The scientists identified the twinning modes on these planes using the expression of twinning shear at free surfaces, predictions of classical deformation twinning theory, and various considerations of twin morphology and crystal structure. To analyze the twin modes, the team used atomic shuffle calculations that allow formation of either a glide plane or a mirror plane at the twin interface. All five of the twin modes exhibited small atomic shuffles. For (001) twins, the team obtained the smallest shuffles using a glide plane at the interface with a displacement vector R = 1/2 [010].

The research results do not uniquely define a twin mode on plane (100), leaving open the possibility that more than one mode operates on this plane. Crystal structure considerations suggest that the relative abundance of twinning modes may correlate with low shear modulus on the twin plane, in the direction of twinning shear and with a possible low-energy interface structure consisting of a xenotime layer (one-half the thickness of a unit cell) that could form at the (100) and (001) twins.

Naturally occurring monazite is typically a reddish brown mineral. It contains the rare earth elements cerium, lanthanum, and neodymium, as well as the radioactive element thorium. Scientists use monazite’s rare earth elements in high-performance magnets; as pigment in ceramics; and in robot motors, X-ray screens, fiber optics, energy-efficient lanthanum lamps, and color television tubes.

They also use monazite in structural ceramics that rely on its unusual combination of properties, including high-temperature stability (melting point 2072°C), compatibility with common structural oxide ceramics, relatively low hardness, and weak bonding with other oxide ceramics. Monazite’s low hardness and weak bonding are especially important both for machined ceramics, enabling material removal, and for fiberreinforced composites, enabling crack deflection and fiber pullout without the oxidation problem prevalent in more commonly used interfaces.

Dr. Randall S. Hay and Dr. Peter S. Meltzer (Anteon Corporation), of the Air Force Research Laboratory’s Materials and Manufacturing Directorate, and Dr. David B. Marshall, of Rockwell Scientific Company, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document ML-H-05-39.