Strengthening in deformation-processed Cu-20% Fe composites

Three Cu-20% Fe composites with different iron powder sizes were fabricated using powder metallurgy processes. The strengths of these composites after extensive deformation processing by rod swaging and wire drawing were shown to be anomalously higher than those predicted by rule of mixtures equations. However, the strengths obey a Hall-Petch type relationship with the iron filament spacings. The strengths of the Cu-20% Fe composites after equivalent deformation processing increased with decreasing initial iron powder size. Comparison of a Cu-20% Fe composite with a similar Cu-20% Nb composite showed that Cu-20% Fe was stronger after an identical degree of deformation processing. This increase in strength of a Cu-20% Fe composite over that of a Cu-20% Nb composite correlated with the greater shear modulus of iron compared to niobium using a barrier model for hardening.     

Cold worked Cu-Fe-Cr alloys

The aim of this project was to investigate the properties of copper rich Cu-Fe-Cr alloys for the purpose of developing a new cost effective, high-strength, high-conductivity copper alloy. This paper reports on the influence of cold work. The age hardening response of the Cu-0.7%Cr-2.0%Fe alloy was minimal, but the resistance to softening was superior to that reported for any commercial high-strength, high-conductivity (HSHC) copper alloy with comparable mechanical and electrical properties. For example, an excess of 85% of the original hardness of the 40% cold worked alloy is retained after holding at 700°C for 1 hour, whereas commercial HSHC Cu-Fe-P alloys have been reported to soften significantly after 1 hours exposure at less than 500°C. The Cu-0.7Cr-2.0Fe alloy would therefore be expected to be more suitable for applications with a significant risk of exposure to elevated temperatures. Optical microscope examination of cold worked and aged microstructures confirmed the high resistance to recrystallization for Cu-0.7%Cr-2.0%Fe. The Zener-Smith drag term, predicting the pinning effect of second phase particles on dislocations in cold worked microstructures, was calculated using the precipitate characteristics obtained from TEM, WDS and resistivity measurements. The pinning effect of the precipitate dispersions in the peak-aged condition was determined to be essentially equivalent for the Cu-0.7%Cr-0.3%Fe and Cu-0.7%Cr-2.0%Fe alloys. A lower recrystallisation temperature in the Cu-0.7%Cr-0.3%Fe alloy was therefore attributed to faster coarsening kinetics of the secondary precipitates resulting from a higher Cr concentration in the precipitates at lower iron content.     

Processing maps for hot-working of powder metallurgy 1100 Al-10 vol % SiC-particulate metal-matrix composite

The hot-working characteristics of the metal-matrix composite (MMC) Al-10 vol % SiC-particulate (SiC_p) powder metallurgy compacts in as-sintered and in hot-extruded conditions were studied using hot compression testing. On the basis of the stress-strain data as a function of temperature and strain rate, processing maps depicting the variation in the efficiency of power dissipation, given by = 2m/(m+1), where m is the strain rate sensitivity of flow stress, have been established and are interpreted on the basis of the dynamic materials model. The as-sintered MMC exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of about 30% at a temperature of about 500°C and a strain rate of 0.01 s^–1. At temperatures below 350°C and in the strain rate range 0.001–0.01 s^–1 the MMC exhibited dynamic recovery. The as-sintered MMC was extruded at 500°C using a ram speed of 3 mm s^–1 and an extrusion ratio of 101. A processing map was established on the extruded product, and this map showed that the DRX domain had shifted to lower temperature (450°C) and higher strain rate (1 s^–1). The optimum temperature and strain rate combination for powder metallurgy billet conditioning are 500°C and 0.01 s^–1, and the secondary metal-working on the extruded product may be done at a higher strain rate of 1 s^–1 and a lower temperature of 425°C.     

Relationship between degassing conditions and tensile properties of Al-20Si-X P/M products

Results are reported which show the effect of different degassing modes on the properties of the Al-20Si-3Cu-1 Mg powder. The paper complements previous papers [1–3] concerning the conventional and modified degassing of the same powder. This research was mainly directed to study the influence of temperature on the tensile properties, ultimate tensile strength, σ_UTS, and elongation, ɛ, of extrudates obtained of Al-20Si-3Cu-1Mg compacts non-degassed, conventionally degassed, and treated by a modified process, namely degassing assisted by flushing with a depurative gas such as argon or nitrogen. The processing of the Al-20Si-3Cu-1Mg P/M powder must include a degassing step which significantly improves the tensile properties, at room and elevated temperatures, of the products of compacted powder with respect to those of the products whose compacts were non-degassed. It is apparent that degassing assisted by flushing with argon or nitrogen gives products with higher tensile properties than those of the products conventionally degassed under optimal conditions of temperature and time and much higher than those of the non-degassed products. The tensile results are in agreement with the theoretical approach to the gas entrapment and evolution of the aluminium powders presented in previous papers.     

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min^–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.     

Comparison of densification and distortion behaviors of W-Ni-Cu and W-Ni-Fe heavy alloys in liquid-phase sintering

This paper aims to correlate the densification and distortion behaviors of liquid-phase sintered 80W-14Ni-6Cu and 80W-14Ni-6Fe heavy alloys with the melting characteristics of the Ni-Cu and Ni-Fe matrices. Differential thermal analysis (DTA) of die-pressed compacts reveals that the melting range of the Ni-Cu matrix is extended from 1235°C to 1453°C by the in situ alloying between elemental Cu and Ni powders, whereas the melting of the Ni-Fe matrix is limited to a narrow temperature range between 1464°C and 1480°C. Dilatometry and furnace sintering tests show that densification due to liquid-phase sintering of 80W-14Ni-6Cu starts at 1287°C and proceeds at a low rate to 1450°C, where full densification without distortion is achieved. In contrast, densification due to liquid-phase sintering of 80W-14Ni-6Fe occurs at a very high rate above 1475°C, and full density can be obtained at 1500°C. For both alloy compacts, distortion is induced by prolonging the sintering time or elevating the sintering temperature after full densification. Crack-like voids develop in the 80W-14Ni-6Cu compact to accommodate the gravity-induced distortion, while spherical pores are dominantly formed in the 80W-14Ni-6Fe compact as a result of water vapor entrapment.     

Microstructure and mechanical properties of biocompatible high density Ti–6Al–4V/W produced by high frequency induction heating sintering

High frequency induction heating sintering method is used for sintering of the metal and ceramics powder. This technique has been used to produce high density compacts, containing as small grains as possible of powders. The alloy of Ti–6Al–4V was modified by addition of 2.5, 5, and 10 wt.% tungsten through powder metallurgy. Ti–6Al–4V/W was prepared by high-energy mechanical milling. The use of the high frequency induction heating sintering technique allows sintering to nearly full density at comparatively low temperatures and short holding times, and therefore suppressing grain growth. Different process parameters such as sintering temperature, and applied pressure have been investigated. The obtained compacts are characterized with respect to their densities, grain morphologies and pore distributions as well as hardness. Ti–6Al–4V/W powder precursors have been successfully compacted and consolidated to densities exceeding 98.8%. The maximum compressive strengths were obtained at sintering temperature 1000 °C for the samples containing 5% W, and at 1100 °C for the samples with 10% W. Maximum hardness was obtained 45 HRC at 1100 °C for 10% W.     

Solid ? liquid phase transformations in hypereutectic P/M Al-Si-Fe-X alloys

In this study, the metastable and stable solid liquid phase transformations of the hypereutectic alloys Al-20Si-5Fe-2Ni (wt%) and Al-17Si-5Fe-3.5Cu-1.1Mg-0.6Zr in as-atomised powder and in as-hot-forged material have been investigated. Differential Scanning Calorimetry (DSC) measurements at high temperatures have been performed. The resultant products have been thoroughly analysed using Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) microanalysis. During solidification large Si, (Al₄FeSi₂), FeNiAl₉, Al₇Cu₂Fe and Q(Cu₂Mg₈Si₆Al₅) are formed. A cooling rate of 5 °C/min and 1 °C/min is too high for the formation of the equilibrium phases (Al₅FeSi) and Al₇Cu₂Fe. The understanding of the sequence of transformations is useful in order to define appropriate processing parameters for these alloys produced by powder metallurgy. The temperature at which the first liquid phase appears during heating at 5 °C/min is 559 °C for the Al-Si-Fe-Ni powder and 506 °C for the Al-Si-Fe-Cu-Mg-Zr powder.     

Properties and Structural Changes of Al-3Cu-2Li Alloy after Thermal Treatment

A quantitative evaluation of microstructure and some mechanical properties of Al-3Cu-2Li (wt.%) alloy submitted to solution annealing at 503°C with succeeded age-hardening at three selected temperatures 163°C, 180°C and 190°C for 17 hours were investigated. With increasing the temperature of artificial ageing increases the strength of the alloy and reaches the maximum at 190°C, what is connected with decreasing of the deformation characteristics. Besides precipitates of Al₂Cu and Al₂CuLi the Cu₃Al particles were found.     

High temperature mechanical properties of low alloy steel foams produced by powder metallurgy

Cu–Ni–Mo and Mo based steel foams having different porosity levels for high temperature applications were produced by the space holder-water leaching technique in powder metallurgy. Steel powders were mixed with binder (polyvinylalcohol) and spacer (carbamide), and compacted. Spacer in the green compacts was removed by water leaching at room temperature and porous green compacts were sintered at 1200 °C for 60 min in hydrogen atmosphere. The successful application of foams at higher temperatures requires a good understanding of their high temperature mechanical properties. Compression tests were carried out on steel foams with different porosities at temperatures varying from room temperature to 600 °C in argon atmosphere. Effect of high temperature on compressive properties of the steel foams was investigated. It was found that the compressive strength of steel foams was greater at elevated temperatures than that at room temperature. This occurs across a range of temperatures up to 400 °C. Beyond this point the compressive strength decreased as the temperature increased. The reason for the enhancement of the compressive strength of Cu–Ni–Mo and Mo based steel foams is expected to be due to the effect of the dynamic age-hardening.     

Characterization of cemented Cu matrix composites reinforced with SiC

In this study, some properties of copper produced by cementation method and its composites reinforced with 1wt%, 2wt%, 3wt% and 5wt% SiC particles, produced by powder metallurgy method, were investigated. Composite powders were pressed by applying an uniaxial pressure of 280 MPa and sintered at temperatures of 700 °C for 2 h embedding in graphite powder. Scanning electron microscope (SEM-EDS), X-ray diffraction (XRD) techniques were used to characterize the Cu and SiC which are dominant components in the sintered composites. Microstructure studies revealed that SiC particles were located around the copper particles. The relative densities of Cu-SiC composites determined by Archimedes’ principle decreased from 98.11% to 90.93% with increasing reinforcement components. Measured hardness of sintered compacts varied from 127 to155 HVN. Maximum electrical conductivity of test materials ranged from 80.17% IACS to 57.76% IACS.     

Hot workability of an Al-Mg alloy AA5182 with 1 wt% Cu

A comparative study of the hot workability of two aluminium alloys, alloy AA5182 used for automotive applications and a variant modified with 1 wt% copper, has been carried out. Hot torsion tests were performed on both alloys subjected to two different heat treatments: a low temperature preheat to 450 °C and a high temperature preheat at 540 °C. The results from the torsion experiments are interpreted in terms of microstructural features. Both treatments produce the same strength, but the high temperature preheat leads to better ductility. This improvement is related to the homogenization of solute elements in the matrix; and, concerning AA5182 + Cu, also to the dissolution of a non-equilibrium Al-Mg-Cu ternary eutectic present in the as-cast microstructure. The precipitation of (Fe, Mn)Al₆ precipitates in the matrix of both alloys is induced by the high temperature heat treatment. Comparison of the results obtained by hot torsion shows that at low deformation rates AA5182 + Cu has better ductility than the classical alloy, but its ductility is lower at strain rates above 0.6–0.8 s^–1. The null ductility transition temperature is lower compared with that in the classical alloy, restricting the range of hot working temperatures. Inside this range the strength of both alloys is approximately the same, although the strain rate sensitivity coefficient is increased by copper additions. The experimental strength values follow the classical sinus-hyperbolic constitutive equation for hot working.     

Processing and microhardness of bulk Cu-Fe nanocomposites

Full-density bulk nanocomposites have been developed in the immiscible Cu-Fe system through a powder metallurgy route. Elemental copper and iron powders were first mechanically alloyed to form single-phase, nanocrystalline, metastable solid solutions, Cu_{100 − x}Fe_x (x = 0 to 100). These solid solutions were subsequently decomposed into Cu/Fe two-phase domains of various volume fractions during the hot consolidation process, forming in situ nanophase Cu-Fe composites. Full-density compacts have been produced at relatively low consolidation temperatures (< 500°C) by employing sinter forging at high applied pressure and a protective atmosphere. Such a consolidation scheme, coupled with enhanced grain size stability due to the unique microstructural evolution sequence involved, retained the grain sizes in the nanometer range for both Cu and Fe grains in the composite products. Fully dense composite specimens exhibit enhanced microhardness as compared with rule-of-mixtures predictions. This enhancement is attributed to interface strengthening at fcc-bcc interphase boundaries.     

大变形Cu-Fe原位复合材料研究

研究了Cu-1Fe(质量分数/%)合金经形变热处理后的组织与性能.结果表明,采用合适的中间热处理工艺,可以明显地提高Cu-Fe合金的强度及导电性,使之具有高强度与高导电性的良好结合.     

Oxidation behaviour of reaction-bonded alumina compacts using an Al88Si12 alloy precursor

The oxidation behaviour of attrition-milled Al₈₈Si₁₂/Al₂O₃ powder mixtures was investigated for the formation of mullite/Al₂O₃ composites by the reaction bonded alumina (RBAO) process. Cylindrical powder compacts were heated at 5°C min^–1 to temperatures between 450 and 1400°C. Oxidation occurred rapidly between ca. 400 and 750°C. Dense, outer reaction layers which formed at the lower temperatures inhibited complete oxidation and led to fracture of the body during continued heating to higher temperatures (above ca. 850°C) While the incorporation of ZrO₂ improved the oxidation of the samples, X-ray analysis indicated that the Si in the alloy reacted with the ZrO₂ to form phases which prevented the formation of mullite at the temperatures used in the experiments.     

Effects of residual carbon content on sintering shrinkage, microstructure and mechanical properties of injection molded 17-4 PH stainless steel

Carbon contamination from the thermoplastic binder is an inherent problem with the metal powder injection molding process. Residual carbon in the compacts after debinding has a strong impact on the sintering process, microstructure, and mechanical properties. In this study, injection molded 17-4 PH stainless steel was debound to two levels of residual carbon, 0.203 ± 0.014 wt% and 0.113 ± 0.008 wt%, by elevating the debinding temperature from 450°C to 600°C. Dilatometry in H₂ atmosphere shows that the 600°C-debound compacts shrink much faster than those debound at 450°C when the sintering temperature rises to over 1200°C. Density measurements for tensile bars sintered between 1260°C and 1380°C confirm the beneficial effect of low residual carbon content on sintering shrinkage. Quantitative metallography reveals that more -ferrite forms along austenite grain boundaries during sintering of the 600°C-debound compacts. In both samples, density gradients across the compact section are correlated with the residual carbon content and corresponding -ferrite formation. Finally, tensile tests show that the 600°C-debound compacts have lower tensile strength but higher ductility than those debound at 450°C. The relevant mechanisms are discussed with a focus on the effects of residual carbon content, -ferrite amount, and porosity.     

Cu-Fe合金的强磁场固溶时效行为

将Cu-15%Fe(质量分数)合金在强磁场中进行不同同溶时效处理,研究了合金的时效行为.结果表明,施加10 T强磁场可以促进第二相Fe枝晶的球化,而且Fe枝晶的形貌受强磁场的球化作用与高温缓慢冷却引起的粗化作用的影响.在Cu-15%Fe合金1000℃同溶处理中,施加10 T强磁场使基体中的Fe含量降低了0.39%.这表明,强磁场在一定程度上促进了Fe在Cu基体中的析出,获得与缓冷相类似的效果;施加10 T强磁场固溶处理并在10 T强磁场作用下经500℃时效处理后,基体中的Fe含量较低.其原因是,施加强磁场后Fe原子的析出规律受温度制度和析出相磁性转变的共同影响.施加强磁场改变了原子的激活能,进而影响了原子的扩散行为.     

Microstructural and mechanical stability of Cu-6 wt. % Ag alloy

The microstructural and mechanical stability of Cu-6 wt. % Ag alloy obtained by cold rolling combined with intermediate heat treatments have been investigated. The stress-strain responses and fracture behavior of Cu-6 wt. % Ag alloy were examined and correlated with the microstructural change caused by thermo-mechanical treatments. The deformation bands stabilized by silver precipitates were observed in heavily rolled Cu-6 wt. % Ag alloy. The highly deformed microstructure stabilized by silver filament was observed to be unstable at temperatures above 200 °C. The strength of Cu-6wt.%Ag alloys were found to decrease remarkably if they were heat-treated above 300°C. The fracture surfaces of Cu-Ag two phase alloys showed typical ductile type fracture. The electrical conductivity did not change appreciably up to the aging temperature of 200°C and increased rapidly at temperatures above 300°C. The increase of the conductivity and the decrease of the strength can be associated with the microstructural coarsening of heavily deformed linear band structure. The difference of the UTS and the conductivity between the rolling direction and the direction perpendicular to the rolling direction (on the rolling plane) were found to be relatively small.     

TEM observatiion of a Cu-Mg age-hardenable alloy

The precipitation processes in Cu-Be, Cu-Co, Cu-Fe alloys have been thoroughly investigated; however, much less attention has been paid to studying the Cu-Mg system. In this work the decomposition of Cu-3.5 wt% Mg alloy during ageing was investigated by means of transmission electron microscopy (TEM). The microstructure of Cu-3.5 wt% Mg alloy aged at 340° C is characterized by the presence of fine dispersed coherent precipitates. On continued ageing the coherent precipitates disappear and a new transition phase with oblate octahedron morphology grows. At temperatures above 34O° C the equilibrium phase is formed by discontinuous precipitation. Ageing of Cu-3.5 wt% Mg alloy at temperatures above 450° C results in the formation of the equilibrium Cu₂Mg phase.     

Dissolution of different graphite grades during sintering of PM steels

The dissolution behaviour of graphite during sintering of Fe–0.8%C and the resulting properties were studied using standard high quality natural graphite and an artificial graphite grade recently developed for ferrous powder metallurgy. Sintering was done in high purity N₂ atmosphere for 60 min, the temperature being varied. It showed that dissolution of carbon takes place mainly in the isothermal temperature range 750–950°C, for the artificial graphite the maximum dissolution rate being observed at a temperature about 50° lower than for the natural graphite. This also affects the properties, especially the hardness, of the specimens sintered in this temperature range while at standard sintering temperatures of about 1120°C both graphite grades yield well comparable results. Therefore, selection of the graphite grade seems to be important especially for pre-sintering of specimens prior to further treatment, dissolution of the graphite increasing the hardness but also improving the inter-particle strength.