Martensitic Transformation And Strain Recovery Characteristic Of Ru Based High Temperature Shape Memory Alloys

The martensitic transformation and strain recovery characteristics have been systematically investigated by optical microscopy, TEM, DSC, XRD, and compression test. The effect of compression deformation on martensite microstructure and internal relations between martensite microstructure and strain recovery characteristics has been illuminated. The effect of adding Fe to martensitic transformation and mechanical properties has been studied. Micromechanism of adding Fe to improve fracture ductility at room temperature of Ru based alloys has been explained.The experimental results show that a two-step phase transformation occurs when cooling Ru₅₀Ta₅₀, Ru₅₃Nb₄₇, and Ru₅₀Nb₅₀ alloys from high temperature to room temperature, formingβ″martensite of monoclinic structure. While one step phase transformation occurs during the cooling procedure of Ru₅₄Ta₄₆, Ru₄₅Ta₅₅, and Ru₄₅Nb₅₅ alloy from high temperature to room temperature,β″martensite of tetragonal structure. For Ru_50-xNb₅₀Fe_x(X=3.5, 7, 14) alloy, two-step phase transformation occurs when cooling these alloys from high temperature to room temperature. Keeping the content of Nb, transformation temperatures drop down with reducing of Fe content. Deformation of Ru based alloys makes that martenstic inverse transformation temperatures increase and phase transformation hysteresises become wider.The study of TEM research shows that thermalβ″andβ′martensite exist at room temperature in Ru based high temperature shape memory alloys. The martensite variants ofβ″andβ′martensites exhibit triangle-like or wedge-like self-accommodating morphology, with alternating regular bands inside; this kind of variants boundary is straight. The twinning relationship between the substructural bands is (101) Type I mode. Compression deformation at room temperature has great influence on martensite microstructure. The main deformation mechanism involved varies as the following sequence: When the specimen deforms, the reorientation and coalescence ofβ′andβ″martensite variants take place. While deformation increases the self-accommodated morphology begins to be broken, injection of the foreign variant into the substructural bands can be found, dislocations can be found in martensite varients. With further deformation, the self-accommodated morphology has been completely broken, uniform variant of martensite is formed, the density of dislocations in martensite variant is increasing and forming winding dislocation.Martensite type has great influence on strain recovery characteristics. After compression deformation and heating,β′martensite shows only one step shape recovery withβ′→βinverse martensitic transformation happening. When deforming at room temperature and heatingβ″martensite, two step shape recovery occurs withβ″→β′andβ′→βtwo-step inverse martensitic transformation happening, and recovery strain ofβ′→βinverse martensitic transformation is good than that ofβ″→β′martensitic inverse phase transformation.The addition of Fe can improve the shape memory effect, the maximal complete recovery strain of Ru₅₀Nb₅₀ is 2%, while the maximal complete recovery strain of Ru₄₃Nb₅₀Fe₇ is 2.5%.The addition of Fe decreases the compression strength and improves the ductility at room temperature of Ru-Nb alloy. The addition of Fe changes the failure mode of Ru-Nb alloy, failure mode of Ru-Nb alloys is brittle intergranulal fracture. failure mode of Ru-Nb-Fe alloys is ductile intragranular fracture. The change of failure mode is the main reason of ductility increment.