Tensile creep and cavitation of monolithic and silicon carbide whisker-reinforced alumina composite produced by slip casting

Weijun Liu, Purdue University

Abstract

High density and purity tensile creep specimens were produced by slip casting and vacuum sintering or sinter/HIPing pure alumina and 6 vol% SiC whisker-reinforced alumina. The whiskers reduced cracking due to drying and handling of the green bodies. The whiskers were found to have a preferred orientation along the tensile axis and a higher apparent number density near the surface. High-temperature creep and cavitation of these materials were studied under constant tensile stresses of 4 to 33 MPa at 1400$\sp\circ$C. The creep exhibited an extended primary transient, reached a minimum and rapidly accelerated to failure. The primary creep is largely attributed to concurrent grain growth. The activation energies from temperature cycling were 470, 540 and 510 kJ/mol for the sintered, HIPed aluminas and composite, respectively. Stress exponents for the sintered and HIPed alumina were 2.2 and 2.4 using minimum creep rate. For the composite, the stress exponent was the same as the monolithic aluminas at low stresses, but became more stress sensitive above 20 MPa. It is argued that the use of minimum creep rates may exaggerate the stress dependence. The creep test results suggest that the creep of all these materials was controlled by the lattice diffusion of aluminum in a Nabarro-Herring mechanism. Almost two orders of magnitude reduction in creep rate and one order of magnitude increase in failure time were found in the composite. The reduced creep rate and longer failure time are attributed to load bearing by the partially aligned whiskers. SEM, TEM and HVEM studies indicated that grain boundary cavitation was the dominant creep damage mechanism in the monoliths, whereas the intersection between the whiskers and alumina grain boundaries were the preferred cavity sites in the composite. Very few subcritical cracks and no dislocations were observed. The float/sink density measurements showed that the cavitation strain contributed 17, 11 and 50% to the failure strain in the sintered, HIPed and composite materials, respectively. The composites were able to "tolerate" significantly greater cavitational damage, especially if their higher initial porosity is considered. All three materials followed the Monkman-Grant relation, but with different M-G constants.

Degree

Ph.D.

Advisors

Solomon, Purdue University.

Subject Area

Materials science|Mechanical engineering|Metallurgy

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