Phase Relations in the Bi–(Pb)–Sr–Ca–Cu–Sc–O System

The phase relations in the Bi–(Pb)–Sr–Ca–Cu–Sc–O system were studied near Bi₂Sr₂CaCu₂O_{8 +} (Bi-2212) and (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} (Bi-2223) between 850 and 930°C. The introduction of Sc led to the formation of a new compound Sr₂ScBiO₆, which coexisted with Bi-2212 and Bi-2223. Using crystallization from a peritectic melt at different cooling rates, we obtained Bi-2212 matrix composites containing finely dispersed Sr_1.9Ca_0.1ScBiO₆inclusions, with T _cattaining 89 K. The T _cof the Bi-2223–Sr_1.9Ca_0.1ScBiO₆superconducting ceramic prepared by solid-state sintering of a Bi–(Pb)–Sr–Ca–Cu–Sc–O precursor was 108.5 K.     

Soft magnetic amorphous Fe–Zr–Si(Cu) boron-free alloys

Amorphous Fe₈₀Zr_xSi_20−x−yCu_y boron-free alloys, in which boron was completely replaced by silicon as a glass forming element, have been prepared in the form of ribbons by using the melt quenching technique. X-ray diffraction and Mössbauer spectroscopy measurements revealed that the as-quenched ribbons with the compositions with x = 6–10 at.% and y = 0, 1 at.% are fully or predominantly amorphous. Differential scanning calorimetry (DSC) measurements allowed the estimation of crystallization temperatures of the amorphous alloys. Soft magnetic properties have been studied by the specialized rf-Mössbauer technique. Since the rf-collapse effect observed is very sensitive to the local anisotropy fields it was possible to evaluate the soft magnetic properties of the amorphous alloys studied. The rf-Mössbauer studies were accompanied by conventional measurements of hysteresis loops from which the magnetization and coercive fields were estimated. It was found that amorphous Fe–Zr–Si(Cu) alloys are magnetically very soft, comparable with those of the conventional amorphous B-containing Fe-based alloys.     

Interfacial microstructure and hardness of nickel (Ni) nanoparticle-doped tin–silver–copper (Sn–Ag–Cu) solders on immersion silver (Ag)-plated copper (Cu) substrates

Sn–Ag–Cu composite solder has been prepared by adding Ni nanoparticles. Interfacial reactions, the morphology of the intermetallic compounds (IMC) that were formed, the hardness between the solder joints and the plain Cu/immersion Ag-plated Cu pads depending on the number of the reflow cycles and the aging time have all been investigated. A scallop-shaped Cu₆Sn₅ IMC layer that adhered to the substrate surface was formed at the interfaces of the plain Sn–Ag–Cu solder joints during the early reflow cycles. A very thin Cu₃Sn IMC layer was found between the Cu₆Sn₅ IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nanoparticles, a scallop-shaped (Cu, Ni)–Sn IMC layer was clearly observed at both of the substrate surfaces, without any Cu₃Sn IMC layer formation. Needle-shaped Ag₃Sn and sphere-shaped Cu₆Sn₅ IMC particles were clearly observed in the β-Sn matrix in the solder-ball region of the plain Sn–Ag–Cu solder joints. Additional fine (Cu, Ni)-Sn IMC particles were found to be homogeneously distributed in the β-Sn matrix of the solder joints containing the Ni nanoparticles. The Sn–Ag–Cu–0.5Ni composite solder joints consistently displayed higher hardness values than the plain Sn–Ag–Cu solder joints for any specific number of reflow cycles–on both substrates–due to their well-controlled, fine network-type microstructures and the homogeneous distribution of fine (Cu, Ni)–Sn IMC particles, which acted as second-phase strengthening mechanisms. The hardness values of Sn–Ag–Cu and Sn–Ag–Cu–0.5Ni on the Cu substrates after one reflow cycle were about 15.1 and 16.6 Hv, respectively–and about 12.2 and 14.4 Hv after sixteen reflow cycles, respectively. However, the hardness values of the plain Sn–Ag–Cu solder joint and solder joint containing 0.5 wt% Ni nanoparticles after one reflow cycle on the immersion Ag plated Cu substrates were about 17.7 and 18.7 Hv, respectively, and about 13.2 and 15.3 Hv after sixteen reflow cycles, respectively.     

Microstructure and precipitation in Al–Li–Cu–Mg–(Mn,Zr) alloys

Abstract

Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al₃Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.     

Failure analysis of K-monel 500 (Ni–Cu–Al alloy) bolts

A bolt manufacturer used several subcontractors to fabricate K-monel 500 bolts. The completed bolts failed mechanical testing, and the manufacturer ascribed the failure to the bar stock. Testing contracted by the supplier of the bar stock showed that it met specifications. Metallographic testing described here shows that the bolts failed because of excessive grain growth and subsequent softening during heat treatment performed on the bolts after they were formed. Alternative heat treatments are discussed.     

Extrusion of AI–Zn–Mg–Cu–Zr alloys

Abstract

Four aluminium alloys of different zinc/magnesium ratio have been studied under various extrusion conditions. The alloys were cast in steel book moulds and subjected to initial thermomechanical treatments. Studies were made of hot extrusions and cold hydrostatic extrusions and in each case the changes in the extrusion parameters were analysed. An attempt has been made to explain some of the extrusion defects which appeared in various extruded sections. The extrusion speed was found to be crucial, since sections developed surface cracks at higher speeds. The extrusion speed was also found to vary inversely with the extrusion ratio, with higher speeds at low ratios. A well defined solute–depleted weld zone was observed on each of the four faces of a square tube extruded using a porthole die. Thermal treatment was not found to improve this weak weld zone. Tubes extruded using a floating-mandrel die withstood pressure testing up to 550 MPa.

MST/43     

Microstructure,mechanical properties and machinability of Cu–Zn–Mg and Cu–Zn–Sb brass alloys

Lead-free alloys have attracted great attentions recently due to the toxic nature of lead for the human body. In this study, low amounts of Mg and Sb were added to the Cu65–Zn35 brass and microstructure, mechanical properties and machinability of samples were compared to Cu65–Zn35 brass. Both Mg and Sb led to the promotion of β′ phase as well as the formation of new ternary copper rich intermetallic particles. It was found that these particles had a significant role in the reduction of the ultimate tensile strength, toughness, work hardening and elongation while increasing the hardness of samples. Results of machinability evaluation of samples showed that the cutting forces were decreased significantly and morphology of chips were improved compared to Cu65–Zn35 brass sample.     

An investigation on microwave sintering of Fe, Fe–Cu and Fe–Cu–C alloys

The powder characteristics of metallic powders play a key role during sintering. Densification and mechanical properties were also influenced by it. The current study examines the effect of heating mode on densification, microstructure, phase compositions and properties of Fe, Fe–2Cu and Fe–2Cu–0·8C systems. The compacts were heated in 2·45 GHz microwave sintering furnaces under forming gas (95%N₂–5%H₂) at 1120 °C for 60 min. Results of densification, mechanical properties and microstructural development of the microwave-sintered samples were reported and critically analysed in terms of various powder processing steps.     

Tensile behaviour of powder metallurgy processed (Al–Cu–Mg)/SiCp composites

Abstract

The tensile behaviour of Al–Cu–Mg alloy matrix composites produced by a powder metallurgy process was investigated as a function of particle size in the as extruded, homogenised, and peak aged conditions. The tensile behaviour of the corresponding matrix alloy which was produced in a similar manner, designated as Control, was also studied. There was a significant increase in the 0.2% yield strength of Control and all the metal matrix composites (MMCs) after homogenisation treatment (53–68%) and peak aging (93–109%), as compared to their values in the as extruded condition. The ultimate tensile strength (UTS) of Control as well as the MMCs also increases considerably after homogenisation treatment (39–70%), however, subsequent peak aging did not result in any further increase in UTS in case of any of the MMCs. It was found that the finer the reinforcement size, the higher the 0.2% yield strength and UTS in all the conditions. On the other hand, ductility decreased considerably after homogenisation treatment and subsequent peak aging. The results are discussed in the light of dislocation strengthening as well as reinforcement damage.     

Liquid-state and solid-state interfacial reactions between Sn–Ag–Cu–Fe composite solders and Cu substrate

The growth kinetics and morphology of the interfacial intermetallic compound (IMC) between Sn–3Ag–0.5Cu–xFe (x = 0, 0.5 wt%, 1 wt%) composite solders and Cu substrate were investigated in the present work. The Sn–Ag–Cu–Fe/Cu solder joint were prepared by reflowing for various durations at 250 °C and then aged at 150 °C. During soldering process, Fe particles quickly deposited in the vicinity of IMC, resulting in the formation of Fe-rich area. Isothermal equation of chemical reaction and phase diagrams were used to explain the effect of Fe on the growth kinetics of IMC during liquid-state interfacial reaction. It was shown that Fe could effectively retard the growth of interfacial Cu₆Sn₅ and Cu₃Sn layers during liquid-state reaction and reduce the size of Cu₆Sn₅ grains. Small cracks were observed in the Cu₆Sn₅ grains after reflowing for 2 min while they were found in the other composite solders reflowing for about 30 min. The Fe tended to suppress the growth of the Cu₃Sn layer during solid-state aging. However, the total thickness of IMCs (Cu₆Sn₅ + Cu₃Sn) for the composite solders with Fe particles was similar to that for SnAgCu without Fe particles.     

Influence of cerium oxide (CeO2) nanoparticles on the microstructure and hardness of tin–silver–copper (Sn–Ag–Cu) solders on silver (Ag) surface-finished copper (Cu) substrates

Nano-sized, non-reacting, non-coarsening CeO₂ particles with a density close to that of solder alloy were incorporated into Sn–3.0 wt%Ag–0.5 wt%Cu solder paste. The interfacial microstructure and hardness of Ag surface-finished Cu substrates were investigated, as a function of reaction time, at various temperatures. After the initial reaction, an island-shaped Cu₆Sn₅ intermetallic compound (IMC) layer was clearly observed at the interfaces of the Sn–Ag–Cu based solders/immersion Ag plated Cu substrates. However, after a prolonged reaction, a very thin, firmly adhering Cu₃Sn IMC layer was observed between the Cu₆Sn₅ IMC layer and the substrates. Rod-like Ag₃Sn IMC particles were also clearly observed at the interfaces. At the interfaces of the Sn–Ag–Cu based solder-Ag/Ni metallized Cu substrates, a (Cu, Ni)–Sn IMC layer was found. Rod-like Ag₃Sn and needle-shaped Cu₆Sn₅ IMC particles were also observed on the top surface of the (Cu, Ni)–Sn IMC layer. As the temperature and reaction time increased, so did the thickness of the IMC layers. In the solder ball region of both systems, a fine microstructure of Ag₃Sn, Cu₆Sn₅ IMC particles appeared in the β-Sn matrix. However, the growth behavior of the IMC layers of composite solder doped with CeO₂ nanoparticles was inhibited, due to an accumulation of surface-active CeO₂ nanoparticles at the grain boundary or in the IMC layers. In addition, the composite solder joint doped with CeO₂ nanoparticles had a higher hardness value than the plain Sn–Ag–Cu solder joints, due to a well-controlled fine microstructure and uniformly distributed CeO₂ nanoparticles. After 5 min of reaction on immersion Ag-plated Cu substrates at 250 °C, the micro-hardness values of the plain Sn–Ag–Cu solder joint and the composite solder joints containing 1 wt% of CeO₂ nanoparticles were approximately 16.6 and 18.6 Hv, respectively. However after 30 min of reaction, the hardness values were approximately 14.4 and 16.6 Hv, while the micro-hardness values of the plain Sn–Ag–Cu solder joints and the composite solder joints on Ag/Ni metallized Cu substrates after 5 min of reaction at 250 °C were approximately 15.9 and 17.4 Hv, respectively. After 30 min of reaction, values of approximately 14.4 and 15.5 Hv were recorded.     

Formation of composite Cu–graphite and Cu–PTFE coatings and their tribological characterization

The Dynamic Chemical Plating (DCP) technique allows production of 2-μm copper films containing particles of graphite or PTFE in 18 and 15 min, respectively, at ambient temperature. DCP yields composites with particle-incorporation fractions of 12% for graphite micro-particles and 22% for PTFE nano-particles. The composite films show excellent tribological properties, acting as self-lubricating coatings with friction coefficients as low as 0.18.     

Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.     

Short transverse fracture of Al–Li–Cu–Mg–Zr extrudate

Abstract

The short transverse fracture toughness of an Al–Li–Cu–Mg–Zr extrudate was determined as a function of aging condition and testing temperature. To elucidate the underlying micromechanisms, the short transverse fracture surfaces of the extrudate were characterised via scanning electron microscopy, grain boundary precipitates and precipitation free zones were identified via transmission electron microscopy, and segregation of elements to grain boundaries was analysed using secondary ion mass spectrometry. Three principal observations were made as follows. First, with increasing aging time, the short transverse toughness of the extrudate increased when tested at room temperature, but decreased at liquid N₂ temperature, whereas with decreasing testing temperature, it remained essentially constant for the underaged condition, and decreased sharply for the peak aged and overaged tempers. Second, in addition to regions exhibiting shallow dimples, smooth ‘featureless’ zones were revealed on the short transverse fracture surfaces, which are intergranular in nature for all the specimens tested. The area fraction of the featureless regions decreased noticeably with increasing aging time when tested at room temperature, and increased markedly with decreasing testing temperature for the peak aged and overaged conditions. Third, segregation of Li, Si, Na, and H was detected for both the underaged and overaged specimens, and also of K for the underaged specimens only. In general, the enhancement of the room temperature short transverse toughness with aging and the negative effect of cryogenic temperature on fracture toughness are in obvious contrast to the in plane toughness behaviour reported in the literature, the featureless character of the short transverse fracture and its connection with poor toughness seldom having been emphasised. Based upon the present study, segregation induced brittleness is proposed as the critical micromechanism responsible for grain boundary weakness, and thus for the poor short transverse fracture toughness.

MST/1829     

Study of rapidly solidified Cu–Cr–RE alloys

Electrical engineering materials of Cu–Cr–RE have been made using the technology of rapid solidification, composite green compacts, extrusion and so on. By means of the analysis of optical metallographs, electron microscopy, physical and mechanical properties as well as electrical properties, and the examining of the hardness, softening temperature, etc. the authors selected Cu–Cr–Y alloy, which has excellent comprehensive properties. The authors have also made a deep study of the chromium and yttrium elements, which affect the structure, the recrystallization temperature, the strength at room and high temperature, the resistivity and contact resistance, and have also compared the properties of the Cu–Cr and Cu–Cr–Y alloy. The results show that rapidly solidified technology and added rare–earth elements not only enhance the fine grain boundary strengthening, but also the second phase strengthening. Cu/Cu–Cr–Y composite material improves the thermal stability and thermal endurance, and also maintains a better electrical conductivity and thermal conductivity.     

Phase Relations in the Cu–Sb–O System

The Cu–Sb–O system was studied by x-ray diffraction and thermal analysis between 700 and 1000°C. The compositions of copper antimonates were refined. Sb₂O₄ was found to exist in two polymorphs above 800°C: -Sb₂O₄ (dominant phase) and -Sb₂O₄. The evolution of phase equilibria with increasing temperature was examined. The isothermal sections of the Cu–Sb–O phase diagram were mapped out using new and earlier reported results.     

Morphological changes and hardness evolution in Cu–30Ni–5Cr and Cu–45Ni–15Cr spinodal alloys

Abstract

The development of increased strength in Cu–Ni–Cr alloys, compared with binary Cu–Ni alloys, is dependent upon heat treatment. These alloys have compositions which permit them to be solution treated at elevated temperature and then aged at a lower temperature, in a two phase field, to produce hardening. Decomposition into two phases may occur by nucleation and growth or by a spinodal reaction, depending on alloy composition and heat treatment temperature. As part of a more extensive study of ternary Cu–Ni–Cr alloys, the decomposition of Cu–30Ni–5Cr and Cu–45Ni–15Cr (wt-%) has been studied in the spinodal range. The evolution of microstructure has been determined together with the coarsening kinetics for the modulated spinodal decomposition products. Specimens rapid quenched from 1050°C, were aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the early stages of aging the modulation wavelength remained constant while the hardness increased continuously. After a certain period of aging, the hardness remained constant at its peak value, while the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.

MST/1733     

Study of rapidly solidified Cu–Cr–RE alloys

Electrical engineering materials of Cu–Cr–RE have been made using the technology of rapid solidification, composite green compacts, extrusion and so on. By means of the analysis of optical metallographs, electron microscopy, physical and mechanical properties as well as electrical properties, and the examining of the hardness, softening temperature, etc. the authors selected Cu–Cr–Y alloy, which has excellent comprehensive properties. The authors have also made a deep study of the chromium and yttrium elements, which affect the structure, the recrystallization temperature, the strength at room and high temperature, the resistivity and contact resistance, and have also compared the properties of the Cu–Cr and Cu–Cr–Y alloy. The results show that rapidly solidified technology and added rare-earth elements not only enhance the fine grain boundary strengthening, but also the second phase strengthening. Cu/Cu–Cr–Y composite material improves the thermal stability and thermal endurance, and also maintains a better electrical conductivity and thermal conductivity.     

Effects of Si and Cu contents on grain size of Al–Si–Cu alloys

The influence of the silicon and copper contents on the grain size of high-purity Al–Si, Al–Cu, and Al–Si–Cu alloys was investigated. In the Al–Si alloys, a poisoning effect was observed and a poor correlation between the grain size and growth restriction factor was obtained. A possible cause of the poisoning effect in these alloys is the formation of a TiSi₂ monolayer on the particles acting as nucleation sites or another poisoning mechanism not associated with TiSi₂ phase formation. In the Al–Cu alloys, a good correlation between the grain size and growth restriction factor was found, whereas in the Al–Si–Cu alloys, the correlation between these two parameters was inferior.     

Bulk magnetic studies on Cu–Mg–Zn ferrites

The ferrite compositions of Cu(0.5-x)MgxZn0.5Fe2O4 were synthesized by thermal decomposition of the solid solution of oxalate complexes obtained by the coprecipitation technique using oxalate precursors. X-ray diffraction (XRD) patterns of all the samples showed a single spinel phase with no detectable impurity phases. The magnetization values were measured by the superconducting quantum interference device (SQUID) technique and the observed variation in magnetization values is attributed to porosity present in these ferrite compositions. The saturation magnetization (Ms) versus temperature curves revealed the curve to be of type Q. The variation of saturation magnetization (Ms) with temperature (T) for composition x=0.25 and x=0.40 exhibited a type Q-curve at a higher field of 1000 G. The monotonic increase in Hc with grain diameter for ferrite compositions under investigation leads to HcD-1. Saturation magnetization (Ms) values of end ferrites sintered at 1000 °C are higher than those of oxalate complexes decomposed at 600 °C. © 1998 Kluwer Academic Publishers