Accession Number : ADA325749

Title :   Micromechanisms of Crack Growth in Laminated Intermetallic Composite Microstructures.

Descriptive Note : Final rept. 1 Sep 93-30 Sep 96,

Corporate Author : CALIFORNIA UNIV BERKELEY DEPT OF MATERIALS SCIENCE AND MINERAL ENGINEERING

Personal Author(s) : Bloyer, Don R. ; Venkateswara-Rao, K. T. ; Richie, Robert O.

PDF Url : ADA325749

Report Date : NOV 1996

Pagination or Media Count : 31

Abstract : A brittle intermetallic, Nb3Al, reinforced with a ductile metal, Nb, has been the focus of a study of the resistance curve (R-curve) and cyclic fatigue-crack growth behavior of relatively coarse laminated composites. With the addition of ^50-125 microns thick Nb layers, the toughness of Nb3Al was increased from ^1 MPaVm to well over 20 MPaVm (after several millimeters of stable crack growth), which was attributed to extensive crack bridging and plastic deformation within unbroken metal phase in the crack wake. The thicker layers provided the best crack-growth resistance although the orientation of these layers, i.e., crack arrester vs. crack divider, did not have a significant effect on toughness properties. Cyclic fatigue-crack growth resistance was also found to be superior in the laminate microstructures compared to monolithic Nb3Al and Nb-particulate reinforced Nb3Al composites. Unlike R-curve behavior, however, both layer thickness and orientation significantly affected cyclic crack-growth rates with the crack-arrester orientations displaying the best properties (superior even to monolithic niobium). In general, the optimal combination of fracture toughness and fatigue resistance was seen in the laminates with the thicker Nb layers aligned in the crack arrester orientation.

Descriptors :   *PLASTIC DEFORMATION, *MICROSTRUCTURE, *COMPOSITE MATERIALS, *CRACK PROPAGATION, *NIOBIUM, METALS, THICKNESS, LAMINATES, RESISTANCE, TOUGHNESS, FRACTURE(MECHANICS), WAKE, FATIGUE(MECHANICS), DUCTILITY, MONOLITHIC STRUCTURES(ELECTRONICS), BRIDGES, ALUMINATES, CYCLIC RATE.

Subject Categories : Laminates and Composite Materials
      Fabrication Metallurgy
      Mechanics

Distribution Statement : APPROVED FOR PUBLIC RELEASE