A molecular dynamics simulation of alloy carbide clusters formation

 The formation of alloy carbide cluster in ferrite was investigated via molecular dynamics simulation, which disclosed the cluster property and formation mechanism. These together provided a better fundamental understanding of the cluster formation and firm information for the evolution of cluster and precipitate in high-strength low-alloy steel.

Influence of cooling rate on the microstructure and corrosion behavior of Al-Fe alloys

The effect of Fe in Al is technologically important for commercial Al-alloys, and in recycled Al. This work explores the use of the novel rapid solidification technology, known as direct strip casting, to improve the recyclability of Al-alloys. We provide a comparison between the corrosion and microstructure of Al-Fe alloys prepared with wide-ranging cooling rates (0.1. °C/s to 500. °C/s). Rapid cooling was achieved via direct strip casting, while slow cooling was achieved using sand casting. Corrosion was studied via polarisation and immersion tests, followed by surface analysis using scanning electron microscopy and optical profilometry. It was shown that the corrosion resistance of Al-Fe alloys is improved with increased cooling rates, attributed to the reduced size and number of Fe-containing intermetallics.

β-Ti grain refinement via α-Precipitation

The present work explores the impact of α precipitates on β recrystallization following hot deformation of Ti-5Al-5Mo-5V-3Cr with grains larger than 1 mm. A single hot rolling pass of 36 pct reduction was conducted on an aged microstructure containing α precipitates at a temperature well below the β transus temperature. After annealing, a uniformly recrystallized structure with a grain size of ~100 µm is formed. The prior β grain boundaries can be readily identified and it is seen that the primary β grains have been replaced by grains displaying a spread of correlated misorientation angles extending up to the highest allowable values. The annealing comprises two stages. The first stage involves normal β subgrain growth limited by the Zener pinning force of the unstable α precipitates. The second stage corresponds to the onset of β recrystallization at the point where the Zener pinning force drops due to dissolution of the α precipitates. This leads to a uniform distribution of site saturated recrystallization nuclei.

Film formation in trihexyl(tetradecyl)phosphonium diphenylphosphate ([P6,6,6,14][dpp]) ionic liquid on AA5083 aluminium alloy

Whilst ionic liquids (IL) have been shown to inhibit corrosion on some reactive metals and alloys by forming a surface film, e.g. Li and Mg, understanding of the interaction between ionic liquids and aluminium is lacking. This research study investigated the viability of film formation on AA5083 Aluminium Alloy by electrochemical treatments in the trihexyl(tetradecyl)phosphonium diphenylphosphate ([P6,6,6,14][dpp]) IL. Two-step anodic treatments were performed on AA5083 in the IL, followed by a comparison of the corrosion behaviour of the IL-treated samples with that of a control. It has been revealed that the two-step IL-treatment led to reduced current densities on AA5083 under cyclic voltammetry scan in the IL before and after the IL-treatment. Lower corrosion rates have been shown on all samples treated in IL at room temperature. Surface characterisation showed a non-uniform porous film on the 50°C IL-treated sample with a film thickness ranging between 37nm and 155nm. The IL-film enhanced the corrosion resistance of AA5083 by protecting the Al-matrix and Fe-rich intermetallic particles (IMPs). Although findings of this study suggest similar IL-film formation as that on Li and Mg, more research needs to be conducted to optimise the electrochemical treatment conditions and ultimately to develop a robust IL-film formation procedure for corrosion protection.

Corrosion behaviour and hardness of in situ consolidated nanostructured Al and Al-Cr alloys produced via high-energy ball milling

This paper presents a hypothesis and its experimental validation that simultaneous improvement in the hardness and corrosion resistance of aluminium can be achieved by the combination of suitable processing route and alloying additions. More specifically, the corrosion resistance and hardness of Al- xCr (x= 0-10 wt.%) alloys as produced via high-energy ball milling were significantly higher than pure Al and AA7075-T651. The improved properties of the Al- xCr alloys were attributed to the Cr addition and high-energy ball milling, which caused nanocrystalline structure, extended solubility of Cr in Al, and uniformly distributed fine intermetallic phases in the Al-Cr matrix.

Precipitation processes in Al-Cu-Mg-Sn and Al-Cu-Mg-Sn-Ag

Microalloying trace elements into aluminum alloys have been shown to improve mechanical properties by altering the precipitation process. Here, trace amounts of Sn and (Sn + Ag) have been added to Al-1.1Cu-1.7Mg (at.%) and the effects have been investigated by a combination of hardness testing and transmission electron microscopy (TEM). Hardness testing shows that the addition of Sn increases the hardness throughout the ageing process, and in combination with Ag, further increases the hardness and shortens the time to reach the peak hardness. The increase in hardness via Sn microalloying is attributed to the homogeneous distribution of S phase (Al2CuMg) precipitates. In the alloy microalloyed with both Sn and Ag, the microstructure is dominated by homogeneously distributed Ω phase (Al2Cu) precipitates in the peak strengthened condition. Given that neither spherical β-Sn precipitates, nor any other obvious nucleation sites for the Ω phase precipitates were observed using TEM, the mechanism for development of such homogeneous precipitation remains to be determined.

Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel

We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (MnγAl anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site.

Investigation of nano-scale precipitation phenomena in Al-Fe and Al-Cu-(Fe) alloys processed via direct strip casting

The use of rapid solidification processes such as direct strip casting (DSC) is a good way to refine the Fe-intermetallics and decrease their detrimental effects. DSC creates out-ofequilibrium supersaturated microstructures. In this work, we explore the precipitation phenomena in direct strip cast Al-Fe and Al-Cu-Fe alloys and related corrosion and mechanical properties. The precipitates are characterised with differential scanning calorimetry and transmission electron microscopy. The corrosion performances are evaluated with immersion tests and weight loss measurements and the yield strength and ductility are estimated with tensile tests. A strong correlation between the microstructure and the bulk properties is revealed with a significant improvement of properties of DSC alloys.

Microstructure and hardness evolution during simulated coiling of a direct strip cast low carbon low niobium steel

This paper examines the impact of coiling temperature and duration on the phase transformation and precipitation behavior of a low carbon and low niobium direct strip cast steel. Coiling was performed at three carefully chosen temperatures: (1) in the ferrite (600°C), (2) during the austenite decomposition (700°C) and (3) in the austenite (850°C). The coiling conditions were found to strongly affect the final microstructure and hardness response, thus highlighting the necessity to judiciously design the coiling treatment. Optical microscopy, and scanning and transmission electron microscopy were used to characterize the microstructural constituents (polygonal ferrite, bainite and pearlite) and the NbC precipitates. Vickers macrohardness measurements are utilized to quantify the mechanical properties. The differences in hardening kinetics for the three different temperatures are shown to come from a complex combination of strengthening contributions.

Multicomponent materials from machining chips compacted by equal-channel angular pressing

Al and Mg machining chip blends were compacted by equal-channel angular pressing with back pressure. By varying the weight fraction of the constituent materials, temperature and processing route, as well as employing subsequent heat treatment, the microstructure and the mechanical properties of the compact were varied. The width of the interdiffusion zone and the formation of intermetallic phases near the interfaces between the two metals were studied by energy-dispersive X-ray spectroscopy and nanoindentation. It was shown that substantial improvement of mechanical properties, such as an increase of strength, strain-hardening capability and ductility, can be obtained. This is achieved by changing the processing parameters of equal-channel angular pressing and the annealing temperature, as well as by optimising the weight fraction of the constituent metals. © 2013 Springer Science+Business Media New York.

A Mg-Al-Nd alloy produced via a powder metallurgical route

A Mg-5 wt.%Al-2 wt.%Nd alloy has been prepared by a powder metallurgical route using a blend of two dissimilar alloy powders. The initial consolidation of the powders was achieved through a single equal channel angular extrusion pass at 150 °C. After heat treatment at temperatures between 420 °C and 530 °C, it was possible to produce a microstructure that consisted of a uniform distribution of Al3Nd and Al11Nd3 precipitates in a magnesium matrix. These precipitates displayed distinct orientation relationships with the matrix. The size and shape of the precipitates depended on the heat treatment temperature and time. © 2009 Elsevier B.V. All rights reserved.

Hot deformation and recrystallization of a metastable beta titanium ingot

Hot deformation behavior and microstructure evolution of a coarse grain metastable beta titanium alloy (Ti-5Al-5Mo-5V-3Cr) was investigated using uniaxial compression testing followed by a subsequent beta annealing treatment. Compression testing was carried out at 720 °C and strain rates between 0.001-10 s-1 on samples with beta annealed condition and aged microstructure containing high volume fraction of relatively large alpha precipitates. The peak load of the aged samples are higher than the non-aged specimens but they show rather similar steady state flow stress. The subsequent beta annealing treatment on the compressed aged samples leads to breaking down the ingot microstructure and formation of a fully recrystallized beta phase with massive grain refinement (order of millimeter to ∼100 μm). However, after annealing such grain refinement is not seen for the non-aged samples except at high strain rates that showed partial and local recrystallization.

Structure and mechanical property variations in Mg-Gd-Y-Zn-Zr alloy depending on its composition and processing conditions

An effect of alloying element content on mechanical properties and precipitate formation in Mg-RE alloys was studied for Mg-8Gd-4Y- 1Zn-0.4Zr (wt%) and Mg-10Gd-5Y-1.8Zn-0.4Zr (wt%). It is shown that small variations in the alloying element concentration can be used to manipulate the alloy microstructure and precipitate formation towards eliminating the asymmetry (tension/compression) and anisotropy of yield stress.

The effect of ageing on the compressive deformation of Mg-Sn-Zn-Na alloy

The influence of ageing on deformation twinning in an extruded Mg-6Sn-3Zn-0.04Na alloy is investigated. In-situ compression tests have been carried out using high resolution synchrotron X-Ray Diffraction (XRD) to measure the influence of precipitates on twining activity. Synchrotron experiments revealed the increase in the critical resolved shear stress of twinning with ageing. The compressive yield strength (along the extrusion direction) of the aged sample increased by ∼ 150% over the non-aged specimen. To obtain statistical insight into the twinning activity, the microstructure of the non-aged and aged samples (200°C, 24 hours) deformed up to ∼1% plastic strain was studied using optical microscopy. A higher number of thinner twins were observed in the microstructure of the aged sample compared to the non-aged sample.

Initiation of plasticity by nanoindentation in Mg alloys

 In this study, nano-indentation technique is employed to investigate the initiation of basal slip and extension twin separately in magnesium. The present method prove useful in studying the influence of solid solution and precipitates on these two deformation modes.