Among several separate challenges, the major one for replacing cobalt in cemented carbides is the difficulty to obtain alternative binder materials with a C-window broad enough to be robustly processed under conventional industrial control on the C content. The C-window is defined as the C content range for which phases that are detrimental to the mechanical properties are avoided. The present paper has two main objectives: first, to show that the processing C-window of Fe-Ni based systems is in fact wider than what thermodynamic equilibrium calculations predict, and that its width can be controlled moderately by tweaking the initial WC grain size and the cooling rate used in the material's processing. Secondly, in case those detrimental phases are not avoided, this work gives insight on how to make their appearance less detrimental for the mechanical properties. The morphology, volume fraction and particle size distribution of the detrimental phases, specifically η6-carbides at low C contents, are investigated to explore desirable combination of hardness and toughness of alternative binder cemented carbides. During this study it was also discovered that in samples with carbon contents below the low-C limit of the C window a carbide with hexagonal lattice known as κ, not commonly seen in cemented carbides, appeared and formed the core of a core-rim structure together with the more common η6-phase. It is believed that the κ-carbide form due to local high concentrations of tungsten during solid state sintering and that it has an impact on the precipitation characteristics of the η6-phase.

Cemented carbides have often been generalized as composite materials composed of hardtungsten carbide (WC) particles embedded in a softer and tougher Cobalt-based matrix. Thisgeneralization has become less accurate over time as alternative binder systems and additionalcarbides (for instance γ-carbides) have been introduced. Such changes have always been drivenby the need to excel in heavy-duty wear applications, as well as, more recently, by the effort tominimize cemented carbide’s social and ecological impact. The development of new grades thatfulfil both goals has been traditionally addressed following a trial-and-error methodology, in whichextensive experimental work must be performed to reach a satisfactory outcome. This leads to longand expensive concept-to-application processes. A more efficient alternative to design sustainablecemented carbides with the required performance is by using the Integrated ComputationalMaterials Engineering (ICME) approach. In this approach models and databases are developed todescribe the relationships between the process-structure-properties-performance (P-S-P-P) of thematerial. These models provide valuable information on the mechanisms ruling the behavior ofthe material at each step in the P-S-P-P chain, which can be used to design new materials moreefficiently.In this work two models are developed, and validated, contributing to an ICME-based frameworkfor the design of cemented carbides. The first model addresses the relationship between the materialstructure and its hardness (property) as a function of temperature. This Hot Hardness model usesstructural information on the mean and variation of the hard phase grain size, and the volume contentof binder to accurately predict the hardness of cemented carbides in a temperature range spanningfrom room temperature up to 1000°C. In addition, the model characterizes the differentmechanisms contributing to softening at elevated temperatures during service. Specifically, theintrinsic softening of the hard phases is in this work described using a Peierls-Nabarro-basedmodel. Further, a method to describe the microstructural rearrangement at high temperatures isproposed.The second model addresses the relationship between the material processing and its structure.The Dynamic C-window model describes the precipitation of detrimental phases in cementedcarbides to redefine the compositional processability limits of these materials, known as the Cwindow.Unlike traditional methods that rely solely on thermodynamic equilibrium calculations,this model also considers the cooling rate and the initial WC grain size, which are influentialprocessing design parameters affecting the width of the C-window. In addition to define a Cwindowthat take kinetics into account, the model also gives insights into the mechanisms rulingthe microstructural evolution during cooling, as well as predicting the particle size distribution ofthe detrimental phases as a function of the considered processing parameters. The modeling resultshave been experimentally validated through the processing and microstructural characterization ofsamples with controlled processing condition. This has allowed to conclude that the C-windowcan effectively be broadened by increasing the cooling rate during processing or/and by increasingthe WC grain size when the application allows it. The implications of this observation on thepotential processing of cemented carbides with alternative binder systems are also described.

Hårdmetaller har ofta generaliserats till ett kompositmaterial av hårda wolframkarbidpartiklar(WC) inbäddade i en mjukare och segare koboltbaserad matris. Denna generalisering har, i taktmed att alternativa bindemedel och ytterligare karbider (γ-karbider) introducerats, blivit alltmermissvisande. Sådana förändringar har drivits av behovet av alltmer krävande industriellaslitageapplikationer, samt av att minimera hårdmetallers miljöpåverkan. Utvecklingen av nyasorter som uppfyller båda dessa mål har traditionellt skett genom en trial-and-error-metod, däromfattande experimentellt arbete måste utföras för att nå ett tillfredsställande resultat. Detta ledertill långa och dyra processer för att gå från koncept till applikation. En effektivare metod för attdesigna hållbara hårdmetaller som har erforderliga egenskaper är genom att använda så kallad”Integrated Computational Materials Engineering” (ICME). I denna metod utvecklas modeller ochdatabaser för att beskriva förhållandet mellan process-struktur-egenskaper-prestanda (P-S-P-P) imaterialet. Dessa modeller ger viktig information om de mekanismer som styr ett materialsbeteende vid varje steg i P-S-P-P-kedjan, vilket i sin tur kan användas för att designa nya materialmer effektivt.I denna avhandling utvecklas och valideras två modeller som bidrar till ett ICME-baseratramverk för design av hårdmetaller. Den första modellen beskriver förhållandet mellan materialetsstruktur och hårdhet (egenskaper) som funktion av dess temperatur. Denna varmhårdhets-modellanvänder strukturell information om hårdfasens kornstorlek och polydispersitet, samt volymsandelbindemedes för att prediktera hårdmetallens hårdhet från rumstemperatur upp till 1000°C.Dessutom beskriver modellen de olika mjukningsmekanismerna som bidrar till minskad hårdhetvid förhöjd temperatur vid användning. Det inre mjuknandet av hårdfaserna beskrivs i detta arbeteav en Peierls-Nabarro-baserad modell. Dessutom presenteras en metod som beskriver denmikrostrukturella omstruktureringen vid höga temperaturer.Den andra modellen som presenteras utforskar förhållandet mellan materialets framställningoch dess struktur. Den kallas för en Dynamic C-window-modell, och beskriver utskiljning avskadliga faser i hårdmetaller i syfte att modifiera begränsningar i sammansättning relaterat tillframställnings-processen, allmänt kallat C-fönster. Till skillnad från traditionella metoder, somenbart förlitar sig på termodynamiska jämviktsberäkningar, beaktar denna modell även viktigaprocessparametrar såsom kylhastigheten och den ursprungliga WC-kornstorleken vilket påverkarbredden på C-fönstret. Utöver att definiera ett C-fönster som tar hänsyn till kinetik ger modellenockså insikter i vilka mekanismer som styr mikrostrukturutvecklingen under kylning. Denmodellerar även partikelstorleksfördelningen av skadliga faser som funktion av deframställningsparametrar som tagits i beaktande. Från modelleringsresultaten och denexperimentella valideringen dras slutsatsen att C-fönstret effektivt kan breddas genom att användahögre kylhastigheter vid framställning av hårdmetaller eller/och genom att öka WC-kornstorlekennär applikationen så tillåter. Konsekvenser av detta för den potentiella framställningen avhårdmetaller med alternativa bindemedel beskrivs också.

There are several semi-empirical models available in literature that correlate the intrinsic hardness of cemented carbides' constitutive phases and certain microstructural parameters, such as mean WC grain size and Co volume fraction, with the hardness of the cemented carbide. Nonetheless, such empirical relations fall short on predicting the behavior of materials other than WC-Co which they were fitted to, limiting their applicability on materials with diverse particle size distributions, alternative binder systems or with additional carbides (γ-carbides). Additionally, current models are limited to the prediction of room temperature hardness. Framed in an Integrated Computational Materials Engineering (ICME) approach, this work proposes several models to be integrated into an already validated semi-empirical approach to describe the hardness of cemented carbides as a function of temperature. First, new microstructural descriptors on the particle and binder size distributions are proposed to enable a better understanding of the influence of polydispersity and of the addition of γ-carbides on the hard-to-soft phase reinforcement. Second, a validated Peierls-Nabarro-based model is used to describe the intrinsic softening of the hard phases with temperature. And finally, the importance of the microstructural changes happening under stress at high temperatures is highlighted and its effect on hot hardness is introduced into the model. These upgrades increase the theoretical and physical base of the modeling tool providing a physical meaning to all the modeling parameters, lowering the need for numerical fitting, making the model more generic and bringing additional information into the micromechanics involved in the softening of cemented carbides.

Cemented carbides are widely used materials in industrial applications due to their remarkable combination of hardness and toughness. However, they are exposed to high temperatures during service leading to a reduction of their hardness. A common practice to damp this softening is to incorporate transition metal carbides in cemented carbide compositions, which keeps the hardness relatively higher when temperature increases. Understanding the underlying mechanisms of this softening is crucial for the development of cemented carbides with optimal properties. In this work, atomic-scale mechanisms taking place during plastic deformation are analyzed and linked to the effect that they have on the intrinsic macro-scale softening of the most common TMC used in cemented carbides grades (TiC, ZrC, HfC, VC, NbC and TaC). The proposed model uses the generalized stacking fault energy obtained from density functional theory calculations as an input to Peierls-Nabarro analytical models to obtain the critically resolved shear stress needed for deformation to occur in different slip systems. Subsequently, this information is used to predict the hardness variation across the temperature service range experienced by cemented carbides in wear applications. In addition to the prediction of hot-hardness for TMC, the obtained results also offer valuable insights into the intrinsic mechanisms governing TMCs deformation. The results facilitate the identification of dominant dislocation types influencing plasticity within distinct temperature regimes, define energetically favorable slip systems, and predict the brittle-ductile transition temperature in these materials. For instance, for group IV carbides at low temperatures, the slip system with a lower GSFE is {110}<11̅0> and around 30% of their melting temperature, the GSFE of partial slip in {111}<12̅1> becomes lower, changing the dominant slip mechanism and characterizing the Brittle-Ductile transition.

Integrated Computational Materials Engineering (ICME) has proved to be an efficient tool for understand-ing the process-structure-property relationships and helping us to design materials. For instance, in cemented carbides manufacturing, one of the most critical parameters is the C-window. It is defined as the C content range for which phases detrimental to the mechanical properties are avoided. This pro-cessing window has been traditionally defined using applied thermodynamics methods. However, the deviation between equilibrium calculations and real manufacturing conditions requires big additional empirical efforts to precisely define the C-window. In this work, an ICME-based approach is proposed to redefine the processability limits of cemented carbides taking the cooling rate and the material's initial powder size into consideration. The method relies on the interactive coupling of several adapted models and tools, to not only set the processability boundaries, but also to study the complex mechanisms inter-play happening along microstructural evolution. A better understanding of these underlaying mecha-nisms leads to new inputs that can be used in the design of cemented carbides. In this regard, it is observed that faster cooling rates or coarser WC grades could be effectively used to prevent nucleation of the detrimental phases enlarging the C-window towards lower C contents. 

Integrated Computational Materials Engineering (ICME) has proved to be an efficient tool for understanding the process-structure-properties relationships and help us design materials. In the case of cemented carbides, ICME can be used to study how well-established relationships are affected by new industry challenges, like the replacement of Co by alternative binder materials. The C-window is one of the most critical parameters in cemented carbides and is defined as the range in C-content for which phases detrimental to the mechanical properties are avoided. For alternative binder materials, the processing window may become very narrow, and a good understanding of its limits becomes crucial. The low-C side of the C-window is limited by the precipitation of η-carbides (M6C/M12C). Here, a systematic and integrated study on the formation of η-carbides as a function of binder chemical composition, cooling rate and microstructure is presented.

Among several separate challenges, the major one for replacing cobalt in cemented carbides is the difficulty to obtain alternative binder materials witha C-window broad enough to be robustly processed under conventional industrial control on the C content. The C-window is defined as the C contentrange for which phases that are detrimental to the mechanical properties are avoided. The present paper has two main objectives: first, to show that theprocessing C-window of Fe-Ni based systems is in fact wider than what thermodynamic equilibrium calculations predict, and that its width can becontrolled moderately by tweaking the initial WC grain size and the cooling rate used in the material’s processing. Secondly, in case those detrimentalphases are not avoided, this work gives insight on how to make their appearance less detrimental for the mechanical properties. The morphology,volume fraction and particle size distribution of the detrimental phases, specifically η-carbides at low C contents, are investigated to explore desirablecombination of hardness and toughness of alternative binder cemented carbides.During this study it was also discovered that in samples with carbon contents below the low-C limit of the C window a carbide with hexagonallattice known as κ, not commonly seen in cemented carbides, appeared and formed the core of a core-rim structure together with the more common η-phase. It is believed that the κ-carbide form due to local high concentrations of tungsten during solid state sintering and that it has an impact on theprecipitation characteristics of the η-phase.

There are several semi-empirical models available in literature that correlate the intrinsic hardness of cemented carbides’ constitutive phases and certainmicrostructural parameters, such as mean WC grain size and Co volume fraction, with the hardness of the cemented carbide. Nonetheless, suchempirical relations fall short on predicting the behavior of materials other than WC-Co which they were fitted to, limiting their applicability on materialswith diverse particle size distributions, alternative binder systems or with additional carbides (γ-carbides). Additionally, current models are limited tothe prediction of room temperature hardness. Framed in an Integrated Computational Materials Engineering (ICME) approach, this work proposesseveral models to be integrated into an already validated semi-empirical approach to describe the hardness of cemented carbides as a function oftemperature. First, new microstructural descriptors on the particle and binder size distributions are proposed to enable a better understanding of theinfluence of polydispersity and of the addition of γ-carbides on the hard-to-soft phase reinforcement. Second, a validated Peierls-Nabarro-based modelis used to describe the intrinsic softening of the hard phases with temperature. And finally, the importance of the microstructural changes happeningunder stress at high temperatures is highlighted and its effect on hot hardness is introduced into the model. These upgrades increase the theoretical andphysical base of the modelling tool providing a physical meaning to all the modeling parameters, lowering the need for numerical fitting, making themodel more generic and bringing additional information into the micromechanics involved in the softening of cemented carbides.