Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 86

Lightweight Materials
The quest to develop lighter structural materials with all the necessary me-
chanical properties normally found only in heavier materials (conventional met-
als, ceramics, etc.) is being pursued on several fronts. These include the strength-
ening of conventional materials, the development of lighter metal alloys, and the
creation of novel composites or hybrids of a number of different material types
combined to create a structural material by design.
Long before the term “nanotechnology” became commonplace, the benefits
of manipulating the grain structure of materials on the nanoscale were known and
exploited. Increased strength and reduced creep (or other irreversible, detrimental
mechanical property changes over time) in finely grained, nanostructured materi-
als is not well established in many polycrystalline materials systems. These
nanoscale materials can provide for stronger, more durable, and more stable
structures. The classic model for how the strength of metals increases as grain size
decreases 163 describes the pileup of dislocations at grain boundaries, producing
stress, which when added to applied stress results in slippage across the boundary.
Smaller grains result in a smaller pileup of dislocations and less stress, with a larger
external force needed to create slippage (and therefore a stronger material).
The goal of developing manufacturing methods and processes that produce
materials with ever-smaller grain size has been pursued in metallurgy for de-
cades. Nanoscale control of the grain structure of lightweight aluminum and
aluminum alloys could be of value especially for aerospace structures. In addi-
tion, improvements in mechanical properties and the allowed operating temp-
erature of titanium and titanium alloys are being pursued using nanoscience
approaches. Large increases in the yield strength of metal alloys are well docu-
mented for nanoscale-grain materials relative to conventional microscale grain
alloys. Methods of refining the grain structure of metal to the 50-nanometer (or
so) scale have been reported. 164 In addition to greater yield strengths, combina-
tions of desirable features such as improved ductility and strength can be de-
signed in, as can hardness for metals and metal alloys composed of nanostructured
materials. However, it can be difficult to avoid grain growth when metals with
such fine grain sizes are subsequently heated. Researchers at AFRL and else-
where are working on methods to increase the thermal stability of nanophase
metals and alloys so that they retain their superior mechanical properties at el-
evated temperatures.
For ceramic materials, a goal has been to enable net-shape manufacturing of
consolidated ceramic nanoparticles (such as titania or alumina) with a shape and
dimensional precision beyond that possible with conventional processes. The
role of nanoscience here is to aid in understanding the behavior of the nanosize
grain boundaries, which can slide under stress without breaking bonds as a result
diffusional healing, an atom transport mechanism that can take place over very