M. Calvo-Dahlborg

Identification of phases in gas-atomised droplets by combination of neutron and X-ray diffraction techniques with atom probe tomography.

Powders of Al(68.5)Ni(31.5) alloy have been produced by gas atomisation and sieved in different grain size families. The resulting families have been analysed by combined neutron and X-ray diffraction in order to investigate the structure and identify the existing phases at the surface and in the bulk of the grains. The weight fraction of the identified phases (Al(3)Ni(2), Al(3)Ni and Al) has been estimated from a profile refinement with the FULLPROF computer codes. An additional phase was observed but could not be identified in the diffraction patterns. Starting from grains less than 5mum in diameter, samples have been shaped by annular focused ion beam into needles that were suitable for atom probe investigations. The structure and morphologies observed by different techniques are compared and discussed. It has also been possible to estimate the crystallite sizes and the strains corresponding to the different phases present in the powders from the refinement of the ND patterns. In addition to Al(3)Ni(2) and Al(3)Ni, a phase of composition close to the nominal one of the alloy was observed in the atom probe measurements. This phase could be one of the decagonal ones referred to in the literature. Small particles of composition close to Al(82)Ni(18) are attributed to the metastable Al(9)Ni(2) phase. The achieved conclusions demonstrate the complementarity of X-ray and neutron diffraction techniques and atom probe tomography to analyse complex structures.

Structural and Microstructural Characterization of CoCrFeNiPd High Entropy Alloys

According to literature, a High Entropy Alloy (HEA) has close to equimolar composition and forms mostly fcc and/or bcc phases as well as solid solutions, i.e. the elements take random occupations on available lattice sites. In this paper we report studies on HEAs of the CoCrFeNiPd system. All alloys have been found to, contrary to what has been reported earlier in literature, consist of four different phases, three of them of fcc type. The relative amounts of the different phases depend on Pd concentration. The different phases seem to be fully interconnected.

Thermal Melt Processing of Metallic Alloys

Using melt superheating as a means to control the structure and properties of metallic alloys has been studied extensively and demonstrated some promising results, though the industrial implementation is limited due to the required high energy for melt heating and holding. The physical mechanisms behind this technology can be divided into two major groups: (1) achieving homogeneous metallic melt with the resultant high undercooling upon solidification and (2) formation of heterogeneous substrates either by formation or transformation of insoluble impurities. In this chapter, we first discuss the structure of melts and its changes with temperature during high-temperature holding. Although mostly of academic interest, these studies demonstrate the complexity of temperature influence on the molten and solidifying melt. Some examples on the effects of the initial melt condition on the solidification microstructures are given as well. After that we consider some practical implications of the changes in insoluble impurities with temperature on the microstructure formed during solidification in some metallic alloys.

Microstructural Evolution in Undercooled Al–8wt%Fe Melts

Containerless rapid solidification of hypereutectic Al–8wt%Fe is investigated experimentally using the Impulse Atomization technique (IA), as well as ElectroMagnetic Levitation (EML) under terrestrial and reduced gravity conditions. The samples were analyzed using scanning and transmission electron microscopy, X-ray and neutron diffraction, as well as electron backscattered diffraction. In both EML and IA, the samples experience some undercooling for the solidification of the primary intermetallic phase, which is likely metastable AlmFe (m = 4.0–4.4). After recalescence, the solidification path then continues with the nucleation and growth of stable Al13Fe4. While Al13Fe4 dominates in EML samples, it becomes minor in favor of AlmFe in IA droplets. The morphology differences of the primary intermetallics growing under terrestrial and microgravity conditions in EML are clear with acicular morphology for the former and a star-like morphology for the latter. The α–Al has a strong texture in microgravity EML and in IA samples while a weak one is observed on terrestrial EML. This difference is attributed to the weaker fluid flow occurring under reduced gravity conditions and in IA droplets.