Determination of the activation energy in a cast aluminium alloy by TEM and DSC

doi:10.1016/j.jallcom.2006.05.132

Journal of Alloys and Compounds

Volume 432, Issues 1–2, 25 April 2007, Pages 241-246

https://doi.org/10.1016/j.jallcom.2006.05.132 Get rights and content

Abstract

The precipitation behaviour and microstructure development of the A319 alloy during ageing were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM and STEM). During T5 treatment, θ′ precipitates with an average size of about 18 nm were observed by TEM. The precipitate sizes increased with ageing temperature and attained an average size of 107 nm. In addition, there was a linear relationship between precipitate growth temperature and the cube of the precipitate size. This indicates that precipitate growth of the A319 alloy belongs to a thermal activated process of the Arrhenius type. The activation energy for the precipitate growth was calculated to be 140.4 ± 13.3 kJ/mol. However, under continuous heating conditions, the activation energy for the precipitate growth obtained by Kissinger plot was determined to be 119.5 ± 8.3 kJ/mol. Allowing for experimental error, both values are comparable and are related to the diffusion of Cu and/or Si in Al.

Introduction

Modern automotive engine developments have led to an increase in the load (mechanical and thermal) exerted upon the different components. The high temperature reached in the fire deck of cylinder heads (200 < T < 300 °C) is the source of thermomechanical fatigue leading to component failure in service. Therefore, understanding the influence of microstructure evolution is very important for the cylinder-head life prediction model. A319, is one of the commercially important alloys used in automotive applications, on account of its excellent casting characteristics and good mechanical properties [1], [2]. This alloy is usually hardened by θ″ and θ′ precipitates (Al₂Cu) which grow in the aluminium matrix [3], [4]. Information on the influence of ageing time and temperature on the morphology and coarsening kinetics of θ′ precipitates would be invaluable in the design of heat treatment of commercial A319 alloy. It also provides a better understanding of the effects of high service temperatures where coarsening of the θ′ occurs and mechanical properties are decreased. In Ostwals ripening, the evolution over time of the size distribution of a large number of dispersed small particles of one phase embedded in a matrix of another phase was formulated by Lifshitz, Slyozov and Wagner, and is widely referred to as the LSW theory [5], [6]. The LSW theory states that the cube of the average particles radius grows proportionally with the ageing time if the mass transport is governed by a diffusion mechanism.

To obtain a better understanding of precipitation in Al–Cu–Si alloys, precipitation experiments were performed on the A319 alloy by using TEM and differential scanning calorimetry (DSC).

Section snippets

Material

The material used was A319 aluminium alloy, containing (in wt.%); 7.0 Si, 3.0 Cu, 0.3 Mg, 0.1 Fe, 0.1 Mn, 0.05 Zn. Eutectic Si modification was performed by using 250 ppm of Sr and Na. The as cast alloy was heat-treated to the T5 condition: air quenched at room temperature, naturally aged for 1 day at room temperature, and then artificially aged at 210 °C for 5 h. With this treatment, the alloy reached peak hardness. After the T5 treatment, other ageing treatments were also carried out in order to

Results

Fig. 1 shows the A319 alloy microstructure in the T5 condition. The microstructure consists of a dendritic aluminium structure (α-Al) containing Cu and aluminium–silicon eutectic, precipitated in the globular form typical of a modified structure, with intermetallic compounds between dendritic arms. Interdendritic space reveals both the iron-containing phases, “Chinese script” α-Al₁₅(Fe,Mn)₃Si₂ [10] (Fig. 1, Fig. 2) and plate shaped β-Al₅FeSi [10], [11], [12] (Fig. 1), as well as the

Discussion

The ageing treatment occurs in two steps: T5 ageing, where θ′ precipitates are present in the aluminium matrix, and following ageing at temperature above 200 °C, where θ′ precipitates are then gradually replaced by the θ, Si and Q precipitates. This study indicates that the precipitation of Si, θ and Q phases strongly overlap. This finding corresponds to DSC experiments performed on the same alloy, which show that the precipitation of Si, θ and Q phases gives rise to a single heat evolution

Conclusions

Precipitate growth kinetics in the A319 alloy has been investigated. The results obtained are summarized as follows:

(1)
TEM investigation on as cast A319 alloy (T5) shows a uniform distribution of θ′ platelets (Al₂Cu) with an average thickness of about 10–15 nm and a length of about 50–100 nm. Furthermore, the presence of θ, Si and Q phases in the dendritic aluminium matrix were detected at ageing temperatures above 200 °C.
(2)
Precipitate growth is controlled by a thermally activated process of the

Acknowledgements

The authors would like to thank the Région Picardie and Montupet SA for financial support. They are grateful to F. Nadaud for useful technical assistance.

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Determination of the activation energy in a cast aluminium alloy by TEM and DSC

Abstract

Introduction

Section snippets

Material

Results

Discussion

Conclusions

Acknowledgements

Mater. Sci. Eng.

J. Alloys Compd.

Metall. Mater. Trans. A

Metall. Mater. Trans. A

Philos. Mag.

J. Mater. Sci.

J. Phys. Chem. Solids

Z. Elektrochem.