Free Energy Calculation of Al-Li-Mg Alloy from Electron Level
Based on Thomas-Fermi model£¬the calculation method of free energy for atom cluster with electric field had been found. The change trend of free energy of Al-Li-Mg alloy under the effect of electric field had been calculated systemically and quantitatively. And following results had been given: mostly the free energy of Al3Li andMg5Al8 at aging temperature is lower than that of free energy at solid solution temperature, but once the negative electric field increased to certain degree there will be opposite result. At aging temperature, the free energy ofAl3Li and Mg5Al8 are lower than that of Al-2.14wt.%Li andAl-2.6wt.%Mg. And comparing with 1420 alloy, the electric field promotes the precipitation of Al3Li and restrains the precipitation of Mg5Al8.
A study on the ageing treatment of 2091 Al-Li alloy with an electric field
Our previous studies [1–3] have revealed that the diffusion and the phase transformation of Al–Li alloy can be influenced by applying an electric field. The experimental results showed the phenomenon of accelerating diffusion of the alloying elements, thereby inducing a phase transformation. The direct effect of electric field on the phase transformation of 2091 Al–Li alloy is investigated in this paper by means of an ageing treatment. The 2091 Al–Li alloy was melted under an argon atmosphere. Table I gives the chemical composition of the alloy. The ingots were hotand cold-rolled to 2 mm thickness. The specimens were given solid solution treatment at 530 8C for 20 min, and then aged at various temperatures and times both with (E  4 kV cmy1) and without an electric field. The mechanical properties were measured using an INSTRON electron tensile machine to investigate the ageing kinetics. The microstructure was observed by means of transmission electron microscopy (TEM). Fig. 1 shows the relationship between the yield strengths and the ageing times (16 h, 24 h, 32 h, 40 h, 48 h, 56 h, 64 h) at different temperatures (150 8C. 160 8C, 170 8C). The ageing treatment in the presence of an electric field retards the ageing kinetics and gives a lower peak strength. Similar phenomena take place at the other ageing temperatures. The times to reach the peak strength tp are fitted to the rate equation for ageing based on the coarsening of precipitate particles [4]:
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.