Temperature Dependence of the Dynamics of Portevin-Le Chatelier Effect in Al-2.5%Mg alloy

Tensile tests were carried out by deforming polycrystalline samples of Al-2.5%Mg alloy at four different temperatures in an intermediate strain rate regime of 2x10-4s-1 to 2x10-3s-1. The Portevin-Le Chatelier (PLC) effect was observed throughout the strain rate and temperature region. The mean cumulative stress drop magnitude and the mean reloading time exhibit an increasing trend with temperature which is attributed to the enhanced solute diffusion at higher temperature. The observed stress-time series data were analyzed using the nonlinear dynamical methods. From the analyses, we could establish the presence of deterministic chaos in the PLC effect throughout the temperature regime. The dynamics goes to higher dimension at a sufficiently high temperature of 425K but the complexity of the dynamics is not affected by the temperature.Comment: 18 pages, 8 figures; accepted in Met. Mater. Trans.

Mechanical Behavior of Non-bonded Fiber Networks in Compression

AbstractThe mechanical behavior of an ensemble of fibers which are not bonded to each other subjected to compression is studied using a numerical model. It is shown that the stress-strain curve is a power law and the strain energy is stored predominantly in the bending deformation mode of the fibers. Hysteresis is observed when the system is subjected to cyclic loading-unloading, with a stable hysteresis loop being obtained after few cycles. The deformation is not smooth; rather it is dominated by instabilities which lead to pronounced stress drops. These instabilities are due to relative fiber sliding and rearrangement and a correlation is observed between stress drops and collective sliding events. The resulting jerky flow is observed even in absence of friction between fibers and is a result of the evolution of the system over a rough energy landscape during compression

Al-SiC nanocomposites produced by ball milling and spark plasma sintering

In this work Al-SiC nanocomposites were prepared by high energy ball milling followed by spark plasma sintering of the powder. For this purpose Al micro-powder was mixed with 50 nm diameter SiC nanoparticles. The final composites had grains of approximately 100 nm dimensions, with SiC particles located mostly at grain boundaries. To characterize their mechanical behavior, uniaxial compression, micro- and nano-indentation were performed. Materials with 1vol% SiC as well as nanocrystalline Al produced by the same means with the composite were processed, tested and compared. AA1050 was also considered for reference. It was concluded that the yield stress of the nanocomposite with 1 vol% SiC is 10 times larger than that of regular pure Al (AA1050). Nanocrystalline Al without SiC and processed by the same method has a yield stress 7 times larger than AA1050. Therefore, the largest increase is due to the formation of nanograins, with the SiC particles’ role being primarily that of stabilizing the grains. This was demonstrated by performing annealing experiments at 150°C and 250°C for 2h, in separate experiments