The preparation of an Al–3·8Fe–3·6Ni–3·8Co engineering material from atomized powders is described. The production route involves cold compaction followed by extrusion at 400–600°C. It is shown that products having acceptable surface finishes may be produced and that the properties of the products are related to the extrusion parameters. The structure is considerably modified during the deformation processing and is much coarser than that existing in the individual powder particles. The properties of the product are superior at room temperature to those of existing non-heat treatable Al alloys and the material maintains its considerable strength at elevated temperatures. PM/0284

Elemental powder compacts of Ni-13.3 wt.% Al, the composition that corresponds to stoichiometric Ni3Al, have been processed by hot extrusion reaction synthesis (HERS) to form a dense rod. A narrow processing window has been determined for the experimental scale rig used. The success of the processing route has been shown to be highly dependent on the initial billet temperature, the heating time of the billet, and the extrusion speed. HERS has been successful at relatively low processing temperatures (less than 600 degreesC) and offers a highly economical method of producing aluminide extrusions. The extruded microstructure consisted of a mixture of ordered gamma prime precipitates in a Ni-rich matrix and grains of twinned martensite. Both a {200} texture and a weaker {111} texture were observed along the extrusion direction. The Vickers hardness of the extruded material was 340 kgf /mm(2). The 0.2% compressive yield stress at room temperature at a constant strain rate of 10(-3) s(-1) was 600 MPa. (C) 2003 Elsevier Ltd. All rights reserved.

The research presented here describes the results of production trials of various nickel, iron and titanium aluminides, using the method of hot extrusion reaction synthesis (HERS). Billets of the elemental powder mixtures have undergone HERS in a specially designed experimental scale extrusion rig. The ease of processing was highly dependant on the container (tooling) temperature, the heating time of the billet, the ram speed during extrusion, the composition and the aluminide system in question. HERS has been successful with aluminides based on the stoichiometric ratios of FeAl and Ni3Al at relatively low container temperatures (less than 600°C). The extruded microstructures consisted of a mixture of intermetallic phases. The short processing cycle led to fine grain sizes and high values of mechanical properties, such as yield strength.

Abstract This paper reports on an investigation of the potential use of high energy ball milling (HEBM) for the production of homogeneous mixtures of titanium/hydroxyapatite powders. This system is of considerable interest for the production of functionally graded material (FGM) components for use in implants in the body. In order for these FGMs to have satisfactory properties homogeneous mixtures of the titanium and hydroxyapatite (HA) powders must be produced prior to consolidation. In this investigation Ti/20 wt-% and 40 wt-% HA powder mixtures were produced by high energy milling for times of 15 minutes and 1 hour. Mixtures were also prepared on a conventional turbula powder mixer for reference purposes. The mixtures were consolidated by either cold pressing, cold isostatic pressing or hot pressing under various conditions. Selected specimens were also subjected to sintering over a range of temperatures (400-1100°C). The resulting microstructures were characterised using scanning electron microscopy, X-ray diffraction, dilatometry, differential thermal analysis and thermogravimetry. Following the HEBM process, the resulting powders consisted of Ti particles coated with continuous surface layers of HA. It had been hoped that the process would produce a homogeneous product consisting of HA particles evenly distributed within the Ti particles and so the results were disappointing. There was some indication that the longer milling time of 1 hour produced a limited amount of HA surface layer breakup but also lead to amorphisation of the HA. A possible way forward may be to use extrusion as the consolidation process as this would provide substantial amounts of shear deformation, which in the past has been shown to be effective in breaking up surface oxide layers. An additional advantage is that the relatively high speed of the extrusion process would minimise the time available for HA amorphisation.

Phase transformation reactions, occurring during heating of as-atomised Al-6.5Fe-1.5V powders, extrusion of the powders, and heating of the as-extruded alloys produced from the powders, have been studied by DSC, XRD and TEM. The DSC studies of the as-atomised powders revealed several phase transformation reactions. The solid solution in zone A decomposed to form metastable phases at 360°C. These metastable phases further transformed to form equilibrium phases at 500°C. The microquasi-crystalline icosahedral (MI) phase particles present in zone A and zone B transformed to equilibrium phases at 500°C. The globular clusters of microquasi-crystalline icosahedral (GCMI) phase particles in zone C transformed polymorphously to icosahedral (I) phase particles at 450°C. These reactions were believed to occur during extrusion of the powders. During heating of the as-extruded alloys produced from coarse powder particles, I phase transformed polymorphously to hexagonal phase at 550°C. The hexagonal phase decomposed to monoclinic Al 45 (V, Fe) 7 and Al 13 Fe 4 phases upon heating for longer times.

Microstructures and their stability in as-atomised Al-6.5Fe-1.5V and Al-6.5Fe-1.5V-1.7Si powders have been investigated using transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDXS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. It was observed that microstructures of the as-atomised powder particles showed a close relationship with powder particle sizes. The as-atomised powders exhibited three types of microstructures, namely 'zone A', 'zone B' and 'zone C'. The 'zone A' type microstructure consisted of very fine and homogeneous distributed precipitates in the α-Al matrix. The 'zone B' microstructure represented the regions consisting of microcellular structures whereas the 'zone C' microstructure represented the regions consisting of coarse cellular structures and globular quasi-crystalline phase particles. Fine powder particles exhibited both 'zone A' and 'zone B' microstructures. The size of 'zone A' decreased with increasing powder particle sizes. The intercellular phases in 'zone B' of both Al-Fe-V and Al-Fe-V-Si were very fine, randomly oriented microquasi-crystalline icosahedral particles. Microstructures of coarse powder particles exhibited both 'zone B' and 'zone C'. The intercellular phases in 'zone C' of Al-Fe-V powders could be Al6Fe, whereas in Al-Fe-V-Si powders they were probably silicide phase. Formation of powder microstructures may be explained by the interactions between the growing α-Al fronts with the freely dispersed, primary phase particles or the solute micro-segregation. Studies using DSC techniques have revealed the microstructural stability of as-atomised powders. There were three DSC exotherms observed in the as-atomised Al-Fe-V powders. The 'zone A' was stable at elevated temperatures and the exotherm peak corresponding to the transformation reactions occurring in 'zone A' was at 360°C. The exotherm peak, which might correspond to the transformation of the globular clusters of microquasi-crystalline icosahedral phase to single-phase icosahedral particles, was at 450°C. The exotherm peak, which may correspond to the formation of Al13Fe4 and Al45(V, Fe)7 phases, was at 500°C. In the as-atomised Al-Fe-V-Si powders, only one exotherm was observed with a peak at 400°C. This exotherm may correspond to precipitation of silicide phase particles.

Powder metallurgy processing involving cold pressing and hot extrusion has been used to fabricate bulk functionally graded materials (FGMs) based on the 2124/SiC/10 p composite system. Two forms of single-core bulk FGMs with circular cross section were fabricated. One form (designated 10SiC-2124) had a central core of unreinforced Al-2124 alloy that was surrounded by a 2124/SiC/10 p reinforced surface layer: the other (designated 2124-10SiC) had a composite core and an alloy surface layer. These forms enabled the effect of the radial graded core on fatigue to be investigated with fatigue crack propagation from either (1) a ductile core to a more brittle region or (2) a brittle core to a ductile region of the FGM. The fatigue crack growth rate was measured using a constant applied stress intensity factor range (δ K =7 MPa √(m) ) technique. Two main fatigue crack growth rates were distinguished corresponding to growth in the core and in the surface layer. The results show that FGMs may exhibit good fatigue crack propagation resistance. For example, when the crack propagated from the brittle core to the tough surface layer, the average fatigue crack growth rate in the Al-2124 core (3.9×10 −6 mm/cycle) was significantly lower than for the Al-2124 alloy (1.5×10 −5 mm/cycle) at a similar δ K value (7 MPa √(m) ), due to the highly tortuous crack path in the 2124/SiC/10 p brittle layer. The 2124/SiC/10 p brittle layer had a lower fatigue crack growth rate (6.6×10 −6 mm/cycle) than the 2124/SiC/10 p conventional composite (7.5×10 −6 mm/cycle) because of the compressive residual stresses in the surface layer. Thus, FGMs could be more acceptable for critical applications than their conventional composite counterparts.

The formation of titanium aluminides from the elemental powders has been investigated. A traditional powder metallurgy route of compaction (by cold isostatic pressing, hot pressing or hot extrusion) followed by heat treatment was compared with the novel technique of hot extrusion reaction synthesis (HERS). The products from these different production methods were characterised by x-ray diffraction and microscopy (light and scanning electron). The intermetallic compound formed under most processing conditions wasTiAl3. Only when there was a rapid increase in temperature to high temperatures, as found in induction heating of compacts or in HERS, were the compounds Ti3Al and TiAl formed.

Experiments were conducted on a miniature and a larger extrusion press to simultaneously form and extrude nickel aluminides (Ni3Al, NiAl) and nickel aluminide composites from elemental powders using a novel process called hot extrusion reaction synthesis. An overview of macroscopic and microscopic processing defects that can arise in this process is presented as well as strategies for overcoming these defects. Extrusion cracking was found to significantly increase with increased nickel content. Higher extrusion die exit temperatures promoted both a reaction converting elemental powders to the desired intermetallic or intermetallic composites and reduced cracking of NiAl extrusions. Processing defects in the form of matrix micro-cracking and reaction layers between intermetallic matrix and SiC reinforcements were also present in the composite material. The reaction always occurs seconds after the material had been extruded, thus bypassing the consolidation stage of extrusion resulting in the presence of reaction induced porosity. A novel high temperature transient window has been identified for the production of pore-free intermetallic and intermetallic composite rods and wires.

Hot extrusion reaction synthesis (HERS) was used to fabricate nickel aluminide/SiC p composites from elemental powder mixtures of nickel, aluminum, and silicon carbide. The effect of extrusion temperature, silicon carbide particle size, and volume content on the developed microstructures and on the peak extrusion pressure was investigated using a miniature extrusion rig. Matrix microcracking and loss of aluminum were observed in the final microstructures. The large surface area to volume ratio of the miniature extrudate “wires” in conjunction with a shorter reaction time at temperature reduced the reaction between the matrix and the SiC reinforcements. Although extrusion should have eliminated reaction synthesis related porosity, considerable levels of porosity were still generally present in the final extrudates, because all the elemental powder extrusions reacted after emerging from the die exit instead of before, thus by-passing the consolidation stage of extrusion. A novel two-stage extrusion method has been identified to overcome this problem.

A mechanical powder metallurgy process involving a vibration stage before cold and hot compaction has been developed to manufacture functionally graded materials (FGMs). The microstructure and mechanical properties of SICp/Al 2124 FGMs produced by this process have been studied. It was found that the vibration stage modified the SiC distribution from a layered structure to a smoothly changing SiC content with position in the sample. The vibration process also improved local homogeneity by breaking up the coarse SiC agglomerates which tended to form in high SiC content regions of the FGMs. Multilayered FGMs exhibited superior toughness to their metal matrix composite counterparts and there were indications that toughness was further enhanced in the continuous SiC gradient FGMs produced by the vibration stage.

Experiments were conducted to examine the feasibility of hot extruding metals on a miniature scale thus providing a quick and cheap method of studying extrusion in general. Aluminium was successfully extruded using a new miniature hot extrusion rig, producing aluminium wires (maximum diameter 2.6 mm) as opposed to rods. A preliminary comparison of extrusion pressures on the miniature rig and on a larger extrusion press was conducted. The effects of temperature, extrusion speed and extrusion ratio on the extrusion pressure were examined for both miniature and larger scale extrusions. Extrusion speed had little effect on extrusion pressure, because the range of speeds examined was too small (due to speed limitations on the larger extrusion press). Both extrusion sizes generally displayed similar dependencies on temperature and extrusion ratio. However, the extrusion pressures for miniature extrusions were found to be always lower than for the larger scale extrusions. This may have been due the evaluation of parameters in the expression used for strain rate in the comparison. Finite element analysis may prove useful in gaining a fuller understanding of the miniaturization process.

Extrusion of the alloy 6063 through bridge dies producing hollow shapes is a problem common in the aluminium industry. This communication reviews previous work on the extrusion of both hollow and solid shapes and presents methods to determine the pressure requirements and assess the difficulty of extrusion of shaped extrusions in general. Material flow through the bridge die is investigated by optical macroscopy and by use of a finite element program. It is shown that the pressure may be predicted by upper bound techniques and that the process has the same thermally activated features as that for rod extrusion. The finite element program appears to predict all the major characteristics of the flow observed macroscopically.

High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a generalised hot working equation. The (α + β) titanium alloy IMI 550 was studied at temperatures in the range 900–950°C, high in the (α + β) phase field, and over a range of strain rates from 4.2 × 10−4 S−1 to 4.2 × 10−2 S−l. The metastable β titanium alloy Ti-10V-2Fe-3Al was studied at temperatures in the range 740–780°C, high in the (α + β) phasefield, and also at 820–950°C in the β field, over the same range of strain rates. A good correlation between the experimental data and the hot working equation was observed for both alloys. Values of the activation energy for hot working have been considered, in conjunction with microstructural evidence, in relation to the dynamic restoration processes taking place. The activation energy for hot working of Ti-10V-2Fe-3Al in the β field has been found to be 185 kJ mol−1, which supports the view that dynamic recovery is the only restoration mechanism in the single phase field. In the(α + β) field, 'effective' activation energy values have been determined for both Ti-10V-2Fe-3Al and IMI 550, and these values were significantly higher than those for β forging, especially in the case of IMI 550. However, for both alloys, the dominant restoration mechanism at temperatures high in the (α + β) field is considered to be dynamic recovery of the β phase.

Elevated temperature true stress-true strain curves have been determined for the isothermal deformation of the (alpha + beta) titanium alloy Ti-4Al-4Mo-2Sn-0.5Si (IMI 550) in both the beta and (alpha + beta) phase fields using hot compression testing. The flow sti ess behaviour of the alloy has been correlated with the microstructural changes taking place during hot deformation. It has been observed that the shape of the hot deformation stress-strain curve is strongly dependent upon the starting microstructure of the material. Dynamic recovery of the beta phase is consider ed to be the dominant restoration mechanism for the temperature ranges studied. (C) 1997 The Institute of Materials.

Elevated temperature true stress-true strain curves have been determined for the isothermal deformation of the metastable β titanium alloy Ti-10V-2Fe-3Al in both the β and (α + β) phase fields using hot compression testing. The flow stress behaviour of the alloy, in two initial microstructural conditions, has been correlated with the microstructural changes taking place during hot deformation. The shape of the isothermal hot deformation stress-strain curve is strongly dependent upon the starting microstructure of the material. The operative restoration mechanism during deformation in the β field isdynamic recovery since β subgrains are readily observed in the as β forged condition. Dynamic recovery of the β phase is also considered to be the dominant restoration mechanism for both microstructural conditions during deformation at temperatures high in the (α + β) field.

The composition, structure, and properties of functionally graded materials (FGMs) vary as a function of position within the material. The extrusion of bulk FGMs, that is FGMs in which the changes occur over distances of many millimetres throughout the crosssection, is described. Graded materials containing up to 30 vol.-%SiCp in a matrix of aluminium alloy Al 2124 were successfully extruded from preforms produced using a cylindrical stacking method employing concentric, cylindrical aluminium foil tubes. However, the extent of the steady state extrusion region was increased by adopting a conical stacking method. For both stacking methods a pure aluminium surface layer was required to reduce the friction associated with the high silicon carbide contents. PM/0729

Recent advances in rapid solidification technology have resulted in the development of aluminium alloys for elevated temperature applications as a suitable replacement for titanium based alloys. Processing of these alloys is an important step in developing optimal properties for elevated temperature use. One such approach is the study of the aspect ratio (width/thickness) effect in extrusion. In the present investigation, gas atomized Al-6.7w/oFe-5.9/oCe powder was extruded into rectangular slabs of aspect ratio approximate to 5:1. For comparison, axisymmetric extrusion (aspect ratio 1:1) was carried out under identical conditions. The room temperature and elevated temperature mechanical properties were evaluated for both die geometry extrudates. The rectangular bars were 'straight' and 'cross' rolled in the ''hot'' condition. Microstructural characterisation was performed using transmission electron microscopy and textures were quantified using X-ray diffraction. The microstructure was homogeneous in the rectangular bar extrudates compared to the axisymmetric round bars due to an improved dispersoid distribution. The straight rolled material exhibited a texture consisting of the ''Copper'' and ''S'' components whereas the Gross rolled product exhibited predominantly the ''Brass'' component with other components such as {110}<2 $($) over bar$$ 23>. The observed texture differences were explained on the basis of the Dillamore and Roberts model. The rectangular bar extrudates do not show an improvement in mechanical properties because texture differences offset the advantage provided by the improved dispersoid distribution.