FEASIBILITY OF PRODUCING DISPERSION-STRENGTHENED CHROMIUM BY BALL-MILLING IN HYDROGEN HALIDES

Abstract

Chromium, with and without 4 vol.-% thoria, and nickel powders were ground to fine powder sizes by ball-milling in gaseous hydrogen halides. After reducing the milled chromium in flowing hydrogen under pulsating pressure at ～680°C, submicron-size powders with 4–500 ppm residual halogens were obtained. The compacted chromium–thoria alloys had interparticle spacings ranging from 2·1 to 6·5 μm. After 100 h at 1318°C the interparticle spacing of the 2·1-μm alloy increased to 5·2 μm. Submicron-size chromium and nickel powders were also obtained by pulsating hydrogen reduction of their chlorides.     

THE PROPERTIES OF IRON CONTAINING A FINE OXIDE DISPERSION

Abstract

Gel precipitation has been used to produce fine dispersions of thoria in iron. The process involves coprecipitation of hydroxides followed by a hydrogen reduction. Iron containing up to 7·65 vol.-% thoria was prepared with particle sizes in two ranges averaging 6 and 110 nm in diameter. The dispersion, stable up to at least 1300°C, raised the tensile and yield strengths by 80% compared with the iron made in the same way while retaining an acceptable level of ductility at ambient temperature and below. Impact testing showed that the presence of thoria did not have an adverse effect on the ductile-brittle transition temperature for thoria contents up to 4·55 wt.-%. It was not possible to estimate the strengthening due to the dispersions from the models of Orowan bowing of dislocations between particles because of the presence of two size ranges of particles in these alloys. The thoria dispersions did not raise the primary recrystallization temperature of the iron but were extremely effective in restricting secondary recrystallization. Grain sizes of <25μm were obtained after 15h at temperatures up to 1400°C. The thoria-containing alloys did not work harden more rapidly than the iron and with thoria contents up to 3 wt.-% could be readily cold worked.     

Effect of composition and milling time on mechanical and wear performance of copper–graphite composites processed by powder metallurgy route

Abstract

Copper–graphite (Cu–Gr) composites with 0, 5, 10 and 15 vol.-% graphite were processed via powder metallurgy route. The effect of composition and milling time on mechanical properties and wear resistance were studied. With increase in vol.-% of graphite, there was decrease in hardness of the composites. However, increasing milling time showed significant increase in hardness of the composites. Compressive strength of the composites containing 5 and 10 vol.-% of graphite was found to be 515 and 393 MPa respectively. The wear tests were carried out using a block-on-ring tribometer at a load of 30 N with varying sliding speed. The wear performance of the composites was found to be better with increase in milling time. The worn surfaces were analysed using FESEM. With increase in graphite content from 5 to 15 vol.-%, the coefficient of thermal expansion of the Cu–Gr composites decreased from 14·1 to 12·2×10^?6/°C.     

ThO2-DISPERSION-STRENGTHENED NICKEL AND NICKEL-MOLYBDENUM ALLOYS PRODUCED BY SELECTIVE REDUCTION

Abstract

A series of Ni-ThO₂ and Ni-12% Mo-ThO₂ alloys, containing from 3 to 9 vol.-% thoria, were prepared by selective hydrogen reduction of mixed submicron oxides. Room-temperature strength properties considerably higher than usual, with good ductility, were obtained. At 982°C, creep-rupture properties were excellent, with significant improvements in strength and stability due to solid-solution strengthening of nickel by 12 wt.-% molybdenum. An investigation was made of the benefits derived from additional cold work after hot extrusion, with and without intermediate annealing treatments. An analysis of the probable strain distribution in oxide-dispersion-strengthened alloys is presented.     

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.     

Effects ofcontaminationonproperties of W-15Cu preparedfrom mechanically alloyed powders

Abstract

To eliminate the contamination of activator elements, such as Fe and Ni, W-15Cu compacts were prepared from mechanically alloyed powders using an attritor with a zirconia tank, balls and agitator arms. Coarse tungsten and copper powders, 9·9 μm and 13·3 μm, respectively, were milled to 1·26 μm composite powders after 145h of milling. The milled powder contained little free copper and was highly combustible in air. After sintering, the 50 vol.-% dense green compacts attained a density of 15·8g cm^-3 or 96·2%. The microstructure consists of uniformly interdispersed tungsten and copper. When stainless steel grinding balls were used, the powder was heavily contaminated with Fe and Ni. The contamination improved the density slightly, but the grain size and the electrical resistivity increased significantly as well. The sintering behaviours of the two composite powders were similar. Most densification occurred during heating before reaching the melting point of copper.     

The Effect of Cooling Rate on the Strength of Sintered Fe—Cu Compacts

Abstract

The effect of cooling rate from the sintering temperature upon the tensile strength of compacts from a mixture of iron and copper powder was investigated. The compacts were pressed at 450 and 390 MPa and sintered in hydrogen at 1120°C for 40 min. The copper content of the compacts varied from 0 to 12%. For alloys with Cu content >4% the tensile strength was found to be strongly dependent upon the cooling rate in the temperature range between 850 and 600°C, with rapidly cooled specimens being considerably stronger. In specimens with 8%Cu the tensile strength increased from 206 to 343 MPa when the cooling rate was increased from 10 to 200 degC min^?1. In specimens with 2%Cu cooling rates above and below 600 degC min^?1 appear to influence the tensile strength. Possible explanations for the observed effects of cooling rate upon tensile strength in sintered Fe–Cu alloys are discussed.     

Mechanical and Thermal Properties of Pulsed Electric Current Sintered (PECS) Cu-Diamond Compacts

In this work, dispersion strengthening of copper by diamonds is explored. In particular, the influence of 50- and 250-nm diamonds at contents of 3 and 6 vol. pct on the mechanical and thermal properties of pulsed electric current sintered (PECS) Cu composites is studied. The composite powders were prepared by mechanical alloying in argon atmosphere using a high-energy vibratory ball mill. The PECS compacts prepared had high density (>97 pct of T.D.) with quite evenly distributed diamonds. The effectiveness of dispersoids in increasing the microhardness was more pronounced at a smaller particle size and larger volume fraction, explained by Hall–Petch and Orowan strengthening models. The microhardness of Cu with 6 and 3 vol. pct nanodiamonds and pure sm-Cu (submicron-sized Cu) was 1.77, 1.46, and 1.02 GPa, respectively. In annealing experiments at 623 K to 873 K (350 °C to 600 °C), the composites with 6 vol. pct dispersoids retained their hardness better than those with less dispersoids or sm-Cu. The coefficient of thermal expansion was lowered when diamonds were added, being the lowest at about 14 × 10^?6 K^?1 between 473 K and 573 K (200 °C and 300 °C). Good bonding between the copper and diamond was qualitatively demonstrated by nanoindentation. In conclusion, high-quality Cu-diamond composites can be produced by PECS with improved strength and better thermal stability than for sm-Cu.     

Current PM literature for engineers and users

Abstract

Fully dense composite materials of M3/2 tool steel reinforced with 5 and 8 vol.-% of niobium carbide were developed using the powder metallurgy route. The consolidated materials exhibited a fine and uniform microstructure consisting of a fine dispersion of carbide particles in a matrix of ferrite/martensite. The 0·2% yield strength up to 600°C was evaluated in compression. The slight increase observed after reinforcement by NbC particles in as hipped materials suggests that the martensite has the major contribution to the strength. After tempering, the reinforced materials showed a moderate increase in yield stress at room temperature with respect to the unreinforced M3/2. This increase is attributed to reinforcement mechanisms associated with niobium carbide. PM/1175     

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

Effect of alloying by transitional refractory metals on the microstructure and thermal stability of Al–5Zn–3Mg alloys

Abstract

The effect of additions of transitional refractory metals on the structure and properties of Al–Zn–Mg alloys, made by ingot and PM routes, was investigated. The strength of the ingot alloys especially is increased by scandium and zirconium. The modifying action of scandium inhibits recrystallisation and precipitation of the fine-grained coherent Al₃(Sc_1–xZr_x) phase. The effect is weaker in PM alloys where the ultra-high cooling rate during high pressure water atomisation produces the fine-grained structure. PM semi-products of the base composition Al–5Zn–3Mg and alloys without scandium are not recrystallised during heating to 500°C, whereas cast alloys of similar composition recrystallised on the hot extrusion stage at 400–450°C. Of the Sc alloys, Al–5Zn–3Mg–0·5Mn–0·7Zr–0·3Sc showed the highest strength (UTS?=?651 MPa, YS?=?596 MPa), whereas of the PM alloys without scandium Al–5Zn–3Mg–0·85Zr–0·22Cr–0·17Ni–0·15Ti alloy showed UTS?=?618 MPa and YS?=?553 MPa. At melt cooling rates of 10⁵–10⁶ K s^–1 the total content of transitional refractory metals must not exceed 1·5–1·7 wt-% and total content (Zn+Mg) should be <8 wt-% at a Zn/Mg ratio of 5:3.     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute     

Microstructure and mechanical properties of SiC particles reinforced Mg–8Al–1Sn magnesium matrix composites fabricated by powder metallurgy

The new type of Mg–8Al–1Sn (AT81) magnesium matrix composites reinforced with different volume fractions (5, 10, 15, 20, 25 and 30 vol.-%) of SiC particles (average size of 10 μm) was fabricated by powder metallurgy. With the increasing volume fraction of SiC particles (SiC_p), the particles gradually show more homogeneous distribution. Compared with the AT81 alloy, the yield strength (YS) and ultimate compressive strength of the SiC_p/AT81 composites are improved simultaneously. With the increasing SiC_p from 0 to 30 vol.-%, the YS and ultimate compressive strength increase from 69 to 239 MPa and 286 to 385 MPa respectively, while the corresponding fracture strain (ε) decreases from 19·3 to 4·8%. The improvement of the YS and ultimate compressive strength of the SiC_p/AT81 composites benefits from the more homogeneous microstructure due to the increase in the SiC particles.     

STUDIES ON THE ACTIVATION-SINTERING OF IRON POWDER

Abstract

A study of the sintering behaviour of iron compacts containing additions of tin up to 1 wt.-% has been made. A tensile strength of 234 MN/m² (34 x 10³ lbf/in²) has been achieved with an optimum tin addition of 0·5 wt.-%, sintering being carried out for 10 min at 1100°C (1373 K) in a reactive halide atmosphere. Combination of the two ‘activating’ techniques (addition of tin and sintering in a reactive atmosphere) permits current properties to be attained at considerably lower sintering temperatures or sintered densities, and is much more effective than when they are applied individually. A tensile strength of 165·3 MN/m² (24 x 10³lbf/in²), achieved by sintering at 1200°C (1473 K) for 10 min with an addition of 0·5 wt.-% tin can be obtained by reactive-sintering the same composition at 900°C (1173 K) for 10 min. Alternatively, the density of the part can be reduced from 6·7 to 6·2 g/cm³ with no loss of strength or elongation. Tin in excess of 0·5 wt.-% causes deterioration in properties under the sintering conditions studied and a reason for this is cited. The improvements in properties are lost also if admixed lubricant is used in the compactionprocess.     

SINTERING KINETICS IN IRON-COPPER ALLOYS WITH AND WITHOUT A LIQUID PHASE

Abstract

Sintering of iron-copper alloys has been studied in the temperature range 950–1250°C. The factors involved include compacting pressure, sintering temperature, sintering time, and atmosphere. The results are interpreted as a decrease in pore volume due to the filling of voids between particles by a diffusion mechanism. An empirical equation of the Arrhenius type, based upon volume change as a function of sintering time, has been derived in order to evaluate the rate constant of the sintering process.

Volume diffusion is considered to be the primary mechanism of material transport in alloys containing 0·5–2·0% copper, when sintered in the range 950–1250°C, and in alloys containing 5·0–10·0% copper, when sintered in the range 950–1050°C. The activation energy derived for the sintering process is 53·4 kcal/mole. Surface diffusion appears to be the operative mechanism of material transport in alloys containing 5·0–10·0% copper, when sintered above the melting point of copper. The activation energy for this sintering process is 32·6 kcal/mole.     

Sintering and aging behaviours of Al4CuXMg PM alloy

Sintering and aging behaviours of Al–Cu–Mg powder metallurgy (PM) alloy produced from elemental powders were examined. After evaluating results from thermal analysis, tests were carried out on Al–4Cu alloys with magnesium contents of 0.5, 1 and 2?wt-% and it was found that additions of 1?wt-% Mg was most effective for enhancing the transverse rupture strength (TRS) of the Al–Cu PM alloys for both as sintered and after a heat-treatment conditions. Grain size reduction in the range of 14–45% was achieved by adding magnesium into Al–Cu system. Analyses showed that produced alloys were composed of Al, Al₂Cu, Al₂CuMg and Al₇Cu₂Fe phases. Differential scanning calorimeter and dilatometer analyses revealed that alloys show swelling behaviour after the eutectic melting reaction at 548°C and swelling rates increasing as a function of magnesium content. Both high hardness value (120 HB) and TRS (650?MPa) were achieved via aging of Al4Cu1Mg alloy for 24 hours.     

Tensile and fatigue behaviour of sinter hardened Fe–1·5Mo–2Cu–0·6C steels

Abstract

In the present work the tensile and axial fatigue behaviour of sintered hardened Fe–1·5Mo–2Cu–0·5C at three density levels (6·8, 7·0 and 7·2 g cm^–3) have been studied. The materials were tested under the as sintered condition, and after tempering at 180 and at 240°C. The results show that steels under the as sintered condition posses a high hardness but a brittle tensile deformation and fracture behaviour. Tempering at 180 and 250°C induces the disappearance of brittleness and tensile fracture is thus ductile although very localised at the necks. Fatigue strength is determined by the resistance of the materials to the internal damage evolution due to the nucleation of small cracks at the pores edges, and their coalescence into a long crack. Tempering induces an increase in the fatigue resistance. The greatest fatigue strength at 2 × 10⁶ cycles is displayed by the steel with a density of 7·2 g cm^–3 and tempered at 180°C.     

Age-Induced Precipitating and Strengthening Behaviors in a Cu–Ni–Al Alloy

Microstructural response and variations in strength and electrical conductivity of a Cu−20 at. pct Ni–6.7 at. pct Al alloy during isothermal aging at temperatures from 723 K to 1023 K (450 °C to 750 °C) were investigated to discuss the age-induced precipitation behavior and strengthening mechanism. At all aging temperatures, fine spherical γ′-Ni₃Al particles were found to nucleate coherently with parent Cu grains by continuous precipitation and then grew gradually by Ostwald ripening. Domains with a high density of twins developed at grain boundaries during aging below 873 K (600 °C) followed by cellular components composed of fiber-shaped γ′-Ni₃Al and Cu solid solution phases at the domain boundaries later. Both the domains and cellular components were suppressed at aging above 923 K (650 °C). The age-induced strengthening principally resulted from fine dispersion of γ′-Ni₃Al coherent particles in the grains. The precipitation strengthening by the fine γ′-Ni₃Al coherent particles exhibited a maximum at an aging temperature of 873 K (600 °C), resulting in excellent mechanical properties such as a high hardness of 340 ± 7 HV and an ultimate tensile strength of 980 ± 14 MPa, which are comparable to those of other commercial age-hardened Cu–Be, Cu–Ni–Si, and Cu–Ti alloys.

     

Effect of iron as alloying element on electrochemical behaviour of 90∶10 Cu–Ni alloy

Abstract

Copper–nickel alloys have been used in many applications in marine environments, because of their excellent corrosion and biofouling resistance. In this study, the effects of iron as an alloying element on the corrosion behaviour of copper–10 wt-% nickel alloy in artificial saline solutions and natural sea water were investigated. Synthetic copper–nickel alloys were prepared in an induction furnace under an argon–7 vol.-% hydrogen atmosphere in cylindrical boron nitride crucibles. They were then homogenised at 950°C for 10 h under the same protective atmosphere. Linear sweep polarisation, cyclic polarisation, Tafel extrapolation and cyclic voltammetry techniques were performed in this investigation. Following the electrochemical measurements, the corrosion products and the passive film were analysed using a field emission scanning electron microscope (FE-SEM), electron probe microanalysis, energy dispersive spectrometry (EDS), wavelength dispersive spectrometry (WDS) and X-ray diffraction (XRD). The electrochemical behaviour of the synthetic Cu–Ni–Fe alloys depends on maintaining iron in a single phase in the solid solution (the maximum amount of iron that can be used was 1·5 wt-%). Quenching improves the electrochemical behaviour of synthetic Cu–Ni–Fe alloys containing relatively high iron content. The outer layer of the passive film is porous in the absence of iron, but when iron is added, the pores disappear and cracks appear. When no sulphate is present in the solution, the passive film formed on synthetic Cu–Ni–Fe alloys consists entirely of chlorides, and Fe₂O₃. In the presence of sulphate, FeS and NiS were detected in the corrosion film.

On a utilisé les alliages de cuivre–nickel à de nombreuses fins en milieux marins, grâce à leur excellente résistance à la corrosion et à l’incrustation biologique. Dans cette étude, on a examiné l’effet du fer en tant qu’élément d’alliage sur le comportement de corrosion d’un alliage de cuivre–10% en poids de nickel dans des solutions salines artificielles et dans de l’eau de mer naturelle. On a préparé des alliages synthétiques de cuivre–nickel dans un four à induction dans une atmosphère d’argon–7% en volume d’hydrogène dans des creusets cylindriques de nitrure de bore. On les a ensuite homogénéisés à 950°C pendant 10 heures sous la même atmosphère protectrice. Dans cette investigation, on a utilisé des techniques de polarisation linéaire à balayage, de polarisation cyclique, d’extrapolation de Tafel et de voltampérométrie cyclique. Après les mesures électrochimiques, on a analysé les produits de corrosion et le film passif en utilisant un microscope à balayage électronique à champs par émission d’ions (FE-SEM), la microanalyse par électrons, la spectrométrie des rayons X à dispersion d’énergie (EDS), la spectrométrie dispersive en longueur d’onde (WDS) et la diffraction des rayons X (XRD). Le comportement électrochimique des alliages synthétiques de Cu-Ni-Fe dépend du maintien du fer en une phase unique dans la solution solide (la quantité maximale de fer que l’on pouvait utiliser étant de 1·5% en poids). La trempe améliore le comportement électrochimique des alliages synthétiques de Cu–Ni–Fe ayant une teneur relativement élevée en fer. La couche extérieure du film passif est poreuse en l’absence de fer, mais lorsque l’on ajoute du fer, les pores disparaissent et des fissures apparaissent. Lorsqu’il n’y a pas de sulfate dans la solution, le film passif formé sur les alliages synthétiques de Cu–Ni–Fe consiste entièrement en chlorures et en Fe₂O₃. En présence de sulfate, on détecte du FeS et du NiS dans le film de corrosion.     

Sintering parameters and mechanical properties of injection moulded aluminium powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded aluminium powder. Gas atomised aluminium powder was injection moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for feedstock. Binder debinding was performed in solvent and thermal method. After debinding, the samples were sintered at different temperatures and times in high purity N₂. Metallographic studies were conducted to determine the extent of densification and the corresponding microstructural changes. The results show that gas atomised aluminium powder could be sintered to a maximum 96·2% of theoretical density. Maximum density, tensile strength and hardness were obtained when sintered at 650°C for 60 min.