Low Expansion Alloys

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Low Expansion Alloys
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• For hundreds of years, engineers have
  experimented with different alloys, in an attempt
  to control heat expansion. In certain
  applications such as vacuum tubes or
  optoelectronics, it is necessary to adhere to set
  expansion requirements over specific temperature
  ranges.
• In these cases, the following materials possess
  the characteristics needed.

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• Alloy 42
• Alloy 52
• Invar 36
• Super Invar 32-5
• Kovar

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• Used in applications where a hermetic seal
  between the metal, glass or ceramic is required,
  controlled expansion alloys are also used in
  vacuum tubes, optoelectronics, lightbulbs and
  other products where a degree of thermal
  expansion takes place (based on the specific
  temperature at any given time).
• Here, Phillip Guthrie of Vulcan Metal Group
  addresses key questions that buyers should
  consider when selecting controlled expansion
  alloys.

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• Q What are the primary controlled expansion
  alloys?
• A The primary controlled expansion alloys
  include Alloy 36, Alloy 42, Alloy 48, Alloy 52
  and Alloy 54, each of which has its own thermal
  expansion coefficient and is used for specific
  applications. Alloy 36, for example, is most
  often used in precision laser equipment,
  instrumentation and electronic devices, where
  Alloy 42 gets the most use in thermostats and
  electrical switches. Alloy 48 is more widely used
  in electrical wiring or in industrial thermostats
  in environments that have to be heated up to 450
  C.
• Q What are some of the other commonly-used
  expansion alloys?
• A Alloy ASTM F-15 is comprised of nickel, cobalt
  and iron and used frequently in machining and
  deep drawing. This is one of the more popular
  controlled expansion options for hermetic sealing
  applications.
• Invar 36 is a low-expansion alloy made of nickel
  and iron. Its typically used in applications
  with minimum expansion requirements. Nickel
  200/201/205/233 are all commercially pure,
  non-alloyed Nickel options that are used in the
  electronics industry. Some of the products that
  are made with this alloy include fuel cells,
  terminals, cathode shields and semiconductor
  supports.
• Nickel 270 is another commercially-pure option
  used by the electronics industry for anode
  plates, passive cathodes and transistor
  enclosures (among other products).

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• Q What other applications can expansion alloys
  be used for?
• A Other uses for expansion alloys include
  compact fluorescent lamps (CFLs), auto lamps and
  additional electronic applications. In most
  cases, controlled expansion alloys are made
  through some combination of Kovar and iron
  nickel. Kovar is an iron-nickel-cobalt alloy with
  a coefficient of thermal expansion similar to
  that of hard (borosilicate) glass, making it
  especially suitable for uses which require a
  matched-expansion seal between metal and glass
  parts.
• In addition, Kovar is frequently used in the
  electronics industry for metal parts bonded to
  hard glass envelopes for such devices as power
  tubes, x-ray tubes, etc., and other applications
  requiring glass-to-metal seals.
• Q What other factors should I consider when
  selecting controlled expansion alloys?
• A Procurement professionals and engineers should
  consider these key points
• 1) What exactly do you want to seal?2) What else
  do we know about the application in question?3)
  What materials are being connected to each
  other?4) What is the CTE of the material that
  were using?5) Does the CTE align with that of
  the selected alloy?

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• The answers to these questions are important
  because the coefficient of thermal expansion
  (CTE) values must align to those of the alloy
  that you select. The CTE indicates the extent to
  which a material expands upon heating, and
  different substances expand by different amounts
  (i.e., exactly how much a specific material will
  expand or contract when it is heated or
  cooled).Its important to understand that over
  the course of small temperature ranges, the
  thermal expansion of uniform linear objects is
  proportional to temperature change.
• Q How else can I narrow down my selection?
• A Once youve made these calculations and
  determined whether the CTE of the material
  matches that of the alloy, you can narrow your
  choices down to a specific material or alloy that
  you need for that application. Note that while
  high-temperature metals like molybdenum and
  tantalum can also be used in areas where
  controlled expansion alloys are needed, for the
  most part youll be selecting among nickel iron
  and Kovar when making procurement decisions
  related to this metal category.