Abstract
The paper was investigated the effect of preliminary multiple martensitic transformations on the microstructure and mechanical and functional stability during subsequent annealing in the range of aging temperatures of the Ti49.051.0 alloy in the coarse-grained state. The structure in the initial state has an austenitic structure with a grain size of 30 ± 5 μm; after TC, the structure is martensite with a grain size of 30 ± 5 μm. According to the results of mechanical tensile tests, thermal cycling leads to an increase in the yield stress, which is associated with the generation and accumulation of dislocations. An increase in the number of cycles to n = 100 led to a slight decrease in the yield stress, which may be due to the saturation effect during thermal cycling. Subsequent aging at T=400 °C after thermal cycling showed that the yield stress increases. At the same time, the results of mechanical tests showed that, in general, the preliminary TC (n = 100) with subsequent aging contributes to an increase in the yield strength and strength. The structure revealed after thermal cycling and subsequent low-temperature annealing confirms the precipitation of aging strengthening particles Ti3Ni4.
Highlights
The NiTi shape memory alloys exhibit excellent characteristics such as shape memory effect and superelasticity and are promising materials for practical applications [1,2,3]
According to the results of mechanical tensile tests, thermal cycling leads to an increase in the yield stress, which is associated with the generation and accumulation of dislocations
The structure of the coarse-grained (CG) Ti49.0Ni51.0 alloy is austenite, which is represented by equiaxed grains with a size of 35 ± 2 μm (Figure 1, a), and aging at T = 400 °C leads to a change in the grain size to 30 ± 2 μm (Figure 1, b)
Summary
The NiTi shape memory alloys exhibit excellent characteristics such as shape memory effect and superelasticity and are promising materials for practical applications [1,2,3]. The most famous methods of obtaining NiTi alloys with nanostructure are methods of severe plastic deformation (for example, cold drawing [15], high pressure torsion [23], extrusion with an equal channel angle [24], repeated cold rolling [25]) followed by heat treatment [15,22]. With such processing, the main task is to keep the grain size in a certain range for high plasticity (deformation with elongation up to 50% [26]), high strength and high impact toughness inherent in NiTi alloys [27]. This work is devoted to the study of this approach during aging both in the low-temperature region and in the field of classical aging
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