Cavity or cavitation is a phenomenon in superplasticity and creep and is significant to the prediction of cavity mechanism. However, compared to cavity growth, cavity nucleation is rarely reported in magnesium alloy, especially Mg-Li alloy. To investigate the cavitation nucleation, a novel Mg-7.28Li-2.19Al-0.1Y alloy has been fabricated by hot rolling and cold rolling; its flow stress and microstructural cavitation at elevated temperatures were investigated by optical microscopy and tensile tester. The maximum elongation to failure of 265.8% was demonstrated in this alloy at a temperature of 623 K and a strain rate of 5.0 × 10 -4 s -1 . Experimental results revealed that cavity or cavitation nucleated at interphase boundary and α-Mg grain boundary in quasi-superplasticity Mg-7.28Li-2.19Al-0.091Y alloy. The present alloy had coarse grain sizes and cavity nucleation was easy to initiate in this coarse-grained alloy. Helmholtz free energy map was plotted to predict the easy and difficulty of cavity nucleation in the present alloy. This work will enhance the workability and understand the fracture initiation behavior in the present α-Mg phase dominated alloy.

Since the rotorcraft can easily be recognized by using the micro-Doppler (m-D) signature of rotor blades, the m-D effect induced by micro-motion dynamics plays an important role in target recognition and classification. However, the existing researches on the rotor blades pay little attention to the mechanism of the time-domain and time-frequency-domain flash phenomena. To comprehensively explain the flash phenomena from physics, the modeling of the rotor blades and the mechanism of the flash phenomena are studied in this paper. Firstly, for the rotor blades, the target cannot be represented as a rigid, homogeneous line nor several points. Taking the scattering coefficients and the interval of adjacent scattering points (the scattering point distribution on the blade) into consideration, the scattering point model of the rotor blade echo is established, and the influence of the scattering point distribution on the radar echo is analyzed as well. The detailed mathematic analysis and comparison results show that the conventional integral model of the rotor blade is only a special case of the scattering point model. Furthermore, In the case where the scattering point model is approximately equivalent to the conventional integral model, the critical interval of adjacent scattering points is deduced by mathematic analysis. Secondly, on the basis of the proposed model above, the physical mechanism of the time-domain and time-frequency-domain flash phenomena is studied from the viewpoint of the electromagnetic (EM) scattering. On the one hand, considering the EM scattering and scattering point distribution, the mechanism of the time-domain flashes is analyzed. Ideally, when the rotor blade is at the vertical position relative to the radar line of sight, i.e., at the flash time, the blade has the strongest echo. At this moment, the radar echo consists of echoes of all scattering points, thus inducing the time-domain flashes. At the non-flash time, the scattering points at the tip of blade and hub of rotor have stronger scattering intensities, so the echo is much weaker than that at the flash time. On the other hand, the time-frequency analysis and the cross range resolution are simultaneously used to analyze the mechanism of the time-frequency-domain flashes in the m-D signature. The m-D signature of the rotor blades consists of three parts: the time-frequency-domain flashes, the sinusoidal Doppler curves, and the zero-frequency band. At the flashes time, the Doppler frequency of adjacent scattering points cannot be distinguished, thus the m-D signature has the frequency band caused by all scattering points, i.e., the time-frequency-domain flashes appear. At the non-flash time, the sinusoidal Doppler curves and the zero-frequency band are caused by the scattering points at the tip of blade induced by the scattering points at the hub of rotor respectively. Finally, the simulation results about the scattering point model with the different intervals of adjacent scattering points show that the effectiveness of the proposed model and the correctness of theoretical analysis.