Effect Of Precipitates In Mg-Zn-La/Ce Alloy On Thermal Conductivity

The addition of alloying elements can improve the mechanical properties of magnesium alloys but reduce the thermal conductivity.The main reason is that after adding alloying elements,the content of solute elements and the second phase in the matrix could scatter the electrons and phonons and reduce the thermal conductivity of Mg alloys.Aging treatment could precipitate the solute atoms from the matrix in the form of precipitates,which is expected to improve the thermal conductivity and mechanical properties of Mg alloys at the same time.However,in the aging treatment process,the influence of the coherent relationship between the precipitates and the?-Mg matrix interface on the thermal conductivity of Mg alloys remains controversial.The Mg-Zn-RE alloys possess well mechanical property.Among RE elements,La and Ce elements are potentially preferred elements for heat-dissipating magnesium alloys due to their low solid solubility in Mg matrix.Therefore,this paper chooses Mg-Zn-La/Ce alloys as the research object to study the influence of precipitation on the thermal conductivity of magnesium alloy.Design and develop high thermal conductivity Mg-Zn-La/Ce alloy.A physical thermal conductivity database of the Mg-Zn-La/Ce system was established by the CALPHAD method combined with experimental data.According to the thermal conductivity database,the distribution of thermal conductivity for Mg-Zn-La/Ce alloys in Mg-rich was predicted and designed.The heat treatment parameters of peak-aged alloys and over-aged alloys were determined by thermodynamic calculations and age hardening curves.The structure and morphology of the precipitated phases in different aging stages are analyzed by TEM.Combined with the measurement results of thermal conductivity,the influence mechanism of alloying elements and precipitation relative to the thermal conductivity of Mg-Zn-La/Ce alloy was analyzed.The corresponding research conclusions are as follows:(1)The establishment of thermodynamic and thermal diffusivity database of the Mg-Zn-La/Ce systems is used to predict the heat capacity,density,and thermal diffusivity of the alloy,and predict the thermal conductivity of Mg-Zn-La/Ce alloys.Using thermal diffusion resistance(RTD)as the parameter,the thermal diffusivities of pure Mg,pure Zn and Mg_98-99.5Zn_0.5-2 alloys were measured.The relationship of RTD with temperature and composition of pure Mg,pure Zn,Mg₇Zn₃?La/Ce Mg₁₂ and?₁-RE_0.08(Mg,Zn)_0.92 was optimized.With the RTD database coupling with thermodynamic database,the thermal conductivity of different Mg-Zn-La/Ce alloys were predicted.The standard and relative errors with the experimental data are±3.0W/(m·K)and 1.6%,respectively.(2)Comparing the effect of adding La/Ce element on thermal conductivity,under the same heat treatment condition,the thermal conductivity of the alloy containing La/Ce is 18.2-30.2 W/(m·K)higher than that of the Mg-Zn binary alloy.Comparing the thermal conductivity of Mg-Zn-La/Ce alloy in different heat treatment condition,the thermal conductivity of the over-aged state alloy own highest thermal conductivity.After peak-aged treatment,the increase in thermal conductivity is 21.1-28.8 W/(m·K).For the alloys at over-aged condition,the increase in thermal conductivity compared with alloys at peak-aged condition is about 0.6-9.8 W/(m·K).The Mg_97.3Zn_2.6La_0.1and Mg_97.4Zn_2.5Ce_0.1 alloys,annealed after 673 K/6 h+473 K/20 h have highest thermal conductivity,and the thermal conductivity is 155.3 W/(m·K)and 148.4W/(m·K).This value is closed to the thermal conductivity of pure Mg about 159.5W/(m·K).(3)The addition of La and Ce forms?₁phase,which reduces the solid solubility of Zn,resulting in a higher thermal conductivity of alloy containing La and Ce than the Mg-Zn binary alloy.Before peak aging,the formation of?₁'-Mg₄Zn₇leads to a decrease in the content of solute elements in the?-Mg matrix and weakens the lattice distortion of the matrix.The thermal conductivity of the alloys improved greatly.After coming into the over-aged condition,the mismatch between(01(?)0)_?'1 and(0001)_Mg in Mg_97.3Zn_2.6La_0.1 and Mg_97.4Zn_2.5Ce_0.1alloys are 0.76%and 0.6%,which means the?₁'is coherent with?-Mg matrix and has little influence on thermal conductivity.