Download Shape-Memory Alloy

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The Superalloys

The superalloys have superlative combinations of properties.

Which must withstand exposure to severely oxidizing environments and
high temperatures for reasonable time periods. Mechanical integrity
under these conditions is critical.

Density is an important consideration because centrifugal stresses are
diminished in rotating members when the density is reduced.

These materials are classified according to the predominant metal in the
alloy, which may be cobalt, nickel, or iron. Other alloying elements
include the refractory metals (Nb, Mo,W, Ta), chromium, and titanium.

In addition to turbine applications, aircraft, these alloys are utilized in
nuclear reactors and petrochemical equipment.
Nickel-Chromium High-temperature Alloys
The main feature of the nickel-chromium alloys is
 Their ability to resist oxidation at elevated temperatures.
 If further suitable alloy additions are made the strength is increased under
conditions of stress at high temperatures.
 Alloys rich in nickel and chromium have a high specific resistance to
electricity, which makes them admirable materials for the manufacture of
resistance wires, and (because of their low rate of oxidation at high
temperatures) heater elements of many kinds capable of working at
temperatures up to bright red heat.
 Further groups of high-temperature, nickel-chromium-base alloys are those
containing iron and, sometimes, molybdenum and tungsten. Of these the
Inconel series has been long established.
 Inconel 600 contains 76Ni; 16Cr; and 7Fe
 Is resistant to many acids and alkalis as well as oxidising atmospheres at
high temperatures.
 Since it retains reasonable strength at high temperatures it is used for
furnace equipment including retorts, muffles, heat-treatment trays, supports
and nitriding boxes
The Refractory Metals

Metals that have extremely high melting temperatures are classified as the
refractory metals.

Included in this group are niobium (Nb), molybdenum (Mo), tungsten (W),
and tantalum (Ta).

Melting temperatures range between (2468 C) for niobium and (3410 C),
the highest melting temperature of any metal, for tungsten.

Interatomic bonding in these metals is extremely strong, which accounts for
the melting temperatures,

In addition, large elastic moduli and high strengths and hardness's, at
ambient as well as elevated temperatures.

The applications of these metals are varied.
1. For example, tantalum and molybdenum are alloyed with stainless steel to
improve its corrosion resistance.
2. Molybdenum alloys are utilized for extrusion dies and structural parts in
space vehicles; incandescent light filaments, x-ray tubes, and welding
electrodes employ tungsten alloys.
3. Tantalum is immune to chemical attack by virtually all environments at
temperatures below and is frequently used in applications requiring such a
corrosion-resistant material.
The Noble Metals

The noble or precious metals are a group of eight elements that have some
physical characteristics in common (noble metals are silver, gold, platinum,
palladium, rhodium, ruthenium, iridium, and osmium).
 They are expensive (precious) and are superior or notable (noble) in properties
 Characteristically soft, ductile, and oxidation resistant.

The noble metals are silver, gold, platinum, palladium, rhodium, ruthenium,
iridium, and osmium; the first three are most common and are used
extensively in jewelry. Silver and gold may be strengthened by solid-solution
alloying with copper; sterling silver is a silver–copper alloy containing
approximately 7.5 wt% Cu.

Platinum is used for chemical laboratory equipment, as a catalyst (especially
in the manufacture of gasoline), and in thermocouples to measure elevated
temperatures
Shape-Memory Alloy
A shape-memory alloy (SMA, smart metal, smart alloy) is an alloy that
"remembers" its original shape and that when deformed returns to its predeformed shape when heated. This material is a lightweight,
The two main types of shape-memory alloys are copper-aluminium-nickel, and
nickel-titanium (NiTi) alloys.
Many metals have several different crystal structures at the same composition,
but most metals do not show this shape-memory effect. The special property
that allows shape-memory alloys to revert to their original shape after heating is
that their crystal transformation is fully reversible. In most crystal
transformations, the atoms in the structure will travel through the metal by
diffusion, changing the composition locally, even though the metal as a whole
is made of the same atoms. A reversible transformation does not involve this
diffusion of atoms, instead all the atoms shift at the same time to form a new
structure, much in the way a parallelogram can be made out of a square by
pushing on two opposing sides.