Materials Science Forum Vols. 492-493

Abstract: The correlation between hardness, H, and yield strength, Y, for hard materials is reexamined using an analytical approach to provide a physical interpretation, which explains the trends observed. Existing analytical predictions using the analogy of the spherical cavity fail to reproduce experimental and finite element results because the accommodation of the intrusion of the indenter due to surface deformation is not taken into account, while experiments made here and elsewhere indicate that the latter can be substantial. A simple modification is proposed, which accounts for this, and allows for a reasonable prediction of the relation between hardness and yield strength for materials, which have a well determined yield point and do not strain harden. The treatment also allows explaining the effect of indenter geometry on measured hardness for such materials.

Abstract: Most important properties which are usually measured for refractories are work temperature and thermal stability. Thermal stability of the alumina based samples was measured using a standard laboratory procedure, the water quench test ( JUS.B.D.8.319.). ImagePro Plus Program was used for image analysis of microphotographs of the samples before, during and after water quench test. Changes at the surface before, and after cycling were given. Mechanical characteristics were considered such as compressive strength, and dynamic modulus of elasticity. It was measured by resonance frequency measurements, as well as ultrasonic velocity. In this work the correlation between microstructure, ulatrasonic velosity and strength on thermal stability of the sample were investigated. The obtained results were used for validation of the model to predict the thermal stability of the refractory specimen.

Abstract: The propagation of cracks in graded materials under monotonic and cyclic loading was investigated via experiment and simulation. Graded alumina/epoxy composite specimens exhibiting a variation in composition from 5% to 65% epoxy, representing a twenty-fold variation in Young’s modulus, across a region of width between 6 and 20 mm, were produced by a multistep infiltration technique. Crack initiation and propagation under monotonic and cyclic four-point bend loading was monitored and crack trajectories and growth rates were measured. Initial crack deflection was observed, in agreement with theoretical and computational predictions in the literature. Cracks exhibited further deviation as they traversed the graded region. Higher deflection angles were observed for specimens with steeper gradients, and for those with cracks initially located closer to the compliant side of the gradient. Homogeneous specimens in the composition range 5% to 55% epoxy were also produced to investigate the composition dependence of mechanical, fracture and fatigue properties for aluminaepoxy composites. Crack propagation resistance appeared to differ between monotonic and cyclic loading, though an increase with crack extension was observed in both cases. The significant variation in measured crack-propagation resistance, for cracks in graded specimens, was accordingly interpreted as a combination of crack-extension effect and spatial variation of both intrinsic and extrinsic crack-growth resistance. A finite element model has been developed to simulate the propagation process, with particular attention paid to crack propagation and deflection criteria. Results from homogeneous specimens were utilised for estimating spatial property distribution and crack-extension effects in the graded specimens. Experimental results for crack path and crack-growth resistance profile show good agreement with modeling predictions.

Abstract: In this work the strength of ceramic laminates is analyzed. Two different architectures with the same surface are studied. One of them, a Al2O3 / Zr(3Y)O2 laminate, contains compressive residual stresses, the other one, a monolithic Al2O3 laminate, has no residual stresses. Residual stresses are estimated analytically on the basis of the materials properties and by an indentation technique. The influence of the residual stress on the strength distribution is investigated. Strength distributions for laminates with compressive stresses at the surface follow a Weibull distribution only under certain conditions.

Abstract: The FGM principle plays an important role in enhancing the efficiency of thermoelectric devices. While a thermoelectric generator (TEG) is typically operating in a large temperature difference, attractive conversion efficiency of a particular semiconductor is restricted to a small temperature range. Hence, when employing a semiconductor with its highest possible efficiency at the respective temperature in the internal temperature field along a stacked TEG, the overall conversion efficiency of the device may be considerably enhanced. Similarly, the FGM principle can be employed for linearization of thermal sensors. The output voltage (response) of the sensor is proportional to the Seebeck coefficient of the material the sensor is made of. Since the Seebeck coefficient is strongly temperature-dependent, the sensor response is not linear with temperature. However, combining in a stack two or more semiconductors which temperature dependence of the Seebeck coefficient are complementary to each other, results in a sensor with linear response (i.e. its output is proportional to the temperature difference, or heat flux, respectively.) Stacking of several materials to each other or grading a semiconducting sample requires a technique which can scan the Seebeck coefficient profiles S(x) along the stack. Accordingly a Seebeck micro-probe technique has been developed for scanning the surface of a sample monitoring S with a resolution of down to 10 µm within the temperature range from -15°C to 60°C. An additional option of such a device is the scanning of the electrical potential along the stack under current flow [1]. Thus, related experimental data on the local profiles of the electrical conductivity and Seebeck coefficient along the stack (or continuously graded FGM) will be available. The apparatus has been automated so that extended areas may be scanned providing two-dimensional images. Additionally, several samples can be scanned in one automatic run.

Abstract: The present paper investigates the creep phenomenon of the functionally graded materials under high temperature environment by the computational micromechanical method (CMM). Based on the real microstructure of the functionally graded interlayer with different component volume fractions, the emulation experiment is implemented for the creep test numerically and the creep parameters are obtained. A further series of simulation works are carried out to investigate the creep phenomenon of FGM interlayers in more detail. Numerical results show that the creep phenomenon is obvious not only for the metal-rich interlayers but also for the ceramic-rich interlayers. The creep property of ceramic/metal interlayer depends on the material’s properties of the ceramic obviously. It is remarkable that the creep strain rate of the ceramic/metal interlayer is larger than the corresponding one of pure metal under the same load when the modulus of the ceramic component is lower than the one of the metal component.

Abstract: In the present work, TZ-3Y20A/Mo multilayer composites were prepared by sedimentation method. Appropriate settling parameters were chosen and green bodies of multilayer composites were obtained by settling TZ-3Y20A powder and Mo powder in turn according to the designed individual layer thickness. Then green laminates were hot pressed at 1773K for an hour under the pressure of 20MPa. Three-point Bending and Single Edge Notched Beam (SENB) tests were adopted to evaluate the flexure strength and fracture toughness of the samples. The results showed that, compared to TZ-3Y20A ceramics, the fracture toughness and the work of fracture increased due to the layered structure. The microstructure and crack deflection of the fracture surface of the laminated composites was also observed by optical microscopy.

Abstract: Al-Al3Zr and Al-Al3Ti functionally graded materials (FGMs) were produced by a centrifugal method from Al-5wt% Zr and Al-5wt% Ti alloys, respectively. Applied centrifugal forces were 30, 60 and 120G (units of gravity). Microstructural characterization was performed to evaluate the intermetallic particles’ distribution and orientation. Knoop hardness tests were carried out, with the indenter’s long diameter normal to the centrifugal force direction. Both the Al3Zr and the Al3Ti intermetallic particles are platelet in morphology. These platelets tend to be oriented normal to the centrifugal force direction. Higher applied centrifugal force increases both the intermetallic platelet volume fraction as well as their orientation in the outer regions of the fabricated FGM rings. Also higher orientation and volume fraction distribution are observed in the Al- Al3Ti FGMs. Knoop hardness measurements in general follow the same trend as the intermetallic particle volume fraction for each sample.