RESUMO
Microstructures and age-hardening phenomena of directly aged (artificial aged) AlSi10Mg alloys fabricated by laser powder bed fusion (LPBF) were characterized using scanning transmission electron microscopy, atom probe tomography, and Vickers hardness testing. The microstructure derived from overlapping melt pools has a full cellular structure consisting of eutectic Si walls surrounding α-Al cells. In the initial stage of aging, solute clusters with density on the order of 1024/m3 were formed in α-Al cells. By prolonging the aging time further, fine Si particles of about 50 nm in diameter precipitated. Before Si precipitation, the hardness of the aged sample was clearly greater than that of the as-built state. With further aging time, the hardness increased further because of the Si precipitation. Cluster analysis revealed that the number density and the size of clusters increased from as-built state by aging, whereas the types of the solute clusters remained almost unchanged by aging. The results indicate that the nanoscale clusters within the α-Al cells, which increase via aging, produce age-hardening effect.
RESUMO
α-Al grains surrounded by Al-Si eutectic as the substructures in AlSi10Mg alloy fabricated by selective laser melting (SLM) were investigated in detail using three-dimensional atom-probe and transmission electron microscopy. Quantitative analysis of α-Al revealed (1) the formation of Mg clusters and Mg-Si co-clusters with a density in the order of 1023 /m3 in α-Al of the as-built state caused by the complicated thermal history, and (2) a supersaturated Si content that significantly exceeded the maximum solubility due to rapid solidification, during the SLM process.
RESUMO
Motor imagery (MI), a mental simulation of voluntary motor actions, has been used as a training method for athletes for many years. It is possible that MI techniques might similarly be useful as part of rehabilitative strategies to help people regain skills lost as a consequence of diseases or stroke. Mental activity and stress induce several different autonomic responses as part of the behavioral response to movement (e.g., motor anticipation) and as part of the central planning and preprogramming of movement. However, the interrelationships between MI, the autonomic responses, and the motor system have not yet been worked out. The authors compare a number of autonomic responses (respiration, heart rate, electro skin resistance) and motoneuron excitability (soleus H-reflex) in elite and nonelite speed skaters during MI. In contrast to the nonelite athletes, MI of elite speed skaters is characterized by larger changes in heart rate and respiration, a greater reliance on an internal perspective for MI, a more vivid MI, a more accurate correspondence between the MI and actual race times, and decreased motoneuron excitability. Two observations suggest that the changes in the autonomic responses and motoneuron excitability for the elite speed skaters are related to the effects of central motor programming: (1) there was no correlation between the autonomic responses for MI and those recorded during mental arithmetic; and (2) mental arithmetic did not significantly alter motoneuron activity. It is suggested that in elite speed skaters, the descending neural mechanisms that reduce motoneuron excitability are activated even when full, vivid MI is performed internally. These inhibitory responses of the motor system may enhance actual motor performance under conditions of remarkably high mental stress, such as that which occurs in the Olympic games.
