SOME OBSERVATIONS ON THE COLD-DRAWING AND ANNEALING BEHAVIOUR OF NICKEL CONTAINING A DISPERSED PHASE OF THORIA

Abstract

Consideration is given to the effects of cold drawing and subsequent heat-treatment on the mechanical properties of extruded nickel-2·5 vol.-% thoria rod, prepared from mechanically mixed nickel and thoria powders. It is shown that the extruded alloy may be reduced > 80% in area using no intermediate annealing treatments, with significant beneficial effect on the room-temperature tensile strength and on the stress-rupture properties at 815°C. The cold-drawn alloy has marked resistance to annealing at temperatures in excess of 1000°C. Comparative data are given for pure nickel. The effects of cold drawing and subsequent heat-treatment on the structure of the alloy are then discussed with reference to changes in the dispersed phase and in the matrix revealed by electron microscopy and by X-ray diffraction studies.     

Influence of extrusion ratio and temperature on microstructure and mechanical properties of 2024 aluminium alloy consolidated from nanocrystalline alloy powders via hot hydrostatic extrusion

Abstract

Nanocrystalline 2024 aluminium alloy powders with an average grain size less than 50 nm, prepared by a unique technique which combines rapid solidification and mechanical milling, were consolidated into bulk material under various technical conditions via hot hydrostatic extrusion and the microstructure and mechanical properties of the consolidated alloy were experimentally investigated. The influence of the two main technical parameters, extrusion ratio and temperature, on the microstructure and mechanical properties of the as extruded alloy is made clear and the reasons why these two parameters had such an influence on the microstructure and mechanical properties of the alloy are also discussed. Furthermore, suggestions are given for rationalising the extrusion ratio and temperature for the consolidation of the nanocry stalline 2024 aluminium alloy powders via hot hydro static extrusion.     

Microstructural investigation of as cast and PREP atomised Ti–6Al–4V alloy

Abstract

A microstructure characterisation of Ti–6Al–4V is conducted for cast, extruded and micrometre sized particles. The plasma rotating electrode process is used to produce spherical Ti–6Al–4V powders from an alloy electrode. The process parameters and their impact on the material properties are described. The effects of electrode rotation speed on the particle size distribution, particle shape and crystal structure are investigated in detail. Optical microscopy and scanning electron microscopy are used for microstructural characterisation. The analysis shows that cast and extruded Ti–6Al–4V alloys have equiaxial α and α+β phase structures, while plasma rotating electrode processed powder from the same alloy compositions has an acicular or martensitic (α) structure. The microstructure scale depends on the particle size. Microhardness measurements are used to assess mechanical property dependence on the microstructure of this alloy. The rapidly cooled alloy particles have much higher hardness than cast or extruded bulk alloy.     

THE PROPERTIES AND SOME APPLICATIONS OF CARBONYL-NICKEL POWDERS

Abstract

Nickel carbonyl, formed by the reaction of carbon monoxide with a nickel concentrate, can be decomposed in a heated space to produce pure nickel in powder form. By varying the decomposition conditions it is possible to achieve a wide range of powders, differing in particle shape and size distribution.

The properties of carbonyl-nickel powders now commercially available or in an advanced stage of development are discussed, and it is shown how such powders can be used for some applications in which specific properties are required.     

Powder Metallurgy Group Meeting,Coventry, 24–26 October 1977: Report of Proceedings

Abstract

The feasibility of producing room temperature superplastic Zn–Al alloys by hot extrusion of gas atomised powders has been investigated. Commercially pure zinc, and Zn–8wt-%Al and Zn–28wt-%Al binary alloys were gas atomised; the resulting powders were cold compacted into cylindrical billets and extruded to form consolidated rod. Two extrusion temperatures (200 and 300°C) were used, chosen to lie on either side of the invariant (eutectoid) temperature of 275°C. It has long been established that in conventional cast alloys rapid quenching from above this temperature is required to produce a microstructure having superplastic properties. (It was anticipated that the 300°C extrusions would contain quantities of near equilibrium eutectoid and thus be unlikely to deform superplastically. The 200°C extrusions were expected to exhibit a non-equilibrium structure that might have potential in terms of superplastic deformation.) The microstructures of the extrudates were investigated by transmission electron microscopy and the mechanical properties established by room temperature tensile testing and Charpy impact testing. PM/0502     

THE PRODUCTION OF METAL POWDERS FROM AQUEOUS SOLUTIONS

Abstract

The principles involved in producing metal powders by hydrogen reduction of metal ions in aqueous solutions are discussed from the thermo-dynamic and the kinetic standpoint. The influence of nuclei and of surface-active agents on the nature of the powders are considered in relation to the production of nickel, cobalt, and copper.

A survey is made of the scale and technology of current processes and the uses to which metal powders may be put are described, with particular reference to the manufacture of metal strip and plates, dispersion-hardened nickel, and composites.

The practicability of controlling the conditions of reduction to yield powders with widely varying combinations of properties is a most valuable attribute of hydrometallurgical methods.     

EFFECT OF PRODUCTION ROUTE ON THE STRUCTURE AND PROPERTIES OF A NICKEL-CHROMIUM-COBALT ALLOY

Abstract

Preparation of an extruded nickel-chromium-cobalt alloy of the Nimonic 90 type, using powders, wires, and conventional cast billets as starting materials, resulted in a wide variety of grain sizes and structures. The form of the oxide/carbide and its distribution varied with the production route. The performance of the materials, as assessed by stress-rupture, fatigue, and impact data, also differed considerably. The stability of the grains was directly related to the distribution of the oxides/carbides and was therefore interrelated to the stress-rupture properties. Fatigue properties were sensitive to a fine general distribution of oxides/carbides and impact properties were sensitive to an intergranular distribution. With the best structure, of a fibrous nature, it was possible to achieve an attractive combination of stress-rupture, fatigue, and impact properties—the last being unusually high.     

High temperature properties of Cu–Cr–Zr alloys processed by conventional inverse extrusion of atomised powders and comparison with Conform consolidated powders

Abstract

Water atomised Cu–Cr–Zr alloy powders were consolidated by inverse warm extrusion and by the commercial continuous rotary extrusion method, Conform. Those alloys consolidated by inverse warm extrusion exhibited enhanced mechanical properties compared with their respective Conform extruded counterparts, when tested at both room and elevated temperatures. The processing parameters adopted in the inverse extrusion experiments resulted in products which retained enough amounts of solutes in solid solution, which in turn, led to improved mechanical properties after aging. Conversely, the excessive adiabatic heat generated in the Conform machine eliminated the saturation effect produced by rapid solidification, negating any possible further improvement on the mechanical properties by aging. The mechanical properties of an inverse extruded Cu–2.8Cr–0.39Zr (at.-%) alloy at temperatures above 450°C were higher than those strengths reported for Cu–Be alloys and comparable to that of Cu–Ta and Cu–Nb composites. Therefore, rapidly solidified Cu–Cr–Zr alloys can be possible candidates for replacing such alloy systems for high temperature applications.     

THE PROPERTIES OF 18Ni 350 MARAGING STEEL PRODUCED FROM ELEMENTAL AND PREALLOYED POWDERS

Abstract

The strength of extruded and heat-treated bar of 18Ni 350 maraging steel produced from prealloyed powder has been found to be comparable to that of conventional, wrought 18Ni 350 maraging steel. Impact toughness was considerably higher. In contrast, properties of bar Obtained from an elemental powder mix were markedly inferior to wrought 18Ni 350 maraging steel. The high toughness of Charpy V-notch specimens produced from prealloyed powder was partly attributed to the presence of continuous, aluminium-rich stringers.This resulted in delamination of the specimens during testing and led to a fibrous fracture. Charpy V-notch specimens produced from the elemental powders also contained stringers. However, these were not continuous and therefore did not cause delamination. The resulting fracture surface was flat and relatively featureless. Electron-microprobe analysis has shown that the morphology of the stringers is largely determined by their composition and the composition at the stringer/matrix interface.     

PREPARATION AND SELECTED PROPERTIES OF CERTAIN DISPERSION-STRENGTHENED LEAD-BASE ALLOYS

Abstract

Dispersion-strengthened lead-copper and lead-aluminium alloys have been prepared by atomizing the alloys from a temperature at which the copper or aluminium was dissolved in the liquid lead. The particles of the resultant powders consisted of a very fine dispersion of a second phase in a lead matrix. The powders were compacted and extruded. The mechanical properties of the extrusions at room and elevated temperature have been determined and their microstructure examined.

At room temperature the alloys had a combination of tensile strength and ductility superior to most other lead alloys, including lead alloys strengthened by a dispersion of lead oxide. The strength of the leadcopper alloys increased with increasing copper content; this was attributed to the closer spacing of the,dispersed copper particles in the extrusions.

The creep of the alloys was investigated at room and at elevated temperatures up to 100°C. Lead-aluminium alloys containing 0·55% aluminium had steady-state creep rates of < 1%/year at stresses up to 2500 lb/in² at 25°C and up to 1200 lb/in² at 100°C. The creep-resistance of the lead-copper alloys was somewhat lower. Their steady-state creep rate at low stresses was proportional to the applied stress, while at high stresses it was proportional to the fourth power of the stress.     

ON THERMAL ACTIVATION DURING EXTRUSION OF METAL POWDERS

Abstract

Four aluminium alloy powders were extruded in the temperature range 250–550° C and the strain-rate/flow-stress/temperature relationship analysed in terms of exponential functions. The results of ～400 experimental runs indicate that the powder process is less strain-rate sensitive than is its conventional counterpart. The process is thermally activated and it appears that the rate-controlling mechanism is the climb of edge dislocations or the migration of jogged screw dislocations.     

Glass-Fibre-Reinforced Aluminium Alloys Formed by Hot Extrusion: Part II: Heat-Treatment

Abstract

The microstructures of rapidly solidified 7050 alloy powders have been observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy and compared with those observed in a cast and homogenized ingot of the same material. The powder alloy was then consolidated by cold compaction followed by extrusion, the resulting structures being compared with those of extruded cast alloys. Finally, the structure and properties of the two materials were compared after heat treatment. It is shown that metastable phases not previously reported are present in the powder particles and that localized melting occurs in the powder alloy at a lower temperature than in the cast alloy; this is the primary cause of blistering, but appropriate modification to the heat treatment will prevent it occurring. Properties comparable to or better than the cast material may be produced by designing a more suitable heat treatment to avoid liquation of this metastable phase. The evidence presented suggests that the degassing procedure employed historically to counteract blistering is probably unnecessary. PM/0322     

NiAl powder alloys: II. Compacting of NiAl powders produced by various methods

The technological properties of granulated NiAl powders produced by gas spraying of melts and NiAl powders produced by calcium hydride reduction (CHR) of mixtures of nickel and aluminum oxides are compared. The possibilities of production of compact workpieces from these powders using hydrostatic pressing, hot pressing, hot isostatic pressing, and hot extrusion are estimated. To improve compressibility, preliminary milling and/or mechanical activation of the powders are proposed. The strength properties of NiAl rods with a diameter of 20 mm extruded from a temperature of 1100°C and made from the granulated powders are slightly higher than those made from the CHR powders. At temperatures higher than 800°C the properties becomes similar. Transition point t _d.b from the ductile to brittle state of samples made from powders sprayed in nitrogen and argon is 100?C150°C higher than those made from the CHR powders. The difference in the mechanical properties is caused by the structural and chemical microheterogeneity of granules (microingots), which is inherited in the rods after hot deformation and annealing at 1200?C1400°C and is (0.67?C0.88)T _m NiAl (T _m is the melting point, K).     

THE PRESSING AND SINTERING PROPERTIES OF IRON POWDERS

Abstract

Some twenty different iron powders are available in Europe for the manufacture of sintered bearings and structural parts. These powders can be grouped under four headings: reduced, atomized, comminuted, and electrolytic. Details are given of the experience gained in testing these types of powder by the methods in common use in the metal-powder industry.

The influence of the data thus obtained on the processing, in particular the pressing and sintering conditions, of iron powders is discussed in detail. Consideration is given to the way in which the properties of the powders affect their sinterability, the pressing operation and tool design, and also the physical characteristics of the finished product.     

THE IMPORTANCE OF SURFACE PROPERTIES AND PARTICLE SHAPE IN THE COMPACTION OF GRAPHITE AND MoS2

Abstract

Adsorptive studies of the surfaces of graphite and MOS₂have shown that these consist of two distinct types of site. The sites on the basal-plane surface differ from those on the edge surface with respect to their relative affinities for different organic compounds. These findings led to the development of grinding techniques to produce graphite and MoS₂ powders possessing different ratios of basal-plane:edge-surface area.

Grinding graphite and MoS₂ in the presence of low-viscosity, volatile hydrocarbons produced very thin flake-like powders, consisting predominantly of basal-plane surface. These fine flakes showed a high affinity for long-chain n-paraffins and were therefore termed oleophilic solids. Grinding under reduced pressure also produced very fine powders, having, however, a more granular structure exhibiting a far lower ratio of basal-plane: edge-surface area. These were termed polar solids to distinguish them from the solids ground in liquid hydrocarbons.

The cold-forming properties of the various powders have been compared under uniaxial compaction. The conversion of synthetic and natural graphite powders to the oleophilic form resulted in marked improvements in both compact strength and modulus. Synthetic graphite converted to the polar form would not form a compact at cold-forming pressures up to 800 MN/m².

The cohesive properties of the oleophilic graphite powders were improved by heating to 900°C in hydrogen. Electrical-resistivity measurements showed that cold-formed oleophilic graphite compacts exhibited a marked anisotropy. The improved cold-forming properties of the powders are ascribed directly to improved cohesion via basal-plane site interactions, coupled with the facility of the flake powders to take up a preferential orientation during compaction in order fully to utilize the extensive basal-plane sites available for cohesion.

The differences between the oleophilic and polar forms of MoS₂ were less marked. It is believed that interparticle cohesive junctions are more readily formed via edge/edge interactions, and basal-plane junctions do not play as important a role in the cohesion of MoS₂ as in that of graphite.

The corrosion and abrasion of metal surfaces by graphite and MoS₂ have been examined. In all cases the powders converted to the oleophilic form showed reduced abrasive and corrosive characteristics when compared to similar powders converted to the polar form. These improvements are believed to result from the reduction of the possibilities of edge interactions with the metal surfaces.     

CHARACTERISTIC PROPERTIES OF COBALT POWDER SUITABLE FOR HARD METAL PRODUCTION

Abstract

Prompted by the fact that knowledge of the essential properties of fine metallic powders, such as Co powder, used in hard metal production is often inadequate, this investigation uses a special working hypothesis. According to this the following properties of such a powder are most important: its SEM image, specific surface area, crystallite size as determined with X-rays, and α/β phase distribution. In the first part of this paper two well-known commercial Co powders are studied; in the second part methods of producing a fine Co powder from a special type of Co oxalate are described. The results show that it is feasible to produce a powder corresponding to the reference powders, provided that the crystal size and form of the oxalate are correctly developed and decomposition conditions (temperature, time) are right.     

THE PROPERTIES OF POROUS NICKEL PRODUCED BY PRESSING AND SINTERING

Abstract

The effects of compacting pressure and of sintering temperature and time on the properties of porous sintered nickel compacts have been studied, using three carbonyl and two reduced nickel powders. For all five powders, the density of the green compacts and the porosity of the sintered compacts were linearly related to the log compacting pressure. Similar relationships with pressure were observed for strength and electrical conductivity.

Photomicrographs of sections through the sintered compacts made from the reduced nickel powders show that there are pores in two different size ranges, originating from the porosity between the original powder particles and the pores within the particles. It is concluded that sintered compacts from all five powders containing 40–50% porosity have adequate strength and conductivity for use in fuel-cell electrodes.     

THE RELATIONSHIP BETWEEN POWDER PROPERTIES AND THE PRESSING AND SINTERING BEHAVIOUR OF BERYLLIUM

Abstract

The paper surveys the existing routes available for the preparation of beryllium powders, and comments on the effect of certain properties, such as particle-size distribution, on the compaction of powders by different techniques. It is emphasized that particle size, degree of oxidation, and overall purity are interdependent variables, and that little attempt has so far been made to study their effects separately.     

SOME EXPERIMENTS IN THE PRODUCTION OF LOW-ALLOY STEEL BY POWDER METALLURGY

Abstract

The properties of alloy steels produced by powder-metallurgy techniques, as compared with those of conventional cast or wrought alloy steels, are reviewed. Factors affecting the production of alloy steels by powder-metallurgy techniques are enumerated, and the importance of adequate mechanical properties coupled with ease of processing is emphasized. The paper covers the preparation and testing of a wide range of low-alloy steels including copper steels, nickel steels, and complex steels; these being made up either from elemental powders or from pre-alloyed or complex powders. The influence of various alloying elements upon the mechanical properties of the steels produced is indicated, and the merits of the various compositions investigated, are discussed.

The response of powder-metallurgy alloy steels to quenching is next determined, and the effects of the elements carbon, manganese, nickel, copper, and molybdenum upon the hardcnability of alloy steels is summarized in the form of depth-hardening curves. The use of precipitation-hardening as a means of improving the properties of low-alloy steels is also investigated for a wide range of copper-containing steels. It is demonstrated that by employing this technique it is possible to obtain very significant improvements in properties, together with some processing advantages.     

THE COMPACTING OF GRAPHITE AND GRAPHITE/URANIUM/THORIUM MIXTURES

Abstract

Fine artificial graphite powders can be cold compacted to give bodies of high density (～ 88% of theoretical), low permeability (B₀ ～10^–14 cm²), and reasonable strength. Such powders, after vacuum annealing, will not compact.

Die-compacted powder has strongly anisotropic properties owing to a high degree of preferred orientation within the compact; this effect is less marked in hydrostatically compacted powder. Minor dimensional changes occur when compacts are annealed in the range 600-1000°C.

The preparation of fuels by incorporation of fissile and fertile materials into graphite powder and cold compacting is described.