Plastic deformation of polycrystalline zirconium carbide

The compressive yield strength of arc melted, polycrystalline zirconium carbide has been found to vary from 77 kg mm² at 1200C to 19 kg mm² at 1800C. Yield drops were observed with plastic strain-rates greater than 3×10^–3sec^–1 but not with slower strainrates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameterm which range from 6.5 to 1500C to 3.8 at 1800C, and the productm ^*(T) was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.     

Hardness anisotropy,deformation mechanisms and brittle-to-ductile transition in carbide

The slip plane for TiC_0.8 VC_0.84 and substoichiometric tantalum carbide has been determined as {110} using microhardness indentation at room temperature. Under the same conditions, HfC_0.98 also slips on {110} but TaC_0.96 slips on {111}. At low temperatures {110} slip is characteristic of the Group IV and substoichiometric Group V transition metal carbides while stoichiometric Group V carbides probably deform preferentially on {111} at all temperatures. This behaviour is explained in terms of two models for the crystal structures of the carbides. The Group IV carbides are described by a close-packed metal lattice whereas the structure of stoichiometric Group V carbides is more open. Various physical and mechanical properties and the effects of changing carbon content have been correlated on the basis of the models. In particular, an explanation of the brittle-to-ductile transition in carbides is proposed.     

Carbon self-diffusion in tungsten carbide

Dense, hot pressed tungsten carbide specimens were used to study the self-diffusion of ¹⁴C into WC in the temperature range 2238 to 2643 ° K. The necessity for extended diffusion anneal times was eliminated by using a submicron sectioning technique, and the diffusion penetration depth was determined by spectrophotometric W^V thiocyanate analysis of the sample sections. The existence of two clearly delineated diffusion mechanisms was demonstrated from the shape of the activity versus penetration curves. The first obeyed a bulk diffusion law, originated at the specimen surface, demonstrated anomalously low diffusion coefficients, dominated to a depth of about 0.5 m, and could be represented by the expression: D _vol=1.90×10^–6 exp–(88,000/RT).Autoradiography demonstrated that the second mechanism was grain-boundary diffusion which dominated at depths greater than 1 m. The Fisher grain-boundary diffusion analysis and the Suzuoka analysis gave apparent grain-boundary diffusion activation energies of 74 Kcal/mole and 71 Kcal/mole respectively. By using an estimated value of the bulk diffusion coefficient, the Suzuoka analysis permitted direct calculation of the grainboundary diffusion coefficient, and can be represented by: D _g.b.=4.57×10² exp–(71,000/RT).     

Plastic deformation mechanisms in poly(acrylonitrile-butadiene styrene) [ABS]

Thin films of two poly (acrylonitrile-butadiene-styrene) [ABS] resins have been strained in tension, and the ensuing deformation has been characterized by transmission electron microscopy. To enhance contrast of the rubber particles, some of the specimens were stained with OsO₄. Films containing only solid rubber particles 0.1 m in diameter show little tendency for crazing. Instead, cavitation of the rubber particles occurs, together with localized shear deformation between the particles along a direction nearly normal to the tensile axis. For specimens containing a mixture of the same small particles plus larger (1.5m diameter) particles containing glassy occlusions, some crazing does occur. Crazes tend to nucleate at the larger particles only. When crazes encounter the smaller particles these cavitate without appearing to impede or otherwise affect the craze growth. The occluded particles also show significant cavitation, with voids forming at their centres at sufficiently high levels of strain. These voids do not seem to lead to rapid craze break-down and crack propagation. In commercial ABS, which typically has both large and small rubber particles, both crazing, nucleated by the large particles, and shear deformation, encouraged by the cavitation of small rubber particles, can be expected to make important contributions to the toughness of the polymer.     

Coarsening of hafnium carbide particles in tungsten

The coarsening behaviour of finely dispersed HfC particles in a W-HfC alloy was investigated by monitoring the growth rate of the particles. An activation energy of 480 kJ mol^–1 was obtained for the process. Diffusion experiments of hafnium in tungsten were conducted at temperatures between 1773 and 2573 K using a secondary ion mass spectroscopy technique to determine the diffusion contribution to the coarsening process. The diffusion process at high temperature is controlled by lattice diffusion with an activation energy of 335 kJ mol^–1 whereas that at low temperature is governed by grain-boundary diffusion with an activation energy of 170 kJ mol^–1. It appears that the coarsening process is controlled by two energy barriers: one dictated by the diffusivity of hafnium and the other by the solubility limit as a function of temperature. The strain energy required to dissociate the carbide particles into individual species was also considered. The effects of the coarsening of HfC particles in a dispersion-strengthened W-0.4 mol% HfC alloy on recrystallization and creep deformation were illustrated using a concerted experimental modelling analysis. Results show that the strengthening effect of the HfC particles is significantly reduced at temperatures above 1800 K, due to particle coarsening.     

Ice-templated porous tungsten and tungsten carbide inspired by natural wood

The structures of tungsten and tungsten carbide scaffolds play a key role in determining the properties of their infiltrated composites for multifunctional applications. However, it is challenging to construct and control the architectures by means of self-assembly in W/WC systems because of their large densities.Here we present the development of unidirectionally porous architectures, with high porosities exceeding 65 vol.%, for W and WC scaffolds which in many respects reproduce the design motif of natural wood using a direct ice-templating technique. This was achieved by adjusting the viscosities of suspensions to retard sedimentation during freezing. The processing, structural characteristics and mechanical properties of the resulting scaffolds were investigated with the correlations between them explored. Quantitative relationships were established to describe their strengths based on the mechanics of cellular solids by taking into account both inter-and intra-lamellar pores. The fracture mechanisms were also identified,especially in light of the porosity. This study extends the effectiveness of the ice-templating technique for systems with large densities or particle sizes. It further provides preforms for developing new natureinspired multifunctional materials, as represented by W/WC-Cu composites.     

Stress induced cavitation in cobalt bonded tungsten carbide

Tensile stress-strain curves of three grades of Co-bonded WC composite have been obtained near 1200° C. Extensive void formation is observed in specimens extended to failure. A qualitative model for deformation based on atomic migration of Co in the bonding Co phase is proposed to account for the observations.     

Coatings made of tungsten carbide and tantalum carbide for machining tools

Continuous evolution of cutting parameters has been a constant requirement for the manufacture of machining tools. This has promoted the development of new technologies to reduce the production time as well as cost without affecting the competitiveness. In the present work, high speed tools have been coated with WC and TaC by means of chemical vapour deposition and tested in a high speed milling machine. Damages on the tips of these cutting tools were analysed by scanning electron microscope. These observations show that the coating is heterogeneous and affected by the type and composition of various phases formed. Results indicate that the coating made from WC is more highly resistant to attrition than that made of TaC. Surfaces that have been machined by the coated tools are evaluated by means of the average roughness parameter     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

A new one-dimensional tungsten carbide nanostructured material

A new medium specific surface area one-dimensional tungsten carbide nanostructure was obtained by the Shape Memory Synthesis method, in which the macrostructural features of a carbonaceous 1D-preformed template were maintained during the carburization of tungsten oxide and determined the resulting carbide morphology.     

Fracture analysis of cobalt-bonded tungsten carbide composites

Specimens of WC-Co were indented to measure the resulting crack size and unindented samples were fractured in three-point flexure to obtain strength and to measure characteristic features on the fracture surface. Fracture toughness was determined using indentation techniques and compared to those determined using fractography. We show that principles of fracture mechanics can be applied to WC-Co composites and can be used to analyse the fracture process.The fracture surfaces were examined by scanning electron microscopy and optical microscopy. Characteristic features observed in glasses, single crystals and polycrystalline materials known as mirror, mist, hackle and crack branching were identified for these composites. We discuss the importance of fracture surface analysis in determining the fracture origins and in the failure analysis of WC-Co composites.     

Sintering study of nanocrystalline tungsten carbide powders

WC powder with an average grain size of 6 nm was obtained after high energy ball milling under protective gas atmosphere. The kinetics of densification was studied during sintering the powder in a dilatometer up to 1450 °C. The microstructure was investigated by TEM and high resolution SEM after various stages of sintering. The green density of the specimens was 45%. Three stages of sintering were defined: (a) rearrangement of particles at low temperature (850 °C) without grain or particle growth, (b) neckformation between powder particles at 1000–1250 °C and initial grain growth at 1200 °C, (c) pore elimination accompanied by massive grain growth at 1300–1450 °C.     

Crack branching and fracture mirrors in cemented tungsten carbide

Fracture mirrors are investigated in several WC-Co grades with the aim of determining crack branching criteria. The size of the mirrors is found to increase with increasing cobalt content and with decreasing carbide grain size. The results are explained in terms of the dependence on cobalt content and grain size of the free surface energy and the elastic strain at fracture.     

Plastic deformation of polypropylene

In a wide range of drawing temperatures (20 to 150° C) and draw-ratios (5 to 20) the axial long periodL _T is a unique function of temperature. It is completely independent of the long period of the starting material which was varied within wide limits (125 to 350 Å). The gradual transformation of the long period observed at small draw-ratio ( between 1 and 5) from the valueL ₀ of an undrawn sample to the limiting valueL _T>L ₀ of highly drawn polypropylene ( 5) could be demonstrated on samples withL ₀ larger thanL _T. The change of long period is abrupt, indicating a discontinuous step in the transformation from the original microspherulitic into the fibre structure.These results may be interpreted in the same manner as in the case of polyethylene. During plastic deformation in the neck the lamellae are broken into small folded chain blocks which are then incorporated into the microfibrils, the basic building element of the fibre structure. The work of deformational forces must so mobilise the chains in the blocks that they get rearranged with a new long period, corresponding to the temperature of drawing.Paper I: The limiting axial long period of drawn polypropylene,J. Polymer Sci. A2 in press.On leave of absence from the Institute of Chemical Physics, Rocosalano, CSIC, Madrid, Spain.     

Microstructure and properties of CVD tungsten carbide from tungsten hexafluoride and dimethyl ether

Tungsten carbide was deposited from tungsten hexafluoride, dimethyl ether, and hydrogen using a horizontal, cold-wall reactor. The effects of substrate temperature, reactor pressure, and reagent ratio on the coating growth rate, morphology, composition, and microhardness were studied. Under most conditions, the solid deposit was primarily W₃C with minor amounts of W. The tungsten carbide growth rate data fit an Arrhenius rate expression for temperatures from 425 to 550°C and had an activation energy of 24kcal/mol at 70mmHg total pressure and a WF₆/DME ratio of 6.3. A variety of surface morphologies and microstructures were observed. The microhardness of the coated substrates increased with coating thickness to a maximum value of 2400kg/mm².