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Abstract: The focus of this study is the influence of irreversible plastic deformation on the martensitic transformation and strain recovery after deformation of the specimen in the martensitic state. Both strain and electrical resistance in TiNi alloy have been measured simultaneously on heating and cooling of the sample which has been preliminarily deformed first in austenite (at 600K) and then in martensite (at 290K). A decrease of the alloy capacity for strain recovery and the change of the martensitic transformation sequence with an increase of the plastic strain have been found. Besides, the experimental data have shown that the electrical resistance varies with a hysteresis on heating and cooling of the sample deformed to 60%. This hysteresis is observed in a wide temperature interval and does not disappear with thermocycles. One can suppose that the internal oriented stresses may be the main cause of the observed phenomena. [Copyright &y& Elsevier] more...

Abstract: The aim of the present study is the investigation of the influence of the preliminary thermo-mechanical treatment and stress on the difference of the temperature intervals of the martensitic transformation and of the strain variation in deformed TiNi alloys. Several techniques were used: differential scanning calorimetry, simultaneous measurement of strain and electrical resistance, and measurement of strain and differential thermal analysis. The obtained data show that a discrepancy between the martensitic transformation temperatures and the strain variation temperatures occurs during the realization of the basic shape memory effects and depends on the stress applied to the sample. There are two main reasons for the observed phenomena: (1) strain variation prior to the transformation can be due to mechanisms of recoverable unelastic deformation other than the transformation; (2) deformation does not occur during the appearance and disappearance of completely twinned B19′ martensite. [Copyright &y& Elsevier] more...

The size and morphology of Ti 3 Ni 4 precipitates, as well as their transformation behavior after isothermal aging, were studied in a Ti-50.7 аt.%Ni shape memory alloy with various B2 austenite grain sizes. A 0.3 mm thick band with an accumulated strain of ε = 44% (e = 0.6) obtained by cold rolling was used. The samples were then solution-treated at 600–800°С for 0.3–1 h with subsequent quenching in water to obtain the structure with different grain size (GS) from 5 to 15 μm. The samples were then subjected to isothermal annealing at 430 °C for 1, 3 and 10 h. The SEM observations were carried out using a JSM-6460LV. The microstructure was studied using a JEOL 2100 TEM. Characteristic temperatures were measured using a “Mettler Toledo” DSC. The results of the microstructure study prove that the GS strongly affects Ti 3 Ni 4 particle size and morphology, as well as the degree of microscale microstructure heterogeneity, transformation kinetics and transformation sequences. This influence is non-unique and depends on the duration of isothermal annealing. After 1 h of aging, the particle thickness and diameter maintain the nanometer range in the whole GS range of 5–15 μm. The increase of aging time to 3 and 10 h leads to localization of nanosized precipitates in the grain-boundary region, with the width of these regions decreasing with increasing GS. After aging for 1 h, the samples with the GS of 5 μm show the B2→R transformation. The increase in the GS to 15 μm and aging duration to 10 h is accompanied by multiplication of martensitic transformations to four stages. The study explains the influence of GS on the precipitated particle size and morphology, transformation kinetics and staging. [ABSTRACT FROM AUTHOR] more...