The Fabrication And Damping Performance Of Light-weight Porous NiTi Shape Memory Alloy Composites Strengthened With In-situ Formed Ti₂Ni Phase

Noise and vibration are widely presented in all kinds of mechanical fields,especially in the high-speed rotating machines,which can cause great harmful to human body and the environment.Now,the most valid way to solve this problem is to adopt the high damping materials.In order to satisfy the stringent requirments,such as light-weight,energy-saving and weather resistance,in the fields of automobile,high speed train,aerospace and military fields,it is imperative to explore the metallic materials with light weight,high strength and high damping capacity.Porous NiTi shape memory alloys(SMAs)have attracted great attentions owing to their great potential in becoming light-weight high damping metallic materials.However,the key problem that the contradiction between the intrinsic damping at parent state and the specific strength for porous NiTi SMAs is not yet to be settled well.In this thesis,light-weight NiTi composites were prepared by introducing in-situ formed Ti2 Ni phase and pores into NiTi SMAs.Furthermore,the effects of Ti2 Ni phase amount,morphology,pore shape and porosity on the microstructure,mechnical properties and damping capacity of the light-weight NiTi SMA composites were systematically investigated,some importants conclusions can be concluded as follows:Firstly,it has been found that the in-situformed Ti2 Ni phase can significantly enhance the specific strength and ductility of dense NiTi SMA composite,and the phase amount and morphology of Ti2 Ni phase are the two key factors.When the Ti2 Ni phase amount is about 20% and the Ti2 Ni phaseswith 5~10μm half-net-like and 100~1000nm particle-like distributed in the NiTi matrix,the strengthen effect can become optimum.At that time,the compressive breaking strength at room temperature can reach as high as 2.9GPa(the specific strength is 450MPa?cm3/g),the strain to fracture can reach 37%,the compressive stress at 130℃ can also reach 280MPa?cm3/g,moreover the intrinsic internal friction at parent phase maintains above 0.008.Secondly,the 2 ~ 5μm particle-like in-situ formed Ti2 Ni phase and 100 ~ 300μm irregular pores were introduced into the NiTi alloys to prepare porous Ti2 Ni /TiNi composite at the same time,the specific strength declines a half,however the intrinsic internal friction increases significantly.Whatever the strain amplitudes,frequencies and cooling/heating rates are,the intrinsic internal friction at parent phase and martensite keep above 0.02 and 0.045 repsectively,which are 150% and 29% enhancement for the dense Ti2Ni/NiTi SMA composite.At the same time,the superelasticity of the porous Ti2Ni/ NiTi composite can reach 2%,and the specific strengthes at both RT and 130℃ are 180MPa?cm3/g and 120MPa?cm3/g,which are better than the porous NiTi SMAs with similar porous structure.It has been found that,the porous Ti2Ni/NiTi composite with the same porosity,the irregular pores help to improve the intrinsic internal friction of both parent phase and martensite,but weaken the specific strength at RT.Finally,the bimodal porous Ti2Ni/NiTi composite with 60% porosity had been prepared successfully,the intrinsic internal friction of which can reach as least 0.06 at a wide temperature range from-80 to 200℃.Moreover,the intrinsic internal friction at parent phase can reach above 0.1,and the internal friction at martensite can reach 0.2,which are the best results for all the reported dense/porous NiTi composites so far.At the same time,the compressive breaking strength at RT can reach 150 MPa,the strain to fracture can also reach 18%,which are 4 and 5 times better than the porous NiTi alloys with similar porous structure.