Savko Malinov

Crystallisation kinetics and phase transformation behaviour of electroless nickel-phosphorus deposits with high phosphorus content

Abstract The effects of continuous heating on the crystallisation kinetics and the phase transformation behaviour of electroless nickel–phosphorus deposits plated on mild steel substrates, with high phosphorus contents of 12 and 16 wt.%, were studied. Both the deposits exhibited an amorphous X-ray profile in the as-deposited condition. The crystallisation temperatures of the deposits increased with decreasing phosphorus content, and increasing heating rate. The activation energies during the crystallisation processes were calculated from the differential scanning calorimetry (DSC) curves of the deposits at heating rates ranging from 5 to 50°C/min. It was found that the activation energy is slightly higher in the deposit with lower phosphorus content. X-ray diffraction (XRD) analyses were also conducted on the deposits after heating processes in the DSC apparatus to 300–800°C at 20°C/min. The sequence of phase transformations was found to be: amorphous phase→intermediate metastable phases+stable Ni 3 P phase (+f.c.c. nickel)→stable Ni 3 P phase (+f.c.c. nickel). Electron microprobe analysis was carried out on the 16 wt.% P deposit and showed that the iron content in the deposit was affected by heating processes. Film growth in the 16 wt.% P deposit has been shown by scanning electron microscopy (SEM) and electron microprobe analysis on the cross-sections of the deposit after heating to 400 and 800°C (at 20°C/min).

Hardness evolution of electroless nickel-phosphorus deposits with thermal processing

Abstract Four samples of electroless nickel–phosphorus (EN) deposits coated on mild steel substrate have been analysed for their hardness changes in relation to the deposit phosphorus contents as well as different heating temperatures at isothermal (100–500 °C for 1 h) and linear heating (to 300–600 °C at 20 °C/min) conditions. It was found that the hardness of the EN samples increased with decreasing phosphorus content at as-deposited condition, and could be enhanced by appropriate heating. The results of Vickers microindentation testing showed that the peak hardness of the EN samples could be achieved after heat-treating at 400–450 °C. This is caused by the formation of intermetallic Ni 3 P stable phase at this temperature range, acting as a function of precipitation hardening. The Knoop microindentation testing on the cross sections of the samples indicated variations of the hardness across the depth (distance from the sample surface towards the sample/substrate interface), but for most samples the tendencies of change were not clear. Scanning electron microscopy analysis has shown that the lamellar structure present in the cross sections of the as-deposited EN samples tends to disappear, and agglomeration occurs when the heat-treating temperature is increased. The concept of kinetic strength ( K s ) is adopted to interpret the kinetic energy ( Q ) of increased hardening effects using the Vickers hardness data after the isothermal experiments, but the result has not been satisfactory.

Synchrotron X-ray diffraction study of the phase transformations in titanium alloys

High-resolution synchrotron X-ray diffraction was used to study the phase transformations in titanium alloys. Three titanium alloys were investigated: Ti–6Al–4V, Ti–6Al–2Sn–4Zr–2Mo–0.08Si and b21s. Both room and high temperature measurements were performed. The room temperature experiments were performed to study the structure of the alloys after different heat treatments, namely as received (AR), furnace cooling (FC), water quenching (WQ) and water quenching followed by ageing. The a, a 0 , a 00 and b phases were observed in different combinations depending on the heat treatment conditions and the alloy studied. A multicomponent hexagonal close packed (hcp) a phase, with different c and the same a lattice parameters, was detected in Ti–6Al–4Vafter FC. High temperature synchrotron X-ray diffraction was used for ‘in situ’ study of the transformations on the sample surface at elevated temperatures. The results were used to trace the kinetics of surface oxidation and the concurrent phase transformations taking place under different conditions. The influence of the temperature and oxygen content on the lattice parameters of the a phase was derived and new data obtained on the coefficients of thermal expansion in the different directions of the hcp a phase, for Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo–0.08Si. D 2002 Elsevier Science Inc. All rights reserved.

Crystallization and phase transformation behaviour of electroless nickel-phosphorus deposits with low and medium phosphorus contents under continuous heating

Electroless nickel-phosphorus deposits with 5–8 wt% P and 3–5 wt% P were analysed for the effects of continuous heating on the crystallization kinetics and phase transformation behaviour of the deposits. The as-deposited coatings consist of a mixture of amorphous and microcrystalline nickel phases, featuring in their X-ray diffraction patterns. Continuous heating processes to 300°C–800°C at 20°C/min were carried out on the deposits in a differential scanning calorimetric apparatus. The subsequent X-ray diffraction analyses show that the sequence of phase transformation process was: amorphous phase + microcrystalline nickel → f.c.c. nickel + Ni3P stable phases. Preferred orientation of nickel (200) plane developed in the deposits after the heating processes. Differential scanning calorimetry of the deposits indicates that the crystallization temperatures increased with decreasing phosphorus content, and increasing heating rate. Crystallization activation energies of the deposits (230 and 322 kJ/mol, respectively) were calculated using the peak temperatures of crystallization process, from the differential scanning calorimetric curves at the heating rates ranging from 5 to 50°C/min. It was found that the deposit with lower phosphorus content has higher activation energy.

Resistivity study and computer modelling of the isothermal transformation kinetics of Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo–0.08Si alloys

Abstract A resistivity technique is used to study the kinetics of β⇒α+β transformation for Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo–0.08Si alloys at isothermal conditions. The kinetics are modeled in the framework of the Johnson–Mehl–Avrami (JMA) theory. The JMA kinetic parameters for different temperatures and conditions of the above transformation are derived. A good agreement between the calculated and the experimental kinetics is demonstrated Two different mechanisms of α-phase formation depending on the temperature of isothermal exposure are proposed. The calculation of the thermodynamic equilibria showed good agreement with the experimental data for the Ti–6Al–4V alloy and disagreement for the Ti–6Al–2Sn–4Zr–2Mo–0.08Si alloy. Based on the resistivity results, time–temperature–transformation diagrams for both alloys are designed.

Titanium alloys : modelling of microstructure, properties and applications

Introduction to titanium alloys Part 1 Experimental techniques: Microscopy Synchrotron radiation X-ray diffraction Differential scanning calorimetry and property measurements. Part 2 Physical models: Thermodynamic modelling The Johnson-Mehl-Avrami method: isothermal transformation kinetics The Johnson-Mehl-Avrami method adapted to continuous cooling Finite element method: morphology of ? to ? phase transformation Phase-field method: lamellar structure formation in ?-TiAl Cellular automata method for microstructural evolution modelling Crystallographic and fracture behaviour of titanium aluminide Atomistic simulations of interfaces and dislocations relevant to TiAl. Part 3 Neural network models: Neural network method Neural network models and applications in phase transformation studies Neural network models and applications in property studies. Part 4 Surface engineering products: Surface gas nitriding: phase composition and microstructure Surface gas nitriding: mechanical properties, morphology, corrosion Nitriding: modelling of hardness profiles and the kinetics Aluminising: fabrication of Al and Ti-Al coatings by mechanical alloying.

Software products for modelling and simulation in materials science

Abstract Models and software products have been developed for modelling, simulation and prediction of different correlations in materials science, including 1. the correlation between processing parameters and properties in titanium alloys and γ-titanium aluminides; 2. time–temperature–transformation (TTT) diagrams for titanium alloys; 3. corrosion resistance of titanium alloys; 4. surface hardness and microhardness profile of nitrocarburised layers; 5. fatigue stress life ( S – N ) diagrams for Ti–6Al–4V alloys. The programs are based on trained artificial neural networks. For each particular case appropriate combination of inputs and outputs is chosen. Very good performances of the models are achieved. Graphical user interfaces (GUI) are created for easy use of the models. In addition interactive text versions are developed. The models designed are combined and integrated in software package that is built up on a modular fashion. The software products are available in versions for different platforms including Windows 95/98/2000/NT, UNIX and Apple Macintosh. Description of the software products is given, to demonstrate that they are convenient and powerful tools for practical applications in solving various problems in materials science. Examples for optimisation of the alloy compositions, processing parameters and working conditions are illustrated. An option for use of the software in materials selection procedure is described.

Application of artificial neural network for prediction of time–temperature–transformation diagrams in titanium alloys

Abstract A model of artificial neural network for simulation of time–temperature–transformation (TTT) diagrams for titanium alloys was designed. Standard backpropagation multilayer feedforward network was created and trained using data from published literature. The influence of aluminium, vanadium, molybdenum and oxygen on transformation kinetics in titanium alloys was assessed on the base of the trained neural network. The results are in good agreement with what is expected from phase transformation theory. Using the model, TTT diagrams for some commercial alloys were predicted. A graphical user interface was created for the use of the model.

Simulation of microhardness profiles for nitrocarburized surface layers by artificial neural network

A model for prediction of the microhardness profiles for nitrocarburized steels was designed. The model is based on a feed-forward artificial neural network. The performance of the model was checked, using data from the published literature as well as authors experiments. Good correspondence between predicted from the artificial neural network (ANN) and experimental data was observed. The influences of the nitrocarburizing parameters and steel composition on the microhardness profile were studied. Using the model the microhardness profiles for some steels after different conditions of nitrocarburizing were predicted. A graphical user interface was created for the use of the model.

Surface gas nitriding of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si alloys

Abstract The paper presents results from thermochemical surface gas nitriding of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si alloys using differential scanning calorimetry equipment. The experiments were carried out in nitrogen atmosphere at temperatures of 950 and 1050°C for 1, 3 and 5 h. Results for the surface hardness, the microhardness profiles and the thickness of the nitrided layers were obtained. Microstructure analyses were made by optical microscopy. X-ray diffraction analyses indicated new phases formed (TiN, TiN0.3, Ti2N and TiO2), depending on the time and the temperature of nitriding. The obtained thickness of the nitrided layers was in the range 60–320 μm depending on the process parameters. The Vickers microhardness along the cross-section depth varied between 610 and 1700.