Single shear lap specimens were subjected to creep, isothermal aging, and thermomechanical fatigue (TMF). Scanning electron microscopy micrographs of previously polished specimens revealed changes in surface morphology. Orientation imaging microscopy was carried out on the same specimens to study the microstructural evolution and crystal orientation changes. As-fabricated joints consistently show a preferred crystal orientation with a few minority orientations with highly preferred misorientations. Alloy additions caused an increase in the number of statistically significant crystal orientations and misorientations. The solidification microstructure was unchanged due to room-temperature creep. Aging caused development and motion of well-defined subgrain boundaries and removal of most minority orientations. TMF causes heterogeneous refinement of the microstructure that accounts for the localized grain boundary sliding in regions of high strain concentration. This study implies that the lead-free solder joints are not polycrystals, but multicrystals, so that deformation is very heterogeneous and sensitive to strain and temperature history.

Single shear lap joints were made with four different solders, Sn-Pb and Sn-Ag eutectic solders, and their composites containing about 20 vol.% in-situ Cu6Sn5 intermetallic phases about 3-8 micrometers in diameter. Two sets of experiments were performed: In the first set, all of the above four solder joints were aged at 150°C for periods ranging to 5000 h and the intermetallic growth was monitored periodically. In the second set, each of the above four solder joints was aged at five different temperatures for 4000 h. The interfacial layers between solders and the Cu substrate were examined using optical and scanning electron microscopy. The growth kinetics of intermetallic interfacial layers formed between solder and Cu substrate was characterized. The effect of in-situ Cu 6 Sn 5 intermetallic phases on the growth rate is discussed. The growth rate of the intermetallic layers in the eutectic Sn-Pb composite was slower for the first 150 h as compared to the eutectic Sn-Pb non-composite. The growth rate of the intermetallic layers were similar for both the eutectic Sn-Ag and eutectic Sn-Ag composite throughout the aging duration. The activation energies for Cu 6 Sn 5 layer growth for the eutectic Sn-Pb and Sn-Ag solder joints are evaluated to be 111 kJ/mol and 116 kJ/mol, respectively. The eutectic Sn-Pb and Sn-Ag composite solder joints exhibit higher activation energies of 161 kJ/mol and 203 kJ/mol.

Objective: To investigate how racial/ethnic density and residential segregation shape the uneven burden of COVID-19 in US counties and whether (if yes, how) residential segregation moderates the association between racial/ethnic density and infections. Design: We first merge various risk factors from federal agencies (e.g. Census Bureau and Centers for Disease Control and Prevention) with COVID-19 cases as of June 13th in contiguous US counties (N = 3,042). We then apply negative binomial regression to the county-level dataset to test three interrelated research hypotheses and the moderating role of residential segregation is presented with a figure. Results: Several key results are obtained. (1) Counties with high racial/ethnic density of minority groups experience more confirmed cases than those with low levels of density. (2) High levels of residential segregation between whites and non-whites increase the number of COVID-19 infections in a county, net of other risk factors. (3) The relationship between racial/ethnic density and COVID-19 infections is enhanced with the increase in residential segregation between whites and non-whites in a county. Conclusions: The pre-existing social structure like residential segregation may facilitate the spread of COVID-19 and aggravate racial/ethnic health disparities in infections. Minorities are disproportionately affected by the novel coronavirus and focusing on pre-existing social structures and discrimination in housing market may narrow the uneven burden across racial/ethnic groups.

Objectives We provide one of the first population-based studies of variation in dementia by marital status in the United States. Method We analyzed data from the Health and Retirement Study (2000–2014). The sample included 15,379 respondents (6,650 men and 8,729 women) aged 52 years and older in 2000 who showed no evidence of dementia at the baseline survey. Dementia was assessed using either the modified version of the Telephone Interview for Cognitive Status (TICS) or the proxy’s assessment. Discrete-time hazard regression models were estimated to predict odds of dementia. Results All unmarried groups, including the cohabiting, divorced/separated, widowed, and never married, had significantly higher odds of developing dementia over the study period than their married counterparts; economic resources and, to a lesser degree, health-related factors accounted for only part of the marital status variation in dementia. For divorced/separated and widowed respondents, the differences in the odds of dementia relative to married respondents were greater among men than among women. Discussion These findings will be helpful for health policy makers and practitioners who seek to better identify vulnerable subpopulations and to design effective intervention strategies to reduce dementia risk.

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