Marek Faryna

Zirconia matrix–tungsten carbide particulate composites manufactured by hot-pressing technique

Zirconia matrix composites with tungsten carbide inclusions were obtained by hot pressing at 1400°C for 40 min. The carbide particle content was changed from 10 to 50 vol.%. All the samples achieved densities higher than 98% of the theoretical value. Elastic properties were measured by ultrasonic methods. Hardness and fracture toughness were estimated by Vickers indentation technique. The microstructure of the composites was determined by TEM observations.

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.

The TZP–Chromium Oxide and Chromium Carbide Composites

Microstructure and mechanical properties of the 2.9(Y) TZP matrix composites with chromium carbide and chromium oxide inclusions were investigated. Two chromium carbides were used, Cr 3 C 2 and Cr 7 C 3 . The TZP matrix powder was prepared by the coprecipitation-calcination technique. Attrition mixing allowed to homogenise the matrix and the additive powders. Coprecipitation was additionally applied in the case of the TZP-Cr 2 O 3 system. Hot pressing at 1400°C for 40min resulted in the higher densification than the pressureless sintering at 1500°C for 120 min. The TZP--carbide systems showed better properties and higher densification in the case of the Cr 7 C 3 additive. It was related to the reaction between the matrix and Cr 3 C 2 . Microscopic observations revealed quite different crack propagation path in the system with the oxide and carbide additives. It allowed to suggest mechanisms of the toughness increase over the value of the pure matrix.

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.

Microstructure and interfacial reactions in the bonding zone of explosively welded Zr700 and carbon steel plates

Abstract The microstructure of an explosive cladding joint formed between parallel Zr700 alloy and carbon steel plates was examined with the use of scanning and transmission electron microscopes equipped with energy dispersive spectrometry. The study focused on near-the-interface microstructural changes and possible interdiffusion between the plates. At the macro-scale, the interfaces were outlined by a characteristic sharp transition, indicating that there was no mechanical mixing between the welded metals in the solid state. At the micro-scale, the melted zones often showed non-uniform swirl-like areas of a similar contrast. The nano-scale analysis revealed that the melted areas were composed of mixed amorphous and nano-crystalline phases. The bonding was always achieved by way of surface melting of the joined materials, which might be invisible for observation methods other than transmission electron microscopy.

SEM and HRTEM study of zirconium-based glass forming alloys cast at various cooling rates

Abstract The microstructure of Zr–Ti–Ni–Cu–X (X=Ag,Al) ingots or melt-spun ribbons was investigated. The ingots were analyzed using a scanning electron microscope with an X-ray energy-dispersive spectroscopy system, and the alloy composition was modified to increase the amount of eutectic on expense of primary crystals. High-resolution electron microscopy observation of thin foils from ribbons of a composition near the eutectic show only extremely small ordered aggregates, while ribbons from alloys near the NiTiCu compound contain nanocrystallites. The addition of silver into Zr–Ti–Ni–Cu alloys results in precipitation of spherical silver crystallites in slowly cooled and melt-spun ribbons. However, their presence has no effect on the alloy’s glass forming ability, as they do not serve as nucleation sites for other phases.

Asymmetric distribution of martensitic variants in non-modulated NiMnGa single crystals

A strong asymmetric distribution of martensitic variants in non-modulated NiMnGa single crystals with respect to austenite is produced during martensitic transformation. A cubic-to-tetragonal transformation occurs with 24 possible variants divided into two groups. The first group with a misorientation of about 6° is composed of the so-called major variants separated by inter-plate boundaries, while the latter comprises minor variants with misorientation of 12.2°. The 6° rotation associated with major variants can also be observed at each inclination point where conjugation boundaries (CBs) occurs. The removal of CBs straightens out the inclined inter-plate boundaries confirming the Müllner–King mechanism.

SEM EBSD and TEM Structure Studies of α-Brass after Severe Plastic Deformation Using Equal Channel Rolling Followed by Groove Pressing

Commercial brass Ms36, 2mm thick was annealed and deformed in 6 passes in dual rolls equipment with attached equal channel equipment (DRECE). Then, material was deformed again using constrained groove pressing (CGP) by pressing of grooves 4.2 mm thick, and the groove angle of 45 deg. The experiment was performed 8 times (pressing out grooves and straightening at room temperature). Both methods allowed deformation without changing of the thickness of the sample, which was almost constant near 2 mm. The tensile experiment have shown the Yield Strength YS after 8x groove pressing of 210 MPa and Ultimate Tensile Strength UTS increased 27% up to 430 MPa. At the same time total elongation decreased from 34 to 15 %. The structure of the material after DRECE 6 passes was investigated using conventional TEM and have shown only rather uniform distribution of dislocations. After additional 8 groove pressing experiment, frequent, narrow deformation twins were observed accompanied by the formation of subgrains. Orientation imaging microscopy performed have shown average grain size after DRECE process near 5 μm, which decreased after 8 processes of groove pressing down to 2.9 μm. The fraction of low angle boundaries (below 5 deg) decreased after groove pressing down to 73% from 85% after DRECE process and annealing, while the fraction of high angle grain boundaries (>15 deg) increased after groove pressing up to 24% from 14%, however the total length of high angle boundaries increased more than 2 times since grain size decreased. The structure studies have shown rather mild effect on the grain refinement of both methods and they have to be modified to obtain material approaching nanosize range.

Microstructure Of The Particulate Composites In The (Y) TZP - WC System

Tetragonal zirconia polycrystals (TZP) show high fracture toughness and strength. But the relatively low hardness of these materials limits their potential applications. Hardness, wear resistance, and fracture toughness could be improved by incorporating hard carbide inclusions into the TZP matrix. Earlier investigations have shown that the additives of the silicon carbide whiskers,1,2 platelets,3 and isometric particles4 lead to improved body properties. Unfortunately, densification of such materials under pressureless conditions was not sufficiently high. Thus, hot uniaxial or isostatic pressing had to be applied.

Structure and Martensitic Transformation in NiMnSn Alloy Ribbons with Partial Sn Substitution by Al

The influence of Al substitution for Sn in Ni44Mn43.5AlxSn12.5-x (x= 0, 1, 2, 3) ferromagnetic shape memory alloy ribbons on phase transformation and microstructure evolution is outlined in this paper. Ribbons produced by melt spinning technique showed fully crystalline structure, however non uniform. Energy dispersive spectroscopy microanalysis (EDS) confirmed the average composition of ribbons in accord with the initial alloys. The higher symmetry parent phase was identified with the aid of X-ray diffraction (XRD) as bcc L21 Heusler type structure. The unit cell parameters were determined applying the XRD profile fitting method. It was observed that with increase of Al content unit cell parameters and in turn unit cell volume decrease. This may be attributed to the fact that Al has a smaller radius compared to Sn, which it was substituted for. Differential scanning calorimetry (DSC) measurements did not allow to detect the martensitic transformation above -150°C.