TITLE: Fracture of TiB₂-AlN Composite in Various Environments

Acta Metallurgica 37 [9] 2313-2320, 1989.

By W.A. Zdaniewski

Abstract - Fracture mechanics and quantitative fractography were used to characterize fracture origins and analyze the fracture mechanism in a TiB₂ -AlN composite. During fracturing in air at room temperature, the composite did not show typically brittle behavior. Thermoelastic mismatch between the unlike phases caused microcracking, and crack branching (mirror formation) during fracture was not observed. A rising resistance curve is presumably operating in the composite. A stress-induced microcrack toughening mechanism was identified which induced the gratest toughness at cryogenic temperatures. As temperature increased, thermoelastic mismatch and fracture toughness decreased until at 1071 K the composite showed typically brittle behaviour accompanied by crack branching.

TITLE: Stereoscopic Fractography of Crack Propagation Phenomena in a TiB₂-AlN Composite

J. Amer. Ceram. Soc. 72 [1] 116-122, 1989.

By W.A. Zdaniewski

Microstructural defects identified in a TiB₂-AlN composite were poorly bonded agglomerates, foreign inclusions and highly strained regions. There were also surface flaws. Fracture surfaces produced in air were highly irregular, suggesting that microcracking cused the composite to behave differently from classical brittle ceramics. Fracture at elevated temperature in Ar or in liquid Al produced the only crack branching observed. No fractographic evidence of slow crack growth in liquid Al was found. Fractographic observations supported a proposed observation of the R-curve behaviour which involves grain bridging across the crack.

TITLE: Degradation of Hot Pressed TiB₂-TiC Composite in Liquid Aluminium

Amer. Ceram. Soc. Bull. 65 [10] 1408-1414, 1986

By W.A. Zdaniewski

Chemical degradation processes were studied in a hot-pressed polycrystalline TiB₂-TiC composite exposed to liquid aluminium at 970º C. The TiB₂ underwent liquid metal embrittlement, while the particulate TiC phase, the grain boundary phase, and Widmanstaetten precipitates reacted with liquid Al and formed Al₄C₃ which is detrimental to the mechanical integrity of the composite. Initially, penetration by liquid Al and chemical reaction with the carbide phase can lead to crack blunting rather than to crack extension. Longer exposure times (>12 h) cause localized swelling at the grain boundaries and interfaces, and result in crack extension. At this stage intergranullar cracking of the TiB₂ predominated over crack blunting. During strength testing, large microstructural defects controlled the strength values and masked the slow crack growth behaviour.

TITLE: Effect of Segregated Cr on Degradation of (Ti, Cr)B₂ Exposed to Liquid Aluminium

J. Electrochem. Soc., 1333 [9] 1777-1781, 1986.  

By W.A. Zdaniewski

A gradual increase in concentration of dissolved Cr was found near the surface of a sintered (Ti, Cr)B₂ electrode. The resulting decrease in the unit cell volume of the Cr-enriched solid solution caused tensile stresses at at the surface of the electrodewhich produced microcracking and thus opened channels for liquid Al to penetrate. Exxagerated grain growth of (Ti, Cr)B₂ was observed near the surface, and there was a subjacent increase in porosity in a narrow zone towards the Cr-depleted interior of the electrode. The effect of stress and microstructural variations on the structural integrityof polycrystalline (Ti, Cr)B₂ exposed to liquid Al environment at 970º C is discussed.
Visit The Electrochemical Society  

TITLE: Role of Microstructure and Intergranular Phases in Stress Corrosion of TiB₂ Exposed to Liquid Aluminium

J. Amer. Ceram. Soc. 68 [11] C309-312, 1985.

By W.A. Zdaniewski

The resistance of several polycrystalline TiB₂ materials to penetration by liquid aluminium at 970º C was investigated, and their microstructures were characterized. The grain-boundary properties of individual diborides rather than the intrinsic properties of TiB₂ are thought to control stress corrosion susceptibility in liquid metal environments.

TITLE: Gamma Radiation Effects on the Strength of a Borosilicate Glass

J. Amer. Ceram. Soc. 66 [5] 311-313, 1983.

By W.A. Zdaniewski, T. Easler and R.C. Bradt.

The effects of ⁶⁰Co gamma radiation on the strength-related mechanical properties of a borosilicate glass were examined. Although the glass darkened cosiderably, only a very slight densification was observed after irradiation to levels of 10⁸ rads. The strength distributions were not appreciably changed by the irradiation, nor was the calculated slow crack growth parameter (N value). Neither did radiation affect the elastic modulus or the fracture toughness of the glass. Gamma radiation does not affect the strength below 10⁸ rads.

TITLE: Effect of Alumina Dispersions on the Thermal Conductivity/Diffusivity and Thermal Stress Resistance of a Borosilicate Glass

J. Mater. Sci. 15, 518-519, 1980.

By D.P.H. Hasselman, W.A. Zdaniewski, J. Swearengen and E. Beauchamp.

Incorporation of a high conductivity crystalline phase (0-35%) in the glass can improve resistance to the initiation and catasrophic nature of thermal fracture in glass. Thermal diffusivity and conductivity were measured from room temperature to 600º C by a laser-flesh technique. The results are interpreted in terms of thermal stress resistance parameters.

TITLE: Effect of Oxidation on the Thermal Diffusivity of Reaction Sintered Silicon Nitride

Amer. Ceram. Soc. Bull. 58 [5] 539-540, 1979.

By D.P.H. Hasselman, W.A. Zdaniewski, H. Knoch and H. Heindrich.

Reaction-bonded silicon nitride was oxidized isothermally in air at T ranging from 300º to 1400º C for a period of 50 hours and the thermal conductivity and diffusivity were measured as a function of oxidation T. A decrease of the conductivity after oxidation in the temperature range of 1100º - 1200º C was observed and it is expected that associated decrease in thermal stress resistance will occur. Therefore, oxidation of reaction-bonded silicon nitride should be kept to a minimum. High density materials can be produced by improved nitridation techniques and post treatment.

TITLE: Thermal Stress Resistance Parameters for Brittle Materials Subjected to Thermal Stress Fatigue

J. Amer. Ceram. Soc. 61 [7-8] 375, 1978.

By D.P.H. Hasselman and W.A. Zdaniewski.