Submicrometer Grain Research Articles

Simultaneous improvement of microstructure and high-temperature tensile properties in CNT-doped Mo-based composites

In this study, molybdenum–hafnium–carbon nanotube (Mo–Hf–CNT, MHC) composites are designed and fabricated using an efficient powder metallurgy method, involving wet dispersion, high-energy ball milling (HEBM) and spark plasma sintering (SPS). The optimal composites have submicrometric grains and nanoparticles uniformly dispersed at the grain boundaries and inside the grains. The composites are characterised using microstructure observations, tensile tests at room temperature and 500 °C as well as hardness tests after annealing at 800 °C-1400 °C. The composites exhibit excellent mechanical properties at 500 °C with an ultimate tensile strength of approximately 1.2 GPa and a total elongation of 15.6%. The MHC composite shows a simultaneous increase in the strength and ductility at 500 °C. Vickers hardness first increases and then decreases with the increase in temperature. The best overall performance is that of the Mo composite containing 2.0 wt % CNT/HfH2. In addition, the formation mechanism of the nanoparticles is discussed in detail. The simultaneous enhancement in the high-temperature strength and ductility can be attributed to the nanoparticles and planar slip. Therefore, this work provides a feasible way to fabricate high-performance Mo-based composites.

Reduced yield asymmetry and excellent strength-ductility synergy in Mg-Y-Sm-Zn-Zr alloy via ultra-grain refinement using simple hot extrusion

An ultrafine-grained (UFG) Mg-RE alloy with superior yield symmetry and strength-ductility synergy was successfully fabricated by simple hot extrusion. The as-extruded specimen was composed of nearly fully recrystallized UFG microstructure with a mean grain size of 0.93 μm and exhibited a nearly random texture (maximum intensity is 1.38). The abundant nano-precipitates and the segregation of alloying elements at grain boundaries played key roles in inhibiting the growth of freshly recrystallized sub-micrometer grains formed during hot extrusion. The mechanical tests revealed that the extruded alloy exhibited an excellent tension/compression yield ratio of 1.03, which was attributed to the suppression of the extension twinning and the extremely weak texture. The suppression of extension twinning mainly resulted from the grain refinement, high concentration of rare earth solute in Mg matrix, precipitates in parts of grains and randomly oriented grains. The main deformation mode of UFG specimen was basal slip in both tension and compression, leading to the similar strain hardening behavior. This work suggests that the high tension-compression symmetry of Mg alloys can be achieved by grain refinement and texture randomization.

Dynamic recrystallization during creep of proton‐conducting barium cerate perovskites

AbstractUniaxial compression tests were conducted on perovskite‐structured barium cerate polycrystals doped with 5‐at.% yttrium and 5‐at.% ytterbium and submicrometric grain sizes, at temperatures between 1423 and 1573 K (0.70–0.78Tm) and different initial strain rates. In contrast to conventional superplastic ceramics, the stress–strain curves show a pronounced and smooth stress drop before the steady state is attained. The stress level and the associated strain at the peak of the curves increase with increasing the deformation rate or decreasing the temperature. This behavior is characteristic of metallic materials with dynamic recrystallization during deformation under warm and hot conditions. Microstructural observations have shown that preexisting twin boundaries, developed during cooling from the sintering temperature because of various crystal phase transformations, are critical in refining the grain structure through interactions with dislocations. The dependences of the peak, steady state, and critical stresses with strain rate and temperature in the present ceramics are adequately described by the constitutive equations used in metallic materials based on the Zener–Hollomon parameter and the work hardening rate.

Alumina-Toughened-Zirconia with Low Wear Rate in Ball-on-Flat Tribological Tests at Temperatures to 500 °C.

An alumina-toughened zirconia (ATZ) material, fabricated using a procedure consisting of the common sintering of two different zirconia powders, was tested using the ball-on-disc method in a temperature range between room temperature and 500 °C. Corundum balls were used as a counterpart. The ATZ composite behaviour during tests was compared with that of commonly used α-alumina and tetragonal zirconia sintered samples. At temperatures over 350 °C, a drastic decrease in the wear rate of the material was detected. SEM analyses proved that, in such conditions, nearly the whole surface of the sliding material was covered with a layer of deformed submicrometric grains, which limited contact with the part of material that was not deformed. The mentioned layer was relatively strongly connected with the material, increased its resistance, and decreased its coefficient of friction. As a reference, commonly used materials, namely commercial alumina and tetragonal zirconia, were tested. The wear parameters of the composite were significantly better than those registered for the materials prepared of commercial powders.

Fabrication and characterization of Cu reinforced with Y-enriched particles following a novel powder metallurgy route

• DS Cu-Y has been obtained by a powder metallurgical route from Cu and yttrium acetate. • TEM measurements reveal the presence of Y 2 O 3 nanometric particles. • The cavities are formed from volatile compounds. • The EBSD analysis indicate that the average grain size is below one micrometer. • The DS Cu-Y yield strength is greater than that of Cu below 473 K. Dispersion strengthened copper alloys have been produced following an innovative powder metallurgy route. Copper and yttrium acetate powders have been mechanically alloyed and posteriorly thermal treated at 923 K for 3 h and 15 h under a hydrogen atmosphere in order to transform the yttrium acetate into Y 2 O 3 . Subsequently, the powders were consolidated by hot isostatic pressing. It has been concluded that the duration of the thermal treatment of the powder is a determining factor in the degree of densification of the alloy. The study of the microstructure by Scanning Electron Microscopy and Electron Backscatter Diffraction has revealed the presence of micrometer and submicrometer grains and nanometric Y-O enriched Cu particles embedded in the copper matrix, the mean grain size being smaller for the sample produced from the powder thermal treated for 15 h. Transmission Electron Microscopy investigations concluded that the nanoparticles exhibit a spherical shape with a size up to 25 nm and correspond to monoclinic Y 2 O 3 . Annealing twins have been also observed, especially in the material produced from thermal treated powder for longer. The mechanical properties have been inferred from Vickers microhardness measurements and compression tests. Below 473 K the yield strengths of the produced materials are greater than that of pure copper and above 473 K are close to them. From the study of the thermal properties of the densest material it has been found that its thermal conductivity remains nearly constant in the temperature range 300–773 K, and its value is around 85% the thermal conductivity of CuCrZr, the reference material for ITER.

Grain Refinement and Improved Mechanical Properties of EUROFER97 by Thermo-Mechanical Treatments

EUROFER97 steel plates for nuclear fusion applications are usually manufactured by hot rolling and subsequent heat treatments: (1) austenitization at 980 °C for 30 min, (2) rapid cooling and (3) tempering at 760 °C for 90 min. An extended experimental campaign was carried out with the scope of improving the strength of the steel without a loss of ductility. Forty groups of samples were prepared by combining cold rolling with five cold reduction ratios (20, 40, 50, 60 and 80%) and heat treatments at eight different temperatures in the range 400–750 °C (steps of 50 °C). This work reports preliminary results regarding the microstructure and mechanical properties of all the cold-rolled samples and the effects of heat treatments on the samples deformed with the greater CR ratio (80%). The strength of deformed samples decreased as heat treatment temperature increased and the change was more pronounced in the samples cold-rolled with greater CR ratios. After heat treatments at temperature up to 600 °C yield stress (YS) and ultimate tensile strength (UTS) of samples deformed with CR ratio of 80% were significantly larger than those of standard EUROFER97 but ductility was lower. On the contrary, the treatment at 650 °C produced a fully recrystallized structure with sub-micrometric grains which guarantees higher strength and comparable ductility. The work demonstrated that EUROFER97 steel can be strengthened without compromising its ductility; the most effective process parameters will be identified by completing the analyses on all the prepared samples.

Single-step laser sintering of YSZ powder-beds to TBC applications

Ceramics are widely employed as thermal insulating materials for thermal barrier coatings (TBC) due to their low thermal conductivity, chemical stability, and high wear and corrosion resistance at high temperatures. The aim of this work was to study the influence of the CO2 laser beam parameters on the single-step irradiation of pre-placed yttria-stabilized zirconia (YSZ) powders on NiCrAlY/AISI 316L substrates. In order to increase the coating’s lifetime and performance, it is proposed a laser sintering of powder-beds (LSP) technique to obtain homogenous YSZ coatings, with controlled surface microstructures. The obtained coatings were characterized by optical microscopy, field emission scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD). The laser intensity and interaction time were the main laser parameters used to control the surface temperature and the combination of these parameters were used to establish a process chart. The LSP resulted in controlled smooth coating surfaces and columnar growth with submicrometric grain size. XRD analyses showed the prevalence of non-transformable tetragonal zirconia, which is known to exhibit higher stability and thermal wear resistance.

Y-PSZ/Bioglass 45S5 composite obtained by the infiltration technique of porous pre-sintered bodies using sacrificial molding

The aim of this work was to obtain porous ceramic parts based on Zirconia stabilized with 5mol.% Yttria (5Y-PSZ), suitable for the infiltration with bioactive glasses, using 3D printed sacrificial polymeric molds. In a first step, honeycomb structured molds were designed with the SolidWorks® software and manufactured by 3D printing using polylactic acid filaments (PLA). These molds were filled with a ceramic mass composed of 5Y-PSZ nanoparticles containing 3wt% polymeric binder and consolidated under pressure of 10MPa and then sintered at 1200 °C-30 min the polymeric molds were consumed. The obtained hexagonal-shaped, porous 5Y-TZP bodies were infiltrated with the bioactive glass 45S5, calcium sodium phosphosilicate, at 1350 °C. The materials were characterized by their relative density, their phase composition by X-ray diffraction analysis, and their microstructure by scanning electron microscopy (SEM-EDS), besides their mechanical properties of hardness and fracture toughness. Pre-sintered 5Y-PSZ substrates exhibit relative density around 75%, and 90% after sintering and Bioglass infiltration. The samples' microstructure is composed of a 5Y-PSZ matrix of sub-micrometric zirconia grains with an average size of 1.0 mm, besides the secondary infiltrated glassy phase homogeneously distributed, with a Ca/P ratio of 1.7, close to the ideal proportion for hydroxyapatite formation. In conclusion, sacrificial molding is an interesting route to obtaining dense Y-PSZ/Bioglass 45S5 composite in a honeycomb format.

Effects of Impedance and Dielectric Loss on the Electromagnetic Shielding Performance of an Ultrathin Carbon Nanotube Buckypaper‐Reinforced Silicon Carbide Nanocomposite

An ultrathin carbon nanotube buckypaper‐reinforced silicon carbide (CNT/SiC) nanocomposite with a thickness of 500 μm and density of 1.78 g cm−3 is prepared by combining a CNT buckypaper preform and an in situ‐synthesized SiC matrix. A uniform CNT distribution and a high CNT content (≈35 vol%) are achieved in the nanocomposite. The SiC matrix is composed mostly of nearly spherical nanograins and submicrometer grains. No residual Si is found. The imaginary permittivity and loss tangent of the nanocomposite throughout the X‐band are 96–257 and 0.44–7.03, respectively. The normalized input impedance is 0.0012–0.0174 over the X‐band. The nanocomposite has an average total shielding effectiveness (SE) of 30 dB throughout the X‐band. The nanocomposite has a reflection coefficient of 0.9651–0.9915, an absorption coefficient of 0.0077–0.0338, and a transmission coefficient of no more than 0.0012 throughout the X‐band, indicating excellent electromagnetic SE. The nanocomposite is well suited for shielding applications, and the SE due to internal absorption (SEA) depends mostly on the impedance matching condition rather than the dielectric loss capacity. The variation with frequency of SEA is not consistent with that of the imaginary permittivity or loss tangent but approximately consistent with that of the normalized input impedance.

Exploring processing, reactivity and performance of novel MAX phase/ultra-high temperature ceramic composites: The case study of Ti3SiC2

The reactivity behaviour between a MAX phase and ZrB2 or WC was explored with the aim of developing novel composites by merging the benefits of the individual constituents. Hot pressing of Ti3SiC2 with 30 vol% ZrB2 at 1450 °C led to notable microstructure re-assessment with formation of inter-locked sub-micrometric boride grains. This composite displayed enhanced hardness and showed a strength over 430 MPa up to 1200 °C, which is a great achievement considering the ductile behaviour of typical pure MAX compounds. However, addition of WC led to a highly porous composite with poor performance. These findings set a first basis for the progress of original light ceramics with combined hardness and failure tolerance over a broad temperature range.

Examination of inverse Hall-Petch relation in nanostructured aluminum alloys by ultra-severe plastic deformation

To have an insight into the occurrence of inverse Hall-Petch relationship in ultrafine-grained (UFG) aluminum alloys produced by severe plastic deformation (SPD), ultra-SPD (i.e. inducing several ten thousand shear strains via high-pressure torsion, HPT) followed by aging is applied to an Al-La-Ce alloy. Average nanograin sizes of 40 and 80 nm are successfully achieved together with strain-induced Lomer-Cottrell dislocation lock formation and aging-induced semi-coherent Al11(La,Ce)3 precipitation. Analysis of hardening mechanisms in this alloy compared to SPD-processed pure aluminum with micrometer grain sizes, SPD-processed Al-based alloys with submicrometer grain sizes and ultra-SPD-processed Al-Ca alloy with nanograin sizes reveals the presence of two breaks in the Hall-Petch relationship. First, a positive up-break appears when the grain sizes decrease from micrometer to submicrometer which is due to extra hardening by solute-dislocation interactions. Second, a negative down-break and softening occur by decreasing the grain sizes from submicrometer to nanometer which is caused by weakening the dislocation hardening mechanism with minor contribution of the inverse Hall-Petch mechanism. Detailed analyses confirm that nanograin formation is not necessarily a solution for extra hardening of Al-based alloys and other accompanying strategies such as grain-boundary segregation and precipitation are required to overcome such a down-break and softening.

Post-Annealing Influence on Magnetic Properties of Rapidly Quenched Ni–Mn–Ga Glass-Coated Microwires

In this article, we showed how the post-annealing process affects the magnetic properties of rapidly quenched Ni-Mn-Ga glass-coated microwires. Microwires with Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">63</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> chemical composition, total diameter D = 54.6 μm and metallic nucleus diameter d = 26.7 μm, were prepared by the Taylor-Ulitovsky technique. The annealing temperature was fixed at 873 K, and the time was 1 and 10 h. The microwires were slowly cooled to room temperature after annealing. Based on thermomagnetic curves and hysteresis loops, we suggest crystallographic texture in the wires that considerably affect magnetic properties. The wires' internal structure indicates nanocrystalline state with radial micrometers and submicrometer grains distribution in the wire annealed during 10 h. A ferromagnetic-to-paramagnetic phase transition occurs around room temperature. No signs of a martensitic transformation were found. The difference in magnetization saturation, remanence, and Curie temperature values for 1- and 10-h annealed microwires indicates the influence of internal stresses and that created by the presence of a glass coating.

Complex interactions between precipitation, grain growth and recrystallization in a severely deformed Al-Zn-Mg-Cu alloy and consequences on the mechanical behavior

Combining submicrometer grain size with nanoscaled precipitation is an attractive approach to achieve high yield stress with reasonable ductility in aluminum alloys. The control of super saturated solid solutions decomposition in ultrafine grain structure during precipitation annealing treatments is however a huge challenge. It raises indeed the general question of competition between precipitation, grain growth and recrystallization. All these microstructure changes are driven by the minimization of the free energy of the system but connected to three different factors, the thermodynamic driving force, the grain boundary energy and the dislocation density, respectively. In this work, we have systematically investigated the interconnection of these mechanisms in an Al-Zn-Mg-Cu alloy processed by high pressure torsion at room temperature to achieve a submicrometer grain size from the solutionized state. Ageing heat treatments were carried out to study the precipitation kinetics using complementary characterization techniques such as in-situ small-angle X-ray scattering, differential scanning calorimetry, transmission electron microscopy and atom probe tomography. Experimental data clearly demonstrate that precipitation starts at much lower temperature in the deformed state and that the precipitation sequence is modified as compared to the classical coarse grain alloy. Beyond the detailed understanding of precipitation mechanisms and the competition with recrystallization and recovery processes, the systematic relationship between microstructural features and the mechanical behavior was also studied. A special emphasis was given on the specific contribution of precipitates nucleated heterogeneously along grain boundaries.

Surface Nanostructuring of a CuAlBe Shape Memory Alloy Produces a 10.3 ± 0.6 GPa Nanohardness Martensite Microstructure.

Severe plastic deformation (SPD) has led to the discovery of ever stronger materials, either by bulk modification or by surface deformation under sliding contact. These processes increase the strength of an alloy through the transformation of the deformation substructure into submicrometric grains or twins. Here, surface SPD was induced by plastic deformation under frictional contact with a spherical tool in a hot rolled CuAlBe-shape memory alloy. This created a microstructure consisting of a few course martensite variants and ultrafine intersecting bands of secondary martensite and/or austenite, increasing the nanohardness of hot-rolled material from 2.6 to 10.3 GPa. In as-cast material the increase was from 2.4 to 5 GPa. The friction coefficient and surface damage were significantly higher in the hot rolled condition. Metallographic evidence showed that hot rolling was not followed by recrystallisation. This means that a remaining dislocation substructure can lock the martensite and impedes back-transformation to austenite. In the as-cast material, a very fine but softer austenite microstructure was found. The observed difference in properties provides an opportunity to fine-tune the process either for optimal wear resistance or for maximum surface hardness. The modified hot-rolled material possesses the highest hardness obtained to date in nanostructured non-ferrous alloys.

Microstructural progression of shear-induced mixing in a CuNi alloy

Shear deformation has been highlighted in multiple research efforts for its ability to impart novel microstructures that demonstrate improvements in mechanical properties. When used to process and densify powdered material, these shear-based consolidation techniques are commonly referred to as friction consolidation (FC). In this paper, the microstructural evolution from compacted Cu and Ni powders to a consolidated Cu0.5Ni0.5 alloy is examined. Various stages of porosity reduction and deformation are shown. Deformation was observed to accumulate preferentially in the more ductile material early in the process, leading to the formation of a tortuous microstructural zone. Porosity reduction was extensive, decreasing from ~65% in the pre-compacted state to ~1% in the fully consolidated alloy. The final consolidated alloy showed a ~2× hardness improvement over the unalloyed, compacted material. Unique aspects of this work include demonstration of FC processing to produce an equiaxed, sub-micrometer grain size in samples within a 0.5 to 2 min processing time. The results point to future opportunities to implement shear deformation during powder densification to expand the range of property outcomes in bulk materials.

Ultrafine‐Grained Metallic Materials and Coatings

Ultrafine-grained (UFG) materials are defined formally as those materials having grain sizes smaller than 1 μm, thereby including both submicrometer grain sizes where the size is within the range of 100 nm to 1.0 μm and true nanostructured materials where the grain size is below 100 nm. These materials have attracted, and are continuing to attract, considerable interest within the materials science community, mainly because they offer the potential for achieving both high strength and a superplastic forming capability. There are now several conferences and symposia organized each year devoted specifically to these materials but there are also other more general meetings where UFG materials are not the main objective but nevertheless they represent an important topic within the overall proceedings of the meeting. The initiative for organizing this Special Section of Advanced Engineering Materials arose from our participation at two different important conferences. The first was the International Baikal Materials Science Forum which was held on July 9-15, 2018 in the Republic of Buryatia (Ulan-Ude and Lake Baikal, Goryachinsk village) in Russia and the second was the 13th International Conference on Superplasticity in Advanced Materials (ICSAM-2018) which was held in Saint Petersburg in Russia on 19-22 August, 2018. At both meetings it was clear that there were many new and interesting developments associated with the topic of UFG materials and the suggestion evolved of inviting several of the major participants at these two meetings to prepare papers for publication in a Special Section of this journal. Invitations were sent out and it is a pleasure to report that they were received enthusiastically by all participants. Accordingly, a collection of papers was assembled for this Special Section covering some of the newer topics within the fields of UFG materials and their associated coatings. The present collection also marks the 10th anniversary of Advanced Engineering Materials Special Issue that was published in 2010 (No 8) with the focus on research of fundamental aspects of nanostructuring of metals and alloys and demonstration of their high innovation potential, thus attracting much interest and high citation frequency in materials science community. This issue continues the discussion of innovation applications of UFG materials. We are grateful to Dr. Sandra Kalveram, Editor-in-Chief of Advanced Engineering Materials, for accepting our proposal for this Special Section and for assisting and coordinating the various papers. We also express our very special thanks to Dr. Elena V. Bobruk of the Institute of Physics of Advanced Materials at Ufa State Aviation Technical University for providing much assistance in communicating with authors, arranging the reviews of manuscripts and for ensuring that all tasks were completed on time. We acknowledge with thanks the authors who accepted our invitation to participate in this Special Section and we are especially grateful that all papers were received within the appropriate deadlines.

Mechanical and Microstructural Analysis of an Ultra-Flexible Nano-Silver Paste Sintered Joint

Soldering using common lead-free solder alloys is still one of the main die attach technology, in particular for applications in power electronics where high temperatures have to be met. However, some newly developed attach technologies promise to offer more interesting features in terms of both mechanical and thermal properties. Among these new methods, sintering of nano-silver particles allows to obtain a high thermal conductivity needed in the assemblies of electronic or optical components, as well as a relatively low elastic modulus for better stress accommodation and enhanced thermo-mechanical reliability. The sintering processing parameters, mainly the bonding pressure, the sintering temperature profile, and the sintering atmosphere, are known to have a critical effect on the properties of the sintered layer, such as its mechanical strength and electrical/thermal performances.In this study, copper substrates are fabricated and assembled by sintering using a nano-silver paste. The objective is to obtain a bonding joint with high mechanical flexbility, capable of addressing the thermomechanical stresses for systems operating under high temperatures. The measured mechanical properties of the sintered material show on the one hand low elastic modulus of the joint which is appropriate for strong difference in thermal expansion between components, and on the other hand sufficient mechanical strength for the assembly. Microstructure analyses reveal a highly porous silver network structure of the joint, with submicrometric silver grains and large micrometric porosities homogeneously distributed.

Self-energy, line tension and bow-out of grain boundary dislocation sources

Experimental observation and numerical modelling of plastic deformation of sub-micrometre grains show that the importance of grain boundary dislocation sources (GBDS) increases as the grain size dg decreases below 1 μm. Grain boundary (GB) ledges and grain boundary dislocations (GBD) define limiting cases of the general concept of disconnections. Although the former have often been identified as potential GBDS, the latter are generally used in models for GBD-nucleation. In absence of pre-existing GBDs, (i.e. boundary ledges or disconnection-free boundaries), conservation of the Burgers vector imposes the creation of a residual GBD-segment when a mobile segment is emitted. This GBD exercises a back-stress on the emitted dislocation, which was analysed using two approaches. The first one calculates the self-energy (SE) of a dislocation loop consisting of a straight segment and a circular arc with arbitrary angle 2α and radius r, providing new results in the elastic theory of dislocations. The second model uses a line-tension (LT) approach, calculating the curvature of the dislocation as a function of the local stress field, defining a prescribed curvature problem. The models provide different details about the activation of GBDS but are remarkably compatible. They predict that the GB-segment formed upon initial bow-out will become shorter until the loop is cut off at the root, leaving a closed dislocation loop within the grain. This defines a GB-equivalent for the classical Frank-Read (FR) source, with a low activation energy for the smallest source lengths.

Compression stress strengthening modelling of a ultrafine-grained equiatomic SPS CoCrFeNiNb high-entropy alloy

High-entropy alloys are known to show exceptionally high mechanical properties, both compression and tensile strength, and unique physical properties, such as their phase stability. These quite unusual properties are primarily due to the microstructure generated by mechanical alloying processes, such as conventional induction arc melting, powder metallurgy, or mechanical alloying. In the present study, an equiatomic CoCrFeNiNb high-entropy alloy was prepared by a sequence of conventional induction melting, powder metallurgy, and compaction via spark plasma sintering. The high-entropy alloys showed uniform sub-micrometer grain microstructure consisted by a mixture of an fcc solid solution strengthened by a hcp Laves phase and a third intergranular oxide phase. The as-cast high-entropy alloys showed an ultimate compression strength (UCS) of ∼1400 MPa, which after sintering and compaction at 1273 K increased up to ∼2400 MPa. Extensive transmission electron microscopy quantitative analyses were carried out to model the UCS. A quite good agreement between the microstructure-strengthening model and the experimental UCS was found.

Correlation Between the Indentation Properties and Microstructure of Dissimilar Capacitor Discharge Welded WC-Co/High-Speed Steel Joints.

The welding of cemented carbide to tool steel is a challenging task, of scientific and industrial relevance, as it combines the high level of hardness of cemented carbide with the high level of fracture toughness of steel, while reducing the shaping cost and extending the application versatility, as its tribological, toughness, thermal and chemical properties can be optimally harmonised. The already existing joining technologies often impart either insufficient toughness or poor high-temperature strength to a joint to withstand the ever-increasing severe service condition demands. In this paper, a novel capacitor discharge welding (CDW) process is investigated for the case of a butt-joint between a tungsten carbide-cobalt (WC-Co) composite rod and an AISI M35 high-speed steel (HSS) rod. The latter was shaped with a conical-ended projection to promote a high current concentration and heat at the welding zone. CDW functions by combining a direct current (DC) electric current pulse and external uniaxial pressure after a preloading step, in which only uniaxial pressure is applied. The relatively high heating and cooling rates promote a thin layer of a characteristic ultrafine microstructure that combines high strength and toughness. Morphological analysis showed that the CDW process: (a) forms a sound and net shaped joint, (b) preserves the sub-micrometric grain structure of the original WC-Co composite base materials, via a transitional layer, (c) refines the microstructure of the original martensite of the HSS base material, and (d) results in an improved corrosion resistance across a 1-mm thick layer near the weld interface on the steel side. A nano-indentation test survey determined: (e) no hardness deterioration on the HSS side of the weld zone, although (f) a slight decrease in hardness was observed across the transitional layer on the composite side. Furthermore, (g) an indication of toughness of the joint was perceived as the size of the crack induced by processing the residual stress after sample preparation was unaltered.