Effect of External Impacts on the Structure and Martensitic Transformation of Rapidly Quenched TiNiCu Alloys

Abstract

TiNi-TiCu quasibinary system alloys with a high Cu content produced by rapid quenching from liquid state in the form of thin amorphous ribbons exhibit pronounced shape memory effect after crystallization and are promising materials for miniaturized and fast operating devices. There is currently no complete clarity of the mechanisms of structure formation during crystallization from the amorphous state that determine the structure-sensitive properties of these alloys. This work deals with the effect of the initial amorphous state structure and crystallization method of the alloys on their structure and phase transformations. To this end the alloy containing 30 at.% Cu was subjected to thermal and mechanical impact in the amorphous state and crystallized using isothermal or electropulse treatment. We show that after all types of treatment in the amorphous state the structure of the alloy remains almost completely amorphous but the characteristic temperatures and enthalpy of crystallization become slightly lower. Isothermal crystallization of alloy specimens produces a submicrocrystalline structure with an average grain size in the 0.4–1.0 μm range whereas electropulse crystallization generates a bimorphic structure consisting of large 4–6 μm grains and 2–3 μm high columnar crystals in the vicinity of the surface. The grains have nanosized plate-like and subgrain structures. The largest grains are observed in thermally activated samples, meanwhile, mechanical impact in the amorphous state leads to the formation of equiaxed finer grains with a less defective subgrain structure and to the shift of the temperature range of the martensitic transformation toward lower temperatures.

Study of Structure and Phase Transformations in Rejuvenated Rapidly Quenched TiNiCu Alloys

“…Treatment in the amorphous state does not change the structure of the specimens to a visible extent within the resolution of SEM. As shown earlier [15,28], the electropulse treatment of amorphous ribbons dramatically changes the crystallization pattern in comparison with isothermal annealing (Figure 9). Cross-sectional SEM images of electropulse-crystallized 25 at.% Cu alloy ribbons show a bimorphic structure: columnar crystals form near the ribbon surface, whereas discrete or clustered large grains with a typical size of 3 to 12 µm and a subgrain structure form in the ribbon bulk (Figure 9a-c).…”

“…Investigations of the fine structure of the alloy were carried out on a JEOL JEM 2100 (JEOL Ltd., Tokyo, Japan) transmission electron microscope (TEM). The specimens for the TEM study were prepared with the focused ion beam (FIB) technique by means of an FEI Strata 201 FIB setup (Thermo Fisher Scientific Inc., Hillsboro, OR, USA) equipped with an Omniprobe ® (Oxford Instruments plc, High Wycombe, UK) nanomanipulator [15]. The XRD analysis was performed using a PANalytical Empyrean diffractometer (Malvern Panalytical plc, Malvern, UK) in Cu-K α radiation at an angle range of 20-90 • at a step of 0.1 • with exposure time 5 s both at room temperature and at 90 • C.…”

“…Their properties largely depend on the structure of the initial amorphous material. We have recently shown [15] that the thermal and deformation treatment of rapidly quenched high-copper TiNi-TiCu system alloys in the amorphous state has a major effect on the structure formation and phase transformations in the alloys, e.g., on the shape and size of grains, the number of defects in the subgrain structure, and the critical temperatures of the martensitic transformation. In this study, the amorphous alloys were subjected to additional heat treatment using the so-called rejuvenation procedure, which involves the structural excitation of amorphous materials and leads to an increase in the enthalpy and free volume, i.e., triggering a reverse aging process [16][17][18].…”