Katarzyna Berent

Crystallographic orientation inhomogeneity and crystal splitting in biogenic calcite

The calcitic prismatic units forming the outer shell of the bivalve Pinctada margaritifera have been analysed using scanning electron microscopy–electron back-scatter diffraction, transmission electron microscopy and atomic force microscopy. In the initial stages of growth, the individual prismatic units are single crystals. Their crystalline orientation is not consistent but rather changes gradually during growth. The gradients in crystallographic orientation occur mainly in a direction parallel to the long axis of the prism, i.e. perpendicular to the shell surface and do not show preferential tilting along any of the calcite lattice axes. At a certain growth stage, gradients begin to spread and diverge, implying that the prismatic units split into several crystalline domains. In this way, a branched crystal, in which the ends of the branches are independent crystalline domains, is formed. At the nanometre scale, the material is composed of slightly misoriented domains, which are separated by planes approximately perpendicular to the c-axis. Orientational gradients and splitting processes are described in biocrystals for the first time and are undoubtedly related to the high content of intracrystalline organic molecules, although the way in which these act to induce the observed crystalline patterns is a matter of future research.

Crystallographic control on the substructure of nacre tablets

Nacre tablets of mollusks develop two kinds of features when either the calcium carbonate or the organic portions are removed: (1) parallel lineations (vermiculations) formed by elongated carbonate rods, and (2) hourglass patterns, which appear in high relief when etched or in low relief if bleached. In untreated tablets, SEM and AFM data show that vermiculations correspond to aligned and fused aragonite nanogloblules, which are partly surrounded by thin organic pellicles. EBSD mapping of the surfaces of tablets indicates that the vermiculations are invariably parallel to the crystallographic a-axis of aragonite and that the triangles are aligned with the b-axis and correspond to the advance of the {010} faces during the growth of the tablet. According to our interpretation, the vermiculations appear because organic molecules during growth are expelled from the a-axis, where the Ca-CO3 bonds are the shortest. In this way, the subunits forming nacre merge uninterruptedly, forming chains parallel to the a-axis, whereas the organic molecules are expelled to the sides of these chains. Hourglass patterns would be produced by preferential adsorption of organic molecules along the {010}, as compared to the {100} faces. A model is presented for the nanostructure of nacre tablets. SEM and EBSD data also show the existence within the tablets of nanocrystalline units, which are twinned on {110} with the rest of the tablet. Our study shows that the growth dynamics of nacre tablets (and bioaragonite in general) results from the interaction at two different and mutually related levels: tablets and nanogranules.

Characterization of the μ and P phase precipitates in the CMSX‐4 single crystal superalloy

A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning‐transmission electron microscopy (STEM) using high‐angle annular‐dark‐field (HAADF) imaging, focussed ion beam‐ scanning electron microscopy (FIB‐SEM) tomography, selected area electron diffraction with beam precession (PED), as well as spatially resolved energy‐dispersive X‐ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), was used to investigate topologically close‐packed (TCP) phases, occurring in the CMSX‐4 superalloy subjected to high temperature annealing and creep deformation. Structural and chemical analyses were performed to identify the TCP phases and provide information concerning the compositional partitioning of elements between them. The results of SEM and FIB‐SEM tomography revealed the presence of merged TCP particles, which were identified by TEM and PED analysis as coprecipitates of the μ and P phases. Inside the TCP particles that were several micrometres in size, platelets of alternating μ and P phases of nanometric width were found. The combination of STEM‐HAADF imaging with spatially resolved EDS and EELS microanalysis allowed determination of the significant partitioning of the constituent elements between the μ and P phases.

Analysis of stresses and crystal structure in the surface layer of hexagonal polycrystalline materials: a new methodology based on grazing incidence diffraction

The multireflection grazing-incidence X-ray diffraction (MGIXD) method is commonly used to determine a stress gradient in thin surface layers (about 1–20 µm for metals). In this article, the development of MGIXD to enable the determination not only of stresses but also of the c/a ratio and the a0 strain-free lattice parameter in hexagonal polycrystalline materials is presented and tested. The new procedure was applied for the results of measurements performed using a laboratory X-ray diffractometer and synchrotron radiation. The evolution of stresses and lattice parameters with depth was determined for Ti and Ti-alloy samples subjected to different mechanical surface treatments. A very good agreement of the results obtained using three different wavelengths of synchrotron radiation as well as classical X-rays (Cu Kα radiation) was found.

Multiphase Ni-Cr-Al Diffusion: Experiment and Simulations

Interdiffusion in two-phase Ni-Cr-Al diffusion couple was studied experimentally and simulated numerically. The diffusion multiples were prepared by hot isostatic pressing, HIP and post-annealing at 1200°C. The concentration profiles were measured with wide line EDS technique – a method suitable to study multiphases. Hence the diffusion paths were determined. The experimental profiles and diffusion paths were compared with numerical results simulated with application of the Darken bi-velocity method implemented to describe interdiffusion in a two-phase zone.

Early Stages of Recrystallization in ECAP-Deformed AA1050 Alloy Investigated by SEM Orientation Mapping

Early stages of recrystallization were observed for the technical purity aluminium alloy (AA1050). The samples were severely deformed by equal channel angular pressing and then annealed to obtain the state of partial recrystallization. The nucleation of new grains and the changes in ‘density’ of low- and high- angle boundaries were analysed using scanning electron microscopy equipped with high resolution EBSD system. After deformation in six passes along route A the alloy contained a structure of flat grains. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. A particular role in the rise of nuclei and the structure spheroidization is attributed to migration of low-angle boundaries.

Interfacial reactions of Sn–Zn–Ag–Cu alloy on soldered Al/Cu and Al/Al joints

ABSTRACT This work shows the effect on the soldering process of the addition of Ag and Cu to Sn–Zn alloys. Soldering of Al/Cu and Al/Al joints was performed for a time of 3 min, at a temperature of 250°C, with the use of flux. Aging was carried out at 170°C for Al/Cu and Al/Al joints for 1 and 10 days. During the aging process, intermetallic layers grew at the interface of the Al/Cu joint at the Cu substrate. Intermetallic layers were not observed during wetting of Al/Al joints. On the contrary, dissolution of the Al substrate and migration of Al-rich particles into the bulk of the solder were observed. The experiment was designed to demonstrate the effect of Ag and Cu addition on the dissolution of Al substrate during the soldering and aging processes. In the solder alloys, small precipitates of AgZn3 and Cu5Zn8 were observed.

Microstructure and Mechanical Properties of Ti/Cu Clads Manufactured by Explosive Bonding at Different Stand-Off Distances

In this study, unalloyed titanium (Gr.1) and deoxidized high phosphorus copper plates were joined through explosive welding process. Different stand-off distances were used to investigate their influence on the microstructure and mechanical properties of the clad fabricated at the same amount of explosive charge. Microstructures near-the-interface were examined with the use of scanning electron microscope equipped with energy dispersive X-ray spectrometer and then microhardness measurements were carried out on the clad. Microstructure examinations showed that with increasing stand-off distance the length and amplitude of the waviness and the quantity of melted zones in areas near-the-interface increase. The inclusions of the melted zones are formed behind the wave crests on titanium side or within the wave vortex. Microhardness measurements indicate a significant increase of both plates microhardness, especially near-the-interface.

Microstructure and Thermal Analysis of As-Cast Ag-Bi-Ni alloys

The calculated liquidus projection of the Ag-Bi-Ni ternary system has been experimentally examined. Alloys were prepared by induction melting, and their microstructure studied by scanning electron microscopy coupled with energy dispersive x-ray spectroscopy. Of the primary solidification phases, (Ni) solidifies over the largest concentration range, although it was found to be narrower than calculated. The range in which Bi3Ni is the primary solidification phase was found to be broader than calculated. Also, the liquid miscibility gap is broader than predicted from assessed thermodynamic parameters. Differential thermal analysis was used to study temperatures of phase transitions of as-cast alloys, and recorded temperatures of melting of Bi3Ni and BiNi phases in ternary alloys agree well with those calculated.