Evidence of the transition from ordered to disordered growth during rapid solidification of an intermetallic phase

Abstract

The dendrite growth velocity during solidification is measured on liquid drops of the intermetallic compound Ni50Al50 undercooled by levitation up to 265 K. A sharp increase of the growth velocity is found at a critical undercooling ∆T * ≈ 250 K. In situ diffraction of synchrotron radiation on levitation-processed samples unambiguously shows a transition from ordered to disordered growth at ∆T * . The sharp interface model is extended to describe the transition from ordered to disordered dendrite growth by taking into account the velocity dependence of the order parameter and the kinetic growth coefficient.

Disorder trapping during the solidification of βNi3Ge from its deeply undercooled melt

“…This is perhaps not surprising given the very low post-recalescence coolig rates that can be achieved during melt fluxing (typically of order 10 K s -1 ) due to the high thermal mass of the flux, crucible, susceptor and heat shield relative to that of the sample. This would be consistent with in situ energy dispersive X-ray diffraction studies using synchrotron radiation on levitation undercooled Ni-Al which revealed that disordered superlattice structures can recrystallise to the ordered state on timescales of only a few seconds [29]. 3 Ge is an ordered intermetallic compound displaying congruent melting in the composition range 22.5-25.0 at.% Ge.…”

Non-Equilibrium Solidification of Undercooled Metallic Melts

“…From this kinetic phase diagram, the kinetic undercooling ∆T K (difference between local equilibrium liquidus and velocity dependent liquidus temperature), k(V) and m(V) are directly inferred. More details of the computations are given in [64]. The results of the computations of dendrite growth velocity as a function of undercooling are given in the upper part of Figure 12 (solid line).…”

“…Since the transition from disordered to ordered phase takes place rapidly [59], a primarily formed disordered B2 phase is not necessarily present in the as solidified sample. In order to obtain unambiguous evidence of the formation of a metastable disordered phase during rapid solidification of the deeply undercooled melts Energy Dispersive X-ray Diffraction (EDXD) has been conducted on levitation processed Al 50 Ni 50 alloys using synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) in Grenoble [64]. Figure 13 (left) shows the results of EDXD on a levitation processed Ni 50 Al 50 sample undercooled by 235 K prior to solidification.…”

Disorder trapping by rapidly moving phase interface in an undercooled liquid

“…If the undercooling increases, the non-equilibrium effect of disorder trapping leads to the formation of a metastable disordered structure. Experimental evidence of disorder trapping has been demonstrated by in situ diffraction studies using synchrotron radiation on levitation-processed samples, in which a transition from ordered to disordered growth at a critical undercooling was unambiguously shown [6]. The phenomenon of order-disorder transition with distinct change of the order parameter is also important in structural transformations with non-monotonic relaxation processes of melts [8] and in liquid-liquid phase separation of undercooled metallic alloys [9,10].…”

“…The disorder trapping was investigated from theoretic perspective by sharp interface models [2,3,6], a diffuse interface model [11] and methods of atomistic simulation [1,12]. In the present work, using an existing approach for fast phase transformations [13], a diffuse interface model results in a system of hyperbolic equations to describe (i) the rapid interface motion and (ii) the transition from ordered to disordered structures formed from undercooled liquids.…”