Particle reinforced metal matrix composites are now being produced commerically, and in this paper the current status of these materials is reviewed. The different types of reinforcement being used, together with the alternative processing methods, are discussed. Depending on the initial processing method, different factors have to be taken into consideration to produce a high quality billet. With powder metallurgy processing, the composition of the matrix and the type of reinforcement are independent of one another. However, in molten metal processing they are intimately linked in terms of the different reactivities which occur between reinforcement and matrix in the molten state. The factors controlling the distribution of reinforcement are also dependent on the initial processing method. Secondary fabrication methods, such as extrusion and rolling, are essential in processing composites produced by powder metallurgy, since they are required to consolidate the composite fully. Other methods, suc...

Particle reinforced aluminium and magnesium matrix composites

Superplasticity—Recent advances and future directions

Dynamic recrystallization (DRX) characteristics of a Mg/3Al/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200� /450 8C and constant strain rates of 1/10 4 � /2/10 4 s 1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1/10 4 s 1 to a strain level of 60%. Grain refinement was less effectiv ea t higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # 2003 Elsevier Science B.V. All rights reserved.

Dynamic continuous recrystallization characteristics in two stage deformation of Mg-3Al-1Zn alloy sheet

The fracture of bulk metallic glasses

Plastic strain localization in metals: origins and consequences

Tensile instability and necking in materials with strain hardening and strain-rate hardening

The deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain. Significant hardening has been recorded in the course of deformation. Microstructural studies on deformed samples indicate the occurrence of simultaneous strain-rate induced grain growth, which explains nearly all of the hardening. A small amount of hardening may also be expected from grain elongation or grain clustering effects. As a result of concurrent grain growth, the strain-rate sensitivity is found to decrease with strain, thus indicating that stress-strain rate behavior determined initially may not be applicable after large amounts of plastic strain. The stressJstrain-rate data obtained from step strain-rate test for a variety of grain sizes, together with the grain growth kinetics plots, provide a means for developing a constitutive description for this material at large strains.

Mechanical behavior and hardening characteristics of a superplastic Ti-6AI-4V alloy

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum.

Effects of strain path changes on the formability of sheet metals

The strain rate sensitivity of flow stress has long been recognized as an important factor in determining superplastic ductility, and its relationship to high tensile elongations is well understood from the mechanics point of view. However, the measurements of this parameter and other properties of superplastic materials are challenging, and quite varied results are observed from different test procedures used. In this paper a discussion of the various characterization methods is presented, and the relationship between the superplastic characteristics and the microstructure is brought forth. The applicability of the uniaxial superplastic properties on the biaxial deformation behavior as in the superplastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.

Influences of material parameters and microstructure on superplastic forming

The strain hardening, strain-rate hardening, and plastic anisotropy properties of metal sheets are normally determined in a tensile test during the nearly uniform deformation prior to the maximum load. Beyond this point, strain nonuniformity leading to a neck is poorly understood in terms of interaction of these material properties with changes in strain-rate and stress-state within the neck, and the resulting load-extension plot. Satisfactory modeling of this problem has been achieved by using a rigid/plastic constitutive law including strain hardening and strain-rate hardening. Progressive cessation of deformation starting from elements in the specimen fillet region toward the center is demonstrated. This effect is shown to generate a strain peak (neck) at the gage length center. The predicted load-extension plots and strain distributions in the neck agree well with experiments conducted on a number of test materials. This work provides a quantitative measure of the influence of various material parameters on tensile ductility and identifies the proper constitutive law for input into mathematical models of more complex forming operations.

Amit K. Ghosh

Papers

Tensile instability and necking in materials with strain hardening and strain-rate hardening

Mechanical behavior and hardening characteristics of a superplastic Ti-6AI-4V alloy

Effects of strain path changes on the formability of sheet metals

Influences of material parameters and microstructure on superplastic forming

A numerical analysis of the tensile test for sheet metals