Properties of ductile cast iron nodularised with multiple calcium–magnesium based master alloy

Abstract

Austempered ductile iron (ADI) is gradually replacing many fabricated and forged steel components in engineering applications. One of its advantages is the combination of good castability, machinability, and mechanical properties with significant savings in cost and weight compared with equivalent steel components. A problem in the production of ADI is the use of expensive and dangerously reactive magnesium as a graphite nodulariser. There is a need to find cheaper, safer, and equally effective substitutes. Results of an investigation of the effectiveness of a multiple calcium–magnesium based master alloy nodulariser and the properties of the ductile iron and ADI produced are reported. Up to 96% nodularity could be obtained using a special Ca–CaC₂–Mg master alloy, compared with 98% using magnesium alone. The mechanical properties were also comparable.     

Thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys

Abstract

The thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys has been studied using TEM, X-ray diffraction, and atom probe analysis. All of the as deposited materials were purely fcc and had grain sizes of 10–20 nm. Heat treatment of nanocrystalline nickel in the range 190–320°C resulted in abnormal grain growth with an activation energy of 122±15 kJ mol^-1. Abnormal grain growth in Fe–50 at.-%Ni was only observed at 400°C but not at 220 or 300°C, where grain growth was very slow. In Fe–33 at.-%Ni, room temperature aging resulted in the formation of large grained areas (～1 µm), some of which transformed to bcc. In heat treated nickel specimens, some evidence of sulphur and carbon enrichment was found at grain boundaries.     

Mesoporous nickel–iron binary oxide nanorods for efficient electrocatalytic water oxidation

The design and fabrication of low-cost,high-efficiency,and stable oxygen-evolving catalysts are essential for promoting the overall efficiency of water electrolysis.In this study,mesoporous Ni1-xFexOy (0 ≤ x ≤ 1,1 ≤ y ≤ 1.5) nanorods were synthesized by the facile thermal decomposition of Ni-Fe-based coordination polymers.These polymers passed their nanorod-like morphology to oxides,which served as active catalysts for oxygen evolution reaction (OER).Increasing the Fe-doping amount to 33 at.％ decreased the particle size and charge-transfer resistance and increased the surface area,resulting in a reduced overpotential (～302 mV) at 10 mA/cm2 and a reduced Tafel slope (～42 mV/dec),which were accompanied by a far improved OER activity compared with those of commercial RuO2 and IrO2 electrocatalysts.At Fe-doping concentrations higher than 33 at.％,the trend of the electrocatalytic parameters started to reverse.The shift in the dopant concentration of Fe was further reflected in the structural transformation from a NiO (＜33 at.％ Fe) rock-salt structure to a biphasic NiO/NiFe2O4 (33 at.％ Fe) heterostructure,a NiFe2O4 (66 at.％ Fe) spinel structure,and eventually to α-Fe2O3 (100 at.％ Fe).The efficient water-oxidation activity is ascribed to the highly mesoporous one-dimensional nanostructure,large surface area,and optimal amounts of the dopant Fe.The merits of abundance in the Earth,scalable synthesis,and highly efficient electrocatalytic activity make mesoporous Ni-Fe binary oxides promising oxygen-evolving catalysts for water splitting.     

The role of microstructure of nickel–aluminium–bronze alloy on its cavitation corrosion behavior in natural seawater

An investigation was carried out in our laboratory to study the effect of the microstructure of nickel–aluminum–bronze (NAB) alloy on its cavitation corrosion behaviour in seawater using a 20-kHz ultrasonic induced cavitation facility. Cavitation tests were made under free corrosion conditions as a function of exposure time in natural seawater. Optical and scanning electron microscopy showed NAB immersed in stagnant seawater suffered from selective corrosion of the copper-rich α phase at boundaries with intermetallic κ precipitates. The κ precipitates and precipitate-free zones did not suffer corrosion. Cavitation made the surface of this alloy very rough, with large cavities or pits, ductile tearing and corrosion of the boundaries of the α columnar grains. In addition, the number of cavities and their size increased with exposure time. Microcracks 5–10 μm long were observed in the α phase adjacent to κ precipitates along the cross-section of the material. Selective phase corrosion and cavitation stresses were considered to be the cause of the cracks observed.     

Synthesis and characterization of cobalt–nickel alloy nanowires

We report on the synthesis and magnetic characterization of ordered arrays of cobalt–nickel alloy nanowires. These alloy nanowires were electrodeposited into the pores of anodic alumina templates. The physical properties of the samples were investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and vibrating sample magnetometer. We found that for the alloy nanowires the field at which the magnetization saturates increases with increasing Co fraction and the saturation field in the normal direction is smaller than the parallel direction, indicating easy magnetization direction normal to wire axis. Nanowires with different compositional ratio of cobalt and nickel showed a nonlinear dependence of coercivity as a function of cobalt concentration. These findings will help tailor magnetic nanoalloys with controlled properties for various applications, such as high density magnetic storage or nanoelectrode arrays.     

Fatigue behavior and plane-strain fracture toughness of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

In this study, the tensile properties, high cycle fatigue behavior and plane-strain fracture toughness of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy were investigated, comparison to that of sand-cast plus T6 heat treated magnesium alloy which named after sand-cast-T6. The results showed that the tensile properties of the sand-cast alloy are greatly improved after T6 heat treatment, and the fatigue strength (at 10⁷ cycles) of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy increases from 95 to 120 MPa after T6 heat treatment, i.e. the improvement of 26% in fatigue strength has been achieved. The plane-strain fracture toughnesses K_IC of the sand-cast and sand-cast-T6 alloys are about 12.1 and 16.3 MPa m^1/2, respectively. In addition, crack initiation, crack propagation and fracture behavior of the studied alloys after tensile test, high cycle fatigue test and plane-strain fracture toughness test were also investigated systematically.     

Preparation of gallium nitride from gallium–magnesium alloy powders

Ga–Mg alloy powders were introduced to synthesise GaN powders with flowing ammonia. When the magnesium percentage in the alloy was 25?wt-% or below and the reaction temperature was about 750°C, all Ga atoms in the alloy were transformed into GaN, and the diameter of the GaN powders was about 40?nm. For the Ga–Mg alloy powders with a diameter ranging from 37 to 75?µm ammoniated at 750°C, only 3?h were spent to transform all the Ga atoms into GaN, indicating that the strategy suggested in this study was effective to produce GaN powders.     

One-pot synthesis of nickel oxide–carbon composite microspheres on nickel foam for supercapacitors

Using glucose and Ni(NO₃)₂ as precursors, the nickel oxide–carbon (NiO–C) composites are directly formed on the nickel foam by a one-pot hydrothermal method. The product presents a spherical morphology with the carbon component in the composite being a non-graphitic phase. The presence of NiO provides additional pseudo-capacitance for the electrode materials and the composites exhibit superior specific capacitance to the pure carbon materials formed on the nickel foam. Apart from that, compared with the nickel sheet substrate, the utilization of the nickel foam benefits for the achievement of higher performance and stability owing to its unique 3D structure. The highest specific capacitance for the NiO–C–nickel foam electrode of 265.3 F g⁻¹ is obtained at a discharge current density of 0.25 A g⁻¹.     

Interfacial structure and mechanical properties of surface iron–nickel alloying layer in pure iron fabricated by surface mechanical attrition alloy treatment

By surface mechanical attrition alloy treatment (SMAAT) and subsequent low temperature anneal treatment, a refined Fe/Ni alloy surface layer, about 50 μm in length, was fabricated on a pure iron plate. Micro-hardness and the friction and wear behavior of alloy surface layers were studied in comparison with those of their SMATed-only nanocrystallization counterpart. The interface microstructure indicated that the nickel powders had been permeated and welded into the pure iron surface in some localized regions by plastic deformation. The SMAAT process includes impacting with high strain velocity, grain refinement and synchronous diffusion. Atomic diffusion has been accelerated by the generation of high density defects through severe plastic deformation. Quick formation of the Fe/Ni intermetallic and solid solution phase alloy layer during SMAAT can be detected. Subsequent annealing treatment further accelerates the diffusion of Ni and Fe elements and leads to the formation of alloy phases. The results of friction and wear tests show that the properties of the alloy layer were remarkably improved. The main reason for this result may originate from its microstructures, i.e. an alloy layer with smaller grains, which reduces the effect of fatigue wear.     

Microstructural characteristics of iron based alloy laser clad on Al–Si alloy

Abstract

The present study investigates some special microstructural characteristics and the diffusion phenomenon occurring in the laser cladding of an iron based alloy (a less commonly used alloy) on an Al–Si eutectic alloy. Among the three microstructural regions, i.e. the clad region, the transitional region, and the post-molten region, the transitional region is non-uniform in both microstructure and composition, in which the aluminium rich area displays a mixture of invisible and snowflakelike morphologies while the iron rich area has an ultrafine lamellalike structure and the transitional zone between them appears as a network structure. Differential scanning calorimetry and X-ray diffraction analysis carried out in the transitional region revealed that the possible phases existing in the three areas are γ-Fe, NiAl, and Fe₂ Al₅ in the iron rich area, α-Al and NiAl₃ in the aluminium rich area and FeAl₃ , Fe₂ Al₅ , and NiAl₃ in the transitional zone, respectively. Furthermore, the solidification process of the pool and the bond mechanism have been discussed in relation to the microstructural characteristics observed in the experiments, the properties of materials and the extent of thermal conduction.     

Plastic deformation behavior of ultrafine-grained Al–Mg–Sc alloy

Ultrafine-grained (UFG) Al–Mg–Sc alloy was obtained by friction stir processing. The UFG alloy was subjected to uniaxial tensile testing to study the tensile deformation behavior of the alloy. An inhomogeneous yielding (Lüdering phenomenon) was observed in the stress–strain curves of UFG alloy. This deformation behavior was absent in the coarse-grained alloy. The Lüdering phenomenon in UFG alloy was attributed to the lack of dislocations in UFG microstructure. A strong dependence of uniform ductility on the average grain size was exhibited by the UFG alloy. Below a critical grain size (0.5 μm), ductility was very limited. Also, with the decrease in grain size, most of the plastic deformation was observed to be localized in necked region of the tensile samples. The negative strain rate sensitivity (SRS) observed for the UFG alloy was opposite of the SRS values reported for UFG alloys in the literature. Based on activation volume measurement, grain boundary mediated dislocation-based plasticity was concluded to be the micro-mechanism operative during plastic deformation of UFG Al–Mg–Sc alloy.     

X-ray photoelectron spectroscopy investigations of zinc–magnesium alloy coated steel

The coating layer composition depth profiles and element chemical states of zinc–magnesium alloy coated steel were investigated by X-ray photoelectron spectroscopy depth profiling. Through the analysis of photoelectron signals and Auger signals of different elements on different depth planes of the coating layer, it can be found that the surface of the coating layer contains MgCO₃, MgO, Mg(OH)₂, metallic Mg, metallic Zn and some complex zinc compounds. Under the surface, there is a Zn₂Mg alloy layer with the thickness of about 300 nm accompanied with MgO and Mg(OH)₂ in the layer. There is a transitional layer with the thickness of about 200 nm between the Zn₂Mg alloy layer and the pure Zn layer, whose components consist of zinc–magnesium alloy without fixed stoichiometry, a little MgO and a little Mg(OH)₂.     

Preparation and magnetic characteristics of mesoporous nickel oxide–silica composites

Mesoporous NiO–SiO₂ (MCM-41) silica-matrix composites with various nickel oxide concentrations (NiO : SiO₂ = 0.025 : 1 to 0.2 : 1) have been produced by oxide cocondensation under hydrothermal synthesis conditions in the presence of cetyltrimethylammonium bromide as a template and (2-cyanoethyl) triethoxysilane as an organosubstituted trialkoxysilane additive. X-ray diffraction data have been used to evaluate the maximum nickel(II) oxide concentration (NiO : SiO₂ = 0.1 : 1) that allows the ordered mesopore structure of MCM-41 to persist in the silica-matrix composites. We have studied the magnetic properties of this material as functions of temperature and magnetic field. The results demonstrate that the magnetic properties of the nanocomposite with NiO : SiO₂ = 0.1 : 1 at low temperatures (T < 20 K) are determined by incomplete spin compensation in the matrix and on the surface of the NiO nanoparticles.     

Corrosion behavior of Ti–Al–N/Ti–Al duplex coating on AZ31 magnesium alloy in NaCl aqueous solution

In this study, Ti–Al–N/Ti–Al duplex coating was deposited on AZ31 magnesium alloy by magnetron sputtering with a Ti/Al composite target. Scanning electron microscopy and Auger electron spectroscopy were applied to investigate the morphology and elemental concentration of the obtained coating, respectively. The top layer was Ti–Al–N film with a Ti:Al:O:N ratio of 0.32:0.84:0.08:1, and the bottom layer was Ti–Al film with a Ti:O:Al ratio of 1.94:0.12:1. Each layer of this coating presented a developed columnar structure. The polarization test and immersion test were used to investigate corrosion behavior of the coated sample in 3.5 wt.% NaCl aqueous solution. The results showed that this duplex coating could protect the substrate effectively in NaCl aqueous solution. Nevertheless, several through-thickness micropores in the coating finally induced the failure of the coated AZ31 in the immersion test.     

Hot deformation behavior of spray-deposited Al–Zn–Mg–Cu alloy

The flow behavior of spray-deposited Al–10.21Zn–2.76Mg–1.45Cu–0.16Zr (wt.%) alloy has been systematically investigated by thermal compression tests with temperature and strain rate ranging from 613 K to 733 K and 0.001–1 s⁻¹, respectively. Microstructural observations revealed that the average grain size of spray-deposited alloy was below 25 μm due to the high cooling rate. Both relatively high temperature and low strain rate could promote the formation of dynamic recrystallization (DRX). The stress level of the alloy decreased with increasing deformation temperature and decreasing strain rate, which could be characterized by a Zener–Hollomon parameter in the hyperbolic-sine equation. Furthermore, the strain-dependent constitutive equation could lead to a good agreement between the calculated and measured flow stresses in the elevated temperature range for spray-deposited alloy. The deformation activation energy for spray-deposited alloy was relatively lower than that of the as-cast alloy owing to ultrafine grains and high supersaturated solid solubility.     

Wear behavior of carbide coated Co–Cr–Mo implant alloy

The wear behavior of a new type of metal carbide surface coating on Co–Cr–Mo implant alloy was studied. The coating was created using a microwave plasma-assisted reaction. Codeposition of impurity diamond film, diamond particles, and soot was prevented by controlling process conditions. Wear tests were carried out using a sapphire ball-on-Co–Cr–Mo disc unidirectional sliding configuration with harsh conditions of high contact stress and slow sliding speed in both no-lubrication, and deionized water lubrication environments. In the case of uncoated Co–Cr–Mo discs, the effect of deionized water lubrication was remarkable and reduced the wear factor by one order of magnitude compared to the no-lubrication tests. The wear factor of carbide coated Co–Cr–Mo discs was slightly smaller than that of uncoated Co–Cr–Mo discs with deionized water lubrication (2.7×10^–6 mm³N^–1m^–1 vs. 4.2×10^–6mm³N^–;1m^–1). The addition of deionized water lubrication did not greatly affect the wear factor of carbide coated Co–Cr–Mo discs. The influence of surface geometry resulting from the brain coral-like surface morphology of carbide layers on wear behavior was analyzed considering stress concentrations and effective contact area.     

Effect of friction stir processing on erosion–corrosion behavior of nickel–aluminum bronze

In the present investigation, effects of Friction Stir Processing (FSP) on Erosion–Corrosion (E–C) behavior of Nickel–Aluminum Bronze (NAB) were studied by weight-loss measurements and surface characterization using an impingement jet test system. After FSP, the initial coarse microstructure of the cast NAB was transformed to a fine structure, and the porosity defects were eliminated. In addition, different FSP structures were produced by each rotation rate. Microhardness measurements showed a marked increase in FSP samples depending upon the FSP parameters. E–C tests were carried out by erodent at kinetic energies about 0.45 μJ and in 30°, 60° and 90° impact angles to simulate actual service conditions. The maximum weight-loss was observed in FSP samples and Scanning Electron Microscopy (SEM) results showed signs of brittle fracture mechanism in FSP samples. By gravimetric analysis, the degree of synergy was evaluated at 0.45 μJ kinetic energy at normal impact angle and negative synergy result implies the presence of a protective film on all sample surfaces.     

Fatigue behavior of friction stir welded Al–Mg–Sc alloy

The effect of Friction Stir Welding on the fatigue behavior of Al–Mg–Sc alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying microstructural features. Crack initiation as well as crack propagation behavior under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermo-mechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.     

Thixoformability of a Mg–Gd–Y magnesium alloy and its mechanical properties

Abstract

A new magnesium alloy with composition of Mg–8·57Gd–3·72Y–0·54Zr (wt-%, GW94) amenable to semisolid forming is presented. A quantitative investigation of its thixoformability in terms of metallurgical parameters in the semisolid state is performed based on Pandat thermodynamic calculation. The optimised working window for thixoforming is from 581 to 605°C, where the volume fraction of liquid does not change significantly with temperature. The alloy is then successfully thixoformed at 600°C, and a typical thixotropic microstructure is obtained. It is shown that significant improvement of mechanical properties is achieved in the thixoformed (TF) GW94 alloy compared to its permanent mould casting counterpart. This is attributed to an obvious decrease in the amount of porosity and fine distribution of the brittle Mg₂₄(Gd,Y)₅ particles in the thixoforming process.     

Study on low temperature brazing of magnesium alloy to aluminum alloy using Sn–xZn solders

In this article, a series of Sn–xZn solders are designed for joining Mg/Al dissimilar metals by low temperature brazing. The effect of Zn content in Sn–Zn solders on microstructure evolution and mechanical properties of the different brazed joints are investigated. The experimental results indicate that Sn–30Zn alloy is identified as the optimized solder. Al–Sn–Zn solid solutions form and disperse in the brazing zone of the Mg/Sn–30Zn/Al brazed joint, decreasing the risk of embrittlement of the brazed joint. The average shear strength of Mg/Sn–30Zn/Al brazed joint can reach 70.73 MPa. The joint fractures in the coarse blocky Mg₂Sn intermetallic phases in the center of the brazing zone.