Study On Temperature-dependent Hardness Of Pure Metals And Thermal Shock Resistance Of Ceramics

Metals and ceramics play an important role in aerospace,energy,electric power and other industrial engineering fields due to their excellent thermal and mechanical properties.With the advancement and development of science and technology,the demand for the materials in service at high temperatures is increasingly stronger,and strict requirements are proposed for the hardness of metals and thermal shock resistance of ceramics under high temperatures.Thus,reasonably characterizing and improving their mechanical properties have been the focus of current research in the field of high temperatures.This thesis studied the theoretical characterization method of the temperature dependence of pure metals' hardness,and ceramics' thermal shock resistance was researched by a combination of experiments and numerical simulations.The main research work is as follows:1)The relationship between yield strength and hardness at different temperature was proposed,and then combined with the temperature-dependent yield strength model of metal materials established by our group,a theoretical characterization model of temperature-dependent hardness for pure metal materials was established.In the model,by selecting two easily obtained hardness values at different temperatures as references,the hardness of pure metals as a function of temperature could be predicted,and the predicted results are in good agreement with the experimental results.This model establishes the quantitative relationship between temperature-dependent hardness,constant pressure specific heat capacity(or Debye temperature),and melting enthalpy.And the material parameters required in the model can be easily obtained from literature,which provides a convenient method for predicting the hardness at different temperatures.2)Using the material thermal shock resistance testing equipment independently developed by our research group,a large-span of initial and target temperature environment can be created.The cooling thermal shock behavior of alumina and zirconia ceramics suffering a large-span thermal shock target temperature from the same thermal shock initial temperature was experimentally studied.Meanwhile,for zirconia ceramics,the effect of specimen size on their thermal shock resistance was researched by experiment.The results showed that for the alumina ceramic specimens which experienced a large-span target temperature cooling thermal shock,their three-point bending strengths at both room temperature and the thermal shock target temperature were all significantly higher than those at the corresponding temperature without experiencing thermal shock.And a method is proposed to improve the strength of ceramic materials at different temperatures,which could provide an effective method to improve the strength of ultra-high materials and structures at different service temperatures.3)Based on the force-heat equivalence energy density principle,a temperaturedependent failure criterion of ceramics was established,and the criterion was embedded into ABAQUS using the USDFLD subroutine to study the cooling thermal shock behavior of zirconia ceramic materials in liquid and gaseous media.The critical temperature difference of crack initiation at different model sizes cooled in a liquid medium was obtained,and the effect of size on their thermal shock resistance was analyzed.At the same time,when cooled in a gaseous medium,it was found that ceramic materials' thermal shock resistance had a dangerous zone related to thermal shock initial temperature.In this dangerous zone,the critical temperature difference is the lowest and their thermal shock resistance is the worst.And then a measure to improve the thermal shock resistance of ceramics under complex thermal shock environment was proposed.