On The Composition, Structures And Properties Of The Re-melting NiTi Shape Memory Alloys

NiTi shape memory alloys have received a considerable research attention and are widely used because they combine special functional properties with a high mechnaical strength.Its unique and excellent proterites depend strongly on the exact chemical composition,processing history and impurity.These result in its high cost and restrict its further development and extensive application.At the same time a mass of leftover material will be produced during the machining for NiTi shape memory alloys appliance.Therefore it is very important to utilize the leftover material of NiTi shape memory alloys and recycle.At the present days,the effective method of metal material recycle is remelting.But there have no reports on the remeling of NiTi shape memory alloys.So the research on the chemical composition,microstructure and properties of the remelting NiTi shape memory alloys has important significance in both the utilizing of its leftover material and recycle.In this paper,ingots with different content of leftover material of NiTi shape memory allos,electrolytic nickel and sponge titanium have been produced via vacuum induction melting in graphite and ZrO₂ crucible respectively at the laboratory conditions.The chemical composition and microstructure of the ingots were researched.On the basis of the laboratory research results,the leftover material of NiTi shape memroy alloys was remelted under the industrial technolgoy via vacuum induction melting.The chemical composition, microstructure,martensitic trandformation behavior,shape memroy effect and mechanical properties of industrial remelting NiTi shape memory alloys under different treatment state have been researched.Carbon and oxygen content have a distinct increase respectively when graphite crucible was used.The increase of carbon mainly comes from the reaction between NiTi high temperature melts and graphite crucible.During the melting the reaction became drastic with the increase of leftover material content in the NiTi high temperature melts.The reaction could be described with a solid state diffusion and liquid state dissolution-precipitation mechanism.At the same time,the higher carbon content in the leftover material of NiTi shape memory alloys than that in the raw materials also has been accumulated alter remelting. Oxygen came from raw material and ambience.During the solidification oxygen tend to get together to the outside of ingots.When the ZrO₂ crucible was adopted the carbon content decreased compared with which the graphite crucible was adopted.Nevertheless it results in the increase of oxygen content and new impurity element zirconium.These are harmful to the NiTi shape memory alloys.It is very low in despite of the impurity element hydrogen and nitrogen content increase slightly with increase of leftover material content.The microstructure of NiTi shape memory alloy after mixed remelting is not uniformity.The impurity content of remelting NiTi shape memory alloys from leftover material is also accordance with the standard although it is higher than that in mix remelting alloys.Distinct thermoelectistic martensitic transformation bebavior exhibited during thermocycling in the remelting alloys from leftover material of NiTi shape memory alloys.TiC is the most impurity phase in the remelting NiTi shape memory alloys.Different shape and size of TiC particles was observed.The TiC particles with large size and quadrate shape tend to distribute at the grain boundary.A few of Ti₄₄Ni₂₅O₃₁oxid with large size was observed in the remelting NiTi shape memory alloys.Preferred orientation appears obviously in the remelting NiTi shape memory alloys after hot rolled.The grain was elongated along the R.D,and the grain crash is more severe than that in the first melting alloys.The distortion grain was recovered by heat treatment,but it is more smaller than that in the first melting alloys.Because the content of carbon and oxygen increased in the remelting NiTi shape memory alloys,and as a consequence TiC and Ti₄Ni₂₆O₃₁are formed in the alloy during remelting and solidification.So the local environment of a TiC and Ti₄Ni₂₆O₃₁particles looses Ti and its Ni content increases.Thus,the average Ni concentration of the matrix of the alloy increases.The quantity of Ti₃Ni₄ precipitate increases.The lattice distortion and local stress in the alloy matrix around the second phase increases.This results in a corresponding increase of resistiance for martensitic transformation during the thermocycle.On the other hand,due to the orientation relationship between martensite/B2 matrix,TiC/B2 matrix at <110> direction,and the semi-coherent of B2/TiC interface,B2 matrix transformation into martensite via shear deformation along <100> direction be restrained by TiC particles.This restraining effect increases with the increase of TiC.All of above results in a complicated martensitic transformation behavior,decrease of martensitic transformation temperature, increase of temperature interval for the martensitic transfomation and its reverse transformation in the remelting NiTi shape memory alloys.Since the second phase in the alloys,when martensitic transformation occurs the lattice of B2 matrix changes but that of second phase is not changed,which affects the shape recovey,the shape memroy effect of remelting NiTi shape memory alloys decreases.The temperature coefficient of the electrical resistivity of the NiTi shape memory alloys is related to the TiC content.The positive TCR is present with TiC content of 0.1at%alloys.When TiC content is 0.5at%the negative TCR appears.Impurity phase and Ti₃Ni₄ precipitate increase and the refined grain result in the increase of strength of the remetling NiTi shape memory alloys matrix.So the elongation of NiTi shape memory alloys decreases after remelting.The effect of impurity on the ductility strain become stronger with the increase of aging temperature.Its ability of bending deformation decreases.It is identified that TiC particles with large size and quadrate shape is the weakness part in the NiTi shape memory alloys via the in-situ observation of fracture surface by SEM. When the load increase the micro-crack appears near the quadrate TiC.Load increase further many micro-cracks spread and then connect and join in one main crak.When the load is high euough the main crack can't exist stably and expand by itsself.As a consequence the fracture appears.The remelting NiTi shape memroy alloys,TiC increase result in the mumber of micro-crack increase during loading,as well as the matrix strengh is higher,exhibits the typical brittle fracture morphology.It is necessary to obtain a appropriate technology of remelting for reducing the content of carbon and oxygen in the remelting NiTi shape memory alloys in the next research.