Ivo Dlouhy

Preparation, microstructure and mechanical properties of metal-particulate/glass-matrix composites

Abstract Composites with a borosilicate glass matrix containing different concentrations of vanadium particles were fabricated by powder metallurgy and hot-pressing. The mechanical properties and fracture behaviour of the composites were assessed by a range of techniques. Youngs modulus, fracture strength in bending, and fracture toughness increased with vanadium content. By virtue of the good interfacial bonding and low residual internal stresses, an effective crack-particle interaction during fracture was achieved. The fracture toughness of composites containing 30 vol. % of vanadium inclusions was approximately 65 % higher than that of the unreinforced glass. Experimental values for the fracture toughness increment were in good qualitative agreement with the predictions of theoretical models in the literature. Extensive plastic deformation of the vanadium inclusions was not found, however. This was attributed mainly to the constraint imposed by the rigid matrix surrounding the particles and to possible embrittlement of the particles during composite fabrication at high temperatures. The brittleness index (B) of the composites was calculated and its relevance for characterisation of the ductile versus brittle behaviour of brittle-matrix composites is discussed.

Determining the fracture resistance of fibre-reinforced glass matrix composites by means of the chevron-notch flexural technique

Abstract Results on fracture toughness determination on SiC (Nicalon®) fibre-reinforced borosilicate glass matrix composites after thermal cycling and thermal shock are presented. The thermal shock tests involved quenching the samples from high temperatures (600–650°C) in a water bath at room temperature. For the thermal cycling tests, the samples were alternated between a furnace at high temperature (700°C) and room temperature for up to 1000 cycles in air. Fracture toughness and work of fracture were measured using the chevron-notched specimen technique. Supported by fracture surface observations, the results were used to assess the microstructural damage in the material after thermal loading. To detect the onset of unstable microcracking during the chevron-notch experiments, an acoustic emission technique was used. The fracture toughness values measured were in the range 18–26 MPam 1/2 , in agreement with literature reports, and they were little affected under the thermal shock and thermal cycling conditions investigated (for low number of cycles). The present results were shown to be in agreement with data of previous studies in which other techniques (Youngs modulus and internal friction determination, fibre push-out test) were used to assess damage development under similar thermal loading conditions. The overall finding is that, for these conditions, no major degradation of the fibre–matrix interfaces occurred, and therefore the material retains its apparent fracture toughness and flaw tolerant behaviour. However, for a high number of thermal cycles in air (>800), severe microstructural damage occurred in the form of porosity development and interfacial oxidation. Under these conditions, the material showed also macroscopic delamination, which made impossible the use of the chevron-notch test.

Service life prediction for refractory materials

Ultrasonic pulse velocity testing and image analysis were used to predict the thermal stability of cordierite–mullite refractories. Two compositions used as substrates in fast firing of porcelain whiteware, characterized by different microstructure and crack propagation behavior, were investigated. Fracture strength and fracture toughness values were obtained from three point bending test and chevron notched specimen technique, respectively. The measurement of the ultrasonic velocity was used to assess the material degradation with increasing number of thermal-shock cycles and specimen damage was monitored using image analysis to obtain further evidence of material degradation. The correlation between thermo-mechanical properties, ultrasonic velocity, microstructure, crack-propagation behavior and thermal-shock resistance was discussed. A remarkable similarity was found between the variation of ultrasonic velocity (when measured through the length of the refractory plates) and fracture strength with number of thermal shock cycles. On the other hand, the development of surface microcracking, as monitored by image analysis, is in good agreement with the variation of KIC with the number of thermal-shock cycles. The variation of the

Thermal shock behaviour of mullite–cordierite refractory materials

\frac{d\sigma_{\rm f}}{dE_{\rm dyn}}

Crack resistance curve in glass matrix composite reinforced by long Nicalon® fibres

ratio with number of thermal-shock cycles shows the highest gradient of the investigated trends and it is proposed as a promising parameter to differentiate refractory materials regarding their different thermal shock behavior. Service life prediction models for refractory plates, from measured values of ultrasonic velocity and surface damage analysis, were proposed and validated.

Influence of Cold-Sprayed, Warm-Sprayed and Plasma Sprayed Layers Deposition on Fatigue Properties of Steel Specimens

Abstract The thermal aging in argon of a commercially available SiC-fiber reinforced glass matrix composite was investigated at temperatures in the range 500–700 °C for exposure duration of up to 1000 h. An inert atmosphere was used to study the effects of temperature alone, thus minimizing and neglecting the effects of oxidation. The mechanical properties of aged samples were evaluated at room-temperature by using four-point flexure strength and three-point flexure chevron-notch techniques. The interfacial properties were determined by push-out indentation measurements using an in-situ SEM indentation apparatus. The fracture toughness values determined by the chevron-notch tests were little affected by the aging conditions and were in the range 19–26 MPa√m . The frictional interfacial shear stress was not affected by the aging conditions either. For the most severe aging conditions investigated (1000 h at 600 °C and 100 h at 700 °C), a significant loss of flexure strength and stiffness of the samples was detected, which has been ascribed to microstructural changes that occurred in the material during aging as a consequence of the softening of the glass matrix. At these aging conditions, a lower interfacial shear stress for fiber-matrix debonding initiation was measured, which may be explained also by the occurrence of matrix softening and void formation.

Micromechanical Aspects of Transgranular and Intergranular Failure Competition

Abstract The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of K Ic measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.

Microstructural control of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si alloy by heat treatment

Sintering of light aluminium alloys powder has been investigated as a way to substitute steels in automotive and aerospace industries. Premix Al-5.5Zn-2.5Mg-0.5Cu composite powder called Alumix 431D was analyzed in this research. Sintering was carried out under ultra high purity nitrogen gas and before reaching sintering temperature, green samples were delubricated at 400°C for 30 min. The powder possesses high sinterability by reaching 96% relative density at 580°C sintering temperature. Formation of liquid phase seems to support achieving high sintering density. Optimum mechanical properties also were obtained under those conditions. T6 heat treatment was done to improve the mechanical properties by formation of precipitation strengthening, and MgZn2 appears to be dominant strengthening precipitate. X-ray diffraction, optical microscopy, and SEM-EDS were used to characterize powder, and sintered and heat treated samples.