Controlled Expansion Alloys and Copper Alloys for RF

Why these alloys are grouped together

These materials are rarely chosen for strength. They are chosen for one physical property: low or matched thermal expansion, electrical and thermal conductivity, or spring behavior. They show up in optical benches, star tracker and sensor mounts, hermetic connectors and feedthroughs, waveguides and RF cavities, grounding springs and EMI gaskets.

Values are from the sources listed at the end of the page and are nominal. For flight hardware, buy to a named specification and require the cert to report the property that matters, not just chemistry.

Invar 36: what low expansion really buys

Invar 36 is an iron-nickel alloy of roughly 36 percent nickel and 64 percent iron. Common grades expand about 1.2 ppm per degree C between 20 and 100 C, compared with roughly 11 to 15 ppm/C for ordinary steels. That is why it appears in telescope structures, optical benches, instrument mounts and large composite layup tools, where the tool must hold shape through an autoclave cycle.

The low expansion only holds over a limited temperature band, and three other behaviors catch designers out:

• Creep and dimensional drift. Invar has a known propensity to creep. Precision parts are usually stress relieved between rough and finish machining, and a stabilizing heat treatment is often specified for optical-grade work. Write it on the drawing.
• Magnetism. Invar is ferromagnetic, with a saturation magnetostriction of about 4 ppm in a 1 tesla field. Near magnetometers or strong magnets that matters, and a nonmagnetic material may be the better choice.
• Joints. An Invar part bolted to aluminum is a bimetallic strip. Flexures or matched-expansion inserts are needed at interfaces, or the low expansion of the Invar part is wasted.

Invar 36 is in MSFC-STD-3029 Table I, high resistance to stress corrosion cracking, including weldments. It is also a nickel-iron alloy whose non-iron alloying content is mostly nickel, which matters for DFARS: the specialty metals definition for nickel and iron-nickel alloys counts alloying metals other than nickel and iron, so check plain Invar against the clause case by case. Invar 36 can also be processed by laser powder bed fusion, though porosity and microstructure affect its expansion.

Kovar and glass-to-metal seals

Kovar is an iron-nickel-cobalt alloy of nominally 29 percent nickel and 17 percent cobalt, designed to match the thermal expansion of borosilicate sealing glass. Published coefficients from 25 C are about 5.5 ppm/C to 200 C, 5.1 to 300 C, 4.9 to 400 C, 5.3 to 450 C and 6.2 to 500 C. That nonlinear curve is the point: it tracks the glass through the sealing cycle, so the seal cools without cracking. It is commonly bought to ASTM F15.

Kovar is used for hermetic connector shells and pins, feedthroughs, microwave and sensor package lids and frames, and leads that pass through glass. Engineering notes:

• The glass and the alloy must be matched as a system. Substituting a different low-expansion alloy without checking the expansion curve against the glass invites cracked seals.
• Kovar is magnetic below its Curie temperature of about 435 C. That can matter near sensitive instruments.
• Kovar packages and pins are commonly plated. Flow the plating spec and thickness to the processor, and confirm whether plating happens before or after the seal.
• Kovar's cobalt content means alloying metals other than nickel and iron exceed 10 percent, so it falls under the DFARS 252.225-7009 specialty metals definition on defense work.

MSFC-STD-3029 also rates Glass Seal 52 (51Ni-49Fe), another sealing alloy, highly resistant to stress corrosion cracking.

Beryllium copper and beryllium safety

The high-strength beryllium coppers, C17200 being the familiar one, contain about 1.6 to 2 percent beryllium and about 0.3 percent cobalt. Age hardened, they reach strengths up to about 200 ksi and Rockwell C 45, the highest of any copper alloy, while keeping useful electrical and thermal conductivity. Uses on aerospace hardware include contact springs, connector parts, grounding fingers and RF-tight gaskets for EMI shielding. Copper alloys are also generally considered non-sparking, which is why beryllium copper tools are used around flammable atmospheres. MSFC-STD-3029 rates C17000 and C17200 in aged conditions highly resistant to stress corrosion cracking.

The issue is beryllium exposure. Inhaling dust, mist or fume containing beryllium can cause chronic beryllium disease, and IARC classifies beryllium as a Group 1 carcinogen. OSHA's general industry standard, 29 CFR 1910.1024, sets these limits:

The standard requires employers to assess exposure, establish regulated areas where exposure exceeds the PEL or STEL, and maintain a written exposure control plan reviewed at least annually. Materials with less than 0.1 percent beryllium by weight are exempt only where the employer has objective data showing exposure stays below the action level. For buyers this has a practical consequence: not every machine shop will cut beryllium copper, and those that do should be able to describe their controls for machining, grinding and cleanup. Ask before you route a beryllium copper job, and confirm the shop's housekeeping keeps beryllium chips out of other jobs' swarf.

Coppers for RF, vacuum and brazed assemblies

For waveguides, RF cavities, cold plates and vacuum parts, the grade of copper matters more than most drawings admit.

• C10100 (OFE): 99.99 percent copper with 0.0005 percent oxygen and a minimum of 101 percent IACS conductivity. Silver counts as an impurity. It has low volatility in ultra-high vacuum and is the usual choice for accelerator, vacuum electronics and high-reliability brazed RF parts.
• C10200 (OF): 99.95 percent copper, 0.001 percent oxygen, 100 percent IACS minimum. Oxygen-free, but with looser impurity limits than OFE.
• C11000 (ETP): 99.9 percent copper with typical oxygen of 0.02 to 0.04 percent, 100 percent IACS minimum. Fine for conductors, but its oxide content is a problem in hydrogen.

Copper containing copper oxide can be embrittled when heated in hydrogen: the hydrogen reacts with the oxide to form steam at grain boundaries, opening voids. Hydrogen atmosphere brazing is common for RF and vacuum assemblies, so specifying ETP for a part that will be brazed in hydrogen is a classic error. Call out OF or OFE copper for any brazed or vacuum-fired copper part.

On stress corrosion, MSFC-STD-3029 lists C11000 up to 37 percent cold rolled in Table I, while common brasses such as C26000 at 50 percent cold rolled are in Table III. Avoid highly cold-worked brass for stressed parts in corrosive environments.

Callouts and certs

• Name the alloy and UNS number and the procurement spec; for Kovar name the sealing glass or the expansion requirement.
• For Invar, state any stress relief or stabilizing heat treatment and when it occurs in the sequence.
• For beryllium copper, state the temper and the age hardening treatment, and whether the supplier ages parts after forming.
• For copper, name the grade (C10100, C10200 or C11000) and any brazing or vacuum firing that follows.
• Require a cert with heat or lot, chemistry and the controlling property, such as conductivity for copper or expansion test data where the spec includes it.

The certs and traceability page covers cert contents and PMI in more detail.

Get low-expansion and copper parts quoted

These parts need shops comfortable with precision stress relief, beryllium exposure controls or oxygen-free copper handling. Send your drawing through a CNC quote request, and we will route it only to suppliers set up for that material, flow down the heat treat, plating and cleanliness requirements, and check the cert package before shipment. See also the materials hub, engineering plastics and nickel superalloys.

Questions

Related

• Aerospace Materials: Selection, Specs and Paperwork
• Engineering Plastics for Flight and Ground Hardware
• Nickel Superalloys and Nickel-Copper Alloys
• Material Certs, Specialty Metals and Lot Traceability

Sources

• Invar, composition, expansion and magnetostriction
• Kovar, composition and expansion by temperature range
• Beryllium copper, composition, strength and hazards
• OSHA 29 CFR 1910.1024, Beryllium
• Oxygen-free copper, grades C10100, C10200, C11000
• NASA MSFC-STD-3029A, Tables I-C, I-D and III-C
• DFARS 252.225-7009 specialty metals definitions