Unexpected behaviour of the crystal growth velocity at the hypercooling limit

The crystal growth velocity is one thermodynamic parameter of solidification experiments of undercooled melts under non-equilibrium conditions, which is directly accessible to observation. We applied the electrostatic levitation technique in order to study the crystal growth velocity $v$ as a function of the undercooling $\Delta T$ for the intermetallic, congruently melting binary alloy NiTi and the glass forming alloy Cu--Zr, as well as for the Zr-based ternary alloys (Cu$_{\mathrm{x}}$Ni$_{\mathrm{1-x}}$)Zr ($x= 0.7, 0.6$) and the Ni-based ternary alloy Ni(Zr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}) (x= 0.5)$. All investigated systems within this work, except the eutectics $Cu_{56}Zr_{44}$ and $Cu_{46}Zr_{54}$, exceeded the hypercooling limit $\Delta T_{\mathrm{hyp}}$ and, remarkably, every $v(\Delta T)$ relation changed significantly at $\Delta T_{\mathrm{hyp}}$. Our results for glass forming CuZr indicate that the influence of the diffusion coefficient $D(T)$ on $v(\Delta T)$ at high undercoolings, as claimed in literature, cannot be the sole reason for the existence of a maximum in the $v(\Delta T)$ behaviour. These observations could make a valuable contribution concerning an extension of growth theories to undercooling temperatures $\Delta T > \Delta T_{\mathrm{hyp}}$. Nevertheless, our finding has direct consequences to various disciplines, as our earth and all living beings are examples for non-equilibrium systems. The scatter of our velocity data is at least two orders of magnitude smaller than measurements performed by former works due to our experimental setup, which allowed precise contactless triggering at a specific undercooling, and our analysis method, which considered the respective solidification morphologies.