The Effect of Severe Plastic Deformation to the Shape Memory Properties of Ni50Ti30Hf20 High Temperature Shape Memory Alloy During Heating-Cooling Cycles Under Constant High Stress MagnitudesWednesday (16.05.2018) 09:50 - 10:15 Part of:
High temperature shape memory alloys have attracted great attention with the need of utilizing these alloys above 100oC. Recently, NiTiHf alloys are promising candidates for especially aerospace applications with their high strength and increasing transformation temperatures with the addition of hafnium. The equiatomic Ni(Ti+Hf) alloy with 20at% Hf content was produced using high purity Ni, Ti and Hf elemental materials via vacuum induction melting under high purity argon atmosphere. Then, the cast alloy was placed in a mild steel can and hot extruded at 9000C with an area reduction of 4:1 for homogenization. The alloy was further subjected to severe plastic deformation via Equal Channel Angular Extrusion (ECAE) following the route C for 2 and 4 passes at 700°C. The transformation temperatures of the extruded and ECAEd samples were measured using Differential Scanning Calorimetry. Transmission Electron Microscope was utilized to investigate the microstructural evolution with ECAE process. Transformation temperatures of hot extruded samples were above 200oC and not affected via the application of ECAE process. The isobaric heating-cooling experiments under incremental constant stress magnitudes were conducted on extruded and ECAEd samples. The constant stress magnitude for each experiment was increased up to 600 MPa. The shape memory properties such as transformation and the irrecoverable strain levels and the transformation temperatures of the extruded and the ECAEd samples were compared for all stress cycles. It was found that there were no noticeable differences in the shape memory properties of samples ECAEd for 2 and 4 passes. However, irrecoverable strain magnitudes decreased from 1.8% down to 1.5% and the transformation strain magnitudes increased from 2.6% to 3% under 600 MPa after ECAE process. These results were attributed to the strengthening of the alloy via severe plastic deformation.