Page 188 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 6 15 Metal Foams

to the original straight shape. Shape-memory alloys can be used to generate motion
and/or force in temperature-sensitive actuators. The behavior of these alloys, also
called smart materials, can be reversible; that is, the shape can switch back and forth
repeatedly upon application and removal of heat. A typical shape-memory alloy
is 55% Ni-45% Ti (Nitmol). Other such alloys are copper-aluminum-nickel,
copper-zinc-aluminum, iron-manganese-silicon, and titanium-nickel-hafnium.
Shape-memory alloys generally have such properties as good ductility, corrosion
resistance, and high electrical conductivity.
Applications of shape-memory alloys include various sensors, eyeglass frames,
stents for blocked arteries, relays, pumps, switches, connectors, clamps, fasteners,
and seals. As an example, a nickel-titanium valve has been made to protect people
from being scalded in sinks, tubs, and showers. It is installed directly into the piping
system and brings the water flow down to a trickle within 3 seconds after the water
temperature reaches 47°C. New developments include thin-film shape-memory al-
loys deposited on polished silicon substrates for use in microelectromechanical
(MEMS) devices (see Chapter 29).

6.|4 Amorphous Alloys (Metallic Glasses)

A class of metal alloys that, unlike metals, do not have a long-range crystalline
structure is called amorphous alloys; these metals have no grain boundaries, and
their atoms are packed randomly and tightly. The amorphous structure first was
obtained in the late 1960s by the extremely rapid solidification of a molten alloy
(Section 11.6). Because their structure resembles that of glasses, these alloys are also
called metallic glasses. Amorphous alloys typically contain iron, nickel, and chromium,
which are alloyed with carbon, phosphorus, boron, aluminum, and silicon. They are
available as wire, ribbon, strip, and powder: One application is for faceplate inserts
on golf-club heads; this alloy has a composition of zirconium, beryllium, copper,
titanium, and nickel and is made by die casting. Another application is in hollow
aluminum baseball bats coated with a composite of amorphous metal by thermal
spraying and is said to improve the performance of the bat.
Amorphous alloys exhibit excellent corrosion resistance, good ductility, high
strength, and very low magnetic hysteresis. The latter property is utilized in the
production of magnetic steel cores for transformers, generators, motors, lamp bal-
lasts, magnetic amplifiers, and linear accelerators. The low magnetic hysteresis
loss provides greatly improved efficiency; however, fabrication costs are signifi-
cant. Amorphous steels are being developed with strengths twice those of high-
strength steels, with potential applications in large structures; however, they are
presently cost prohibitive. A major application for the superalloys of rapidly solid-
ified powders is the consolidation into near-net shapes for parts used in aerospace
engines.

6.l 5 Metal Foams

Metal foams are material structures where the metal consists of only 5 to 20% of
the structure’s volume, as shown in Fig. 6.3. Usually made of aluminum alloys (but
also of titanium, tantalum, and others), metal foams can be produced by blowing air