Quantitative Microstructural Analysis Research Articles

Grain Growth Behavior During Microwave Annealing of Silicon Nitride

AbstractA comparative study of grain growth behavior in silicon nitride under conventional and microwave annealing is presented. Microwave annealed specimens showed a faster growth rate as indicated by the quantitative microstructural analysis. The phenomenon was used in combination with seeding techniques to develop a silicon nitride exhibiting a bi-modal microstructure. Microwave annealing was carried out using a microwave radiation frequency of 28 GHz.

A microstructurally based method for stress estimates

A method is described in which quantitative microstructural analysis is used to estimate the local stress and strain states occurring within near-surface layers due to frictional contact. Quantitative estimates of local stress and strain have applications in friction and wear models, in finite element analysis of sliding interfaces, and as a basis for formulating and evaluating models on a local scale. This method is illustrated for three cases of dry sliding on nominally flat surfaces. Sliding tests were performed on a flat plate friction tester, developed at Sandia, which used copper friction samples and a steel test platen. The evolving friction coefficients were measured as a function of normal load and sliding speed. Microstructural analyses included both scanning and transmission electron microscopy (SEM and TEM) of the cross-sectioned friction samples. The sliding-induced dislocation substructures were quantitatively characterized and measured as a function of subsurface depth and normal load. Two simple relationships between the size scale of the dislocation substructure and the flow stress were used to estimate the material properties and the stress state as a function of depth and normal load.

Influence of Si3N4 interface chemistry on both grain morphology and fracture resistance

Quantitative microstructural analysis was performed on Si 3 N 4 materials doped with 5 wt% Y 2 O 3 and 5 wt% Sc 2 O 3 as sintering aids. Two different processing routes were utilized to achieve complete densification: (i) gas-pressure sintering of Si 3 N 4 -starting powders (SSN) and (ii) post-sintering of reaction-bonded Si 3 N 4 (SRBSN). Apart from quantitative evaluation of grain diameter and aspect ratio of the matrix grains, the fracture toughness was determined. While the two Y 2 O 3 -doped materials (SSN, SRBSN) showed identical K IC -values, a large discrepancy in fracture toughness was found for the Sc 2 O 3 -doped Si 3 N 4 (ΔK IC =5 MPa√m). SEM crack propagation studies and additional TEM interface characterization showed no significant difference between these materials. Quantitative microstructure analysis, however, revealed a pronounced variation in grain diameter and aspect ratio after thermal treatment. A correlation between interface chemistry and both resulting microstructure and fracture resistance is discussed

Microstructural Design of Si3N4 Based Ceramics

ABSTRACTParameters controlling the size and aspect ratio of elongated Si3N4 grains are discussed, based on the assumption that only pre-existing β-Si3N4 particles of the starting powder grow. Powder mixtures of α-rich and β-rich Si3N4 were prepared In order to study the microstructural development. The resulting microstructures were analyzed by quantitative microstructural analysis determining the distribution of the length and aspect ratio of the Si3N4 grains. Subsequently, the Influence of the sintering conditions on grain growth was analyzed In relation to mechanical properties. A high Weibull modulus and the non-catastrophic failure during thermal shock of coarse-grained materials Is attributed to an R-curve behaviour. Finally, the influence of sintering additives on the mechanical properties was studied. The Importance of phase relationships between the matrix and the grain boundary phase Is discussed for Si3N4 with Yb2O3 additives. It Is demonstrated that the oxygen content of Si3N4 powder must been taken Into account In order to devitrify defined secondary phases and to achieve a high degree of crystallization. A reduction in the amount of additives does not necessarily Improve the properties as high temperature strength and creep data Indicate.

Digital probes for three-dimensional microstructural analysis

The issue of function-property correlation studies in quantitative microstructure analysis is introduced. Modern digital stereological techniques are described and their role in such studies is discussed. Surface and volume estimation algorithms are presented in relation to the corresponding geometrical sampling probes utilized for analysis. The procedures are illustrated in both biomedical and metallurgical samples and the potential in further studies is discussed.

Fatigue crack propagation in weathering weldable steel KT 315 Si

Complex loading conditions are typical for structural materials, especially for steels. Fatigue crack propagation measurements during bend loading were carried out with different load ratios on a newly developed weathering steel KT 315 Si. This steel indicates an improved corrosion resistance in comparison with the weathering steel KT 315. Scanning electron microscopy of the fracture surfaces and quantitative microstructure analysis by means of light microscopy were applied to elucidate relations between microstructure and mechanical properties. An important result of the investigation is that the steel KT 315 Si reveals a lowered mean stress sensitivity of the fatigue threshold value ΔK th in comparison with that of other ferritic-pearlitic steels of the same strength level

Quantitative microstructural analysis of a continuous fibre composite

A micrograph of a carbon fibre-reinforced PEEK prepreg has been analysed to determine the fibre distribution and the results compared with the probability density of the theoretical distribution obtained using a spatial statistical model. The close agreement indicates that the model may be used with confidence for the predictive modelling of high performance continuous fibre composites of around 50% volume fraction.

Fracture toughness characterization of a weldment in a microalloyed steel using resistance curves

Various weldment regions, e.g. the weld zone, heat-affected zone (HAZ) and parent metal, are characterized in terms of strength and fracture toughness parameters, J integral and crack-tip-opening displacement (CTOD) δ after manual metal arc weld to a microalloyed steel plate. The critical J values are obtained by three methods, namely the ASTM Standard E 813-81, the stretched-zone width measurement with superposition on the R curve and the proposed method based on the experimental data. On the contrary the critical δ values are determined by four methods: (i) British Standard 5762; (ii) superposition of the blunting line on the R curve; (iii) stretched-zone width measurement and superposition on the R curve; (iv) the proposed method. A linear relationship between the CTOD and stretched-zone width with a slope of 0.43 is obtained in the present study. The crack growth toughness values are also obtained from the R curves. The crack initiation toughness values obtained by the above methods are compared. The study focuses on the J- δ relationship for the weld metal and the HAZ. Quantitative microstructural analysis and fractographic studies have also been conducted to rationalize the observed toughness values in the weld metal, HAZ and the parent metal.

The solid‐state flow of polymineralic rocks

Polymineralic rocks display three end‐member types of mechanical and microstructural behavior: (1) strong minerals form a load‐bearing framework that contains spaces filled with weaker minerals; (2) two or more minerals with low relative strengths control bulk rheology and form elongate boudins; (3) one very weak mineral governs bulk rheology, while the stronger minerals form clasts. A concept of strain energy partitioning in polyphase aggregates allows the expression of the bulk strength of a rock in terms of its strain rate, temperature, and the volume proportions and rheological properties of its constituent minerals. Bimineralic strength versus composition relations predicted with this model are highly nonlinear and mimic the rheological behavior of experimentally deformed bimineralic aggregates. Quantitative analysis of microstructures in quartzo‐feldspathic tectonites suggests that foliation development leads to a significant reduction in rock strength. This is due to decreased stress concentration within the deforming aggregate as matrix and clast grain size is reduced and the average spacing between strong minerals increases.

Quantitative analysis of cast microstructures

If it is to be incorporated into an analytical model, the microstructure of a cast material must be described in unambiguous, quantitative terms. Three-dimensional microstructures have useful stereological properties which are common to features of any geometry and can be determined from measurements on metallographic sections. This paper features examples for determining microstructural fineness (e.g., dendrite arm spacing), constituent phase coarsening during heat treatment, and distribution analysis of graphite nodule sizes in ductile iron.

On image analysis and micromechanics

We give an illustration of how quantitative morphological analysis of microstructures may help to develop a micromechanical approach to investigate the mechanical behaviour of materials. * In the first part, we recall the main morphological parameters that account for a quantitative description of microstructures from a statistical point of view : correlation functions of order n, moments Q(B). These parameters may be measured on materials by means of image analysis, or may be calculated for certain classes of random models of structures, in order to sum up available data on the microstructure. * When studying mechanical properties, it may be useful to develop specific measurements. This will be illustrated by three examples in the case of fracture mechanics : - a self-consistent approach allowed to relate coke strength to its porous structure ; - spatial relationship between crack paths and underlying mineralogical structure were looked for with appropriate measurements in a study of iron ore sinter degradation during reduction ; - image analysis simulations of crack propagation in porous graphite were developed to explain the role of intergranular macropores on toughness and strength anisotropy.

Microstructure-thermomechanical-property correlations of two-phase and porous materials

The paper basically deals with the aim, to a get a better scientific insight into the effects of microstructure and properties of multi-phase and porous materials and to use the results technologically for tailoring those materials. First the theory of microstructure-property correlations including both, the bound concept and the model concept, is described using conductivity and Youngs modulus of elasticity as property examples. Since in the frame of the theoretical derivation no fitting factors have been permitted to be introduced into the bound equations and constitutive equations the determination of the microstructural factors by quantitative microstructural analysis is demonstrated in the second part of the contribution. By comparing measured and calculated property values for porous ceramics, graphite and metals as well as cermets, metalpolymer and polymer-ceramic-composites the equations are tested for engineering conditions. Finally the dependences of the thermal conductivity and Youngs modulus of elasticity on porosity are used to predict the thermal shock resistance of porous glass and to compare the results with experimental values.

Determination of contrast in automatic quantitative microstructure analysis

For the automatic quantitative measurement of microstructures in general television‐based systems are used. To guarantee a defined accuracy of the measurement, these systems need a minimum contrast between different phases. This contrast is calculated for different numbers of phases and measuring conditions. The result can be used to calculate type and thickness of interference layers to realize these contrasts.

Preparation, characterization and thermal diffusivity of γ-LiAlO 2

γ-LiAlO 2 powder has been prepared by reaction of aqueous LiOH with Al and calcination at 1225 K. Samples of different porosities (10 to 22%) were fabricated by pressing and sintering at temperatures of 1625–1675 K. The sintered samples were characterized by chemical and X-ray analysis. Pore-concentration, -shape, and -orientation were determined by quantitative microstructural analysis. The porosity dëpendance of the thermal diffusivity was investigated. Over the temperature range from room temperature to 1275 K the thermal conductivity follows an equation of the type γ = 1 (A + BT)+CT 3 − D . The discrepancy between earlier published data of the thermal conductivity and results from this work is discussed on the background of different production processes for the γ-LiAlO 2 powder. Furthermore the linear thermal expansion of γ-LiAlO 2 was measured and compared with earlier data. The following equation is recommended for the temperature range 293 < T < 100 K: caseΔl l 0 = 4.317 × 10 −3 + 1.550 × 10 −5T− 6.459 × 10 −9T 2+ 3.889 × 10 −11T 3 .

Quantitative microstructural analysis of reaction-bonded silicon nitride

Quantitative analysis of the essential microstructural parameters of injection-moulded reaction-bonded silicon nitride was performed. The effect of silicon grain size on the porosity, the pore size distribution, the degree of reaction and the proportions of the alpha and beta modifications and the grain size of these phases was studied under constant processing conditions. With increasing silicon grain size the degree of reaction and the total porosity and density remain unaffected. The percentage of beta phase, the grain size of alpha and beta modifications as well as the mean pore diameter increase with increasing grain size of the starting powders. The effect of the structural parameters on the room-temperature strength is discussed.

CHARACTERIZATION OF THE DEGREE OF MIXING IN LIQUID-PHASE SINTERING EXPERIMENTS

Homogeneity of mixing plays an important role in liquid-phase sintering. In order to describe quantitatively the dependence of shrinkage on the degree of mixing, six different tungsten-copper powder mixtures were prepared. These powder blends were either presintered or sintered in the presence of liquid phase. The degree of mixing in both the presintered and liquid-phase sintered samples was determined by quantitative microstructural analysis. The developed method is well able to characterize the different state of mixing in the six powder blends on a microscopic scale. It is shown that a close relation exists between the microscopically characterized degree of mixing and densification during liquid-phase sintering.

The fundamental equation of quantitative microstructural analysis

The complete form of the fundamental equation of quantitative microstructural analysis of multiphase aggregates is formulated and presented. Six new theorems are derived and combined with earlier work by Delesse, Rosiwal and Thompson to form the overall fundamental equation. It is shown that the volume fraction of any constituent phase in a uniform multiphase aggregate with a random-curvature surface is exactly equal to its areal, lineal, or point fractions, measured either on the randomly curved surface of the solid; on its surface-projected normal image; or averaged over planar sections of the multiphase aggregate. It is found that the fundamental equation applies equally well to common solids with geometrical symmetry (polyhedral, spherical and cylindrical solids) provided the phase of interest is of uniform distribution, and that such solids are sampled from the parent matrix at random. The proposed theory can be used for the determination of spacial phase distribution of non-uniform multiphase aggregates. Several applications of the fundamental equation to quantitative fractographic analysis are also discussed with emphasis on plane-strain void fracture in ductile materials.

Relation between Glass Transition Temperature and Composition of Ethylene Propylene Copolymers

Abstract Vulcanizate properties and processability of ethylene propylene copolymers (EPR) are known to be sensitive to variations in polymer microstructure. A basic understanding of the practical performance of EPR requires a knowledge of this microstructure and means for evaluating it. This study was concerned with determining the utility of a previously proposed method for quantitative analysis of microstructure in EPR. The influence of crystallinity effects on the glass transition temperature (Tg) of this polymer was also examined. A major objective of this work was to determine whether, as suggested by literature data, a minimum can exist in the relation between Tg and composition of EPR. If so, it would seriously impair the proposed method for quantitative microstructure analysis. Data relating Tg and composition were obtained by differential thermal analysis (DTA) and infrared analysis (IR) of EPR fractions obtained by gradient elution fractionation. An apparent minimum was observed in the curve defined by this data. Analysis of this minimum suggests that it is due to nonrandom microstructure. In the samples examined, it was evident in the 25 to 40 mole per cent propylene range. Low temperature annealing studies indicate that the minimum is not from crystallinity although there is some indication that small amounts of crystallinity can be induced in these systems by annealing. Analysis of the main Tg interval in a variety of EPR samples showed that three types can be distinguished. These appear to correspond to random, stereoblock, and random-block polymers. Further, a main Tg near −58° C. was observed over the 20 to 74 mole per cent propylene range in some nonrandom systems. Because of this, it is concluded that the proposed system for quantitative microstructure analysis in EPR is generally inapplicable to nonrandom systems. However, the data indicate that useful information about EPR microstructure can be obtained by the techniques described.