Effect of Ag addition on the corrosion properties of Sn-based solder alloys

Corrosion properties of three different Sn-Ag lead free solder alloys have been investigated in 0.3 wt% Na₂SO₄ solution as corrosive environment. As cast solder alloy was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Volume fractions of the Ag₃Sn in the solders were determined by image analysis technique. Pitting potential and corrosion potential for the alloys were determined by potentiodynamic tests. Electrochemical impedance spectroscopy (EIS) was carried out to measure the film and charge transfer resistance. Alloys with lower Ag content have been found as better corrosion resistance material.

Nanoscale precipitation in advanced high strength steels

Precipitation strenghthening is one of the most important approaches for enhancing the strenght of microalloyed steels. This study has made a significant contribution in understanding the nucleation and growth mechanism of nanoscale interphase precipitates in steel during commercial processes. Atom Probe Tomography revealed the existence of nanoscale clusters with precipitates that then dictate the final strength.

Effects of tool positioning on joint interface microstructure and fracture strength of friction stir lap Al-to-steel welds

Friction stir lap welding (FSLW) experiments have been conducted to study the effects of tool positioning on microstructures formed in the Al-to-steel interface region and on joint strength, defined as maximum applied force over the width (F m/w s) of the test sample, of the welds. Various pin positioning and speed conditions were used in the FSLW experiments followed by microstructure examination on the interface regions and tensile-shear testing on the welds, including an examination on crack propagation in mixed stir zone. It was found that when the pin was close to the bottom steel piece, Al-to-steel reaction occurred resulting in intermetallic outbursts formed along the interface. This represents the case of incomplete metallurgical joint. When the pin was lowered to just reach the steel, a thin and continued interface intermetallic layer formed. Evidences and consideration on growth kinetics have suggested that the layer could only remain thin (≤2.5 μm) during FSLW. This layer could bear a high load during tensile-shear testing and the adjacent aluminium deformed and fractured instead. The resulting F m/w s was high. When the pin penetrated to steel, F m/w s reduced due to brittle fracture being dominant inside mixed stir zone. Evidences have shown that the amount of penetration and speed condition during FSLW do not have large effects on F m/w s.

Electrochemical etching of aluminium alloy in ionic liquids

AA5083 aluminium alloy has been shown to be partially passivated by a 2-step anodic pre-treatment in Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P6,6,6,14][NTf2]) ionic liquid. Surface characterisation revealed that an electrochemical etching process had occurred, comparable to acid etching of aluminium. Scanning electron microscopy/energy dispersive x-ray spectroscopy results have established that magnesium dealloyed from the Mg2Si intermetallic particles and metal fluorides were deposited onto the remaining Mg2Si sites, which subsequently led to decreased anodic corrosion kinetics (to one third of the control) as well as an increase in the corrosion and pitting potentials. This unique electrochemical etching process offers a simple and quick method to improve the corrosion resistance of an aluminium alloy as it leads to a more uniform surface, in terms of defect size and distribution, compared to conventional acid etching. This process has the potential to be used as a pre-treatment to inhibit corrosion of AA5083 alloy.

Inhibition of corrosion on AA2024-T3 by rare earths & mercaptoacetate

The use of the aluminium alloy AA2024-T3 has long been associated with a strong vulnerability to localised corrosion. Dealloying and pitting corrosion can occur on and around intermetallic particles when exposed to aggressive environments such as sodium chloride electrolytes. Specific combinations of rare earths and organic compounds have demonstrated strong synergistic inhibition on the AA2024-T3 alloy. This work has focused on rare earths and organic compounds containing thiol functional groups. It is believed that the sulphur in the thiol group can form protective films over the surface of copper-rich intermetallic particles due to the affinity between copper and sulphur. Previous studies with the multiwell tests have identified that solutions containing sodium mercaptoacetate provided strong inhibition at pH 3 and 6. This work presents the initial findings from the polarisation tests and constant immersion corrosion experiments in the presence of sodium mercaptoacetate.

Grain refinement of an extruded Mg alloy via na microalloying

The effect of 0.3 wt pct Na on the microstructure of extruded alloy Mg-2Sn-1Zn is examined. We report that Na stabilizes the Mg2Sn phase, resulting in its precipitation during extrusion under conditions where a solid solution is otherwise expected. This effect appears to be thermodynamic in nature and is different from the kinetic enhancement of low- temperature aging reported by Mendis et al. [Phil. Mag. Letters, 86 (2006), 443]. The precipitates of the current study enable useful refinement of the grain size.

Analysis of strengthening in AA6111 during the early stages of aging: atom probe tomography and yield stress modelling

In this work, a series of aging treatments has been conducted on AA6111 alloy samples for various times at ambient temperature (so-called natural aging) and at temperatures between 60 and 180 °C (artificially aged). The time at artificial ageing was chosen such that samples with approximately the same yield stress were produced. The microstructures of these alloy samples have been carefully characterized using atom probe tomography together with advanced cluster-finding techniques in order to obtain quantitative information about the changes in distribution of both the solute clusters and early-stage precipitates that are formed. The size distribution of clusters has been mapped onto the glide plane and then the stress necessary for a dislocation to pass through the range of obstacles has been estimated using an areal glide model where the dislocation–obstacle interaction strength has been assumed to be related to the obstacle size on the glide plane. It is demonstrated that the contribution of cluster strengthening during artificial aging at higher temperatures is dominated by the high number density of small clusters (Guinier radius <1 nm), whereas the situation during room temperature natural aging is more complex.

Cryo-rolling and formability of 2024 aluminium

Sheets of precipitate hardenable 2024 aluminium have been processed by rolling at liquid nitrogen temperature in order to refine the microstructure. A number of different aging/heat treating procedures have been utilised that have resulted in significantly different mechanical properties. The cryo-rolled material was heat treated at 150 °C for varying times and the resulting mechanical properties evaluated as a function of this holding time. The resulting properties were found to be strongly influenced by precipitates that formed either during the aging step, rolling process or the subsequent heat treatment. The formability of the cryo-rolled and heat treated material has been investigated using a limiting dome height test (Erichsen cupping test).

Corrosion of mg alloy ZE41 and its mitigation using ionic liquids

Magnesium alloy ZE41 (Mg-Zn-RE-Zr), which is used extensively in the aerospace industry, possesses excellent mechanical properties albeit poor corrosion resistance. This work investigates the mechanism of corrosion, and the interaction between the grain boundary intermetallic phases, the zirconium (Zr)-rich regions within the grains and the bulk Mg rich matrix in both the as-cast and heat-treated conditions. The results of optical and scanning electron microscopy (SEM) show the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The Zr-rich regions play a distinct role in the early stages of corrosion with this alloy. The second part of this work investigates the interaction of two different ionic liquids (ILs) with the surface of the ZE41 alloy. ILs based on trihexyltetradecylphosphonium (P 6,6,6,14) coupled with either diphenylphosphate (DPP) or bis(trifluoromethanesulfonyl) amide (Tf 2N) have been shown to react with Mg alloy surfaces, leading to the formation of a surface film that can improve the corrosion resistance of the alloy. The interaction of the ILs with the ZE41 surface has been investigated by optical microscopy and SEM. Surface characterization has been performed using Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). The surface characterization and microscopy revealed the preferential interaction with the grain boundaries and grain boundary phases. Thus the morphology and microstructure of the Mg surface seems critical in determining the nature of the interaction with the IL. The corrosion protection of the IL films formed on the ZE41 surface was investigated by SEM and potentiodynamic polarisation.

Impact of grain microstructure on the heterogeneity of precipitation strengthening in an Al-Li-Cu alloy

The effect of grain microstructure on the age-hardening behavior is investigated on recrystallized and un-recrystallized Al-Cu-Li alloys by combining electron-backscatter-diffraction and micro-hardness mapping. The spatial heterogeneity of micro-hardness is found to be strongly dependent on the grain microstructure. Controlled experiments are carried out to change the pre-strain before artificial ageing. These experiments lead to an evaluation of the range of local strain induced by pre-stretching as a function of the grain microstructure and results in heterogeneous formation of the hardening T1 precipitates.

A study of phase behavior and conductivity of mixtures of the organic ionic plastic crystal N-methyl-N-methyl-pyrrolidinium dicyanamide with sodium dicyanamide

We report on the thermal, structural and conductivity properties of the organic ionic plastic crystal (OIPC) N-methyl-N-methyl-pyrrolidinium dicyanamide [C1mpyr][N(CN)2] mixed with the sodium salt Na[N(CN)2]. The DSC thermal traces indicate that an isothermal transition, which may be a eutectic melting, occurs at ~ 89 °C, below which all compositions are entirely in the solid phase. At 20 mol% Na[N(CN)2], this transition is the final melt for this mixture, and a new liquidus peak grows beyond 20 mol% Na[N(CN)2]. The III- > II solid-solid phase transition continues to be evident at ~- 2 °C. The microstructure for all the mixtures indicated a phase separated morphology where precipitates can be clearly observed. Most likely, these precipitates consist of a Na-rich second phase. This was also suggested from the vibrational spectroscopy and the 23Na NMR spectra. The lower concentrations of Na[N(CN)2] present complex 23Na MAS spectra, suggesting more than one sodium ion environment is present in these mixtures consistent with complex phase behavior. Unlike other OIPCs where the ionic conductivity usually increases upon doping or mixing in a second component, the conductivity of these mixtures remains relatively constant and above 10- 4 S cm- 1 at ∼ 80 °C, even in the solid state. Such high conductivities suggest these materials may be promising to be used for all solid-state electrochemical devices.

Study of growth mechanism of TiC cluster in ferrite via molecular dynamics simulation

Abstract Atomistic simulations were used to investigate the evolution process of titanium carbide clusters to mature precipitates in ferrite. The typical kinetic of carbide cluster growth was studied in detail through analyzing the atomic interactions of a carbide cluster with scattered carbon atoms. The driving force required for cluster growth was calculated along with the atomic diffusivity in the iron matrix, exploring the change in response as two main growth steps. The growth kinetic improved the understanding of precipitate evolution at the atomic level.

Investigation of precipitate hardening of slip and twinning in Mg5%Zn by micropillar compression

A Mg-5%Zn alloy has been aged to form c-axis rod precipitates which are known to increase strength. Micropillar compression tests were carried out in the precipitate-free and aged samples to investigate the effects of these precipitates on twinning and slip in magnesium alloys. Basal slip, pyramidal slip and {101¯2} twinning were selectively activated by compressing micropillars in the [112¯3], [0 0 0 1] and [112¯0] orientations, respectively. It has been found that precipitation causes moderate hardening of the basal slip system, and also significantly increases the work hardening rate. The compression of [112¯0] initiated twinning, but the present experiments were dominated by twin nucleation, rather than growth. It was found that the effect of precipitation on twin nucleation was negligible. Precipitation had little effect on specimens compressed in the c-axis.