Metastable Undercooled State Research Articles

Remarkable undercooling capability and metastable thermophysical properties of liquid Nb84.1Si15.9 alloy revealed by electrostatic levitation in outer space.

The stable manipulation, high undercooling, and thermophysical property measurement of the liquid Nb84.1Si15.9 refractory alloy were successfully achieved by the electrostatic levitation technique on board the China Space Station. By controlling the superheating temperature, a maximum liquid undercooling up to 421K (0.18 TL) was obtained in the space environment, and two distinct solidification paths with different recalescence features were realized at metastable undercooled states. The liquid density and the ratio of specific heat to emissivity were measured in a wide temperature range from 1841 to 2346K, which displayed linear and quadratic relations vs temperature, respectively. The liquid emissivity was further deduced from the specific heat of the liquid alloy calculated by molecular dynamics simulation. In addition, both the density and structural characteristics of the undercooled liquid alloy were also analyzed by MD calculations.

Atomic-scale structural signature of dynamic heterogeneities in metallic liquids

With sufficiently high cooling rates, liquids will cross their equilibrium melting temperatures and can be maintained in a metastable undercooled state before solidifying. Studies of undercooled liquids reveal several intriguing dynamic phenomena and because explicit connections between liquid structure and liquids dynamics are difficult to identify, it remains a major challenge to capture the underlying structural link to these phenomena. Ab initio molecular dynamics (AIMD) simulations are yet especially powerful in providing atomic-scale details otherwise not accessible in experiments. Through the AIMD-based study of Cr additions in Al-based liquids, we evidence for the first time a close relationship between the decoupling of component diffusion and the emergence of dynamic heterogeneities in the undercooling regime. In addition, we demonstrate that the origin of both phenomena is related to a structural heterogeneity caused by a strong interplay between chemical short-range order (CSRO) and local fivefold topology (ISRO) at the short-range scale in the liquid phase that develops into an icosahedral-based medium-range order (IMRO) upon undercooling. Finally, our findings reveal that this structural signature is also captured in the temperature dependence of partial pair-distribution functions which opens up the route to more elaborated experimental studies.

Specific heat measurement of stable and metastable liquid Ti–Al alloys

The specific heats of liquid Ti-20at.%Al and Ti- 51at.%Al alloys are determined to be 33.01 ± 2.75 and 31.27 ± 2.91 J mol −1 K −1 in the stable superheated and metastable undercooled states by using an electromagnetic levitation drop calorimeter. The experimental temperature ranges are 1733-2133 K and 1511-1948 K, and maximum undercoolings of 230 (0.12 TL) and 242 K (0.14 TL) are achieved, respectively. On the basis of the experimental re- sults, the specific heat dependence on the composition is ob- tained for binary Ti-Al alloys.

Dendritic solidification of undercooled melts: mushy zone recalescence dynamics

The nucleation and growth of crystals in deeply undercooled melts leads to rapid dendritic growth accompanied by recalescence. The recalescence temperature provides an internal degree of freedom if the thermodynamic system is closed to all energy and mass transfers over relevant observational time scales. Precision adiabatic recalescence experiments on both pure and binary undercooled melts were conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable undercooled state, and also sets bounds on the microstructural parameters of the mushy zone, i.e. the volume fraction of the phases, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: (1) a short time scale ( ca. less than 1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with propagation of the dendrites and release of latent heat; (2) an intermediate time scale ( ca. 10 3–10 4 s) of slowly rising temperature, associated with coarsening of the mushy zone; (3) a long time scale ( ca. days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface. Interpretation of these experiments is provided in an attempt to understand the overall process of non-equilibrium freezing of undercooled alloy melts.

Thermal recalescence and mushy zone coarsening in undercooled melts

Precision adiabatic recalescence experiments on both pure and binary undercooled melts have been conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable under-cooled state (creating a mushy zone) and also sets bounds on the microstructural parameters of the mushy zone, namely the volume fraction of the phases, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: a short time scale (less than ∼1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with the propagation of the dendrites and release of latent heat; an intermediate time scale (∼103–104 s) of slowly rising temperature, associated with coarsening of the mushy zone; and a long time scale (days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface.