Research in monazite deformation twinning benefits science and defense.

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: (1) growth twinning, (2) transformation twinning, or (3) 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 bodycentered 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.