High strength and good electrical conductivity in Cu–Cr alloysprocessed by severe plastic deformation

S.V. Dobatkin a,b, J. Gubicza c, D.V. Shangina a,b,n, N.R. Bochvar a, N.Y. Tabachkova b

a A.A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii Ave. 49, 119991 Moscow, Russiab National University of Science and Technology “MISIS”, Laboratory of Hybrid Nanostructured Materials, Leninskii Ave. 4, 119049 Moscow, Russiac Department of Materials Physics, Eotvos Lorand University, Pazmany Peter Setany 1/A., Budapest, Hungary

a r t i c l e i n f o

Article history:Received 19 March 2015Accepted 29 March 2015Available online 6 April 2015

Keywords:Cu–Cr alloysHigh pressure torsionUltrafine-grained structureHardnessElectrical propertiesX-ray techniques

a b s t r a c t

Ultrafine-grained (UFG) microstructures in Cu–Cr alloys were processed by high pressure torsion (HPT).The improved hardness was accompanied by a reduced electrical conductivity due to the large amount ofgrain boundaries. The effect of heat-treatment after HPT-processing on the hardness and the electricalconductivity was studied for different chromium contents (0.75, 9.85 and 27 wt%). For low Crconcentration (0.75%) the electrical conductivity increased considerably above 250 1C, however thehardness decreased concomitantly. At the same time, for high Cr content (9.85% and 27%) the hardnesswas only slightly reduced even at 500 1C, while the electrical conductivity increased to a similar level asbefore HPT due to grain boundary relaxation and decomposition of Cu–Cr solid solution. Our studydemonstrates the capability of SPD-processing and subsequent heat-treatment to achieve a combinationof high strength and good electrical conductivity.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Grain refinement via severe plastic deformation (SPD) is aneffective tool for improving the mechanical performance of chro-mium, zirconium and hafnium bronzes [1–8]. The formation ofultrafine-grained (UFG) microstructures in copper–chromiumalloys resulted in an improvement in durability, wear resistanceand fatigue strength [2,9] which are important service propertiesin their applications for resistance welding electrodes and switch-ing devices [10]. One of the most effective SPD method in grainrefinement is high pressure torsion (HPT) [1]. Former papers[11,12] have shown that the application of HPT can reduce thegrain size down to 10 nm, thereby achieving significant hardeningin high-chromium alloys. At the same time, SPD-processingusually yields the reduction of electrical conductivity due to thehigh density of lattice defects, such as grain boundaries. Tailoringthe microstructure by an additional heat-treatment after SPD mayyield an improvement of electrical conductivity while reservingthe high strength. Most probably, chromium content in Cu has aconsiderable influence on the effectiveness of these procedures inthe achievement of the optimal properties. In this work, we demo-nstrate the capability of HPT-processing and subsequent heattreatment in obtaining a combination of high strength and good

electrical conductivity in chromium bronzes with different chro-mium contents of 0.7%, 9.85% and 27%.

2. Materials and methods

Copper alloys containing 0.75%, 9.85% and 27% chromium (in wt%) were chosen for the study. Before deformation Cu–0.75%Cr andCu–9.85%Cr samples were subjected to hot forging at 800 1C, thenthe forged specimens were heat-treated by two different ways:(i) annealing at 1000 1C for 2 h and cooling in air to roomtemperature (RT), and (ii) annealing at 1000 1C for 2 h and waterquenching to RT. The samples processed by the first and secondroutes are referred to as “annealed” and “quenched” specimens,respectively. The Cu–27%Cr alloy was studied in the as-cast state. Allthe samples with 10 mm in diameter and 0.6 mm in thickness wereseverely deformed by HPT at RT and a rate of 1 rpm under a pressureof 4 GPa. The deformation was performed in a “groove” with thedepth of 0.2 mm. The final sample thickness was 0.3 mm. Thenumber of turns was 5 which corresponds to a true strain ofεE4.8 at the half-radius of the disks. The HPT-processed sampleswere heat-treated at temperatures ranging from 50 to 600 1C with astep of 50 1C and a holding time of 1 h at each temperature.

The microhardness was measured using a 402 MVD InstronWolpert Wilson Instruments tester. The measurements were madeat a distance of 2.5 mm from the sample center (i.e. at the

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2015.03.1440167-577X/& 2015 Elsevier B.V. All rights reserved.

n Corresponding author.E-mail address: shanginadaria@mail.ru (D.V. Shangina).

Materials Letters 153 (2015) 5–9

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half-radius of the disks). The resistivity was measured using a BSZ-010-2 micro-ohmmeter at RT. The resistivity was calculated andtransformed into electrical conductivity according to InternationalAnnealed Copper Standards (IACS). The microstructure was obser-ved using JEM-2100 transmission electron microscope. Thin foils forelectron microscopy were prepared by ion polishing with a GATAN600 unit. The crystallite size, the dislocation density and the twin-fault probability were obtained by X-ray line profiles using a high-resolution rotating anode diffractometer (Nonius, FR591) withCuKα1 radiation (wavelength: λ¼0.15406 nm). The line profileswere evaluated by the Convolutional Multiple Whole Profile(CMWP) fitting method [13].

3. Results and discussion

Fig. 1 shows that HPT-processing yields significant hardening inall alloys and the hardness increases with increasing Cr content. Thiseffect can be explained by the higher dislocation density and thesmaller grain size due to the pinning effect of chromium on latticedefects, either these atoms are in solid solution or they are inprecipitates. The grain structure in some selected specimens isshown in Fig. 2 and the grain size values are listed in Table 1. Theeffect of the initial treatment before HPT on the grain size wasmarginal. In quenched Cu–0.75%Cr, Cu–9.85%Cr and Cu–27%Cr alloysprocessed by HPT the average grain size values were �209, 143 and

Fig. 1. Temperature dependence of microhardness (a,c,e) and electrical conductivity (b, d, f) of Сu–0.7%Cr (a,b), Cu–9.85%Cr (c,d) and Cu–27%Cr (e,f) alloys.

S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–96

40 nm, respectively. X-ray line profile analysis also revealed that thelattice defect structure only slightly depends on the initial state(annealed or quenched), while it is very sensitive on the alloyingelement concentration (see Table 1). With increasing Cr contentfrom 0.75% to 27% the crystallite (subgrain) size decreased fromabout 60 nm to 36 nm, while the dislocation density and the twin-fault probability increased from 38�1014 m�2 to 163�1014 m�2and 0% to 1.2%, respectively.

The alloys after HPT have a reduced electrical conductivity,mainly due to the presence of Cr atoms in solid solution (especiallyfor initially quenched states) and increase of the amount of grainboundaries and another lettice defects. Chromium atoms in solidsolution reduce the electrical conductivity much more stronglythan in the form of precipitates [5]. This effect yields a higherelectrical conductivity of the annealed specimens compared to the

quenched samples. The solubility limit of Cr in Cu is about 0.75 wt%at the temperature of the intial heat-treatment (1000 1C), thereforein the quenched states the solute Cr concentration cannot exceed thisvalue even if the nominal Cr content is higher (e.g. 9.85%).

Heat-treatments were applied in order to improve the electricalconductivity in the HPT-processed Cu–Cr alloys. The change of thehardness and the electrical conductivity as a function of the heat-treament temperature is shown in Fig. 1. It can be seen that thehardness decreased or remained at the same level while the electricalconductivity rised with increasing temperature. For both initiallyannealed and the quenched Cu–0.75%Cr alloys processed by HPTthe hardness decrease is marginal up to about 250 1С. This can beexplained by the lack of considerable grain growth, as revealed by thecomparison of Fig. 2a and b. These images show that in the quenchedand HPT-processed Cu-0.75%Cr alloy the grain size increased only

Fig. 2. Microstructure of the quenched Сu–0.7%Cr alloy after HPT (а) and heat-treatment at 250 1С (b), quenched Сu–9.85%Cr alloy after HPT (c) and heat-treatment at500 1С (d and e), as-cast Сu–27%Cr alloy after HPT (f and g) and heat-treatment at 500 1С (h). A nanosized Cr particle is shown in (e).

S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9 7

from �209 to �245 nm due to the heat-treatment at 250 1С. Fig. 1bindicates that above 250 1С a significant increment in the electricalconductivity took place through the supersaturated solid solutiondecomposition (for initially quenched state) and the grain growth(for both initially annealed and the quenched states).

Fig. 1 shows that the thermal stability of the HPT-processedCu–9.85%Cr alloys is better than that for the Cu–0.75%Cr compositionsince the higher Cr content has a stronger retarding effect on graingrowth. After the heat-treatment at 500 1C the grain size onlymoderately increased from �143 nm to �229 nm (see Fig. 2c and d).

Therefore, the decrease of the hardness is marginal even athigh temperatures while the electrical conductivity is considerablyincreased, as shown in Fig. 1c and d. The strongly improvedelectrical conductivity (by 38% IACS) in initially quenched alloyafter HPT can be explained by chromium precipitation and grainboundary relaxation.

Fig. 2e reveals the appearance of a dispersed Cr particle with thesize of about 20 nm. In the annealed and HPT-processed alloy grainboundary relaxation during heat-treatments has a major effect onelectrical conductivity, while it has only marginal influence onhardness.

It has been shown that the specific resistance of grain bound-aries in Cu decreases from 5.5�10�16Ωm2 to 2�10�16 Ωm2due to grain boundary relaxation during annealing [14]. Consider-ing the grain boundary relaxation and the slight grain growth at500 1C an electrical conductivity increase of about 13%IACS ispredicted which is in accordance with the experimental resultsfor annealed and HPT-processed sample.

For HPT-processed Cu–27%Cr alloy considerable softening was notobserved up to 500 1C (see Fig. 1e) since the grain structure remainedvery fine. As revealed by the TEM images in Fig. 2f–h the heating upto 500 1C leads to an increase in the grain size from 40 to 96 nm. Atthe same time, at 500 1C the electrical conductivity increased to thelevel characteristic for the as-cast state (see Fig. 1f).

Reducing the level of electrical conductivity down to 20%IACSduring HPT indicates a possible occurrence of deformation-inducedsupersaturated solid solution. Thus, a further increase in electricalconductivity upon heating occurs both due to decomposition of solidsolution, as well as grain boundary relaxation and moderate grain-growth.

Table 2 summarizes the hardness and the electrical conductiv-ity values in the initial and the HPT-processed states, and afterheat-treatments at 250 1C for the Cu–0.7%Cr alloy, and at 500 1Cfor the Cu–9.85%Cr and Cu–27%Cr alloys. It can be seen that HPT-processing and subsequent heat-treatment at appropriate tem-peratures in Cu-alloys with high Cr content yields better hardnessand electrical conductivity than in the initial states.

4. Conclusions

1. HPT-processing in copper-chromium alloys leads to a signifi-cant hardening due to the formation of UFG microstructure.With increasing chromium content the microhardness risesfrom about 1700 to 2700 MPa due to the reduction in averagegrain size from �209 to �40 nm as well as the increase of thechromium content, the dislocation density and the twin-faultprobability. The electrical conductivity is reduced with increas-ing Cr content due to the higher amount of grain boundariesand Cr alloying atoms.

2. The heat-treatment after HPT results in a gradual decrease ofthe hardness and an increase in electrical conductivity. For highCr contents (9.85% and 27%) an appropriate selection of theheat-treatment temperature enables the preservation of thehigh hardness while the electrical conductivity increased con-siderably. The significant improvement in the electrical con-ductivity can be explained by Cr precipitation and grainboundary relaxation. Our study demonstrates the capability ofHPT-processing and subsequent heating for obtaining bothhigh hardness and electrical conductivity in Cu–Cr alloys.

Acknowledgment

The work was supported by the Russian Foundation for BasicResearch (Project 13-08-00102), the Ministry of Education andScience of the Russian Federation (Project no. 14.A12.31.0001) andthe Hungarian Scientific Research Fund, OTKA, Grant no. K-109021.

References

[1] Valiev RZ, Zhilyaev AP, Langdon TG. Bulk nanostructured materials: funda-mentals and applications. Hoboken: TMS, Wiley; 2014.

[2] Vinogradov A, Ishida T, Kitagawa K, Kopylov VI. Effect of strain path onstructure and mechanical behavior of ultra-fine grain Cu–Cr alloy produced byequal-channel angular pressing. Acta Mater 2005;53(8):2181–92.

Table 2Electrical conductivity and microhardness of the alloys.

Alloy Treatment Initial state HPT HPT and heatingn

HV (MPa) %IACS

HV (MPa) %IACS

HV (MPa) %IACS

Cu–0.7%Cr

Quenching 804738 39 1740774 34 1830736 35Annealing 1000736 86 1612738 61 1435730 72

Cu–9.85%Cr

Quenching 1270765 37 2140722 29 1753744 67Annealing 12687104 63 2107787 54 18927137 76

Cu–27%Cr

As-cast 1402779 41 2698790 20 26387109 42

n The heating temperatures are 250 1C for the Cu–0.7%Cr alloy, 500 1C for theCu–9.85%Cr and Cu–27%Cr alloys.

Table 1The parameters of the microstructure obtained by TEM and X-ray line profile analysis: the grain size, the area-weigthed mean crystallite size (〈x〉area), the dislocation density(ρ) and the twin boundary probability (β).

Alloy Treatment Grain size (nm) 〈x〉area (nm) ρ (1014 m�2) β, (%)

Cu–0.7%Cr Quenching þHPT 209 5876 3874 070.1AnnealingþHPT – 6476 3874 070.1

Cu–9.85%Cr QuenchingþHPT 143 5277 7577 0.370.1AnnealingþHPT – 4775 7178 0.370.1

Cu–27%Cr HPT 40 3675 163710 1.270.1

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http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref1http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref1http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref2http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref2http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref2

[3] Wang QJ, Xu CZ, Zheng MS, Zhu JW, Du ZZ. Fatigue crack initiation lifeprediction of ultra-fine grain chromium–bronze prepared by equal-channelangular pressing. Mater Sci Eng A 2008;496(1-2):434–8.

[4] Shangina DV, Bochvar NR, Dobatkin SV. The effect of alloying with hafnium onthe thermal stability of chromium bronze after severe plastic deformation. JMater Sci 2012;47(22):7764–9.

[5] Shangina DV, Gubicza J, Dodony E, Bochvar NR, Straumal PB, NYu Tabachkova,et al. Improvement of strength and conductivity in Cu-alloys with theapplication of high pressure torsion and subsequent heat-treatments. J MaterSci 2014;49(19):6674–81.

[6] Dopita M, Janecek M, Kuzel R, Seifert HJ, Dobatkin S. Microstructure evolutionof CuZr polycrystals processed by high-pressure torsion. J Mater Sci 2010;45(17):4631–44.

[7] Mishnev R, Shakhova I, Belyakov A, Kaibyshev R. Deformation microstructures,strengthening mechanisms, and electrical conductivity in a Cu–Cr–Zr alloy.Mater Sci Eng A 2015;629:29–40.

[8] Dobatkin SV, Shangina DV, Bochvar NR, Janeček M. Effect of deformationschedules and initial states on structure and properties of Cu–0.18% Zr alloyafter high-pressure torsion and heating. Mater Sci Eng A 2014;598:288–92.

[9] Purcek G, Yanar H, Saray O, Karaman I, Maier HJ. Effect of precipitation onmechanical and wear properties of ultrafine-grained Cu–Cr–Zr alloy. Wear2014;311(1–2):149–58.

[10] Lamperti A, Ossi PM, Rotshtein VP. Surface analytical chemical imaging andmorphology of Cu–Cr alloy. Surf Coat Technol 2006;200(22–23):6373–7.

[11] Sauvage X, Jessner P, Vurpillot F, Pippan R. Nanostructure and properties of aCu–Cr composite processed by severe plastic deformation. Scr Mater 2008;58(12):1125–8.

[12] Bachmaier A, Rathmayr GB, Bartosik M, Apel D, Zhang Z, Pippan R. Newinsights on the formation of supersaturated solid solutions in the Cu–Crsystem deformed by high-pressure torsion. Acta Mater 2014;69:301–13.

[13] Ribárik G, Gubicza J, Ungár T. Correlation between strength and microstruc-ture of ball-milled Al–Mg alloys determined by X-ray diffraction. Mater Sci EngA 2004;387–389:343–7.

[14] Qian LH, Lu QH, Kong WJ, Lu K. Electrical resistivity of fully-relaxed grainboundaries in nanocrystalline Cu. Scr Mater 2004;50(11):1407–11.

S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9 9

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High strength and good electrical conductivity in Cu–Cr alloys processed by severe plastic deformationIntroductionMaterials and methodsResults and discussionConclusionsAcknowledgmentReferences