  Pressure-induced phase transformations (PTs) between numerous phases of Si,
the most important electronic material, have been studied for decades. This is
not the case for plastic strain-induced PTs. Here, we revealed in-situ various
unexpected plastic strain-induced PT phenomena. Thus, for 100 nm Si,
strain-induced PT Si-I to Si-II (and Si-I to Si-III) initiates at 0.4 GPa (0.6
GPa) versus 16.2 GPa ($\infty$, since it does not occur) under hydrostatic
conditions; for 30 nm Si, it is 6.1 GPa versus $\infty$. The predicted
theoretical correlation between the direct and inverse Hall-Petch effect of the
grain size on the yield strength and the minimum pressure for strain-induced PT
is confirmed for the appearance of Si-II. Retaining Si-II at ambient pressure
and obtaining reverse Si-II to Si-I PT are achieved, demonstrating the
possibilities of manipulating different synthetic paths.
