Abstract
Influence of thermomechanical treatment (deformation, thermal cycling treatment in the temperature range of martensitic transformations B2-B19’) on the TiNi alloys’ mechanical behaviour and fracture was studied. Different states were considered, they are initial coarse-grained (CG), ultrafine-grained (UFG) after ECAP (with a grain size of 200 nm), the state after ECAP and cold upsetting by 30% - UFG state with high dislocation density. It was shown that thermal cycling causes some increase in dislocation density, strength and microhardness in all the states. Thermal cycling of UFG alloys allows forming the states with non-equilibrium grain boundaries, with additional dislocations of “phase hardening”. The nature of the fracture was analysed in the TiNi alloy in various states.
Highlights
The TiNi-based alloys relate to a class of functional materials with shape memory effect (SME) caused by thermoelastic martensitic transformations [1,2,3,4]
It is of special interest to establish the influence of the UFG state of TiNi alloys on the structural changes during thermal cycling through martensitic transformation temperatures, to determine the role of grain boundaries in dislocation generation at phase martensitic transformations, and to establish the possibility of mechanical and functional properties enhancement of these materials via phase hardening
In the initial CG state at room temperature the Ti49.3Ni50.7 has an austenite structure with a grain size of about 50±5 μm according to scanning electron microscope (SEM) analysis (Fig. 1a)
Summary
The TiNi-based alloys relate to a class of functional materials with shape memory effect (SME) caused by thermoelastic martensitic transformations [1,2,3,4]. HPT technique enables producing TiNi samples in the amorphized state, subsequent annealing may result in formation of an NC structure with a grain size starting from 20 nm [6]. It is of special interest to establish the influence of the UFG state of TiNi alloys on the structural changes during thermal cycling through martensitic transformation temperatures, to determine the role of grain boundaries in dislocation generation at phase martensitic transformations, and to establish the possibility of mechanical and functional properties enhancement of these materials via phase hardening. We studied the mechanical behavior and fractography after various treatments
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