Elsevier

Acta Materialia

Volume 46, Issue 12, 24 July 1998, Pages 4257-4271
Acta Materialia

Melting of Pb particles embedded in Cu–Zn matrices

https://doi.org/10.1016/S1359-6454(98)00094-9Get rights and content

Abstract

The effect is reported of interface structure and in particular the existence of steps at the interface on melting and superheating of nanometre-size Pb particles (5–65 nm) embedded in Cu–Zn f.c.c. matrices (of two compositions, Cu–11% Zn and Cu–36% Zn) using high resolution and in-situ transmission electron microscopy and differential scanning calorimetry (DSC). The particles are shown to exhibit a cube on cube orientation relation. The shape of the Pb particles embedded in the Cu–Zn matrix of low Zn concentration (11 at.%) is found to be a truncated octahedron bounded by 111 and 100 facets. The in-situ microscopy and DSC thermograms show direct evidence of superheating of the Pb particles in this alloy only after repeated thermal cycling. High resolution electron microscopy reveals that the particle–matrix interfaces in the as solidified alloys contain steps of heights ranging from 0.42 to 1.9 nm on the {111} planes. The steps vanish after repeated cycling to yield a perfectly faceted equilibrium shape and can be correlated with the appearance of superheating. On the other hand, the Pb particles dispersed in the matrix of higher Zn concentration in the as melt spun state show three distinct size dependent shapes. In all cases, the bounding planes do not reveal prominent steps and show considerable superheating. A change of melting behaviour is observed in the second and subsequent heating with the majority of the Pb particles melting near the bulk melting temperature. The microstructure after the second cycle shows that many of the particles are sheared by fine plates having a twin relation with the matrix which form in the matrix during thermal cycling between 340 and 410°C leading to a prominent stepped Pb–matrix interface.

Introduction

The solid–liquid phase transition has been studied extensively both theoretically and experimentally[1]. Lindemann[2]proposed that melting is associated with the vibrational instability of the crystal. His theory states that the whole crystal collapses into a liquid when the mean amplitude of vibration of atoms reaches a critical fraction of the interatomic distance. The amplitude of vibration of atoms is usually higher at the surface than in the bulk, and this indirectly suggests that atoms at the surface are more prone to melting. Recent experiments on a single crystal of Pb[3]have established that melting can initiate at the surface below the bulk melting point of Pb and the phenomenon is anisotropic in nature. The {111} facets of Pb do not melt below the bulk melting point[4]and a Pb crystal entirely bounded by 111 facets superheats by about 3 K[5].

An experimental study of melting of nanometre-size crystals of free Au, Ag, Bi, In and Pb[6]showed that the melting point of a crystal decreases with the size of the particle. In contrast, a considerable amount of superheating is reported in nanoparticles embedded in a matrix7, 8, 9, 10, 11. A large superheating of approximately 470 K was observed for nanocrystals of Ar embedded in an Al matrix[12], where the amplitude of vibration of atoms is reported to be depressed at the solid–solid interface. The observation of a difference in the melting behaviour of free and embedded nanoparticles suggests a role of the interface in initiating melting. It is not always true that particles embedded in a matrix are superheated. In fact a considerable depression of melting point is observed for nanometre-size Pb embedded in amorphous alumina[13]and Bi in a glassy Al–Fe–Si matrix[14]. In these experiments, large supercooling during solidification was also observed, indicating a difficulty in wetting of the matrix by the solid.

The importance of the nature of the interface in controlling melting has recently been realized. The superheating of nanocrystals of Pb embedded in Zn[10]is attributed to the change in the morphology of the particle which modifies the particle–matrix interface. The presence of epitaxy of closed packed planes and directions can promote the condition for superheating and this is demonstrated in the case of Pb embedded in Al, Cu and Ni matrices[11]. The present work reports on the melting and solidification behaviour of nanometre-size Pb particles embedded in f.c.c. Cu–Zn matrices with particular emphasis on the characterization of the interface and elucidating its role in the process of melting.

The interaction between Zn and Pb is repulsive, whereas that between Cu and Zn is highly attractive and easily forms a solid solution[15]. The binary Cu–Pb system shows a large miscibility gap[16]in both the liquid and the solid state. The addition of Zn widens the miscibility gap. The solid solubilities of Cu and Zn in Pb are also very small at room temperature. In the present case the melting behaviour of the nanometre-sized embedded Pb particles subjected to thermal recycling above and below the melting point of lead is studied. Experiments were also carried out to study the influence of the defects like dislocations, introduced at the particle–matrix interface by deformation, on melting.

Section snippets

Experimental

Approximately 5 g of Cu–Zn–Pb alloys were prepared in an arc melting furnace which was evacuated to 133 Pa and then filled with Ar to a pressure of 0.5 MPa before melting. Two alloys of nominal composition (in at.%) Cu–13.7Zn–4Pb (alloy 1) and Cu–43Zn–3Pb (alloy 2) were prepared. The alloys were remelted in a quartz crucible using an induction furnace and injected with an excess pressure of 150 mbar on to a Cu wheel rotating with a surface velocity of 30 m/s. Both the melting and melt spinning

Dispersion of Pb in Cu–11% Zn matrix (alloy 1)

Rapid solidification resulted in a very fine dispersion of Pb particles (radius 10–60 nm) in the f.c.c. α matrix of Cu–Zn. Fig. 1(a) and (b) show the morphology of the particle viewed along the 100 and 111 zone axes of the matrix. The corresponding diffraction patterns are shown in the inset of Fig. 1(a) and (b). The size distribution of the particles is shown in Fig. 1(c). The distribution is narrow with approximately 80% of the particles in the range of 15–35 nm. A well-defined orientation

Discussion

The present investigation aims at exploring the role of the interface in the melting behaviour of the nanometric Pb particles embedded in Cu–Zn alloys. The results strongly suggest a strong correlation between the nature of melting and the nature of the matrix–particle interface. In the following the implication of the observations is discussed. However, to put the melting results into proper perspective, the microstructural features are briefly discussed first. Following this the phenomenon of

Conclusions

A very fine dispersion of Pb particles ranging from 5 to 65 nm is obtained in Cu–Zn matrices through rapid solidification. The particles show a definite orientation relation with the matrix. The melting point is enhanced when the interface becomes sharp. The presence of dislocations at the interface does not influence the melting behaviour. However, steps at the interface drastically reduce the superheating ability of the particles. The recycling and the deformation improve the sharpness of the

Acknowledgements

One of the authors (R.G.) would like to thank Alexander von Humboldt Foundation for the financial support. Professor Ranganathan is also thanked for many stimulating discussions.

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