Influence of the Thermal Treatments on the Structural ...

D

Journal of Materials Science and Engineering A 3 (4) (2013) 256-262

DAVID

PUBLISHING

Influence of the Thermal Treatments on the Structural Relaxation in a Metallic Glass Al80Fe8Mo7Ni5 Mohamed Abo-Elsoud1 and Awad Al-Rashdi2 1. Materials Science Laboratory, Physics Department, College of University, Al Gunfuda, Umm Al Qura University, Saudi Arabia 2. Chemistry Department, College of University, Al Gunfuda, Umm Al Qura University, Saudi Arabia Received: January 09, 2013 / Accepted: February 06, 2013 / Published: April 10, 2013. Abstract: In the present paper, the study of metallic glass Al Fe Mo Ni which shows the amorphous state is altered by structural relaxation and crystallization processes. The variations of the Doppler broadening of the positron annihilation line-shaped S-parameter were reported. The behavior of the annihilation line-shape shows reversible and non-reversible contributions. Crystallization of the sample appears as a three-stage process. The corresponding crystallization process consists of only one activation enthalpy, Eai. The mean value obtained for Eai is: Eam = (3.65  1.14) eV. X-ray diffraction showed that the crystallization was a complex involving growth of the Al Fe Mo crystalline phase along with crystallization of the amorphous matrix Al phase at T  500 C. Electron micrograph with image analysis of the scanning electron microscopy pictures confirmed the presence of fcc-Al and Al Fe Mo crystalline phase. Key words: Structural relaxation, metallic glasses, phase transitions, crystallization, Doppler broadening positron annihilation.

1. Introduction

state has been described both in terms of topological

A number of papers on the study of metallic glasses show the amorphous state is altered by structural relaxation and crystallization processes. Amorphous aluminum metallic glasses systems are often produced in ternary compositions containing Al-RE-TM (RE = rare earth, TM = transition metal) due to their good glass formability and have been found to have bending ductility of about 180 in wire samples [1, 2]. For example, Al-Y-Fe systems have good formability, due to negative heats of mixing and the large ( 12%) atomic size differences [3, 4] of the constituent elements. The addition of a second transition metal to these

alloys

to

form

quaternary

systems,

Al-RE-TM1-TM2, can further enhance the glass formability [5]. Positron annihilation behavior in the amorphous

short-range

ordering

(TSRO)

and

chemical

short-range ordering (CSRO) at the basis of the structural

relaxation

mechanisms

[6].

During

crystallization the positron behavior is determined by the phase diagram of the amorphous and crystallized alloy system. The superposition of reversible and non-reversible variations has been reported for metallic glasses by many authors [3, 7, 8]. The reversible effect was ascribed to CSRO, while the non-reversible effect was assigned to TSRO. The TSRO is usually related to the loss of excess free volume in the as-quenched amorphous alloy [7]. The CSRO processes occur by atomic jumping [3, 8]. Essentially,

all

prior

Doppler

broadening

measurements [9] have been performed using either slow

positron

beams

or

wide-energy-spectrum

positron beams from radioactive sources. Two parameters S (for shape), and W (for wings) [10] are

Corresponding author: Mohamed Abo-Elsoud, Ph.D., associated professor, research fields: nano-materials and quasi-crystal alloys. E-mail: [email protected].

usually used to characterize the annihilation peak. The S-parameter is more sensitive to the

Influence of the Thermal T Treattments on the e Structural Relaxation R in a Metallic Gllass Al80Fe8Mo M 7Ni5

2577

coo oling of the sample, s durinng isothermaal heating off the sample and at room tem mperature on an annealedd sam mple. The heaat treatment w was performeed in vacuum m -3 (10 Pa), a prrogrammed furnace wass used. Thee mettallic glass of o the presennt study, Al80 8 Fe8Mo7Ni5, wass commerciallly obtained iin the form of o ribbons off

Fig. 1 Dop ppler broadeniing line-shapee from which h the S-parameter is i defined. Thee limits g1, g2, g3 are arbitraary to a certain degrree, but have to be the same for all annihilaation lines analyzed d.

annihilation with low w momentum m valence and unbound ellectrons. Thhe S-parameeter defined by Mackenzie, et al. [11] as a the ratio of o the integraation over the cenntral part of thhe annihilatioon line to the total t integration. Diffraction peaks are analyzed a throough common fittting procedurres, which ressult in parameeters like the ceentre of graavity and thhe width of the distribution. Fig. 1 shows s Dopppler broadenning line-shape from f which the t S-parameeter is calcullated using the folllowing equattion: S where,

(1) x d , and xcc is the centeer of

the peak. This papeer reports on kinetic transsformations from f the amorphoous to the cryystallized statte in the mettallic glass Al80Fee8Mo7Ni5 allooy. The effeccts of the therrmal treatments on the structural relaxation and crystallizatioon have been b studiedd by posiitron annihilation. The influencce of crystalline phases onn the crystallizatioon of the amorphous phase will be discussed.

2. Experim ments Doppler-bbroadening poositron annihhilation technnique (DBPAT) have h been peerformed durring heating and

2.6 cm wide and 60 m m thick. Eacch measuredd specimen consissted of two sttacks of five layers of thee sam mple materiall, with a possitron sourcee sandwichedd betw ween them. For F the Dopppler-broadeniing measurem ments duringg the isochronal annealing a off the sample the positronn sou urce of 1 m Ci C free carrieer 22NaCl waas evaporatedd from m an aqueouus solution of sodium chloride c andd dep posited betweeen two thin Al foils (7.5 5 m), whilee for measuremennts during hheating and cooling andd during isotherm mal heating of the samplee a thin 22Naa sou urce was used as positron so ource. Thee Dop ppler-broadenning line-shhape of thee 511 keV V positron annihillation radiatioon was meassured with a hyp perpure Ge detector d and a measuring g chain withh zero o and gain digital stabbilization. The T Dopplerr broadening lineshape wass characterizzed by thee S-parameter as defined d by Mackenzie, et al. a [11]. For F the measuurements on the isochronaally annealedd sam mple each storred spectrum contained at least 1  105 cou unts in the 511 keV peak. The sample remained forr two o hours at eaach step of thhe annealing temperature.. Forr the measureements duringg continuouss heating andd coo oling of the sample the storage of each e Dopplerr broadening specctrum lasted oone hour. Thee temperaturee chaange rate waas about 3 C/h. The data d for thee linee-shape specttra were anallyzed by usin ng the SP-01 com mputer prograam [12]. Characterizati C ion of the saamples from m the variouss stattes (as-cast, annealed or after being tested) wass don ne by X-ray diffraction w with CuK α radiation r andd scan nning electrron microsccopy examination withh observationss. elecctron miccrograph In-situu high h-temperature X-ray difffraction (HT TXRD) wass con nducted durinng isothermal annealing of o the testedd

258


sample usingg CuK α radiiation. The diffraction pattterns were measuured at 490 C, beloow the prim mary crystallizatioon temperatuure, in a He atmosphere a evvery 5 min using the position sensitive deteector.

Thee results, shoown in Fig. 55, confirmed the presencee of fcc-Al f and Al A 7Fe8Mo phase transition.

3. Results and Discusssion The valuees of the S-pparameter measured m at rooom temperature for an isochhronally anneealed samplee are plotted in Fig. 2 as function off the anneaaling temperature. The mainn goal of the isochrronal annealing experimentss is to point p out the non-reversibble transform mations resuulting from the heat treatm ments. The behavior b of the S-param meter determines temperature ranges whicch can be rellated to the diffe ferent phasess. The S-paarameter sligghtly

parameter obttained for an n isochronallyy Fig.. 2 The S-p annealed sample is plotted d versus th he annealingg tem mperature.

decreases tiill 300 C, and drastically goes down between 300 C and 5000 C. It recoovers at 500 C. The tempeerature deppendence off the Dopppler broadening line-shape showed several s posiitron annihilationn states durinng the kineticc transformattions from the am morphous to the fully crrystallized state. Fig. 3 preesents the evolution e off the diffracction patterns forr in-situ tim me-resolved high-tempera h ature X-ray measuurements durring the isothhermal anneaaling for 100 min m at 490 C to furrther study the crystallizatioon process. On O the diffracction patternss the first sign off the crystalliization onsett only appearrs at 2  41. Itt shows the formation annd growth off the crystalline phase p from thhe amorphouus Al matrix. Fig. 4 presents the t changes in i integratedd peak intenssities of crystallinne phase Al7Fe F 8Mo duringg the anneal. The peak appearrs after 25 miin and continuues to grow until u about 75 min m of annealling, after whhich it seem ms to saturate. Thhe results shhow that cryystallization is a combinationn of the crysstallization of the amorphhous Al matrix annd the formattion and grow wth of crystallline phases [13, 14]. X-ray diffraction d w performeed at was room-tempeerature with wider  angle coverrage, using thee same method ass the innitial room-tempeerature XRD measuremennts, to verifyy the phases preseent after the in-situ isothhermal annealling.

Fig.. 3 In-situ higgh temperaturre X-ray diffra action patternss mea asured duringg isothermal annealing att 490 C on n Al800Fe8Mo7Ni5 forr 100 min show wing that the crystallization n is a combination of crystallizatiion of the amo orphous phasee and d crystalline ph hase formation.

Fig.. 4 Integratted peak inteensity of duriing in-situ higgh-temperaturre X-ray diffrraction at T = 490 C showing an a intensity oof the Al7Fe8Mo M phase with h incrreasing time.


2599

The T behavior of the S-paraameter during g heating andd coo oling experim ments is com mpatible with h the resultss obtaained for an isochronally i annealed sam mple (Fig. 6).. It consisted c of heating h from m room tempeerature up too

Fig. 5 XRD D pattern for the Al80Fe8Moo7Ni5 sample after annealing at 490 4 C for 1000 min.

300 0 C (A-B), cooling-dow wn to room temperaturee from m 300 C (B--C), heating ffrom room tem mperature upp to 490 C (C C-D), and cooling-dow wn to room m tem mperature from m 490 C (D D-E). The minima m in thee curv ves of Figs. 2 and 6 aree not at exactly the samee tem mperature; thhis is due to differen nt annealingg con nditions. The curve of thhe S-parameteer as plottedd verssus the saample tempperature incrreases untill satu uration below w 200 C. IIt suddenly decreases too show several phase p transitiions, the maain of whichh

Fig. 6 The S-parameter S o obtained from continued heaating and cooling experiments is plotted aggainst the sam mple temperature.

occurring at 3000 C, at 3900 C and at 490 C. Thee bran nches B-C and D-E off Fig. 6 ressulting from m coo oling-down shhow linear tenndencies. Thee variation off the annihilation lineshape is caused both by b reversiblee and d non-reversible effects [[7, 8]. Since the thermall exp pansion coeffficient is possitive in mosst metals thee bran nches B-C annd D-E of Figg. 6 can also be related too the changes induuced by therm mal expansion n [15, 16]. In n conformityy with the eleectron diffracction patternss meaasured [17, 18] 1 and SAD DP’s electron n micrographh showed in Fig. 7, 7 the behavior of S parameeter at 300 C C can n be related to t the formaation of MSII crystallites.. Thee feature of thhe curve betw ween 390 C C and 490 C C can n be related too the appearaance of MSIII crystallites..

Fig. 7 E Electron micrrograph of a metallic glass Al80Fe8Mo7Nii5 alloy aged at 490 C foor 1 h. The SAD pattern in th he inset showss diffuse ringss from amorphous phase and few w sharp spots from f crystallin ne phase.

Thee drastic recoovering at 4990 C can bee ascribed too the appearance of o the SIII struucture. By B image analysis a of the scanniing electronn miccroscope (SEM M) pictures (Fig. (8a)), wee deduce thatt the sample has low amount oof volumic po orosity beforee ann nealing, but after testingg at differen nt annealingg tem mperatures, thhis amount tto roughly in ncreased andd has high volum mic porosity ddue to the ch hange in thee stru uctural relaxxation (Fig.. (8b)). In n fact, thiss pheenomenon is induced byy temperaturre annealing.. Thu us, the mechanism that makes the t kineticss tran nsformation possible at hhigh temperaature in thiss allo oy system iss presumablyy related to th he formationn

260

Influence of the Thermal Treatments on the Structural Relaxation in a Metallic Glass Al80Fe8Mo7Ni5

of pores [19]. This behavior may be assisted by the

(a)

(b) Fig. 8 (a) SEM images taken on the sectioned sample of a metallic glass Al80Fe8Mo7Ni5 alloy in as-cast state (a1) after aged at 200 C, (a2) 300 C, (a3) 390 C and (a4) 500 C; (b) Porosity (%) for an annealed sample as a function of the ageing temperature.

Fig. 9 A cross cut through some fitted isothermal curves S(t). Table 1

The time

Cross cut at 0.4885 0.4880 0.4875 0.4870 0.4850 0.4860

phase transition at T  500 C due to the induced structural relaxation as explained on the basis of processes associated with the release of stored transformation apparent activation enthalpy. These processes cause a redistribution of pores and atomic jumping in the network at transformation and formation of crystalline phases in the metallic glasses [3, 19-21]. To determine the activation enthalpy, the values of annealing time corresponding to a horizontal cross cut through the isothermal curves obtained with the S-parameter can be used as shown in Fig. 9. In the case of metallic glasses the Arrhenius rate equation may involve heterogeneous reactions, since transformation from the amorphous to the crystallized state is rather complex in such material. From the integration of the Arrhenius rate equation for heterogeneous reactions between two well defined states of S(t)(So = S(tso = 0) and Si = S(tsi)): S S dS dt . exp E ⁄kT S g S S where, g(S) is a function of S. Eai is the apparent activation enthalpy, k is Boltzmann’s constant, T is the absolute temperature, and t is the time. It can be written: ln C C ⁄T where C1 and C2 = Eai/k are constants represent the concentration of positron annihilation sites. Plotting the In(tSi) as a function of 1/T one finds a linear relationship. The values of C1 and C2 can be evaluated using a simple least-squares fitting [22]. Several horizontal cross cuts Si have been made through the corresponding to each Si are given in Table 1. The

values at a cross cut

through some isothermal curves.

200 1.965 4.951 5.701 12.623 -

h at temperature 300 390 1.590 1.215 1.223 0.942 2.002 1.502 1.370 1.022 2.005 1.513 4.537 1.220

250 1.724 2.891 4.346 6.692

470 1.242 0.632 1.352 1.023

490 0.230 1.220 0.642

Apparent activation enthalpy eV 3.40  0.30 3.68  0.07 3.64  0.08 3.71  0.14 3.66  0.07 3.82  0.17


corresponding values for the activation enthalpy Eai are also given in the same Table 1. For the cross cuts Si between 0.4885 and 0.4860 one can observe that the Eai values are comparable. We conclude that the corresponding crystallization process of metallic glass consists of only one activation enthalpy. The mean is: Eam  (3.65  0.14) eV.

value obtained for

4. Conclusions The structural relaxation and crystallization with the phase transitions in the metallic glass Al80Fe8Mo7Ni5 remain complex. From the continued and cooling experiments, crystallization of the sample appears as a three-stage process. The S-parameter behaves in the same way from both results of the isochronal experiments and of the continued heating and cooling experiments.

and growth of crystalline phases Al Fe Mo along with the crystallization of the amorphous Al matrix phase.

Acknowledgments The authors would like to thank prof. Dr. Emad A. Badawi, El-Minia University-Egypt, for permitting the use of laboratory facilities for studying and measurements the samples in positron annihilation spectroscopy. E-mail: [email protected].

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[3]

The behavior of the S-parameter supports the idea that

positrons

are

trapped

by

defects

and

inhomogeneities inherently present in the as-received metallic

glass

Al80Fe8Mo7Ni5.

The

annihilation

characteristics of positrons are very sensitive to phase

[4] [5]

transition. The variation of the annihilation line-shape shows reversible and non-reversible contributions. The reversible behavior can be related to the changes

[6]

induced by thermal expansion. The non-reversible variation is due to chiefly to phase transitions. The first and second phase transitions show up by the lowering of the S-parameter values at 300 C and at 390 C, indicating densification mechanisms. The third stage of crystallization which is marked by an increasing of the S-parameter at 490 C can be ascribed to an introduction of more attractive traps for

[7] [8]

[9] [10]

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261

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[13]

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