HighP–TNano-Mechanics of Polycrystalline Nickel

We have conducted highP–Tsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume–pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify “micro/local” yield due to high stress concentration at the grain-to-grain contacts and “macro/bulk” yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highP–Tconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni

Role of Al2O3 particulate reinforcements on precipitation in 2014 Al-matrix composites

This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Precipitation in commercial aluminum alloy 2014, without and with alumina particulate reinforcements, was studied using microhardness, electrical resistivity, differential scanning calorimetry (DSC), and transmission electron microscopy. The precipitation sequence in 2014 Al was confirmed to be [alpha][sub ss] [yields] [alpha] + GPZ [yields] [alpha] + [lambda][prime] [yields] [alpha] + [lambda][prime] + [theta][prime] [yields] [alpha] + [lambda] (AlCuMgSi) + [theta] (CuAl[sub 2]). Reinforcement addition decreased the time to peak hardness, but also reduced the peak matrix microhardness. This was traced to a decrease in the amount of [lambda][prime] formed in the composites. Further, it was observed that while Guinier-Preston (GP) zone and [theta][prime] formations are accelerated in the composites, [lambda][prime] precipitation is decelerated. The acceleration is attributable primarily to enhanced nucleations resulting from an increase in the matrix dislocation density due to coefficient of thermal expansion (CTE) mismatch between the matrix and the reinforcements, whereas the deceleration is associated with a decrease of low-temperature solute diffusivity due to absorption of vacancies at dislocations and interfaces. It was also observed that the degree of overall acceleration in hardening and the reduction in peak matrix microhardness with reinforcement addition decreased with decreasing aging temperatures. The causal relationships of these observations with the associated mechanisms are discussed.Non

MICROSTRUCTURAL ANALYSIS OF FRACTURE-TOUGHNESS VARIATION IN 2XXX-SERIES ALUMINUM-ALLOY COMPOSITES REINFORCED WITH SIC WHISKERS

The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study. Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less. Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance, l*, when manganese-containing particles are absent. 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions. The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too. Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition.open1118sciescopu