Materials Science & Engineering A 582 (2013) 219–224

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Materials Science & Engineering A

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n CorrMetal RChina. T

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Effect of continuous extrusion on the microstructure and mechanicalproperties of a CuCrZr alloy

Hui Feng, Haichang Jiang, Desheng Yan, Lijian Rong n

Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China

a r t i c l e i n f o

Article history:Received 23 May 2013Received in revised form3 June 2013Accepted 8 June 2013Available online 20 June 2013

Keywords:Continuous extrusionDynamic recrystallizationPrecipitateStacking fault energy

93/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.msea.2013.06.031

espondence to: Division of Materials for Specesearch, Chinese Academy of Sciences, 72 Wenel.: +86 24 23971979; fax: +86 24 23978883.ail address: ljrong@imr.ac.cn (L. Rong).

a b s t r a c t

A Cu–0.16Cr–0.12Zr alloy with an initial grain size of about 400 μmwas extruded by continuous extrusionforming (CEF), where severe plastic deformation and precipitation process occurred. Scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM) were employed to examine themicrostructure and morphology of the precipitates. Experimental results show that a notable grain sizereduction to sub-micron scale is obtained through continuous dynamic recrystallization and precipitatesmaintain a fine and disperse morphology after the CEF process. These two features are considered as theeffective ways to improve the strength and ductility of the CuCrZr alloy after cold deformation andsubsequent aging process without a significant decrease of electrical conductivity.

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1. Introduction

Taking the excellent mechanical properties, high electricalconductivity, and high thermal conductivity, CuCrZr alloys arebeing considered as the primary candidate material for applica-tions, such as the International Thermonuclear ExperimentalReactor first wall, electrical contact, and the trolley contact wirefor high speed railway, etc. [1–3]. Due to the work hardening incold deformation process and precipitation hardening in agingtreatment, CuCrZr alloy could gain high tensile strength. So, highercontents of Cr and Zr are required to reach better mechanicalproperties. However, this method will bring about metallurgicalproblems and inevitability decrease the electrical conductivity.Thus, it is required to improve the mechanical properties of theCuCrZr without an increase of alloying element contents [2,4,5].

Continuous extrusion forming process [6] is a severe plasticdeformation technology, and is considered as an alternative way toimprove the strength without losing electrical conductivity sig-nificantly. In addition, the advantage of producing significantlong continuous products, which is hard for the conventionalforming processes, makes the CEF process competitive in indus-trial production. The CEF technology has been adapted to improvethe strength of the CuMg alloys, which are used for high speedrailway contact wires, and the grain size can be refined to about

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ial Environments, Institute ofhua Road, Shenyang 110016,

2 μm through recrystallization [7]. Besides, the electrical conduc-tivity of Cu–Mg alloys is not decreased significantly after the CEFprocess. However, relevant researches on the application of con-tinuous extrusion on CuCrZr alloys almost have not been con-ducted up to now because the CEF process is complicated for theprecipitation of hardened alloy, which not only involves the grainrefinement, but also influences the precipitation process.

In the present study, the CuCrZr rods were continuallyextruded on the TLJ400 equipment. The microstructures, mechan-ical properties and electrical conductivities of the alloys producedby CEF process are compared with that by the conventionalprocess.

2. Experiments procedure

The CuCrZr alloy was produced in a vacuum induction furnaceby adding electrolytic copper, CuCr intermediate alloy and pure Zr.The billets were hot extruded into a diameter of 20 mm. Thecompositions of experimental alloys are analyzed and listed inTable 1.

After solution annealing (1233 K for 2 h), the as-hot-extrudedrods were subjected to the corresponding processing, respectively:

(A)

Cold drawing (75%) and aging (723 K for 2 h); (B) CEF process, cold drawing (75%) and aging (723 K for 2 h).

For the CEF process, the feedstock with a diameter of 20 mmwas solution treated, and was extruded into a rod of 25 mm in

Table 1Chemical compositions of the CuCrZr alloys (wt%).

Alloy Cr Zr Cu

Cu–0.16Cr–0.12Zr 0.16 0.12 Bal.

Fig. 1. Microstructure of Cu–0.16Cr–0.12Zr alloy after solution treated.

Fig. 2. OM and SEM microstructures showing intense shear bands (SB)of Cu–0.16Cr–0.12Zr alloy after the CEF process.

H. Feng et al. / Materials Science & Engineering A 582 (2013) 219–224220

diameter using the TLJ400 copper continuous extrusion machine.The wheel speed is 4 rpm and the preheating temperature is723 K. Samples were sliced parallel to the extrusion direction sothat the deformed structure could be characterized.

Using the Schlumberger 7081 precision digital voltmeter withan accuracy of 10 nV, the electrical resistance was determined bymeasuring the voltage of samples with a constant current of 50 mA.The relative electrical conductivity (% IACS) was deduced fromelectrical resistance according to Ref. [8]. Microstructural character-ization was performed along the drawing direction by opticalmicroscopy (OM) and the FEG-SEM (JEOL 7001F) equipped withan EBSD (HKL system). The 700 μm thick foils used for transmissionelectron microscopy (TEM) observation were cut from the colddrawn bar after aging treatments and then were mechanicallygrounded to 50 μm, followed by two-jet electro-polishing at 243 Kin a 30% nitric/methanol acid solution. Particle sizes, distributionand coherency were observed on a JEM 2010 transmission electronmicroscopy with an operating voltage of 200 kV.

3. Experimental results

3.1. Microstructure

Fig. 1 shows the microstructure of Cu–0.16Cr–0.12Zr alloy aftersolution treated at 1233 K for 2 h. A homogeneously distributedand coarse grained microstructure can be found in the alloy with agrain size of about 400 μm. Within the grains, annealing twins canbe observed clearly.

The microstructure of Cu–0.16Cr–0.12Zr alloy after the CEFprocess is shown in Fig. 2. Large equiaxed grains obtained fromsolution treatment disappeared, instead, a deformed morphologywith intense shear bands (SB) could be clearly recognized, whichcan be attributed to the shear deformation induced by the frictionbetween the rod and the extrusion die. At a larger magnificationshown in Fig. 2(b), it could be noticed that shear bands inclined to

each other are composed of largely refined grains and the relativelysmooth zones between the shear bands consist of larger grains.

The Cu–0.16Cr–0.12Zr alloy after the CEF process was subjectedto a 75% cold drawing and an aging treatment at 723 K for 2 h(Process B), while the alloy without the CEF process subjected tothe same processing and heat treatment (Process A), as illustratedin Section 2. Fig. 3 shows the final microstructure of CuCrZr alloysafter Processes A and B, respectively. It can be seen in Fig. 3(a) thatthe grain spacing of Cu–0.16Cr–0.12Zr alloy after Process A reachesdozens of micrometers. For coarse grained alloys, dislocationmovement and twinning are the primary deformation mechan-isms during the cold drawing. When subjected to a large enoughstrain, macro-shear bands develop due to severe local deforma-tion. While in contrast, the uniform ribbon-like grains elongatedalong drawing direction was found in the Process B sample, with amean ribbon width of about 1 μm (Fig. 3(b)), which helps toimprove the ductility of the alloy.

3.2. Mechanical properties

The mechanical properties before and after the CEF process arelisted in Table 2. Due to the severe plastic deformation, both theyielding and ultra strength are improved drastically, meanwhile

Fig. 3. Longitudinal sectional morphology of CuCrZr alloy after Process A (a) and B(b) at different magnifications.

Table 2Mechanical properties of Cu–0.16Cr–0.12Zr alloy before and after CEF process.

Yield strength(MPa)

Ultra strength(MPa)

Elongation (%)Conductivity

(% IACS)

Solutiontreated

72 218 48.0 38.5

After CEF 375 415 23.6 44.8

Table 3Final mechanical properties of Cu–0.16Cr–0.12Zr alloy after Processes A and B.

Yield strength(MPa)

Ultra strength(MPa)

Elongation (%)Conductivity

(% IACS)

Process A 510 550 15.7 78.7Process B 570 590 17.2 77.6

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the elongation drops apparently. Besides, the conductivity showsan improvement after continuous extrusion. It is widely acceptedthat the electrical conductivity is strongly influenced by the latticedistortion originated from the change of degree of solute super-saturation. Then it could be deduced that the precipitationoccurred to release the supersaturation of the matrix during thecontinuous extrusion, regarding the relatively high temperature inthe process.

The final mechanical properties of CuCrZr alloys after ProcessesA and B are shown in Table 3. It can be noticed that the Cu–0.16Cr–0.12Zr alloy subjected to continuous extrusion gains higher tensilestrength and elongation than the alloy without continuous extru-sion process, i.e. 590 MPa and 17.2% versus 550 MPa and 15.7%.Meanwhile, the decrease of electrical conductivity is negligible. In

fact, the alloy after Process B possesses a larger amount (sub)grainboundaries than that after Process A. However, boundaries have aweak effect on the electrical conductivity. For example, when thegrain size was refined to nano-scale, the conductivity of pure Cudecreased by only about 3% IACS [9]. Thus, it is reasonable that theelectrical conductivity of CuCrZr alloy after Process B is slightlylower than that after Process A.

4. Discussion

4.1. Grain refinement of continuous extrusion

The EBSD result of continuous extruded rod is shown in Fig. 4.The microstructure is elongated along the extrusion direction.Most of the grains are very fine and in a sub-micron scale.According to the EBSD data, grains are divided into recrystallized,sub-structured and deformed ones depending upon the grain-to-grain misorientation angles and marked by different colors. Thus,it can be seen in Fig. 4 that the sub-structured grains (marked byyellow regions) are dominating here, with a fraction of about 75%according to EBSD analysis. Meanwhile, the recrystallized grains(marked by blue regions) can also be found, with a fraction close to20%. And there are still about 5% grains defined as deformed ones.The EBSD results demonstrate that the CuCrZr rod was incomple-tely dynamic recrystallized during the continuous extrusionprocess.

4.1.1. Grain refinement mechanismFig. 5 shows the dominant subgrain formation process in the

specimen, which includes the following process: first, dislocationsaccumulate (Fig. 5(a)); then dislocations tangle under furtherstraining (Fig. 5(b)); at a certain strain level, dislocations will berearranged at the dislocation tangles area to minimize the totalsystem energy, thus part of dislocations annihilate resulting in theformation of sub-boundaries (Fig. 5(c)). The corresponding selectedarea electron diffraction (SAED) pattern of Fig. 5(a), similar to that ofa copper single crystal, indicates that the misorientation across theaccumulated dislocations is negligible, and the SAED patterns ofFig. 5(b) and (c) show small misorientations between adjacentgrains.

It has been proved that the stacking fault energy (SFE) plays animportant role in determining the plastic deformation behavior offace centered cubic metals [10]. Pure Cu, especially CuCrZr alloy,possesses a relatively low SFE. Lower SFE implies that the partialdislocations will be separated farther, which will induce moredislocation tangles to restrain dislocation movement because crossslip will be very difficult and even forbidden in some cases. Withfurther straining, these dislocation tangles act as dislocationsources, which promote the piling ups of dislocations, resultingin the morphology with high density of dislocations.

As the continuous extrusion process provides the accumulatedstrain, dislocations annihilation and rearrangement occur forminimizing the total system energy, which leads to the formationof sub-boundaries. Then, part of the sub-boundaries transformedinto high angle grain boundaries and the recrystallization volumefraction is about 20% according to Fig. 3. In nature, this dislocationmovement process typically characterizes the continuous dynamic

Fig. 4. EBSD maps of continuous extruded rod: (a) color-coded grain orientation asdetermined by EBSD; (b) the same area as in (a) showing recrystallization fraction: theblue region represents recrystallized grains, yellow region represents sub-structuredgrains and the red region represents deformed grains. (For interpretation of referencesto color in this figure legend, the reader is referred to the web version of this article.)

H. Feng et al. / Materials Science & Engineering A 582 (2013) 219–224222

recrystallization (CDRX) [11]. Previous studies have shown that thealloy with lower SFE could reach a smaller grain size after recoveryand recrystallization than those with high SFE [12]. For example,the grain size of pure Cu after equal channel angular process witha true strain of about 8–12 is about 200 nm while that of pure Alis just 400–700 nm [13,14]. In this work, the grain size aftercontinuous extrusion is less than 1 μm (Fig. 5), which is reasonableconsidering that the true stain is only 2–3 yet the SFE ofCu–0.16Cr–0.12Zr alloy is slightly lower than pure Cu.

Fig. 5. TEM images illustrating the formation of sub-grains: (a) dislocation accu-mulation, (b) dislocation tangles, and (c) dislocation rearrangement. The insets arethe corresponding SADPs.

4.1.2. Strengthening effect of grain refinementThe mechanical properties of metallic materials are strongly

affected by their grain sizes. A general relationship betweenstrength and grain size was proposed by the Hall–Petch equation[15,16]:

sy ¼ s0 þ kyd1=2

where sy is the friction stress, s0 and ky are the constants and drepresents the grain size. According to Ref. [10], ky of Cu–0.16Cr–0.12Zr alloy could be estimated as 130 MPa μm1/2. In this study, theaverage grain size before and after the continuous extrusion areabout 400 μm and 1 μm respectively. Regarding the relatively

weaker pinning effect of low angle boundary than that of highangle grain boundary, the strength improvement due to grain sizereduction is approximately 90 MPa.

Fig. 6. Precipitate morphology of CuCrZr alloy after the CEF process.

Fig. 7. Comparison of micro-hardness between the technologies with and withoutCEF process. In the diagram, ‘ST’ represents solution treated. (For interpretation ofreferences to color in this figure, the reader is referred to the web version of thisarticle.)

H. Feng et al. / Materials Science & Engineering A 582 (2013) 219–224 223

It could be noticed in Table 2 that the strength improvementcaused by CEF process is about 200 MPa, much higher than thestrengthening effect by grain refinement, 90 MPa. The main reasonis that the CEF process involves not only the process of grainrefinement but also the precipitation, which will be discussed below.

4.2. Precipitates

The Cu–0.16Cr–0.12Zr alloy was solution treated prior to thecontinuous extrusion process, so dynamic precipitation took placeduring the extrusion process. Fig. 6 shows the precipitate mor-phology of the CuCrZr alloy after continuous extrusion. Theprecipitates are homogeneously distributed with an average sizeof about 10 nm, which are similar to the partial size of those ofpeak age treated. According to the Refs. [5,17–19], the precipitatesare indexed to be Cr, CrCu2Zr and Cu4Zr, and are coherent with thematrix. Though the temperature reported inside the extrusionchamber is above 1000 K [20], the maintenance of a fine pre-cipitate morphology may be attributed to the short time ofextrusion process and to the water cooling after the extrusion.

4.2.1. Effect of precipitate on the morphology and strength incrementThe precipitates have a great effect on the morphology of the

CuCrZr alloy after continuous extrusion. The coherent precipitatesare distributed at the (sub)grain boundaries as well as inside thegrains, as shown in Fig. 5. They could effectively pin sub-grainboundaries and dislocations, which significantly prohibit thetransition from low angle boundaries to high angle boundariesand promote the accumulation of dislocation inside the grains. Asa result, only about 20% of the grains recrystallized and most of thegrains are elongated, as shown in Fig. 4.

Besides the effect on the morphology, the precipitationstrengthening of the precipitates could be estimated by [21,22]:

sCOH ¼Mε3=2Grfb

� �1=2

where M is a numerical constant ranging from 2.6 to 3.7, ε denotesthe misfit strain at the coherent interface, G the shear modulus ofthe matrix, b the modulus of Burgers vector of matrix dislocationand r the radius of precipitates and f their volume fraction.Parameters of the CuCrZr alloy are obtained from Refs. [23–25], i.e. G¼44 GPa, b¼2.56�10−10 m and ε¼0.015. The volume fractionof nano-scale precipitates was roughly estimated through the

measurement of the area fraction of precipitates in TEM images.Thus, after continuous extrusion, the volume fraction is about 0.5%in this study. Based on the value above, the maximum strengthen-ing effect in the studied material is about 80 MPa.

Besides the grain refinement and precipitation strengthening,the CEF process also introduces work hardening effect. As demon-strated in Fig. 4, part of the grains are deformed. Work hardening, aswell as grain refinement and precipitation strengthening, explainsthe strength improvement in Table 2 after the CEF process.

4.2.2. Precipitating process with and without continuous extrusionFor Process A as mentioned in Section 2, precipitation only

occurs and completes in the aging process after cold drawing.While with the introduction of continuous extrusion (Process B),the precipitation process was divided into two steps: precipitationin continuous extrusion process and in aging process, respectively.Fig. 7 shows the comparison of the micro-hardness increasebetween the technologies with and without continuous extrusion.For the traditional process (depicted in black), the micro-hardnessshows a remarkable improvement, about 40 HV, during the agingtreatment after solution treatment and cold drawing. While forthe technology with continuous extrusion (depicted in red), asstrengthening particles precipitate in the continuous extrusionprocess (verified by the conductivity increment in Table 2 andprecipitate morphology in Fig. 6), the micro-hardness incrementduring aging treatment is relatively weak, about 10 HV.

However, the hardness of the alloy with CEF process after agingis about 10 HV higher, which is consistent with the tensile result. Itcan be deduced that grain refinement has a great effect on theimprovement of mechanical properties. Although no evidenceshows the two-step precipitation makes contribution to thestrength increment, the fine and homogeneously distributedprecipitates after continuous extrusion help the alloy maintain ahigh level of strength. Besides, it should be noticed in the diagramthat only about 1 h is needed to reach peak aging hardness for thealloy with continuous extrusion process while 2 h for the alloywithout continuous extrusion process. Shorter peak aging timemay be attributed to the previous precipitation during the CEFprocess and to the residual stress after continuous extrusion,which is of great significance for improving the efficiency ofindustrial scale production.

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5. Conclusions

1.

Cu–0.16Cr–0.12Zr alloy with continuous extrusion formingprocess reaches an high strength of 590 MPa and anelongation of 17.2%, which are 40 MPa and 1.5% higher thanthat of the alloy with the traditional hot working process,respectively. Meanwhile, the electrical conductivity has anegligible drop from 78.7% to 77.6% IACS.

2.

Ultrafine (sub)grains of sub-micron scale are obtained bycontinuous dynamic recrystallization after continuousextrusion process, which has great contribution to theimprovement of strength and ductility.

3.

During the continuous extrusion process, the precipitatesmaintain a fine size and disperse distribution, whichrestrains the dynamic recrystallization procedure, and aswell helps the alloy to reach a high level of strength.

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