I^– Ions as Obstacles to Dislocation Motion in NaCl:I^– Single Crystals

Abstract

Strain-rate cycling tests associated with the ultrasonic oscillation were conducted for the purpose of investigation on the interaction between dislocation and I – ions during plastic deformation of NaCl:I – (0.5 mol% in the melt) at 77 K to room temperature. The relative curves of stress decrement ( Δt ) due to the oscillation and strain-rate sensitivity ( ) have stair-like shape for NaCl single crystals doped with I – at low temperatures. There are two bending points and two plateau regions. λ decreases with Δt between the two bending points. τp at Δt of first bending point and λ p between λ at first plateau place and at second one depend on the dopant ions as weak obstacles to dislocation motion. Not only temperature dependence of τ p and λ p but also τ p versus V (activation volume) reflects the interaction between dislocation and I – ions. On the basis of the data ( i.e. τ p and λ p ) analyzed in terms of the relative curves of Δ t and λ , the activation energy, G 0 , for the overcoming of dislocation from the dopant ion is found to be 0.47 and 0.53 eV for NaCl:Br – and NaCl:I – , respectively. This result that G 0 for NaCl:I – is somewhat larger than for NaCl:Br – leads to the phenomenon that I – ions are slightly stronger than Br – ones as weak obstacles to dislocation motion because of the difference between isotropic strains around I – ion and around Br – in NaCl single crystal. Furthermore, the values of τ p0 and T c are also obtained for the two kinds of specimens. τ p0 and T c are the value of τ p at absolute zero and critical temperature at which τ p becomes zero.