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Aluminium alloys containing additions of iron and cerium are among the alloys being developed as potential replacements for titanium based alloys for moderately high temperature applications. Development of these alloys is possible using rapid solidification technology, which results in a very fine distribution of dispersoids in the aluminium matrix. The microstructures of two rapidly solidified high temperature alloy powders of composition (wt-%) Al–6·7Fe–5·9Ce (alloy A) and Al–6·2Fe–5·9Ce–1·63Si (alloy B) have been characterised using transmission electron microscopy and the results are explained on the basis of some of the major solidification parameters, such as nucleation undercooling and recalescence. It was observed that most of the powder particles in the +10 to −20 μm size range contained both microcellular and cellular regions, which could be explained in terms of an initial large undercooling followed by recalescence. The decomposition of the powder microstructure after exposing the pow...

An Al-5% Cr-2% Zr (wt%) alloy for elevated temperature service has been rapidly solidified (RS) by gas atomization and planar flow casting (PFC). A comparative study of the atomized powder and the planar flow cast ribbon has been performed during consolidation by extrusion. Particular attention has been given to the microstructural changes during fabrication and the influence of RS particulate type on extrusion performance, microstructure and mechanical properties. Microstructurally it has been shown that the more homogeneous PFC ribbon has advantages over the atomized powder and can improve mechanical properties. However, optimization of the thermomechanical processing of the PFC ribbon is necessary to achieve significant performance benefits in comparison to the atomized powder.

Three Al-Cr alloys containing additions of Zr and Fe have been fabricated via cold compaction and hot extrusion. The decomposition of the powder microstructure and the subsequent coarsening during thermomechanical treatment have been studied. Detailed electron microscopy investigations were performed at different locations of partially extruded billets at 450 °C. The microstructure of the dead metal zone reflects the effect the induction heating exerts on the as-atomized powder microstructure. In the low Cr alloys, Al-4Cr-1 Fe and Al-5Cr-2Zr, decomposition of the rapidly solidified microstructure commences at the rich intercellular network, whereas the microstructure of the Al-6.43Cr-1.67Zr alloy remains relatively unaffected. Within the deformation zone the precipitation kinetics are affected by the shearing and the temperature rise. The cells and the powder particles are aligned along the extrusion direction. Precipitation is taking place within the primary segregation-free areas, observed in Al-4Cr-1 Fe and Al-5Cr-2Zr alloys, whereas in the Al-6.43Cr-1.67Zr alloy, decomposition of the powder microstructure starts at the Cr-rich intermetallic particles. The as-extruded microstructure is fibrous and heterogeneous. The heterogeneity of the as-extruded microstructure is a result of the microstructural variation observed within different size powder particles and within individual ones.

Paniculate alumina reinforced Al-4Cr-1 Fe alloys were fabricated from rapidly solidified aluminium alloy powder and commercially purchased alumina powder by traditional powder metallurgical techniques involving powder mixing and cold compaction followed by hot extrusion. The tensile tests at ambient temperature indicated a considerable improvement in the mechanical strength at the expense of ductility and modulus. Poor values of modulus were explained by the presence of porosity in the composites. The high temperature mechanical properties of the matrix, tested at 350 °C after prolonged exposure to the test temperature under static air conditions, were intrinsically poor. Additions of the filler material, alumina particles, up to a weight fraction of 15% did not improve the high temperature performance of the matrix substantially. Possible causes for this are discussed and alternatives proposed.

A study of the structural changes occurring during the processing of a rapidly solidified powder aluminium alloy has been performed. Considerable microstructural differences have been noted from among different powder particles and from within the same particle. Three typical microstructures were observed; small particles exhibited a microcellular structure, medium-sized particles a dual morphology of microcellular and cellular structure, whilst in the large particles coarse intermetallics were formed during solidification, acting as nucleation sites for a cellular aluminium structure. Apart fromα-Al three other phases were detected in the powder microstructure, Fe4Al13, (Fe, Mn)Al6 and an unidentified phase termed “F”. The formation of both spheres and needle particles was promoted at high temperatures irrespective of the type of phase (Fe4Al13 or (Fe, Mn)Al6). It has been shown that the heterogeneity of the microstructural features was maintained during subsequent consolidation via extrusion. The influence of heating and deformation modifies the microstructures but was insufficient to produce a uniform microstructure. The decomposition behaviour of the different microstructures has been examined in detail and the heterogeneities in the extrudate could be related to the powder microstructure and ultimately the solidification behaviour during atomization. The microstructural heterogeneity of the extrudates was characterized by the formation of long bands which normally contained two types of structure. Bands with a high volume fraction of needle-like precipitates and bands with a high volume fraction of spheroids originated from the microcellular and coarser cellular structure, respectively.

A comparative study of three rapidly solidified Al–Mg powder alloys containing additions of chromium, iron, and manganese has been performed. Fine, air atomized powder of the three compositions was cold compacted and extruded after preheating, using an induction heater, to temperatures in the range 300–500°C. The Al–Mg–Cr alloy exhibited the best hot working characteristics: the iron and manganese bearing materials were less satisfactory because the extrudates exhibited surface cracking owing to hot tearing. The data from the Al–Mg–Cr alloy showed that the extrusion pressure can be related to the extrusion ratio and the temperature compensated strain rate. During extrudate tensile testing, discon-tinuous yielding was observed in the iron and manganese bearing alloys. All three alloys exhibited improved Young's modulus, but over the range of temperatures studied (20–300°C) the chromium and iron alloys displayed the best combination of strength and ductility. PM/0359

Experiments have been carried out to determine the relationship between processing, microstructure, and mechanical properties of an Al–6Mg–3Cr powder alloy consolidated by extrusion. Attention is focused on the microstructure of the powder and the subsequent changes that occurred during thermomechanical consolidation to a fully dense product. The powder microstructure was composed of aggregates of three equilibrium phases (Al18Mg3Cr2, Al3Mg2, and Al13Cr2) which were uniformly distributed within an Al matrix. Preheating to the extrusion temperature by induction had very little effect on the microstructure. However, consolidation of the powder accompanied by significant structural changes occurred during passage through the extrusion shear zone. The extruded microstructure was found to be inhomogeneous with coarse and fine bands of precipitates throughout. The inhomogeneities of extrudate microstructure have been related to the original powder and were caused by the wide range in powder particle siz...