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Tensile strength and fracture toughness of two magnesium metal matrix composites

Reinforcement of magnesium alloys with ceramic particulates has engineered a new family of materials that are marketed under the trade name metal-matrix composites. Rapid strides in the processing of these materials during the last two decades have provided the necessary impetus for their emergence and use in structural and automotive-related components. In this article we report the results of a study aimed at understanding the role of particulate reinforcements on the tensile, compressive, impact deformation and overall fracture behaviour of magnesium alloy discontinuously-reinforced with siliconcarbide particulates and saffil alumina short fibres. An increase in reinforcement content was observed to have a marginal influence on strength and detrimental influence on ductility compared to the un-reinforced counterpart. Micro-cracking in the metal matrix coupled with failure of the reinforcing phase (i.e. particulates or shortfibres), both independently dispersed and in clusters, dominated the fracture sequence at the microscopic level. The deformation and final fracture behaviour of the composite materials are discussed in light of the concurrent and mutually interactive influences of microstructural effects; deformation characteristics of the composite constituents,nature of loading and local stress state.

Magnesium alloys are very suitable for applications that require materials with high strength-to-weight ratio. However, the use of magnesium alloys is limited due to their low elevated temperature properties. Magnesium matrix composites are the possible alternatives. The present work involved the production and subsequent property evaluation of AM100 magnesium alloy and its alumina short fibre reinforced composites. Studies on microstructure, hardness, density, stiffness, tensile properties, impact strength, wear resistance and corrosion resistance were carried out. Results indicate the significant improvement in the properties achieved by making composites. The findings also highlight the dominant roles of the base alloy matrix and the fibre volume fraction in determining the above properties.

Squeeze casting is a near-net-shape process that is rapidly gaining popularity due to its simplicity and effciency. During the past few years Al alloys and Zn alloys have been produced through squeeze casting and their mechanical properties have been studied [1, 2].

brusive Jet Machining (AJM) or Micro Blast Machining is a non-traditional machining process, wherein material removal is effected by the erosive action of a high velocity jet of a gas, carrying fine-grained abrasive particles, impacting the work surface. The AJM process differs from conventional sand blasting in that the abrasive is much finer and the process parameters and cutting action are carefully controlled. The process is particularly suitable to cut intricate shapes in hard and brittle materials which are sensitive to heat and have a tendency to chip easily. In other words, AJM can handle virtually any hard or brittle material. Already the process has found its ways Into dozens of applications; sometimes replacing conventional alternatives often doing jobs that could not be done in any other way. This paper reviews the current status of this non-conventional machining process and discusses the unique advantages and possible applications.

Zinc-aluminium cast alloys (ZA alloys) exhibit good castability and mechanical properties but these alloys lack creep resistance and high temperature stability. One solution to improve these properties is to reinforce with ceramic particles or fibres, to result in MMCs. MMCs can be produced using casting technique involving infiltration. A systematic investigation was taken and this paper discusses the salient findings of the study on the ZA-27 alloy based MMCs produced through squeeze casting. (Reinforcing fibers: SAFFIL (chopped alumina) or mullite.)

An experimental investigation was carried out to Characterise the performance of four types of cast iron under adhesive wear, erosive wear, cavitation erosion and low frequency thermal cycling. Results indicate that vermicular graphite iron has the best rating among the cast irons investigate,based on the overall performance as well as cost consideration.

In recent years, compacted graphite cast iron was a good substitute for several industrial applications that involve wear. The adhesive wear characteristics of compacted graphite iron and flake graphite iron were investigated in this work. It was found that the coefficient of friction is lower for compacted graphite iron. For both cast irons, the influence of microstructure and graphite morphol. on the wear process was studied and discussed in this paper. Compacted graphite iron seems to have good potential as a substitute material for applications involving adhesive wear.

Zinc-aluminum alloys (which are known as the alloys of the 80's) have found several newer applications in recent years. The microstructural features of MMCs based on zinc-27% aluminum alloy (ZA-27) were studied with a view to analyze the interfacial bonding and the distribution of different elements on the matrix. The findings are briefly discussed.

The mechanical alloying process has been reviewed in this paper. The research and development on nickel, iron, aluminum, titanium, and copper base MA alloys, and commercially available alloys are discussed.

Vermicular graphite cast iron is a new addition to the family of cast irons. Various methods for producing vermicular graphite cast iron are briefly discussed in this paper. The mechanical and physical properties of cast irons with vermicular graphite have been found to be intermediate between those of gray and ductile irons. Other properties such as casting characteristics, scaling resistance, damping capacity and machinability have been compared with those of gray and ductile irons. Probable applications of vermicular graphite cast irons are suggested.

The fractographic features of unalloyed and alloyed compacted graphite (CG) cast irons which failed in tension and in fatigue testing modes are reported. For comparison, the features of gray and ductile iron are also included. Generally, brittle fracture features were observed. The investigation led to the conclusion that fracture surface details of compacted form of irons are a blend of gray and ductile irons. Heat treatment and alloying affect the properties as well as the failure mode.

Use of $Na_2SiF_6$ powder and ester liquid as hardeners with Na silicate was studied. In each binder system, studies were conducted on the setting characteristics of sands bonded with different binders and hardener levels. Sand mixures were analyzed by using SEM and x-ray diffraction to determine the nature of the bond formed. Experiments were conducted to assess permeability, friability, knockout, and thermal properties of the sand mixts. $Na_2SiF_6$ and ester containing 90 diacetin and 10% triacetin were hardeners for Na silicate bonded sands.

The heat pipe is an innovative engineering structure characterized by its capacity to transfer large quantities of heat through relatively small cross-sectional areas with very small temperature differences; it also possesses high thermal conductance and low thermal impedance. In recent times, heat pipes in various forms and designs have found a wide variety of applications. This paper briefly presents the basic concepts of heat pipes, recent innovations in design and their applications.

Compacted graphite cast iron (CG cast iron) has some advantages over ductile and gray irons in production and use. Cast irons with compacted graphite can be produced either by deliberate undertreatment with nodularizers or by a simultaneous treatment with nodularizers and antinodularizers. Experimental investigations show that repetitive production is possible using either of the above methods. The properties of CG cast irons thus produced were assessed and the mechanical properties correlated with the aspect ratio of graphite. The CG cast irons have mechanical properties, machinability, wear resistance, and corrosion characteristics intermediate to those of gray and ductile irons. The reason for the intermediate behavior of the CG cast irons is explained based on the microstructure and the graphite shape. Using SEM studies, attempts were made to study the morphological and growth behavior of the compacted graphite. It is confirmed that, in CG cast iron, graphite growth takes place perpendicular to the basal plane of graphite.