Microstructural characterization of the crystallization sequence of

a severe plastically deformed Al-Ce-Ni-Co amorphous alloy

Zs. Kovács1,a, P. Henits1, A. P. Zhilyaev2,3, N.Q. Chinh1 and Á. Révész1,4

1Department of General Physics, Eötvös University, Budapest, H-1518, P.O.B. 32, Budapest,

Hungary2Dept. Mech.& Astronautical Eng., Naval Postgraduate School, Monterey, CA 93943-5100, USA3On leave from Institute for Metals Superplasticity Problems, RAS, 39 Khalturin St., Ufa, 450001

Russia4Dept. Phyics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250,

USA

aCorresponding author: kovacszs@metal.elte.hu

Keywords: primary Al nanocrystallization, Al-based metallic glass, high pressure torsion(HPT)

Abstract. Discs of Al85Ce8Ni5Co2 amorphous alloy were severely deformed by high pressuretorsion. Severe plastic deformation exceeding equivalent strain of 8.2 induces the formation ofnanocrystalline fcc-Al in a more stable residual amorphous matrix. Calorimetric and X-raydiffraction measurements revealed that the deformed outer part of the disc crystallizes into amixture of equilibrium phases during the first thermal event. However, in the amorphous ribbon thesame crystalline mixture develops only after the second stage.

Introduction

It is well known that conventional aluminum alloys are often used as light-weight components inengineering applications, particularly in the aerospace industry. Recently, it has been found thatrapidly quenched Al-based alloys exhibit excellent mechanical properties [1,2]. In most cases,during the annealing of glassy Al-rich amorphous precursors containing rare earth and transitionmetal additions, primary crystallization of α -Al takes place with an average size of 8-10 nm [3-5].Generally, the crystallization sequence in these Al-rich systems strongly depends on theconcentration of the alloying constituents, e.g. single primary α -Al precipitation in Al-Ce-Ni alloysis not observed above 4-6 at.% of Ce [6]. Recently, severe plastic deformation inducednanocrystallization, e.g. by high pressure torsion (HPT) [7] has received enhanced attention. Duringthe HPT processing, a porosity- and impurity-free disc-shaped sample is strained by torsion underhigh pressure [8]. It was found that the application of this method can induce defect-free Al-nanocrystal development and growth mainly in the shear bands with an extremely high density andhomogeneous distribution [7].

Fully amorphous melt-spun ribbon with nominal composition of Al85Ce8Ni5Co2 has been re-cently studied in detail due to its high thermal stability, improved mechanical properties and highcorrosion resistance [9]. Calorimetric measurements revealed a remarkable glass-transition followedby a two-stage crystallization process. However, in-situ synchrotron radiation experiments have

Materials Science Forum Vols. 519-521 (2006) pp 1329-1334Online available since 2006/Jul/15 at www.scientific.net© (2006) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.519-521.1329

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shown that this alloy lacks of any primary crystallization of the α -Al phase [10]. High pressure tor-sion of the Al85Ce8Ni5Co2 amorphous ribbon induced the formation of primary α -Al nanocrystals inthe perimeter of the disc, on contrary to joint formation of several phases during continuous heatingof the metallic glass [11]. Furthermore, it was found that the deformation induced crystallizationprocess exhibits athermal nature [11].

In the present work, high temperature behavior and microstructure of an Al85Ce8Ni5Co2 HPTdisc have been studied. Due to the non-homogeneity of the applied stress, radial dependence ofsome quantities was also examined.

Experimental

An ingot was prepared by induction melting a mixture of high purity (99.9%) Al, Ce, Ni and Cometals. Al85Ce8Ni5Co2 ribbon was obtained using a single roller melt spinning technique in inertatmosphere with a Cu wheel rotating at peripheral velocity of 39 m/s. The ribbon was cut into smallpieces (flakes), and then placed between anvils and subjected to HPT deformation with five wholerotations, under hydrostatic pressure of 6 GPa. The HPT process results in several porosity freediscs with a diameter of 10 mm and thickness of about 120 μm. Further details on the HPTtechnique are given elsewhere [8]. Generally, accumulated shear strain for torsion deformation at aradius r can be represented by

2r N r

L L

θ πγ ⋅ ⋅= =, (1)

formula, where θ , N and L are the rotation angle in radians, number of rotations and the thicknessof the disc, respectively. Corresponding equivalent strain can be expressed as [12]

( )1/ 22

= 2 3 log 14 2

γ γε

+ +

. (2)

The accumulated shear strain due to geometrical constrains during deformation determines the elon-gation and shrinkage of the particles. In order to investigate the radial dependence of the microstruc-ture and thermal behavior, the HPT disc was fragmented into small pieces and systematically select-ed into three groups according to their distance (0-1/3, 1/3-2/3 and 2/3-1 in fraction of the radius)from the center of the disc. Hereafter, these selections will be denoted as Sector A, B and C. The es-timated average equivalent strain values for Sector A, B and C are 0-7,7-8.2 and 8.2-9, respectively.

Microstructures were examined by X-ray diffraction (XRD) using a special double crystaldiffractometer with negligible instrumental broadening. A fine focus rotating copper anode (Nonius,FR 591) was operated at 45 kV and 80 mA. The symmetrical Ge (220) primary monochromatorproduced monochromatic CuK1 radiation. The crossection of the beam impinging the specimen was 0.1x2 mm2. Diffractionpatterns were registered by FUJI Imaging Plate (BAS MS2025). Distance between specimen andimaging plate was selected to 340 mm to cover the required angular ranges by two flat imagingplates. A gap in the XRD patterns around 50 deg is due to geometrical reasons, i.e., covering theXRD spectrum by two imaging plates.

A Perkin Elmer power compensated differential scanning calorimeter (DSC) was used to in-vestigate the thermal behavior and crystallization applying continuous heating experiments per-formed at scan rate of 40 Kmin-1. All measurements were carried out under argon atmosphere. Thetemperature and the enthalpy were calibrated by using pure In and Al. Each measurement was fol-lowed by a second run in order to obtain the baseline.

1330 Aluminium Alloys 2006 - ICAA10

Results and Discussion

Figure 1 shows the XRD patterns corresponding to the as-quenched Al85Ce8Ni5Co2 ribbon and thedisc obtained by HPT, ε>8.2 (Sector C). The pattern of the ribbon consists of a broad symmetrichalo indicating a typical homogeneous amorphous phase. Severe plastic deformation of theamorphous phase results in the appearance of additional crystalline peaks superimposed on the halo.These crystalline peaks coincide with the fcc-Al Bragg peaks. Supplementary transmissionelectronmicroscopy study on the microstructure reveals the formation of high density α-Alnanocrystals with size of 5-10 nm at the perimeter of the HPT disc [11]. The phenomena ofnanocrystal formation during severe plastic deformation of amorphous alloys was observed inseveral alloys [13-15]. Specially, amorphous Al88Y7Fe5 alloy deformed by HPT shows the formationof fcc-Al nanocrystallines [7]. Recently, the thermal or athermal nature of the deformation inducedprecipitation process has been discussed in the Al85Ce8Ni5Co2 amorphous alloy [11].

30 40 50 60 70 80 90

(b)

(a)

Inte

nsi

ty (

a.u

.)

2Θ (deg)

Figure 1: XRD patterns of the as-quenched (a) and plastically deformed (b) Al85Ce8Ni5Co2 alloy.The symbol ▼ denotes the Bragg-positions of α -Al.

Figure 2 shows continuous heating DSC curves of the as-quenched ribbon and the differentsectors of the HPT disc. The curve of the as-quenched ribbon exhibits a visible glass transition (Tg)and two distinct exothermic calorimetric events. The inner regions of the HPT disc (Sector A and B)show three distinct peaks with characteristic temperatures of T1, T2 and T3, while the first peakdisappears in Sector C. It should be noted that the as-quenched ribbon and Sector C sample exhibita simple two stage crystallization sequence on contrary the DSC curves corresponding tointermediate deformations.

The XRD patterns of the as-quenched and plastically deformed samples (Sectors A, B andC) obtained after linear heating above the first crystallization peak (592 K), are shown in Figure 3.Experimental results reveal that in the as-quenched ribbon α -Al, orthorhombic Al11Ce3,orthorhombic Al3Ni and unidentified metastable phase form simultaneously, in accordance withprevious studies [9]. On contrary, the XRD pattern of the deformed samples exhibit broaddiffraction peaks corresponding to a moderate crystal formation, especially, the most highlydeformed sample (Sector C) shows the presence of only fcc-Al Bragg peaks similarly to the as-deformed state. This is in agreement with the lack of the first crystallization DSC-peak (Fig. 2).

Materials Science Forum Vols. 519-521 1331

550 600 650

T3

T3

T2

T2

T2

T1

T1

Sector C

Sector B

Sector A

as-quenchedH

eat f

low

(a.

u.)

Temperature (K)

exo T1

T2

T3

Tg

temperatures selected for annealing

Figure 2: DSC curves of the as-quenched Al85Ce8Ni5Co2 ribbon and different sectors of the HPTdisc obtained at 40 Kmin-1 heating rate.

30 40 50 60 70 80 90

*?

?

*

*

*

**** *

***

*

***

?

?

?

?

??

?

?

?

?

??

as-quenched

Sector A

Sector B

Sector C

Inte

nsity

(a.

u.)

2Θ (deg)

α-Al Al

3Ni

Al11

Ce3

unknown

Figure 3: XRD patterns of the as-quenched, and plastically deformed (Sector A,B and C)Al85Ce8Ni5Co2 alloy after linear heating up to 592 K.

1332 Aluminium Alloys 2006 - ICAA10

According to a gradual decrease of the XRD peak intensities (Fig. 3) and a monotonousdecrease in the area of the first DSC peak (Fig. 2) with increasing deformation, Sector A and B canbe considered as a mixture of the as-quenched and “deformed states” (Sector C).

In order to characterize the high temperature behavior of the deformed state, heat treatmentswere carried out in DSC at heating rate of 40 Kmin-1 up to temperatures of 592, 612, 618 and 624 Kas indicated by arrows in Fig. 2. As it can be seen in Figure 4, a slight decrease in the intensities offcc-Al peaks takes place from 592 to 612 K indicating thermal dissolution of the athermally inducedα -Al phase in the deformation state. Simultaneously, the intensity of the amorphous halo decreasesand at 612 K it completely disapears. According to the XRD pattern, the initial composite of α -Aland amorphous alloy decomposes into a mixture of extremely small nuclei of crystalline phases atthis temperature. Heat treatments above the T2 crystallization peak (618 and 624 K) lead to theformation of similar phase mixture (α -Al, hexagonal Al3Ce, orthorhombic Al3Ni, monoclinicAl9Ce2, as well as unidentified phase) obtained in equilibrium of the as-quenched ribbon by the heattreatment up to 823 K [9]. Interestingly, inspite of the presence of the T3 DSC peak, no significantmicrostructural change takes place in the deformed state from 624 to 823 K.

30 40 50 60 70 80 90

?

624 K

618 K

612 K

592 K

?

????

??

?

Inte

nsity

(a.

u.)

2Θ (deg)

α-AlAl

3Ni

Al9Co

2

Al3Ce

unknown

Figure 4: XRD patterns of the plastically deformed state (Sector C) of Al85Ce8Ni5Co2 amorphousalloy after linear heating up to 592, 612, 618 and 624 K.

Summary

High pressure torsion above equivalent strain of ε=8.2 induces nanocrystalline fcc-Al formation inamorphous Al85Ce8Ni5Co2 alloy resulting in a more stable deformation phase in the perimeter of thedisc. Calorimetric and microstructural investigations reveal that this deformation state crystallizesinto a mixture of equilibrium phase during the first thermal event. However in the amorphousribbon, the same crystalline mixture develops only after the second stage. Inner part of the discexhibit the mixture of the as-quenched amorphous and deformation states at ε<8.2.

Materials Science Forum Vols. 519-521 1333

Acknowledgements

The authors gratefully acknowledge the support of INTAS 03-51-3779 project. The work haspartially been supported by the Hungarian Scientific Research Fund (OTKA) under grant T-043247.Zs. K. is grateful for the support of the Postdoctoral Research Program (D048461) of the HungarianScientific Research Fund. The authors are grateful to Dr. L.K. Varga for providing the ingot. Á. R.acknowledges the Hungarian-American Enterprise Scholarship Fund (HAESF) for support. Wewish to thank to K. Nyilas for his assistance in the XRD measurements.

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Aluminium Alloys 2006 - ICAA10 10.4028/www.scientific.net/MSF.519-521

Microstructural Characterization of the Crystallization Sequence of a SeverePlastically Deformed Al-Ce-Ni-Co Amorphous Alloy

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