SOLID STATE PHASE TRANSFORMATIONS IN AA8090 Al-Li ALLOYS

Abstract

Detailed microstructural studies have been carried out on characterisation of commercially important Aluminium-Lithium base alloys AA8090C and AA8090A (herein after referred as 8090C and 8090A, respectively). These alloys were developed and produced by British ALLAN, and had the following chemical compositions (wt.%): Alloy Li Cu Mg Zr Al AA8090C 2.20 1.00 0.70 0.04 Balance AA8090A 2.42 1.00 0.73 0.12 Balance Most of the investigations have been restricted to alloy 8090C. The alloy 8090A was used mainly for comparative studies on the morphology of AI3Zr (R') precipitates. Characterisation of the as-received alloy 8090C Alloy 8090C was received in the form of 3 mm thick rolled sheet, in. T81 heat treated condition*'. The optical microstructure of the alloy showed a recrystallised grain structure. The morphology of the grains was equiaxed near the sheet surface, and elongated at the centre. Coarse inclusions aligned in the rolling direction were observed. Energy dispersive X-ray spectroscopy in both SEM and TEM showed these inclusions to be intermetallic compounds containing Fe, Si, Mg and Cu. *' T81 : Solution treated, cold stretched 1-3 % and underaged. Transmission electron microscopy revealed. , that the major precipitating phase was the spherical, fully coherent AI3Li (S') with L12 superlattice structure. The average size of S' was found to be around 1 Onm. The S' precipitation extended almost up to the grain boundaries. Composite particles with Al3Zr (p') at the centre and circular envelopes of S', resembling the bull's eye structure, were observed. These spherical (i' particles also had the L12 superlattice structure and are coherent with the matrix. In addition to the spherical 13' which forms a bull's eye structure, faceted (i' was also present. The faceted 13' particles were not enveloped by 8'. In, addition to the various precipitates, a dislocation substructure was observed within the grains. Grain boundaries were found to be decorated by Al6CuLi3 (T2) phase. Formation of AI3Zr (R') precipitates The discovery of faceted 13' in AI-Li based alloys led to a detailed , investigation on their shape and orientation relationships with the matrix. Alloy 8090C was chosen for these studies, since it showed the greatest number of faceted 13' particles. Further, the specimens were used in the solution treated condition so as to avoid interference from those precipitates which usually form during ageing. Since the particle size was of the order of -3O nm, the microdiffraction technique was used to examine the orientation relationship (OR) between the (i' particles and the matrix. As mentioned above, the spherical 13' particles were fully coherent, with a cube-on-cube OR with the matrix. On the other hand, the faceted 13', while still having the ordered L12 structure, were oriented differently with the matrix. Electron diffraction studies revealed that they had a twin-related orientation relationship with the matrix. This twin-related OR is represented as,