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Ceramic films and interfaces: Chemical and mechanical properties

Description: In this progress report we begin by highlighting our work on the enhancement of interfacial cohesion in alumina by small additions of transition metals. A possible mechanism by which grain boundaries in ceramic can be strengthened is proposed towards the end of this report and is further elaborated in the continuation proposal. The results support the view that the addition of zirconia lowers the interfacial energy in alumina. The lower interface energy has the effect of (1) increasing interfacial cohesion, (2) increasing the activation energy for diffusional transport required for superplastic deformation and sintering, and (3) delaying nucleation in the crystallization of an amorphous alloy of alumina and zirconia.
Date: December 1, 1991
Creator: Raj, R.
Partner: UNT Libraries Government Documents Department

Ceramic films and interfaces: Chemical and mechanical properties. Progress report

Description: In this progress report we begin by highlighting our work on the enhancement of interfacial cohesion in alumina by small additions of transition metals. A possible mechanism by which grain boundaries in ceramic can be strengthened is proposed towards the end of this report and is further elaborated in the continuation proposal. The results support the view that the addition of zirconia lowers the interfacial energy in alumina. The lower interface energy has the effect of (1) increasing interfacial cohesion, (2) increasing the activation energy for diffusional transport required for superplastic deformation and sintering, and (3) delaying nucleation in the crystallization of an amorphous alloy of alumina and zirconia.
Date: December 1, 1991
Creator: Raj, R.
Partner: UNT Libraries Government Documents Department

Creep failure analysis for ceramic composites containing viscous interfaces

Description: This paper describes an experimental and theoretical study of the creep fracture of advanced ceramic composites under steady axial tension. Such composites consist of a high fraction of elongated ceramic grains, varying substantially in aspect ratio and embedded in a glassy matrix phase. For creep testing, a model test system was prepared, which consisted of well-aligned elongated mica platelets ({approximately} 60 vol%) and residual glass phase ({approximately} 40 vol%) in its final heat-treatment stage. The creep curves of several specimens under various applied loads and at a temperature (800 C) higher than the T{sub g} of the glass matrix ({approximately} 650 C) were obtained up to creep fracture. Micrographs of the creep fracture surfaces revealed substantial grain pull-out and cavitation in the matrix phase with virtually no transgranular fracture. The objective of this work is to simulate the creep response and fracture based on the accumulation of localized void growth and microstructural parameters, using a computational mechanics technique, called viscous break interaction (VBI), developed to compute stress fields around strongly interacting fractures or voids in composites with fibrous microstructures. To simulate the creep process up to fracture, a Monte Carlo model is developed which couples VBI with a statistical description of grain length. Both the experimental and simulation results show that random lengths and random overlap of the aligned grains naturally lead to (i) local and microstructure-sensitive damage evolution up to ultimate failure and (ii) substantial variation in failure times of seemingly identical specimens.
Date: September 1, 1998
Creator: Beyerlein, I. J.; An, L. & Raj, R.
Partner: UNT Libraries Government Documents Department

Metal-ceramic composites for hostile environment applications

Description: The authors have developed a new metal-ceramic composite made from vanadium metal (V) and a non-stoichiometric magnesio-aluminate spinel ceramic. Three vanadium-spinel compositions, 40-60, 50-50, and 60-40 (by volume) were prepared by hot pressing mixtures of commercial powders. The properties of these composites were determined by measuring coefficient of thermal expansion, hardness, elastic constants, and fracture toughness. Radiation damage studies were performed on 50-50 vanadium-spinel composite samples using 1.5 MeV Xe{sup +} ions, with samples held both at 20 K and at room temperature. Room temperature irradiated samples exhibited very little change in microstructure, indicating that this composite has radiation damage resistance qualities such as resistance to volume swelling under particle bombardment. This feature, as well as other properties reviewed in this paper, suggest that vanadium-spinel composites are attractive structural materials for fusion reactor design.
Date: July 1, 1996
Creator: Uestuendag, E.; Sickafus, K.; He, Y.; Schwarz, R.B.; Panda, P.C. & Raj, R.
Partner: UNT Libraries Government Documents Department