Initial transients during solid/liquid phase transformations in a temperature gradient

doi:10.1016/j.jcrysgro.2007.03.001

Journal of Crystal Growth

Volume 304, Issue 1, 1 June 2007, Pages 281-286

https://doi.org/10.1016/j.jcrysgro.2007.03.001 Get rights and content

Abstract

Melting and resolidification processes during the initial transients in the mushy zone of a metal alloy in a steep temperature gradient have been studied. Changes of microstructure, temperature distribution and solute concentration in the mushy zone are characterised, and microstructural changes in grain size and morphology are quantified. All phenomena occur before stationary gradients are reached in an early initial transient period which is studied in detail for the first time. A stationary gradient is attained in an unexpectedly short time. The mechanisms leading to the observed changes of the microstructure and to the acceleration of mass transport out of the mushy zone are discussed.

Introduction

In the mushy zone of a metal alloy, several types of melting/resolidification processes have been described in the literature. Examples are dendrite arm coarsening [1], liquid film migration (LFM) [2], [3] and temperature gradient zone melting (TGZM) [4]. All these processes are based on simultaneous, but spatially slightly separated melting and solidification processes. They can be distinguished by the underlying driving forces. While dendrite arm coarsening is initiated by the curvature distribution, LFM and TGZM are driven by differences in local composition or temperature, respectively. During simultaneous melting and resolidification latent heat is at the same time released and consumed at the corresponding interfaces. Redistribution of heat thus does not control the movement of the interfaces—combined they are highly mobile.

Even though melting and solidification are phase transformations involving the same (solid and liquid) phases, thermodynamically they cannot be treated symmetrically, as was pointed out in detail in Ref. [5].

In a recent experimental study [6], microstructure evolution in the mushy zone of a binary Al–Cu alloy placed in a steep temperature gradient (up to 12 K/mm) was investigated. The evolution of a macroscopic gradient in solute concentration, and coarsening of the initial equiaxed fine grained microstructure was observed. The reduction of supersaturation in the solid was suggested as the relevant driving force. In contrast to the study in Ref. [6], the focus on the present work is the initial transient in a equiaxed microstructure. Due to the high driving force and the highly mobile solid/liquid interfaces in the mushy zone, the coarsening of the microstructure occurs on shorter time scales than e.g. in isothermal solid/liquid coarsening experiments [7].

In the present study, a detailed and quantitative analysis of microstructure and concentration distribution in an initially equiaxed microstructure is presented for the first time. Besides grain size and morphological analysis, macroscopic and microscopic concentration measurements were carried out to characterise macroscopic and local changes in concentration as compared to the initial concentration, with the focus on short holding times. Consequently, early initial transients that occur during the formation of microstructure and concentration gradients are quantified. The quantitative analysis gives insight into the role of different mechanisms on which microstructure evolution is based in the present study.

Section snippets

Experimental procedure

Experimental set-up and procedure are described in detail in Ref. [6] and are only outlined here. Rods of Al–Cu alloys (5–7 wt% Cu) were partially melted in a steep temperature gradient (max. 12 K/mm). After different holding times (with 5, 10 and 15 min shorter than in Ref. [6]), the samples were water quenched and metallographically analysed. The solute concentration was measured using energy dispersive X-ray analysis (EDX) along longitudinal sections. One hundred and fifty points were measured

Results

In Fig. 1, longitudinal sections along the cylinder axis of the three samples with different holding times (5, 10 and 15 min) are shown. In the very left part of the samples $(x \approx 0 mm)$ , the initial equiaxed fine grained microstructure can be seen. This region remained completely solid at all times during the experiment. At the location where the temperature reached the solidus temperature $(T_{S}, x \approx 0.5 mm)$ and above, the samples were at least partially melted. The right part $(x > 9.5 mm)$ of the samples was

Discussion

The results show clearly that the mushy zone in the temperature gradient was subject to complex phase transformation processes, involving both microstructural and compositional changes.

The evolution of the concentration gradient in the mushy zone as shown in Fig. 3 requires mass transport of solute atoms out of the mushy zone. Due to the short time scales in the order of magnitude of minutes, liquid diffusion alone is not sufficient to cause composition changes over a length of 10 mm. Using the

Summary

In the present study the initial transients during the gradient formation of a binary alloy in a temperature gradient have been investigated. Melting and resolidification lead to microstructural changes such as coarsening of the grains and to a change of the grain aspect ratio in the mushy zone. To the knowledge of the present authors these microstructural changes are quantified here for the first time. After 10 min holding time the grain size in transversal direction is more than doubled. In

Acknowledgment

Financial support by the Deutsche Forschungsgemeinschaft (DFG) under Grant number 235409 is gratefully acknowledged.

References (15)

M. Kuo et al.
Acta Metall. Mater.
(1991)
M. Buchmann et al.
J. Crystal Growth
(2005)
H. Nguyen-Thi et al.
J. Crystal Growth
(2003)
T.Z. Kattamis et al.
Trans. AIME
(1967)
Y.J. Baik et al.
Acta Metall
(1985)
W.G. Pfann
Trans. AIME
(1955)
M. Rettenmayr et al.
Mat. Sci. Forum
(2006)

There are more references available in the full text version of this article.

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¹: Now with the GKSS Research Centre, Geesthacht, Germany.

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Initial transients during solid/liquid phase transformations in a temperature gradient

Abstract

Introduction

Section snippets

Experimental procedure

Results

Discussion

Summary

Acknowledgment

Acta Metall. Mater.

J. Crystal Growth

J. Crystal Growth

Trans. AIME

Acta Metall

Trans. AIME

Mat. Sci. Forum