Ingenieria

1. INTRODUCTION.

The governing criterion for the selection of materials for aerospace vehicles has continuously been performance. Materials that contribute to improved performance are of great importance to the aerospace industry in particular and the transportation industry in general. Performance improvements include increased payload, longer range, greater speed, reduced fuel consumptionand lower total life cycle costs. Historically, aluminum alloys have dominated the materials market for aerospace applications. However, in recent years, the advantages of high strength aluminum alloys have de-creased relative to composite materials and other novel lightweight metallic materials.

There exists a critical need for structural materials to be not only cost-effective, but also toprovide superior nigh temperature strength, reduced density and improved stiffness properties. These requirements, coupled with an increased emphasis on efficiency and reliability, have engendered considerable interest in the development and use of improved and novel techniques for the production of aluminum alloys having properties superior to those of existing commercial aluminum alloys. Payoffswill result from weight savings of structural components, which in turn will lead to increased payload and improved service life. While continuous casting of aerospace aluminum alloys has long been the dominant production method, it is limited by the slow rates of solidification which generate the large degrees of segregation and the coarse microstructures normally associated with casting processes.Finer microstructures and reduced microsegregation, beyond the levels possible with continuous direct-chill casting, can lead to reduced homogenization times during solution heat treatment and to superior fracture-related properties. These refinements can best be achieved by rapid solidification processing.

Early research and development efforts on powder metallurgy (P/M) aluminum alloysyielded encouraging results, and techniques such as rapid solidification and mechanical alloying have focused attention on the prospects for the creation of new engineering alloys which have properties superior to those currently available. The highly attractive combinations of microstructures and properties achievable trough rapid solidification (RS) have prompted the the study and application of rapidsolidification technology as a means of not only improving the behavior of existing alloy systems but developing novel alloy compositions.

It was not long after Pol Duwez reported continuous solubility in the copper-silver (Cu-Ag) and gallium-tin-germanium (Ga-Sb-Ge) eutectic alloy systems, that attention was drawn to the family of aluminum alloys. In fact, successful at-tempts to processhigh strength aluminum alloys through powder metallurgy methods followed as early as 1961. This study set the course for all subsequent work on aluminum-iron (Al-Fe) and other rapidly solidified, dispersion-strengthened alloys.

New nonequilibrium alloy phases at eutectic compositions in the silver-germanium (Ag-Ge) and gold-silicon (Au-Si) systems were observed during early studies. The nextdecade saw these observations extend to other alloy systems and give rise to a spectrum of methods for obtaining a rapid quench. Most activity during the I970s centered on the development and potential application of metallic glasses, but in recent years much interest has shifted to rapidly solidified crystalline alloys. As a consequence of this renewed interest in rapid / solidification technology,rapidly solidified aluminum alloys have advanced to various stages of development, and continue to receive much attention as novel compositions with improved mechanical properties expand the horizon for potential applications. With over a hundred potentially useful alloying additions, it is not surprising that aluminum is the basis of an extensive family of commercially prominent materials...