Microstructure Evolution And Failure Mechanism Of A Cobalt-based Alloy Under Thermal Shocks

Cobalt-based alloy is a traditional superalloy,which is widely and irreplaceably used in generation equipment and aerospace equipment.The cobalt-based alloy in this research is prepared by hot isostatic pressing?HIP?technique.It has excellent high temperature strength and corrosion resistance.Due to the rapidly heating and rapidly cooling between elevated temperature and room temperature during the working condition,the microstructure evolution and failure mechanism of the cobalt-based alloy in the thermal shock test is essentially to be well studied.This thesis comprehensively studied this question in aspects of oxidation,mechanical properties,microstructures associated with stress.The static oxidation products and the static oxidation kinetics were analyzed.The changes of compressive mechanical properties of this cobalt-based alloy before and after static oxidation test were analyzed.At the same time,high temperature tensile test and high temperature fatigue test under static oxidation condition were carried out and the results were analyzed.The results showed that the effect of static oxidation on mechanical properties was not obvious.The macro-mechanical properties and micro-mechanical properties of this cobalt-based alloy before and after thermal shocks were tested by means of in-situ tensile test and nano-indentation test.It was found that the large area of damage?oxidation crack?on the cross section of the specimen was the main factor for the decrease of the macro-mechanical properties of the cobalt-based alloy.Residual stress,dislocation density and micro-crack were the main factors that account for the change of micro-mechanical properties.These three factors were analyzed by means of Suresh model and EBSD analysis.It was found that the carbides in the cobalt-based alloy were preferential cracked in the process of in-situ tension test.The cohesive zone model and Cohesive unit were adopted to simulate the tensile fracture behavior of the cobalt-based alloy.The simulation results showed that the diffusion layers around the carbides had a direct effect on the tensile properties of the cobalt-based alloy.Based on the analysises of the changes of microstructure and grain boundary diffusion in the cobalt-base alloy samples that subjected to thermal-shock treatment,it was found that there were elements diffusion around the carbides and precipitates of M₆C and M₂₃C₆during the process of thermal shocks.Both the precipitates and the interfacial diffusion distances increased with the increase of the thermal-shock temperatures and the number of thermal shocks.What's more,microcracks were be found again on the carbides inside the samples subjected to thermal shocks.A polycrystalline plastic model was established to simulate the formation of microcracks during thermal-shock process.The diffusion layer between WC particles and Co matrix was simulated as the Cohesive unit;the internal stress level and stress distribution generated by the difference of the thermal expansion coefficients of the matrix and the carbides in the process of heating and cooling were simulated.During the heating process of thermal shocks,the Mises stress was seriously concentrated on the internal of WC carbides.However,the Mises stress was seriously concentrated at the interface of WC/Co matrix during the cooling process of thermal shocks.These highly stressful concentration locations were prone to be damaged.The simulation results elaborated the phenomenon of microcracks initiation at the interfaces and the hard phases during thermal-shock process.The initiation and propagation of cracks during thermal shocks were studied by comparison the thermal-shock oxidation with the static oxidation tests.Stresses aroused by oxidation,which relating to the volume changes of oxidation,the mismatch of thermal expansion coefficients?CTEs?between oxides and the matrix,and the temperature gradient between the matrix and the internal oxides,were analysed and calculated.These stresses promoted the nucleation of thermal-shock cracks.The large amount of internal oxides after thermal shocks reduced the density of the alloy and accelerated the propagation of cracks.It was found that the crack growth rate in thermal shocks satisfied the classical crack growth equation of Paris'law and the crack growth rate was dominated by thermal stress.The growing mechanism of"finger"like surface oxide in thermal-shock tests was discussed.Based on the Stokes-Herring-Suo model,the stress generated by oxidation-diffusion-creep during oxidation was analyzed and calculated.The shear-lag mechanics model was used to analyze the relationship between the degree of the oxidation stress and the growing behavior of the oixde finger.