Mechanisms Behind The Spontaneous A-site Metal Whisker Gtowth On MAX Phases

The MAX phases are a family of nanolaminate ternary carbide or nitride ceramics,which combine some properties of metals,such as low hardness,machinability,and good electrical and thermal conductivity,with those of ceramics,such as high elastic moduli,and oxidation,corrosion and irradiation damage resistance.The MAX phases have shown great promise in applications involving electric contact materials,high temperature structural materials and nuclear materials.However,the A-site metal whisker growth on MAX phases poses a great challenge to the reliability of MAX phases.Therefore,a total understanding of the mechanisms behind is urgent to solve the problems associated with the growth of metal whiskers on MAX phases.In addition,metal whisker growth on both MAX phases ceramics and metals are atomic motion in essence.Consequently,investigating the mechanisms behind whisker growth on MAX phases will also assist the understanding of the general whiskering problems in other materials.With the combination of first-principles calculations and experimental studies,Ti₂SnC,a typical MAX phase,is chosen to study the mechanisms behind spontaneous metal whisker growth on MAX phases from the point of atomic motion.The main finding are as follows:Sn atom source for the spontaneous Sn whisker growth on Ti₂SnC/Sn system has been investigated by combining stable isotope tracing approach and inductively coupled plasma mass spectrometry?ICP-MS?.Acid dissolution method is used to dissolve the whiskers,and ¹¹⁸Sn and ¹²⁰Sn are selected as the analytic elements.Firstly,a reliable ICP-MS test method has been built by analyzing the composition of the Sn powder and the Sn whiskers grown on Ti₂SnC/Sn system.Then,the Sn atom source for the whiskers grown on Ti₂SnC/¹²⁰Sn has been studied.It is found that Ti₂SnC and elemental ¹²⁰Sn provide 67%and 33%Sn atoms for whisker growth,respectively,when the molar ratio of them is 1:0.15.Besides,when the Ti₂SnC/A?A?Sn?system has whiskering phenomenon,the composition of the whiskers is Sn and all the Sn atoms come from the Ti₂SnC lattice.The results clarify the Sn atom source for Sn whisker growth on Ti₂SnC/Sn system,i.e.,the Sn atoms come from both the Ti₂SnC lattice and the elemental Sn.According to the formation energy calculation of Ti₂AC?A=Al,Si,P,S,Ga,Ge,Cd,In,Sn,Pb,Bi?phases,the interaction between A element and Ti₂SnC has been simulated.Then the whisker growth behavior and whisker composition in Ti₂SnC/A systems has been experimentally studied.When the substitution behavior is feasible in Ti₂SnC/A?A?Sn?system,the A atoms will diffuse into the Ti₂SnC lattice to substitute the Sn atoms,and the substituted atoms would have higher chemical potential.The existence of chemical potential gradient,together with the energy released during the substitution process,provide driving force for Sn atom diffusion.Otherwise,the Sn atoms in Ti₂SnC would maintain the equilibrium state,and there is no driving force for Sn atom diffusion.In Ti₂SnC/Sn system,Sn atoms in elemental Sn will diffuse into the Ti₂SnC lattice driven by the concentration gradient.The Sn atoms in supersaturation state would have higher chemical potential,and thus provide driving force for diffusion.For example,Ti₂SnC/Sn and Ti₂SnC/Ga systems are capable of substitution,while the substitution behavior is difficult to occur in Ti₂GaC/Sn and Ti₂SnC/Cd systems.As a result,Sn whisker growth is observed on Ti₂SnC/Sn and Ti₂SnC/Ga bulks,while,no whiskers are observed on Ti₂GaC/Sn and Ti₂SnC/Cd bulks.Finally,a relationship between the interaction of Ti₂SnC/A and the whisker growth behavior has been established,and the driving force for whisker growth has been revealed to come from the interaction between Ti₂SnC lattice and elemental A.The crystallographic condition and mass transport channel for Sn whisker growth has been studied by the planar disregistry calculation of Ti₂SnC/Sn,simulation on the formation and migration energies of vacancies in Ti₂SnC,together with the FIB-SEM/TEM characterization of Ti₂SnC/Sn whisker interface.According to the simulation results,the Sn vacancy with the formation energy of 1.72 eV is the easiest vacancy to form in Ti₂SnC,and the diffusion activation energy of Sn is calculated to be 0.60 eV.Therefore,a low diffusion barrier is expected for Sn atoms to diffuse along the Sn layer in Ti₂SnC.The Ti₂SnC/Sn whisker interface orientation relationship is close to the low interface energy orientation.In addition,the whisker root is not connected with the elemental Sn distributed in the substrate,and Sn atoms diffused from the Ti₂SnC substrate and combined with the whisker root has been observed at the Ti₂SnC/Sn whisker interface using HRTEM.The results indicate that Sn whiskers tend to form on the locations where the interface energy between Ti₂SnC and elemental Sn is low;elemental Sn,who plays the role of whisker nucleus in whisker formation process,is necessary for Sn whisker growth;Sn atoms feeding Sn whisker growth mainly migrate along the Sn atom layer in Ti₂SnC.The whisker morphology formation mechanism has been studied by the combination of surface energy calculation of?-Sn,in situ SEM observation of Sn whisker growth,and Sn whisker morphology analysis.The surface energy of?-Sn is anisotropic.When Sn atoms diffuse out of the substrate,Sn whiskers bounded by low-energy planes form to minimize the total energy.In contrast,the whiskers maintain the striated morphology inherited from the whisker nucleus in the environment containing oxygen due to the confining effect of the nano-sized oxide film on whisker surface.In addition,alternate growth of striated and faceted segments happens when the Sn whisker is alternately cultivated in air and vacuum,and all the striated segments share the same cross-sectional contour.Consequently,the typically striated whisker morphology is confirmed to inherit from the whisker nucleus and be reserved by the confinement of the surface oxide film.In conclusion,from the point of atomic motion,the findings in this thesis reveal the Sn atom source,driving force,interface microstructure and mass transport channel,and the morphology formation mechanism behind Sn whisker growth on Ti₂SnC.Therefore,a comprehensive understanding of the physics behind is achieved,which will also provide theoretical basis for metal whisker mitigation.