Effects of solid-state annealing on the interfacial intermetallics between tin-lead solders and copper

The interfacial intermetallics between Cu and solder were studied for four Sn-Pb compositions at the annealing temperatures of 125°C, 150°C, and 175°C for up to 30 days. The η-phase (Cu₆Sn₅) layer formed during reflow continues to grow during annealing. An additional layer of ɛ-phase (Cu₃Sn) forms at the η/Cu interface after an incubation annealing time. The thickness results fit a power-law relationship against time with average exponents 0.69 and 0.44 for the η phase and the ɛ phase, respectively. On prolonged annealing, the proportions of the individual phases in the total layer reach a steady state.     

Investigation on the Impact of Seasonally Frozen Soil on Seismic Response of Bridge Columns

This paper presents the development of numerical models that investigated the seismic response of a simple two span prototype bridge system during warm and frozen temperatures. Models from both temperature conditions were subjected to a range of seismic intensities to examine the effect of seasonal freezing on the response of the system. Stiffness characteristics were defined using cyclic models of a bridge pier that were previously developed and validated using results from an experimental program on identical full-scale column-foundation units, which were tested during the summer and winter months. Dynamic characteristics of the seismic models were defined using approaches found in the literature. Frozen conditions increased the maximum bending moment and shear force demands for all seismic intensities, with nonlinearity in the column/foundation reducing the difference between the peak responses at higher intensities. At the depth of maximum foundation shear for the frozen model, demand was three times higher than the unfrozen for the 500-year return period and twice during the 2,500-year event. This is significant as one will assume shear is not critical at this location if the effects of frozen conditions are ignored. Apart from the smallest intensity event, increased peak lateral displacements were developed by the warm model down the length of the column and foundation. However, the displacement demand to capacity ratio was higher at the column top for the frozen model, exceeding the capacity during the 2,500-year return period event.     

A simplified analysis method for characterizing unbonded post-tensioned precast wall systems

In recent years, a significant effort has been placed towards understanding the lateral load behavior of unbonded post-tensioned precast wall systems because they are easier to construct and they exhibit desirable response characteristics, including self centering feature and minimal damage when subjected to large lateral displacements. However, the conventional section analysis cannot be applied to these walls because of the nonexistence of strain compatibility condition between the unbonded post-tensioning steel and concrete at the section level. This paper presents a simplified analysis method to characterize the monotonic response of unbonded post-tensioned precast wall systems and provides verification for the method using available test data. It is shown that the proposed simplified analysis method not only captures the lateral load behavior of walls, but also accurately quantifies the neutral axis depth, elongation in the post-tension steel, and deformation of the connector used in jointed wall systems as a function of lateral drift.     

Low‐Symmetry Monoclinic Phases and Polarization Rotation Path Mediated by Epitaxial Strain in Multiferroic BiFeO3 Thin Films

A morphotropic phase boundary driven by epitaxial strain has been observed in lead‐free multiferroic BiFeO₃ thin films and the strain‐driven phase transitions have been widely reported as iso‐symmetric Cc‐Cc by recent works. In this paper, it is suggested that the tetragonal‐like BiFeO₃ phase identified in epitaxial films on (001) LaAlO₃ single crystal substrates is monoclinic M_C. This M_C phase is different from the M_A type monoclinic phase reported in BiFeO₃ films grown on low mismatch substrates, such as SrTiO₃. This is confirmed not only by synchrotron X‐ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal‐like phase lie in the (100) plane, not the (11 0) plane as previously reported. A phenomenological analysis is proposed to explain the formation of M_C Phase. Such a low‐symmetry M_C phase, with its linkage to M_A phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO₃ films and shed light on a complete understanding of possible polarization rotation paths and enhanced multiferroicity in BiFeO₃ films mediated by epitaxial strain. This work may also aid the understanding of developing new lead‐free strain‐driven morphotropic phase boundary in other ferroic systems.     

Concept and Finite-Element Modeling of New Steel Shear Connectors for Self-Centering Wall Systems

Self-centering precast concrete walls have been found to provide excellent seismic resistance. Such systems typically exhibit low energy dissipation, requiring supplementary dissipating components to improve their seismic performance. Mild steel shear connectors can provide an economical energy dissipating element. The design and analysis of steel shear connectors for a new precast wall system has been undertaken. A series of finite-element analyses were conducted to investigate the behavior of different types of connectors. Emerged from these analyses is a oval-shaped connector (O-connector) that provided satisfactory force-displacement behavior and appeared well suited for the new wall system in high seismic regions. An extensive experimental test program was then conducted to verify the performance of the chosen O-connector, which confirmed the expected response with sufficient energy dissipation. The experimental data demonstrated good correlation with the finite-element model developed, providing satisfactory confidence in the finite-element technique used for the development of the different connectors.     

Effect of Ta nanobarrier in magnetron sputtering of Nd-doped SrBi2Ta2O9 thin films

SrBi2Ta2O9 (SBT) is a bismuth layered perovskite with attractive ferroelectric properties for random access memory applications. Our previous studies showed that Nd-doped SBT (SNBT) thin films exhibited an improved remnant polarization and reduced coercivity. This paper concentrates on the effect of Ta nanobarrier in between the SNBT and the Pt layers. Without the nanobarrier, severe bismuth diffusion is revealed by the secondary ion mass spectroscopy. However, with a nano layer (up to 2 nm) of Ta metal, the interfacial diffusion is effectively suppressed even at 800 degrees C. Details of the composition profiling, film crystallinity and remnant polarization are discussed in view of the nanobarrier thickness.     

Microstructural phase evaluation of high-nitrogen Fe–Cr–Mn alloy powders synthesized by the mechanical alloying process

In this study, the formation of Fe18Cr8MnxN alloys by mechanical alloying (MA) of the elemental powder mixtures was investigated by running the milling process under nitrogen and argon gas atmospheres. The effect of the milling atmosphere on the microstructure and phase contents of the as-milled powders was evaluated by X-ray diffraction and transmission electron microscopy. The thermal behavior of the alloyed powders was also studied by differential scanning calorimetry. The results revealed that in the samples milled under nitrogen, three different phases, namely ferrite (α), austenite (γ), and a considerable amount of amorphous phase are present in the microstructure. In contrast, in the samples milled under argon, the structure contains the dominant crystalline ferrite phase. By progression of MA under the nitrogen atmosphere, the ferrite-to-austenite phase transformation occurs; meanwhile, the quantity and stability of the amorphous phase increase, becoming the dominant phase after 72 h and approaching 83.7 wt% within 144 h. The quantitative results also showed that by increasing the milling time, grain refinement occurs more significantly under the nitrogen atmosphere. It was realized that the infused nitrogen atoms enhance the grain refinement phenomenon and act as the main cause of the amorphization and α-to-γ phase transformation during MA. It was also found out that the dissolved nitrogen atoms suppress the crystallization of the amorphous phase during the heating cycle, thereby improving the thermal stability of the amorphous phase.