the Fe₂O₃–Al composite powders in the plasma flame were Fe–Al–O ceramic melt and Fe melt. Fe was not always formed in the reaction products of each composite particle, and the formation of Fe was dependent on the composition distribution and the reaction kinetics process of each composite particle. The composition inhomogeneity, discontinuity and porousness of the composite particles resulted in the difference of the reaction kinetics of each composite particle and the composition difference of the droplets. Hercynite solid solution, Fe, Al₂O₃ and FeAl phases were formed by non-equilibrium solidification of the Fe–Al–O ceramic melt. For the smaller size droplet, when it spread on the substrate, the spreading droplet was quickly chilled to form equiaxed grains with size of 100–200 nm. Nano-sized or submicron equiaxed grains, nano-sized columnar grains and cellular structure were formed in the larger spreading droplet. Fe rich hercynite solid solution nucleated and grew preferentially and then Al rich hercynite solid solution grew, which form composition segregation in the columnar grains. The main phases in the composite coating were hercynite solid solution.     

Computer simulation of grain growth and macrostructure development during solidification

Abstract

A Monte Carlo computer simulation technique, previously applied to the simulation of a number of solid state processes involving microstructural evolution, has been employed to simulate and represent pictorially grain growth and grain interactions during solidification. By careful control of the number, location, and time of origin of grain nuclei it has been possible to simulate a wide range of features of grain structure in castings. Also, in support of an existing statistical argument, it has been demonstrated that the columnar to equiaxed transition occurs when the volume fraction of equiaxed grains ahead of the columnar interface is 0·50.

MST/912     

Oxide microstructure and deuterium ingress in Zr-2.5Nb CANDU® pressure tubes

Abstract

The microstructures of oxides grown on the inside surface (water-side) of Zr–2.5Nb pressure tubes removed from CANDU® reactors were characterised by TEM and correlated with deuterium ingress. Oxide cross-sections consisted of two structurally distinct regions: a columnar oxide region next to the metal/oxide interface, and an outer coarse equiaxed oxide region. Near the metal/oxide interface, the microstructure consisted of columnar grains without overt porosity. Away from the interface, the oxide consisted of coarsened equiaxed grains with (100)_m twins, grain-boundary cracks and nanopores. The oxide microstructures on various pressure tubes differ in the proportion of equiaxed grains. Electron micrographs suggest that a larger proportion of equiaxed grains is associated with higher deuterium uptake. The predominance of grain-boundary cracks in equiaxed oxides indicates that they are likely more permeable to water than columnar oxides.

Energy dispersive X-ray analyses revealed substantial amounts of Fe–K_α (and Mn–K_α) in the equiaxed oxide grains at the outermost surface. Energy Dispersive X-ray mapping of Fe–K_α and detection of the Mn-K_α (produced by neutron activation of ⁵⁴Fe and subsequent decay of ⁵⁵Fe) in the absence of external excitation, unequivocally demonstrated that the iron had accumulated in the oxide during reactor operation. The Fe concentration was highest near the outermost region, and decreased inwards towards the metal/oxide interface. These results are consistent with permeable equiaxed oxides picking up considerable amounts of Fe at the outermost region from the heavy water coolant.     

Effect of high energy ball milling on solid state reactions in Al–25 at.-%Ni powders

Abstract

The effect of high energy ball milling on the solid state reactions between aluminium and nickel in Al–25 at.-%Ni powders has been investigated using scanning electron microscopy, thermal analysis techniques, and X-ray diffractometry. It has been observed that the microstructure of the powder particles evolves in three stages: stage I is the formation of entrapped nickel particles in the aluminium matrix structure; stage II is the formation of an Al–Ni multilayered structure; and stage III is the formation of Al₃Ni single phase. The temperature required to activate the reaction between aluminium and nickel during heating decreases by more than 200 K as the powder particle microstructure evolves from the entrapped particle structure to the multilayered structure, and then it decreases gradually with decreasing nickel layer thickness. The nucleation and lateral growth of Al₃Ni phase at the Al/Ni interfaces occurs at much lower temperatures than those required for the transverse growth of Al₃Ni. The fraction of Al₃Ni formed through nucleation and lateral growth at the interface is almost linearly proportional to the interfacial area. The activation energy for nucleation and lateral growth of Al₃Ni at the Al/Ni interfaces is independent of nickel layer thickness, but the activation energy for transverse growth of Al₃Ni decreases substantially with decreasing nickel layer thickness. The latter is attributed to the observation that the nickel layers are thinned by plastic deformation and thus contain an increasingly higher density of dislocations.     

Preparation of rod-shaped BaTiO3 powder

Rod-shaped BaTiO₃ powder particles have been prepared from rod-shaped TiO₂ ·nH₂O and BaCO₃ in molten chloride. The morphology of BaTiO₃ particles was studied referring to the effects of the chemical species of the starting titanium compound, amount of chloride, particle size of the titanium compound and reaction conditions, and the preparation condition of rod-shaped BaTiO₃ has been determined: i.e., large TiO₂ ·nH₂O particles were heated at 700°C in molten salt with an equal amount of BaTiO₃. This condition was effective in suppressing the formation of BaTiO₃ by a solution-precipitation process as well as the deformation of either TiO₂ ·nH₂O or BaTiO₃, which are responsible for the formation of equiaxed BaTiO₃ particles. The obtained rod-shaped BaTiO₃ particles had a cubic symmetry. Electron diffraction analysis showed that the following topotactic relation is retained; 0 1 0_{potassium tetratitanate} 0 1 0_{hydrated titania} 1 0 0_anatase 1 0 0_{barium titanate}     

Mechanism of transition from columnar to equiaxed zone in ingots

Abstract

A new theory for the transition from columnar to equiaxed crystals has been developed. It is proposed that free crystals are formed in the liquid ahead of the growing front and that these crystals grow in the liquid as a result of natural convection. When the free crystals are sufficiently large or sufficiently numerous they physically block the growth of the columnar crystals by adhering to the solidification front. The growth rate of the free crystals is determined by the cooling rate of the mould and the kinetics of the solidification process. The theory indicates that the higher the cooling rate and the slower the solidification process the easier the transition. The calculations are compared with experimental observations on a large steel ingot.

MST/272     

Decomposition of deformed ferrite in a duplex stainless steel

Abstract

The decomposition of deformed ferrite in a duplex stainless steel (U44L) has been examined using transmission electron microscopy. During annealing, austenite precipitation was extremely rapid and pinned the developing ferritic subgrain structure. Hence, the ferrite recrystallized via a continuous mechanism. Deformation of the ferrite, and to a lesser extent the austenite during growth/coarsening of the austenite, led to a ‘secondary recovery’ process. Subgrain coalescence, within the ‘primary’ ferrite subgrain structure and in the austenite particles, was observed after long term anneals: this process took place by the dissolution of the sub-boundaries formed during the secondary recovery process. In addition to the formation of austenite, M₂₃C₆, σ, and martensite were also observed.

MST/311     

Structure refinement of pure Mg under pulsed magnetic field

Abstract

A pulsed magnetic field (PMF) was introduced into the solidification of pure Mg. Fine uniform equiaxed grains are acquired in the whole ingot from the PMF treatment, in contrast with the coarse columnar grains observed in conventional casting, and the average grain size is refined to 260 μm with a 200 V PMF treatment. Pulsed magnetic field increases melt convection during solidification, and the violent agitation causes warmer liquid to fracture the tip of columnar dendrites or to break off dendrite branches to promote the formation of an equiaxed structure, with the broken pieces transported into the bulk liquid acting as nuclei. In addition, the uniform temperature field resulting from the stirring increases the likelihood of nuclei survival. The Joule heat effect also participates in the structure refinement. The pure Mg produced with a 200 V PMF treatment exhibits improved mechanical properties, such as the ultimate compressive strength (227 MPa) and fracture strain (33·2%).     

Effect of substrate preset temperature on crystal growth and microstructure formation in laser powder deposition of single-crystal superalloy

A successful repair of single-crystal components needs to avoid the stray grain formation and achieves continuous epitaxial growth of columnar dendrites in the repaired zone. In this study, the effect of substrate preset temperature on crystal growth and microstructure formation in laser powder deposition of single-crystal superalloy was studied through an improved mathematical model and corresponding experimental approaches. The results indicated that the variation of substrate preset temperature between ?30?°C and +210?°C changes the molten pool morphology little, but obviously affects the columnar-to-equiaxed transition conditions. The preheating of substrate facilitates the stray grain formation and enlarges the primary columnar dendrite arm spacing, while the situation for precooling of substrate is opposite. Under the specific processing conditions, the critical condition for continuous epitaxial growth is that the substrate preset temperature T_sub?≤?+90?°C. When the substrate preset temperature T_sub is below +90?°C, the height ratio of melting depth to total height of the molten pool is larger than that of stray grain, ensuring that stray grains can be completely remelted and the continuous columnar dendrites during the multi-layer laser powder deposition process on (001) surface of single-crystal substrate can be achieved.     

Aluminium and Al–4·5Cu alloy end chill: structural observation and heat flow analysis

Abstract

End chill experiments were performed on aluminium and Al–4·5Cu (wt-%) in order to study the effect of melt superheat (20–150 K), chill material (copper, iron, or sand), and specimen length (890–230 mm) on the type and size of macrostructure. Increasing melt superheat increases the length of columnar zone, which is shorter for the alloy than for the commercial purity metal. The columnar fraction increases with the thermal conductivity of the chill material and the heat transfer coefficient. The results are correlated with the temperature gradient, solidification rate, and growth rate obtained from a heat flow model. The columnar to equiaxed transition is found to occur at a critical temperature gradient and growth rate. These critical values differ with alloy composition. The grain size of columnar and equiaxed grains is found to follow a power relationship with solidification rate.

MST/1709     

Nanocoating of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on ZrO<Subscript>2</Subscript> powder and its effect on the sintering behaviour in ZrO<Subscript>2</Subscript> ceramic

ZrO₂ powder was coated with Al₂O₃ precursor generated by a polymeric precursor method in aqueous solution. The system of nanocoated particles formed a core shell-like structure in which the particle is the core and the nanocoating (additive) is the shell. A new approach is reported in order to control the superficial mass transport and the exaggerated grain growth during the sintering of zirconia powder. Transmission electron microscopy (TEM) observations clearly showed the formation of an alumina layer on the surface of the zirconia particles. This layer modifies the sintering process and retards the maximum shrinkage temperature of the pure zirconia.     

Development of gradient microstructure in the lattice structure of AlSi10Mg alloy fabricated by selective laser melting

To identify the microstructural features of the lattice structures of Al alloys built via the selective laser melting(SLM)process,AlSil OMg alloy with a body-centered cubic(BCC)-type lattice structure was prepared.Characteristic microstructures comprising melt pools with several columnarα-Al phases with<001>orientations along the elongation direction and surrounded by eutectic Si particles were observed at all portions of the built lattice structure.In the node portions of the lattice structure,a gradient microstructure(continuous change in microstructure)was observed.The columnarα-Al phases were observed near the top surface of the node portion,whereas they became coarser and more equiaxed near the bottom surface,resulting in softening localized near the bottom surface.In the strut portions of the lattice structure,the columnarα-Al phases were elongated along the inclined direction of struts.This trend was more prevalent near the bottom surface.Theα-Al phases became coarser and more equiaxed near the bottom surface as well.The aforementioned results were the basis of a discussion of the development of the gradient microstructure in lattice-structured Al alloys during the SLM process in terms of thermal conductivities at the boundaries between the manufactured(locally melted and rapidly solidified)portions and adjacent(unmelted)alloy powder.     

Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings

The microstructural features of high-temperature sintered and CaO-MgO-Al₂O₃-SiO₂ (CMAS) corroded air plasma sprayed Y₂O₃ stabilized ZrO₂ (YSZ) thermal barrier coatings (TBCs) under the thermal gradient condition were comparatively studied. As-sprayed YSZ has a lamellar structure and the lamellae are composed of closely aligned columnar crystals. The sintered and the CMAS corroded YSZ coatings maintain the t’-ZrO2 phase as the as-sprayed YSZ coating. The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grains coarsening. After the CMAS corrosion, the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of striped Zr₂Y₂O₇. The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating along with significant infiltration of amorphous CMAS and anorthite formation in the bottom layer owing to the high contents of Ca and Al.     

Morphological and frictional behavior of sputtered MoS2 films

From the texture and growth patterns of sputtered MoS₂ films deposited onto substrates, three regions can be distinguised: (1) a ridge formation region, (2) an equiaxed transition zone and (3) a columnar-fiber-like structure. The lubricating properties of sputtered MoS₂ films can be visually identified with respect to optical changes before and after rubbing. The orientation of the surface microcrystallites is identified, and the change in optical properties is explained. In sliding contact the sputtered film tends to break up at the base of the columnar region. Effective lubrication occurs with the film remaining on the substrate. This film is 0.18–0.22 microm thick.     

Effect of calcium carbonate particle size on formation and morphology of calcium hexaboride powder synthesized from condensed boric acid-poly(vinyl alcohol) product

Calcium hexaboride (CaB₆) powder was synthesized by carbothermal reduction via the transient boron carbide (B₄C) formation starting from a condensed boric acid (H₃BO₃)-poly(vinyl alcohol) (PVA) product. The effect of the size of calcium carbonate (CaCO₃) particles on the formation behavior of CaB₆ and the obtained particle morphology was investigated in this study. CaB₆ powder was prepared using CaCO₃ powders with microsize or nanosize particles. The CaB₆ formation reaction was accelerated at lower temperature and shorter heat treatment time when using nanosize CaCO₃ particles. The complete formation of CaB₆ was achieved at 1400?°C for 3?h in an Ar flow. Furthermore, CaB₆ powder with nanosize particles was obtained. The precursor powder obtained using nanosize CaCO₃ particles transiently formed fine B₄C and calcium borate particles, which are reactive species of CaB₆, leading to the facile formation of fine CaB₆ particles.     

Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.     

Synthesis of nano (Ti,W)C powder with preferred orientation and twin boundary structure

(Ti,W)C is a novel additive for high performance cermets. In this study, (Ti_0.88W_0.12)C with the lowest formation energy is synthesized by carbothermal reduction-carbonization in Ar. The starting materials included WO_2.72 with one-dimensional nanostructure, TiO₂ and carbon black. The phase transition temperatures were established by thermal analysis. XRD analysis results disclose that once TiC is formed at a temperature over 1220 °C, W atoms begin to diffuse into the TiC lattices, which is independent of the existing form of tungsten. At a condition of 1500 °C for 180 min, W and C atoms from the decomposed W₂C and WC are fully dissolved in the TiC lattices. Under such a TiC-centered atomic reconfiguration environment, the as-synthesized powder is featured with a BET particle size of 76 nm and texture coefficients TC₍₁₁₁₎ of 1.53 and TC₍₂₀₀₎ of 1.33. Results from SEM and HRTEM reveal that the roughly equiaxed powder particles have characteristics of readily identified twin boundary structure and stacking faults. Microscopic inhomogeneity of W solution atoms is discussed. The revelation of the easily identified twin boundary structure and stacking faults is of great significance to the hard phase regulation for high performance Ti(C,N)-based cermets.     

High ductilities in physically vapor-deposited nickel

Two thick films of physically vapor-deposited Ni were prepared on either side of the Movchan-Demchishin T₁ transition temperature. Both deposits were fine grained and columnar, but the lower temperature deposit contained a fine dispersion of pores. During high temperature deformation, the fully dense deposit behaved erratically owing to abnormal grain growth. The porous deposit transformed to a microstructure consisting of fine equiaxed grains interspersed with voids. This transformed structure deforms by grain boundary sliding. The measured activation energy is 116 kJ mol^?1 corresponding to grain boundary self-diffusion. The creep stress exponent is found to be 3, and the grain size exponent is -1. This porous physically vapor-deposited Ni attains steady state elongations that are far superior to those of conventionally processed or larger-grained Ni.