Thermal stability and microstructure of some rapidly solified Al-Cr-Zr alloys

Abstract

The thermal stabi'ity and mierostnicture of some rapidly solidified aluminium alloys which had been prepared by different inpid solidification routes have been examined. These include an AI-5Cr-2Zr (wt.%) alloy, which was in the following forms : atomised and sintered powder, chopped sheet produced by melt spinning, sintered extruded chopped sheet and three melt-spun ribbons prepared under different conditions. An AI-l.99Mn-0.80Mg-0.60Cr 0.40Zr-().l3Fe-0.05Ti (wt.%) alloy, which had been produced as twinrolled sheet, was also examined. The alloys havëjbccn investigated by differential scanning calorimetry (DSC), Dilatometry, x-ray diffraction (XRD). microhardness testing, optical, scanning (SEM) •nid transmission electron microscopy (TEM). . The investigation» the as-atomised powder and the as-received chopped sheet of the AI-5Cr-2Zr (wt.%) alloy showed that lie morphology of the powder iwas spherical whereas the chopped sheet was irregular. XRD investigations of both materials revealed the presence of AljZr and AliT'r. phases. DSC studies of these materials shewed the presence of two exothermic re n iions. The low temperature reaction was related to the precipitation of a zirconium-rich prase of type AI.,Zr and the high temperature reaction to the precipitation ol a chromium-rich phase of type AljiCiy Microharduess measurements of the sintered powder and chopped sheet showed a peak hardness in the temperature range 350-360°C. TEM studies revealed the presence of chromium-containing spherulites in 'lie powder sintered at 205°C. in addition to the A11 jCi' phase. Electron diffraction patterns of the sphemlitcs exhibited five-fold symmetry, i. e. icosahedral point symmetry. Optical microscopy of the extruded chopped sheet showed that it was more compact than the sintered material. XRD revealed the presence of AI,Zr and AlnCc phases. DSC exhibited a broad exothermic peak which was related to the precipitation of a chromium-rich phase. Optical microscop* observation of the as-spun ribbons of the A!-5Cr-2Zr (wt.%) alloy showed that rapid cooling by the substrate resulted in a microstructure which varied from the chilled side to the unchilled side of the ribbonJ Two distinct /.ones were observed: zone A near the substrate and zone B near the air side. XRD studiessof the as-spun ribbons revealed the presence of the equilibrium AlÿCty phase. DSC exhibited an exothermic peak, which was related to the precipitation of the chromium-rich phase TEM investigations revealed the presence of spherulites in the as-solidified state and the aged samples. These particles were themally stable as they were observed even in samples aged at 500°C. Selected area diffraction patterns taken from the spherulites, clearly displayed the six five fold, ten three-fold and fifteen two-fold symmetry elements associated with icosahedral symmetry. Ageing at temperatures between ,450 and 500X for 30 minutes resulted in the formation of needle or plate-like particles of the Al/Cr pliqse. I Optical observation» of the AI-!.99Mn-0.80Mg-0 60Cr-0.49Zr-0.l3Ec-0.05 I’i (wt.%) twin-rolled sheet showed that it exhibited a band structure across its thickness. These bands were parallel to the “rolling" direction. XRD revealed the equilibrium AIÿMn phase in addition to the a-aluminium matrix. DSC exhibited a broad pe ik which was probably associated with the precipitation of a manganese-rich phase. TFAfstudies confiAicd the presence of the AlfiMn phase in the as-rolled sheet, in heat treated samples some zirconium-containing precipitates were observed in addition to the AUMti phase. EDX analysis revealed the presen* of some chromium and iron in the AI6Mn phase. Dilatomelric of thelibbons and the rolled sheet at low temperature 380-300K. showed the presence of anisotropy in the roll d sheet. In the case of the ribbons, the behaviour at low temperature is seen by the presence of uniisuaflft eaks in the aluminium alloys.