For the first time, this study discovers a new type of planar defect within WC grains in cemented carbides with TiC addition. Its unique feature lies in the stable existence of a single layer of metallic Ti atoms inside the ceramic phase grains. This planar defect is caused by the ordering of foreign atoms distributed on certain crystal planes of the matrix phase, and it is significantly different from known planar defects such as phase boundaries, grain boundaries, twin boundaries, stacking faults, and interfacial structures.

The composition, structure, and crystallographic characteristics of this planar defect are characterized in detail at the atomic scale, and its energy state and stability are evaluated through computational simulations. The study finds that the single Ti atomic layer facilitates the nucleation of new WC crystals along the normal direction of its basal plane. Due to the disturbance of the heterogeneous atomic layer, the deposition of W and C atoms deviates from their lattice positions in the perfect crystal lattice, resulting in changes in the arrangement of W-C atomic configurations in the nucleated and grown WC grains.

It is confirmed that regulating the distribution density of this planar defect can alter the mechanical properties of cemented carbide materials, and the optimal mechanical properties can be achieved under the condition of an appropriate planar defect density. This study provides a new approach to improving the mechanical properties of materials by introducing and regulating planar defects inside grains.

Research Background

In addition to phase boundaries and grain boundaries, typical planar defects in polycrystalline materials mainly include twin boundaries, stacking faults, and complexions. Planar defects within grains, such as twin boundaries and stacking faults, are usually caused by deformation.

Planar defects formed at phase boundaries or grain boundaries, such as complexions, are generally generated by segregation of dopant elements, phase decomposition, precipitation, etc. Doping is widely used in material preparation to improve the performance of the matrix. Additives can exist at grain boundaries in the form of element segregation or precipitation of second-phase particles, or transform into solute atoms, clusters, or precipitates within grains. When additives exist at grain boundaries in a certain form, they can reduce interfacial energy, thereby inhibiting grain growth. When distributed inside grains, they play an important role in strengthening the matrix through solid solution (substitutional or interstitial) or dispersed precipitation. WC-Co cemented carbide is a typical cermet composite material, widely used as tool and die materials in industry.

The grain growth inhibitors used in its preparation are mainly refractory metal carbides, such as VC, Cr₃C₂, TiC, NbC, and TaC. Existing studies have shown that most grain growth inhibitors are transformed into (W,M)Cₓ complexions (where M is a refractory metal and 0<x<1) during the dissolution-precipitation process during sintering. These complexions exist at phase boundaries or grain boundaries in the form of thin films containing several atomic planes.

Ti and Co exhibit different segregation and precipitation behaviors at WC grain boundaries, which can form highly asymmetric superstructures at WC/WC grain boundaries. Another form of grain growth inhibitors in cemented carbides is dissolution in the Co binder phase or formation of solid solutions in WC. In this case, refractory metals usually exist as substitutional solute atoms disorderly distributed in matrix grains. However, whether refractory metal atoms have other forms of existence and their effects on the microstructure and properties of materials remain unclear. In this study, a special type of planar defect is discovered within WC grains for the first time.

This defect is caused by the ordered arrangement of Ti atoms on certain WC crystal planes in TiC-added WC-Co cemented carbides. It is significantly different from layered phases with ordered structures (such as B2 and L1₂) commonly found in superalloys and intermetallic compounds, nor is it a MAX phase with ordered or partially ordered layered structures. This study characterizes the composition, microstructure, crystallographic features, and interfacial energy states of such intragranular planar defects at the atomic scale, investigates their formation mechanism, and compares it with that of other planar defects (such as complexions and stacking faults). Furthermore, the influence of this special planar defect on the mechanical properties of cemented carbides is studied.

Innovation Points

(1) Discovery of a new type of intragranular planar defect

The defect is caused by the ordered arrangement of foreign atoms In WC-Co cemented carbides containing TiC, which were sintered at temperatures ranging from 1340 to 1490 °C, special planar defects were found within WC grains. These defects exist in two forms, as shown in Figure 1(a) and (b). For Type I, the WC crystals on both sides of the interface have the same orientation, meaning the two parts of WC share the same zone axis. For Type II, it is equivalent to a part of the WC crystal rotating 180° around the interface normal and combining with another part of WC that initially has the same orientation. The aforementioned intragranular planar defects differ from the Σ13a-type WC grain boundaries (as shown in Figure 1c). Specifically, a typical Σ13a grain boundary is formed by the orientation relationship between two WC grains: (0001)WC//(0001)WC and [10-10]WC//[1-210]WC. In contrast, the WC crystals on both sides of the intragranular planar defects discovered in this study have consistent orientations, as shown in Figure 1(d). HAADF-STEM images and energy dispersive spectroscopy (EDS) analysis results indicate that these planar defects are Ti-rich, as illustrated in Figure 1(e).

 

(2) Revealing the crystallographic characteristics and formation mechanism of the planar defect at the atomic scale on WC grains

Combined with experimental and theoretical calculation results, the single Ti layer within WC grains is a stable structure formed by atomic diffusion after the decomposition of (W,Ti)Cₓ complexions existing at WC/Co phase boundaries. During the dissolution-precipitation process in liquid-phase sintering, the single Ti layer provides nucleation sites for the growth of WC crystals.

 

First-principles calculations were performed on various interface structures composed of the Ti layer and adjacent W and C atomic layers. It was found that both types of planar defects exhibit the lowest interface energy when following the W–C–Ti–C–W arrangement, indicating that the planar defect is in the most stable state when the single Ti layer is connected to the C atomic layer of the WC crystal, as shown in Figure 2(a). This is consistent with the results of TEM observation and analysis.

 

In addition, calculations were conducted to predict the possibility of forming planar defects with the same structure in WC grains by other refractory metals (such as V, Zr, Nb, Mo, and Hf). The results showed that the interface energy of planar defects induced by the single Ti layer is significantly lower than that induced by other elements, as shown in Figure 2(b). This explains why the aforementioned planar defects were found in cemented carbides with TiC addition, while such defects are rarely observed in those with VC, ZrC, NbC, Mo₂C, or HfC addition.

 

 

(3) Preparation of high-strength and high-toughness cemented carbides by regulating the density of intragranular planar defects

The density of intragranular planar defects in WC grains of the prepared cemented carbides can be regulated by adjusting TiC particle size, addition amount, and sintering process parameters. Under an appropriate planar defect density, the prepared cemented carbides exhibit a transverse rupture strength (TRS) of 4840±230 MPa and a fracture toughness (KIc) of 14.2±0.5 MPa·m¹/². Their comprehensive mechanical properties are superior to those of previously reported cemented carbides with similar Co content but without such intragranular planar defects.

 

Intragranular planar defects in baño can hinder the long-range movement of dislocations and stacking faults, effectively improving the strength and toughness of cemented carbides by strengthening WC grains and inhibiting the propagation of transgranular cracks.

 

Resumen

In this study, a new type of planar defect induced by a single layer of Ti atoms was discovered within the ceramic phase (WC grains) of TiC-added WC-Co cemented carbides, and the regulation of its distribution density was achieved. Combined with atomic-scale microstructural characterization and simulation calculations, the composition, structure, crystallographic characteristics, and formation mechanism of this planar defect were revealed.

This planar defect exhibits high stability, which can hinder the long-range movement of stacking faults and dislocations, and inhibit the propagation of transgranular cracks. With an appropriate distribution density of intragranular planar defects in WC, the prepared cemented carbides simultaneously achieve high transverse rupture strength and fracture toughness.

This work demonstrates that special intragranular planar defects can be induced by a single layer of foreign metal atoms in covalent crystals. Introducing and regulating such planar defects within hard phase grains provides a new approach to improving the mechanical properties of ceramic matrix composites.

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