Dispersion Of Second Phase Particles Research Articles

Observations on the Role of Interfacial Phenomena in Materials Processing.

Examples of the role interfacial phenomena can play in materials processing are highlighted in the description of a number of recent research studies conducted at Imperial College. Interfacial phenomena play critical roles in increasing or retarding the rates of chemical reaction and in promoting or hindering wetting and dispersion of phases in each other.The reduction of ilmentite to iron and titanium carbide or titanium oxycarbide has been studied with the ultimate aim of achieving separation of the titanium and its subsequent conversion to pigment grade titanium dioxide. The need to achieve good separation of iron from other reaction products is then a prime concern. The effects of reducing conditions on the wetting of titanium carbides and oxycarbides by iron alloys has therefore been studied. It seems that associative adsorption of titanium and carbon may be responsible for the observed effects of dissolved titanium and carbon on the wetting of TiC by liquid iron alloys.As a result of this work a further project has been generated involving the identification of conditions for achieving good dispersions of refractory carbides including titanium carbide in iron alloys. The major motivation behind this work was the desire to develop a cheap casting based process for the production of iron based metal matrix composites capable of producing near net shape products. As a result of this work a novel rapid testing technique for the assessment of the wettability and compatibility of potential filler materials with liquid metal matrices has been developed. The technique employs levitation and quenching of liquid metal drops containing added filler materials to permit assessment of alloy composition, filler coatings and temperature on matrix/filler interactions. The levitation technique has been further utilised in a study of the conditions required for dispersion or non dispersion of second phase particles in liquid superalloys. In this case the cleanliness of the superalloys achieved during recycling procedures is determined by the ease with wich inclusions can be removed. Some observations with ternary oxides also indicate the importance of the associative adsorption phenomenon.The importance of interfacial considerations has also been highlighted by our studies on the production of aluminium-titanium-boron grain refining master alloys from fluoride fluxes. Entrapment of the products can result from emulsification occurring during the reduction reactions. A detailed study of this phenomenon has been conducted using a modified sessile drop technique. The results obtained indicate the critical role that interfacial tension plays in determining the ease of metal-flux separations and in determining whether products are dispersed in the metal or slag.The kinetics of metal-salt reactions were also found to be of importance. Fast transfer of Ti and B to the metal results in the build up to TiAl3. TiB2 or AlB12 at the interface. These compounds when present at the interface can be wet by the flux and result in emulsion formation. Inhibition of emulsification can be achieved by the presence of surface active elements such as magnesium and calcium.The role of interfacial phenomena in influencing the kinetics of the reduction of slags has been studied in an investigation of the kinetics of alkali metal oxide release from silicate melts. The kinetics of K2O and Na2O release during heating in graphite crucibles has been studied from binary alkali oxide-silicon dioxide melts and from a wide range of CaO-Al2O3-SiO2 slags. The contribution of the wetting of the graphite by the slag towards influencing reaction kinetics represents a notable feature of the study.

Determination of microtexture of initial stages of secondary recrystallisation in Fe–3 wt-%Si by etch pitting

AbstractRegular grain oriented silicon iron, produced by a two stage cold rolling technique, has been the subject of many investigations owing to its economic significance as sheet steel for transformer cores. In particular, .many investigations of the texture development through the various processing stages have been carried out by calculation of the orientation distribution functions. The final processing stage is a high temperature box annealing, by which discontinuous grain growth (secondary recrystallisation), controlled by a fine dispersion of second phase particles, leads to a very sharp Goss texture. To investigate the conditions for secondary recrystallisation, material from two stages of processing has been examined: the primary recrystallised material and the material just before the start of secondary grain growth. Utilising the etch pit method – a well established and powerful technique for single orientation determination – in addition to grain size measurements, both applied to scanning el...

On microstructural residual stresses in particle reinforced ceramics

Residual microstructural stresses are shown to influence the crack propagation in particle reinforced ceramics. The dispersion of second phase particles with different coefficients of thermal expansion generates a spatially fluctuating stress field. In this paper deterministic and statistical solutions are considered for the determination of residual stresses. Numerical results are presented for SiC-platelet reinforced Si 3 N 4 and correlated to mechanical properties. It is concluded that the toughening mechanisms of particle reinforced composites are influenced decisively by the spatially fluctuating stress field. Der Rißfortschritt in partikelverstärkten Verbundwerkstoffen wird entscheidend von den mikrostrukturellen Eigenspannungen bestimmt. Verschiedene Berechnungsverfahren zur Bestimmung der Eigenspannungen werden vorgestellt und mit grundlegenden Überlegungen zum Rißfortschritt in keramischen Verbundwerkstoffen verknüpft. Am Beispiel von SiC-Platelet verstärktem Si 3 N 4 wird der Einfluß von räumlich fluktuierenden Eigenspannungen auf die Verstärkungsmechanismen diskutiert. On montre ici que les contraintes résiduelles dans la microstructure jouent un rôle important dans la propagation des fissures pour les céramiques renforcées par des particules. La dispersion de ces particules comme seconde phase ayant un coefficient de dilatation thermique différent génère un champ de contraintes spatiales oscillantes. On rapporte dans cet article des solutions statistiques et précises pour définir le rôle des contraintes résiduelles. On présente des résultats numériques concernant les céramiques Si 3 N 4 renforcées par des plaquettes en SiC et la corrélation avec les propriétés mécaniques. Enfin, on a pu conclure que les mécanismes de propagation de fissures, de ces composites dépendent fortement du champ de contraintes spatiales oscillantes.

Redistribution of Ce and La during processing of Ce(La)-TZP/Al2O3 composites

Abstract The dispersion of second-phase particles, platelets or fibres in a ceramic body offers possibilities of considerable improvements in mechanical properties. In such second-phase reinforced composites the materials properties are to a large extent controlled by the structure of the internal interfaces, i.e. the phase and grain boundaries. The microstructure/microchemistry of aluminadispersed ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP/20A) were characterized by means of analytical TEM/STEM techniques. Local variation in the concentration of Ce stabilizer as well as of La additives and Si impurities during processing of the composite ceramics were studied by EDS X-ray analysis with high spatial resolution. Emphasis was put on the segregation of Ce and La in interface regions. Evidence was found for the presence of a vitreous intergranular phase situated in small pockets at triple grain junctions. The addition of small fractions of La 2 O 3 in TZP resulted in interreaction between matrix constituents and Al 2 O 3 particles. In this reaction a stable crystalline Al-Ce-La-Zr mixed oxide phase is formed, effectively reducing the fraction of residual vitreous intergranular phase, resulting in improved fracture toughness in this material.

Redistribution of particles during casting of composite melts: Effects of buoyancy and particle pushing

When a melt containing a dispersion of second phase particles is solidified, the initial distribution of the particles can change due to three phenomena, namely, buoyant motion of the particles, pushing of the particles by the moving solidification front, and by convection currents in the melt. This paper presents a computer simulation model using which, the net redistribution due to the combined effect of the first two phenomena can be predicted. the existing theory for calculating the critical velocity for particle pushing is extended to include the effect of the buoyancy force and a numerical correlation is developed for easy calculation of the critical velocity. This correlation is incorporated into a computer programme which tracks the position, velocity and direction of the solidification front as well as the position of each particle in the melt as a function of time. The final positions of the particles describe the distribution of the particles in the solidified material. Predicted distributions for various heat extraction rates and particle sizes are presented for a system of silicon carbide particles in a pure aluminium melt solidifying unidirectionally as well as multidirectionally in cylindrical moulds. It is shown that for any heat extraction rate there is an optimum particle size which gives the maximum uniformity of distribution in the solidified material.

Reactive Synthesis of NbAl3 Matrix Composites

AbstractNbA13 matrix composites were synthesized “in-situ” via reactive hot compaction (RHC) of elemental powders. It was found that the simultaneous application of pressure during synthesis was effective in attaining a near-theoretical density matrix at relatively low temperatures and pressures. Using this technique, two types of composites were produced: (1) matrices containing a uniform dispersion of second phase particles (either Nb3Al or Nb2Al with an Nb core or Nb2Al) and (2) matrices reinforced with coated or uncoated ductile Nb filaments. It was found that a limited amount of toughening is obtained using the first approach, while composites containing coated Nb filaments exhibited a significant increase in the ambient temperature fracture toughness. In this paper, various aspects of RHC processing of NbAI3 matrix composites, the effect of initial stoichiometry and powder size on the microstructure, as well as the mechanical behavior of the composites will be discussed.

Flow stress and structure of age-hardened Cu-0.4wt.%Cr alloy after large deformation

In this paper the results of an investigation of the structure and mechanical properties of age-hardened and heavily deformed Cu-0.4wt.%Cr alloy are presented. It was found that the structure and shape of the σ0.2vs.εr curve is strongly influenced by the dispersion of second-phase particles. The presence of small, highly dispersed particles causes the development of a uniformly distributed dislocation structure as a result of retarding the rearrangement dislocation process. The development of this structure leads to a passive dislocation background and causes the localized flow bands to take the main role in the deformation. As a result a step-like σ0.2vs.εr curve in which each step on the curve corresponds to the formation and development of one-band family is obtained. For a low dispersion of particles the recovery process leads to a cellular dislocation structure and to partition of the strain between the cellular dislocation background and the bands. In the latter case the σ0.2vs.εr curve is smooth. The appearance of localized flow bands causes a rapid decrease in the strain-hardening rate.

Influence of precipitate dispersion and texture differences through the sheet thickness on selective grain growth in silicon-iron

Two steels having slightly different chemical compositions were transformed by a conventional two-stage cold rolling process. Different dispersions of second phase particles (grain growth inhibitors) were obtained by controlling the Mn/Cu ratio in the steels, which ultimately came to have significantly different core losses as a consequence of a different magnetic permeability and the size of the secondary recrystallized grains. After primary recrystallization and just before secondary recrystallization, the steels were analyzed for grain size distribution, texture, and particle size evolution. The size of every single grain was classified in relation to its orientation in different positions through the sheet thickness. The evolution of grain size distribution for grains near (±10°) the {110}〈001〉 was found to differ from that for grains with the other orientations. This is in agreement with the theory of grain growth in the presence of texture and special orientation relationships between classes of grain having different orientation. Magnetic properties were related to the starting conditions (frequency and dispersion around the exact Goss orientation) of grain growth after primary recrystallization. The amount of inhibitor and the effect of process parameters (like temperature of intermediate annealing, partition of cold rolling degree, etc.) were found to play a decisive role in improving the intensity of {110}〈001〉 grains and in reducing their dispersion after primary recrystallization.

On the Mechanism of Secondary Recrystallization in Grain Oriented Silicon Iron

The final Goss texture in grain oriented silicon iron develops by a secondary recrystallization process in presence of second phase particles.A sheet of Fe-3%Si was annealed in a gradient furnace in order to establish the exact temperature of the beginning of abnormal growth.Measurements of grain size distributions of the different orientations and in particular of the {110} 〈 001 〉 grains (within a cone of ±10° width around the rolling direction) were performed at different stages of grain growth. The so identified Goss grains show a different growth kinetic in comparison with the matrix. The results are analysed and discussed in the framework of the statistical model of grain growth assuming a texture selection process operating.The agreement between experimental and theoretical results is quite good and allows to state that the main mechanism for the secondary recrystallization in silicon iron is a selective process of grain growth which is controlled by the presence of an inhibitor (dispersion of second phase particles). As a consequence an initial larger grain size for {110} 〈 001 〉 oriented grains is not a necessary condition to get the final Goss texture.

Dislocation-particle interactions during high temperature deformation of two-phase aluminium alloys

Aluminium-silicon, and aluminium-nickel alloys containing various dispersions of secondphase particles have been deformed in compression at elevated temperatures. It is found that there is a sharp transition in the mechanical behaviour, microstructure, and microtexture, at a critical temperature or strain rate, which is dependent on particle size. The results are compared with various models of stress relaxation at particles, and it is shown that both interface and bulk diffusion controlled mechanisms are operative. The relevance of the transition to the hot working of two-phase alloys is discussed.

Grain Growth Behavior of a Rapidly Solidified Fe-10Cr-8A1 Alloy

AbstractA rapidly solidified Fe-10Cr-8Al alloy that contained a dispersion of very fine MnS and Al203 particles was produced by a gas atomization/twin-roll quenching technique. The RS particulates were consolidated by hot isostatic pressing, and the consolidated alloy was hot forged, and then hot and cold rolled. Grain growth took place during primary recrystallization of the alloy; however, once primary recrystallization was complete, the dispersion of second phase particles was effective in pinning alloy grain boundaries.

Influence of DSPP on the properties of AgI

For this investigation AgI was used as a matrix and η-Al2O3(0.05 μm) or ∝-Fe2O3 (0.6 μm) were taken as dispersed second phase particles (DSPP). The conductivity in β phase was increased remarking by the addition of DSPP. The maximum increment appears at 40 m/o for η-Al2O3 and 30 m/o for ∝-Fe2O3. The conductivity of β-AgI(40 m/o η-Al2O3) is three orders of magnitude higher than that of pure β-AgI at room temperature. The activation energies are obviously decreased. The conductivity in ∝-AgI containing DSPP decreases and the activation energy increases. It is found that DSPP influences the phase transition, especially ∝ to β transition. As a result the ∝ to β phase transition temperature of AgI(50 m/o η-Al2O3) is about 17 °C lower than that of pure AgI. Mössbauer spectrum of AgI(∝-Fe2O3 covered with 57Fe) has been studied. It is suggested from the results that both bulk and interfacial effects exist in AgI (∝-Fe2O3).

The high-temperature strength of commercial-purity alpha uranium

The effects of adding less than 500 ppm of aluminum, iron, and silicon on the high-temperature properties of alpha uranium are characterized. Adding aluminum or adding iron and silicon increased the flow stress in all cases and also increased the activation energy if a fine dispersion of second-phase particles was retained. On solution treating the additives in the beta region and retesting the alpha phase, the flow stress increased by about 50 pct—a result of much larger alpha-phase grain size and dispersion-hardening effects. Strong crystallographic textures developed by rolling did not significantly affect flow stress.

7Li NMR INVESTIGATIONS IN AMORPHOUS IONIC CONDUC-TOR Li2B2O4

In order to understand the mechanism of the ionic conductivity enhancement during pre-crystallization process, we have performed 7Li NMR investigations on amorphous ionic conductor Li2B2O4. The temperature dependence of 7Li NMR spectra is studied. It is found that 7Li NMR spectra in crystalline, amorphous samples and amorphous sample with small amounts of crystallites are very different from each other. An additional peak is observed to overlap 7Li NMR absorption line on amorphous sample with small amounts of crystallites. This phenomenon is similar to that in LiCl (or LiI) containing dispersed secondphase particles. 7Li NMR investigations on amorphous ionic conductor B2O3-0.7Li2O-0.7LiCl is also made, the results are similar to that mentioned above. It is concluded that they are all caused by the interfacial effect between amorphous matrix and the crystallites.

7Li NMR INVESTIGATIONS IN LiCl WITH DSPP

In order to understand the mechanism of the ionic conduetivity enhancement in LiCl (or LiI) containing dispersed second-phase particles (DSPP), we have performed 7Li NMR investigations on both line width and spin-lattice relaxation time (T1) at 35 MHz in temperature range of 173-483 K. An additional peak was observed to overlap the resonance absorption line. The signal-to-noise ratio was obviously increased while T1 decreased. The profile of this peak changes with type and content of DSPP and temperature. It is concluded that the ionic eonductivity enhancement and the appearing of the additional peak are both given rise by the DSPP of submicron size. The high conduction in the space charge region near the interface between LiCl and DSPP is suggested as a possible mechanism for the ionic conductivity enhancement and the appearing of the additional peak.

Creep of dispersion-strengthened alloys

The importance of dispersed second-phase particles in providing strengthening of alloys both at room temperature and at high temperatures has long been recognized and the effects of dispersoids have been the subject of many research studies. These studies have led to a considerable understanding of the role of particles but inconsistencies still remain both in the experimental observations and in the interpretation of these observations. The purpose of this review is to examine the characteristics of high temperature creep of dispersion-strengthened alloys from the viewpoint of the possible mechanism involved. Three mechanisms of plastic deformation are distinguished and discussed. Specifically, these are examined in relation to oxide-dispersion-strengthened nickel-based alloys as these have potentially good high temperature corrosion resistance and useful creep resistance to high fractions of their melting points. These attractive properties at temperatures in excess of about 1000°C, where the useful creep strength of creep-resisting steels and superalloys rapidly diminishes, offer what at present appears to be the most technologically feasible development of materials for engineering applications at temperatures of 1100°C and above.

Deformation Twinning in Iron Alloys Containing Dispersed Particles

AbstractAcoustic emission has been used to detect the onset of twinning in iron polycrystals containing dispersions of second phase particles. A new explanation of the grain size dependence of the twinning stress is offered based on the grain size dependence of the twinning intensity. The grain size dependence of the initial twinning stress and the 0.2 % offset stress are similar; however, at larger grain sizes the rate of twin formation is higher which leads to anomalously large values for the grain size dependence of the macroscopic twinning stress.Twinning appears to occur due to the attainment of a large local stress concentration and thus the twinning stress is independent of the dispersion of second phase particles although the density of twins may differ in single phase and two phase materials and large second phase particles may result in twin fragmentation. The results indicate major differences between the influence of second phase particles and deformation by slip and twinning. Resume On a util...

Composition, Microstructure and Mechanical Properties of Flame Sprayed Molybdenum Coatings

SummaryFlame sprayed molybdenum coatings applied to ferrous engineering components can combat wear and fretting action effectively. The micro-hardness of coatings was shown to increase (up to 1700 HV) as the oxygen partial pressures of the oxy-gas flame increase. ‘Soft’ and ‘hard’ coatings have been examined in cross section by optical and scanning electron microscope techniques. Different etching characteristics were noted in the layered structure. At a magnification of ~ × 20 K, regions were revealed in the hard coatings where the etchant had apparently removed a fine dispersion of second phase particles. EDAX microanalysis indicated that the molybdenum content of the softest layers was 92·6% and 87·1% in the harder regions, whilst X-ray diffraction data revealed that the oxide dispersed throughout the coating was MoO2. Heat treatment of the coated samples at 1150°C coarsened the second phase oxide dispersion, a 50% reduction in microhardness resulted. The possibility of ultra fine grain sizes and dispersions of fine oxide particles are seen as the major strengthening agents in the coatings. The influence of internal oxide distribution on wear and fretting behaviour is considered to be important.

Effect of a second phase particle on the rate of growth of fatigue cracks

The effect of the shear modulus, size and dispersion of second phase particles on rate of growth of fatigue cracks in two-phase materials is determined using the kinetic theory approach. The results obtained explain the experimental observations in pearlitic steels.

Grain-Size Control in Steels

An account has been given of the methods of grain-size control in steels. The importance of particle pinning in inhibiting grain coarsening has been described and some consideration given to the solubities and coarsening characteristics of the particles themselves. Grain-size control by hot working has also been considered, where the recrystallization characteristics of the austenite are important in connection. with the grain size of the ferrite-transformation product. The effects of second-phase particles on the recrystallization of cold-worked carbon steels have also been considered. The importance of the integrated nucleation rate on the final grain size has been demonstrated, independent of whether the ferrite grains are formed by transformation or by recrystallization.The mechanism of secondary recrystallization has been described, and it is suggested that this process can be caused by the presence of critically dispersed second-phase particles.