Tensile deformation and fracture behavior of CuZn5 brass alloyat high temperature

M. Sharififar, S.A.A. Akbari Mousavi n

School of Metallurgy and Material Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 28 April 2013Received in revised form11 November 2013Accepted 14 November 2013Available online 22 November 2013

Keywords:Electron microscopyNon-ferrous alloysBulk deformationFractureHardening

a b s t r a c t

Alpha brass alloys are widely used for production of rectangular waveguides because of their low bulkresistivity. In this paper, the microstructure, tensile deformation and fracture behavior of CuZn5 brassalloy were investigated. The strain rate sensitivity and its relation to post-uniform deformation in tensiletest and correlation between strain hardening exponent (n) and temperature were examined. The resultsshow that strain hardening exponent decreases from 0.5 to 0.4 with increase in test temperature from250 to 450 1C. Tensile fracture mechanisms of as-extruded CuZn5 brass alloy were studied over a range oftemperatures from 300 to 450 1C and range of strain rates from 0.01 to 0.4 1/s by means of scanningelectron microscope (SEM) and Atomic Force Microscope (AFM). The results show that different fracturemechanisms operate in different temperature and strain rate ranges. While transgranular dimple fractureis dominant at 300 1C and 0.4 1/s, the dominant fracture mechanism at 450 1C and 0.01 1/s is cleavagefacets. Precipitations and grain boundary sliding at high temperature may be the mechanism of ductilitydrop. Dynamic strain ageing (DSA) did not occur since none of the manifestations of DSA are observed.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Rectangular waveguides are the one of the earliest type of thetransmission lines that is used for microwave frequencies. They areused in many applications. Typically waveguide is made of brass,copper, silver, aluminum, or any metal that has low bulk resistivity.Comprehensive information about metallurgical factors of rectan-gular waveguides can be achieved from ASTM B372 standard [1].CuZn5 is one of the alloys used to produce rectangular wave-guides. The rectangular waveguides can be produced by hotextrusion process. The rectangular waveguides were undergonemicrowave heating during service. The maximum temperature thematerials experienced are about 450 1C. Therefore, the study wasconducted to consider the tensile deformation and fracture beha-vior of CuZn5 brass alloy between 250 and 450 1C.

So far many researchers studied the deformation behavior andmechanical properties of Face Centered Cubic (FCC) alloys. Mahmudi[2] studied the post-uniform deformation behavior of AA5050,AA3105 and AA8014 aluminum alloys and showed that the extentof post-uniform deformation achieved for different alloys was relatedto their strain rate sensitivity. In addition, few investigations werecarried out to study the work hardening behavior of FCC alloys such

as copper [3], stainless steel [4,5] and brass [5,6]. Recent studiesrevealed that in FCC polycrystalline alloys of low stacking faultenergy (SFE), the strain hardening behavior is distinctly differentfrom that of materials of higher SFE [7,8]. Strain hardening rateversus strain plots for low SFE alloys show four-stage strain hard-ening response that is related to forming and decreasing of twining.This trend is observed in some FCC alloys such as 70/30 brass and 316stainless steel [9]. However, FCC polycrystalline alloys of high SFEshow two-stage strain hardening response in the strain hardeningrate versus strain plots [10]. Rohatgi et al. [10] studied the strainhardening response of Cu and Cu–Al alloys. They showed that instage 1 the strain hardening rate decreased sharply by increase ofstrain and in stage II hardening rate was almost constant.

So far many researchers studied the fracture surfaces for thetensile test sample and considered the correlation between thefracture mechanism and ductility of the material. Sajjadi andZebarjad [11] studied the correlation between change of ductilityand fracture surface for Ni–base super alloys at different tempera-tures. It was shown that the tensile fracture behavior was inagreement with the strength and ductility variation of the alloywith temperature. Void formation was found to be a dominantfracture mechanism though transgranular and intergranular frac-ture and cleavage were also seen as other fracture mechanismsoperating in specific temperature ranges. Wouters et al. [12]studied the fracture mechanisms occurred due to strain ratevariations. They discussed various fracture mechanisms operatingunder various ranges of strain rates.

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0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.msea.2013.11.051

n Corresponding author. Tel.: þ98 21 82084096; fax: þ98 21 88006076.E-mail addresses: m.sharififar@ut.ac.ir (M. Sharififar),

akbarimusavi@ut.ac.ir (S.A.A. Akbari Mousavi).

Materials Science & Engineering A 594 (2014) 118–124

Use of Atomic Force Microscope (AFM) is widely used incorrosion and surfaces engineering studies [13,14]. Shyam et al.[15] used AFM to study the slip line in the fatigue fracture surfaceof nickel–base super alloy. Guerrero et al. showed 3D images ofsemi-crystalline polypropylene fracture surface using AFM [16].However, the analysis of metallic fracture surfaces with AtomicForce Microscope has not been studied so far to the best of ourknowledge.

Brasses are commercial standardized materials with well-known properties reported in engineering handbooks. Few spe-cific studies were carried out on the brass alloys. In [17], which isin Japanese language, only the effect of Zn-content and strain rateon fracture elongation, reduction of area and n-value werediscussed and there was no study on the temperature effect andalso there was no study on the tensile fracture mechanisms of thebrass alloys investigated. Moreover, in the study of [18], thetemperature range is 470–770 K, which is in the range of thisstudy; however, the strain rate effect was not studied in this paper.No study, to date was found to the best of our knowledge to studyboth the effects of hot deformation, strain rate and fracture. Insummary, study of hot deformation of CuZn5 alloy at temperaturerange of 250–450 1C and strain rate of 0.001–0.4 1/s was notstudied at the same time to the best of our knowledge. In addition,as described in the introduction of the paper, the analysis ofmetallic fracture surfaces with Atomic Force Microscope was notinvestigated so far.

In this paper, the microstructure and tensile deformation of as-extruded CuZn5 brass alloy was investigated. In addition, tensilefracture mechanisms were studied by means of SEM and AFM atdifferent temperatures and strain rates.

2. Experimental procedure

The material used in this study was alpha brass alloy knownaccording to UNS standard C21000 [19]. The chemical compositionof CuZn5 alloy used is tabulated in Table 1.

Metallographic samples were cut from the as extruded rectan-gular profile of the alloy in longitudinal direction. The samples weremounted initially and polished on progressively finer grades ofsilicon carbide paper using copious amounts of water as lubricant.Subsequently, the samples were polished using 0.3 μm alumina-based polishing compound suspended in water. Grain morphologyand other microstructural features were revealed using an etchantmixture of 1.5 ml nitric acid, 15 ml distilled water and 1.5 mgr FeCl3.The polished and etched specimens were examined by optical andscanning electron microscopes (SEM).

Subsized flat tensile test specimens with 100 mm overalllength, 25 mm gauge length, 10 mm width of grip section, 6 mmwidth of gauge and 2 mm thickness according to ASTM-E08 [20]were provided using a wire cut machine from the as-extrudedmaterial in longitudinal direction.

Uniaxial tensile tests were performed up to failure on a fullyautomated mechanical test machine (SANTAM) with furnace andtemperature control system. Tensile tests carried out at thetemperatures of 250, 300, 350, 400 and 450 1C. Before each test,the specimen was soaked at the test temperature for 15 min so asto achieve stability with the environment. The strain rate wascontrolled by a crosshead displacement rate. Tests were performed

at crosshead displacement rates of 1.5, 15, 150 and 600 mm/mingiving nominal strain rate values of 0.001, 0.01, 0.1 and 0.4 1/s,respectively. The stress and strain measurements were recordedon a PC-system.

Fracture surfaces of the fully deformed and failed tensile sampleswere examined by scanning electron microscope (SEM) to investigatemechanisms governing tensile fracture. Samples for SEM observationwere obtained from the fully deformed tensile specimens by section-ing parallel to the fracture surface and cleaning the fracture surfaceusing acetone solution and ultrasonic system. Quantitative analysis offracture surface was carried out by Nanosurf Easyscan 2 Atomic ForceMicroscope (AFM) using the non-contact mode.

3. Results and discussion

3.1. Microstructure

The microstructure of the as-extruded material in the long-itudinal direction is shown in Fig. 1. The as-extruded material

Fig. 1. Optical microstructure of CuZn5 at 250� .

Fig. 2. SEM image of annealing twins at 2000� produced during air colling afterextrusion process.

Table 1Chemical compositions of CuZn5 brass alloy used in this study (wt%).

ISO no. Copper Zinc Lead Ni Sb Bi Al

CuZn5 Base 4.9151 0.0078 0.0172 0.0503 0.0569 0.0145

M. Sharififar, S.A.A. Akbari Mousavi / Materials Science & Engineering A 594 (2014) 118–124 119

revealed a single phase and equiaxed grain structure with averagegrain size of 30 μm that was obtained with Clemex Vision Soft-ware. Moreover, the annealing twins are shown in Fig. 2. The

annealing twins were created during cooling stage in air subsequentto hot extrusion process. Mechanical twins tend to be lens-shapedwith sharp edges which reduces the long range elastic strains andthe annealing twins have flat sides and they are formed as aconsequence of growth accidents during recrystallization of

250300

350

400450

250300 350

400

250

400

300

350

450

250

300

350400

450

Fig. 3. (a) True stress–true strain curve at strain rate of 0.001 1/s. (b) True stress–true strain curve at strain rate of 0.01 1/s. (c) True stress–true strain curve at strain rate of0.1 1/s. (d) True stress–true strain curve at strain rate of 0.4 1/s.

Fig. 4. Variation of ultimate tensile strength (sUTS) with strain rate at varioustemperatures.

350°C

400°C

450°C

300°C

250°C

Fig. 5. The magnitude of m with strain rate at various temperatures.

250

300

350

400 450

Fig. 6. Strain hardening rate–strain curve of CuZn5 alloy in _ε¼ 0:01 1=s.

Fig. 7. Relationship between n and temperature at _ε¼ 0:01 1=s.

M. Sharififar, S.A.A. Akbari Mousavi / Materials Science & Engineering A 594 (2014) 118–124120

deformed cubic-close packed metals such as copper, austenitic iron,and nickel [21,22].

3.2. Hot tensile behavior

The true stress–strain curve of the CuZn5 brass alloy achievedat 250, 300, 350 and 450 1C and 0.001, 0.01, 0.1 and 0.4 1/s areshown in Fig. 3a–d, respectively.

The results showed the degradation of ultimate tensile strength(sUTS) with increase in test temperature. Fig. 4 shows that increaseof temperature from 250 to 450 1C results in decrease of sUTSabout 40–50%. The mechanism responsible for the decrease inultimate tensile strength may be due to activation of moredislocation slip systems in higher temperature [21]. According tothe literature, temperature increase leads to activation of new slipsystems. At high temperatures, the thermal energy may assist thedislocations to overcome obstacles at lower values of applied

Fig. 8. (a) Fracture surface at 300 1C and 0.01 1/s. (b). Fracture surface at 350 1C and 0.01 1/s. (c) Fracture surface at 400 1C and 0.01 1/s. (d). Fracture surface at 450 1C and0.01 1/s. (e). Fracture surface at 400 1C and 0.1 1/s. (f) Fracture surface at 400 1C and 0.4 1/s.

M. Sharififar, S.A.A. Akbari Mousavi / Materials Science & Engineering A 594 (2014) 118–124 121

stress than that required at 0 K. Under such conditions, an increasein temperature will reduce the flow stress [23]. However, there isno considerable difference between the true stress–strain curvesachieved at constant temperature for various strain rates. Accord-ing to Eqs. (1) and (2), the strain rate sensitivity (m) of the alloy isequal to the slope of Log s– Log _ε in constant temperature.

s¼ Cð_εÞmjε;T ð1Þ

m¼ d log sd log _ε

� �� Δ log s

Δ log _ε

� �ε;T

¼ log s2� log s1log _ε2� log _ε1

¼ log ðs2=s1Þlog ð_ε2=_ε1Þ

ð2Þ

Fig. 5 shows the magnitudes of m at various temperatures. Themagnitudes of m are in the range of 0.002–0.006 that are very lowmagnitudes. The results show that change of strain rate has nosignificant effect on the strength properties of the CuZn5 at 250–450 1C temperature ranges.

It is accepted that post-uniform deformation is mainly con-trolled by strain rate sensitivity [2]. The strain rate sensitivityinfluences the resistance to necking. Under the circumstanceswhere a region in a deforming body undergoes thinning whilethe strain rate increases, the strain rate hardening causes to slowdown the rate of deformation, thereby resisting strain localization.The resistance of materials to necking results in an enhanced post-uniform elongation and a more gradual rate of loss of load-carrying ability. This matter can be elaborated by comparing thepost-uniform deformation area in Fig. 3a–d and the magnitude ofstrain rate sensitivity (m) calculated from Fig. 5.

3.3. Strain hardening

It is reported that in high SFE Face Centered Cubic (FCC) alloys,the ds/dε�ε plot shows two stages [10]. In stage I, the strainhardening rate decreases sharply with increase of strain and instage II exhibits an almost constant strain hardening rate [10].Fig. 6 shows the strain hardening rate–strain curve of CuZn5 alloyfor _ε¼ 0:01 1=s. Fig. 6 shows that: (1) increase of temperature doesnot change the strain hardening mechanism of the alloy sincethere is no difference between the trend of the strain hardeningrate-strain curves obtained at various temperatures and (2) strainrate hardening slightly decreases with temperature. This matter inturn shows that at high temperature slip systems are moreactivated. Therefore, the variation of strain rate hardening withtemperature is almost constant.

In Fig. 7 relationship between work hardening exponent (n)and temperature for _ε¼ 0:01 1=s is plotted. n Is the magnitude ofstrain in the point that curves ds/dε�ε and s�ε intersected. It isclear that the work hardening exponent decreases near a constantslope from 0.5 to 0.4 with increase in test temperature from 250 to450 1C.

3.4. Tensile fracture

The fracture surfaces of the CuZn5 brass alloy undergonetensile testing at different temperatures and strain rates wereexamined to correlate the fracture characteristics with ductilityproperties. Top views of fracture surfaces of the alloy are shown inFig. 8a–f.

Observation of fracture surfaces for most specimens indicates alarge number of dimples that is the characteristic of ductilefracture. Of course in various temperatures and strain rates, the

Fig. 9. Correlation between temperature and %El of tensile test specimens in0.01 1/s.

Fig. 10. AFM images of fracture surface in 0.01 1/s: (a) 300 1C and (b) 450 1C.

Fig. 11. Correlation between strain rate and % El in temperature of 400 1C.

M. Sharififar, S.A.A. Akbari Mousavi / Materials Science & Engineering A 594 (2014) 118–124122

change in fracture surfaces and mechanisms can be seen. Fig. 9shows the correlation between temperature and %El of tensile testspecimens for strain rate of 0.01 1/s.

It is shown that within an unexpected trend, the elongationof the CuZn5 brass alloy decreases in the temperature range of300–450 1C. Some researchers studied the relation between duc-tility drop and temperature increase for some alloys. It wasmentioned in [24] that the dynamic strain aging is the reasonfor ductility drop. However, in [25], the grain boundary siding isdescribed as a reason for ductility drop. Formation of microscopiccavities around inclusions and precipitations on grain boundariesare other reasons of ductility drop as mentioned in [26,27].Moreover, presence of melted metal in the field is another reasonof ductility drop as illustrated in [12,28]. In this study, followingthe previous investigations, because of very low amount of leadand bismuth in the CuZn5 chemical compositions and also becauseof no any confirmation in the stress–strain curves to dynamicstrain aging, and because of no detected crack at the fracturesurface, it is more probable that the grain boundaries precipita-tions and grain boundary sliding at high temperature is thedominant mechanism for ductility drop. However, detection ofdominant mechanism of ductility drop needs more study andwould be elaborated in further study. According to above investi-gations and [29], the ductility drop of alloys at some temperatureranges is due to dynamic precipitation and grain boundary sliding(GBS). It can be seen from Fig. 8b and c that with increase oftemperature from 350 to 400 1C, no change in fracture mechanismoccurs. At 300 and 350 1C, it is clear that the fracture surfacescontain large dimples indicating a ductile transgranular mode offracture (Fig. 8a and b). The samples fractured at 400 and 450 1Cshow cleavage fracture surface with little plasticity (Fig. 8c and d).Comparison between Figs. 8b–d and 9 shows that there is a goodaccordance between change in fracture mechanism and ductilitydrop in temperature range of 300–450 1C. Typical AFM images atstrain rate of 0.01 1/s are shown in Fig. 10 which illustrates themorphology of fracture surface. It is clear that in a 25�25� μm2

area, the fracture surface at 450 1C (Fig. 10b) is smoother than thatat 300 1C (Fig. 10a). The difference between peak and throughpoints in a 25�25 μm2 of fracture surface at 300 1C is 6.15 μm andat 450 1C is 1.75 μm, respectively. The reason of this difference isattributed to the forming of deep dimples in ductile fracture at300 1C and cleavage facet in brittle fracture at 450 1C.

A similar observation can be seen at 400 1C and strain rates of0.001–0.4 1/s. Fig. 11 shows the correlation between strain rateand %El in temperature of 400 1C. It is shown that the elongation ofthe alloy increases with increase of strain rate.

Fig. 8c, e, and f shows that with increase of strain rate from 0.01to 0.4 1/s, the mechanism of fracture converts from brittle fracture

to ductile fracture. The fracture surface at 0.01 1/s shows cleavagefracture surface (Fig. 8c) and that at 0.4 1/s shows large dimplesindicating a ductile fracture (Fig. 9f). Comparison between curve ofelongation–strain rate (Fig. 11) and fracture surfaces (Fig. 8c, e andf) shows a good accordance between change in fracture mechan-ism and ductility variation of the alloy in strain rate range of 0.01–0.4 1/s and temperature of 400 1C. AFM images of strain rates 0.01and 0.4 are shown in Fig. 12a and b. The difference between peakand through points in a 25�25 μm2 of fracture surface at 0.01 1/sis 2.65 μm and that at 0.4 1/s is 10.7 μm, respectively. Similar toFig. 10, forming of cleavage faces at 0.01 1/s strain rate and deepdimples at 0.4 1/s strain rate and constant temperature is thereason of variation of fracture surface between Fig. 12a and b.

Dynamic strain aging (DSA) is a phenomenon occurring atintermediate strain rates and temperatures, and arises due tointeraction of mobile dislocation with solute atoms. The variousmanifestations of DSA are as follows: (1) a plateau or a hump inthe strength, (2) a peak in the work hardening, (3) negative strainrate sensitivity, (4) a ductility minimum with variation withtemperature, and (5) serrated plastic flow in DSA temperaturerange [24]. From Fig. 3a–d, it can be seen that there is no serratedplastic flow at any given temperature range. Moreover, any plateauor a hump in the strength cannot be seen. The curve of n as afunction of temperature (Fig. 7) also does not display any sharpmaxima. In addition, negative strain rate sensitivity cannot be seen(Fig. 5). Only a ductility minimumwith variation with temperaturecan be seen from strain–stress curves, therefore DSA cannot beidentified as a reason for cleavage type fracture under the specifiedconditions. It was mentioned in [30] that dynamic strain aging didnot occur in the temperature range of 250–450 1C for the dualphase steel. They showed that at any temperature range thatdynamic strain aging occurs, some of the above manifestations canbe observed for same time. Following the above discussions, sincenone of the above mentioned manifestations was observed, there-fore, DSA cannot be the reason for cleavage fracture for the CuZan5material considered.

4. Conclusions

The aim of this investigation was to study the tensile behaviorand fractography of the as-extrude CuZn5 brass alloy deformed bytension at different temperatures and strain rates. The correlationbetween strain rate sensitivity and post-uniform deformation andalso relation between strain hardening and temperature wereinvestigated. It is shown that strain hardening exponent (n)decreases from 0.5 to 0.4 with increase in test temperature from250 to 450 1C. SEM and AFM investigations of the fractured tensile

Fig. 12. AFM images of fracture surface in 400 1C: (a) 0.01 1/s and (b) 0.4 1/s.

M. Sharififar, S.A.A. Akbari Mousavi / Materials Science & Engineering A 594 (2014) 118–124 123

specimens indicated that the fracture mechanisms operating at eachcondition are different. The occurrence of a given mechanismdepends on the test temperature and strain rate. It was shown thatthe tensile fracture behavior was in agreement with the ductilityvariation of the CuZn5 brass alloy with temperature and strain rate. Afracture mechanism involving void formation occurs over a specificrange of experimental conditions. Cleavage fracture is the othermechanism operating in a specific temperature and strain rate range.Precipitations and grain boundary sliding at high temperature maybe the mechanism of ductility drop. Dynamic strain ageing (DSA) didnot occur since none of the manifestations of DSA are observed.

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