Recently, Professor Xiaoyan Song’s team from Beijing University of Technology has made important progress in studying the influence of hard phase WC grain morphology on the stress-strain distribution and mechanical properties in the microstructure of cemented carbides under different types of loads. The research work, titled “Mechanical behavior of cermets with different morphology of ceramic grains,” was published in the latest 2024 issue of *Acta Materialia*. Dr. Jinghong Chen and Master Yulu Yang are the co-first authors, while Associate Professor Hao Lu and Professor Xiaoyan Song serve as the co-corresponding authors. This marks Dr. Jinghong Chen’s second first-author publication in *Acta Materialia* in the research direction of material computational simulation.

?Introduction to Cited Articles

This paper is another contribution to the study of the structure-property relationship in cemented carbides by the research team, following their previous works:

1. Regulation of interfacial phase stability in cemented carbides (Acta Mater. 2018, 149, 164?178,https://doi.org/10.1016/j.actamat.2018.02.018)

2.New methods and principles for enhancing the distribution of characteristic grain boundaries in cemented carbides(Acta Mater. 2019, 175, 171?181, https://doi.org/10.1016/j.actamat.2019.06.015)

3.Accurate analysis of residual thermal stress (RTS) in cemented carbides and its mechanism affecting material mechanical behavior (Acta Mater. 2021, 221, 117428, https://doi.org/10.1016/j.actamat.2021.117428)

Article link: https://doi.org/10.1016/j.actamat.2024.119649

 

About the Research

Research Background of WC grain morphology

For cemented carbides, a typical composite material, the influence of WC grain morphology on the mechanical behavior of the material, especially representative mechanical properties such as strength and toughness, remains unclear. Furthermore, cemented carbides prepared by powder metallurgy inevitably contain high-magnitude and complexly distributed residual thermal stress (RTS), and its accurate quantitative description is a common challenge in the field of cermet composites. Moreover, when working tools manufactured from cemented carbides are in use, the interaction between as-prepared RTS and external loads inevitably affects the material’s mechanical behavior and service performance. Therefore, studying the coupling effect of WC grain morphology and as-prepared RTS on the stress-strain response during material loading is of great significance for comprehensively understanding the mechanical behavior, failure mechanism, and performance enhancement approaches of cemented carbides in practical applications.

 

Research Methods

In this study, a method for constructing finite element models of real microstructures and phase distributions in cermet composites was proposed. Combining finite element simulation with quantitative analyses such as transmission electron microscopy (TEM) observation and X-ray diffraction (XRD) experiments, the influence of WC grain morphology on RTS characteristics and the mechanical behavior and properties of materials under external loads was analyzed. This provides important scientific evidence for optimizing microstructural stress-strain distribution through regulating grain morphology to achieve strengthening and toughening of cermet composites.

Fig. 1 Microstructural characteristics and finite element models of WC-10wt.%Co cemented carbides with different WC grain morphologies: (a) SEM image of E-WC-Co; (b) SEM image of P-WC-Co; (c) Schematic diagram of equivalent elliptical area method; (d) Probability distribution of WC grain geometric shape factors

 

Through the design of preparation methods and regulation of process parameters, WC-10wt.%Co cemented carbides with equiaxed and plate-like WC grain structures were successfully prepared, respectively. Based on EBSD experiments, a method for constructing finite element models was developed to reproduce the real microstructure, morphological and mechanical characteristics of cemented carbides, which can reflect the local stress field, strain field and properties of actual materials.

 

Research Findings

The researchers found that the compressive RTS level is higher in the WC region near the acute dihedral angle of WC/Co. In contrast, the RTS in WC near the WC/Co phase boundary away from the vertex of the acute dihedral angle mainly exhibits tensile stress. Furthermore, a higher geometric shape factor leads to more narrow WC connections in the cemented carbides with plate-like WC grain structures, where the WC regions have highly concentrated compressive RTS.

Fig. 2 RTS distribution in WC-10wt.%Co cemented carbides with different WC grain morphologies: (a) Transverse RTS distribution nephogram of WC grains in E-WC-Co; (b) Axial RTS distribution nephogram of WC grains in E-WC-Co; (c) Transverse RTS distribution nephogram of WC grains in P-WC-Co; (d) Axial RTS distribution nephogram of WC grains in P-WC-Co; (e) Transverse RTS distribution nephogram of Co phase in E-WC-Co;

(e) Transverse RTS distribution nephogram of Co phase in E-WC-Co; (f) Axial RTS distribution nephogram of Co phase in E-WC-Co; (g) Transverse RTS distribution nephogram of Co phase in P-WC-Co; (h) Axial RTS distribution nephogram of Co phase in P-WC-Co

 

It was found that the average RTS of WC grains presents an asymmetric dumbbell-shaped distribution with respect to the WC geometric shape factor. When the WC grain geometric shape factor is between 3 and 5, the average RTS distribution range of WC grains is narrow, and the average RTS of WC grains is in a compressive stress state. In addition, both finite element simulation and X-ray diffraction experimental results show that the WC grain morphology has little effect on the phase-averaged RTS of cemented carbides.

 

Fig. 3 The variation law of the average residual thermal stress (RTS) of WC grains with the WC geometric shape factor.

 

Fig. 4 Experimental determination of RTS in WC phase of cemented carbides with different WC grain morphologies: E-WC-Co

 

The compressive and tensile properties of cemented carbides exhibit significant asymmetry. The presence of RTS improves the compressive strength of both E-WC-Co and P-WC-Co. Compared with E-WC-Co, P-WC-Co has a relatively lower compressive yield strength but shows larger strain after yielding without obvious hardening. The presence of RTS reduces the tensile yield strength of E-WC-Co, while having little effect on the tensile strength of P-WC-Co.

 

Fig. 5 Strain responses of WC-10wt.%Co cemented carbides with different WC grain morphologies and initial stress states during compression and tension: (a) Compressive stress-strain curve of E-WC-Co; (b) Compressive stress-strain curve of P-WC-Co.

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Conclusions

The study shows that the compressive deformation of cemented carbides can be divided into three stages: the elastic stage, the elastoplastic stage, and the plastic stage.

1.In the initial elastic stage of loading, the hard ceramic phase WC and the ductile metal phase Co undergo independent elastic deformation.

2.In the elastoplastic stage, plastic deformation occurs preferentially in the metal phase Co, while the ceramic phase WC remains in an elastic deformation state.

3.In the plastic stage, due to the much larger plastic strain in the metal phase Co than in the ceramic phase WC, strain gradients and strain localization are likely to occur in the Co region near the WC/Co phase boundary. These strain gradients and localization increase with the applied load, leading to an increase in dislocation density and strain hardening.