Influence of Plastic Deformation on Magnesium Precipitation in LiF Crystals

Abstract

In alkali halide crystals with divalent cation ions substitutionally incorporated in the lattice, impurity precipitation is preceeded by an agglomeration of point defects including as a first step the formation of impurity-vacancy dipoles (1 to 5).The nuclei of the new phase are to be formed from impurity-vacancy clouds. The initial precipitates are too small to be observed by light microscopy and too scarce for detection by the X-ray diffraction techniques. One method which can be used for investigation of precipitation processes is the determination of bulk density e (6) since the cation vacancies necessary to maintain charge neutrality will not remain in the crystal lattice after precipitation of divalent impurities. Besides, the volume expansion caused by relaxation around the impurity ion must disappear (6).In the work reported here this method was used for studying the influence of defects (dislocations + point defects) on the decomposition of rather dilute solid LiF: Mg solutions.We have used LiF crystals (containing 0.03 and-0.15% Mg) grown by the Kyropoulos method which were annealed at 750 "C for 48 h and subsequently cooled with a rate of 5 per hour. The crystal density was not influenced by this heat treatment. 3The specimen@ were prepared by cleavage of crystals to dimensions of 4 x 4~1 2 mm .The samples were deformed by compression along [ 0011 direction with the constant straining rate of 10 -4 s-I. Plates (about 1 x 2~2 mm ) were cut from the middle part of the deformed specimen. These plates were subjected to a pulsating isochronal

Precipitation in nickel-doped sodium chloride crystals

“…l a . Only one region of the solid solution decomposition is seen, the impurity precipitation resulting in a density decrease contrary to previous data [2,3,5, 61, where an increase was reported. It is worth noting that the temperature of the minimum density (at about 370 "C) is close to that of the maximum light scattering (355 "C) as reported in [7].…”

Flotation study of Ca2+ and Sr2+ precipitation in KCl crystals

“…2a). A similar dependence was observed earlier for undeformed crystals of KC1:Sr containing 1.4 x lW4 Sr2+ but with significantly more prolonged annealings (5…”

“…Something siiiiilar had been observed in metals where a change in density owing to formation of Guinier-Preston zones is opposite to that for incoherent precipitation [8]. Moreover, 1,iF:Mg crystals behave similarly to KCl:Ca, namely the crystal density goes through a maxiiiiuiu after a slight decrease [5]. The increase in density occiirs in the temperature region where according to X-ray investigations the stable phase of MgFz is formed whereas the decrease corresponds t o precipitation of the metastable Suzuki phase…”

“…a) Precipitation in region I, b) the influence of air-doping on the kinetics in the three regions. The relative density change is plotted against ageing time a t temperatures:(1) 70, (2) 80, (3) 90, (4) 270,(5) 400 "C. The dark points relate to air-doped crystals (treated 2h a t 720 "C)…”

Precipitation in strontium-doped sodium chloride crystals

“…The present paper is concerned with Sr precipitation in doped NaCl crystals occurring both a t elevated and a t room temperature, as determined by crystal density measurements using the flotation method [6]. Also, we examined the yield stress as a function of impurity concentration and thermal history of crystals since mechanical properties are strongly dependent on the impurity state.…”