Preperation And Properties Of The Microwave Sintered Porous NiTi Shape Memory Alloy With Light Weight And High Strength And High Damping Capacity

As a new type of structural material with intelligence and function,the NiTi shape memory alloy has been widely applied in aerospace,mechanical engineering,civil engineering and biomedical engineering due to its excellent mechanical properties,unique shape memory effect and superelasticity,good wear resistance and corrosion resistance and high damping performance.However,its high density(~6.45 g/cm3)is a very unfavorable factor when it is applied to aerospace structures or working at a high speed operation mechanical structure device.In this thesis,porous NiTi shape memory alloys with low density,high mechanical strength,high damping capacity and excellent shape memory effect were prepared by microwave sintering technique combined with pore-forming agent technology.The porous NiTi alloys could be used for vibration control,noise reduction,impact and shock buffering and energy absorption etc.for the aerospace industry.The effects of pressing pressures,microwave sintering temperatures and the Mg pore-forming agent contents on the microstructure of porous Ni Ti alloy were investigated.Furthermore,the mechanical properties,phase transformation behavior,superelastic behavior and damping capacity of porous NiTi alloy samples prepared without adding pore-forming agent or adding Mg or NH4HCO3 as pore-forming agent,were comparatively evaluated as well as their corrosion behavior in 3.5 wt.% Na Cl solution.The results showed that the optimum pressing pressure and microwave sintering temperature used to prepare porous NiTi shape memory alloys with Mg pore-forming agent are 775 MPa and 900 ℃,respectively.The porous NiTi alloys were mainly composed of B2 phase and a little B19? phase.The samples prepared in the same conditions with the NH4HCO3 pore-forming agent were composed of not only B2 phase and B19? phase,but also contained a little second phase of Ti2 Ni and Ni3 Ti phases.With increasing the contents of pore-forming agents,the diffraction peaks of the main phase increased,while the second phases weakened.The pore sizes of the samples prepared without adding pore-forming agent were less than 50μm,those of the samples prepared with Mg pore-forming were almost less than 100μm,while the pore size distribution of the samples prepared with NH4HCO3 pore-forming was mainly in the range of 100~150μm.The average density of the porous NiTi alloy sample prepared with 3wt.% Mg pore-forming agent was 3.86 g/cm3,decreased by 40.2% compared to that of the dense NiTi.The average compressive strength and bending strength were 1880.9 MPa and 209.6 MPa respectively,approximately to those of the samples prepared without adding pore-forming agent,which are 1897.6 MPa and 208.0 MPa respectively.With increasing the contents of pore-forming agents,the compressive strength and bending strength of the porous NiTi alloy samples prepared with Mg pore-forming increased first,and then decreased,while the compressive strength and bending strength of the samples prepared with NH4HCO3 pore-forming linearly decreased.The DSC curves on cooling of the porous NiTi alloys fabricated by two different pore-forming agent were changed from two steps phase transformation to one step phase transformation,while the DSC curves on heating of them all showed the multi-step phase transformation.When the pre-strain was less than 10%,the superelastic recovery rate of the two different porous NiTi alloy samples all reached more than 60.0%.The superelastic recovery rate of the samples prepared with Mg pore-forming with the porosity of 43.7% or prepared with NH4HCO3 pore-forming with the porosity of 42.9% can reach more than 90.0% after treated by 4 times superelastic training in 8% pre-strain.The internal friction of the porous NiTi alloys decreased with increasing the vibration frequency.The samples prepared with Mg foreforming possessed higher internal friction values and lower storage modulus values compared to those prepared without adding pore-forming agent or those prepared with NH4HCO3 fore-forming.The maximum internal fiction value of the Mg pore-forming porous NiTi alloy can be up to 0.165,which is 0.65 times higher than that of the dense NiTi alloy(0.100).The corrosion resistence of the samples prepared with Mg poreforming in 3.5wt.% solution had no obvious different from the sample with the approximately porosity prepared with NH4HCO3 pore-forming.After the friction and wear tests under the Si3N4 ball as the pair grinding part,the wear volume and the wear volume rate of the samples prepared with Mg pore-forming were less than those of the samples prepared with NH4HCO3 pore-forming,which indicated that the former had a better wear resistence than the latter.