The compressive deformation and impact response of a magnesium alloy: influence of reinforcement

https://doi.org/10.1016/j.msea.2004.09.070Get rights and content

Abstract

Reinforcement of magnesium alloys with ceramic particulates has engineered a new family of materials that are marketed under the trade name metal matrix composites. In this paper is reported the results of a study aimed at understanding the role of particulate reinforcements on compressive deformation and impact response of a magnesium alloy discontinuously-reinforced with silicon carbide (SiC) particulates. An increase in carbide particulate reinforcement content in the magnesium alloy metal matrix was observed to have only a marginal influence on compressive strength and impact energy absorption when compared to the unreinforced counterpart. Microcracking in the metal matrix coupled with failure of the reinforcing SiC particulates both independently dispersed and in clusters dominated the fracture sequence at the microscopic level. The mechanical response of this composite material is discussed in light of the interactive influences of intrinsic microstructural effects, deformation characteristics of the composite constituents, nature of loading and local stress state.

Introduction

Use of magnesium has progressively grown in the time spanning the last two decades, and magnesium continues to make rapid strides and even an impact in a spectrum of automotive products. This is primarily attributed to the lightness of magnesium, i.e. 30.0% lighter than aluminum, 75.0% lighter than zinc, and 80.0% lighter than steel [1], [2]. Besides, magnesium has the highest strength-to-weight ratio (σ/ρ) of any of the commonly used metals [1]. A few of the other noteworthy advantages of magnesium include: good castability, high die casting rates, electromagnetic interference, shielding properties, part consolidation, dimensional accuracy and excellent machinability, all factors, which promote its utilization in automobile-related products [2]. Increasing demands for a material having high specific strength (σ/ρ) and specific stiffness (E/ρ) have resulted in the development and emergence of lightweight metal matrix composites based on magnesium alloys. These materials offer improved strength, stiffness and wear resistance over the conventional unreinforced counterpart [3], [4].

A class of metal matrix composite materials, which have emerged to attract attention and use in automotive components, space applications and even consumer-related products, are the magnesium alloy matrices discontinuously-reinforced with ceramic particulates. The presence of a discontinuous reinforcement phase in a continuous magnesium alloy metal matrix can result in properties not attainable by other means, thus enabling an extension of the range of possible applications [5], [6]. The ability to achieve noticeable improvements in mechanical properties is largely dependent on the concurrent influence of (a) intrinsic properties of the composite constituents, and (b) size, shape, orientation, volume fraction, and distribution of the carbide particulate reinforcement phase in the metal matrix [7], [8], [9], [10], [11], [12], [13]. From a design perspective, the attractiveness in preferring and choosing a discontinuously-reinforced (DR) magnesium alloy metal matrix for many applications stems from an improvement in specific modulus, i.e. the density compensated increase in elastic modulus. Emerging applications for the DR magnesium alloy based MMCs involve automotive and aerospace components such as automotive pulleys, sprockets, and cylinder liners and aircraft engine castings [14], [15].

Presence of discontinuous particulate reinforcements in a magnesium alloy metal matrix tends to alter the microstructure of the material during secondary processing when compared to the unreinforced monolithic counterpart [12], [16]. The objective of this paper is to record the influence of discontinuous particulate reinforcements and materials processing on mechanical behavior of a magnesium alloy reinforced with fine particulates of silicon carbide. The compressive deformation and impact response behavior of the composite material is discussed in light of the interactive influences of reinforcement effects on microstructure, deformation characteristics of the composite constituents, and key mechanisms governing fracture.

Section snippets

Materials

The material chosen for this investigation is magnesium alloy AZ92 (Mg–Al–Zn). The nominal chemical composition (in wt%) of the alloy is summarized in Table 1. The step-wise sequence followed for producing the magnesium alloy, using pure metal ingots as the starting raw material, was optimized following a series of experiments. Details of the technique are summarized in the flow chart shown in Fig. 1. Commercially pure magnesium was chosen as the base metal. The ingots of magnesium were first

Production of castings of the base (unreinforced) alloy (AZ92)

A master ingot of alloy AZ92 was charged into a small refractory crucible kept in an electric resistance furnace along with 1.0 wt% flux. Chemical composition of the flux used for this magnesium alloy is given in Table 2. Based on composition of the master alloy, the constituent elements were added to obtain the desired composition. Melting was done in an electric resistance furnace, which was set at a temperature of 680 °C. Both the die and punch used in squeeze casting operation of the melt

Initial microstructural characterization

Test specimens taken from both the unreinforced (AZ92) alloy and the composite counterparts (AZ92/SiC/xxp-T6) were mechanically ground on progressively finer grades of silicon carbide impregnated emery paper using copious amount of water as the lubricant. The ground specimens were then finish polished, using alumina suspended in distilled water as the lubricant, to near mirror-like surface finish. To reveal the key microstructural features the mechanically ground and polished specimens were

Initial microstructure of alloy AZ92

The two primary constituents of alloy AZ92 are aluminum and zinc. In Fig. 4, Fig. 5 are shown the microstructures of the die cast and squeeze cast AZ92 alloy. Phase diagram of the binary alloy (Mg–Al) suggests initiation of solidification to occur at around 600 °C. This necessitates that the alloy be superheated sufficiently above 600 °C in order that the melt has both fluidity and castability during casting practice. Further, the phase diagram suggests that cooling the molten metal below 600 °C

Conclusions

Based on a study on understanding the influence of ceramic particulate reinforcements on mechanical behavior of magnesium alloy AZ92 the following are the key findings:

  • (1)

    The overall grain size of the squeeze cast alloy AZ92 was much finer when compared to the die cast counterpart. The intrinsic influence of squeeze casting on grain structure development is attributed to the high initial rate of heat extraction.

  • (2)

    Aging response of the ceramic particulate-reinforced magnesium base alloy (AZ92),

Acknowledgements

All of the authors extend profound thanks to the unknown reviewer for his meticulous and thorough review of the initial draft of the manuscript. The corrections, comments, suggestions have been carefully incorporated in the revised version resulting in a strengthened manuscript.

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