Precision alloys with a set thermal linear expansion coefficient

A thermal linear expansion coefficient (TLEC) is a physical value that characterizes the relative change of linear sizes of the body with the temperature increase by 1 °К under constant pressure.

Precision alloys with set TLEC are materials with a regulated TLEC value within a certain temperature range, i.e. these alloys guarantee a certain change of the product linear sizes within a set temperature range.

The considered materials are rather durable and very plastic. This allows making a wide range of products.

Alloys with a set TLEC are based on several chemical systems. Magnetic materials are based on iron-nickel (Fe-Ni) and iron-chromium (Fe-Cr). They have a minimal, low and medium coefficient. Non-magnetic materials are based on chromium (Cr) systems with such alloying agents as iron (Fe), manganese (Mn), rhenium (Re), tantalum (Ta), etc. They provide a minimal and low coefficient. A medium thermal coefficient is peculiar for nickel-based alloys with such additives as molybdenum (Mo), tungsten (Wo), chromium (Cr), manganese (Mn); zirconium-based alloys with such additives as титана (Ti); titanium-based alloys with such additives as molybdenum (Mo) and vanadium (V). Manganese-nickel-copper systems (Mn-Ni-Cu) have a high TLEC.

By magnetic properties alloys with a set TLEC are divided into two large groups: magnetic and non-magnetic materials. Each group contains materials with minimal, low, medium and high coefficient.

Table 1

Group	Grade	General specification	Intended use
Ferromagnetic alloys
With a minimal TLEC	36Ni 36Ni-VI 32NiCoCu 32NiCo-VI 32NiCo-EB 36NiCr 35NiCoTi 35NiCoMn 39Ni 36NiMn6 36NiMnTi	TLEC below 3.5·10-6 K-1 under temperature not exceeding 100 °С	For instrument parts used in metrology and geodesy, as part of thermostatic bimetals, for basic devices of gas lasers, pipelines for cryogenic liquids, etc. The alloy is chosen considering its thermal coefficient, mechanical properties, stability towards phase changes within the working temperatures and loads
With low TLEC	42Ni 42NiN-VI 44Ni 46Ni 38NiCo	TLEC 3.0-7.0·10-6 K-1 up to the temperature of expansion line bend (Тb). The thermal expansion sharply increases above Тb	For connecting to ceramics with solder alloy, cements, glues, for bases of nickel and other meshes as constructional materials with set TLEC
	30NiCuCo 29NiCo 30NiCoV	TLEC 4-7·10-6 K-1 and low thermal expansion above the bend temperature	Widely used for leak-tight connections with С39-1, C48-1, С52-1, С59-3 and other glasses.
With medium TLEC	34NiCo 31NiCo 24NiC 33NiCo	TLEC 5.5- 8.0·10-6 K-1 up to the bend temperature and minimal TLEC above the bend temperature	For connections with ceramics using 22ХС and polikor, leucosapphire and other similar solder alloy connections with melting temperature 800-1000 °С
	38NiCoCu 47NiCr 48NiCr 47NiЗCr 47NiCrB 50NiЗCrAl 18CrTiV 52Ni 47NiCu	TLEC 7-12·10-6 K-1	For connecting with С80-1, С72-1, С76-4, С95-3, С94-1, С93-2, С93-4, С90-1 and other soft glasses
	58Ni-VI	TLEC 11·10-6 K-1	For line scales with temporally constant sizes
Non-magnetic alloys
With minimal TLEC	95CrCo 96Cr	TLEC 1-6·10-6 K-1 below 100 °С	For producing parts with high size stability, increased hardness, corrosive stability and absence of magnetic field distortion
	75NiMo-VI 80NiMoW 80NiMoWCr3 76NiCrWMn	TLEC 10-15·10-6 K-1 below 900 °С	For connections with “soft” glasses and ceramics, as a constructional material with a set TLEC
	93TsTi	TLEC 6·10-6 K-1 below 700 °С
	72TiV 75TiMo	TLEC 8-10·10-6 K-1 below 900 °С
With high TLEC	56CuMnNiCr 70MnNiCuCr	TLEC above 15·10-6 K-1 from 20 °С to the melting temperature	For parts with thermal expansion that matches the similar value of aluminium alloys, active parts of thermostatic bimetals

The chemical composition, production technology and some other peculiarities are the main factors that impact on the material properties. The properties should be considered within the groups described in Classification section. See Table 1 for the TLEC.

TLEC is the most practically important property of the precision alloys that belong to the considered group. Its values can be divided into minimal, low, medium and high.

Technological, physical, mechanical and chemical properties are also important. The first ones determine the product type and production method. Others determine the operational properties (maximum working temperatures, mechanical load, environment, etc.). In general, all alloy groups have a good plasticity (so that they can be pressurized) and durability.

The main grades of precision alloys with the set TLEC are given in Classification section, Table 1. The chemical composition and other requirements are regulated by GOST 10994-74.

Alloys with TLEC about 10^-6 K^-1 and lower are used in metrology, cryogenic and radioelectronic equipment and geodesy. TLEC values close to zero are required for providing the high accuracy of the instruments, creating stable length references, gas lasers and construction of non-return pipelines for pumping liquid gases.

Most configurations of vacuum-tube, gas-discharge and semi-conductor devices have metal junctions with a non-organic dielectric (e.g., glass, ceramics, mica). They should be extremely vacuum-tight. The TLEC of most of them is lower than ordinary metals and alloys have. For making tight junctions of glass, ceramics or semi-conductors with alloys, the couple should have a corresponding TLEC within the process and operational temperature ranges. The allowable deviations of the TLEC of connected materials depend on the junction structure, oxide film properties, junction quality and material plasticity. In case of a large difference between the TLEC of the alloy and non-organic dielectric, the voltage leads to cracks in the junctions and loss of tightness during the device assembly operation.

Ferromagnetic materials with a minimal TLEC are applied in precision instruments industry for producing instrument parts, in metrology, geodesy, components of thermostatic bimetals, in basic devices of gas lasers, in constructing pipelines for cryogenic liquids, etc. The alloys are chosen considering its TLEC, mechanical properties of stability to phase changes within working temperatures and loads. Materials of this group with low and medium TLEC are applied in several vacuum-tube devices (receiving valves, magnetrons, klystron tubes, teletrons, etc., tight leads and cases of semi-conductor devices). The alloys are chosen on the basis of thermal expansion properties of their non-organic dielectrics or other materials used for soldering so as considering the requirements towards physical and mechanical alloy properties. The considered materials are also used in the precision instruments industry as components of devices and line scales in machine tool engineering.

Due to radioelectronics development a need of non-ferromagnetic metal materials with a minimal TLEC has appeared. Iron-nickel based alloys can not be used in some devices due to their ferromagnetism as it forms a magnetic field in such devices. So a need in non-ferromagnetic materials with a minimal TLEC has appeared. The alloys are used with matched and non-fully matched junctions with metals, alloys, ceramic materials and glasses (including for junctions working under high temperatures), for components of thermostatic bimetals, for fixed length reference used for optical mechanics dilatometer.

Materials with a set TLEC are produced in form of sheets and flat bars (flat products) so as wire, rods and pipes (round products). The certain application determines the used type of blanks.