Structure and Phase Transformations of Nanophases Embedded in Solids

Funded by the DOE Office of Basic Energy Sciences

Ulrich Dahmen, Velimir Radmilovic Co-Principal Investigator

This program is aimed at understanding the fundamental features that underlie the behavior of nanoscale phases embedded in a solid matrix and their role in the evolution of microstructure in materials. Because of the scale and nature of such microstructures, electron microscopy is an integral part of these investigations - as an analytical tool as well as a subject of technique development. The goal is to understand and ultimately gain control of the structure, distribution and shape of nanophase inclusions by establishing the basic relationship between crystallographic variables and microstructural features. Such relationships are put to use both analytically, to examine the structure of particles and defects, and synthetically, to produce new and unique microstructures with defect configurations reflecting composite symmetries. The ability to observe the kinetics and mechanisms of phase transformations directly by electron microscopy and to correlate the effects of size and shape with measurable local properties is a key element of this research. The fundamental principles established using model alloy systems are employed in the design and testing of new materials such as Al-based alloys of interest for energy-related technologies.


CURRENT PROJECTS

Core-shell precipitate structures

After developing core-shell precipitates in Al alloys, with a Zr-rich shell surrounding a Sc-rich core, we have recently demonstrated a new path to producing monodisperse core-shell inclusions in a solid matrix using solid-state reactions. A uniform distribution of particles with a Li-rich shell a few nanometers thick around a Sc-rich core can be made reproducibly by exploiting kinetic and thermodynamic parameters. This approach can be applied to range of alloys and could lead to a new class of dispersion-strengthened materials.

Left: Dark field image of an Al–Li–Sc–Zr alloy. Cubic Al3(LiScZr) forms a thin shell between the Al3(ScLi) core and the metastable cubic Al3Li outer shell. Right: HREM image showing ordered structure of complex Al3(ScLiZr) core/shell precipitate.

Radmilovic et al. Monodisperse Al-3(LiScZr) core/shell precipitates in Al alloys. Scripta Mater 58 529-32 (2008)
Tolley et al. Segregation in Al-3(Sc,Zr) precipitates in Al-Sc-Zr alloys. Scripta Mater 52 62125 (2005)

Liquid inclusions in Al

In previous work under this program, it was found that at the nanoscale, topotaxially aligned Pb particles embedded in Al follow a sequence of magic sizes and exhibit significant size-dependent superheating. When liquid particles are sufficiently small, they are found to undergo Brownian motion inside the solid Al matrix. By careful investigation of the size-, temperature- and composition-dependence, we are currently investigating whether the same mechanism of step nucleation at the solid-liquid interface that is responsible for the kinetic limit of shape equilibration also controls the Brownian motion.

Prokofjev et al. Effect of Morphology on the Mobility of Nanosized Liquid Pb Inclusions in Solid Al. Diffusion and Defect Data 264, 55-61 (2007)

Johnson et al. Brownian motion of liquid lead inclusions along dislocations in aluminum. J Mater Sci 40, 3115-19 (2005)

Strain-compensated nanoclusters

We have found the first conclusive experimental evidence for pre-precipitation nano-clusters of Si–Ge in a ternary Al–Si–Ge alloy. These observations confirm the hypothesis that such clusters form due to atomic mismatch strain compensation between the Si and Ge atoms in an Al solid solution. Similar combinations of solute elements with opposite misfit could be used to form strain-compensated clusters in systems where elastic stresses prevent phase separation. Application to different host alloys may be useful in achieving novel microstructures that use these clusters as templates for heterogeneous nucleation.

Left: (A) An 8%Si + Ge isoconcentration surface of the ternary alloy aged for 15 min at 160°C. Four spherical Si- and Ge-enriched regions are clearly evident in this volume. The corresponding two-dimensional concentration maps of the same volume for Si and Ge are shown in (B) and (C); size of the volume analyzed: 100 x 50 x 10 nm. Right: high resolution micrograph showing multiply- twinned Si-Ge precipitate after loss of coherency.

Radmilovic et al. Strain-compensated nano-clusters in Al-Si-Ge alloys. Scripta Mater 54, 1973-78 (2006)