Search

Biomedical Ti-22Nb-6Zr, Ti-18Zr-15Nb, and Ti-41Zr-12Nb (at.%) shape memory alloys were subjected to cold rolling (CR) and subsequent post-deformation annealing (PDA). The evolutions of phase and structure states, crystallographic texture, and crystallographic limit of recovery strain were studied using EBSD, TEM, and XRD analyses. The study found that CR (e = 1.5) and PDA at 800 °C for 30 min results in fine- and coarse-grained structures. Severe CR (e = 3.0) and PDA at 550 °C for 5 min results in a recrystallized, equiaxed, predominantly ultrafine-grained structure of the β-phase with a small amount of low-angle boundaries. Increasing the degree of CR from moderate (e = 0.3) to severe (e = 3.0) results in a favorable strong {111}ββ recrystallization texture. Alloys with low Zr content are more susceptible to this type of crystallographic texture formation during TMT, primarily due to a higher Nb content. The Ti-41Zr-12Nb alloy shows the highest crystallographic limit of recovery strain (εrmax ≈ 6%). The limit decreases to ≈5% (for Ti-18Zr-15Nb) and ≈3% (for Ti-22Nb-6Zr) when transitioning from high- to low-Zr alloys. The transition of Ti-Zr-Nb alloys from coarse-grained to ultrafine-grained structures of the β-phase and a decrease in grain size do not affect the crystallographic limit of recovery strain in the studied grain size ranges.

For colloids of NaYF4:Yb3+/Tm3+ nanoparticles in DMSO, by the method of time-resolved luminescence spectroscopy with nanosecond pulsed excitation at a wavelength of 975 nm, the photophysical processes that determine the course of kinetic curves have been revealed. It has been found that the luminescence rise time decreases with an increase in the concentration of activators and sensitizers due to the increase in the efficiency of energy transfer from sensitizers to activators. The cross-relaxation of the excited states of activators provides a decrease in the luminescence decay time with an increase in the concentration of activators and a constant concentration of the sensitizer. There is no correlation between the time of luminescence decay with the change in the concentration of sensitizers and the constant concentration of activators due to the competition of the processes of energy back transfer from activators to sensitizers and the “feeding” of activators by excitations coming from remote sensitizer ions.

The potential medical Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys strengthened using high-pressure torsion (HPT) were investigated in this work. HPT led to a significant refinement of the microstructure of both alloys with the formation of an ultrafine-grained structure (UFG). The average grain size after HPT was ~700–800 nm for both alloys. The formation of the UFG structure led to an increase in the ultimate tensile strength of up to 401 ± 16 and 482 ± 12 MPa for the Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys, respectively. Additionally, a variation in ductility of the Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys of up to 56.3 ± 16.9% and 4.4 ± 0.6%, respectively, was also observed, apparently due to textural changes. HPT led to a small increase in the degradation rate of the alloys after 1 day of incubation in the medium. However, an increase in the incubation period of up to 30 days slowed down the degradation process and leveled the difference between the initial and HPT-treated state of the alloys. HPT did not affect the cytotoxicity of the Zn-1.7%Mg-0.2%Zr alloy and contributed to the reduction of hemolysis. Thus, the processing of the Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys using HPT accelerated their biodegradation without compromising their biocompatibility.

The effect of such an additional promising alloying element as samarium on hot-extruded Mg–Y–Gd–Zr alloys is investigated. The microstructure, kinetics of aging during the decomposition of a supersaturated Mg solid solution, and the mechanical properties of the alloys are studied. The differences of the recrystallization processes that occur in hot-extruded alloys with various contents of samarium (0, 1.7, 2.5%) are demonstrated. After hot extrusion, Mg–Y–Gd–Zr and Mg–Y–Gd–Sm–Zr alloys are additionally hardened during aging due to the decomposition of a supersaturated Mg solid solution. At the same time, samarium changes the nature of this hardening. The alloys with samarium are hardened faster, and the maximum hardness is achieved with shorter aging exposures. The mechanical properties of hot-extruded Mg–Y–Gd–Zr alloys with samarium addition are determined at room and elevated up to 300 °C temperatures. The efficiency and expediency of using samarium both as a separate alloying element and as a partial replacement of more expensive rare-earth elements in alloys with yttrium and gadolinium are shown.

Alloys of the quasibinary TiNi-TiCu system manufactured by melt quenching in the form of thin 20–50 μm ribbons have proven to show good potential as materials for the fabrication of micromechanical devices. At high cooling rates (about 106 K/s), this method allows producing high-copper (more than 20 at.%) amorphous alloys which exhibit an excellent shape-memory effect after crystallization. Their properties are known to largely depend on the crystallization conditions and the structure of the initial amorphous material acting as a precursor for the formation of crystal phases. It has been shown recently that the rejuvenation procedure (cryogenic thermocycling) of metallic glasses is one of the most promising methods of improving their properties. In this study, we investigated for the first time the effect of cryogenic thermocycling of rapidly quenched amorphous TiNiCu on the initial state, as well as on structure formation and the phase transformation patterns of subsequent crystallization conducted using various methods. The effect was analyzed utilizing the methods of scanning and transmission electron microscopy, X-ray diffraction analysis, and differential scanning calorimetry. The results show that rejuvenation treatment slightly reduces the glass transition and crystallization onset temperatures and moderately changes the sizes of structural features (grains, martensite plates), the quantity of the martensite phase, and the characteristic temperatures and enthalpy of the martensitic transformation.

Rotary swaging (RS) of alloy Mg–1.03Zn–0.66Ca (ZX11) was shown to refine the average grain size to 4.5 ± 1.2 µm in a longitudinal section and 4.8 ± 0.9 µm in a transverse section. In addition, a small amount of Mg2Ca particles about 300 nm in size and Mg6Zn3Ca2 particles with a size of about 100 nm was detected. This resulted in pronounced strengthening: the yield strength and the ultimate tensile strength rose to 210 ± 8 MPa and 276 ± 6 MPa, respectively, while the elongation hardly decreased (22.0 ± 1.8% and 18.3 ± 2.9% before and after RS). Furthermore, RS led to an increase in the fatigue limit of the alloy from 120 MPa to 135 MPa and did not impair its resistance to chemical corrosion. The studies in vitro showed that ZX11 induces hemolysis without inhibiting the viability of peripheral blood mononuclear cells and has a more pronounced cytotoxic effect on tumor cells in comparison with non-transformed cells. No significant difference of the latter effect between the initial and the deformed states was observed.

A study of the effect of high-pressure torsion (HPT) on the structure, phase composition, corrosion resistance, mechanical properties and bioactivity in vitro of Zn-1%Mg-0.1%Ca alloy was performed. It was shown that HPT leads to refinement of the alloy microstructure with the formation of recrystallized α-Zn grains with an average size of 750 ± 30 nm, and grains of a mixture of different phases with a size of 38 ± 7 nm. In addition, precipitation of Ca-enriched particles ~20 nm in size was observed. X-ray phase analysis showed that the Zn-1%Mg-0.1%Ca alloy consists of five phases (Zn, Mg2Zn11, MgZn2, CaZn11 and CaZn13), whose volume fraction does not change after HPT. It was found that HPT does not lead to a deterioration in the corrosion resistance of the alloy. At the same time, HPT leads to an increase in the yield stress of the alloy from 135 ± 13 to 356 ± 15 MPa, the ultimate tensile strength from 154 ± 5 to 416 ± 31 MPa, and the ductility from 0.4 ± 0.1 to 5.5 ± 2.8%. No significant increase in hemolytic activity, bactericidal activity, and the ability to colonize the surface of the alloy by cells was revealed during the conducted studies. Additionally, there was no significant difference in these parameters in comparison with the control. However, HPT contributes to a decrease in the cytotoxicity of the alloy by an average of 10% compared to the annealed alloy. The conducted studies allow us to conclude that the Zn-1%Mg-0.1%Ca alloy is promising material for the development of biodegradable orthopedic medical implants.

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.

The effect of equal channel angular pressing (ECAP) on the microstructure, texture, mechanical properties, and corrosion resistance of the alloys Mg-6.0%Ag and Mg-10.0%Gd was studied. It was shown that ECAP leads to grain refinement of the alloys down to the average grain size of 2–3 μm and 1–2 μm, respectively. In addition, in both alloys the precipitation of fine particles of phases Mg54Ag17 and Mg5Gd with sizes of ~500–600 and ~400–500 nm and a volume fraction of ~9% and ~8.6%, respectively, was observed. In the case of the alloy Mg-6.0%Ag, despite a significant grain refinement, a drop in the strength characteristics and a nearly twofold increase in ductility (up to ~30%) was found. This behavior is associated with the formation of a sharp inclined basal texture. For alloy Mg-10.0%Gd, both ductility and strength were enhanced, which can be associated with the combined effect of significant grain refinement and an increased probability of prismatic and basal glide. ECAP was also shown to cause a substantial rise of the biodegradation rate of both alloys and an increase in pitting corrosion. The latter effect is attributed to an increase in the dislocation density induced by ECAP and the occurrence of micro-galvanic corrosion at the matrix/particle interfaces.