Effects Of Pore Structural Parameters On The Superelasticity Of A Beta-type Ti-based Shape Memory Alloy Sheet With Pores

New ?-type porous Ti-based shape memory alloys(SMAs)have a low elastic modulus,good biocompatibility and a mimic structure of human bones,etc.,which show the potential application prospects as the implanting materials.However,due to the presence of the pores in the porous alloys,it can cause the stress concentration,which may decrease the superelasticity of the alloys.Deep study on the effects of the stress concentration caused by the pores on the phase transformation and micro-deformation of the porous SMAs is helpful in revealing the mechanisms for the unique performances of the porous SMAs.Thus,in the present thesis,a shape memory alloy,Ti-7.5Nb-4Mo-2.5Sn,was taken as the research object.The deformation process of the alloy sheets with and without pores were monitored in-situ by using a digital image correlation(DIC)technique,and the effects of the pore structure parameters on the strain distribution,the stress-induced martensite phase transformation and super-elasticity of the alloys were investigated.The main contents and results are summarized as follow:(1)the Ti-7.5Nb-4Mo-2.5Sn shape memory alloy was fabricated by using a vacuum argon arc melting method.After cold rolled with the reduction of 93%and annealing treatment at 800?,the microstructure of the alloy mainly consists of ?phases with the averaged grain size of 50?m.The alloy exhibit the superelasticity at room temperature.The critical stress for the stress-induced martensites,the tensile strength and the elongation are 238MPa,500MPa and 27%,respectively,and the strain recovery rate under the pre-strain of 3%is 57%.(2)The effects of the pore diameter on the superelasticity of the alloy samples with the single-pore were studied.The results indicate that,during the tensile deformation,the stress concentration appears around the pore,which can affect the martensite transformation process in the area.With the increase of pore size,the degree of the stress concentration increases,leading to the localization of the martensite transformation.After unloading,many residue plastic deformation and deformation twins can be observed in the areas around pores,resulting in the decrease of the superelasticity of the alloy.(3)The effects of the distance between the two pores on the superelasticity of the alloy samples with double pores were investigated.It is found that,with the increase of the the distance between the two pores,the critical stress of the stress-induced martensite phase transformation decreases gradually,and the superelasticity of the alloy decreases firstly and then increase The reasons lie in:with the increase of the distance between the two pores,the interaction of the stress fields between the two pores gradually decreases,leading to decrease in the degree of local deformation,and thus to the improvement of the superelasticity of the alloy.(3)The effect of the pore arrangements on the superelasticity of the alloy samples with the four-pore specimens and the double-pore were studied.The results indicate that the pore arrangement has an important effect on stress field distributions in the areas around the pores.Rational design of the pore arrangement in the porous alloy can decrease the degree of the stress concentration in the areas around pores,leading to the improvement of the superelasticity of the porous alloys.