Microstructure and Phase Formation in a Rapidly Solidified Laser-Deposited Ni-Cr-B-Si-C Hardfacing Alloy

In this study, microstructural evolutions and phase selection phenomena during laser deposition of a hardfacing Ni-Cr-B-Si-C alloy at different processing conditions are experimentally investigated. The results show that even minor variations in the thermal conditions during solidification can modify the type and morphology of the phases. Higher undercoolings obtained at faster cooling rates suppressed the primary borides and encouraged floret-shape mixtures of Ni and Cr5B3 via a metastable reaction. Variations in the boride phases are discussed in terms of nucleation-and growth-controlled phase selection mechanisms. These selection processes also influenced the nature and proportion of the Ni-B-Si eutectics by changing the amount of the boron available for the final eutectic reactions. The results of this work emphasize the importance of controlling the cooling rate during deposition of these industrially important alloys using laser beam or other rapid solidification techniques. (C) The Minerals, Metals & Materials Society and ASM International 201

Experimental and Numerical Modeling of Segregation in Metallic Alloys

International audienceElectromagnetic levitation (EML) has been used as an experimental technique for investigating the effect of the nucleation and cooling rate on segregation and structure formation in metallic alloys. The technique has been applied to aluminum-copper alloys. For all samples, the primary phase nucleation has been triggered by the contact of the levitated droplet with an alumina plate at a given undercooling. Based on the recorded temperature curves, the heat extraction rate and the nucleation undercooling for the primary dendritic and the secondary eutectic structures have been determined. Metallurgical characterizations have consisted of composition measurements using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry and the analysis of SEM images. The distribution maps drawn for the composition, the volume fraction of the eutectic structure, and the dendrite arm spacing (DAS) reveal strong correlations. Analysis of the measurements with the help of a cellular-automaton (CA)-finite-element (FE) model is also proposed. The model involves a new coupling scheme between the CA and FE methods and a segregation model accounting for diffusion in the solid and liquid phases. Extensive validation of the model has been carried out on a typical equiaxed grain configuration, i.e., considering the free growth of a mushy zone in an undercooled melt. It demonstrates its capability of dealing with mass exchange inside and outside the envelope of a growing primary dendritic structure. The model has been applied to predict the temperature curve, the segregation, and the eutectic volume fraction obtained upon single-grain nucleation and growth from the south pole of a spherical domain with and without triggering of the nucleation of the primary solid phase, thus simulating the solidification of a levitated droplet. Predictions permit a direct interpretation of the measurements

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