Monday, 11 October 2004 - 1:50 PM

This presentation is part of : Graduate Student Competition

Strain-induced phase transformations and chemical reactions under high pressure: a micro scale three-phase model

Oleg Zarechnyy and Valery I. Levitas. Texas Tech University, 7th & Boston, ME dept. TTU, Lubbock, TX 79409-1021

Strain-induced structural changes under high pressure, which include phase transitions and chemical reactions, are considered. A three-phase system with possible direct and reverse structural changes between all phases is studied. The main mechanism of structural change is the nucleation on strong defects (e. g. dislocation pile-ups and shear band intersections) generated during plastic flow. Simple strain-controlled kinetic equations are thermodynamically derived, which take into account the difference in plastic strain in each phase due to the different yield stress of the phases. A stationary solution for these equations is derived and analyzed. The particular case of structural change between two phases inside of inert matrix is considered as well. The model is applied to explain some mechanochemical phenomena observed under compression and shear of materials in diamond or Bridgman anvils. Thus, stationary solution explains zero pressure hysteresis observed experimentally. A clarification is also provided to explain why a nonreacting matrix with a yield stress higher (lower) than that for reagents significantly accelerates (slows down) the reactions, but does not change the stationary solution. Results are applied to interpret phase transformation between phases I (strong semiconductor, diamond-cubic structure), II (ductile metal tetragonal structure) and III (strong semiconductor bcc structure) of geranium and silicon.

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