Mechanical properties of the high-entropy alloy Ag0.5CoCrCuFeNi at temperatures of 4.2–300K M. A. Laktionova, E. D. Tabchnikova, Z. Tang, and P. K. Liaw Citation: Low Temp. Phys. 39, 630 (2013); doi: 10.1063/1.4813688 View online: http://dx.doi.org/10.1063/1.4813688 View Table of Contents: http://ltp.aip.org/resource/1/LTPHEG/v39/i7 Published by the AIP Publishing LLC. Additional information on Low Temp. Phys. Journal Homepage: http://ltp.aip.org/ Journal Information: http://ltp.aip.org/about/about_the_journal Top downloads: http://ltp.aip.org/features/most_downloaded Information for Authors: http://ltp.aip.org/authors Downloaded 09 Sep 2013 to 132.174.255.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://ltp.aip.org/about/rights_and_permissions LOW-TEMPERATURE PHYSICS OF PLASTICITYAND STRENGTH Mechanical properties of the high-entropy alloy Ag0.5CoCrCuFeNi at temperatures of 4.2–300K M. A. Laktionovaa) and E. D. Tabchnikova B. I. Verkin Institute of Low-temperature Physics and Engineering, National Academy of Sciences of Ukraine, pr. Lenina 47, Kharkov 61103, Ukraine Z. Tang and P. K. Liaw University of Tennessee, Knoxville, Tennessee 37996, USA (Submitted December 12, 2012; revised February 12, 2013) Fiz. Nizk. Temp. 39, 814–817 (July 2013) The plastic deformation behavior of the high-entropy alloy Ag0.5CoCrCuFeNi produced in an argon-arc melt is studied for the first time at low temperatures (down to 4.2 K). Lowering the temperature from 300 to 4.2 K leads to an increase in the nominal yield strength from 450 to 750 MPa while the degree of plasticity of the alloy remains on the order of 30% over the entire range. The strain rate sensitivity is measured for deformations e� 2% by strain rate cycling. Assuming thermally activated plasma deformation, the activation volume for movement of dislocations is calculated for e� 2% and is found to vary from 122b3 at 300 K to 35b3 at 30 K, where b is the Burgers vector.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4813688] 1. Introduction The mechanical properties of a new class of materials, cast multicomponent alloys, have recently been under active study,1–3 especially at room temperature and above. These alloys, known as high-entropy alloys (HEA) consist of at least five metallic elements, each with a concentration rang- ing from 5 to 35 at. %.4,5 It is assumed that the stability of these structures is a result of their high entropy of mixing. A reduction in the free energy owing to the high entropy of mixing makes it possible to obtain simple structures in multi- component alloys.5–7 For this reason HEA are more ther- mally stable and are more durable than traditional alloys.8,9 It has been found1 that the high-temperature strength of HEA is determined by the choice of elements in the alloy, with their different atomic radii, melting temperatures, diffu- sion coefficients, and concentrations. The mechanical prop- erties of HEA and the relation of these properties to their microstructure at temperatures ranging from room tempera- ture to 1200 �C have been discussed widely in the litera- ture.1,6,10,11 The alloy Ag0.5CoCrCuFeNi (the concentrations of the elements are in molar ratios) has been studied in great- est detail. It has been found that the structure of this alloy consists of a solid solution with an fcc lattice consisting of two phases with different chemical compositions. The matrix phase has regions that are enriched in copper, in the shape of dendrites of size 1–10 lm containing nanocrystalline inclu- sions with sizes of 5–10 nm. High strength and plasticity are well combined above room temperature.10 This kind of alloy has potential as a structural material, but there are no data on the plastic properties of HEA below room temperature. Thus, experimental studies of the mechanical properties (strength and plasticity) of these alloys at cryogenic tempera- tures will be of some interest. This paper is a study of the mechanical properties of the HEA Ag0.5CoCrCuFeNi with decreasing temperature from 300 to 4.2 K. Studies of the low-temperature mechanical properties of this alloy are of basic as well as applied importance. 2. Experimental technique The initial blanks were obtained by fusing of compo- nents (purity at least 99 at. %) in an arc furnace with a puri- fied argon atmosphere. The resulting 2-mm-diam rod was cut into cylindrical samples of length 4 mm. In order for the loaded ends to be mutually parallel, they were polished in advance. Deformation was produced on a rigid strain machine (machine strength �7� 106 N/m) by uniaxial com- pression at a rate of 4� 10�4 s�1 over temperatures of 4.2–300 K. Intermediate temperatures in the 77–300 range were obtained by blowing nitrogen vapor through a shower that encompassed the sample and in the 4.2–77 K range by cooling the sample in helium vapor. The resulting "load- time" diagrams were converted to "stress-plastic deforma- tion" (stress-strain) curves r(e). r was defined as the ratio of the load to the initial cross sectional area of the sample. e was calculated as the ratio of the change in length of the sample resulting from plastic deformation to its initial length. Loading of the samples was stopped at strains of �20%–30% for all temperatures. At strains of �2% the rate sensitivity of the deformation stress, Dr=D ln _e, was meas- ured; for this the strain rate _e was increased by a factor of 4.4. 3. Results and discussion Figure 1 shows typical strain curves r(e) for various temperatures from 300 to 30 K. It is clear that in this temperature range the r(e) curves have the staged character typical of coarse grained fcc met- als.12 After a parabolic initial stage for e� 1% the r(e) curves undergo a transition to a linear hardening stage. High 1063-777X/2013/39(7)/3/$32.00 VC 2013 AIP Publishing LLC630 LOW TEMPERATURE PHYSICS VOLUME 39, NUMBER 7 JULY 2013 Downloaded 09 Sep 2013 to 132.174.255.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://ltp.aip.org/about/rights_and_permissions plasticity of the samples was observed over the entire tem- perature range. Fracture of the samples was not observed up to strains of �30%, when loading was stopped. The transition from the parabolic initial stage to the lin- ear hardening stage for the different temperatures shows up clearly in Fig. 2, where the rate of strain hardening dr/de is plotted as a function of the plastic deformation e. The plastic undergoes a transition from smooth to dis- continuous in the strain curves at temperatures below 15 K. The inset to Fig. 1 shows some typical discontinuous strain curves in coordinates r(e) measured at 4.2 and 7.5 K. At 4.2 K the stress jumps set in for strains e¼ 1.2%, and at 7.5 K for 4.5%. The size of the stress jumps Dr increases with increasing strain; thus at 4.2 K it increases from 20 MPa for e� 2% to 67 MPa for e� 23%. We note that at cryogenic temperatures a change in the form of the plastic flow from smooth to discontinuous is typical of many polycrystalline fcc alloys.13,14 Figure 3 shows the temperature dependence of the yield strength r0.2 for temperatures of 4.2–300 K. At 300 K the nom- inal yield strength is 450 MPa, which is considerably higher than the analogous values for coarse grained polycrystalline fcc alloys such as Cu-11.8 at. % Al (�70 MPa), Cu-20 at. % Ni (�50 MPa)9 and Al-3.8 at. % Li (�150 MPa).15 A comparison of the data in Fig. 3 and data on the temperature dependence of the elastic modulus E for an alloy of similar composition16 shows that the yield strength of Ag0.5CoCrCuFeNi alloy increases with falling temperature more rapidly than the elastic modulus. Figure 4 is a plot of the rate sensitivity of the normal de- formation stress, Dr=D ln _e, as a function of temperature for e� 2%. Dr=D ln _e decreases monotonically as the tempera- ture is lowered from 300 to 30 K. The observed temperature dependences of r0.2 and Dr=D ln _e are intrinsic to thermally activated character of plastic deformation. Based on that assumption, the activation volume V for motion of the dislocations (for e� 2%) was calculated for different temperatures using the formula8 VðTÞ ¼ kT D ln _e DsðTÞ ; (1) where k is the Boltzmann constant, T is the absolute temper- ature. and Ds(T)¼Dr/m is the change in the shear stress (m¼ 3 is the Taylor factor for an fcc lattice). V was found to decrease with decreasing temperature from 122b3 at 300 K to 35b3 at 30 K (the Burgers vector b¼ 2.54� 10�10 m for complete {111}h110i slide dislocation systems6). Once the temperature dependence of V (for e� 2%) is known, it is possible to obtain a first order estimate of the FIG. 1. Typical stress-strain curve r(e) for different temperatures with com- pression strain. The inset shows the discontinuous stress-strain curves for 7.5 and 4.2 K. FIG. 2. The hardening coefficient dr/de as a function of plastic deformation e at different temperatures. FIG. 3. Temperature dependence of the nominal yield strength r0.2. FIG. 4. Temperature dependence of the rate sensitivity of the normal defor- mation stress, Dr=D ln _e, for e� 2%. Low Temp. Phys. 39 (7), July 2013 Laktionova et al. 631 Downloaded 09 Sep 2013 to 132.174.255.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://ltp.aip.org/about/rights_and_permissions length L of a dislocation segment participating in the ther- mally activated motion, L ¼ V bd ; (2) where d is the width of the barrier to be overcome thermally. Taking d� b, we get L� 31� 10�9 m (for 300 K) and L� 8.9� 10�9 m (for 30 K). Taking the density of disloca- tion pinning points to be q¼ 1/L2, for q we find 1.04� 1015 m�2 (for 300 K) and 1.26� 1015 m�2 (for 30 K). These values are an order of magnitude higher than their analogs for a number of coarse grained fcc alloys.15 Identifying the microscopic mechanisms responsible for plastic deformation at low temperatures, however, will require further experimental study of the effect of plastic de- formation on the alloy microstructure, obtaining detailed dependences of its thermal activation parameters on defor- mation and temperature, etc. 4. Conclusions The plastic deformation behavior of the high-entropy alloy Ag0.5CoCrCuFeNi has been studied at temperatures of 4.2–300 K for the first time. Within this temperature interval, the samples had high plasticity. Fracture of the samples was not observed at plas- tic deformations exceeding 30%. It was found that this alloy has high durability. The nom- inal yield strength increases from 450 to 750 MPa over tem- peratures of 300–4.2 K. At temperatures below 15 K the form of the stress-strain curves r(e) changes from smooth to discontinuous. Assuming a thermally activated plastic deformation pro- cess for e� 2%, we have calculated the activation volume for thermally activated motion of dislocations. It decreases from 122b3 to 35b3 as the temperature is lowered from 300 to 30 K. These studies have shown that the high-entropy alloy Ag0.5CoCrCuFeNi can be used over a wide range of temperatures, from 800 �C10 to 4.2 K as a material with a unique combination of high durability and plasticity. a)Email:
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