Precipitation of bcc nanocrystals in bulk Mg–Cu–Y amorphous alloys

Coexistent amorphous and nanoscale bcc-Mg7Li3 phases were found in slowly quenched alloys of the Mg65–xLixCu25Y10 system containing 3 to 15 at.% Li. The crystallization behavior of these alloys has been studied. The grain size of the nanocrystalline bcc phase which is formed ranges from 2 to 20 nm. The volume fraction of nanocrystalline phases as well as the grain size of the nanocrystals increase as the Li composition increases. Transmission electron microscopy studies suggest that the alloy exhibits phase separation in the undercooled liquid state and that the nucleation and growth of the bcc-nanocrystals is related to the phase separation. Some characteristic thermal properties of the glassy phase are presented, and the composition dependence of Tg and Tx is discussed. It is concluded that the addition of a small amount of Li is essential for the production of a bcc nanocrystalline phase in the Mg–Li–Cu–Y system

Temperature Control via Affine Nonlinear Systems for Intermediate Point of Supercritical Once-Through Boiler Units

For the operation of the supercritical once-through boiler generation units, the control of the temperature at intermediate point (IPT) is highly significant. IPT is the steam temperature at the outlet of the separator. Currently, PID control algorithms are widely adopted for the IPT control. However, PID cannot achieve the optimal performances as the units’ dynamic characteristic changes at different working points due to the severe nonlinearity. To address the problem, a new control algorithm using affine nonlinear system is adopted for a 600 MW unit in this paper. In order to establish the model of IPT via affine nonlinear system, the simplified mechanism equations on the evaporation zone and steam separator of the unit are established. Then, the feedback linearizing control law can be obtained. Full range simulations with the load varying from 100% to 30% are conducted. To verify the effectiveness of the proposed control algorithm, the performance of the new method is compared with the results of the PID control. The feed-water flow disturbances are considered in simulations of both of the two control methods. The comparison shows the new method has a better performance with a quicker response time and a smaller overshoot, which demonstrates the potential improvement for the supercritical once-through boiler generation unit control