STRUCTURE AND PROPERTIES OF PM Al-7Cr ALLOY PREPARED BY RAPID SOLIDIFICATION

摘要

Rapidly solidified melt spun ribbons from Al - 7% Cr alloy were prepared under various conditions of melt spinning. As-solidified ribbons mostly exhibited heterogeneity represented by structural gradient due to non-uniform freezing conditions at the substrate side and free surface. The material at the substrate side has a typical segregation-free structure. On the other hand, in the microstructure close to the free surface, many primarily crystallised, variously shaped formations were found. X-ray diffraction and electron diffraction revealed that rapidly solidified material contains supersaturated (Al) solid solution and various less or more stable intermetallics. Investigations of micro structural development induced by thermal influence revealed that metastable phases formed due to rapid solidification transformed to those more stable and the development finished with the achievement of an equilibrium at approx. 375 °C, being represented by the (Al)-solid solution and the Al_(13)Cr_2 aluminide. Inhomogeneities of rapidly solidified ribbons were reflected in different annealing behaviour of various samples. Consolidation of rapidly solidified material has been carried out by hot extrusion at temperatures 375 °C and 450 °C, respectively. Material extruded at lower temperature had higher ultimate tensile and yield strengths, at an elongation of 10%. The fracture surfaces of specimens showed a ductile morphology with deep dimples and large plastic deformation. Annealing of the material at the consolidation temperatures indicated that the properties of the alloy can be slightly modified by thermal treatment up to the maximum after annealing for 1-2 hours.