Our studies of the precursor decay in pure FCC and BCC metals revealed striking differences in the behavior of the two groups of solids. In FCC metals (Al, Cu, Ag) the decay τHEL (h) (HEL, Hugoniot elastic limit) is found to be smooth; over a wide temperature (RT - melting) range and propagation distances h = 0.03-3 mm it may be reasonably well fit by a two-parameter function, τHEL(h)=τHEL0(h/h0)-α with α = 0.35-0.7. Moreover, the growth of τHEL with temperature implies that the motion of dislocations in these metals is controlled by phonon viscous drag. In the case of any of seven studied BCC metals (Ta, Nb, V, W, Mo, Cr, Fe) the dependence τHEL (h) is not smooth. At relatively small, less than 1-1.5 mm, propagation distances the decay of elastic precursor wave in BCC metals is similar to that in FCC ones, i.e. the exponent α also lies somewhere between 0.35 and 0.7. But as soon as the elastic wave propagates beyond this, 1-mm, threshold the character of the decay changes dramatically; the value of the decay exponent α abruptly becomes smaller than 0.1. Such change of the decay rate corresponds to the transition of the control of plastic deformation from phonon viscous drag to thermally activated (slower but much energy-saving) generation of dislocation double-kinks. The stress τHEL* at which the transition takes place at given temperature is the Peierls stress τPof the metal. Our studies of seven above mentioned BCC metals show that the dependences of their normalized Peierls stress τP(T)/τP(T=0) on normalized temperature T/Tm are described by a single function. This should be considered as an evidence of similarity of the dislocations core structure in BCC metals. Financial support from the Israel Science Foundation (Grant 197/15) and the Israeli Ministry of Defense (Grant 87576411) is gratefully acknowledged.
|Journal||APS March Meeting 2018|
|State||Published - 2018|