Relationship between hardness and strength characteristics of Cu–Cr microlayer composite material at elevated temperatures

Abstract

We investigate the structure, hardness, strength, plasticity and fracture character of Cu-Cr microlayer composite material for electric contacts with prescribed microlayered structure and chemical composition in the temperature range from 290 to 1070 K. The correlation relationships between the hardness and strength characteristics have been established. Notation σ u -ultimate strength, MPa σ 0 2 . -offset yield stress, MPa δ -relative elongation, % δ pr -relative uniform elongation, % ψ -reduction of the cross-sectional area, % HV -Vickers hardness, MPa T -thermodynamic temperature, K U -plastic strain activation energy (enthalpy), eV ′ A -constant characterizing the material and strain rate c -proportionality factor, c H = σ H -hardness, MPa σ -characteristic of strength, MPa k -the Boltzmann constant G -shear modulus, MPa HV m -mean value of Vickers hardness, MPa S -root-mean-square deviation, MPa w -coefficient of variation, % ΔHV -reliable estimate at a 0.95 confidence level, MPa a b , -regression coefficientsIntroduction. Copper and chrome composites are widely used as the most efficient electric contact materials for arcing contacts of arc-extinguishing chambers in vacuum circuit breakers. In addition to the conventional methods of powder metallurgy, these materials are produced by high rate electron-beam evaporation/condensation of copper and chrome from separate water-cooled crucibles and layer-by-layer condensation on a metal substrate (a rotating steel disc). The electron-beam physical vapor deposition (EBPVD) technology makes it possible to produce, within one production cycle, sheet copper-chrome composite materials with a given microlayer structure and chemical composition, and condensates with the content of gaseous impurities no higher than that in the initial material, even in the case of evaporation of active metals such as chrome. One more peculiar feature of this method is that it allows the creation of combined contacts with a working layer of an arc-extinguishing composite material obtained by physical vapor deposition, which is cohesive with the copper substrate that provides heat removal from the working layer.The optimum content of chrome in these composites that provides the most favorable combination of electrical, mechanical and chemical characteristics of the material is from 30 to 40 mass%, the tensile strength of composites being 400 to 550 MPa, hardness from 1600 to 1800 MPa, and the resistivity not exceeding 2 5 10 8 . ⋅ − Ω ⋅ m. A high-temperature annealing of the condensed materials Cu-(30 to 40 mass%) Cr is responsible for the decrease in their strength and resistivity down to 30%, but, in fact, does not influence the hardness. Owing to the specific structure of these materials, their unique physico-mechanical and operating characteristics are formed. At present, condensed Cu-Cr microlayer composite materials have been produced as pilot lots by the "Gekont" Research and Production Enterprise (Vinnytsya, Ukraine) [1][2][3][4][5][6][7].In the process of operation, materials of co...

Microlayered Composite Materials on Basis of Copper, Refractory, Rare-Earth Metals, and Carbon for Electrical Contacts and Electrodes

“…The production of such materials became possible after the creation of electron-beam facilities for high-speed evaporation of composite components from separate water-cooled crucibles with a subsequent layer-by-layer condensation of the regulated vapor flows on the revolving metallic substrate [8]- [17]. The apparent advantages of the electron-beam technology, which allowed obtaining the new-generation composite materials for electrical contacts, include:…”

Relationship between composition, structure, and mechanical properties of a condensed composite of copper–tungsten system

“…Mechanical characteristics were determined at room temperature (in air) and at 870 K (in vacuum no worse than 0.7 mPa) based on tensile test results for standard flat specimens with a design test portion length of 15 mm, width of 3 mm, and thickness of 1.2-2.0 mm [16]; the tests were performed on a Mod. 1246-R test machine [17] according to the State Standards GOST 1497-84 and GOST 9651-84, respectively.…”

“…The Cu- (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) wt.% W specimens exhibit an undulating fracture and delamination ( Fig. 4a and 4c) corresponding to the tungsten enrichment of the surface layers oriented towards the bath.…”

“…The Cu- (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) wt.% W condensed composites with a laminar structure, which have a fairly high mechanical and electrophysical characteristics, can be recommended for break-contact components in switchgears for medium-and high-current electrical networks. The composites with a cone-shaped structure (with a tungsten concentration varying from 20 to 40 wt.%), which feature high strength and hardness especially at elevated temperatures, are suitable for the fabrication of working surfaces of electrodes for contact welding machines as well as electrodes for electrical discharge machining for metallic materials including hardmetals.…”