Creep behavior of AS41 alloy matrix nano-composites

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Abstract

Nano-alumina reinforced AS41 magnesium alloy matrix composites with 2 wt% and 5 wt% Al2O3 were fabricated by combined stir casting and ultrasonic processing in order to distribute the nano-alumina particles uniformly in the melt. Indentation creep tests were performed on the nano-composites and the as cast AS41 alloy at four different temperatures of 448 K, 473 K, 498 K, and 523 K and three different stresses of 109.2 MPa, 124.8 MPa and 140.4 MPa. The creep curves obtained in these tests show that creep resistance increased with the addition of nano-Al2O3 into the magnesium alloy, and nano-composites with 5 wt% Al2O3 exhibited highest creep resistance. The values of obtained stress exponent range from 3 to 3.5, 3.55 to 6, and 4 to 6.5 for the as-cast AS41 alloy, and AS41/2 wt% Al2O3, and AS41/5 wt% Al2O3 composites, respectively. Thus, dislocation creep is the dominant creep mechanism. The calculated activation energies for as-cast AS41, AS41/2 wt% Al2O3 and AS41/5 wt% Al2O3 are approximately 66.17 kJ/mol, 83.77 kJ/mol, and 88.3 kJ/mol, respectively. The activation energy values for the creep of composites are close to that of the pipe diffusion (92 kJ/mol) of magnesium.

Introduction

Nowadays the lightest metal that is used for structural applications is magnesium and its alloys. Magnesium alloys have a wide range of applications in automotive sector and aerospace sector because of their light weight, good specific strength, good machinability, excellent castability and also very good dimensional stability. Magnesium alloys suffer from some drawbacks like high tendency for oxidation, poor cold workability, and high thermal expansion coefficient. High temperature properties of magnesium alloys are not so good because of their low melting temperature. At high temperature, deformation mainly occurs by slip which occurs on basal and non-basal planes and by sliding of grains [1].

AZ91 is the most widely used magnesium alloy. However this alloy is not good for high temperature applications because of the presence of low melting Mg17Al12 intermetallic in the matrix. Volkswagen discovered Creep resistant AS41 alloy in the 1970s [1]. This alloy is mainly used for engine blocks and powertrain applications. Its creep resistance is mainly because of high melting Mg2Si intermetallic phase, which is present as Chinese script in the α-Mg matrix [2]. Yet its range of application is limited to 373 K. Dispersion hardening is an inexpensive option to overcome this shortcoming of AS41 alloy. Micron-sized ceramic particles of Al2O3, SiC, TiO2 etc. and also their fibers are widely used to fabricate magnesium alloy MMCs [3], in which strength is increased but the ductility decreases, which limits their applications. MMCs reinforced with nano-sized ceramic particles have high strength as well as good ductility [4], [5]. Zhang et al. [6] and Chen et al. [7] explained that nano-particles improve the creep resistance and other mechanical properties by the Orowan strengthening mechanism. These nano-particles hinder the dislocation movement by acting as obstacles in the path of dislocations. The cohesion between the matrix and reinforced particles is more when nano-sized particles are used and this results in improved mechanical properties [7]. Among other things, mechanical properties of the MMCs reinforced with nano-particles depend mainly on the distribution and interparticle distance of these nano-particles. Stir casting is an inexpensive and widely used method for producing MMCs reinforced with micron-size ceramic particles. However, nano-particles have much higher surface energy due to which, they form clusters in the melt. It is reported that the ultrasonic cavitation technique can be effectively used to disperse the nano-particles evenly [8], [9]. The ultrasonic processing technique is also best to refine the grain size and for evenly distributing the intermetallic phase throughout the matrix [10], [11].

In this work it is endeavored to improve the creep resistance of AS41 alloy for usability above 373 K by introducing the nano-alumina particles into the matrix. Hybrid processing involving stir casting combined with ultrasonic processing is employed to fabricate the particle reinforced alloy matrix composite. In this paper, nano-composites containing 2 wt% and 5 wt% Al2O3 nano-particles are produced and their creep behavior is investigated and compared with that of the AS41 alloy. Underlying creep mechanisms are discussed.

Section snippets

Experimental methods

Commercial AS41 magnesium alloy with nominal composition of 4 wt% aluminum, 1 wt% silicon and balance magnesium was used. Nano-alumina reinforced magnesium alloy metal matrix composite was fabricated by stir casting together with ultrasonic processing in an electric resistance furnace. The schematic of the experimental setup is shown in Fig. 1. The ultrasonic processing unit (Model VCX 1500 from Sonics and Materials, USA) consists of a 1500 W electric power supply, a 20 kHz acoustic generator, an

Microstructural analysis

Fig. 2 shows the SEM micrograph of as cast AS41 alloy and X-ray energy spectra obtained from different microstructural features A–C. The compositions at different locations are semi-quantitatively determined from EDS and the estimated phases are shown in Table 1. In as cast AS41 alloy, Mg17Al12 and Mg2Si intermetallic phases are observed in the α-Mg matrix. Mg2Si phase which is a high melting point (~1102 °C) FCC phase is present as ‘Chinese script’ around and inside the grain boundaries of α-Mg

Conclusions

The creep behavior of AS41 alloy and Al2O3 nano-particle reinforced AS41 alloy matrix composites produced by hybrid processing involving stir casting and melt ultrasonication is investigated. The creep behavior is studied at various temperatures ranging from 448 K to 523 K and three different stresses (109.2 MPa, 124.8 MPa, and 140.4 MPa). Following conclusions are drawn:

  • 1.

    Creep resistance of nano-composites is higher than that of the as cast AS41 alloy. Nano-composite with 5 wt% alumina is the most

Acknowledgments

The research funding provided by the Department of Science and Technology, Ministry of Science and Technology, Government of India (Grant no. SR/S3/ME/0024/2007) is gratefully acknowledged.

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