Correlation of microstructure and magnetic properties in Cu–Co–Ni alloys

The present study concerns correlation of microstructure and magnetic properties of nanocrystalline binary 50Cu–50Co and ternary 50Cu–25Co–25Ni (wt%) alloys prepared by ball milling and subsequent isothermal annealing of the ball milled alloys. High resolution transmission electron microscopic (HR-TEM) investigation has shown deformation-induced microstructural features. Field emission scanning electron microscopy (FE-SEM) has revealed a distinct change in morphology of as-milled CuCoNi alloys after annealing. Differential scanning calorimetric (DSC) and X-ray diffraction (XRD) analysis have revealed that annealing of the CuCoNi alloy above 350 °C results into precipitation of nanocrystalline Co (fcc) in the CuNi matrix by spinodal decomposition. It is also demonstrated that isothermal annealing of the ball milled alloys in the temperature range between 350 and 650 °C significantly influence the magnetic properties, e.g. coercivity (H_c), remanence (M_r) and magnetic saturation (M_s) due to annihilation of defects such as stacking and twin fault along with dissolution and/or precipitation of magnetic phases in the Cu-rich matrix.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

Precipitation characteristics of μ-phase in wrought nickel-base alloys and its effect on their properties

Thermal exposures consisting of 1–16000 h at 540, 650, 760, and 870°C were used to study the susceptibility of selected nickel-base alloys to precipitation of -phase and its effect on mechanical strength and corrosion resistance. Analytical electron microscopy and X-ray diffraction were used to characterize the -phase. A -phase of the type Mo₆Ni₇ in nickel-base alloys was found to be stabilized by critical concentrations of iron in an excess of about 3 wt%. Generally, the -phase had a characteristic defect structure consisting of twins and stacking faults, and it exhibited a preferential tendency for precipitation at existing molybdenum-rich carbide particles within the alloy matrix and at grain boundaries. Precipitation of -phase was found to produce a moderate loss of room-temperature tensile ductility; however, it resulted in a considerable degradation of impact toughness and corrosion resistance. In contrast, it had no significant effect on elevated temperature tensile properties. A correlation was found to exist between the Ni/Fe + Co ratio as well as the Mo + W content of the alloy and susceptibility to precipitation of -phase.     

Effect of pre-deformation on the precipitation process and magnetic properties of Fe–Cu model alloys

The effect of pre-deformation on the precipitation process and magnetic properties of Fe–Cu model alloys was investigated. These specimens simulate irradiation embrittlement of nuclear reactor pressure vessel (RPV) steels. Fe–1 wt%Cu alloys with and without pre-deformation in solid-state solution were thermally aged at 773 K for various times and the evolution of hardness, conductivity, and microstructure were investigated. Pre-deformation enhanced Cu precipitation and caused precipitation at dislocations. The coercive force tended to decrease for the pre-deformed specimen and the underlying mechanism is discussed. The results obtained are related to the magnetic characteristics of irradiated RPV steels.     

Investigation of the effect of composition on microhardness and determination of thermo-physical properties in the Zn–Cu alloys

Zinc–copper of (99.99%) high purity alloys were directionally solidified upward with different compositions, C_o, Zn–(0.7,1.5, 2.4 and 7.37) wt.% Cu under two different solidification conditions (G = 3.85 K/mm, V = 0.0083 mm/s and G = 8.70 K/mm, V = 0.436 mm/s) using a Bridgman type directional solidification apparatus. The measurements of microhardness of directionally solidified samples were made by using a microhardness test device. The dependence of microhardness (HV) on composition was analyzed. According to these results, it has been found that the values of HV increase with the increasing Cu content (C_o). Variation of electrical resistivity (ρ) and electrical conductivity (σ) with the temperature were also measured by using a standard d.c. four-point probe technique. The enthalpy of fusion (ΔH) and specific heat (Cp) of the Zn–Cu alloys were determined from heating curve during the transformation from solid to liquid phase by using differential scanning calorimeter (DSC).     

The effect of Ta on structure and magnetic properties in Fe–N films

Microstructure and magnetic properties of Fe–Ta–N alloy films near the eutatic composition were studied. The four systematic alloy films with different Ta content were prepared by reactive sputtering. The dependence of structures and magnetic properties on Ta and annealing were investigated by VSM and X-ray diffraction. It is found that Ta atoms replace Fe in α-Fe lattice and have strong affinity for nitrogen, which inhibits the formation of γ-Fe₄N phase in Fe–Ta–N films. The TaN phase precipitates in grain boundaries and suppresses the growth of α-Fe(N) crystalline during annealing. Coercivity varies with the change of microstructure.     

Superelastic anisotropy characteristics of columnar-grained Cu–Al–Mn shape memory alloys and its potential applications

The columnar-grained (CG) Cu–Al–Mn shape memory alloy samples possess a strong < 001>-oriented texture along the solidification direction (SD) and straight low-energy grain boundaries fabricated by unidirectional solidification technique. When the angle between tensile direction and the SD ranged from 0° to 90° at the tensile tests, the superelasticity of samples changed in a “V” shape and showed a large anisotropy. Meanwhile, the martensite transformation critical stress of the CG Cu–Al–Mn samples increased from 258.5 MPa for 0° to 521.9 MPa for 45°, and then decreased to 324.3 MPa. The large anisotropy of the superelasticity was attributed to the combined effects of grain orientation and grain boundaries, wherein the influence of the grain boundaries had an obvious dependence on orientation. The potential applications of CG Cu–Al–Mn alloys as anisotropic shock isolators and dampers in high rise buildings and precision instruments were also proposed.     

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.     

Effect of aluminum on precipitation hardening in Cu–Ni–Zn alloys

The effect of aluminum on the precipitation hardening of Cu–Ni–Zn alloys with varying aging temperatures and times was investigated in this article, in the hope to achieve better mechanical properties. Vickers hardness, tensile, and electrical conductivity tests were carried out to characterize the properties of the Cu–Ni–Zn alloys with or without an addition of aluminum subjected to different aging treatments. The results show that an addition of 1.2 wt% aluminum can play an influential role in the precipitation hardening of the Cu–Ni–Zn alloys. For example, it can increase the peak hardness from 58 Hv for the solution-treated Cu–10Ni–20Zn alloy to 185 Hv for the solution-treated Cu–10Ni–20Zn–1.2Al alloy during aging at 500 °C. The yield strength, tensile strength, and electrical conductivity of the Cu–10Ni–20Zn–1.2Al alloy subjected to suitable treatments under prior cold-rolled and aged conditions can reach 889 MPa, 918 MPa, and 10.96% IACS, respectively, being much higher than those of the relevant alloy without aluminum and comparable to those of the Cu–Be alloys (C17200 and C17510). According to the transmission electron microscope observations, it was found that formation of nanosized precipitates with the L1₂-type ordered lattice results in precipitation hardening, and an orientation relationship of [011]_\textp//[011]_\textm [011]_{\text{p}}//[011]_{\text{m}} and (100)_\textp//(200)_\textm (100)_{\text{p}}//(200)_{\text{m}} exists between the precipitates and the α-Cu matrix.     

Microstructural simulation in spinodally-decomposed Cu–70 at.%Ni and Cu–46 at.%Ni–4 at.%Fe alloys

The microstructure simulation of spinodal decomposition was carried out in the isothermally-aged Cu–70at.%Ni and Cu–46at.%Ni–4at.%Fe alloys using the phase field method. The numerical simulation was based on the solution of the Cahn–Hilliard nonlinear partial differential equation by the explicit finite difference method. A slow growth kinetics of phase decomposition was observed to occur in the aged Cu–Ni alloy. The morphology of decomposed phases consisted of an irregular shape with no preferential alignment in any crystallographic direction at the early stages of aging in this alloy. The growth kinetics rate of phase decomposition in the aged Cu–46 at.%Ni–4 at.%Fe alloy was appreciably faster than that in the aged Cu–70 at.%Ni alloy. In the case of the aged Cu–46 at.%Ni–4 at.%Fe alloy, an irregular shape of the decomposed phases was also observed at the early stages of aging. A further aging caused the change of initial morphology to a cuboid and/or plate shape of the decomposed Ni-rich phase aligned in the elastically-softest crystallographic direction <100> of Cu-rich matrix.     

Review of effect of P and Sr on modification and porosity development in Al–Si alloys

Abstract

The benefits of Sr additions to Al–Si alloys to modify the eutectic are often impaired by the development of porosity, sometimes to the degree that benefits are negated. Experimental reports are reviewed in this paper, suggesting an explanation in terms of the oxide population in the melt. The unmodified silicon particles are nucleated by AlP, which has in turn nucleated on oxide bifilms. The oxide bifilms, which are essentially cracks, are straightened by the crystalline growth of Si particles, leading to increased crack size and consequently reduced mechanical properties. The addition of Sr improves properties by suppressing the formation of Si on bifilms and thereby preventing the straightening of the pre-existing cracks. Si is now forced to precipitate at a lower temperature as a coral-like eutectic. Unfortunately, the bifilms are now freed (the primary Si particles no longer exist to grow around and sequester the bifilms), remaining in suspension in the liquid metal, allowing them to act to block interdendritic flow and aid the initiation of the formation of pores, countering the benefits of the improved structure.     

Mechanical anisotropy and stress–strain rate characteristics in superplastic deformation of titanium alloys

Abstract

Uniaxial tension and compression tests have been carried out on two titanium based alloys, Ti–6Al–4V in the form of extruded tubes and forged plates and Ti–3Al–10V–2Fe sheets, to study anisotropic behaviour during superplastic deformation. The following were observed: (i) originally round cross-section became elliptical after deformation; (ii) the flow stresses and strain rates were dependent on the orientation of the specimens; and (iii) the strain anisotropy became less severe as the strain rate increased. These characteristics of anisotropy were related to the original microstructure (e.g. the mechanical fibring of the α grains) and the microstructural evolution during superplastic deformation. New constitutive equations for describing anisotropic superplastic deformation have been proposed to explain the effect of strain rate or stress on anisotropy.     

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.     

Nucleation and growth of precipitates in Al–Cu–Mg–Ag alloys

Abstract

Adding small amounts (<1wt-%) of both magnesium and silver to an aluminium alloy containing about 4 wt-% Cu causes precipitates with a hexagonal structure (Ω-phase) to form on {111} planes of the aluminium lattice. Precipitation of θ′ on {100} planes may also occur, the relative proportions of the two types of precipitate being dependent on the levels of magnesium and silver, e.g. ~0·7 wt-% of each element almost entirely suppresses θ′ formation. Even when θ′ does form in parallel with Ω-phase, on prolonged aging it tends to dissolve in favour of Ω growth. Using an X-ray technique to establish foil thickness, the relative amounts of Ω and θ′ precipitate have been measured as a function of aging time, analysis of the data showing that growth is diffusion controlled with an activation energy of 136± 15 kJ mol^?1.

MST/648     

Hexagonal martensite decomposition and phase precipitation in Ti–Cu alloys

The mechanical behavior of Ti–Cu alloys can be improved by controlling Ti₂Cu precipitation. In eutectoid alloys, such precipitation can be achieved by the decomposition of martensite in response to aging heat treatment. The purpose of this work is to discuss the evolution of precipitates during the decomposition of hexagonal martensite in Ti–Cu alloys. First, samples with near-eutectoid compositions were prepared in an arc furnace equipped with a non-consumable tungsten electrode and water-cooled copper hearth under a high purity argon atmosphere. After chemical homogenization at a temperature in the beta field, the samples were water-quenched and examined by differential scanning calorimetry and high-temperature X-ray diffraction. The results indicate that rapidly quenched near-eutectoid Ti–Cu alloys present Ti₂Cu precipitates. Regardless of the cooling rate applied, such precipitation is unavoidable. No evidence of beta phase stabilization was found in the rapidly quenched samples. Precipitation temperatures of coherent and incoherent phases of 415 °C and 550 °C, respectively, were determined from the differential scanning calorimetry measurements. Ti₂Cu precipitation was examined in situ by high temperature X-ray diffraction experiments. The total decay of martensite was found to occur above 575 °C. Vickers hardness testing of aged samples revealed a correlation between phase precipitation and hardening.     

Microstructure,mechanical properties and shape memory effect of Cu–Hf–Al–Ni alloys

The influence of hafnium element’s incorporation on a Cu–xHf–13.0Al–4.0Ni (wt-%) (x?=?0.5, 1.0 and 2.0) high-temperature shape memory alloy was investigated systematically. The results show that the matrix of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys is 18R martensite, and an orthorhombic-structured Cu₈Hf₃ phase is formed and distributed at the grain boundaries. The grain size is significantly reduced with increasing Hf content. The mechanical properties of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys are improved by Hf doping due to the combination of refinement strengthening, solid solution strengthening and second phase strengthening. After heating under pre-strain of 10%, the shape memory effect of the Cu–1.0Hf–13.0Al–4.0Ni alloy reaches 5.6%, which is obviously higher than that of the Cu–13.0Al–4.0Ni alloy.     

Martensitic transformation kinetics and shape memory effect in Co–Ni alloys

Abstract

The shape memory effect (SME) and martensitic transformation kinetics in Co–Ni alloys were studied. The degree of SME decreased with increasing Ni content, and was proportional to the pre-existing ? martensite content, suggesting that the SME in Co–Ni alloys is related to the coalescence of the pre-existing ? plates. Thermal cycling (α ?) increased the ? martensite content, and the SME became greater with an increasing number of thermal cycles. The martensitic transformation kinetics of Co–Ni alloys can be expressed as Y = 1- exp[-0.00526(M _s-25)], where Y is the volume fraction of ? martensite and M _s is the starting temperature of martensitic transformation (°C).     

Review on microstructure evolution in Sn–Ag–Cu solders and its effect on mechanical integrity of solder joints

The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of “Pb-free” alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb–Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn–yAg–zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed.