Thermodynamic analysis of metal segregation in dual phase high entropy ceramics

Equilibrium Gibbs'free energy calculations were used to determine metal segregation trends between boride and carbide solid solutions containing two metals that are relevant to dual phase high entropy ceramics.The model predicted that Ti had the strongest tendency to segregate to the boride phase followed by Zr,Nb,Mo,V,Hf,and Ta,which matches experimental results of measured compositions.The ratio of a metal in the carbide phase to the content of the same metal in the corresponding metal boride had a linear trend with the change in standard Gibbs'free energy of reaction for a metal carbide reacting with B_(4)C to produce its corresponding metal boride and carbon.The proposed model was used to predict the changes in standard Gibbs'free energy for CrC→CrB_(2) to be−260 kJ and WC→WB_(2) to be 148 kJ,which indicates that Cr has the strongest segregation to the boride and W has the strongest segregation to the carbide.The proposed model can be used to estimate the segregation of metals in dual phase high entropy boride-carbide ceramics of any boride/carbide ratio or metal content.

Multi-phase(Zr,Ti,Cr)B_(2) solid solutions:Preparation,multi-scale microstructure,and local properties

Multi-phase ceramics based on ZrB_(2),TiB_(2) and doped with CrB_(2) and SiC were prepared by powder metallurgy and hot pressing to explore the possibility of obtaining multi-scale microstructures by super-saturation of complex(Zr,Ti,Cr)B_(2) solid solutions.Core-shell structures formed in TiB_(2) grains,whereas ZrB_(2) appeared to form a homogeneous solid solution with the other metals.Precipitation of nano-inclusions within both micron-sized borides was assessed by transmission electron microscopy and thermodynamics elucidated the preferential formation of boride inclusions due to the specific sintering atmosphere.In addition,atomic size factors explicated the precipitation of CrB_(2) nano-particles into ZrB_(2)-rich grains and of ZrB_(2) nano-particles into TiB_(2)-rich grains.The hardness of the constituent phases measured by nanoindentation ranged from 36 to 43 GPa.

From thermal conductive to thermal insulating:Effect of carbon vacancy content on lattice thermal conductivity of ZrC_(x)

Lattice thermal conductivities of zirconium carbide(ZrC_(x),x=1,0.75 and 0.5)ceramics with different carbon vacancy concentrations were calculated using a combination of first-principles calculations and the Debye-Callaway model.The Gruneisen parameters,Debye temperatures,and phonon group velocities were deduced from phonon dispersions of ZrC_(x) determined using first-principles calculations.In addition,the effects of average atomic mass,grain size,average atomic volume and Zr isotopes on the lattice thermal conductivities of ZrC_(x) were analyzed using phonon scattering models.The lattice thermal conductivity decreased as temperature increased for ZrC,ZrC_(0.75) and ZrC_(0.5)(Zr2 C),and decreased as carbon vacancy concentration increased.Intriguingly,ZrC_(x) can be tailored from a thermal conducting material for ZrC with high lattice thermal conductivity to a thermal insulating material for ZrC_(0.5) with low lattice thermal conductivity.Thus,it opens a window to tune the thermal properties of ZrC_(x) by controlling the carbon vacancy content.