Page 190 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

or pressed under vacuum at high temperature within a hot isostatic press. A great
deal of consumer products are fabricated using these techniques; high-tech plastics
and other composite parts represent a significant market share for powder metallur-
gical materials. This is especially the case since the soaring gas prices and stringent
environmental regulations dictate the design of lighter vehicles, to improve gas
consumption. Outside of automotive and aerospace [5] applications, other uses are
prevalent such as parts for air-conditioner and refrigerator compressors, permanent
magnets, [6] and even monetary coinage. [7]
There are a number of attractive benefits for powder metallurgy:
1. Lack of machining eliminates scrap losses
2. Facile alloying of metals
3. In situ heat treatment is useful for increasing the wear resistance of the finished
material
4. Facile control over porosity and density of the green and sintered material
5. Fabrication of complex/unique shapes which would be impractical/impossible
with other metalworking processes
6. Rapid solidification process extends solubility limits, often resulting in novel
phases
Although we have described powder metallurgy as being an ideal process, without
limitations or hazards, it does pose some serious safety risks and limitations. The
majority of metallic powders and other finely divided solids are pyrophoric, mean-

ing that they will spontaneously ignite in air at temperatures below 55 C. Unlike
black powder, which contains both the fuel (C and S) and oxidizer (potassium
nitrate), it is not immediately apparent why metallic powders would ignite, since
both key components are not present within the powder matrix.
There are two primary reasons for this pronounced reactivity. The extremely large
exposed surface area of powders relative to the bulk results in rapid oxidation upon
exposure to air, especially for metals that form stable oxides such as aluminum,
potassium, zirconium, etc. Also, there is enhanced internal friction among the
individual micron- or nanosized individual particulates comprising the powder.
Simply pouring the powder onto a table will yield sparks that may or may not be
visible to the naked eye. Indeed, if one does not physically see the spark, he or she
will soon know if there was one! As you would imagine, both the pulverizing and
pressing steps in powder metallurgy are especially dangerous, as the particles are
forced into contact with one another and the equipment surfaces (another purpose
for an added lubricant during compaction). NASA recently published a technical
[8]
paper that describes the production of rocket propellants ; this is definitely worth a
read, to find out how one prepares mixtures of such reactive components.

3.2. METALLIC STRUCTURES AND PROPERTIES

We are now in a position to investigate a question that will be posed throughout this
textbook: What is the relationship between the microstructure of a material, and its
overall properties? If our world wishes to stay on its current path of unprecedented