Dieter M. Herlach

Metastable Solids from Undercooled Melts

Undercooled melts of metals and alloys possess an excess free energy. This opens up a variety of solidification pathways from the liquid to the solid with the benefit that a great number of metastable materials all of different physical properties can be directly produced from the undercooled melt. Undercooling is therefore a very efficient experiment parameter for the design of materials of advanced properties. We apply containerless processing by levitation melting of metallic materials to undercool them below their melting temperature. Owing to the complete avoidance of heterogeneous nucleation on container walls deep undercoolings are achieved in the order of 20% of the liquidus temperature of the respective materials investigated. The freely suspended drop gives the extra benefit to combine levitation processing with suitable diagnostic means do directly observe the entire process of non-equilibrium solidification of an undercooled melt starting with the nucleation of different crystallographic phases to rapid crystal growth of metastable microstructures. In the present study, the concept of electromagnetic levitation is introduced to observe the rapid solidification process of an undercooled melt utilizing different diagnostic means. Examples are shown for the formation of metastable phases solidified directly from the undercooled melt. A hardmagnetic intermetallic phase is nucleated in undercooled Nd-Fe-B alloys circumventing the peritectic reaction, which always involves soft magnetic -Fe. Metastable ferritic steels are produced in the regime of the phase diagram of Fe-Ni-Cr in which the austenitic steel is thermodynamically stable. The dendrite growth velocity is measured as a function of undercooling. Such measurements give inside to the conditions of the formation of supersaturated solid solutions and disordered superlattice structures in intermetallics. Undercooling is also a very efficient parameter to produce very grain refined materials as demonstrated by levitation experiments on various metallic alloys. Interestingly, two critical undercoolings are identified at which both grain size and grain morphology changes abruptly like a phase transition.

Undercooling and Solidification of Si by Electromagnetic Levitation

Abstract Pure Si droplets were containerlessly undercooled using an electromagnetic levitation method. An undercooling up to 330 K prior to solidification has been reproducibly achieved for bulk samples in size of 10 mm. A transition from faceted growth at lower undercoolings to continuous growth at higher undercoolings was observed through analyses of changes in phase morphologies on the surface of the samples. The transition was caused by existence of a large kinetic undercooling. The nucleation frequency and the crystal/melt interfacial energy are discussed within the frame of Spaepen’s model in terms of the structure of the interface.

Mechanism of formation of the anomalous eutectic structure in rapidly solidified Ni–Si, Co–Sb and Ni–Al–Ti alloys

Abstract Electromagnetic levitation technique was applied to crystallize containerlessly bulk melts of eutectic Ni–Si, Co–Sb and Ni–Al–Ti alloys at various undercoolings and cooling rates. Complementary, the drop-tube technique was used to solidify droplets of the same alloys at high cooling rates as splats. At small undercoolings, a regular lamellar eutectic microstructure was found which undergoes a partial transition above a critical undercooling to an anomalous eutectic structure. This critical undercooling depends on the cooling rate. The transition is explained by a fragmentation of primary grown eutectic lamellae, with the reduction of the interfacial energy as its driving force. Fragmentation models are applied to describe quantitatively the observed processes.

Liquid-liquid phase separation in undercooled Co-Cu alloys

Abstract The liquid–liquid phase separation in undercooled Co–Cu alloy melts has been investigated by differential thermal analysis in combination with glass fluxing technique over a composition range of 16.0–87.2 at.% Cu. The DTA signals, obtained during isochronous cooling, indicate that this separation process is exothermic and proceeds till the rapid solidification of Co-rich liquid phase occurs. The metastable miscibility gap that was determined directly and reproducibly from the onset temperatures of this process is slightly shifted to the Cu-rich side and roughly symmetrical about a Cu concentration of 53 at.%. The measured critical temperature of phase separation is 1547±1.5 K and is about 108 K below the corresponding liquidus temperature. In the present measurements the separated Co-rich liquid always solidified prior to the Cu-rich phase, which crystallized near the peritectic temperature. Lower surface tension and better wetting properties of the Cu-rich liquid phase with glass flux are responsible for the Co-rich phase to be always encased by the Cu-rich phase. In addition, thermodynamic calculations have been accomplished leading to a binodal line, which is in sufficient agreement with the experimental results.

Grain refinement through fragmentation of dendrites in undercooled melts

Abstract The widely observed microstructural transitions from coarse dendritic to fine equiaxed grains in solidification of undercooled melts have been puzzling for many years. Here we describe recent progress made in solving this puzzle. We review the basic elements of a simple model that assumes that these transitions result from the fragmentation of dendrites by remelting during the period following recalescence when the interdendritic melt solidifies. Furthermore, we review the results of detailed experiments on Ni–Cu alloys specifically designed to test the predictions of this model. A good overall quantitative agreement is obtained between the model predictions and the experimental observations. In particular, the model predicts the existence of four grain refinement transitions, three of which are observed experimentally. It also describes the dependence of the microstructural transition undercoolings on the cooling rate that follows recalescence and on the alloy composition, in agreement with the experimental findings.

Undercooling and solidification behaviour of melts of the quasicrystal-forming alloysAl–Cu–Fe and Al–Cu–Co

Abstract Al–Cu–Fe, Al–Fe and Al–Cu–Co melts of different compositions were undercooled by containerless processing in an electromagnetic levitation facility. The phase selection during solidification from the undercooled melt was determined by direct measurements of the temperature changes during recalescence. Complementarily, the phase selection and microstructure development was studied by scanning- and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD) on the as-solidified samples with the undercooling and the alloy composition as experimental parameters. For comparison, rapidly quenched samples of the same alloys were produced by splat-cooling and investigated by TEM and XRD. The undercooling results were analyzed within the framework of classical nucleation theory. The activation threshold for the nucleation was found to be small for the icosahedral quasicrystalline phase in Al–Cu–Fe, medium for the decagonal D-phase in Al–Cu–Co and crystalline phases with polytetrahedral symmetry elements (Al13Fe4 and Al5Fe2), but large for the cubic phase of Al50(CuCo)50 with non-polytetrahedral crystalline symmetry. These results are explained assuming of an icosahedral short-range order that prevails in the undercooled melt and gives rise to an interfacial energy decreasing with increasing degree of polytetrahedral order in the solid nucleus.

Growth of lamellar eutectic dendrites in undercooled melts

Electromagnetic levitation is used to undercool bulk samples of eutectic Ni 21.4 at % Si alloys. Large undercoolings ΔT up to ΔT=220 K are achieved by containerless processing of the melts. Crystal growth velocities are measured as a function of undercooling. The growth kinetics during solidification of the eutectic alloys is controlled by atomic diffusion. A maximum in the relation of the growth velocity on undercooling is observed which is due to the progressively decreasing diffusion coefficient counteracting the enhancement of the driving force of crystallization with increasing undercooling. Microstructure analysis of as-solidified samples reveals colonies of eutectic lamellae enveloped by a nonplanar growth front of a dendritelike morphology. The experimental data are analyzed within current models of crystal growth taking into account a negative temperature gradient in front of the solidification front. While the thermal undercooling causes a dendriticlike morphology of the solidification front, co...

Colloids as model systems for metals and alloys: a case study of crystallization

Metallic systems are widely used as materials in daily human life. Their properties depend very much on the production route. In order to improve the production process and even develop novel materials a detailed knowledge of all physical processes involved in crystallization is mandatory. Atomic systems like metals are characterized by very high relaxation rates, which make direct investigations of crystallization very difficult and in some cases impossible. In contrast, phase transitions in colloidal systems are very sluggish and colloidal suspensions are optically transparent. Therefore, colloidal systems are often discussed as model systems for metals. In the present work, we study the process of crystallization of charged colloidal systems from the very beginning. Charged colloids offer the advantage that the interaction potential can be systematically tuned by a variation of the particle number density and the salt concentration. We use light scattering and ultra-small angle x-ray scattering to investigate the formation of short-range order in the liquid state even far from equilibrium, crystal nucleation and crystal growth. The results are compared with those of equivalent studies on metallic systems. They are critically assessed as regards similarities and differences.

Rapid solidification of Cu84Co16 alloy undercooled into the metastable miscibility gap under different conditions

Abstract The Cu 84 Co 16 alloy melt processed by differential thermal analysis, electromagnetic levitation and drop tube experiences a liquid phase separation when it is undercooled into the metastable miscibility gap. The phase separation and coagulation processes are mainly controlled by the degree of undercooling, cooling rate and convection level in the containerless states. Disperse structures have been formed in droplets solidified during free fall in the drop tube.

Undercooling-dependent solidification behavior of levitated Nd14Fe79B7 alloy droplets

Abstract Bulk Nd 14 Fe 79 B 7 alloy droplets were processed using electromagnetic levitation technique for the purpose of studying their metastable solidification behavior at significant melt undercoolings. The results show that γ-Fe solid solution, Nd 2 Fe 14 B compound and the metastable Nd 2 Fe 17 B x compound were solidified as primary phase in sequence of increasing bulk undercooling level. The critical undercoolings were determined to be 45 K and 60 K, respectively. Following primary γ-Fe formation, the Nd 2 Fe 17 B x compound was solidified peritectically prior to the Nd 2 Fe 14 B compound. However, primary Nd 2 Fe 14 B formation was quite predominant over the whole sample volume. In case of primary Nd 2 Fe 17 B x formation, the Nd 2 Fe 14 B compound was solidified also in a peritectic manner. The metastable Nd 2 Fe 17 B x compound was found to decompose into α-Fe plus Nd 2 Fe 14 B during the post-solidification process. The phase selection mechanisms were discussed in terms of in-situ observations on the solidification process.