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Strain-induced disorder, phase transformations, and transformation-induced plasticity in hexagonal boron nitride under compression and shear in a rotational diamond anvil cell: In situ x-ray diffraction study and modeling
The Journal of Chemical Physics (2006)
  • Valery I. Levitas, Texas Tech University
  • Yanzhang Ma, Texas Tech University
  • Javad Hashemi, Texas Tech University
  • Mark Holtz, Texas Tech University
  • Necip Guven, Texas Tech University
Abstract

Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure(∼10GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostaticpressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity (TRIP) was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure (by a factor of 3–4) while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.

Keywords
  • Center for Mechanochemistry and Synthesis of New Materials
Publication Date
2006
Publisher Statement

This article is from The Journal of Chemical Physics125 (2006):044507, doi:10.1063/1.2208353. Posted with permission.

Citation Information
Valery I. Levitas, Yanzhang Ma, Javad Hashemi, Mark Holtz, et al.. "Strain-induced disorder, phase transformations, and transformation-induced plasticity in hexagonal boron nitride under compression and shear in a rotational diamond anvil cell: In situ x-ray diffraction study and modeling" The Journal of Chemical Physics Vol. 125 (2006)
Available at: http://works.bepress.com/valery_levitas/27/