5:15 PM - 7:15 PM
[SCG62-P11] Deformation-induced phase transition of enstatite by micro-Vickers hardness testing
Keywords:enstatite, Deformation-induced phase transition, the micro-Vickers test
Damage theory has been proposed in geophysics, which suggests that localized damage in rock causes deformation and strain localization from the damage (Bercovici and Ricard, 2014Nature). Micro-Vickers tests were performed on enstatite polycrystals (Mg2Si2O6), a type of pyroxene, to investigate the load- and grain size-dependence of the deformation-induced phase transition in order to understand the physical processes of how damage causes changes at the crystal scale. Enstatite has polymorphs at room pressure: protoenstatite (PEn T = 985-1557°C) is stable at high temperatures, and clinoenstatite (CEn T <500°C) is stable at low temperatures, resulting in a -6% volume change, and deformation occurred preferentially due to the volume change associated with the phase transition.
In the micro-Vickers test, a pyramid-shaped indenter was forced against a polished specimen, held at a constant load for 15 seconds, and then the indenter was removed. High stress (~10 GPa) was applied to the specimen by applying a load of 0.1-20 N to the area(7.6~2660µm2). Initial samples with 100% PEn and a ratio of PEn to CEn of 91:9% were prepared by varying the sintering temperature and time. Our experimental results showed that the volume fraction of CEn, fCEn, increased with increasing load range from 0.1 to 10 N load, but the fCEn did not change at 20 N load. The microstructural analysis investigated that the fCEn increases with the increase of time as compared to immediately after the indentation test. In particular, the fCEn increased significantly near the indentation. These features indicate that the deformation-induced phase transition is load-dependent and that the phase transition from PEn to CEn is affected by the fCEn in the undeformed sample (grain size of the initial sample). When the fCEn >> 50%, the samples were collapsed after a period of time and became powders due to the volume change induced by the back-transformation of PEn as seen in our previous study (Tasaka and Iwago, 2024PCM). This property could be applied to analogous to investigate crystal-scale microstructural changes when damage is applied.
Our experimental results suggests that deformation occurred preferentially due to phase transitions in minerals. Deformation by the Vickers test occurs by low-temperature plasticity (Peierls mechanism). The phase transformation of pyroxene analyzed in this study is similar to the phase transformation from wadsleyite to ringwoodite in the mantle transition zone (520 km depth), including similar volume change rates of phase transformation and the phase transformation associated with low-temperature plastic deformation at low temperatures and under high stress. The microstructural changes during the transformations observed in this study are likely to be useful analogs when exploring the physical properties of minerals produced during phase transformations in Earth’s interior.
In the micro-Vickers test, a pyramid-shaped indenter was forced against a polished specimen, held at a constant load for 15 seconds, and then the indenter was removed. High stress (~10 GPa) was applied to the specimen by applying a load of 0.1-20 N to the area(7.6~2660µm2). Initial samples with 100% PEn and a ratio of PEn to CEn of 91:9% were prepared by varying the sintering temperature and time. Our experimental results showed that the volume fraction of CEn, fCEn, increased with increasing load range from 0.1 to 10 N load, but the fCEn did not change at 20 N load. The microstructural analysis investigated that the fCEn increases with the increase of time as compared to immediately after the indentation test. In particular, the fCEn increased significantly near the indentation. These features indicate that the deformation-induced phase transition is load-dependent and that the phase transition from PEn to CEn is affected by the fCEn in the undeformed sample (grain size of the initial sample). When the fCEn >> 50%, the samples were collapsed after a period of time and became powders due to the volume change induced by the back-transformation of PEn as seen in our previous study (Tasaka and Iwago, 2024PCM). This property could be applied to analogous to investigate crystal-scale microstructural changes when damage is applied.
Our experimental results suggests that deformation occurred preferentially due to phase transitions in minerals. Deformation by the Vickers test occurs by low-temperature plasticity (Peierls mechanism). The phase transformation of pyroxene analyzed in this study is similar to the phase transformation from wadsleyite to ringwoodite in the mantle transition zone (520 km depth), including similar volume change rates of phase transformation and the phase transformation associated with low-temperature plastic deformation at low temperatures and under high stress. The microstructural changes during the transformations observed in this study are likely to be useful analogs when exploring the physical properties of minerals produced during phase transformations in Earth’s interior.