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Abstract: The microstructure and adhesion strength of the Sn–9Zn–xAg lead-free solders wetted on Cu substrates have been investigated by differential scanning calorimetry, optical microscopy, scanning electron microscopy, energy dispersive spectrometry and pull-off testing. The liquidus temperatures of the Sn–9Zn–xAg solder alloys are 222.1, 226.7, 231.4 and 232.9°C for x =2.5, 3.5, 5.0 and 7.5wt%, respectively. A flat interface can be obtained as wetted at 350°C at a rate of 11.8mm/s. The adhesion strength of the Sn–9Zn–xAg/Cu interfaces decreases from 23.09±0.31 to 12.32±1.40MPa with increasing Ag content from 2.5 to 7.5wt% at 400°C. After heat treatment at 150°C, the adhesion strength of the Sn–9Zn–xAg/Cu interface decreases with increasing aging time. [Copyright &y& Elsevier]

The effect of Si content on the intergranular corrosion resistance of SUS 309L stainless steels was investigated by 10 wt.% oxalic acid test, 65 wt.% boiling nitric acid test, double loop electrochemical potential reactivation (DLEPR) method, optical microscope (OM) and scanning electronic microscope (SEM). A maximum corrosion rate, weight loss of 2.80 g m−1 h, was obtained when SUS 309L stainless steel containing 0.73 wt.% Si was subjected to heat treatment at 800 °C for 1 h. It shows that the Cr depletion zone formed due to σ phase precipitation. The same result is obtained in DLEPR test and the ditch microstructure after oxalic test was found in samples that showed the minimum corrosion rate. [Copyright &y& Elsevier]

Abstract: This microstructural evolution of the δ/σ/γ phase transformations in SUS 309LSi stainless steel has been investigated as aged under different nitrogen atmospheric ratios from 0% up to 80%. The results showed that the increment of the nitrogen/air ratio accelerated the δ→γ phase transformation and that of δ→σ was inhibited. There are four precipitation sites of σ-phase at 800°C, which were δ/γ interface boundary, triple junction, corner, and cellular. On the other hand, the δ/σ phase transformation was the dominant factor that caused the precipitation of σ-phase. The mapping analysis indicated that the δ-ferrite and σ-phase in the SUS 309 stainless steel were Si-rich and Cr-rich phases, respectively. It leaded to the decrease of enrichment of Cr when the nitrogen/air ratio increased. The microstructural observation also exhibited the phase transformations among δ-ferrite, σ-phase and γ-phase were very different under various conditions. [Copyright &y& Elsevier]

The influence of 5 GPa-level high pressure on the microstructure and mechanical properties of Al–7Si–3Cu-0.4 Mg and Al–7Si–3Cu-0.4Mg-0.5Ge alloys is investigated. It is found that α-Al dendrites disappear owing to the enhanced growth interfacial stability induced by high pressure. Unconventional nanocrystalline eutectics emerge in the intragranular region, while Si phases in all forms of eutectics display significantly modified structures. The pressure-induced influence on the microstructure evolution of alloy is further explored concerning thermodynamics and kinetics changes. After the direct aging treatment, fine Si particles precipitate densely and uniformly along the <100> Al directions in the Al matrix in both alloys. The APT analysis shows that these Si precipitates are enriched with Mg and segregated by Cu. The role of enhanced solid solubility and solute interactive effect on the formation of nanoscale Si rods is demonstrated. The Al–7Si–3Cu-0.4 Mg alloy fabricated under 5 GPa exhibits the highest compressive strength of 863.6 MPa and microhardness of 214.3 HV after aging. The findings reported in this paper provide a potential way to develop aluminum-silicon-based alloys with improved performance by introducing high pressure. [ABSTRACT FROM AUTHOR]

The combination of excellent electrical conductivity and low cost makes copper a good selection for interconnect materials. However, rapid oxidation of Cu nanoparticles especially at high temperatures is a fatal demerit. To improve the oxidation resistance and realized mass production, Cu@Ag core-shell submicron particles were prepared using commercial copper oxide particles through a low temperature reduction method and subsequent electroless-plating to form Ag shells. Thermal analytical results suggest that Ag coated Cu particles show improved anti-oxidation ability. Mixed with Ag submicron particles obtained from thermal spray pyrolysis, the electrical resistivity of the sintered Cu-Ag composite pastes reaches 10.4 μΩ cm under a reductive atmosphere. Under the bonding pressure of 10 MPa at 275 °C for 30 min, robust Cu to Cu bonding can be achieved with the Cu-Ag composite pastes, for which the shear strength of the joints reaches 32.7 MPa, and it remains 28.2 MPa as the bonding pressure is reduced to 5 MPa. It was also demonstrated that the joints thus formed have superior elevated temperature strength, and excellent reliability subjected to high temperature storage at 250 °C as well as thermal cycling ranged from −65 °C to 150 °C. [ABSTRACT FROM AUTHOR]

The influence of high pressure on the microstructural evolution and mechanical properties of Al–Si–Cu–(Ge)–(Mg) alloys was investigated. With the increased solidification pressure, the secondary dendrite arm spacing (SDAS) of Al decreases, and the eutectic Si undergoes a transformation from the coarse acicular morphology to the coral-like morphology. The well-refined microstructure is attributed to the accelerated nucleation rate and the restrained crystal growth rate under high pressure. The morphological transformation of eutectic Si is ascribed to the synergetic effect of the high undercooling and Cu segregation at the outer surfaces of Si. Besides, the study shows that multiple nano-sized particles emerge in the alloys solidified under high pressure due to the high solid solubility of elements and increased nuclei. These nano-sized particles act as dominating strengthening phases leading to enhanced hardness and compressive strength compared with alloys solidified under atmospheric pressure. The as-cast alloy MG solidified under 4 GPa has the highest mechanical properties with a hardness of 135.2 HV and compressive stress of 721 MPa. The strengthening contribution of variable nano-sized particles was discussed. These results indicate that high pressure can be used as an effective method to introduce structural refinement and nano-sized strengthening phases, thus providing a potential way to improve mechanical properties of Al–Si–Cu-based alloys. [ABSTRACT FROM AUTHOR]