Shock-Induced Martensitic Transformation of Highly Oriented Graphite to Diamond
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SHOCK-INDUCED MARTENSITLC TRANSFORMATION OF HIGHLY ORIENTED GRAPHITE TO DIAMOND David. J. ERSKINE and William. J. NELLIS Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550 ABSTRACT Shock-wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond-like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa. INTRODUCTION Because of their great technological importance, the investigation of the carbon phases and how they transform from one to another under pressure is an important field of materials research. A significant part of this research is accomplished using shock-waves, since in these experiments uniform and accurately determined pressures can be applied over large sample volumes. It is well known that diamond is formed by the shock compression of graphite. This process, which occurs in microseconds, happens naturally in the impact of meteors[1,2], within products of explosives[3,41, and by explosive compression of powders[5,6]. However, an important issue is whether the shock-induced phase transition of graphite to diamond is martensitic or diffusive. The relation between the crystal structures of graphite and diamond indicate that the phase transition should be fast and martensitic if shock pressure is applied perpendicular to the graphitic basal plane[7,8]. Since the lattice planes of graphite are loosely coupled, small amounts of shear stress naturally existing in the shock state are expected to induce displacive shear motion between the planes so as to produce the diamond stacking sequence. Recently, several groups have measured the stability of the graphite lattice under static compression[9-12]. These studies all support that the graphite lattice is unstable above -20 GPa and that the transformation to the new phase is reversible. However previous shock compression studies of graphite presented an inconsistent picture. A consequence of a martensitic transformation is a well-defined transition onset pressure. Shock studies[13-15] using pressed porous graphite samples indicate a phase transition near 20 GPa. In contrast, other studies using pyrolytic graphite (a quasi single-crystalline form) have observed phase transitions near 34 GPa[161,45 GPa[13] or have failed to see it below 50 GPa[17]. Since pyrolytic samples are more ideal crystalline graphite than porous samples, this wide variety of transformation pressures previously seemed inconsistent. Our preliminary measurements[18] on pyrolytic graphite of two different crystalline grades having different orientational order indicate a strong sensitivity of the transformation on graphite microstructure. Thus we believe the elevated transition pressures observed with pyrolytic samples of earlier studies were
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