Microstructure,Mechanical Properties And Thermal Conductivity Of Mg-Sm(-Mn) Alloy

Magnesium alloy has many advantages,such as low density,high specific strength and specific stiffness,good electromagnetic shielding,excellent thermal conductivity and so on.It is widely used in aerospace,transportation,electronic communication and other fields.In recent years,with the rapid development of science and technology,the integration of electronic components is getting higher and higher,and the cost of continuous improvement of performance is that the heat of the motherboard is getting larger and larger,especially small portable electronic devices such as mobile phones and tablets.Therefore,it is required that the shell material of such equipment should have good thermal conductivity while taking into account a certain strength.Such shell material can protect the internal precision components while playing a similar heat dissipation effect of the soaking plate to ensure the normal operation of electronic equipment.Therefore,it is of great significance to the research and development of magnesium alloys with certain mechanical properties and thermal conductivity,but how to achieve the synergistic improvement or relative balance of the two properties still has certain challenges.Although pure magnesium has excellent thermal conductivity,its mechanical properties are seriously insufficient and cannot be directly used as a load-bearing structural component.In the same alloy system,the mechanical properties and thermal conductivity are often balanced with each other.The addition of any metal element in pure magnesium will more or less scatter electrons,resulting in a decrease in the thermal conductivity of the material.In this study,the rare earth Sm element,which has a sharp decrease in solid solubility with the decrease of temperature,is selected for alloying.The lower solid solution atom content is beneficial to reduce the influence on the thermal conductivity.The cost of Sm element is low,and it has the advantages of small proportion but significant strengthening effect.It has become an important alloying element in the strengthening process of magnesium alloy.Two kinds of alloys were designed in the experiment,one is based on the binary Mg-5Sm alloy,and the other is the ternary Mg-5Sm-1Mn alloy.The significance of adding Mn element is that it can refine the grain size,and Mn element does not form any intermetallic compound with Mg,and it is almost not dissolved in the Mg matrix.Under the premise of having as little influence on the thermal conductivity as possible,it plays a certain strengthening role in the mechanical properties of the alloy,and plays a horizontal comparison with the binary Mg-5Sm alloy.This study is mainly divided into three parts:as-cast and heat treatment state,extrusion deformation and aging state,and extruded pretensile secondary deformation and aging state,to explore the influence of changes in the microstructure of alloy under different process conditions on the mechanical and thermal conductivity properties.The mechanical properties of the two alloys are relatively low in the as-cast and heat-treated states,and the mechanical properties of the as-cast alloys are enhanced to some extent by the treatment of solid solution,but the content of Sm atoms in the matrix is increased by the treatment of solid solution,resulting in a sharp decline in the thermal conductivity.After the subsequent aging treatment,due to the precipitation of Mg₄₁Sm₅phase,Thermal conductivity has been improved again;After extrusion deformation of the alloy,the strength and elongation of the material are greatly improved compared with that of the cast state due to the effect of fine grain strengthening.After aging heat treatment of the extruded alloy,although aging hardening will lead to a decrease in the material elongation,the strength of the material is further improved due to the precipitation of a large number of Mg₄₁Sm₅nanoparticles.At the same time,as the content of Sm atoms in solid solution in the matrix decreases,the hindrance effect of defects on electron movement decreases,so the thermal conductivity is also improved.Finally,the extruded alloy is pre-stretched secondary deformation and aging heat treatment.After the pre-stretched treatment with 10%shape variable,the grain size and grain boundary area of the extruded alloy do not change greatly,and a large number of dislocation defects are introduced at the microscopic level,which provide more nucleation sites for the aging precipitated phase.Compared with the pure extruded and aging state,The phase of Mg₄₁Sm₅precipitated by aging is more dense.Combining the treatment method of pre-tensile secondary deformation with the study of mechanical and thermal conductivity,although more dislocation defects may decrease the thermal conductivity while improving the material strength,the thermal conductivity will be improved due to the precipitation of denser Mg₄₁Sm₅phase.Combining these two influences,The final result is that the strength of the material can be further improved without any decrease in the thermal conductivity of the two alloys.The final yield strength of the Mg-5Sm alloy is 209 MPa,the elongation is 17%,the thermal conductivity is103 W/m·K,and the final yield strength of the Mg-5Sm-1Mn alloy is 230 MPa.The elongation is 14%and the thermal conductivity is 95 W/m·K.Transverse comparison between the two alloys shows that the grain size of ternary Mg-5SM-1Mn alloy is finer than that of binary Mg-5Sm alloy either as cast or as extrusion-modified after the addition of Mn element,so the strength of ternary Mg-5Sm-1Mn alloy is relatively high.However,due to the relatively large grain boundary area of ternary Mg-5Sm-1Mn alloy,At the same time,there are nano-Mn particles in the matrix,and the scattering effect on electrons is stronger than that of the binary alloy,and the mean free path of electrons decreases,so the thermal conductivity decreases compared with the binary alloy.