Martensitic Transformation And Shape Memory Effect Of TiNiGa Shape Memory Alloys

In present work,the influence of composition on microstructure,martensitic transformation behavior,mechanical properties,one-way shape memory effect,ion release behavior and electrochemical behavior of Ti₅₀Ni₅₀,Ti₅₀Ni_50-xGa_x?x=0.4,1,2 at.%??Ga substitutes Ni?and Ti_50-xNi₅₀Ga_x?x=0.4,1,2 at.%??Ga substitutes Ti?alloys are investigated by using optical metallography microscope,X-ray diffraction analyzer?XRD?,scanning electron microscopy?SEM?,differential scanning calorimetry?DSC?,tensile testing,bending test method,immersion method,electrhemical method.The influencing mechanism is also analyzed based on the experimental results.The results show that a B2?B19?martensitic transformation is observed in Ti₅₀Ni₅₀alloys upon cooling and heating.The microstructure of Ti₅₀Ni₅₀ alloys consists of B19?martensite.For the alloys with Ga substituting Ni,the microstructure consists of B19?martensite and irregular?Ti,Ga?₂Ni phase at room temperature.For the alloys with Ga substituting Ni,the microstructure consists of B2 phase and irregular?Ti,Ga?₂Ni phase at room temperature.The substitution of Ga for Ni does not change the transformation type,while the substitution of Ga for Ti results in a two-stage transition,B2?R?B19?martensitic transformation.When Ni is substituted by Ga,the transformation temperatures are first raised slightly and then dropped slightly with the increase of Ga content.When Ti is substituted by Ga,the transformation temperatures decrease gradually with the increase of Ga content.Meanwhile,R phase transformation becomes more obvious.At the temperature of A_f+30?,the tensile strength increases and the elongation decreases with the increase of Ga content,regardless of that Ga substitutes Ni or Ti.For the alloys with Ga substituting Ni,only the plastic deformation of the parent phase occurs when the Ga content is 0.4at.%.Stress-induced martensitic transformation occurs when Ga is increased to 1at.%and 2at.%.However,for the alloys with Ga substituting Ti,only plastic deformation of the parent phase occurs and there is no stress-induced martensitic transformation.The shape memory recovery ratio is slightly reduced due to the substitution of Ga for Ni,while significantly improved by the replacement of Ga for Ti.With the increase of pre-deformation,the shape recovery rate of all alloys decreases gradually.The sample with0.4at.%of Ga substituting for Ti exhibits the best shape memory effect.The shape recovery ratio reaches 83%even when the pre-deformation is 20%.Ga ion releases are less than that of Ni ion in both Ti50Ni50-xGax?x=0.4,1,2 at.%?and Ti_50-x0-x Ni₅₀Ga_x?x=0.4,1,2 at.%?alloys.The release rate of Ga and Ni ions shows time dependence and decreases with the increase of immersion time.The substitution of 0.4at.%Ga for either Ni or Ti,respectively,could suppress the dissolution of Ni ions.The electrochemical test in artificial saliva results show that,except Ti48Ni50Ga2,the other five alloy exhibit the higher self-corrosion voltage and lower self-corrosion current density than Ti50Ni50,which means that the corrosion resistance of these alloys is higher than Ti₅₀Ni₅₀.