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Composition, microstructure, Vickershardness and activation energies ofCo–Cu alloys fabricated by arc meltingtechniqueA.M. Mebed a , Alaa M. Abd-Elnaiem a , Tesleem B. Asafa b & M.A.Gaffar aa Department of Physics, Faculty of Science, Assuit University,Assuit 71516, Egyptb Department of Mechanical Engineering, King Fahd University ofPetroleum and Minerals, Dhahran 31261, KSAVersion of record first published: 12 Apr 2012.

To cite this article: A.M. Mebed , Alaa M. Abd-Elnaiem , Tesleem B. Asafa & M.A. Gaffar (2012):Composition, microstructure, Vickers hardness and activation energies of Co–Cu alloys fabricated byarc melting technique, Phase Transitions: A Multinational Journal, 85:12, 1079-1090

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Phase TransitionsVol. 85, No. 12, December 2012, 1079–1090

Composition, microstructure, Vickers hardness and activation energies

of Co–Cu alloys fabricated by arc melting technique

A.M. Mebeday*, Alaa M. Abd-Elnaiema, Tesleem B. Asafab and M.A. Gaffara

aDepartment of Physics, Faculty of Science, Assuit University, Assuit 71516, Egypt;bDepartment of Mechanical Engineering, King Fahd University of Petroleum and Minerals,

Dhahran 31261, KSA

(Received 10 January 2012; final version received 17 February 2012)

We have determined the phase transition for the Co-20 and -30 at.% Cu alloysfabricated by arc melting technique, from the binodal to the two phases �þL aswell as the peritectic transitions, using differential thermal analysis (DTA). Weequally studied the effects of aging treatment, ranging from 3 to 35 h, on the alloysamples using scanning electron microscopy (SEM) and Vickers hardness (HV).The activation energies of these alloys are equally determined using fiveestablished models. Our results show that for aging time up to 15 h, within thespinodal region at 773K, the hardness value for Co-20 and -30 at.% Cu alloysoscillates reaching a local maximum at the aging time of 8.5� 0.5 h. After 20 h ofheat treatment, the HV for Co-20 at.% Cu alloy diminishes significantly whilethat of Co-30 at.% Cu effectively stabilizes at 241MPa. The activation energiesfor the peritectic transformation based on Ozawa model are estimated to be 2465and 2680 kJmol�1 for Co-20 and -30 at.% Cu, respectively.

Keywords: spinodal; bionodal; phase transformation; differential thermalanalysis; Co–Cu alloy; Vickers hardness

1. Introduction

The Co–Cu alloy system is well known as a peritectic system, but shows a metastablemiscibility gap in the undercooled liquid state [1,2]. For sufficiently large undercoolingduring melting the binodal, which separates the undercooled melt and the metastableregion, is crossed and the melt separates into a Co-rich (L1) and a Cu-rich phase (L2).Though, a few studies have been conducted to understand the kinetics of the phaseseparation and the accompanying microstructures, however, large discrepancies in theposition of the metastable miscibility gap still exist in the literature [2–10].

In the homogeneous materials, the phase change may be initiated by nucleation-growthor spinodal decomposition [11–14]. Spinodal decomposition is similar to an age hardeningreaction which involves quenching and subsequent heat treatment. However, instead offorming precipitates by a conventional nucleation and growth mechanism, regularvariations in composition occur in the lattice with an extremely fine spacing between them.

*Corresponding author. Email: abdu_55@yahoo.comyOn leave for: Al-Jouf University, Skaka-2014, KSA.

ISSN 0141–1594 print/ISSN 1029–0338 online

� 2012 Taylor & Francis

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The two constituents have the same crystal structure but different lattice parameters. Theresulting strain hardening leads to good mechanical properties. With increasing in agingtime the rate of precipitation of the second phase increases [15,16].

Differential thermal analysis can be considered as one of the best techniques to studythe phase transitions though other methods like microscopic observations and X-raydiffraction exist [17–24]. The popular use of DTA is not without the possibility forcareful control of the heating and thermal exchanges between the actual and thereference samples. In this work, we employed DTA to determine the unstable region andthe metastable region in miscibility gap. In addition, we studied various aspects ofphase transition for the Co-20 and -30 at.% Cu systems covering the temperature range upto 750K.

2. Experimental procedures

Few specimens of Co-20 and -30 at.% Cu alloys were prepared by means of arc meltingtechnique in a purified and dried argon atmosphere following the procedure discussedelsewhere [20]. Various samples ranging from 0.25 to 4mm in thickness were obtainedfrom the specimens. To investigate their thermal stability, all the samples wereencapsulated in an evacuated quartz tube and heated in the furnace set at 773K for 3 hand then cooled to the room temperature. The samples’ Vickers hardness (HV) values weremeasured and SEM micrographs were taken as discussed below. Then, the samples wereput back into the furnace set for the same temperature and dwelled for 3 h. They were thenice-quenched for about 5min. The HV were determined and SEM micrographs weretaken. This procedure was repeated until the total dwelling/aging times were 3, 5, 8, 11, 14,17, 20, 25, 30, and 35 h.

In order to observe their microstructures at each stage, some of the samples weremechanically polished and chemically etched in a solution of 5 g FeCl3, 10mL HCl and100mL alcohol at 273K. The microstructures were then observed through JOEL-JSM-5400LV SEM manufactured in Japan. Then, energy dispersive spectrometry (EDS)measurement was performed on the samples without etching to determine the averageelemental compositions of the as-prepared samples.

The values of the HV were measured with microhardness tester manufactured byMicromet, Adolph I. Buehler Inc., USA. The hitherto polished samples’ surfaces wereplaced under a tester load of 0.2 kg and held for 30 s. The hardness was estimated from theindentation depth and the hardness value of each sample was taken from the arithmeticalmean of 16 measured indentations. Because the indentations were measured randomly,these hardness values could well reveal the hardness distribution over the sample surface ata given dwelling time. The transition behaviors of the as-prepared samples were observedwith Shimadzu DTG-60H Differential Thermal Analyzer. To do this, samples of 20mgweight were placed in a platinum holder suspended from the arm of the balance by meansof a quartz wire and were observed under non-isothermal conditions with 10–40Kmin�1

heating rates.We, thereafter, used the results of the DTA thermograms to study the dependence of

the peritectic temperature TP on the heating rate � and subsequently for the evaluation ofthe activation energy of the peritectic transformation. We applied the non-isothermalthermo-analytical models of Ozawa, Takhor, Kissinger, Starink, and Madahevan toestimate the activation energies of the alloys [22–27]. These models are based on theAvrami treatment of the transformation kinetics and define an effective crystallization rate

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coefficient with Arrhenian temperature dependence [26,27]. These models are summarizedin the following sentences.

For a temperature T, Ozawa [22] modeled the relationship between the heating rate �and the volume fraction x, of the dissolved element as in the following equation:

ln � ¼ �1

n ln½� lnð1� xÞ��

E

RTp

� �þ ln T 2

� �þ const:

� �ð1Þ

where E is the effective activation energy describing the overall transformation process andn the reaction order which depends on the mechanism of nucleation and growth. Theactivation energy can be determined from the slope of a plot of ln (�) against (1/TP).Takhor’s model [23] takes the form:

ln�

Tp � T0

� �¼

1

n

� �ln

n

n� 1

� þ ln Koð Þ �

E

RTpð2Þ

Similar to the procedure for evaluation of the activation energy in Ozawa’s model, theactivation energy based on Takhor’s model can be estimated at a room temperature T0

from the plot of lnð �Tp�T0Þ against 1

Tp. In a seemingly simpler form, Kissinger presented a

model in the form of the following equation [24]:

ln�

T2p

!¼ �

E

R

� �1

Tp

� �þ CK ð3Þ

where CK is a constant. More recently, Starink [25] showed that Equation (4) can be usedto accurately determine the activation energy of the individual process.

ln�

T1,92p

!¼ �A

E

R

� �1

Tp

� �þ Cs ð4Þ

where A¼ 1.0008 and CS is a constant that depends on the reaction kinetic model. E can beestimated from the slope of the plot of lnð �

T1:92pÞ versus ð 1Tp

Þ. The approaches proposed byChan et al. [26] and Mahadevan et al. [27] can equally be used.

ln�

Tp

� �¼ �

E

R 1Tp

� þ lnðKoÞ ð5Þ

where R, Em, and Ko are the gas constant, activation energy, and frequency factor,respectively. By plotting ln(�/TC) against (1/TC) the Em can be obtained. The frequencyfactor Ko, as a decomposition parameter, measures the probability of forming new nucleiby the nucleation-growth mechanism.

3. Results and discussion

3.1. The elemental compositions of the as-prepared samples

Figure 1(a) shows the EDS chart for Co-20 at.% Cu. The chart exhibits peakscorresponding to Co and Cu elements. The average Co and Cu compositions, using the kfactor of each element, are estimated to be 86.5% and 13.5%, respectively. For Co-30at.% Cu alloy, the average composition of Co is found to be 73.5% and 26.1% for Cu andits EDS chart is shown in Figure 1(b). These results imply that the Co compositions are

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increased by 8.5% and 5% for Co-20 and -30 at.% Cu alloys, respectively, at the expenseof Cu compositions which are lowered by 32.5% and 13% for the two alloys. Thedifferences between the as-prepared and the observed elemental compositions can beattributed to the spinodal phase decomposition during the heat treatment process.

3.2. The HV

The HV values for the Co-20 and -30 at.% Cu binary alloys annealed at 773K for 3 h withdifferent thickness are shown in Figure 2. Annealing at 773K was considered because itfalls within the spinodal decomposition region of the phase diagram as indicated inFigure 3. It is observed that the value of HV is independent of the thickness within theexperimental error. This result is in agreement with the fact that the HV is thickness

Figure 1. EDS chart for the (a) Co-20 at.% Cu and (b) Co-30 at.% Cu alloys.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

50

100

150

200

250

300

350

400

Har

dnes

s (H

V)

Thickness (mm)

Co-20CuCo-30Cu

Figure 2. HV vs. thickness for Co-20 and -30 at.% Cu alloys annealed at 773K for 3 h.

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independent. It is also found that the HV for Co-20 at.% Cu is higher than that of Co-30

at.% Cu. Analysis of variance of these results indicates that the HV values are significantly

different between the two alloys. The 95% confidence intervals for the mean difference of

25.14MPa are 6.08 and 44.21. Since the confidence interval does not enclose zero,

accordingly the HV difference is significant at the 0.05 level. The F- and p-statistics of 8

and 0.0134 equally supports this conclusion. For the samples annealed for 5 h, the

conclusion is similar to that of 3 h, but only for the magnitudes of the statistical

parameters. The HV values are generally higher with mean difference of 40MPa and the

95% confidence interval of 27.53 and 52.47. The F- and p-statistics are estimated to be

51.08 and 0.00003 which indicate that the difference in the HV is more significant.The HV of the Co–Cu alloys based on the aging times are shown in Figure 4. It is

observed that HV for Co-20 at.% Cu alloy are generally higher than those of Co-30 at.%

Cu alloy in the entire range of aging times considered. For aging time up to 15 h, the

changes in the hardness values for the two alloys oscillate significantly. At the aging time

of 8.5� 0.5 h for the two samples, a local increase in the hardness is recognized. These

0 5 10 15 20 25 30 35220

230

240

250

260

270

280

290

300

HV

(MP

a)

Aging Time at 773 K (h)

HVCo20Cualloy HVCo30Cualloy

Annealed samples

Figure 4. Aging time (at 773K) dependence of HV for Co-20 and -30 at.% Cu alloys. The horizontalgrids are to serve as guides for the eyes.

Figure 3. Phase diagram of Co–Cu [1], with the calculated spinodal and the binodal lines [21]. Thesolid squares represent the annealing conditions.

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values are approximately 280 and 240MPa for Co-20 and -30 at.% Cu, respectively.Beyond 20 h of aging time, the hardness values are observed to decrease significantly forCo-20 at.% Cu while remaining relatively stable at 241 MPa for Co-30 at.% Cu alloys. Itis observed that the HV for all the samples are lower than their annealed counterparts.

The behavior of the hardness value may be explained in terms of precipitation andcoarsening mechanisms. The initial dendritic microstructures of the alloys at the beginningof the heat treatments, within and out of the miscibility gap, are partially dissolved duringaging. This activates the spinodal decomposition and precipitation process is initiated. Thealloy strength increases to a value above the HV obtained after 9 h of aging due to theexistence of the fine precipitates. For increased aging time up to 16 h, the precipitatescoarse forming more large precipitates that have opposite effect on the alloy strength. It isnoticeable that as the Cu content increases the HV decreases. Apparently, the coarseningprocess has almost completed after 20 h for the case of Co-30 at.% Cu, while precipitatesin Co-20 at.% Cu are still coarsening. The simulation results on Cu–Co alloys confirm thissuggestion [21]. Therefore, HV values after 20 h of aging time decreases significantly forCo-20 at.% Cu, while it is relatively stable for Co-30 at.% Cu.

The significant reduction in the hardness values for the Co-30 at.% Cu alloy withincreasing aging time compared to those of Co-20 at.% Cu alloy is due to the increase inthe coarsening process.

Figure 5. SEM microstructure of Co-20 at.% Cu alloy at heat treated at 773K (a) annealed (b)aging time of 3 h (c) aging time of 8 h, and (d) aging time 11 h.

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3.3. Microstructure of the annealed samples

The SEM images of the samples after annealing at 773K and for aging time of 3, 8, and

11 h are shown in Figure 5(b)–(d), respectively. For the sake of comparison, the

morphology of the as-prepared sample (Figure 5a) is also included. The as-prepared

sample was undercooled into the metastable miscibility gap and the evaluation of the

microstructure evolution during this process is significant to the understanding of the

phase transformation. During cooling, the liquid melt is transformed into a dendritic

structure with Co-rich L1-phase and a Cu-rich L2-phase through growth and coagulation.

The separation to two phases by spinodal decomposition is considered to be responsible

for the wide gap obtained in the DTA curve of Figure 6. With increasing aging time, L2

dissolves partially into L1 phase and accordingly phase decomposition is expected at the L1

and L2 boundaries and L1/L2 interface. This increase in L2 phase at the expense of L1

phase was already discussed in Section 3.1.

800 1000 1200 1400

DT

A s

igna

l (ar

b. u

nits

)

Temperature(K)

Ts-b

Tp

Tb-α+L

(3)

(2)

(1)

Figure 6. The DTA curves performed at different heating rates [(1) 10Kmin�1, (2) 30Kmin�1, and(3) 40Kmin�1] for as prepared Co-20 at.% Cu alloys.

Figure 7. SEM microstructure of Co-30 at.% Cu alloy (a) after annealing at 773K for 3 h and(b) after annealing at 773K for 8 h.

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Figure 7(a) and (b) are the SEM micrographs of the annealed Co-30 at.% Cu sample at773K and aging time of 8 h, respectively. Similar to the finding in Co-20 at.% Cu alloy,Co-30 at.% Cu alloy contains dendritic structure of the Co-rich L1-phase and a Cu-richL2-phase. In addition, several particles of Cu are found in the Co-rich phase and may beresponsible for the higher hardness observed at the aging time of 8 h compared to that of5 h as shown in Figure 4.

3.4. Phase transition of the annealed samples

The DTA curves of the phase transition for Co-20 at.% Cu alloy at the heating rate of 10,30 and 40Kmin�1 are shown in Figure 6. The transition temperature Ts-b corresponds tothe onset of the phase transformation from the unstable spinodal phase to the metastablebinodal two-phase regions. The values of the Ts-b at the heating rates 10, 30 and40Kmin�1 are about 1080, 950 and 980K, respectively. Apparently, this transitionprocess is stretched over an extended temperature interval of about 550K, as estimatedfrom the phase diagram for Co-20 at.% Cu alloy of Figure 3 [1]. The peritectictemperature (Tp) for Co-20 at.% Cu alloy is manifested on the DTA traces as a sharp peakin the range 1374–1383K for the different heating rates. The transition temperature fromthe binodal to the two phases �þL (Tb-�þL) is found to be about 1450K at rate 40 for Co-20 at.% Cu alloy.

As the aging time increases, the interface comes closer to the calculated spinodal line dueto the systematic change in the solubility limit with the particle size. This conclusion is logicalsince in metastable equilibrium with different aging temperature, the concentrations changecontinuously with the boundary line of the metastable miscibility gap. The DTA resultsmight indicate the continuity of the decomposition kinetics at the boundary betweenunstable and the metastable regions which is in accordance with our previous work [20].

From the phase diagram of Co-30 at.% Cu alloy shown in Figure 8, the magnetic andthe peritectic transition temperatures are observed to lie inside the spinodal region [1]. Thetemperature for the magnetic transition is about 1310K while it is 1385K for the peritectictransition. From the DTA chart of Figure 8, the peritectic temperature (Tp) is found to be

900 1000 1100 1200 1300 1400 1500

DT

A s

igna

l (ar

b. u

nits

)

Temprature(K)

Tc

Tp

Ts-b

(3)

(2)

(1)

Figure 8. DTA curves performed at different heating rates [(1) 10Kmin�1, (2) 20Kmin�1, and (3)30Kmin�1] for as prepared sample of Co-30 at.% Cu alloys.

1086 A.M. Mebed et al.

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1378, 1382, 1384, and 1386.5K for heating rates of 10, 20, 30, and 40Kmin�1,

respectively. The characteristic onset temperature of this process is designated as Tp

on the DTA chart of Figure 8. It is worth to mention that the peritectic transition for the

Co-30 at.% Cu alloy has been observed at 1385K at low cooling rates between 50 and

70K s�1 [28].Three peaks at 1410, 1414, and 1417K for different heating rates of 10, 20, and

30Kmin�1, respectively, are identified. These corresponding temperatures are suggested to

be for the transition from the spinodal to the binodal region which agrees well with those

calculated from phase diagram [1]. The characteristic onset temperature of this process is

designated as Ts-b on the DTA chart of Figure 8. No indication of magnetic transition is

found in the DTA chart and further study is in progress to explain the reason for this

absence.

4. Kinetic parameters for the peritectic transition

Based on Equation (1), as deduced by Ozawa, plots of ln (�) versus (1/TP) for Co-20 and

-30 at.% Cu alloys can be adjusted to excellently fit the experimental data (Figure 9). In

such a case, the values of Ep estimated from the slopes of the fitted straight lines are 2465

and 2680 kJmol�1 for Co-20 and -30 at.% Cu alloys, respectively. Applying

Equations (2)–(5) to the same experimental data and follow the same procedure as

discussed in Section 2, the activation energies of the alloys are obtained for different

models as presented in Table 1. A good indicator for the selection of the best models is the

goodness of fit. Ozawa model has the best goodness of fit and thus the values of its

activation energy are taken for the alloys. The frequency factor Ko (from Equation (5)) is

calculated from the intercepts along the vertical axis and are found to be 5.8� 1090 and

1.01� 1099min�1 for Co-20 and -30 at.% Cu alloys, respectively.

0.721 0.722 0.723 0.724 0.725 0.726 0.727 0.728

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ln(α

)

1000/Tp[K-1]

y=-296.6x+218 R2=0.9933

y=-322.5x+236 R2=0.9814

Figure 9. Plot of ln(�) vs. (1/Tp) of Co-20 at.% Cu alloy (solid triangle) and Co-30 at.% Cu (solidsquare) alloy. The scattered points are the experimental data and the solid lines are the linear fits tothe experimental points.

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The overall nucleation rate constant (K) at any temperature T in the entire region of

the endothermic peak is usually assumed to have an Arrhenian temperature dependence as

in the following equation:

K ¼ Ko exp �E

RT

� �: ð6Þ

Substituting the activation energy Ep and the frequency factor Ko, the value of K as a

function of T can be estimated for each heating rate. The calculated value of K(T) for Co-

20 at.% Cu alloys at a constant heating rate is plotted as lnK versus 1/T as shown in

Figure 10. From Figure 10, it is noticed that the decomposition rate constant K is faster at

higher heating rates than at lower heating rates. In other words, the Co-rich phase is being

thermally formed more rapidly at higher temperatures.

Table 1. Comparison between the average activation energiesfor the peritectic transition of the Co-20 and -30 at.% Cualloys.

Alloy Method

Activationenergy

(kJmol�1)

Co-20 at.% Cu Ozawa 2465Mahadevan 2453Kissinger 2442Starink 2441Takhor 2450

Co-30 at.% Cu Ozawa 2680Mahadevan 2669Kissinger 2657Starink 2657Takhor 2666

0.7265 0.7270 0.7275 0.7280 0.7285 0.7290

208.7028

208.7029

208.7030

208.7031

208.7032

208.7033

208.7034

208.7035

ln K

1000/T [K-1] 1000/T [K-1]

(a)

0.7222 0.7224 0.7226 0.7228 0.7230 0.7232 0.7234

208.70445

208.70450

208.70455

208.70460

208.70465

208.70470

208.70475

208.70480

208.70485

ln K

(b)

Figure 10. Temperature dependence of the nucleation rate constant for Co-20 at.% Cu alloy atconstant heating rate of (a) 10Kmin�1, (b) 40Kmin�1. The solid lines are taken as guides to the eye.

1088 A.M. Mebed et al.

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5. Conclusions

The following conclusions can be drawn based on the results.

(1) The DTA results confirmed that there is no distinction between metastable andunstable states but a wide step throughout the metastable region driven by theactivation energy required for the nucleation of new precipitates.

(2) The Tp for Co-20 at.% Cu alloy is manifested in the DTA traces by a sharp peak ata temperature in the range 1373–1386K for different heating rates. The transitiontemperature from the binodal to the two combined phases �þL (Tb-�þL) is foundto be about 1450K at rate of 40Kmin�1 for Co-20 at.% Cu alloy.

(3) For the Co-30 at.% Cu alloy the magnetic and the peritectic transitions are bothexisting inside the spinodal temperature region. Temperature of the magnetictransition is about 1310K while it is 1385K for the peritectic transition. Weobserved that the magnetic peak disappeared from the DTA charts.

(4) The value of HV is independent of the sample thickness within the experimentalerror. The value of HV is higher for Co-20 at.% Cu alloy than that for Co-30 at.%Cu alloy for different heat treatments.

(5) The presence of the fine precipitates in Cu–Co alloy is responsible for higherhardness in the alloys. Larger precipitates are formed at longer aging time and HVdiminishes.

(6) The growth rate of cobalt precipitates increase with the cobalt concentration in thealloy.

(7) The activation energies were determined by five different models and their resultsare similar.

References

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[3] A. Munitz, S.P. Elder, and R. Abbaschian, Supercooling effects in Cu-10 Wt Pct Co alloys

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