The Study On Microstructure Evolution Behavior During Sintering Process And Zn Strengthening Of Mg-Mg₂Sn Composites

As a lightweight and environmentally friendly new structural material,magnesium alloy is receiving wide attention.Enhancing strength has always been a focus and difficulty in the field of magnesium alloys.The magnesium alloys are compounded with different types of reinforcements to obtain magnesium matrix composites,which can greatly improve the strength.Compared with other reinforcements,particle reinforcements have great advantages due to their economical advantages.In addition,the in-situ method for preparing the magnesium matrix composites overcomes the disadvantages of poor interface bonding and interfacial reaction caused by the external addition method,and it is more advantageous for obtaining the materials with excellent performance.The mechanical alloying method has the unique value of being able to obtain the particle reinforcement in situ.The nano-sized Sn powder and micro-sized Mg powder were used as raw materials,Mg-Mg₂Sn composites were prepared by mechanical alloying and hot pressing sintering,and the mechanical properties were increased with the increase of milling time.In this paper,the reasons for the properties enhancement of Mg-Mg₂Sn composites were firstly investigated.According to the DSC results of different milling time Mg-Sn mixed powders,the quenching experiments of the cold-compacts corresponding mixed powders were carried out to simulate sintering processes.Then,in order to obtain the magnesium matrix composites with better performance,the Zn element was introduced into the Mg-Sn system,and the influence of Zn and subsequent heat treatment on the microstructure and properties of the Mg-25Sn-xZn material was investigated.The main findings are as follows:After mechanical alloying of Mg-Sn mixed powder in a short time（5h）,the intermetallic compound Mg₂Sn can be formed in situ.As the mechanical alloying time increases,the content of Mg₂Sn in the Mg-Sn mixed powder increases,and the structure is more uniform.There is a good interfacial bond between the Mg₂Sn phase and Mg in the milled powder.During the temperature rise of the Mg-Sn cold-compact,reverse eutectic reaction of Mg₂Sn and Sn,melting of Sn,the first stage reaction of Sn with Mg to form Mg₂Sn,and the second stage formation of Mg₂Sn occurs in sequence.Meanwhile,as the mechanical alloying time increases,the amount of Mg₂Sn generated in the first stage increases,and the Mg₂Sn formation temperature decreases.Then the Zn element is introduced into the Mg-Sn system.As the Zn content increases,the strength of the sintered Mg-25Sn-xZn bulk material increases first and then decreases.Among them,the Mg-25Sn-6Zn bulk material has the best performance,its micro Vickers hardness reaches 160HV,the yield strength is 388MPa,the compressive strength is 497MPa,and strain is 7.5%.The yield strength and compressive strength were increased by 43.2%and23.0%,respectively,compared to the Zn-free Mg-25Sn bulk material.After heat treatment,the mechanical properties of Mg-25Sn-xZn bulk materials are enhanced to some extent.Compared with the sintered material,the Mg-25Sn-4Zn bulk material is most obviously strengthened after being kept at 380°C for 16h and 175°C for 14h.Its hardness reaches 165HV,the yield strength reaches 493MPa,and the compressive strength reaches 545MPa,but the strain is only 6.4%.