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Phase transformations in the ag-20.1 to 27 at o/o a1 alloys. Cambal, Ludvik

Abstract

The anisothermal and isothermal phase transformations in the Ag-20.1 to 27 at% Al alloys were examined. The cooling rates employed during continuous cooling had a distinct effect on the phase transformations. In the 24, 25, 26 and 27 at% Al alloys the low temperature transformations required cooling rates of 0.3-0.6°C per minute in order to obtain the phases predicted from the phase diagram. The 20.1 and 23 at% Al alloys exhibited much less μ-phase at room temperature after cooling at 0.3°C per minute than anticipated from the phase diagram. The μ-phase developed within the (ξ + μ) field in the 24 and 25 at% alloys resembles Widmannsfatten plates. The WidmannstStten structure was also present in the 25 at% alloy at room temperature. During the transformation ξ →(ξ → μ) in the two phase field the parent ξ matrix develops a polygonized substructure. This substructure is interpreted in terms of dislocation rearrangements in the accommodation region adjacent to the μ-phase plates. It .was found that the quenched β-phase exhibits precipitation hardening during low temperature ageing. At 200°C the μ-phase grows in the β-matrix as spherical particles. The radius of these particles is a linear function of time. Thus the β → μ transformation was classified as an interface controlled process. The kinetic data obtained during the isothermal transformation of the quenched ξ-phase into the μ-phase at 160 and 200°C indicatesthat the overall growth rate of this reaction is also interface controlled. However, the growth characteristics and the surface distortion accompanying the ξ → μ transformation indicate that more than one atomic process operates during the ξ → μ reaction. The X-ray and electron microscopy analyses suggest that there is not a random distribution of the Ag and Al atoms in the μ-phase. It .appears that the μ-phase contains layers rich in Ag or Al atoms. The regular stacking of: these layers may give rise to a long range superlattice as suggested by the X-ray diffraction patterns. The designation of the μ-phase as an isomorphic β-Mn structure seems incorrect. Electron transmission microscopy of the.isothermally produced μ + ξ structures in the 24 at% alloy established that there is a crystallographic relationship between the parent and the product. The structure relationship between the Widmannstatten μ-phase and the ξ-matrix in the 25 at% alloy was also obtained using X-ray Laue techniques. ξ-phase single crystals transformed into μ-phase polycrystals. The orientation of the μ-crystallites was not completely random. Although, a high degree of randomness was indicated when using X-ray D.S. techniques on fine rods. "Fragmentation" of the ξ-phase single crystal into a polycrystal takes place during room temperature deformation of the quenched ξ-phase. Relatively small amounts of deformation introduces numerous twin lamellae into the ξ-phase. Although the ductility of the deformed ξ-phase decreased rapidly with increasing deformation, no presence of a strain induced transformation product was detected using X-ray methods. The ξ → μ isothermal transformation bears a strong resemblance to the ξ → ξ transformation in the Ag-Zn alloy.

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