Mechanical properties of Zr57Ti5Al10Cu20Ni8 amorphous and partially nanocrystallized alloys

MATERIAIS

SCIEMCE & ENClBlEERlHG A

ELSEVIER

Materials Science and Engineering A226-228

Mechanical

(1997) 874-871

properties of Zr,,Ti,Al,,Cu,,Ni, amorphous and partially nanocrystallized alloys

L.Q. Xing a,*, D.M.

Herlach a, M. Cornet b, C. Bertrand b, J.-P. Dallas b, M.-F. Trichet b, J.-P. Chevalier b

n Instim Fiii Raumsimulation, DLR, Postfaclz 906058, 51140 Cologne, Germmy ’ Cenwe d’Etudes de Chimie M~tallurgiqueCNRS, 15, we Georges UFbain, F-94407 Wry SW Seine Cedex, Frame

Abstract Zr,,Cu,,Al,ONisTi, amorphouscylinder-sof 8 mm diameterswere cast with copper mouids.The mechanicalproperties of the amorphousalloys obtainedwere measuredby compressiontests.The as-castamorphousalloys show a high yield stress of about 1560MPa and fracture stressof 1650MPa. Structural relaxation, through annealingbelow the glasstransition temperature,for example,at 573K for 40 min, increases thesevaluesto about 1700and 1760MPa, respectively.Annealing of the amorphousalloy in the supercooledliquid region causesinitial crystallization, but the residualamorphousphasebecomesrelatively stable. The partially crystallized amorphousalloy, for examplewith about 40% (vol) crystalline phase,still showsexcellent strength, with a yield stressof 1700MPa and a fracture stressof 1850 MPa. Both the as-castand annealedamorphousalloys show ductile deformation with serratedflow before final fracture. These results indicate that amorphous alloys with such crystallization behaviour can be quite safely hot-worked near the glasstransition temperatures.0 1997Elsevier ScienceS.A. Keyworrls: Bulk amorphous alloys; Mechanical property of amorphous alloys; Crystallisation

1. Introduction Besides the excellent properties at room temperature, e.g., high strength and ductility, etc., amorphous alloys have a unique property, that is, very low deformation stress and superductility near the glass transition temperature. This particular property may make amorphous alloys processable like oxide glasses by pressing, drawing and blowing to various shapes. However, hot working near the glass transition temperature may lead to the crystallization of the amorphous alloys. The properties of the hot-worked amorphous alloys are dependent of the microstructures of the worked alloys which are closely associated with the crystallization behaviors of the amorphous alloys. Some amorphous alloys may become very brittle after crystallization or partial crystallization, e.g., the Fe-B-Si amorphous alloy [l-3]. whilst others show improved mechanical properties after controlled partial crystallization, e.g., the AI-Ni-Y amorphous alloy [4,5]. * Coresponding author. 0921.5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(96)10810-S

This paper presents the crystallization characteristics of the Zr,,Cu,,Al,,Ni,Ti, bulk amorphous alloy and its mechanical properties in both the as-cast state and after annealing.

2. Experimental

procedure

Ingots of the Zr&u,,Al,,Ni,Ti,

alloy, with mass

ranging from 30 g to 4’7 g, were prepared by melting appropriate amounts of Zr, Ti, Al, Cu and Ni with electromagnetic induction in a water-cooled copper crucible under He atmosphere. These ingots were then remelted by electromagnetic levitation under He atmosphere and allowed to drop into a copper mold to form amorphous cylinders. The cylindrical copper mold has an outer diameter of 45 mm and an inner casting hole

of 8 mm diameter. The amorphous nature of the cast samples was examined by X-ray diffractometry (Co K], radiation) and differential scanning calorimetry (DSC). The crystallization enthalpy was measured by integrating the crystallization peaks separately, and the relative accuracy due to the definition of the integration limits

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is within & 5%. Transmission electron microscopy was carried out using a Topcon 002B high resolution microscope operating at 200 kV. The mechanical properties were measured by compression at room temperature. The test specimens were cylinders of size of 3 mm diameter by 6 mm length machined from the cast cylinders.

3. Results 3.1. Mechanical properties in the amorphous state Fig. l(a) and (b) show the compressive stress-strain curves of the Zr,,Cu,,Al,,J$Ti, amorphous alloy in the as-cast state and after annealing at 573 K for 40 min (i.e. below the glass transition temperature). Both specimens are in a fully amorphous state. The annealed specimen has undergone structural relaxation. These stress-strain curves show the following features: elastic regime, followed by yielding, plastic deformation and fracture. This deformation behaviour is characteristic of large elastic deformation, slight work hardening and distinct serrated flow during plastic deformation. The fracture occurs along the maximum shear plan which is declined by about 45” to the compression direction. Fig. 2 shows the fracture surface of the as-cast amorphous alloy. Well developed vein patterns appear in the fracture surface. This fracture morphology indicates deformation of ductile nature. The as-cast amorphous alloy shows a moderate Young’s modulus of 65 GPa, a high yielding stress of 1560 MPa and fracture stress of 1650 MPa. When combined with densities of 6.6 g cme3, the specific strength reaches the very high value of 240 KPa m3 Kg- ‘. Annealing below the glass transition temperature for structural relaxation, for example annealing at 573 K for 40 min, causes the Young’s modulus to increase up to 68 GPa, but a distinct increase of the yielding stress and fracture stress, with values of about

Fig. 2. Fracture surface of the as-cast Zr&u,,,A1,,Ni,Ti, alloy.

amorphous

1700 and 1760 MPa respectively. There are no significant changes in the modes of elastic and plastic deformation, and serrated flow also occurs after the annealing treatment leading to structural relaxation. 3.2. Crystallization amorphous alloy

behavior and thermostability

of the

When the as-cast Zr,,Cu,,Al,,Ni,Ti, amorphous alloy undergoes continuous heating at 0.33 K s- ‘, the alloy shows a glass transition temperature Tg of about 650 K, initial crystallization temperature TX of about 705 K, a supercooled liquid interval AT, = TX - Tg of 55 K and three crystallisation peaks, as shown in Fig. 3(a). When the amorphous alloy is isothermal annealed around T,, crystallization occurs after a period of incubation, but the crystallization becomes very slow after a period of initial crystallization. Fig. 4 shows an isothermal DSC curve of the amorphous alloy annealed at 673 K for 75 min. The amorphous alloy was heated to 673 K at 1.33 K s-l and then maintained for the isother-

0.33Kls 2000 _ Zr57Ti~All0C~,oNi8

cc) 120 min. 50% WSt. 7TGziG;;

1500 - b: 1

annealing al 573 K for40 minulcs

- c: -

annealing at 673 K for 40 minutes

600

650

: 7[ 30

750 Temperature

Fig. 1. Compressive stress-strain curves of the amorphous Zr,,Cu,,Al,,,Ni,Ti, alloy in the as-cast state and after annealing.

800

850

900

9

iK)

Fig. 3. DSC curves of the Zr,,Cu,,Al,,Ni,Ti, amorphous alloy in the as-cast state and after annealing at 673 K for various times.

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0

10

20

30

40

50

GO

Fig. 4. Isothermal DSC curve of the Zr,,Cu,,Al,,Ni,Ti, alloy annealed at 673 K.

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amorphous

ma1 annealing. Crystallization began after about 3 min of incubation, lasted for about 37 min and then became very sluggish (with no measurable heat output). Fig. 3(b) and (c) shows the DSC curves of the amorphous alloy after the isothermal annealing at 673 K for various time. It is deduced, by comparing the crystallization enthalpy values (in Fig. 3), that the crystallization volume fraction reaches about 40% after annealing for 30 min and increases only to 50% after annealing for 120 min. This is consistent with the exothermic effect shown in Fig. 4. The glass transition temperature of the residual amorphous phase does not change significantly from that of the original amorphous alloy after the initial crystallization. To confirm this interpretation (based on an analysis of the DSC scans), both X-ray diffraction and transmission electron microscopy have been carried out. Fig. 5 shows the X-ray diffraction pattern of the amorphous alloy after the isothermal annealing at 673 K for 40 min. It indicates a residual amorphous phase together

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Fig. 6. Fracture Surface of the Zr,,Cu,,Al,,NisTi, annealed at 673 K for 40 min.

amorphous alloy

with crystalline phases. High resolution transmission electron microscopy also shows that the alloy annealed at 673 K for 40 min remains partially amorphous [6]. 3.3. Mechanical properties partial c~ystallisation

of the amorphous alloy after

Fig. l(c) shows the compressive stress-strain curves of the partially crystallised amorphous alloy in comparison with that of the as-cast alloy (Fig. l(a)). The isothermal annealing was carried out at 673 K for 40 min. The volume fraction of crystallised phase was about 42%, as deduced by comparing the crystallization enthalpy value with that of the as-cast amorphous alloy. The partially crystallized amorphous alloy shows a higher yielding stress and fracture stress than the as-cast amorphous alloy, with values of about 1700 and 1850 MPa, respectively. The Young’s modulus increases a little up to about 68 GPa. The value of the elastic deformation shows no distinct change, but the amount of plastic deformation decreases after the partial crystallization. The serrated deformation behavior remains indicating that the partially crystallized amorphous alloys with about 42% volume fraction of crystalline phases still deforms in a manner typical of amorphous alloys. Fig. 6 shows the morphology of the fracture surface of the partially crystallised amorphous alloy. It still shows the typical vein patterns of the amorphous deformation, but the diameters of the vein patterns are reduced as compared with those of the as-cast amorphous alloy (Fig. 2).

4. Discussion 40

50

60

70

80

90

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Fig. 5. X-ray diffraction patterns of the Zr,,Cu,,Al,,Ni,Ti5 amorphous alloy after isothermal annealing at 673 K for 40 min.

The crystallisation of the classic amorphous alloys usually proceeds completely after the exothermic peak in the isothermal DSC curves, for example, the Fe-&B,

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Science and Engineering A226-228

Fe-B amorphous alloys etc. The crystallisation kinetics of such amorphous alloys are usually described by the Johnson-Mehl-Avrami equation [7-91. However, the Zr,,Cu,,All,,Ni8TiS amorphous alloy remains partially amorphous after the rapid initial crystallisation which shows an exothermic peak in the DSC curves of the isothermal annealing (Fig. 4). The crystallization peaks in the DSC curves can be associated with the formation of different phases El01 or with the crystallisation of separate amorphous phases. When the amorphous alloy is annealed at 673 K, the exothermic peak shown in Fig. 4. is associated with the first peak in Fig. 3(a). The formation of the crystalline phases during the first exothermic peak is likely to lead to a composition change between the crystalline phase and the residual amorphous phase. This, in turn, seems to lead to a more stable residual amorphous phase. Furthermore, when crystalhsation corresponding to the second peak occurs, the crystallization temperature shifts towards higher temperature which is consistent with the existence of a more stable residual amorphous phase. This behaviour is similar to the crystallization of the Al-NiY amorphous alloy [4] but unlike classical crystallization behaviour where either annealing below the glass transition temperature or partial crystallization causes the crystallization temperature to shift towards lower temperature, for example, the Fe-Si-B amorphous alloy [71* Classical amorphous alloys, e.g. Fe-Si-B etc., are ductile in the as-cast state, hut become brittle after partial crystallisation [l-3]. The high strength of the partially crystallized Zr,,Cu,,Al,,Ni,Ti, amorphous alloy is attributed to its microstructures. High resolution electron microscopy shows that the microstructures consists of nanocrystals embedded in an amorphous matrix [6J. The serrated flow (Fig. l(c)) and the fracture morphology (Fig. 6) indicate that the deformation behaviour of the annealed alloy are still dominated by the amorphous matrix. The precipitated tie crystals may act as barriers to the shear of the amorphous matrix, and hence increase the strength of the alloy and decrease the extent of the plastic deformation as well as the characteristic size of veins observed in the fracture surface. Finally, the annealed amorphous alloy becomes brittle when the crystalline fraction is above about 50%. The high thermal stability of the residual amorphous phase and the unique mechanical properties of the Zr,,Cu,,Al,,Ni,Ti, amorphous alloy in the partially crystallized state indicate that this kind of amorphous alloys can be quite safely used in hot-working pro-

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cesses. For example, parts of various shapes could be obtained by shaping near the glass transition temperature where the alloys have very good viscous flow and low deformation stresses.

5. Conclusion The Zr,,Cu,,Al,,Ni,Ti, amorphous alloy shows a high yield stress of about 1560 MPa and fracture stress of 1650 MPa in the as-cast state. Annealing below the glass transition temperature leads to structural relaxation and increases these values to about 1700 and 1760 MPa, respectively. Annealing the Zr,,Cu,,Al,,Ni,Ti, amorphous alloy near the glass transition temperature cause an initial crystallisation with about 40% volume fraction crystals, but the residual amorphous phase becomes quite stable subsequently. The partially crystallized amorphous alloy, for example with about 40% (vol) crystalline phase, still shows excellent strength with a yield stress of 1700 MPa and a fracture stress of 1850 MPa. This results indicate that the amorphous alloys with such crystallisation behaviors can be quite safely hot-worked near the glass transition temperatures.

Acknowledgements Thanks are due to P. O&in, A. Dezellus and J.-L. Bonnentien for the alloy preparation, J. Devaud-Rzepski for assistance with the electron microscopy and G.P. Giirler for the DSC measurement. L.Q. Xing is very grateful for the financial support of the Alexander-vonHumboldt Foundation.

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