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Precipitates in an aged Al-Cu-Mg-Ag alloy have been characterised using atomic-resolution energy-dispersive X-ray spectroscopy (EDXS) scanning transmission electron microscopy (STEM) and high-angle annular dark-field (HAADF) STEM. A metastable precursor phase, designated Ω′, which forms as thin plates on {111} α planes of the α-Al matrix phase, is found for Ω precipitate which is the key strengthening constituent in the Al-Cu-Mg-Ag alloy. The Ω′ phase has a hexagonal structure that is distinguishably different from that of the Ω precipitate, and this hexagonal structure is isomorphous to that of the single unit-cell thick T 1 precipitate plates in Al-Cu-Li alloys. Our proposed hexagonal structure is also supported by density functional theory (DFT) calculations. One interesting feature of Ω′ phase is that its thickness is invariably one unit-cell height, ∼0.915 nm thick. In contrast to the commonly accepted view that shear is not involved in diffusional phase transformations, the formation of Ω′ phase involves an appreciably large shear component, which is equivalent to two 30° Shockley partial dislocations, and the magnitude of this shear component is significantly larger than the dilatational strain associated with the α-Al → Ω′ structural transformation. With continued ageing, the metastable Ω′ phase transforms in-situ into the Ω phase. Compared to the direct α-Al → Ω structural transformation, the existence of Ω′ considerably lowers the energy barrier to the nucleation of Ω via the reduction of the shear strain. [Display omitted] [ABSTRACT FROM AUTHOR]

The stress-aging method is proposed to improve the dimensional stability of Al-Cu-Mg (2024 aluminum) alloy. Both the reduction of residual stress and the acceleration of precipitates are investigated during the stress-aging process. It is observed that some S′ particles precipitate in the alloy during stress-aging. With the external stress increase to 100 MPa, the residual stress and thermal strain (ɛ c ) decrease, while the micro-yield strength increases significantly. This is attributed to that the dislocations are effectively pinned by the uniformly distributed precipitates and the stored elastic energy diminishes remarkably, which leads to a considerable improvement of the dimensional stability of 2024 aluminum alloy. However, when further increases the external stress to 150 MPa, due to the coarsened precipitates, the residual stress and thermal strain increase, while the micro-yield strength decreases slightly. [ABSTRACT FROM AUTHOR]

• Deformation mechanism under shock compression of an additively manufactured HEA has been revealed. • Higher strain rate promotes the phase transitions in the HEA. • Elemental segregation of Mn and Ni strongly influences the phase transitions in the HEA. [Display omitted] Shock compression of an additively manufactured high-entropy alloy, Cr 10 Mn 10 Fe 60 Co 10 Ni 10 , is investigated via gas-gun plate impact. The postmortem sample shocked to 11.3 GPa is characterized with electron back-scattered diffraction and transmission electron microscopy; two phase transitions are observed, the face-centered cubic (FCC) to hexagonal close-packed (HCP) transition, and the FCC to body-centered cubic (BCC) transition. The HCP and BCC phases with different variants are observed following specific orientation relationships with the FCC matrix. The phase transitions are strongly affected by elemental segregation during the additive manufacture (the HCP and BCC phases are absent in the Mn and Ni enriched areas), and one possible reason is the increase of local stacking fault energy caused by the Mn and Ni enrichment. [ABSTRACT FROM AUTHOR]

In this study, two different microstructures were obtained after subsolvus solution treatment (910 °C + 550 °C) and supersolvus solution treatment (1010 °C + 550 °C) in TC21 alloy. The relationship of microstructure with room and high temperature properties was studied. The results showed that at room temperature, both samples after two treatments had high hardness. At room temperature, the subsolvus sample had high tensile strength of 1070 MPa and moderate plasticity and impact toughness, while the supersolvus sample was hard and brittle. The high strength and toughness of subsolvus sample was owing to its clear basket-weave structure within finer β grains which contributes to fine grain strengthening and the increase of crack tortuosity. Poor plasticity and toughness of supersolvus sample could be mainly attributed to its incomplete basket-weave structure, coarse β grains, Widmannstatten α and some coarse α-phase in the grain boundary. Both samples showed excellent high temperature tensile properties and stability at 400 °C, the tensile strength was still as high as 1000 MPa while plasticity was significantly improved. With the tensile temperature increasing to 600 °C, the alloys showed obvious softening as the tensile strength significantly decreased and elongation increased. • TC21 alloys after subsolvus heat treatment showed an obvious basket-weave structure. • The TC21 alloy after subsolvus treatment showed high fracture strength of 1070 MPa with the elongation of 8 %. • Both subsolvus and supersolvus samples showed excellent high temperature tensile properties at 400 °C and 600 °C. • The tensile strength was still as high as 1000 MPa while plasticity was significantly improved during 400 °C tensile. • The tensile strength significantly decreased to about 500 MPa as elongation increased during 600 °C high temperature tensile. [ABSTRACT FROM AUTHOR]

A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt%) alloy is subjected to a series of thermal and mechanical treatments involving solution treatment, artificial ageing to peak hardness, high pressure torsion (HPT) and a second artificial ageing. During HPT precipitates dissolve and during the final post-HPT ageing, the supersaturated solid solution decomposes and solutes segregate at grain boundaries. By employing this T6 + HPT + T5 treatment, the hardness increases to 156 ± 1 HV, which is much higher than that achieved by any other reported combination of thermal and thermo-mechanical processing of Mg alloys. The ultra-high hardness is due to the combined effects of solute segregation, grain refinement and high dislocation density. [ABSTRACT FROM AUTHOR]

Three texture-free CoCrNi-based medium-entropy alloys (MEAs) with similar grain sizes, CoCrNi, (CoCrNi) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 , are investigated under plate impact loading. Free surface velocity histories are obtained along with microstructure characterizations of postmortem samples. The influences of the addition of minor elements Al and Ti on the microstructures, dynamic mechanical properties and underlying deformation and damage mechanisms of MEAs are elucidated. Compared to the single-phase CoCrNi MEA, spherical nanosized L 1 2 precipitates and micron sized body-centered-cubic phase are found in (CoCrNi) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 , respectively. The addition of minor elements Al and Ti leads to an increase in the dynamic yield stress approximately by 70% (to 1.0 GPa) and 130% (to 1.4 GPa) for (CoCrNi) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 , respectively, due to the solid solution strengthening and precipitate strengthening. Multiple deformation mechanisms are observed. Planar dislocation glide and stacking faults are the dominant deformation mechanisms for all the MEAs. However, deformation twinning is deactivated in (CoCrNi) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 with small additions of Al and Ti. For spallation, damage is ductile in nature for CoCrNi and (CoCrNi) 94 Al 3 Ti 3 , but brittle for (CoCrNi) 90 Al 5 Ti 5 . Under similar shock stress, the spall strength of (CoCrNi) 94 Al 3 Ti 3 is comparable to that of CoCrNi, ∼ 150% higher than that of (CoCrNi) 90 Al 5 Ti 5 because intergranular brittle fracture activation does not require substantial plastic deformation. The minor addition of Al and Ti in medium-entropy alloys significantly affects their dynamic mechanical properties, deformation and damage mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Drop impact reliability has become an important criterion when assessing the reliability of portable electronics. Finite element software such as ABAQUS is used to simulate solder joints during drop-impact tests and determine the drop-impact reliability of the product. This modelling requires the rate-dependent constitutive properties of the solder in the range of strain rates between 0.1 s−1 to 300 s−1. Unfortunately, the constitutive properties of most solder materials in this range of strain rates have not been characterized. This paper presents the mechanical properties of four solder alloys — 63Sn37Pb, Sn1.0Ag0.1, Sn3.5Ag, and Sn3.0Ag0.5Cu at strain rates between 0.005 s−1 and 300 s−1. The lead-free solder alloys tested exhibited greater strain rate sensitivity compared to 63Sn37Pb. In addition, both the lead-free solders with high Ag content (≥3%) exhibited much higher flow stresses compared to 63Sn37Pb. The higher flow stress could explain the lower reliability of these joints in drop tests as a result of shifting of the failure site from the ductile solder to the brittle intermetallic compound [Wong EH, Rajoo R, Mai Y-W, Seah SKW, Tsai KT, Yap LM. Proc. 55th Elec. Comp. & Tech. Conf. 2005;1202–9]. Based on the data obtained, Sn1.0Ag0.1 seems to be the most suitable lead-free alloy to replace eutectic tin-lead in products where drop-impact performance is required. [Copyright &y& Elsevier]