Comparison of the thermal properties of AM20 and AS21 magnesium alloys

Abstract

The present work is a study of the thermal properties of AS21 and AM20 magnesium alloys in the temperature range of 20–380 °C. The thermal diffusivity and the thermal expansion coefficient (CTE) were measured and the thermal conductivity was calculated. The thermal diffusivity and the thermal conductivity of the alloys increase with increasing temperature. The temperature dependency of the CTE in the first thermal cycle differs from that in the following cycles for both alloys. Pre-deformation of the AS21 alloy in compression leads to an increase in CTE in a certain temperature range, whereas pre-deformation in tension causes a decrease in CTE.

Introduction

Magnesium alloys, owing to their low density and high specific stiffness, are used as lightweight structural materials in various applications. Mg–Al-based alloys such as the AM and AZ series offer high specific stiffness and strength at room temperature. However, the AM and AZ type magnesium alloys exhibit low creep strength and hence, are not suitable for service at temperatures of 150–200 °C. Si addition may improve the thermal properties of Mg alloys at elevated temperature [1], [2]. Therefore, for applications in service conditions above 150 °C, the Mg–Al–Si (AS series) alloys may be considered as creep resistant magnesium alloys [3], [4]. Silicon in the AS series alloys forms stable Mg₂Si precipitates. The stable Mg₂Si phase can be present in two morphologies: Chinese-script type and massive type. The formation of the stable phase Mg₂Si with its fine dispersion in the AS21 (Mg–2Al–1Si) alloy – (reduction in aluminium content decreases the amount of the Mg₁₇Al₁₂ phase in the alloy) – is why AS21 exhibits better creep strength than the commercial AM and AZ type magnesium alloys [4]. The Mg₂Si phase has low density, high hardness and a high melting temperature. It is interesting to note that the density of Mg₂Si is 1.88 g cm⁻³, which is close to that of Mg. On the other hand, the values of the CTE and thermal conductivity of Mg₂Si at room temperature are 7.5 × 10⁻⁶ K⁻¹ and 8.0 W m⁻¹ K⁻¹, respectively; they are much lower than those for Mg (25 × 10⁻⁶ K⁻¹ and 156 W m⁻¹ K⁻¹). The Mg₂Si phase has a higher strength at elevated temperatures than Mg₁₇Al₁₂ and the Mg₂Si phase has a higher thermal stability than the Mg₁₇Al₁₂ phase.

The coefficient of thermal expansion of alloys and metal matrix composites (MMCs) is an important thermomechanical property. Structural components in service can be subjected to temperature changes and it may be important to minimise dimensional changes. Thermal conductivity plays an important role in the performance of alloys and MMCs in structures. The values of thermal conductivity may be essential for the selection of alloys for certain applications. However, very few papers give information about thermal properties of Mg-based alloys [5], [6], [7].

The aim of the present study is to measure the thermal properties CTE and thermal diffusivity of AS21 alloy in a wide temperature range and to calculate its thermal conductivity. This paper also reports temperature dependencies of the CTE, and thermal conductivity for the AM20 alloy as obtained elsewhere [7]. An attempt is made in the present study to account for the difference in the thermal properties behaviour between the AS21 and AM20 alloys.