Evidence for a transition from diffusion-controlled to thermally controlled solidification in metallic alloys

Abstract

Dendrite growth velocities have been measured as a function of undercooling in droplets of dilute alloys of B in Ni undercooled by up to 306 K using an electromagnetic levitation technique. At a critical undercooling, depending strongly on the B concentration, a steep rise of the dendrite growth velocity is observed. The analysis of the results within current theories of dendrite growth gives evidence for a transition from diffusion-limited to purely thermally controlled growth with partitionless solidification as a consequence.Rapid solidification processing is a well-established method for the preparation of metastable materials, giving access to a range of materials properties, in particular supersaturated solid solutions. Here the crystal-growth velocity is the decisive parameter governing the trapping of solute in the solvent material. ' Rapid solidification is usually achieved by rapidly quenching the melt in techniques such as melt spinning or atomization. However, such experiments do not allow direct investigations of the growth process. They are restricted to analyses of the already solidified products, which are subject to considerable modification of the material through, e.g. , aging processes.Alternatively, rapid solidification conditions prevail even at slow cooling rates of bulk melts, provided that a substantial amount of undercooling is achieved prior to solidification. The high driving force for crystallization accumulated in this way leads to rapid crystallization, which can be observed quantitatively. In the present case, an electromagnetic levitation technique has been used to avoid heterogeneous nucleation on container walls, with the extra benefit of a suspended drop accessible to direct observation and even external stimulation of solidification.The dominant crystal-growth mechanism in undercooled melts is dendritic growth, which has attracted recent attention, both theoretical ' and experimental. "' In particular, measurements of the dendrite growth velocity as a function of undercooling on levitation undercooled Cu-Ni alloys' have revealed that nonequilibrium effects at the crystallization front play a decisive role in rapid solidification. The measurements indicated a velocity dependent partition coe%cient as predicted by theory at high solidification velocities, '" in agreement with laser heating investigations. ' In this work, we report measurements of dendrite growth velocities as a function of undercooling on levitation undercooled dilute alloys of Ni-8 which, in contrast to previously investigated alloys, are characterized by a small equilibrium partition coeScient kE «1. Such a system exhibits an extremely high sensitivity of the partition coeScient on the growth velocity, therefore allowing a direct observation of a theoretically predicted transition' ' from diffusion-controlled dendrite growth to the thermally controlled regime. This is evidenced by the observation of a sudden rise in the growth velocity at a critical undercooling temperature 5 T'. Undercoolings above the crit...

Non-Equilibrium Solidification of Undercooled Metallic Melts

“…The concept of solute trapping is schematically illustrated in Figure 8. Solute trapping during rapid dendrite growth of undercooled melts has been demonstrated in previous investigations of both completely miscible solid solutions, such as Cu-Ni [51], and alloys with complex phase diagrams in the region of dilute concentration, such as Ni 99 B 1 alloy [52]. This has been further supported by equivalent investigations on the dilute Ni 99 Zr 1 alloy in which the dendrite growth velocity has been measured as a function of undercooling using the CPS.…”

Parameter-free test of alloy dendrite-growth theory

“…2,4,5 In addition to a kinetic undercooling, deviations from chemical equilibrium at the s/l interface appear in rapid growth of solutal dendrites in alloys. 3,6 Kinetically suppressed partitioning at the interface is well described by the continuous growth model for solute trapping of Aziz and coworkers. [7][8][9] In the dilute concentration limit of alloys the velocity (V) dependence of the partition coefficient (k) is given by k͑V ͒ϭ…”

“…Qualitatively similar behavior in the velocity versus undercooling relation was previously observed in dilute Ni-B alloys. 6 For an analysis of the experimental data we apply current theories of alloy dendrite growth in undercooled melts. 2,4,5 Correspondingly, the total undercooling ⌬T is expressed as the sum of four different terms,…”

“…The collision limited growth model has been shown experimentally to be a good approximation for modeling the interface undercooling of Ni-based alloys forming fcc structure. 3,6,19 The interfacial tension is estimated using the negentropic model developed by Spaepen 20,21 ϭ␣…”

“…This effect has been studied for dilute Ni͑B͒ and is interpreted as a transition from solutally controlled growth at small undercoolings to a pure thermally controlled dendrite at high undercoolings. 6 In the present work an electromagnetic levitation technique is used to undercool bulk melts of Ni 99 Zr 1 and to measure the growth velocity as a function of undercooling. A large undercooling range becomes accessible by this technique owing to the complete avoidance of container wall induced heterogeneous nucleation.…”