As-cast Counterpart Research Articles

Stress Corrosion Cracking Behavior of Fine-Grained AZ61 Magnesium Alloys Processed by Equal-Channel Angular Pressing

The effect of equal-channel angular pressing (ECAP) on stress corrosion cracking (SCC) behavior of a cast AZ61 Mg alloy was investigated in distilled water (DW) using the slow strain rate tensile test (SSRT) at a strain rate of 1 × 10−6 s−1. The fine-grained alloy after ECAP showed a greater SCC susceptibility but a higher ultimate tensile strength, compared with the as-cast counterpart. The results were attributed to refined grains, high-density dislocations and increased proportion of high-angle grain boundaries induced by severe plastic deformation, as well as isolated fine β-phase particles transiting from net-like β-phase.

Microstructure and Wear Properties of Electron Beam Melted Ti-6Al-4V Parts: A Comparison Study against As-Cast Form

Ti-6Al-4V (Ti64) parts of varying thicknesses were additively manufactured (AM) by the powder-bed-based electron beam melting (EBM) technique. Microstructure and wear properties of these EBM-built Ti-6Al-4V parts have been investigated in comparison with conventionally cast Ti64 samples. Sliding wear tests were conducted using a ball-on-disc micro-tribometer under ambient conditions. Experimental results reveal that EBM-built Ti64 samples exhibited higher microhardness and an overall larger coefficient of friction as compared to the as-cast counterpart. Of interest is that the corresponding specific wear volumes were lower for EBM-built Ti64 samples, while the as-cast Ti64 showed the poorest wear resistance despite its lower coefficient of friction. Wear mechanisms were provided in terms of quantitative microstructural characterization and detailed analysis on coefficient of friction (COF) curves.

Effect of impurity N2 concentration on the hydriding kinetics of Na-doped Mg–Ni alloys

Here, we report on the hydriding behaviour of various Mg–Ni alloys in the presence of N2 gaseous impurities using in situ synchrotron XRD. We find that a N2 content as low as 0.5 mol.% can completely inhibit the hydriding of pure Mg2Ni whereas the Na-modified equivalent remains relatively unaffected even at N2 concentrations of up to 5 mol.%. The activated hypoeutectic Mg–Ni alloy demonstrates excellent hydriding kinetics compared to the as-cast counterpart under 1 mol.% of N2. Surface analysis shows it is the electropositive nature of a Na-modified Mg–Ni surface that facilitates selective adsorption of N2 while leaving metallic Ni available for H2 adsorption/dissociation. The results are important in designing Mg–Ni alloys with tailored properties that are suitable for use in hydrogen storage/purification/separation applications.

On the formation of AlSi10Mg single tracks and layers in selective laser melting: Microstructure and nano-mechanical properties

Selective laser melting (SLM) is a relatively new manufacturing technique that can be used to process a range of materials. Aluminum alloys are potential candidates for SLM but are more difficult to process than the titanium alloys more commonly used with this technique. This is because of the former’s physical properties that can result in high levels of porosity in the final parts. Although the majority of studies to date into the processing of Al alloys by SLM have considered the development of load bearing objects, in particular porosity reduction and mechanical characterization of the parts, it is also important to study the single tracks formed during the process. This paper studies the effect of changing the scan speed on the formation of fusion lines and single tracks from an Al alloy, as well as their overlap to form a single layer. The geometrical features of the melt pools as well as the boundaries of continuity and/or irregularities were defined and showed dependence on scan speed. Keyhole mode melting domination was observed. The scan tracks and layers were porosity-free suggesting pores to form with layer accumulation. Investigations showed that increasing the layer thickness should be avoided as it promoted defects. Energy dispersive X-ray (EDX) mapping was implemented to compare the chemical composition distribution in the SLM material and its as-cast counterpart. A fine microstructure with homogenous distribution of the alloying elements was observed. Nanoindentation and EDX were used to establish an understanding of the hardness profile across melt pools of single tracks and their interrelation to the chemical composition. The elemental distribution yielded uniform high nano-hardness with no spatial variation across the SLM material.

Substantial enhancement of hydrogen permeability and embrittlement resistance of Nb30Ti25Hf10Co35 eutectic alloy membranes by directional solidification

As-cast Nb30Ti35−xHfxCo35 (x=0, 10, and 17.5) alloys consist of a fully lamellar eutectic structure, where the substitution of Ti by Hf induces higher hydrogen solubility and permeability, but poorer embrittlement resistance. Different as-cast alloys were found to be subject to embrittlement failure after hydrogen permeation for 88.3h (x=0), 71.1h (x=10) and 16.2h (x=17.5) at 673K. In contrast, directionally solidified (DS) Nb30Ti25Hf10Co35 exhibits a substantial enhancement of hydrogen permeability and embrittlement resistance. Typically, DS samples solidified at 1μm/s show the highest permeability of 4.83×10−8molH2m−1s−1Pa−0.5 at 673K, which is 1.72 times that of its as-cast counterpart and more than three times that of pure Pd. This sample does not fail during hydrogen permeation for 120h at 673K. The high permeability and large embrittlement resistance of DS samples are attributed to their modified eutectic microstructure, which induces a high overall hydrogen diffusivity and a high tolerance to lattice expansion. The present work demonstrates that the directional solidification technique has a high potential to produce high-performance eutectic alloy membranes for hydrogen separation.

Evolution of shear bands, free volume, and structure in room temperature rolled Pd40Ni40P20 bulk metallic glass

The evolution of the shear band, free volume, and structure in room temperature rolled Pd 40 Ni 40 P 20 bulk metallic glass was investigated. It was found that the average shear band density increases monotonously with increasing strain. For the room temperature rolled Pd 40 Ni 40 P 20 bulk metallic glass with a strain of 99%, a high density of shear bands with an average spacing of 31 nm was observed. The absolute free volume content was determined based on the free volume model and found to increase monotonously with increasing strain. The free volume content in the room temperature rolled Pd 40 Ni 40 P 20 bulk metallic glass with a strain of 99% is 34% higher than its as-cast counterpart. Neither phase separation nor crystallization occurs in all the deformed samples. The coordination number of the first coordination shell decreases and the degree of disorder of atomic arrangement increases with increasing strain.

Superior high tensile elongation of a single-crystal CoCrFeNiAl0.3 high-entropy alloy by Bridgman solidification

The crystallographic orientation, tensile behavior, fracture mechanism, hardness, and elastic modulus of a single-crystal CoCrFeNiAl0.3 high-entropy alloy (HEA) successfully synthesized by Bridgman solidification are investigated in detail. The growth direction of the single-crystal product mainly focuses on the <001> orientation. An ultimate tensile elongation of about 80% is achieved in the single-crystal alloy, accompanied by a large work-hardening exponent and a shear-fracture mode. A quantitative Hall–Petch relationship for the current HEA can be obtained as σy=171.65+0.73/d. The single-crystal sample displays a relatively-lower elastic modulus than the as-cast counterpart, indicating the anisotropy of elastic modulus. The outstanding tensile ductility for the single-crystal product is attributed to (1) low-angle grain boundaries and thus less distance to dislocation motion; and (2) single <001> crystallographic orientation and therefore less plastic-strain incompatibility.

Stress-induced phase transformation characteristics and its effect on the enhanced ductility in continuous columnar-grained polycrystalline Cu–12 wt % Al alloy

The microstructure evolution and stress-induced phase transformation (SIPT) characteristics of continuous columnar-grained (CCG) polycrystalline Cu–12wt%Al alloy during the tensile deformation process were investigated to understand the ductility enhancement mechanism, which shows a tensile elongation of 28%, nine times as high as that of the conventional as-cast polycrystalline counterpart. The results show that the CCG alloy has highly-oriented 〈001〉β texture and straight low-energy grain boundaries (GBs) along solidification direction, which significantly promote grains compatibility during plastic deformation. Additionally, besides β1'→α1' transformation, β1'→γ1'→α1' SIPT was also observed simultaneously in the CCG polycrystalline Cu–12wt%Al alloy, which is different from single crystalline and conventional polycrystalline counterparts. As a consequence of the special phase transformations, high phase transformation plasticity (15–20%) and prominent subsequent plastic deformation (8–13%) of the phase transformation product α1' martensite can be obtained.

Hydrogenation-induced microstructure evolution in as cast and severely deformed Mg-10 wt.% Ni alloy

We determined the kinetics of hydrogen absorption of the hypoeutectic Mg-10 wt.% Ni alloy in the as-cast state and after processing by four passes of equal channel angular pressing (ECAP). While during the first hydrogenation cycle the ECAP-modified alloy exhibited faster absorption than its as-cast counterpart, this advantage was lost after the second hydrogenation cycle; parity was regained after six cycles. We attributed these differences in the hydrogen absorption kinetics to the formation of large (tens of micrometers) faceted Mg crystals observed during the first hydrogenation cycle. These crystals were significantly larger in the ECAP-modified alloy than in its as-cast counterpart. We discussed the growth of large Mg crystals during hydrogenation in terms of self-diffusion of Mg atoms driven by the metal-hydride transformation stress. The larger size of these crystals in the ECAP-processed alloy was attributed to the acceleration of diffusion by ECAP. Our metallographic studies revealed a number of microstructural changes in the alloys upon hydrogenation, such as cracking, accumulation of plastic strain in large Mg crystals, and re-distribution of the dispersed particles of Mg2Ni phase in the partly hydrogenated alloys.

Plasticity enhancement of Zr-based bulk metallic glasses by direct current electropulsing

Direct current electropulsing was used to improve the plasticity of (Zr53Cu30Ni9Al8)99.5Si0.5 bulk metallic glasses. After the electropulsing treatment, the specimen showed reductions in both the glass transition and the crystallization temperatures while retaining its amorphous structure, and both Young's modulus and the hardness decreased while the nanoindentation loading curve became more serrated. Using the bond-interface method and Vickers indentation, the treated specimen showed more branching of semi-circular shear bands and less radial shear bands compared to its as-cast counterpart. The possible plasticity enhancement mechanism of the electropulsing treatment was also discussed.

Microstructure and Mechanical Property of an ECAP Processed Al-Mg&lt;sub&gt;2&lt;/sub&gt;Si &lt;i&gt;In Situ&lt;/i&gt; Composite by Using a Combined Route

Microstructure and mechanical property of a hypoeutectic Al-Mg2Si composite processed by equal channel angular pressing up to eight passes in a combined route 2A+4BA+2A were investigated. The results show that the initial developed eutectic Mg2Si was significantly refined into submicrometer-scale particles and distributed homogeneously in the Al matrix, which together with the refinement of Al matrix leads to a much higher ductility with the elongation to failure up to 24% and a significantly enhanced ultimate tensile strength of 284MPa in the processed composite, increased by 2300% and 70%, respectively, compared to those in its as-cast counterpart.

The effect of equal channel angular pressing on hydrogen storage properties of a eutectic Mg–Ni alloy

A multipass equal channel angular pressing (ECAP) technique was applied to as-cast eutectic Mg Ni alloy to refine its microstructure down to sub-micrometer size of Mg and Mg 2Ni grains. This grain refinement was achieved after 10 ECAP passes. The Mg grains showed supersaturation in Ni, which was non-homogeneously distributed across the grains. All samples studied exhibited a gravimetric hydrogen storage capacity of about 6 wt.%. The pressure-composition isotherms for the hydrogenation of as-cast and ECAP processed alloys were determined. It was shown that equilibrium hydrogen desorption pressure increases with increasing number of ECAP passes, the alloy processed by 10 ECAP passes exhibiting approximately 50% pressure increase over its as-cast counterpart. The ECAP processed alloy exhibited an excellent hydrogen desorption kinetics, desorbing 5 wt.% of hydrogen in less than 5 min at the temperature lower than 573 K. It was also shown that in terms of hydrogen desorption pressure the ECAP treated Mg Ni alloy outperforms the alloys of similar composition nanostructured by alternative processing techniques.

Characterization of stress-annealed amorphous CoFeBSi ribbons by GMI and inductance spectroscopy

The effect of tension annealing on Co 66Fe 4Si 12B 18 amorphous alloys was studied by means of magnetoinductance measurements. Samples annealed under stress manifested a strong decrease in transverse permeability and relaxation frequency, as compared with their as-cast alloy counterpart. Samples subsequently subjected to a further relaxation regime showed moderate improvements on both parameters. These results are interpreted on the basis of the axial anisotropy induced by the stress-annealing conditions and its influence on the low-frequency magnetization processes of the alloys.

Isothermal annealing induced embrittlement of Zr 41.25Ti 13.75Ni 10Cu 12.5Be 22.5 bulk metallic glass

Zr 41.25Ti 13.75Ni 10Cu 12.5Be 22.5 bulk metallic glass has been isothermally annealed at four chosen temperatures for different holding times. Compared with its as-cast counterpart, the annealed alloys deform from ductile to brittle evident both in the morphology of fracture surface and the data of fracture stress and strain. From the point of view of classical fracture mechanics, the annealing induced embrittlement is considered to be due to the loss of plastic deformation ability in the crack tip zone resulting from the decrease of free volumes in the annealing process. The presence of nano-scale precipitates embedded in the amorphous matrix is found to increase the viscosity of the system and undermine the mechanical properties of the annealed alloys.

Isothermal annealing induced embrittlement of Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glass

Zr 41.25 Ti 13.75 Ni 10 Cu 12.5 Be 22.5 bulk metallic glass has been isothermally annealed at four chosen temperatures for different holding times. Compared with its as-cast counterpart, the annealed alloys deform from ductile to brittle evident both in the morphology of fracture surface and the data of fracture stress and strain. From the point of view of classical fracture mechanics, the annealing induced embrittlement is considered to be due to the loss of plastic deformation ability in the crack tip zone resulting from the decrease of free volumes in the annealing process. The presence of nano-scale precipitates embedded in the amorphous matrix is found to increase the viscosity of the system and undermine the mechanical properties of the annealed alloys.