The coercivities of mixtures of two ferromagnetic sponges in WC-Co cemented carbides

Evidence is presented to show that the coercive fields of mixtures of two ferromagnetic sponges behave according to the predictions of the Gerlach Principle for mixtures of two non-interacting systems of ferromagnetic particles. A consequence is that the behaviour of the coercive force of one of the sponges, although interconnected and interacting magnetostatically, is indistinguishable from that of a single system of non-interacting ferromagnetic particles. Only a knowledge of the internal demagnetising factors, the magnitudes of which influence the absolute magnitudes of the coercive fields, can distinguish the two systems. This conclusion is supported by further evidence that coercive fields of mixed ferromagnetic sponges are the same whether in contact, i.e. when short range interactions operate, or whether separated by 5 mm, i.e. when long range interactions operate. An experimental method for characterising component coercivities in mixtures of sponges is also presented.     

Methods for atom probe analysis of microgradients in functionally graded cemented carbides

Methods to prepare needle-shaped specimens for atom probe field ion microscopy from near surface regions have been developed. The material used was a cemented carbide with a composition gradient towards the surface, but the method is equally applicable for other materials. The preparation technique involves dimple grinding, electropolishing and focused ion beam (FIB) milling. The use of FIB milling allows for specimen preparation of materials which due to the preferential etching of different phases are difficult to electropolish. The technique also allows for preparation of specimens at well defined depth from the sample surface, selection of phase to be analysed, and to sharpen and re-use already analysed specimens.

Atom probe analyses of the near surface zone region in a gradient sintered WC–Ti(C,N)–TaC–Co cemented carbide are presented.     

Magnetic behaviour of cobalt sponges in cemented carbides

The measurement and analysis of magnetisation curves, including anhysteretic and initial curves of heterogeneous and anisotropic materials (sintered tungsten carbides of constant grain size containing 3–25 wt% Co and Co-rich alloys) are presented. Néel's law of approach to saturation magnetisation for ferromagnetics containing non-magnetic inclusions was compared with the results from an empirical Lamont-Frölich-Kennelly equation and the standard law of approach to saturation magnetisation. Internal demagnetising factors N_if, where f refers to the magnetic fraction obtained from inverse anhysteretic susceptibilities (H ≈ 0) X_af^-1 were compared with those from high-field inverse susceptibilities A_f (LFK equati on) and calculated high-field demagnetising factors a/M_sf (Néel theory); all obtained as a function of volume fraction V of WC. A_f and a/M_sf both increase to a maximum at V ≈ 0.9 and then decrease. A_f is twice the magnitude of a/M_sf at the maximum, having a smaller difference at lower V. A_f is also twice the magnitude of N_if being an increasing function of applied field and is interpreted to be a high-field demagnetising factor. Assuming the Néel equation to account for voids, the LFK equation is interpreted to measure the effects of voids, of demagnetisation by hard directions in hcp Co (at low V) and of demagnetising fields in Bloch walls in very thin Co films at high V. The Néel equation was also fitted directly to the results, showing that the theory derived for isotropic materials is inadequate in dealing with large magnetocrystalline anisotropies. A relationship between H_c, j_r (remanence ratios) and N_if for presintered (pressed powder) and sintered specimens with a range of Co contents and WC grain sizes was established.     

Geometric and Crystallographic Characterization of WC Surfaces and Grain Boundaries in WC-Co Composites

Electron backscattered diffraction has been used to determine the orientation of WC crystals in a WC-Co composite and atomic force microscopy has been used to measure the shapes of planar sections of the same crystals. A stereological analysis has been used to determine that {101¯0} prism facets and the {0001} basal planes are the WC surfaces that are most frequently in contact with Co. Further, the WC habit is an approximately equiaxed trigonal prism bound by three prism facets and two basal facets. An analysis of 15,600 grain boundaries shows that certain interfaces occur with a frequency that is much higher than would be expected in a random distribution and that the grain boundary habit planes also have {101¯0} and {0001} orientations. Eleven percent of all the observed WC-WC interfaces are 90° twist boundaries about [101¯0]. Two types of boundaries with a 30° rotation about [0001], a twist and an asymmetric tilt, comprise 3% of the population.     

Femtosecond laser ablation of cemented carbides: properties and tribological applications

Laser ablation with fs laser pulses was performed in air on cobalt cemented tungsten carbide by means of a Ti : sapphire laser (800 nm, 100 fs). Small and moderate fluences (2, 5, 10 J/cm²) and up to 5×10⁴ pulses per irradiated spot were used to drill holes with aspect ratios up to 10. Cross-section cuts from laser-irradiated samples were produced and they were analysed with optical microscopy and SEM. EDX analyses were carried out on selected zones. Quasi-cylindrical holes were found for 2 J/cm², whereas for 5 and 10 J/cm² irregular shapes (lobes, bottoms wider than hole entrances) were found to occur after a given number of incident pulses. Layers with modified structure were evidenced at pore walls. SEM revealed a denser structure, while EDX analyses showed uniform and almost similar contents of W, C, and Co in these layers. As a direct application, patterning of coated WC-Co was carried out with 2 J/cm² and 100 pulses per pore. The resulted surfaces were tribologically tested and these tests revealed an improved friction and wear behaviour. PACS 42.65.Cs; 79.60.Ds     

Abnormal grain growth of WC with small amount of cobalt

Abnormal grain growth (AGG) often appears in the synthesis of cemented tungsten carbide (WC), especially where ultra-fine powder is used. In this study, it was observed that AGG was strongly affected by the amount of Co. The obvious AGG was found to occur when the amount of Co was between 0.2 and 0.9 wt%. Careful examination of the etched samples found crack-like defects in abnormal grains, which were indications of grain boundaries in these large grains. Therefore, it is suggested that the considerable AGG of WC at lower Co concentrations be attributed to a grain boundary re-entrant edge (GBRE)-assisted two-dimensional nucleation process. Grain coalescence during sintering was the source of the grain boundaries. Due to the selective wetting of (0001) WC planes with the Co liquid, those grain boundaries were parallel to the (0001) planes resulting in platelet-like large grains with high aspect ratio. The amount of Co would affect the number of the wetted (0001) planes and the probability of grain boundary formation, and hence affect the AGG. The correspondence of the calculated weight percentage of Co required surrounding all the planes of WC grains and the experimental upper limit of the range supported this mechanism.     

Experimental investigations and DICTRA simulations on formation of diffusion-controlled fcc-rich surface layers on cemented carbides

Wear resistant fcc-rich surface layers were produced on cemented carbides by nitridation of W-Ti-Ta-Nb-Co-C compositions at 1400 °C in nitrogen atmosphere. A 15 ± 3 μm thick (Ti,Ta,Nb,W)(C,N) top-layer formed on the surface of the cemented carbides. The driving force for formation of the fcc-rich layers was the difference in nitrogen activity between the sintering atmosphere and the cemented carbide bulk, which promoted in-diffusion of nitrogen and out-diffusion of Ti, Ta and Nb. The diffusion-controlled process was modeled by DICTRA considering that all diffusion occurred in the liquid binder phase of a dispersed system model with a labyrinth factor of λ(f) = f. Good agreement between experimental and simulations regarding layer thickness, phase fraction distribution and element profiles was obtained for the presented model.     

Relation between grain shape and fractal properties in random Apollonian packing with grain rotation

We extend the common theme of random Apollonian packing of circles to consider orientable grains with a noncircular shape. Systems of up to 10(6) grains are examined for a range of polygonal and elliptical shapes using both the random Apollonian packing model and the new rotational random Apollonian packing model which takes into account the extra rotational degree of freedom of noncircular grains. We identify the constraining length D_c that limits growth of the grain during the packing process and find that a universal relation exists between grain shape and the scaling properties of the system.     

Effect of texture and grain shape on ultrasonic backscattering in polycrystals

An ultrasonic backscattering model is developed for textured polycrystalline materials with orthotropic or trigonal grains of ellipsoidal shape. The model allows us to simulate realistic microstructures and orthotropic macroscopic material textures resulting from thermomechanical processing for a broad variety of material symmetries. The 3-D texture is described by a modified Gaussian orientation distribution function (ODF) of the crystallographic orientation of the grains along the macroscopic texture direction. The preferred texture directions are arbitrary relative to the axes of the ellipsoidal grains. The averaged elastic covariance and the directional anisotropy of the backscattering coefficient are obtained for a wave propagation direction arbitrary relative to the texture and grain elongation directions. One particular application of this analysis is the backscattering solution for cubic crystallites with common textures such as Cube, Goss, Brass and Copper. In our analysis, in the texture-defined coordinates the matrix of elastic constants for cubic crystallites takes the form of orthotropic or trigonal symmetry. Numerical results are presented, discussed and compared to the experimental data available in the literature illustrating the dependence of the backscattering coefficient on texture and grain shape.     

Effect of adding non-ferromagnetic nanoparticles to grain boundary on coercivity of sintered Nd-Fe-B magnet

In this work, we investigated the influence of additional compounds of Nd-Cu-Al, Dy-Nb-Al, Dy-Zr-Al and Nb-Cu-Al on coercivity of sintered Nd-Fe-B magnets. The additional nanoparticles with size in the range of 40–80 nm was mixed with the micrometer Nd-Fe-B powder before sintering process. The results show that the coercivity of the sintered Nd-Fe-B magnets can be improved by introducing additional nanoparticles to their grain boundaries. The improvement of the coercivity of the magnets is clearly dependent on composition and fraction of the additional compounds. While the Dy-Nb-Al, Dy-Zr-Al and Nb-Cu-Al compounds degrade the coercivity of the sintered Nd-Fe-B magnets, the Nd-Cu-Al nanoparticles considerably improve this quantity. The coercivity the sintered Nd_16.5Fe₇₇B_6.5 magnets has been enhanced about 40% by adding 3 wt% of the Dy-free compound of Nd₄₀Cu₃₀Al₃₀.     

The effects of grain shape and frustration in a granular column nearjamming

We investigate the full phase diagram of a column of grains near jamming, as a function of varying levels of frustration. Frustration is modelled by the effect of two opposing fields on a grain, due respectively to grains above and below it. The resulting four dynamical regimes (ballistic, logarithmic, activated and glassy) are characterised by means of the jamming time of zero-temperature dynamics, and of the statistics of attractors reached by the latter. Shape effects are most pronounced in the cases of strong and weak frustration, and essentially disappear around a mean-field point.     

Effects of the degree of grain alignment on the hard magnetic properties of sintered NdFeB magnets

r increases monotonously with an enhancement in the degree of grain alignment (i.e. decrease in the orientation coefficient σ), whereas the intrinsic coercivity _jH_c decreases monotonously. This dependence leads to a non-monotonic behaviour of the magnetic-inductive coercivity _bH_c and a saturation of the maximum energy product (BH)_max. Received: 28 August 1997/Accepted: 12 January 1998     

Coercivity enhancement of sintered Nd-Fe-B magnets by grain boundary diffusion with Pr80-xAlxCu20 alloys

A grain boundary diffusion (GBD) process with Pr_80-xAl_xCu₂₀ (x = 0, 10, 15, 20) low melting point alloys was applied to commercial 42M sintered Nd-Fe-B magnets. The best coercivity enhancement of a diffused magnet was for the Pr₆₅Al₁₅Cu₂₀ GBD magnet, from 16.38 kOe to 22.38 kOe. Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary (GB) phase, optimizing the microstructure to enhance the coercivity. The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains. Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process. Meanwhile, the introduction of Al can effectively promote the infiltration of Pr into the magnet, which increases the diffusion rate of rare-earth elements within a certain range. This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.     

Connection between deformation texture and deformed grain shape in commercially pure iron after rolling

A definite connection was established in [1] between the compression texture of iron and deformed grain shape and orientation in relation to the direction of compression.Analysis of deformed grain shape is utilized in the present work to explain texture peculiarities in rolled iron.     

The influence of grain shape,friction and cohesion on granular compaction dynamics

This article is a review of our recent and new experimental works on granular compaction. The effects of various microscopic parameters on the compaction dynamics are addressed, in particular the influence of the grain shape, the friction and the cohesion between the grains. Two dimensionnal and three dimensionnal systems are analysed. And the role of dimensionality will be emphasized. Theoretical and numerical investigations provide additional informations about that phenomenon. Indeed numerical models permit us to study the influence of some parameters not easily accessible experimentally. Our results show that the above mentioned parameters have a deep impact on the compaction dynamics. Anisotropic grains lead to two different compaction regimes separated by a “burst" of the packing fraction. Friction is observed to modify how the grains are arranged in the pile. This is confirmed by numerical simulations. Cohesive forces between particles inhibit compaction and lead to extremely low values of the packing fraction.     

Multifractal analysis of grain pattern formation in LZT ceramics

LZT ceramics with medium ferrostiffness were obtained. Multifractal parameterization of the ceramic grain pattern at different stages of preparation was performed. The effects of pattern formation and sintering conditions on ferrostiffness are discussed. Possibilities for optimizing the production of ceramics and preventing destructive processes are shown.     

Synthesis of carbides in the arc plasma

A method of synthesis of iron, silicon, titanium, and tungsten carbides in an argon arc between graphite electrodes is developed. It is found that iron, silicon, and titanium atoms introduced into the interelectrode gap cause the voltage drop across the gap to decrease, whereas tungsten atoms increase this voltage. The X-ray diffraction analysis of synthesized powders reveals carbide and graphite crystallites. Their relative concentrations are estimated from the intensities of the respective reflection peaks. The morphological examination of the synthesized particles shows that titanium carbide particles are the largest and iron carbide ones are the finest. Raman spectra taken of the powders confirm the presence of graphite and carbide crystallites.     

Quantitative analysis of surface donors in ZnO

At low temperatures, typically up to 30 K or even higher, the electrical properties of bulk ZnO samples are nearly always dominated by a conductive near-surface region. Here we show that a single, low-temperature Hall-effect measurement, say at 20 K, and a reasonable assumption regarding the upper limit of the surface compensation ratio, yields a value of surface donor concentration N_D,surf accurate to within about a factor two. Examples are given for bulk materials grown by the vapor-phase, melt, and hydrothermal processes.