Microstructure And Properties Of Biomedical NiTi Based Shape Memory Alloys By Vacuum Arc Melting

The equiatomic ratio NiTi shape memory alloy has unique shape memory effect,superelasticity,biocompatibility and excellent mechanical properties,and has high application value in the field of medical devices.With the advancement of society and the improvement of medical level,the quality and performance of the equiatomic ratio NiTi shape memory alloy under high temperature and low temperature conditions have higher requirements.Therefore,it is our current task to develop a NiTi-based shape memory alloy that can satisfy a wider use environment.In this paper,the development of NiTi-based shape memory alloy with excellent properties was studied,and the microstructure,phase transition behavior and mechanical properties of the alloy were analyzed.Firstly,the control group of the cquiatomic ratio NiTi shape memory alloy was set up.Secondly,the influence of the content of single addition of Cr,V and Fe on the microstructure,phase transition behavior and mechanical properties of NiTi alloy was studied systematically.Based on the experimental results,The effects of Fe content on microstructure,phase transformation behavior and mechanical properties of the alloy were investigated by adding Fe and V elements.The ingredient formula of Ni49.25Ti49.25Fe1.OV0.5 alloy was optimized by comparison analysis.The main conclusions drawn are as follows:(1)When Cr,V and Fe are added in a single element,the grain refinement in NiTiCr alloy increases with the increase of element content,and the solid solution strengthening effect is gradually enhanced,the precipitated phase of HCP structure Cr2Ti is gradually connected at the grain boundary by points to Lines,subgrains appear in the microstructure;the medium-axis microstructure of the NiTiV alloy is slightly refined,and the solid solution strengthening effect of the V atoms is gradually enhanced,and the V-containing precipitates phase enriched along the grain boundary in the alloy are changed from the FCC structure NiTi2 to the BCC structure elemental V phase;The grain size of the equiaxed structure of NiTiFe alloy is gradually increased,and the solid solution strengthening effect is gradually enhanced.(2)When a single element of Cr,V and Fe is added,the phase transition temperature decreases with the increase of element content,and the influence of Cr and Fe elements on the phase transformation temperature of the alloy is more significant,and the degree of subcooling causes the phase change driving force to increase,and the phase transition enthalpy decreases gradually during the temperature rise and fall process;in the NiTiCr alloy,the low temperature martensite phase atom activity is lower,and the phase transition enthalpy during the heating process is greater than the temperature drop process;in the NiTiV and NiTiFe alloys,due to the presence of V and Fe added elements has increased the stability of the B2 phase,the phase transition enthalpy in the alloy cooling process is greater than the heating process.(3)When Cr,V and Fe are added in a single way,as the influence of solid solution strengthening and the presence of high-strength and high-hardness HCP structure Cr2Ti in the grain boundary,the hardness of NiTiCr alloy increases,and the hardness of the alloy reaches a peak of 357.69Hv when the Cr content is 3.5 at.%,which is 27.01%higher than that of the NiTi alloy;In the NiTiV alloy,due to the formation of the BCC structure elemental V soft phase,the V content is in the range of 0.5-3.5 at.%,and the hardness is basically maintained between 200.00-225.00Hv,which is 20.10-28.98%lower than the equiatomic ratio NiTi alloy;Due to the solid solution strengthening mechanism of Fe,the hardness of the NiTiFe alloy showed a sharp rise in positive correlation,when the Fe content was 3.5 at.%,the hardness of the alloy reaches a peak of 469.91 Hv,which is 66.86%higher than that of the NiTi alloy.(4)When Fe and V are added in combination,the phase transition temperature of the alloy decreases as the Fe content increases due to solid solution strengthening of Fe,the martensitic transformation temperature is lower than-120℃,and the phase change driving force was increased provided by the degree of subcooling,and the phase transition enthalpy decreases as a whole.The stability of the B2 phase increases due to the reverse defect of Fe and the atomic relaxation effect of Ti,the phase transition enthalpy during the cooling process is greater than the temperature rise.When the Fe content in the alloy is more than 0.5 at.%，the solid solution strengthening effect of Fe causes the hardness of the alloy to rise sharply,and when the Fe content increases to 1.0 at.%,the hardness of the alloy is 282.11Hv,which reaches the level of the equiatomic NiTi alloy and continues to increase the Fe content,due to the formation of the BCC structure elemental soft phase V，according to the short plate effect,the solid solution strengthening effect of Fe reaches the limit,and the hardness does not change significantly.