A study on the microstructure and phase transformation of electroless nickel deposits

Electroless Ni-7.4 to 10% P deposits obtained from acidic nickel sulphate baths with sodium hypophosphite as a reducing agent were analysed by transmission electron microscopy, X-ray diffraction and thermal analysis. The deposits could be represented better by a microcrystalline structure composed of 4 to 5 nm fcc Ni-P solid-solution grains rather than an amorphous structure. The deposits also had the (111) texture, which persisted in nickel grains even after phase separation of nickel and Ni₃P by heating in the case of the low nickel content, whereas the texture approached the random orientation with increasing phosphorus content. The phase transformation temperature was independent of the phosphorus content.     

Phase transformation behaviour of a low carat gold alloy for porcelain bonding during continuous heating

The phase transformation associated with hardening in a low carat gold alloy for porcelain bonding during continuous heating was studied by resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopies. A short-range order structure, based on Au-Pd, was produced at a lower temperature. At an elevated temperature, two types of precipitation were observed. One is a nodular reaction consisting of an L1₂ ordered structure based on Pd₃Sn and a disordered FCC structure at grain boundary. The other is a formation of spherical L1₂ ordered precipitates based on Pd₃Sn within the grain followed by Ostwald ripening. A fine dispersed precipitate of the latter contributed to the remarkable hardening.     

Characterization of phase transformation behaviour and microstructural development of electroless Ni-B coating

Phase transformation behaviour of amorphous electroless Ni-B coating with a targeted composition of Ni-6 wt% B is characterized in conjunction with microstructural development and hardness. Microscopic observations of the as-deposited coating display a novel microstructure which is already phase separated at multiple length scales. Spherical colonies of ∼5 μm consist of 2-3 μm nodular regions which are surrounded by ∼2-3 μm region that contains fine bands ranging from 10 to 70 nm in width. The appearance of three crystalline phases in this binary system at different stages of heat treatment and the concomitant variation in hardness are shown to arise from nanoscale fluctuations in the as-deposited boron content from 4 to 8 wt%. High temperature annealing reveals continuous crystallization up to 430 °C, overlapping with the domain of B loss due to diffusion into the substrate. The implications of such a microstructure for optimal heat treatment procedures are discussed.     

Structure and phase transformation behaviour of electroless Ni–P composite coatings

This paper addresses the structural characteristics and phase transformation behaviour of plain electroless Ni–P coating and electroless Ni–P–Si₃N₄, Ni–P–CeO₂ and Ni–P–TiO₂ composite coatings. The X-ray diffraction patterns of electroless Ni–P–Si₃N₄, Ni–P–CeO₂ and Ni–P–TiO₂ composite coatings are very similar to that of plain electroless Ni–P coating, both in as plated and heat-treated conditions. Selected area electron diffraction (SAED) patterns obtained on the Ni–P matrix of Ni–P–Si₃N₄, Ni–P–CeO₂ and Ni–P–TiO₂ composite coatings exhibit diffuse ring patterns resembling the one obtained for plain electroless Ni–P coating. Phase transformation behaviour studied by differential scanning calorimetry (DSC) indicates that the variation in crystallization temperature and the energy evolved during crystallization of plain electroless Ni–P coating and electroless Ni–P–Si₃N₄, Ni–P–CeO₂ and Ni–P–TiO₂ composite coatings is not significant. The study concludes that incorporation of Si₃N₄, CeO₂ and TiO₂ particles in the Ni–P matrix does not have any influence on the structure and phase transformation behaviour of electroless Ni–P coatings.     

Structural and phase transformation behaviour of electroless Ni-W-Cr-P alloy coatings on stainless steel

Ni-W-Cr-P alloy coatings were prepared by electroless deposition on stainless steel. The effects of heat-treatment on the structure and phase transformation behavior, microhardness of the Ni-W-Cr-P alloy coatings were mainly investigated. XRD analysis shows that as-deposited Ni-W-Cr-P coatings were microcrystalline; the precipitation of Ni₃P was observed at the annealing temperature of 400°C; after heat-treatment at 500°C the crystallization of Ni₃P and Ni was near completion; at 600°C new phase Cr_1.12Ni_2.88 was observed and Ni₃P began to decompose. Cr_1.07Fe_18.93 and Ni₁₇W₃ were formed when heated at 700°C, and Ni₃P was not found. With increasing temperature to 800°C, FeNi₂P and Cr₄Ni₁₅W were the only two dominant phases. The experimental results reveal that with an increase in the annealing temperature, micro-hardness of the Ni-W-Cr-P alloy coatings increased, reached the maximum value at 700°C, and then decreased slightly. Annealing temperature dependence of the microhardness and corrosion resistance of the coatings was also observed. The related strengthening mechanism in the electroless deposited Ni-W-Cr-P alloy coatings is also discussed.     

Tensile behaviour of as deposited and heat-treated electroless Ni-P deposits

Electroless Ni-P coatings were deposited on mild steel substrates and the effect of heat-treatment on their structure and tensile behaviour was studied, with the following conclusions. The as-deposited electroless Ni-P coating is amorphous and it remains amorphous up to 300 °C. At 400 °C the coating becomes crystalline and consists of a Ni₃P matrix containing areas of metallic nickel. For the selected coating/substrate thickness ratio, the contribution of the coating in the tensile properties of the coating-substrate system is negligible as expressed by the values of yield strength, ultimate tensile strength and fracture strain in mild steel substrates and coated as-deposited and heat-treated specimens. Extensive cracking of the coating accompanied by spalling was occurred during the tensile tests. The density of cracks was found to increase close to the fracture surface of the tensile specimen and with increasing heat-treatment temperature. The cracks observed on the surface of the coatings are believed to form due to the inability of the brittle coating to accommodate the strain generated in the ductile substrate. Their orientation to the tensile axis is in close relation to the structure of the coating and the failure mechanism that is dictated by this structure. The first cracks on the surface of the coatings were found to form after the yield strength of the tensile specimen has been reached and plastic deformation of the substrate takes place. Their density increases with the accumulation of strain up to fracture.     

Compressive behaviour of aluminium foams at low and medium strain rates

Compressive behaviour of CYMAT aluminium foams with relative densities ranged from 5% to 20% has been studied experimentally in this paper. An MTS machine is employed to apply a compressive load at strain rates of 10⁻³–10⁺¹ s⁻¹ to these closed-cell aluminium foams. It has been found that the plateau stress is insensitive to the strain rate and is related to the relative density by a power law. Deformation is not uniform over the whole sample: it first occurs in the weakest band, followed by the next weakest bands after the first one has been completely crushed.     

Crystallization of leucite as the main phase in aluminosilicate glass with low fluorine content

The paper presents the results of the study of aluminosilicate glass that contains 0.77 wt% of F^– anions. The results show that thecritical size of glass particles at which the surface mechanism ofcrystallization changes to volume crystallization is in the vicinityof 0.075 mm in this glass. The temperature of the maximum nucleationrate was determined to be T _n = 680°C, and it ishigher than the transformation temperature T _g. Atcrystallization temperatures up to T _c = 950°C,the crystal phase of Lucite is formed, alone. Above this temperature,besides the main phase of lucite, two other phases, diopside andphlogopite appear. The activation energy of crystallization E _a = 319 ± 23 kJ/mol was determined. The results alsoshow that the activation energy does not depend on thecrystallization mechanism in this glass. The value of the Avramiparameters is n = 1.45. Microstructural studies confirmed thefact that volume crystallization of leucite takes place in thisglass. The leucite crystals grow in form of equiaxial dendrites withpronounced anisotropy along the direction ‹001›.Kinetical and microstructural studies show that the process ofleucite crystal growth occurs at the smooth atomic scale-facetedcrystal/glass interfaces and that it is controlled by volumediffusion. During the isothermal heat treatment the structure ofdendrites changes.     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Correlation of the phase transformation accompanying the hydration of calcium aluminate with ultrasonic wave behaviour

Observations of changes in the amplitude and velocity of the ultrasonic wave transmitted through a hydrating high alumina cement were found to have no corresponding changes in X-ray phase analyses. A continuous X-ray scan of an hydrating sample showed no evidence for the completion of hydration. X-ray patterns from a sample aged for two months and from one aged for 24 hours were basically identical.     

Effects of preheating treatment on repeated martensitic transformation under cyclic heating and cooling

Hot-worked die steel is usually used under conditions requiring repeated heating and cooling. The repeated transformation of austenite to martensite or vice versa may occur on the surface of the hot-worked die. The characteristics of repeated martensitic transformation and the effects of preheating treatments have been investigated using a Formaster-F fully automatic transformation dilatometer. The results show that the volume fraction of martensite, f _m, formed in 750 °C 20 °C repeated cycling is changed with the cycling number, (N), and the preheating treatment has an important influence on the kinetics of martensitic transformation.     

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T _a. It was found that upon lowering T _a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.     

Transformation of phosphorus in intermittent aerated biofilter under aerobic continuous feeding with long backwashing intervals

A combined fixed-film system composed of anaerobic biofilter, aerobic biofilter, and intermittent aerated biofilter (IABF) is developed for enhanced biological phosphorus removal (EBPR) treating domestic wastewater. This work presents details on the performance of IABF under aerobic condition, where phosphorus-accumulating organisms are accumulated. Analysis on distribution of phosphorus in both the bulk and the biofilm indicates that the PAOs-rich biofilm is characterized by a high activity, a strong P capacity, and a good adaptability of fluctuations in aerobic continuous loading. An innovative means for P removal slows down accumulation of P in biofilm. As a result, removal of P-rich biomass is no longer a key limitation of EBPR performance in the biofilm system. Long backwashing interval is practicable in IABF under aerobic continuous feeding regime.     

Model for cooling and phase transformation behaviour of transformation induced plasticity steel on runout table in hot strip mill

Abstract

A mathematical model has been developed considering non-symmetric cooling in the thickness direction of strip on a runout table (ROT). In order to solve a one-dimensional transient heat transfer equation including the heat evolved from phase transformation, a finite element method was applied, coupled with thermodynamic and kinetic analyses. The heat capacities of each phase and heat evolution owing to phase transformation were obtained from thermodynamic analysis of the Fe–C–Mn–Si system using Thermo-Calc. The phase transformation kinetics of a transformation induced plasticity (TRIP) steel were derived by using continuous cooling experiments and thermodynamic analysis. Heat transfer coefficients of strips on the ROT were, by applying an inverse method, determined from actual mill data under various cooling conditions. Using the developed model, temperature–time variations of plain carbon and TRIP steels on the ROT were calculated. The calculated results were in good agreement with the actual mill data. In addition, quantitative phase evolution during cooling could also be predicted by the model. From this analysis, the optimum cooling pattern on a ROT for the production of TRIP steel could be determined.     

Precipitation behaviors of carbides and Cu during continuous heating for tempering in Cu-bearing medium C martensitic steel

The precipitation behaviors of carbides and Cu during continuous heating for tempering were investigated in Cu-bearing medium C martensitic steel by means of dilatometry, electrical resistivity, and transmission electron microscopy. The addition of 1.5 wt% Cu suppressed carbide precipitation during quenching from 900 °C, resulting in a large amount of solute C atoms in virgin martensite. The addition of Cu increased both the finish temperature of ε-carbide precipitation and the amount of ε-carbide precipitates during continuous heating. The precipitation of cementite was retarded and the amount of cementite precipitates increased by the addition of Cu. Retarded cementite precipitation in the Cu-bearing steel was attributed to sluggish Cu partitioning from cementite particles to the martensite matrix, the hindrance to the migration of cementite interfaces by Cu particles, and the slowed diffusions of C and Fe atoms. Cu precipitation was accelerated by cementite precipitation because cementite interfaces and the high Cu concentration near cementite particles provided nucleation sites for Cu precipitation. The hardness of the tempered Cu-bearing steel was higher than that of the tempered Cu-free steel at the temperatures of over 300 °C due to both Cu precipitation hardening and retarded cementite precipitation.     

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10?nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.