AFRL scientists have significantly advanced the understanding of a phenomenon called deformation twinning, a major materials deformation mechanism that is particularly important at low temperatures and high strain rates. Working with industry, laboratory researchers successfully identified five deformation twin modes in monazite, a complex mineral with low symmetry. They were able to explain the existence of these modes using fundamental principles that should ultimately prove useful for the prediction of deformation twinning in more complex systems. These studies help scientists obtain the knowledge required to create better tools for analyzing the composition and application potential of minerals and other natural materials essential both to the development of national defense systems and to the research and development of dynamic new commercial products.
Twinning, a natural phenomenon in crystal alignment, originates in one of three ways:
- growth twinning,
- transformation twinning, or
- deformation twinning.
Growth twinning is a result of an accident that occurs during crystal growth, wherein a new crystal forms on the face of an existing one. Transformation twinning is a strain accommodation mechanism associated with phase transformations induced by pressure or temperature. Deformation twinning is also a strain accommodation mechanism; it is specific to conditions of applied stress (see figure on next page). While deformation twinning is a common plasticity mechanism in body centered cubic metals, the phenomenon is less well understood in more complex materials despite its prevalence at low temperatures and high strain rates.
AFRL scientists worked with Rockwell Scientific Company (Thousand Oaks, California) to deform polycrystalline monazite at room temperature using a spherical indenter. Using transmission electron microscopy in 70 monazite grains, the team successfully identified five deformation twin modes on several planes. Plane (100) was by far the most common, planes (001) and (120) were less common, and plane (122) was rare; the team further identified kinks in the (120) twins as irrational (483) twin planes.
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 .
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 fiber reinforced 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.
Air Force Research Laboratory Technology Horizons Magazine
This article first appeared in the April, 2006 issue of Air Force Research Laboratory Technology Horizons Magazine.
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