Determination of thermal diffusivity and thermal conductivity of Mg–Al alloys

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Abstract

The thermal diffusivity and thermal conductivity of magnesium–aluminium alloys AM20, AM50 and AM60 were investigated in the temperature range from 20 to 300 °C. The thermal diffusivity of AM20 increases with increasing temperature upto 160 °C, above this temperature the increase is less marked. The change in the slope of the temperature variation of the thermal diffusivity is probably caused by a reduction of the amount of Mg17Al12 phase. The values of the thermal diffusivity and of the thermal conductivity of the solid solution are lower than those obtained when the alloys contain two phases. The measured values are compared with those calculated by transformed Nordheim rule.

Introduction

The most widely used magnesium alloys are based on the Mg–Al system. The Mg–Al alloys exhibit good mechanical properties. The AM20, AM50 and AM60 alloys (with 2, 5 and 6 wt.% of Al, respectively, and 0.2–0.5 wt.% Mn) form a series of high-purity alloys with reduced iron content. An addition of manganese is required to control the corrosion behaviour. AM50 and AM60 alloys are used for applications where good ductility and high fracture toughness are required. The improved properties arise because of a reduction in the amount of Mg17Al12 phase around grain boundaries [1].

In contrast to the mechanical properties there is only limited information concerning thermal properties, especially thermal conductivity. The values of the thermal conductivity of AM20 and AM50 alloys are presented in a computerised magnesium information system of Norsk Hydro a.s [2]. The temperature dependence of the thermal conductivity was obtained by calculation from electrical resistivity. On the otherhand, the temperature dependence of thermal conductivity of AM60 alloy was measured by a stationary comparative method (DYNATECH) which is presented in [3]. The AM20, AM50 and AM60 alloys are generally used in the as cast conditions. After non-equilibrium cooling during preparation, the microstructure consists of hypoeutectic Mg–Al solid solution, eutectic supersaturated α solid solution and eutectic β (Mg17Al12) phase. If the as cast alloys or alloys after solution heat treatment and subsequent quenching are heated, the transformation of the supersaturated solid solution occurs partially to equilibrium precipitate β. Heat treatment is able to completely dissolve the β-phase. Recently, an overview on the development of the microstructure in Mg–Al alloys has been published [4]. The presence of the Al8Mn5 intermetallics (as particles or as agglomerates of particles) in AM alloys has been reported by Aghion and Bronfin [5]. The influence of the microstructural changes on the thermal conductivity has been reported for AM100 alloy; different values of the thermal conductivity were measured for the as cast alloy and for the alloy after T4 and T6 heat treatment [6].

It is known from our former studies [7], [8] that the thermal conductivity and the thermal diffusivity of Mg alloys depend on their thermal history. For example, the thermal diffusivity of AZ91 alloy (after T4 heat treatment) measured at room temperature increased about 8% after a heating cycle upto 300 °C [7]. From results obtained by measurements on isochronally annealed samples at various upper temperatures it follows that an increase in the thermal diffusivity occurs above 200 °C. Similar results were found for composites based on AZ91 alloy and for QE22 alloy and its composites [8]. An increase of the thermal diffusivity was explained in all cases by purification of the matrix due to precipitation.

It can be seen from the phase diagram of the Al–Mg system (Fig. 1) [9] that changes in the microstructure of AM20, AM50 and AM60 alloys may also be expected during heating. The concentration of Al in solid solution increases with increasing temperature and, therefore, it may be expected a decrease of the thermal conductivity and thermal diffusivity at the temperature at which the phase boundary is reached.

The main aim of this work is to study the dependence of thermal diffusivity and thermal conductivity of AM20, AM50 and AM60 alloys on temperature and their thermal history. The results will be discussed with respect to the influence of the phase transition on both properties.

Section snippets

Experimental

The materials for the present study were three magnesium–aluminium alloys AM20, AM50 and AM60. Their nominal composition in weight percent is the following: AM20 (Mg–2Al–0.5Mn), AM50 (Mg–5Al–0.3Mn) and AM60 (Mg–6Al–0.3Mn). The alloys supplied by the Technical University of Clausthal were prepared by squeeze casting.

The thermal conductivity was measured by a comparative stationary method using TCFCM-DYNATECH device (USA) in argon atmosphere upto 300 °C. Cylindrical samples for thermal

Results

The temperature dependences of the thermal diffusivity for the three AM alloys are shown in Fig. 2. Nearly the same results were obtained for the second and third measurements on all measured samples as can be seen for AM20 alloy. For the sake of clarity only results obtained in the first and second runs of measurements of AM50 and AM60 are shown in Fig. 2. The difference in the values measured in the first and further runs are small but very well reproduced for all studied samples. The largest

Discussion

The thermal diffusivity and thermal conductivity of selected Mg alloys have been measured on the as-cast and on heat-treated material. The as-cast and the quenched alloys have a non-equilibrium microstructure and their heating leads at first to the equilibrium state. There are differences between the values of thermal properties of the as-cast materials measured during the first and second heating (Fig. 2, Fig. 4). The higher thermal diffusivity and thermal conductivity estimated in the second

Conclusions

The investigation of thermal diffusivity and the thermal conductivity of the magnesium alloys AM20, AM50 and AM60 showed that the values of these properties are sensitive on the microstructure of the alloys. The solid solution of Al in Mg has lower thermal conductivity than alloys where the Mg17Al12 phase is present. The phase transitions lead to a change in the slope of the temperature dependence of the thermal diffusivity and the thermal conductivity. The results confirm that the thermal

Acknowledgements

The authors wish to thank Professor P. Palček and Dr B. Hadzima (University of Žilina) for providing the optical micrographs of AM20 alloy. The authors acknowledge financial support from the Grant Agency of the Czech Republic under grant 106/99/1717 and the Grant Agency of the Academy of Science of the Czech Republic under Grant A2041203.

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