Volume Fraction Of Second Phase Research Articles

Effect of the Iron Ion Doping in LaCoO<sub>3</sub> Perovskite, Both in Powders and in Sintered Samples Obtained from Combustion Reaction and Solid State Route

The aim of this work is to show the effect of the iron ion doping in LaCoO3 perovskite, both in powders and in sintered samples obtained from combustion reaction and solid state route. The phase formation and particle morphology and particle size distribution of the powders were analysed by XRD, SEM and sedimentation techniques, respectively. Relative density, microstructure (secondary phases and grain size) and pore size distribution of LaCo1-xFexO3 sintered ceramics were investigated by SEM/EDS and Hg porosimetry analysis. Although LaCo1-xFexO3 powders obtained from the combustion reaction exhibited smaller grain sizes when sintered at high temperatures, they showed a higher volume fraction of secondary phases. The presence of these crystalline phases in addition to the desired perovskite affected the microstructure acting as grain growth inhibitors by grain boundary pinning. It is believed that by observing three grain junction pores that the LaFeO3 phase has a smaller dihedral angle than LaCoO3. This fact would explain why LaFeO3 presented a smaller driving force for sintering with a higher tendency of pore and inclusion coarsening at higher temperatures (1400°C).

Effect of Sintering Temperature on the Superconducting Properties of MgB<sub>2</sub> Superconductor Co-Added with a High Concentration of Si and C

In this study, as much as 10 and 15 wt.% nanosized silicon and carbon (Si+C) were reacted with (Mg+2B) at 650°C and 850°C, respectively, for 1 hour. The phase formation, surface morphology and superconducting properties of these samples were evaluated. The relative peak intensity as calculated from the XRD patterns indicates the formation of large Mg2Si volume fraction at low sintering temperature. MgB4 phase was detected in the samples sintered at high temperature as a result of Mg deficiency. The C substitution level as estimated from the lattice parameters, was shown to increase in the samples reacted with a higher amount of (Si+C) at high temperature. Scanning electron micrograph showed that (Si+C) co-addition had refined the grain size and improved the grain coupling of MgB2. The superconducting transition temperature was found to decrease with increasing addition level. The superconducting transition width was also broadened because of a large volume fraction of secondary phases. The improved field dependent critical current density at both 5 K and 20 K is accounted to enhanced scattering by C substitution and grain boundary pinning.

(Invited) Capabilities of Analytical Transmission Electron Microscopy for the Analysis of Structural, Chemical and Electronic Properties Exemplified by the Study of Y-Doped (Ba,Sr)(Co,Fe)O3-δ

Transmission electron microscopy (TEM) is a standard technique for microstructure studies with a resolution that meanwhile has reached values better than 0.1 nm. Essential is the capability of obtaining images and diffraction patterns from the same small specimen region which facilitates crystal structure determination of small precipitates. Particularly unique is the possibility to perform atomic-scale chemical analyses by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). Information on the electronic material properties like, e.g., valence states or bond configurations between neighbor atoms is obtained by the electron near edge structure of ionization edges in EELS spectra. In addition to these well-known capabilities, many more imaging modes and evaluation techniques are available which yield additional material properties and render electron microscopy an extremely powerful technique. We will illustrate the capabilities of analytical TEM for the example of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) which is a promising and highly complex material for oxygen separation membranes. BSCF suffers from degradation of the oxygen diffusivity at temperatures below 840 oC which is attributed to the decomposition of the cubic phase into several secondary phases [1,2]. Composition and crystal structure analyses of the different phases combined with Co-valence state determination sheds light on the origin of the instability of the cubic phase. Indispensable is the determination Co-valence state in the cubic and hexagonal BSCF phases which required the introduction of a new evaluation technique due to the overlap of the Co-L2,3ionization edges with ionization edges of other elements in BSCF [3]. Doping with Yttrium has been used to stabilize the cubic Sr1−x Y x Co1−y Y y O3−δ phase [4] which also appears to be a promising strategy for BSCF. Substitution of B-site Co- and Fe-cations in BSCF by Y3+-cations with a fixed valence state should lead to the stabilization of the Co-valence and, hence, the stabilization of the cubic BSCF phase. The volume fraction of secondary phases is indeed substantially reduced in Y-doped BSCF. Only the hexagonal BSCF phase is observed which is confined to grain boundaries (Fig. 1). In contrast, a large volume fraction of different secondary phases is found in undoped BSCF (Fig. 2). A closer inspection suggests, however, that the suppression of secondary phases in Y-doped samples could be partially related to the lack of nucleation sites rather than the stabilizing effect of Y-doping. To achieve an improved understanding of the effect of Y-doping, the occupation sites of Y-cations were analyzed by atom location by channeling enhanced microanalysis (ALCHEMI) as proposed by Spence and Taftø [5]. ALCHEMI analyses demonstrate that Y-cations occupy A- and B-sites. Despite substitution of B-site cations, 25 % to 40 % of the Y-cations occupy A-sites depending on the doping level. This indicates the complex effect of Y-doping in BSCF which will be discussed.

Promotion effect of secondary phase particles on grain refinement of deformed Mg–Y–Nd–Zn alloy

The promoting effect of secondary phase particles on the refinement of matrix grains in a series of Mg–Y1.5–Nd0.6-Zn.x (x=0, 0.2, 0.4, 0.8, 1.3) alloys deformed by friction stir processing (FSP) was investigated in this study. The volume fraction of secondary phase in the alloys increased with increment of Zn, and most of the secondary phases were broken into nano-sized particles and dispersed in the matrix during thermal mechanical processing. The resulting fine particles had a remarkable promoting effect on the grain refinement of the Mg matrix during mechanical processing, thermal deformation, and cooling with increasing volume fraction of the secondary phase. For example, the grains of Mg–Y1.5–Nd0.6–Zn1.3 alloy were refined to an average size of 28nm, compared with the more than 100nm grain size of the Mg–Y1.5–Nd0.6 alloy.

Effects of secondary phase and grain size on the corrosion of biodegradable Mg–Zn–Ca alloys

The bio-corrosion behaviour of Mg-3Zn-0.3Ca (wt.%) alloy in simulated body fluid (SBF) at 37°C has been investigated using immersion testing and electrochemical measurements. Heat treatment has been used to alter the grain size and secondary phase volume fraction; the effects of these on the bio-corrosion behaviour of the alloy were then determined. The as-cast sample has the highest bio-corrosion rate due to micro-galvanic corrosion between the eutectic product (Mg+Ca2Mg6Zn3) and the surrounding magnesium matrix. The bio-corrosion resistance of the alloy can be improved by heat treatment. The volume fraction of secondary phases and grain size are both key factors controlling the bio-corrosion rate of the alloy. The bio-corrosion rate increases with volume fraction of secondary phase. When this is lower than 0.8%, the dependence of bio-corrosion rate becomes noticeable: large grains corrode more quickly.

Effect of Zr addition on the microstructure, phase transformation and mechanical property of Ni50Mn25Ga17Cu8 alloy

This study investigated the microstructure, phase transformation and mechanical property of Ni50Mn25Ga17Cu8−xZrx (x=0, 4 and 8) alloys. The microstructure of alloys changed from a single martensitic phase to a dual phase consisting of martensitic/austenitic matrix and second γ phase after substituting Cu with Zr and the volume fraction of second phase increased with increasing Zr content. The martensitic transformation temperature and transformation enthalpy were reduced after adding Zr, which was related to the introduction of second phase that changed the composition and reduced volume fraction participating in martensitic transformation of the alloy. The decrease of transformation hysteresis after introducing the second phase was thought to be caused by the martenstic structure change of the matrix from tetragonal to orthorhombic that reduced the lattice deformation of austenite→martensite transformation. The mechanical strength of the Cu8 alloy was increased firstly after substituting 4 at% Cu by Zr and then decreased after completely substituting Cu by Zr. The change of mechanical performance was related to the fracture character of the alloys. The intergranular fracture of matrix and second phase dominated in the Cu8 and Zr8 alloys, respectively, but the fracture of Cu4Zr4 alloy was determined by a mixture of intergranular fracture of second phase and transgranular fracture of matrix.

Strength evolution and the development of crystallographic preferred orientation during deformation of two-phase marbles

We conducted conventional triaxial compression and confined torsion tests on calcite rocks at 1023K and equivalent strain rates between 10−6 and 10−3s−1, to investigate the influence of variations in volume fraction and geometry of a second phase on the evolution of strength and CPO. Nominally dense, two-phase marbles were made by hot-isostatic pressing of mixtures of calcite and amorphous carbon. The carbon grains were either spheres or splinters with known size distributions, were chemically inert, and are nearly rigid. Adding 1vol.% spheres caused only small changes in strength or CPO. Strengths were consistent with the activation of slip on f101¯1, the hardest system in calcite. Adding 10vol.% splinters increased the strength at low strains and strain rates but had smaller effects at high strains or rates. The CPO of rocks containing splinters were similar to those in pure samples, but with reduced intensity. When 10vol.% spheres were added, the aggregate strength under confined torsion was less than that of the pure material. Distinct and intense CPOs developed. The reduced stresses were consistent with those necessary for slip on systems weaker than f. Viscoplastic self-consistent calculations used to model the texture evolution required slip on f101¯1 and c1¯21¯0 to match the CPO of pure samples; results were consistent with the inclusion of slip on r1¯21¯0. In samples containing 10vol.% spheres deformed in torsion, slip activity on the harder systems was decreased. The exact cause of the strength reduction in these experiments is not understood, but it is possible that adding smooth spheres enhanced grain-boundary sliding, thereby decreasing activity on f101¯1. Thus, shape and volume fraction of second phases may be important in determining CPO and strength; and it is important to consider the loading geometry.

Micromechanics based constitutive modeling of martensitic transformation in metastable materials subjected to torsion at cryogenic temperatures

Special class of multi-phase structural members with graded microstructure can be obtained at cryogenic temperatures as a result of controlled transition from the parent phase to the secondary phase. The required features are obtained via the mechanism of diffusionless plastic strain induced phase transformation from purely austenitic to martensitic lattice (γ→α′). Many ductile materials are known to behave in metastable way when strained at extremely low temperatures. Among them the austenitic stainless steels are often used to construct components of superconducting magnets, cryogenic transfer lines and other cryogenic systems. The multiscale constitutive model developed to describe the plastic strain induced phase transformation at very low temperatures involves plastic hardening where two important effects play fundamental role: (1) interaction of dislocations with martensite inclusions and (2) increase of resultant tangent stiffness due to the evolution of harder martensite within the softer austenite. The micro-mechanism of interaction of dislocations with martensite inclusions is based on the Orowan scenario. The other mechanism takes into account constantly evolving proportion between the primary and the secondary phase and involves the relevant homogenization scheme. Both effects contribute to strong nonlinear hardening that occurs as soon as the phase transformation process begins. The constitutive model has been used in order to obtain a new closed form analytical solution for the case of torsion of round bars at extremely low temperatures. Moreover, numerous experiments involving quasistatic and cyclic loads were carried out in order to trace the profile of phase transformation. An ultimate proof is presented that a functionally graded structural member, characterized by the required profile of volume fraction of both phases, can be obtained. The profile of volume fraction of secondary phase (martensite) is checked by means of 2 independent methods: hardness measurements and verification by using a ferritscope (magnetic induction).

An analytical model for secondary phase dissolution kinetics

An analytical model for dissolution kinetics of secondary phase particles upon isothermal annealing has been proposed. Considering the interactions of solute diffusion fields in front of the secondary phase/matrix interface upon dissolution, a Johnson–Mehl–Avrami type equation, subjected to necessary modification, was derived, in combination with a classic dissolution model for single-particle system. Compared with the semiempirical dissolution models, which are used to fit the experimental results and phase-field method simulation, the current model follows an analogous form, but with the time-dependent kinetic parameters. Distinct from the model fitting work published recently, the current model is derived from the diffusion-controlled transformation theory, while the modeling quality is guaranteed by the physically realistic model parameters. On this basis, the current model calculation leads to a clear relationship between the secondary phase volume fraction and the time. Accordingly, model predictions for isothermal θ′ dissolution in Al–3.0wt%–Cu alloy and silicon dissolution in Al–0.8wt%–Si alloy were performed; good agreement with the published experimental data has been achieved.

Influence of technological factors on dispersion strengthened materials deformation mechanism studied by 'in-situ tensile test in SEM'

Direct observation of deformation process until formation of fracture on extreme small samples of composite in SEM is interesting from practical and interpretation aspects. In the present work the effects of composite composition, volume fractions of secondary phases as well as the matrix grain sizes (micro and nano size) on deformation and failure processes of Al and Cu-based composites were evaluated. Experimental materials were prepared by the different powder metallurgy methods. Interpretation of results was based on the quantification of the physical parameters of material phases depending on the preparation methods and on the analysis of deformation processes using the ‘in-situ tensile test in SEM’. The mechanism of deformation and fracture for each composite system was estimated and models of fracture were proposed.

Microstructure-level modeling and simulation of the flexural behavior of ceramic tool materials

The flexural behavior of ceramic tool materials is investigated by both experiments and numerical simulation. The Voronoi Tessellation hybrid random algorithms are utilized to construct a microstructure model to simulate the flexural behavior of ceramic. The secondary phase volume fraction, the nano-scale particle volume fraction, the grain centroid distribution, and the grain diameter distribution of the materials are considered in the model. The flexural strength of ceramic materials is calculated via the simulation of three-point bending tests. The effects of average grain diameter, nano-scale particle and secondary phase volume fraction on the flexural strength of ceramic materials are systematically studied. The numerical simulation results show good agreement with the experimental results.

Microstructure and Corrosion Properties of Mg-Zn-Ca-Nd Alloy for Biomedical Application

The microstructure and corrosion properties of the as-cast Mg-Zn-Ca-Nd alloys were investigated in this study. The results showed that the alloy was mainly composed of α-Mg solid solution and the secondary phases of MgZn, Ca2Mg6Zn3 and Mg41Nd5. The portion of precipitated phases was increased with the increment of Nd content. Both electrochemical measurement and immersion test indicated that Mg-4Zn-0.5Ca-1.0Nd had the best corrosion resistance in SBF solution. This is attributed to a combination of (1) the uniform microstructure of alloy, and (2) a lower volume fraction of second phases. In the Mg-4Zn-0.5Ca-3.0Nd alloy, the corrosion rate might slow down due to finer dendrite cells were surrounded by the more continuous second phases along the grain boundary and in the α-Mg matrix.

Effects of Sn addition on the microstructure and mechanical properties of as-cast, rolled and annealed Mg–4Zn alloys

The effects of Sn addition on the microstructure and mechanical properties of as-cast, as rolled and subsequent isothermal annealed Mg–4Zn alloys are investigated. Tensile and Rockwell hardness tests are conducted to evaluate the mechanical properties of the as-cast alloys. Improvements of the mechanical properties in Sn-containing alloys are attributed to the increasing volume fraction of secondary phases and modifying the type of intermetallic phases. Additions of Sn increase the twinning density and modify the annealing texture in as-rolled Mg–4Zn alloys. In comparing rolled-annealed Mg–4Zn to Mg–4Zn–3Sn alloy, numerous small particles are dispersed in the matrix of rolled-annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles with a size of about 175nm. The rolled-annealed Mg–4Zn–3Sn alloy exhibits high ultimate tensile strength, yield strength and elongation of 273MPa, 159MPa and 16.2%, respectively. The UTS and YS are about 16% and 53% higher than Sn-free alloy.

Extended finite element modeling of crack propagation in ceramic tool materials by considering the microstructural features

A microstructure model is established to study failure of ceramic materials. The model is based on the Voronoi Tessellation in which random functions are employed to take into account the secondary phase volume fraction, the grain centroid distribution, the grain diameter distribution, and the grain boundary volume distribution of the materials. Extended Finite Element Method (X-FEM) is utilized to simulate the crack propagation considering the influence of microstructure of ceramic, which can obtain high accuracy without the need for re-meshing compared with the standard finite element method. The influences of grain diameter, secondary phase volume fraction and grain boundary volume fraction on the fracture toughness of ceramic materials have been systematic studied. The results presented in this article are useful to improve toughness and failure resistance of ceramic materials.

Influence of Sc addition on microstructure and transformation behaviour of Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy

Vacuum-arc melted Ni24.7Ti50.3Pd25.0 and Ni24.7Ti49.3Pd25.0Sc1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti2(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (Mf) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni24.7Ti49.3Pd25.0Sc1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni24.5Ti50.0Pd25.0Sc0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C.

Effect of cooling rate on the microstructure and mechanical properties of sand-casting Mg–10Gd–3Y–0.5Zr magnesium alloy

The present study investigated the effect of cooling rate on the microstructure and mechanical properties of sand casting Mg–10Gd–3Y–0.5Zr alloy with a cooling rate range of 0.7–3.6°C/s. When the cooling rate was increased, the average grain size of α-Mg decreased from 59μm to 39μm, the volume fraction of the second phase increased from 17.6% to 24.5%, and the eutectic compound exhibited continuous network instead of coarsening discontinuous network of plate-shaped microstructure. Energy-dispersive X-ray test results suggested that solute content of RE elements (Gd and Y) was low within the area near the grain boundary of α-Mg with high cooling rate, and solid-solution reaction occurred in the sand casting process. In addition, Vickers hardness (HV) testing indicated that HV increased with the increasing cooling rate, and the volume fraction of second phase (V) played a main role in hardness of the alloys. There was a linear relationship between them, the fitting function was HV=1.25V+63.05, R2=0.9989. Tensile strength test showed that both of ultimate tensile strength (UTS) and tensile yield strength (TYS) first increased and then decreased with the increasing cooling rate. Based on fracture observations, the types of fracture surface characteristics are correspondingly classified into three modes, transgranular, dimple-like fracture and intergranular fracture with the increasing cooling rate.

Influence of Y/CeMM ratio on the microstructure and mechanical properties of Mg95Zn2(Y,CeMM)3 alloys

The influence of Y/CeMM ratio on the microstructure and mechanical properties at room temperature of extruded Mg95Zn2(Y,CeMM)3 alloys was evaluated. The volume fraction of second phases, mainly intermetallic Mg12RE and long period stacking ordered phase (LPS–phase), was close to 45% for all alloys but the volume fraction of LPS–phase particles was increased as the Y/CeMM was raised. The strength and ductility of the alloys were increased with increasing the volume fraction of LPS–phase because this phase assumed the stress released from cracking of Mg12RE particles during plastic flow.

The role of second phases in the corrosion behavior of Mg–5Zn alloy

The MgxZny second phases in Mg–5Zn alloy were adjusted by T4 and T6 treatment, and then the role of second phases in the corrosion behavior of Mg–5Zn alloy was investigated. The dimension of the MgxZny second phases is only some hundred nanometers. They present discrete distribution on the boundaries of the as-received sample and continuous distribution in the T6 sample. The T4 sample in absence of second phases exhibits the best corrosion resistance while the T6 sample with the largest volume fraction of second phases exhibits the worst one, which can be attributed to the micro-cathodic effect of second phases.

Variation of Microstructure and Properties of Re-7075 Al Alloy after Homogenizing Annealing

The variation of microstructure and properties of RE-7075 Al alloy after homogenizing annealing were studied by some modern analytical instruments which including OM(optical microscope),SEM(scanning electron microscopy),EDS(energy dispersive spectroscopy ), Brinell hardness and conductivity meter of SIGMASCOPE SMP10 type in this experiment. The results shows that the alloy′s microstructure was uniform ,reduced the volume fraction of second phase,increased supersaturation of matrix and eliminated gradually dendritic segregation in homogenizing annealing state. The primary phase of matrix microstructure contain α(Al),T(Al2Mg3Zn3) and grain boundary microstructure include α(Al),Al2Cu,AlCuMg4La4 after homogenizing annealing.Compared with as cast condition,the conductivity and the hardness reduced after homogenizing annealing.

Mathematical Modeling of Behaviour a Liquid Steel Flow and a Non-Metallic Inclusions in the One Strand Wedge Type Tundish with a Subflux Turbulence Controller and a Low Dam

The numerical modelling technique is successfully used for simulation of liquid metal movement and behaviour of non-metallic inclusions in the continuous casting devices. The CFD (Computational Fluid Dynamic) method allows information on physical, chemical and hydrodynamic phenomenon in the metallurgical processes to be obtained. In the continuous steel casting process, where the tundish performs the function of a device batching steel to the mould, the monitoring especially of non-metallic inclusions is very essential because a quantity of non-metallic inclusions in the liquid steel limit the quality of final steel product. The paper presents the results of computing simulation of behaviour a liquid steel flow and non-metallic inclusions in the one-strand tundish with stopper rod system. The subject of simulation was a one-strand tundish of a nominal capacity of 30 tons. The tundish is used in the polish steel mill to process of continuous casting slabs. In the paper authors analysed two tundish i.e. tundish with a low dam and tundish with a subflux turbulence controller and a low dam. The Ansys-Fluent® program was used for solved mathematical model of casting process. The computer simulations were performed for unsteady and nonisothermal conditions. The population balance model was used to description of non-metallic inclusions growth process. As a result of computations, fields of liquid steel flow, fields of secondary phase volume fraction, fields of volume fraction for particular bins and non-metallic inclusions growth curve were obtained.