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PRODUCT DATA of 08: Miscellaneous Alloys
Material08: Miscellaneous Alloys
General InformationA metal is classed as miscellaneous if it does not fall within another declared materials list (DML) category in ECSS-Q-70. Also included in this section are comments on metal-based materials that are either prohibited or should be approached with caution for space applications.
Use in SpacecraftLight alloys based on magnesium and beryllium are used in some primary and secondary structures.
Plating appears in many applications (e.g. electronics, thermal control and corrosion protection) and calls mainly for silver and gold.
"Memory alloys" based on titanium and nickel can find uses as actuators:

In addition to standard conventional alloys, more recent material developments include:
  • reinforced alloys (metal matrix composites - MMC) consisting of magnesium alloys reinforced with carbon fibres;
  • lithium additions to conventional magnesium alloys;
  • reinforced silver alloys.
Main CategoriesMiscellaneous metals include, but are not limited to:
  • magnesium alloys;
  • beryllium and Be-alloys (see 02 for Be-Cu alloys);
  • refractory alloys;
  • superalloys,which as a group include cobalt-, iron- or nickel-based alloys (see .03 for Ni-based superalloys);
  • mercury;
  • plating materials: e.g. cadmium, zinc, tin, gold, silver and osmium.

NOTE This chapter also includes comments on metal-based materials that are either prohibited or can be used cautiously for space applications.
Processing and AssemblyMagnesium alloys are available as wrought forms or for casting. Care shall be taken in storing magnesium alloys due to their tendency to corrode. Machining shall be performed with special precautions to prevent ignition and burning of swarf.

Sophisticated techniques and rigorous safety procedures shall be applied during processing of beryllium to avoid the formation and release of beryllium oxide,
Metal particles and compounds which are toxic. Consequently, the majority of beryllium components and structures are produced by companies dedicated to working with these materials.

Superalloys are processed following recognized aerospace procedures or other appropriate industry standards.

Specialist methods for processing refractory metals and alloys are applied.

During processes when metals with known or suspected toxicity problems are involved, appropriate safety equipment shall be used for operatives and appropriate procedures followed for collection and disposal of waste.

PrecautionsThe metallic components proposed for use inmost spacecraft shall be screened to prevent failures resulting from SCC. Three alloy ratings were derived: high-resistance, moderate-resistance and low-resistance to SCC (these are listed in Tables 1, 2 and 3 respectively of ECSS-Q-ST-70-36C). The alloys listed in Table 1 should be used for space applications. For alloys listed in Table 2 or 3 a detailed justification for space use shall be provided, demonstrating that SCC testing according to the standard method detailed in ECSS-Q-ST-70-37C took place (method incorporates constant load and alternate immersion in 3,5 % NaCl solution).
  • Magnesium alloys
    • Dusts of magnesium and its alloys are flammable; requiring special safety measures. Some magnesium alloys (with thorium) can have a slight residual radioactivity.

  • Beryllium and Be-alloys
    • Beryllium is used in its pure form, but is both brittle and difficult to fabricate as well as being fairly toxic. This metal is produced by powder metallurgy involving hot isostatic processing and component parts should be initially rough machined, heat treated to remove major residual stresses and then fine machined. A final chemical etching treatment should be performed to remove 0,1 mm from the surface of machined parts. This generally removes mechanical damage such as subsurface microcracks and deformation twins.
    Beryllium and Beryllium oxide dust and vapours are toxic: special precautions shall be taken when work is done on this material.

  • Miscellaneous
    • Refractory alloys are generally selected for extreme high-temperature applications where other metals cannot be used. However, engineering data on refractory alloys are limited, especially under the extreme environments encountered on spacecraft.
    • Nickel-based and Cobalt-based superalloys possess various combinations of high-temperature mechanical properties and oxidation resistance up to approximately 550 ºC. Many of these alloys also have excellent cryogenic temperature properties.
    • Some metals, such as cadmium and zinc, are rather volatile and should not appear in space hardware. Platings of these metals, as well as tin, are known to grow whiskers both in air and under vacuum. They should be excluded from all spacecraft and ground-support equipment. Porous platings are potential sources of danger and this occurs frequently with gold plate over silver.

      • NOTE Cadmium in NiCd batteries is acceptable.

    • Osmium oxide is toxic: special precautions shall be taken when work is done on this material.
Hazardous and PrecludedMercury and mercury-containing compounds can cause accelerated cracking of aluminium and titanium alloys. It is therefore a prohibited substance for the manufacture of aerospace structures and subsystems.

Specialized safety equipment and procedures for the collection and disposal of dust and debris shall be used for operatives working with toxic materials, such as beryllium and osmium, and for materials with a risk of ignition and burning, such as magnesium.

In electronic assemblies, tin-, silver- and gold-plating on terminals of PCBs is removed in order to achieve an approved tin-lead finish. Soldering directly to gold finishes is unacceptable and de-golding processes are used. In unavoidable use of gold-finishes, such as in RF circuitry, selective plating processes are used for soldered connections.

Effects of Space environmentVacuum affects volatile metals, such as cadmium and zinc. These metals sublime readily at temperatures over 100 ºC and 150 ºC respectively, and can form conductive deposits on insulators or opaque deposits on optical components. Oxide layers slow down the process of evaporation when they are thick enough and not cracked. All metals in contact under vacuum conditions or in inert gas have a tendency to cold weld. This phenomenon is enhanced by mechanical rubbing or any other process which can remove or disrupt oxide layers. It is particularly intense for pairs of cubic-lattice metals which alloy readily.
  • Radiation at the level existing in space does not modify the properties of metals.
  • Temperature problems are similar to those encountered in technologies other than space, but are complicated by the difficulty of achieving good thermal contact in vacuum and the absence of any convective cooling.
  • Atomic oxygen in low Earth orbit attacks some metals, such as silver (solarcell interconnectors) and osmium (extreme-UV mirrors).
Some Representative ProductsEuropean sources of beryllium are: SAGEM (F), Royal Ordnance Factory (UK), Heraeus (D), Brush Wellman (UK and D); Superalloys: Aubert and Duval (F)
Magnesium alloys: Magnesium Elektron (UK).
Procurement to internationally recognized specifications isrecommended, such as ISO,MIL Specs, B.S., SAE., DIN or AFNOR specifications.
The materials listed in Table A-9 (from ECSS-Q-ST-70-36), can be considered.