Lepidiota Stigma Scale Video reconstruction

3D reconstruction of plasma-FIB sectioning of Lepidiota Stigma scales.Result of final year project. Stuart Robertson of the Loughborough Materials Characterisation Centre (https://www.lboro.ac.uk/research/lmcc/) performed the sectioning with project student Romy Owen who did all the reconstruction. Simon Martin looked on in admiration. </div

Evaluation of metallurgical risk factors in post-test, advanced 9%Cr creep strength enhanced ferritic (CSEF) steel

9wt.% Cr steels are widely used in the design and fabrication of thick section components in combined cycle or coalfired applications for working temperatures of 600~650°C. This family of materials possesses a martensitic microstructure stabilized by precipitates. The presence of nitrides, inclusions or evolution of second-phase particles may increase the metallurgical risk to creep. The chemical composition and microstructural evolution of 9wt.% Cr steels contribute to thermal stability and long-term performance. In some specialist alloys, Ta is added to the composition which causes the formation of fine MX precipitates which are only present at the nanometre scale in tempered martensite, which hinder the recovery of dislocations and the migration of laths to extend creep life. However, the presence of large Ta-containing particles or inclusions in the 9wt.% Cr steels may have a detrimental effect on its creep performance, as they may act as preferred sites for cavity nucleation. To fully appreciate the development of damage in these steels, it is necessary to link the pre- and post-test conditions, evaluate damage in the parent metal, develop procedures that provide consistency of results, and obtain statistically relevant data. The evolution of the Ta-containing phase has been tracked and quantified using a variety of correlative characterization approaches. Utilizing focused ion beam microscopy and two-dimensional electron-based microscopic characterisation, three-dimensional tomography has identified a strong relationship between creep cavities and Tacontaining phases from the early stages of creep.</p

Defect formation and mitigation in Cu/Cu joints formed through transient liquid phase bonding with Cu-Sn nanocomposite interlayer

 Joining based on transient liquid phase bonding (TLPB) has a great prospect in microelectronic packaging. However, this process is plagued with its low throughput. In this work, we designed and utilised a preformed CuSn nanocomposite interlayer (Cu-Sn NI) to speed up the TLPB process. Comparing with the uses of a Sn interlayer only, the preformed Cu-Sn NI enabled the bonding process to be accelerated by >20 times. Furthermore, instead of columnar Cu6Sn5 grains, Cu-Sn intermetallic compounds joints formed with Cu-Sn NIs were mostly filled by refined equiaxed Cu6Sn5 grains with an average size of ~1.6 μm. As a result, the shear strength of IMC joints achieved with Cu-Sn NI was found to be higher than those bonded with Sn interlayer. However, various kinds of defects, one of the main drawbacks in TLPB, were still found from this unique bonding process, which is likely to deteriorate the performances and long-term reliability of resultant TLPB joints. This study aims to observe and analyse various defects formed during TLPB with Cu-Sn NI, hence propose possible mitigation solutions to prevent their occurrence and consequently improve the reliability of the joints obtained by TLPB with Cu-Sn NIs.  </p

Microstructural and mechanical characteristics of Cu-Sn intermetallic compound interconnects formed by TLPB with Cu-Sn nanocomposite

Transient liquid phase bonding (TLPB) is a promising technology for three-dimensional integration of circuits (3D IC), but it can be slow and less productive. A novel Cu-Sn nanocomposite interlayer (Cu-Sn NI) composed of Sn matrix with an embedded Cu nanowire array prepared by electrodeposition can significantly accelerate the bonding process, approximately by 20 times. Bonding time with a Cu-Sn NI can be as short as ∼2 min to achieve a full Cu-Sn intermetallic compound (IMC) joints, whereas it can take ∼60 min with a pure Sn interlayer of the same thickness under the same bonding conditions (250 °C). Unlike the columnar Cu6Sn5 grains commonly formed with Sn interlayer, refined equiaxed Cu6Sn5 grains with an average size of ∼1.6 µm are found to be formed with Cu-Sn NI. Such grain refinement has significantly contributed to the improvement of shear strength of IMC joints formed with Cu-Sn NI (23.1 ± 3.3 MPa), higher than those bonded with pure Sn interlayer (17.9 ± 2.1 MPa). The underlying mechanisms of the new TLPB process and the formation of finer microstructure when bonding with Cu-Sn NIs are also illuminated and validated based on the experimental observation.</p

Thermo-mechanical characteristics and reliability of die-attach through self-propagating exothermic reaction bonding

Self-propagating exothermic reactions (SPER) provide intense localized heat sufficient for bonding metals or alloys with minimal heat excursion to the components, which shows great potential for the die attach in power electronics packaging. However, the reliability of such formed joints is yet to be fully understood owing to a wide range of defects involved in the instantaneous propagating reaction and heating/cooling. In this work, the finite element analysis is performed to understand the thermal transfer and mechanical responses of materials to the SPER bonding for the die attach of Si device onto direct bonded copper (DBC) substrate with Sn-3.0Ag-0.5Cu solder. The simulation has been validated using the temperature distribution in SPER bonding, which shows a good agreement with the actual measured results. Moreover, a systematic investigation on the mechanical responses due to thermal mismatch reveals their effects on the thermal stress of interfaces and bonding reliability

Microstructure characterisation of electromagnetic pulse welded high strength aluminium alloys

Electromagnetic pulse welding is a high-velocity impact joining process employed with the intention of forming fast and effective solid-state bonds. Electron microscopy techniques, including SEM and TEM, revealed that bonding was not fully accomplished in the solid state; instead, local melting can occur. These locally melted areas likely occur around the point of first contact during the welding process and are associated with a debonded region that runs alongside or through the centre of melted zones. Microstructural characterisation showed dispersoid-free regions, columnar grains, epitaxial growth, and localised increases in O, Fe, Si, and Mn content in locally melted areas. This region contrasts with the solid-state bonded region, in which the interface exhibited sub-micron grains.</p

Further enhancement of thermal conductivity through optimal uses of h-BN fillers in polymer-based thermal interface material for power electronics

Due to the demand of miniaturization and increasing functionality in power electronics, thermal dissipation becomes a challenging problem for thermal management and reliability. To enable effective heat transfer across the interconnect interfaces, thermal interface materials (TIMs) are required. Electrically insulating TIMs are primarily polymer-based composites which use conductive fillers to enhance thermal conductivity (TC). In this study, the optimal hybrid filler constituents, achieved through mixing spherical and platelet h-BN particles with different ratios, in polymer-based TIM was predicted using finite element (FE) simulations. The underpinning mechanisms of the variation in TC of the TIMs were analyzed from the temperature distribution patterns and micro heat flux paths. Results showed that with the same total volume fraction of h-BN, mixed spherical and platelet h-BN fillers of a certain ratio can further improve the thermal properties of the TIMs compared with those with spherical or platelet h-BN particles alone

Investigation of thermal effect on solidification in Sn/Cu interconnects during self-propagating exothermic reaction bonding

Self-propagating exothermic reaction (SPER) of alternating metal nanolayers provides intense localised heat that allows bonding of metals or alloys under ambient temperature. However, the formation of the bonds through the rapid heating and cooling confined at the bonding interfaces is a non-equilibrium process, and the thermal effect on solidification and manufacturing reliability is yet to be understood. In this work, the Cu/Sn-nanofoil-Sn/Cu interconnects (where Sn is a solder layer) prepared via SPER of Ni/Al nanofoil are studied by numerical simulations and experiments to understand the thermal transfer and its effect on the solidification. It has been found that the SPER completes within a few milliseconds, the temperature at solder/Cu interface can be higher than the melting point of solder, and the cooling rate can be as high as 1.5 × 107 ◦C/s, the maximum temperature gradient can reach 5.40 × 107 ◦C/m. The microstructure predicted by simulation agrees well to the experimental results: the columnar dendrites are formed in the solder during the cooling stage, and the columnar structures prefer to form and grow in the solder region due to the high cooling rate

Structural integrity and damage of ZrB₂ ceramics after 4 MeV Au ions irradiation

Ultra-high temperature ceramics have been considered as good candidates for plasma facing materials due to their combination of high melting point, high strength and hardness, high thermal conductivity as well as good chemical inertness. In this study, zirconium diboride has been chosen to investigate its irradiation damage behavior. Irradiated by 4 MeV Au2+ with a total fluence of 2.5 × 1016/cm2, zirconium diboride ceramic shows substantial resilience to irradiation-induced damage with its structural integrity well maintained but mild damage at lattice level. Grazing incident X-ray diffraction evidences no change of the hexagonal structure in the irradiated region but its lattice parameter a increased and c decreased, giving a volume shrinkage of ∼0.46%. Density functional theory calculation shows that such lattice shrinkage corresponds to a non-stoichiometric compound as ZrB1.97. Electron energy-loss spectroscopy in a transmission electron microscope revealed an increase of valence electrons in zirconium, suggesting boron vacancies were indeed developed by the irradiation. Along the irradiation depth, long dislocations were observed inside top layer with a depth of ∼750 nm where the implanted Au ions reached the peak concentration. Underneath the top layer, a high density of Frank dislocations is formed by the cascade collision down to a depth of 1150 nm. All the features show the potential of ZrB2 to be used as structural material in nuclear system

Formation of twins in AlON material and its effects on the Vickers hardness and fracture toughness

Aluminum oxynitride (γ-AlON) powders were synchronously synthesized by carbothermal reduction-nitridation (CRN) and high-temperature solid state reaction (SSR) methods. Twin structures existing in γ-AlON powders and transparent AlON ceramics were investigated by systematically employing electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). It was found that the twin structures in synthetic powder and transparent ceramics exhibit quite different microstructural features; binary twin structures were widely observed in powders and ceramics, whereas the sandwich-like twin structure was produced only in transparent ceramics. High-resolution electron backscattered diffraction (HREBSD) was used to determine the residual elastic stress distributions in AlON powders and ceramics. Discrepancies in the magnitude and distribution of the residual stress between binary twins and sandwich-like twins were fully clarified. The formation of two kinds of twins in powders and ceramics was discussed in detail. The Vickers hardness and its corresponding indentation size effect (ISE) were studied in CRN-AlON and SSR-AlON ceramics. The effects of the twin structure on the Vickers hardness and fracture toughness were investigated simultaneously