Kinetic Disordering of Intermetallic Compounds Through First- and Second-Order Transitions by Rapid Solidification

Abstract

During rapid solidification of intermetallic compounds, the atoms may not have time to find the lowest-energy sites in the crystal, resulting in the growth of a solid with partially or completely suppressed chemical order. A kinetic model has been developed for “disorder trapping” during rapid solidification, which predicts the long range order parameter, composition and temperature at the interface of a chemically ordered phase as functions of interface velocity and liquid composition. The model predicts that as the solidification velocity increases, the long range order parameter decreases. Beyond a critical velocity the order parameter is zero. The predicted transition to solidification of a disordered solid is discontinuous for thermodynamically first-order cases, and continuous for thermodynamically second-order cases.

Ni₂TiAl (L2₁) and Ni₃AI (L1₂), which in equilibrium are ordered to their melting points, have been kinetically disordered by rapid solidification following pulsed laser melting. The order-disorder transitions are second-order and first-order, respectively. The interface velocity and order parameter can be followed with nanosecond resolution by monitoring the reflectivity and lateral resistance of a thin film sample during and immediately after solidification. X-ray diffraction and Transmission Electron Microscopy (TEM) indicate that metastable fcc Ni₃Al was retained by quenching. In rapidly solidified bulk Ni₂TiAl TEM reveals a fine array of antiphase boundaries, indicating that the material formed from the melt with the unstable B2 structure and subsequently transformed to the equilibrium Heusler structure during cooling to room temperature. In rapidly solidified Ni₂TiAl thin films, electron diffraction indicates that the unstable bcc structure was formed directly from the melt and retained upon cooling.