Estudo da obtenção de compósitos metal-metal e ligas metálicas por extrusão em canal angular de pós elementares

Abstract

Severe plastic deformation of metals was initially developed for obtaining ultrafine grained microstructures; however, researchers have also been using these techniques for powder consolidation and the obtaining of metal-based composites. In this work, Equal Channel Angular Pressing (ECAP) was used as a technique for the consolidation of from blended elemental powders and posterior obtaining of alloys and metal-metal composites after heat treatment. In an initial work, blended elemental powders of iron, chromium, nickel and manganese (Fe-25Cr-20Ni-2Mn wt.%) were consolidated by ECAP in a Φ = 120° die by using 1 and 2 passes by route A. Both samples were after subjected to heat treatment at 1100°C for one hour. Optical and electron microscope analysis have demonstrated green densification of 97% after one pass and 98.4% after two passes. Heat treatment led to densification of about 99% for both samples. Vickers hardness of all metals was measured and substantial improvement was observed, mostly in the first ECAP pass. Interdiffusion between adjacent particles was observed by SEM/EDX analysis of samples after heat treatment, indicating the potentiality of these routes for alloying. In a second study, the effect of temperature was investigated on the consolidation of blended elemental powders of aluminum and copper. A Φ = 90° die was used and consolidation were performed at room (1 and 4 passes by route Bc) and cryogenic temperature (1 pass). For cryogenic consolidation, the die was submerged in liquid nitrogen. The characterization of the samples revealed that cryogenic consolidation led to the formation of denser and harder microstructures, that were comparable to the one obtained after four passes at room temperature. Reference samples of commercially pure aluminum and copper were subjected to tensile tests at room and cryogenic temperatures, where a simultaneous increase in strength and ductility were observed at cryogenic temperature. Such results indicate that a partial suppression of the dynamic recovery mechanisms, at cryogenic temperature, may have allowed higher plastic deformation by the inhibition of plastic instabilities. EBSD analysis of ECAPed samples have revealed a microstructure of submicrometric grains (0,1 μm-1 μm) for both metals, as well as the development of a higher fraction of high angle grain boundaries for higher equivalent strains. After heat treated at 500 °C for 15, 30, 45 and 60 minutes, the Al-Cu samples have presented a microstructure of intermetallics and partially dissolved Al and Cu particles. Initially, the Al2Cu (θ) phase was formed, followed by Al4Cu9 (γ1) phase and an intermediate area with variable chemical composition (δ + ζ2 + η2). Longer heat treatments have led to the thickening of the γ1 + δ + ζ2 + η2 layer and the formation of microcracks along the microstructure, due to volumetric shrinking associated to the formation of Cu-rich intermetallics.