Accession Number:

ADA179146

Title:

Transformation Toughening of Ceramics.

Descriptive Note:

Annual rept. no. 1, 1 Sep 85-31 Aug 86,

Corporate Author:

ROCKWELL INTERNATIONAL THOUSAND OAKS CA SCIENCE CENTER

Personal Author(s):

Report Date:

1987-01-01

Pagination or Media Count:

130.0

Abstract:

In situ optical observations during mechanical loading have been used to investigate R-curve behavior in a series for transformation toughened ceramics and to examine characteristics of the tetragonal-to-monoclinic transformation that is responsible for toughening. In high strength and high toughness materials, the strength is dictated by the R-curve and is independent of pre-existing flaws. The results explain observed strength-toughness relations in several zirconia systems peak strength obtained at intermediate toughness and are qualitatively consistent with predictions of fracture mechanics analysis of transformation toughening. Important insight into other microstructural influences of strength and toughness, and into factors that influence initiation and stabilization of the martensitic transformation were also obtained. Mechanisms of grain growth control, critical for fabricating transformation toughened ceramics, in the two-phase region of the Zirconium oxideYttrium oxide system were investigated by measuring phase partitioning and microstructural development during sintering of a metastable precursor with a very uniform initial distribution of Y2O3 additive. From the results it is suggested that partitioning begins with small compositional fluctuations between grains, and that strain energy resulting from lattic parameter mismatch inhibits grain boundary movement and hence grain growth. High strength zirconia fibers were fabricated by drawing from an acetate precursor. The fibers had uniform diameters in the range 1-5 micrometers and, the compositions that exhibited grain growth control grain size or 0.2 micrometers, strengths up to 2.6 GPa were obtained.

Subject Categories:

  • Ceramics, Refractories and Glass

Distribution Statement:

APPROVED FOR PUBLIC RELEASE