V. J. Tennery

Structure-Property Correlations for TiB2-Based Ceramics Densified using Active Liquid Metals

The compound TiB2 has numerous exceptional properties including high hardness, high melting temperature, high electrical conductivity, and nonreactivity with various liquid metals. These make it an attractive candidate for technological applications, such as cutting tools, valve trim for erosive environments, and cathodes in Hall-Heroult cells for aluminum smelting. In general, such applications require fabrication of TiB2 into various shapes, and the latter requirement dictates the production of high-density polycrystalline ceramic bodies. The preparation and characterization of TiB2-based materials have already been the subject of numerous previous works, exemplified by the references (1–9). These include data on the phase equilibria in TiB2 containing systems,1,3,9 the wettability of TiB2 by various metals,1,3,4 and the results of property determinations on TiB2-based specimens.

Fracture strength characterization of tubular ceramic materials using a simple c-ring geometry

The potential application of the c-ring compression specimen for strength determinations involving tubular ceramic samples was investigated. Mathematical descriptions of the stress distributions generated within the c-ring were derived using several approaches. The resulting equations were used in conjunction with a Weibull analysis to predict a relationship between failure probability and fracture strength. Finally, the validity of the c-ring test was examined for a SiC ceramic by comparing the c-ring strength data with that obtained from four-point bend samples.

Growth and properties of Ni20.3Ti2.7B6 (τ-phase) crystals☆

Single crystals of Ni20.3Ti2.7B6 1 cm in diameter were grown from the melt by the Czochralski method at 1200–1215°C. Growth parameters and crystal properties are characterized.

Dynamic fatigue behavior of two SiC and a SiCp reinforced Al2O3 at elevated temperatures

Abstract Dynamic fatigue behavior of a siliconized SiC, a sintered β-SiC, and a SiCp (silicon carbide particle)-reinforced Al 2 O 3 formed by directed oxidation of Al metal, which are considered for use in heat exchangers in coal-fired power plants, were evaluated at 1100 and 1400 °C in air. Four-point flexure specimens were tested at five stressing rates from 37 MPa s −1 to 0.0001 MPa s −1 resulting in total times to failure up to 1200 h. Thirty specimens of each material were tested at the fast-fracture condition and 10 specimens were tested at the four dynamic fatigue conditions at each temperature. At 1100 °C none of the materials exhibited any loss of strength as a function of stressing rate and very little tendency to creep was observed. At 1400 °C the sintered β-SiC exhibited no strength loss, while the siliconized SiC showed a significant loss of strength and some signs of creep at stressing rates less than 0.01 MPa s −1 . The SiCp reinforced Al 2 O 3 exhibited extensive creep at stressing rates ranging from 0.01 to 0.0001 MPa s −1 at 1400 °C, in fact at the slower stressing rates the creep was dominant and the specimens could not be brought to fracture in the four point flexure fixtures. Extensive fractography showed that the failure mode for the sintered β-SiC was indeed a fast-fracture mode at all temperatures and stressing rates, the specimens mostly failing from pores in the microstructure. The siliconized SiC failed partly from pores and partly from metal inclusions at 1100 °C and fast stressing rates, and at 1400 °C at slower stressing rates slow crack growth was observed to occur with the Si-metal inclusions as starting points. The failure modes in SiCp reinforcedAl 2 O 3 changed from fast fracture from residual Al-alloy rich areas to a creep failure at intermediate stressing rates at 1400 °C.

Application of electron microscopy for the analysis of the mechanical behavior of advanced ceramics)

The microstructure and fracture surfaces of two commercially available silicon nitride ceramics have been characterized using techniques of scanning and analytical transmission electron microscopy. These results have been correlated to mechanical property data (obtained from static and dynamic fatigue tests using four-point flexural test methods both at room temperature and at elevated temperature). For one of the materials, the results clarified failure mechanisms. For the other, the results showed how processing variables can affect microstructure (and thus mechanical behavior), and emphasized particularly that mechanical tests of billet-derived specimens may be unreliable in predicting the mechanical behavior of production components.Copyright