Elastic behavior and strength of Al2O3 fiber/Al matrix composite and implications for equation of state measurements in the diamond anvil cell

Abstract

To examine pressure relationships in a mixed phase assemblage, we performed room temperature/high pressure radial x-ray diffraction measurements on a controlled-geometry bimaterial composite consisting of oriented Al 2O3 fibers embedded in an aluminum matrix. Lattice strains of each material were measured as a function of orientation with respect to the fiber alignment, as a function of orientation with respect to the major principal stress axis of the diamond cell, and as a function of pressure of up to 15 GPa. The results show that Al and Al2O 3 both support differential stresses, with Al supporting between -0.06(45) and 0.32(65) GPa and Al2O3 supporting between 1.4(3) and 4.9(9) GPa. The hydrostatic pressures determined from the average lattice strains of Al and Al2O3 are not in general equal, with the pressure of Al2O3 higher than that of Al by an average of 0.5(4) GPa throughout the measured range. The geometric relationship between the composite material and the principal stress axis of the diamond cell plays a role in establishing both the absolute and relative strain responses of the composite sample. A comparison of the two composite geometries under the same diamond cell compression shows that when the fibers are oriented vertically along the diamond cell axis, the differential stress supported by Al2O3 is 3.1(5) GPa, at a pressure of 9.35(42) GPa. The corresponding values for Al are much lower: 0.09(18) GPa (differential stress) and 8.67(04) GPa (hydrostatic pressure). When the fibers are oriented horizontally along the radial direction, the pressure supported by Al and Al2O3 is more similar: 9.63(15) vs 9.48(35) GPa. The differential stress supported by both materials is higher: 0.32(65) for Al and 4.9(9) for Al2O3. Understanding the strength and elastic behavior of an intermixed phase assemblage is vital for the interpretation of mineral behavior at high pressures and temperatures. Many in situ measurements of high pressure mineral phase stability and elasticity are performed using intermixed phases - the unknown and a reference marker. Measurement of properties relies on the assumptions that the reference material has an accurate and well-calibrated equation of state and that the pressures of the two materials are identical in the high pressure sample chamber. This latter assumption is clearly violated in our experiments. Therefore, it is important to account for potential pressure effects due to sample geometry when making in situ x-ray measurements of equations of state and phase transformations. © 2006 American Institute of Physics.