Grain boundary wetting by a solid phase; microstructural development in a Zn–5 wt% Al alloy

doi:10.1016/j.actamat.2004.06.011

Acta Materialia

Volume 52, Issue 15, 6 September 2004, Pages 4537-4545

https://doi.org/10.1016/j.actamat.2004.06.011 Get rights and content

Abstract

A systematic study of grain boundary wetting by a solid phase was carried out for the first time. The microstructure of Zn–5 wt% Al polycrystals was studied in the temperature range 250–375 °C. The Al-rich phase formed either chains of separated lens-like precipitates or continuous layers at the Zn-rich phase/Zn-rich phase grain boundaries upon annealing at different temperatures. The contact angle at the intersection between the Al-rich phase/Zn-rich phase interphase boundaries and the Zn-rich phase/Zn-rich phase grain boundary decreased with increasing temperature. It became zero at a certain temperature, and remained zero above this solid-state wetting temperature, i.e., a continuous Al-rich phase layer covered the Zn-rich phase/Zn-rich phase grain boundaries. The fraction of wetted grain boundaries increased with increasing temperature and was independent of annealing time. The growth of the Al-rich phase at the grain boundaries is controlled by volume diffusion in the matrix phase.

Introduction

Decomposition of supersaturated solid solutions is a very important process occurring upon heat treatment of materials, because thereby the microstructure of the alloy can be changed and thus the properties of the material can be controlled.

Precipitation at grain boundaries can occur easier than in the bulk, because the reduction of grain-boundary area and thus grain-boundary energy favours nucleation [1]. Two morphologies may be distinguished for second-phase (β) particles at a grain boundary (GB) in the matrix (α):

(1)
If the GB energy of grains in the matrix α per unit area, σ_GB^αα, is smaller than two times the energy per unit area of the α/β interphase boundary (IB), σ_IB^αβ, formed upon precipitation of the β particle on the α/α GB, then the growing β particle tends to reduce its surface, and develops a lens-like shape characterized by a contact angle θ (see Fig. 1(d)). If the surface tensions at the junction of the GB and the IBs are balanced (in “equilibrium”), it holds $σ_{GB}^{αα} =2σ_{IB}^{αβ} cos θ/2$ [2]. This equation holds for isotropic interfacial energies and for a stress-free state.
(2)
If σ_GB^αα>2σ_IB^αβ, the growing β particle tends to increase its surface: a layer of β-phase covers continuously (“wets”) the α/α GB (Fig. 1(e)). In other words: the α/α GB is unstable in contact with the growing β particle and the “equilibrium” (see above) contact angle θ is nil.

Layers of solid second phases fully covering the GBs in a matrix have been observed in many systems. Important examples are Fe₃C layers at GBs in ferritic and austenitic steels [3], [4], layers of Cu at GBs in sintered W polycrystals [5], [6], [7], [8], α-Zr layers at GBs of β-(Zr, Nb) [9] and Bi layers at GBs in Cu [10]. Such continuous GB layers of a solid second phase can have either detrimental effects (e.g. enhanced brittleness) or favourable effects (e.g. improved plasticity).

The GB wetting (covering) by a layer of a solid second phase is analogous to the conventional GB wetting by a liquid second phase [11], [12], [13], [14], [15], [16], [17], [18]: cf. Fig. 1(b)–(e). The value of the temperature is decisive for the occurrence of wetting. With increasing temperature both the GB energy σ_GB(T) and the IB energy σ_IB(T) decrease normally. If, at sufficiently low temperature 2σ_IB>σ_GB and, upon increasing temperature, the temperature dependencies σ_GB(T) and 2σ_IB(T) intersect (Fig. 1(g)), then a GB wetting proceeds at the temperature T_w of intersection and at temperatures higher than T_w (cf. points (1) and (2)). Starting from a relatively low temperature, upon increasing the temperature the contact angle θ decreases down to θ=0 at T_w. Above T_w the contact angle remains θ=0 (Fig. 1(c) and (e)). The tie-line of GB wetting, i.e. the tie-line connecting the phases in equilibrium at T_w, can be drawn in the two-phase regions (α+L) and (α+β) of the phase diagram for the cases of liquid and solid GB wetting (Fig. 1(a)). At and above this tie-line the second, liquid or solid, phase forms a layer separating the crystals. GBs with a relatively low energy possess a relatively high T_w. In polycrystalline materials a spectrum of GBs with different energies exists. Therefore, in polycrystals a range of T_w values occurs: from T_wmin to T_wmax. Corresponding tie-lines at T_wmin and T_wmax for wetting by a liquid phase have been presented in the Al–Sn, Al–Mg and Al–Zn phase diagrams [15], [17], [18]. Above T_wmax all GBs are completely wetted. At temperatures between T_wmin and T_wmax only a fraction of the total number of GBs is wetted. Below T_wmin all GBs are not wetted, and the second phase appears at the GBs only as (chains of) isolated particles. Peculiar electrical and mechanical properties of materials can be caused by a layer of a second phase fully covering the GBs. This has relevance especially for nanocrystalline materials that have a large volume fraction of GBs. Tie-lines as mentioned can then be very useful with a view to practical applications.

The “wetting” phenomenon, although described above deliberately in a general way, has until now been discussed and observed in the literature with respect to wetting by a liquid phase. The purpose of this contribution is to demonstrate that “wetting” by a solid phase can occur and be explained on the basis of the same thermodynamic background. The following predictions can thus be made in advance:

•
Transition from incomplete coverage (wetting) of a single GB by a solid second phase to complete coverage (wetting) of that GB with increasing temperature at a certain, solid-state wetting temperature, T_ws.
•
Dependence of T_ws on the GB energy (low T_ws for high σ_GB^αα and vice versa).
•
Increase of the fraction of GBs covered by a solid phase from 0 to 100% with increasing temperature from T_wsmin to T_wsmax.

These phenomena of solid-phase wetting have been studied and discussed for the first time in this work. To this end the Al–Zn system has been chosen. For this system the occurrence of conventional liquid-phase wetting was shown recently for the two-phase ((Al) + L) region of the Al–Zn phase diagram [18]. The current study demonstrates the occurrence of solid-phase wetting and, as a final result, provides the tie-line for first occurrence of solid-phase wetting in the Al–Zn phase diagram.

Section snippets

Experimental details

A Zn–5 wt% Al alloy was produced by melting the corresponding amounts of Zn (99.999 wt%, Hereaus) and Al (99.9995 wt%, VAW Aluminium AG), and subsequent cooling down under vacuum (residual pressure of about 10⁻⁵ Pa). Slices (2 mm thick) of the alloy were prepared by sawing, grinding and chemical etching for 15 s (using a 5 wt% HF aqueous solution). Next, these samples were sealed in evacuated silica ampoules (residual pressure of approximately 10⁻⁴ Pa) and annealed in a tube furnace at several

As-cast microstructure

The as-cast microstructure of the as-prepared Zn–5 wt% Al alloy, i.e. after solidification and cooling down to room temperature, is shown in Fig. 3. Eutectic colonies of Al-rich phase and Zn-rich phase lamellae, formed at the eutectic temperature T_e=381 °C during solidification, can be observed clearly (Fig. 3(a); see Fig. 2 for the Zn-rich part of the Al–Zn phase diagram). EPMA indicates the existence of (only) two phases in these alloys, namely, the Al-rich and Zn-rich solid solutions. In the

Final remark

An important difference between GB wetting by a liquid phase and GB wetting by a solid phase can be indicated. In general in a two-phase (α+β) region of a (binary) phase diagram α/α, β/β and α/β interfaces can occur. The composition of the binary Zn–Al alloy of this study was such that in the two-phase region analysed (see Fig. 2) β/β (i.e. Al-rich phase/Al-rich phase) GBs were practically not observed (see Section 3.1). Now consider an alloy of composition involving that a relatively large

Conclusions

•
The transition from incomplete coverage of Zn-rich phase/Zn-rich phase grain boundaries by an Al-rich solid phase to complete wetting of the grain boundary by a continuous Al-rich phase layer occurs upon increasing the temperature at a critical temperature of solid-state wetting, T_ws.
•
The critical temperature T_ws is different for grain boundaries and grain-boundary areas with different energies: T_ws decreases for increasing grain-boundary energy.
•
The tie-line for the minimal temperature of

Acknowledgements

This investigation was partly supported by the German Research Foundation (DFG), NATO Linkage Grant (Contract PST.CLG.979375), the German Federal Ministry of Education and Research (BMBF), INTAS (Contract 03-51-3779), and the Russian Foundation for Basic Research (Contract 04-03-32800).

References (25)

J.W. Christian
The theory of transformations in metals and alloys, part I
(1975)
C. Herring
Surface tension as a motivation for sintering
I.I. Novikov
Theory of thermal treatment of metals
(1986)
H. Schumann
Metallography
(1991)
Y.E. Geguzin
Physics of sintering
(1984)
V.N. Eremenko et al.
Sintering in the presence of liquid phase
(1968)
V.V. Skorokhod et al.
Poroshk. Metall.
(1986)
W.J. Huppmann et al.
Acta Metall.
(1975)
M.J. Iribarren et al.
Defect. Diff. Forum
(2001)
I. Apykhtina et al.
Defect. Diff. Forum
(2001)

N. Eustathopoulos et al.

J. Crystal. Growth

(1976)

E.I. Rabkin et al.

Acta. Metall. Mater.

(1991)

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Grain boundary wetting by a solid phase; microstructural development in a Zn–5 wt% Al alloy

Abstract

Introduction

Section snippets

Experimental details

As-cast microstructure

Final remark

Conclusions

Acknowledgements

The theory of transformations in metals and alloys, part I

Surface tension as a motivation for sintering

Theory of thermal treatment of metals

Metallography

Physics of sintering

Sintering in the presence of liquid phase

Poroshk. Metall.

Acta Metall.

Defect. Diff. Forum

Defect. Diff. Forum

J. Crystal. Growth

Acta. Metall. Mater.