S. Dubinskiy

Crystal lattice of martensite and the reserve of recoverable strain of thermally and thermomechanically treated Ti-Ni shape-memory alloys

X-ray diffraction has been used to study shape-memory alloys of composition Ti-(49.73–51.05 at %) Ni subjected to quenching and thermomechanical treatment (TMT) by the scheme “cold deformation (e = 0.3–1.9) + postdeformation annealing (200–500°C) to provide different defectness of the parent B2 austenite. For the quenched alloys, the concentration dependences of the lattice parameters of the B19′ martensite, maximum lattice strain upon martensitic transformation, the crystallographic orientation of the lattice in single crystals, and the reserve of recoverable strain in polycrystals have been determined. The lattice parameters of martensite formed from polygonized, i.e., nanosubgranular, or from nanocrystalline austenite differ from the corresponding parameters of quenched martensite formed from recrystallized austenite, and their difference increases with increasing defectness of the parent-austenite lattice. An increase in the defectness of the austenite lattice is accompanied by a decrease in the reserve of recoverable strain. The deformation of the existing martensite or the formation of stress-assisted martensite under the anisotropic action of external stresses changes the interplanar spacing and the thermal expansion coefficient in different crystallographic directions but does not affect the averaged lattice parameters near the Ms-Mf interval and the reserve of recoverable strain.

Mechanical Properties of Thermomechanically-Processed Metastable Beta Ti-Nb-Zr Alloys for Biomedical Applications

Metastable beta-titanium alloys combine exceptionally low Youngs modulus and high biocompatibility, thus attracting special interest in the prospect of their application as biomedical implant material. In this work, Ti-21.8Nb-6Zr (at.%) ingots were manufactured by vacuum argon melting followed by hot isothermal pressing. The obtained ingots were thermomechanically processed using the following TMP sequence: a) cold rolling (CR) from e=0.37 to 2 of the logarithmic thickness reduction; and b) post-deformation annealing (PDA) of between 450 and 700°C (10’…5 h for 600°C and 1 h for other temperatures). The influence of the TMP on the alloy’s mechanical properties under static and cyclic loading was studied.

Structure formation during thermomechanical processing of Ti-Nb-(Zr, Ta) alloys and the manifestation of the shape-memory effect

The formation of structure during thermomechanical processing by the regime of cold plastic deformation by rolling and postdeformation annealing (PDA) and its influence on the mechanical properties of Ti-Nb-(Zr, Ta) shape-memory alloys (SMAs) have been investigated. A moderate strain (e ≈ 0.3) leads to the formation of a developed dislocation substructure in the β phase. With going to severe plastic deformation (e ≈ 2), a nanocrystalline structure can locally be formed without the amorphization of the structure. There are also present α″-, α-, ω phases in the deformed alloys. When the PDA (1 h) is performed below 450°C, the structure of the β phase changes only slightly. Above 450°C, a polygonized substructure is formed in the β phase, which is nanosubgrained at an annealing temperature of 500°C and transforms completely into a submicron one at 600°C. In the case of severe plastic deformation to e ≈ 2, in this range of annealing temperatures, high-angle misorientations of blocks are also observed. The recrystallization of the β phase in the Ti-Nb-(Zr, Ta) SMAs develops at temperatures above 600°C. The presence of the ω phase is detected at temperatures of up to 550°C. The lattice parameters of the strain-induced α″ martensite formed in the Ti-Nb-Ta alloy are independent of the PDA temperature in the range from 600 to 900°C, where the polygonized substructure transforms into the recrystallized structure of the β phase. The range of PDA temperatures that are most favorable for the manifestation of the effect of superelasticity in the Ti-Nb-(Zr, Ta) alloys is in the vicinity of 600°C.

Functional fatigue behavior of superelastic beta Ti-22Nb-6Zr(at%) alloy for load-bearing biomedical applications

Ti-22Nb-6Zr (at.%) alloy with different processing-induced microstructures (highly-dislocated partially recovered substructure, polygonized nanosubgrained (NSS) dislocation substructure, and recrystallized structure) was subjected to strain-controlled tension-tension fatigue testing in the 0.2...1.5% strain range (run-out at 10^6 cycles). The NSS alloy obtained after cold-rolling with 0.3 true strain and post-deformation annealing at 600 °C showed the lowest Youngs modulus and globally superior fatigue performance due to the involvement of reversible stress-induced martensitic transformation in the deformation process. This NSS structure appears to be suitable for biomedical applications with an extended variation range of loading conditions (orthopedic implants).

Manufacturing, Structure Control, and Functional Testing of Ti–Nb-Based SMA for Medical Application

This paper focuses on the development and characterization of Ni-free shape memory alloys, more specifically, Ti–Nb-based alloys for biomedical applications. It starts by describing the smelting technology used to produce small and medium size ingots of selected compositions. Thermomechanical treatments: structure interrelations are discussed next. Finally, the results of their mechanical, electrochemical, and in vitro cytotoxicity testing are presented to allow a general assessment of the mechanical, chemical, and biological aspects of compatibility of these alloys, and of the methods to control their functional properties.

Investigation of the structure stability and superelastic behavior of thermomechanically treated Ti-Nb-Zr and Ti-Nb-Ta shape-memory alloys

The superelastic parameters of Ti-Nb-Ta and Ti-Nb-Zr alloys, such as Young’s modulus, residual strain, and transformation yield stress after thermomechanical treatment (TMT), were determined during cyclic mechanical tests using the tension-unloading scheme (maximum strain 2% per cycle, ten cycles). The superelastic parameters and the alloy structure have been studied by electron microscopy and X-ray diffraction analysis before and after testing and after holding for 40 days, as well as after retesting. The Young’s modulus of the Ti-Nb-Ta alloy decreases from 30–40 to 20–25 GPa during mechanocycling after TMT by different modes; however, it returns to its original magnitude during subsequent holding for 40 days, and changes only a little during repeated mechanocycling. The Young’s modulus of the Ti-Nb-Zr alloy changes insignificantly during mechanocycling, recovers during holding, and behaves stably upon repeated mechanocycling. Surface tensile stresses arise during mechanocycling, which facilitate the development of martensitic transformation under load, orient it, and thereby promote a decrease in the transformation yield stress and the residual strain. The enhancement of the level of initial strengthening stabilizes the superelastic behavior during mechanocycling.

Martensitic Transformations and Mechanical and Corrosion Properties of Fe-Mn-Si Alloys for Biodegradable Medical Implants

The Fe-Mn-Si alloys are promising materials for biodegradable metallic implants for temporary healing process in the human body. In this study, three different compositions are considered (Fe23Mn5Si, Fe26Mn5Si, and Fe30Mn5Si, all in wt pct). The phase composition analysis by XRD reveals ε-martensite, α-martensite, and γ-austenite in various proportions depending on the manganese amount. The DSC study shows that the starting temperature of the martensitic transformation (Ms) of the alloys decreases when the manganese content increases (416 K, 401 K, and 323 K (143 °C, 128 °C, and 50 °C) for the Fe23Mn5Si, Fe26Mn5Si, and Fe30Mn5Si alloys, respectively). Moreover, mechanical compression tests indicate that these alloys have a much lower Young’s modulus (E) than pure iron (220 GPa), i.e., 145, 133, and 118 GPa for the Fe23Mn5Si, Fe26Mn5Si, and Fe30Mn5Si alloys, respectively. The corrosion behavior of the alloys is studied in Hank’s solution at 310 K (37 °C) using electrochemical experiments and weight loss measurements. The corrosion kinetics of the Fe-Mn-Si increases with the manganese content (0.48, 0.59, and 0.80 mm/year for the Fe23Mn5Si, Fe26Mn5Si, and Fe30Mn5Si alloys, respectively). The alloy with the highest manganese content shows the most promising properties for biomedical applications as a biodegradable and biomechanically compatible implant material.

A comparative study of structure formation in thermomechanically treated Ti-Ni and Ti-Nb-(Zr, Ta) SMA

The processes of structure formation in Ti-Ni and in Ti-Nb-Zr, Ti-Nb-Ta shape memory alloys (SMA) under thermomechanical treatment (TMT) were studied. The TMT comprised cold rolling with true strains from e=0.25 to 2 and post-deformation annealing. Differences in these processes between two groups of alloys are considered. The main conclusions are as follows: nanostructures created by TMT are useful for radical improvement of the SMA functional properties, and an optimum nanostructure (nanocrystalline structure, nanosubgrained structure or theirmixture) should be selected by taking into account other structural and technological factors.

Influence of omega-phase precipitation hardening on the static and dynamic properties of metastable beta Ti-Nb-Zr and Ti-Nb-Ta alloys

The influence of thermomechanical processing on the Ti-21.8Nb-6Zr (TNZ) and Ti-19.7Nb-5.8Ta (TNT) (at%) alloys’ structure, phase composition, mechanical and functional properties is studied. Both alloys possess polygonized dislocation substructure (average subgrain size  100 nm), and manifest superelastic behavior at room temperature and recovery stress generation during constant-strain temperature scanning experiments. After aging treatment, both alloys were -phase precipitation hardened, but their mechanical behavior was impacted differently -- it was detrimental for TNZ and beneficial for TNT. The different impact of aging heat treatment on the mechanical behavior of these alloys is explained by the differences in the -phase nucleation rate, precipitates’ size, shape, volume fraction and distribution, and by their effect on the alloys’ critical stresses and transformation temperatures.

In Situ X-Ray Study of Phase Transformations in Ti-Nb-Based SMA under Variable Stress-Temperature Conditions: Preliminary Results

The technique and preliminary results of in situ X-ray diffraction analysis of the martensitic transformation in the newly developed Ti-Nb-Zr SMA for biomedical application are presented. To perform the in situ analysis, an original tensile stage, powered by a Ti-Ni SMA actuator and fit within the “TTK450” thermal chamber of a “PANalytical X’Pert Pro” diffractometer is designed, manufactured and validated. The tensile stage working principle and analysis methodology are described in detail. Preliminary results obtained during in-situ X-ray analysis of the phase transformations in Ti-Nb-Zr SMA are also presented.