A comparative study of primary Al precipitation in amorphous Al87Ni7La5Zr by means of WAXS, SAXS, TEM and DSC techniques

doi:10.1016/j.actamat.2004.07.005

Acta Materialia

Volume 52, Issue 17, 4 October 2004, Pages 5031-5041

https://doi.org/10.1016/j.actamat.2004.07.005 Get rights and content

Abstract

The crystallisation behaviour of melt spun amorphous ribbon of Al₈₇Ni₇La₅Zr alloy was studied. The transformed fraction of the α-Al phase which forms in a primary reaction from the amorphous matrix was determined by means of WAXS, SAXS and DSC measurements. There is a marked difference in the results given by different techniques. Transformed fractions obtained with XRD refer to the real crystal weight (volume) fraction which is lower than that obtained from DSC. The difference may be accounted for by the heat released due to matrix homogenisation, which is diffusion controlled and overlaps with this stage of crystallisation. Crystal shape, size and distribution were analysed by TEM and SAXS obtaining a clear picture of the microstructure made of fine α-Al crystals grouped in clusters.

Introduction

Al based amorphous alloys can be obtained by rapid solidification of systems containing a transition metal (TM) and a rare earth (RE) element. Once they are properly annealed, copious Al nanocrystals form dispersed in the amorphous matrix. The nanocrystalline alloys with such microstructure attracted interest because of promising mechanical properties. Values of tensile strength up to 1000 MPa for amorphous and 1600 MPa in case of nanocrystalline alloys in ribbon form were reported with respect to 550 MPa of the best conventional Al alloys [1], [2], [3]. Such excellent mechanical properties were found for various systems containing 80–92 at.% Al with additions of RE and TM up to 20 at.%. Compositional studies have shown that the alloy Al₈₇Ni₇Gd₆ is one of the easier glass former [4]. Following this finding, the same elemental proportion was retained in studies of devitrification behaviour and mechanical properties in Al₈₇Ni₇RE₆ alloys using Nd, Ce, Sm and La as RE elements [5], [6], [7], [8]. When employing different rare earth elements in the same amount as Gd, a variation in transformation mechanism is induced: there are two stages when RE = La or Ce, whereas they become three when RE = Nd, Sm or Gd. The first stage in the latter case is due to formation of nanocrystalline Al followed by a metastable intermetallic. For the former alloys the primary formation of a metastable intermetallic phase was found instead of Al. However, when the RE content was slightly reduced the precipitation of primary Al was again observed. Moreover, a recent literature report suggests that separation within the glassy phase could be a possible precursor stage to Al nanocrystallisation [9]. Therefore addition of 1 at.% Zr was devised [10] for replacing La in the base Al₈₇Ni₇La₆ alloy. This element has strong negative enthalpies of mixing with Al and Ni, but positive with La. This might force a change in local order within the glassy phase.

Key factors determining the mechanical properties of partially crystallised Al alloys are the transformed fraction of crystalline phase, crystal size and crystal distribution. As for transformed fractions, they are predominantly derived from DSC experiments after suitable annealing of the material. However, these values are only rarely accompanied with corresponding crystalline transformed fractions obtained either from TEM or XRD studies [11]. In this work, emphasis was therefore put on a comprehensive study of primary transformation with DSC, TEM and XRD techniques. DSC and XRD are bulk techniques providing integral quantities which must be extracted from careful heat calibration on one hand and proper analysis of patterns in the other. Crystal size and crystal distributions are usually obtained from statistics of TEM observations. Dark field images are used to count crystals effectively, however the results are affected with systematic error arising from the fact that some crystals are not in exact diffraction condition. On the other side, bright field TEM imaging suffers with lower contrast and problems to separate overlapped crystals. Further factors limiting the reliability of TEM statistical methods at higher transformed fractions were described in [11].

This paper is aimed at giving information not only on the new amorphous alloy, its devitrification steps and kinetics of primary transformation but at providing a careful comparison of the results obtained with different experimental techniques.

Section snippets

Experimental

A master alloy of composition Al 87 at.%, Ni 7 at.%, La 5 at.% and Zr 1 at.% was prepared by arc melting precisely weighted elements of high purity. Melting was carried out under Ar protective atmosphere using a getter made of rare earth elements. A lump of master alloy was melt spun onto a hardened Cu wheel in a protective atmosphere obtained by repeated evacuation and purging of the apparatus with Ar. The piece of master alloy was contained in a SiO₂ crucible lined with a coating of rare

Phase transformations from the amorphous state

A DSC curve obtained on continuous heating at 20 °C min⁻¹ the as-quenched Al₈₇Ni₇La₅Zr ribbon is shown in Fig. 1 spanning the temperature interval from room temperature up to 600 °C. In contrast to the two-step reaction of Al₈₇Ni₇La₆, reported elsewhere [5], [6], [7], [8], DSC curves exhibit a more complex four-step devitrification process [8]. Several distinct stages (relaxation, primary crystallisation and multiple transformation) can be localised on each DSC trace. The experimental separation

Discussion

After substitution of a small amount of La with Zr to the Al₈₇Ni₇La₆ alloy, the ability to form a glass on rapid quenching is still retained. The crystallisation mechanism is, however, modified in that the primary phase is α-Al in the quaternary alloy whereas it is a metastable compound in the base ternary alloy. The addition of Zr also has effect on the crystallisation of the matrix which occurs in a single reaction in Al₈₇Ni₇La₆ whereas a series of reactions are found in Al₈₇Ni₇La₅Zr. Three

Conclusions

The study of crystallisation behaviour of melt spun amorphous ribbon of the Al₈₇Ni₇La₅Zr alloy has evidenced that the primary Al phase forms with crystals grouped in clusters contrary to the usual homogeneous dispersion found in most such materials. This is due to the substitution of one La atom with Zr possibly acting as inoculant for Aluminium.

The transformed fraction was determined by means of DSC and XRD. It has been shown that the DSC method overestimates transformed fractions, except for

Acknowledgment

This work was funded by the Research Training Network of the European Commission “Nano Al” HPRN-CT-2000-00038.

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¹: On leave from: Department of Materials Engineering, Slovak University of Technology, Trnava, Slovakia.

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A comparative study of primary Al precipitation in amorphous Al87Ni7La5Zr by means of WAXS, SAXS, TEM and DSC techniques

Abstract

Introduction

Section snippets

Experimental

Phase transformations from the amorphous state

Discussion

Conclusions

Acknowledgment

Progr Mater Sci

Mater Sci Eng A

Scripta Mat

Scripta Mater

Intermetallics

Nanocrystalline aluminium alloys

Adv Eng Mater

Mater Sci Forum

Mater Trans

Acta Mater

A comparative study of primary Al precipitation in amorphous Al₈₇Ni₇La₅Zr by means of WAXS, SAXS, TEM and DSC techniques