Phase Transformation Behavior And Mechanical Properties Of Co(Nb) Doped TiNi Shape Memory Alloys

Strain glass is formed by doping excessive defects into alloys that can undergo martensitic transformation to restrain long-range ordered martensitic transformation.Previously reported strain glass alloys are mainly obtained by doping one of point defects,line defects and bulk defects.Recently,it has been found that strain glass transition can occur in Ti Ni Pt nanocrystalline alloy by co-doping Pt atoms with large atomic radius and plane defects.In this paper,by introducing Co element into nanocrystals,we study whether co-doping of Co atoms with smaller atomic radius and grain boundaries can cause strain glass transformation in nanocrystals TiNiCo.In addition,it has been found that TiNiCo alloy has Elinvar effect,which is mainly due to the offset of modulus hardening caused by nano-domain growth and modulus softening caused by phase transformation.In this paper,the effect of doping elements on the Elinvar effect was studied by adding Nb elements to regulate the nano-domain in TiNiCo alloy.In this paper,TiNiCo（Nb）alloys were prepared by severe deformation of cold drawing and low temperature crystallization annealing.The phase transition behavior was studied by means of DMA,DSC and electrical resistivity,the microstructure evolution was studied by TEM and XRD,and the mechanical properties were tested by tensile machine.The results show that the strain glass transition does not occur in the nanocrystalline TiNiCo alloy doped with 5 at.%and 10 at.%Co elements,instead the B2→R phase transition occurs.However,the average structure of nanocrystalline TiNiCo alloy doped with 15 at.%Co element remains unchanged,and the frequency dependence of storage modulus was not detectable due to the temperature limitation of DMA test,so it is difficult to judge whether strain glass transition occurs in nanocrystalline Co15 alloy.With the increase of annealing temperature,B2→R→B19’phase transition occurs in Ti_50.5Ni_44.5Co₅ alloy,and B2→R phase transition occurs in Ti₄₉Ni₄₆Co₅,Ti₅₀Ni₄₀Co₁₀ and Ti_50.5Ni_39.5Co₁₀ alloys.The DMA test results show that the modulus of Ti₅₀Ni₃₅Co₁₅ alloy is almost unchanged over the temperature range of30℃～200℃,which is consistent with the previous reports.The XRD results show that the coarse-grained Ti₅₀Ni₃₅Co₁₅ alloy has B2→R phase transition below-150℃,which is different from the strain glass transition reported in the literature.The nano-domains were observed at-80℃by TEM observation.It is inferred that the Elinvar effect of the alloy is caused by the modulus softening caused by the increase of nano-domain volume fraction during cooling and the traditional hardening caused by the anharmonic vibration of atoms.The modulus of the alloy with Nb element did not soften dramatically at low temperature,and the modulus of Ti₄₇Ni₃₃Co₁₈Nb₂ alloy kept almost unchanged over the range of-130℃～200℃.Compared with the alloy without Nb,the temperature range of Elinvar effect was doubled.The XRD test showed that the structure of the alloy did not change even when it was cooled down to-180℃.Mechanical properties show that the recoverable strain of nanocrystalline Ti_50.5Ni_44.5Co₅ alloy was 7.44%,which is higher than 3.88%of Ti₄₉Ni₄₆Co₅ alloy,while there is no superelasticity in nanocrystalline alloy doped with 10 at.%Co element.The alloy doped with 5 at.%and 10 at.%Co elements has better superelasticity at low temperature,and the maximum recovery strain can be up to 7.95%.The recoverable strain of nanocrystalline Ti₅₀Ni₃₅Co₁₅ alloy was 3.06%,and the maximum elongation in coarse-grained state was 97.04%.The strength of the alloy increases obviously after adding Nb element.Compared with the sample annealed at 400℃for 20 min,the yield strength of Ti₄₅Ni₃₆Co₁₅Nb₄ alloy is 520 MPa higher than that of Ti₅₀Ni₃₅Co₁₅ alloy,and the yield strength of the sample annealed at 600℃is still above 1 GPa.