To shed light on the origin of the loss of functional properties of NiTi with temperature increasing above 100 degrees C, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical tensile testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by nanoscale orientation mapping in TEM. The stress induced martensitic transformation generating recoverable transformation strain as well as plastic strain is not completed at the end of the upper stress plateau. The higher is the test temperature, the larger is the volume fraction of retained austenite as well as the plastic strain. The martensite variant microstructures in NiTi wire deformed up to the end of the stress plateau at 120 degrees C contain partially detwinned single domains of (001) compound twin laminate filling entire grains. It is proposed that the stress induced martensitic transformation proceeds via habit plane interface between austenite and second order laminate of (001) compound twins and that the martensite promptly reorients and deforms plastically by dislocation glide in the [100](001) slip system. When the wire is loaded further beyond the end of the stress plateau, the stress induced martensitic transformation continues and the martensite deforms plastically. It is concluded that the observed gradual loss of superelastic functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the loss of resistance of the stress induced martensite to the plastic deformation under increasing stress.

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

Functional behavior of nanocrystalline NiTi shape memory wires having various austenitic microstructures was investigated by thermomechanical testing and TEM analysis of martensite microstructures in deformed wires. Three yet not very well known or understood phenomena are reported and discussed in this work. First, it is observed that martensitic NiTi wire heated under low applied stresses elongates several percent before it shortens due to reverse martensitic transformation. This phenomenon is explained as being due to thermally induced martensite reorientation proceeding via motion of interfaces between (001) compound twin domains in the microstructure of selfaccommodated B19’ martensite. Second, it is shown that martensite stabilization by deformation (upward shift of As temperature after deformation in martensite) is not caused by plastic deformation and lattice defects introduced by deformation as frequently argued in the literature but that it is due to evolution of martensite variant microstructures in polycrystal grains with increasing applied strain. Third, it is found that i) no plastic strains are generated by martensitic transformation proceeding in the absence of external stress, ii) plastic strains, which are generated by martensitic transformations proceeding under external stress, are significantly larger than plastic strains generated during reorientation or detwinning processes in martensite, iii) plastic strains generated while the wire shortens during reverse transformation on heating are significantly larger than plastic strains generated while it elongates upon forward transformation on cooling. It is claimed that generation of unrecovered plastic strains accompanies all possible martensitic transformation/reorientation/detwinning/ processes in NiTi, though in different extents. Unrecovered plastic strain accompanying martensitic transformation proceeding under external stress are largest. It is proposed that plastic deformation proceeds in oriented martensite phase via [100](001) dislocation slip. This coupled deformation mechanism constitutes a TWIP/TRIP like deformation enabling functional fatigue of NiTi at low stresses below the yield stress for plastic deformation of martensite.

Constitutive behavior of engineering materials is typically characterized by stress–strain curves from isothermal tensile and/or compression tests until fracture. Strain reversible behavior of martensitically transforming shape memory alloys (SMA) is additionally characterized by cyclic stress–strain and strain-temperature curves limited to temperatures and stresses, at which the recorded strain responses are reversible in closed loop cycles. In this work focussing coupled martensitic transformation and plastic deformation of NiTi, we deformed nanocrystalline NiTi SMA wire in isothermal and isostress tensile tests beyond the temperature and stress limits stemming from the requirement on the strain reversibility in closed loop cyclic tests. Stress–strain-temperature responses of NiTi wire in such tests were recorded and analysed. To detect and characterize deformation mechanisms activated in performed thermomechanical loads, electric resistance and dynamic elastic modulus of the wire were evaluated in-situ during tensile tests. Martensite variant microstructures and lattice defects in austenite evolving upon heating deformed NiTi wire under 750 MPa stress were analyzed by post mortem transmission electron microscopy. Stress-temperature diagram showing critical stress–temperature conditions for activation of 5 different deformation/transformation processes in thermomechanically loaded NiTi was constructed from the results of isothermal and isostress tests and the recorded stress–strain-temperature responses were discussed based on this diagram

The aim of the paper was a comprehensive experimental study of the martensite stabilization effect (MSE) in NiTi alloys under various conditions to find the mechanism behind this phenomenon. The MSE was studied in the quenched Ni50Ti50 and Ni51Ti49 alloys subjected to pre-deformation to various strains in three ways: deformation in the martensite state, cooling under stress through a temperature range of the forward transformation and deformation in the austenite state due to the stress-induced martensite formation. It was found that after pre-deformation by the martensite reorientation, the MSE was observed even after a small pre-strain. After pre-deformation by cooling under stress or by stress-induced transformation, the MSE was small if the pre-strain was less than 5%; otherwise, the value of this effect was significant. A new mechanism of the martensite stabilization effect was considered. It was assumed that the damage to the martensite boundaries during the pre-deformation (without plastic strain) decreased their mobility and a larger thermodynamic force needed to be provided to move the interfaces during the reverse transformation.

The influence of heat treatment on the structure and martensitic transformation in the 5-layered NiTi sample deposited by wire arc additive manufacturing on Ti substrate was studied. The Ti2Ni phase was found on the boundary of the Ni-rich NiTi grains after annealing that was not typical for NiTi alloys produced by conventional techniques. This was due to the as-built sample contained Ti-rich NiTi phase in the interdendritic space. During annealing, this phase decayed to the NiTi and Ti2Ni phases and led to the formation of the Ti2Ni precipitates on grain boundaries. Annealing led to homogeneous composition of the NiTi phase within the layer but kept it to be different in various layers. This was caused by the Ti2Ni and Ni4Ti3 (Ni3Ti2) precipitates formed during annealing, but the diffusion of the Ti and Ni elements between layers hardly occurred. Annealing decreased the difference in Ni concentration in the NiTi phase of the 3rd to 5th layers that decreased the temperature range of the martensitic transformation. It was shown that the properties of the NiTi sample deposited to Ti substrate after post-production annealing are close to the properteis of the NiTi sample deposited on the NiTi substrate.

The influence of the chemical composition of a NiTi alloy on the martensite stabilization effect was studied. The Ni- 50.0 at. %Ti, Ni - 49.5 at. % Ti and Ni - 49.0 at. % Ti alloys were quenched from 900 degrees C (10 min) into water and after this heat treatment, the alloys underwent the B2 <-> B19' transformation on cooling and heating without the R phase formation. The martensite stabilization effect was observed in NiTi alloys regardless of the chemical composition and value of the preliminary strain. The value of the martensite stabilization effect was measured as the difference between the temperatures that were measured during the first and the second heating. When the residual strain was less than 2.5%, the martensite stabilization effect values were close to each other in all studied alloys. Otherwise, if the residual strain exceeded 2.5%, the martensite stabilization effect values in the Ni- 50.0 at. % Ti and Ni - 49.5 at. % Ti alloys were larger than in the Ni - 49.0 at. % Ti alloy. It was shown that there was no correspondence between the values of the martensite stabilization effect and the irreversible plastic strain that appeared in the samples during the preliminary deformation. The martensite stabilization effect was found in the Ni - 49.0 at. % Ti alloy after preliminary deformation up to 10% or less that was not accompanied by a plastic strain. Thus, it was shown that the plastic strain was not the main reason for the martensite stabilization effect. A new hypothesis was assumed that a loss in the coherency of the interface that was caused by the martensite reorientation and detwinning during the preliminary deformation might be responsible for an increase in the temperatures of the reverse transformation that occurred on the first heating. (C) 2019 Elsevier B.V. All rights reserved.