S. D. Prokoshkin

Specific features of the formation of the microstructure of titanium nickelide upon thermomechanical treatment including cold plastic deformation to degrees from moderate to severe

X-ray diffraction, electron microscopy, microhardness measurements, and differential scanning calorimetry have been used to investigate the formation of the dislocation substructure and nanocrystalline and amorphous structures in Ti-Ni shape-memory alloys depending on the degree of cold deformation by rolling and post-deformation annealing. The moderate deformation (e = 0.25) leads to the formation of a developed dislocation substructure; with an increase in the deformation to e = 2, the dislocation substructure is gradually substituted by a mixed nanocrystalline and amorphous structures. The residual martensite completely disappears as the deformation increases in the interval of e = 2−3 or upon annealing in the interval of 200–300°C. Annealing at 400°C after a moderate deformation leads to the formation of a polygonized (“nanosubgrain”) dislocation substructure in austenite. As the initial deformation increases to e = 2, this structure is gradually substituted by a nanocrystalline structure of austenite. Annealing after deformation to intermediate degrees (e = 0.75−1.0) results in the formation of a mixture of nanocrystalline and submicrocrystalline polygonized structures.

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.

Effect of Equal Channel Angular Pressing and Repeated Rolling on Structure, Phase Transformations and Properties of TiNi Shape Memory Alloys

The nanostructured TiNi-based shape-memory alloys were synthesized by multi-step SPD deformations – ECAP plus cold rolling or drawing. It is found that the SPD processing changed the morphology of the martensite and temperature of martensite transformation. Also, we found that the mechanical and shape memory properties can be enhanced by forming nanostructures in these alloys. SPD processing renders higher strength, higher yield dislocation strength and in results - higher recovery stress and maximum reverse strain of shape memory.

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).

Functional Properties of Nanostructured Ti-50.0 at % Ni Alloys

Ti-50 at % Ni alloy wire is subjected to cold-rolling (true strain e=0.3-1.9) and post-deformation annealing (200–700°C range). For all levels of cold work, the maxima of recovery strain and stress are obtained after annealing in the 350–400°C range. For the moderately-to-high cold-worked material (e=0.3-0.88), this annealing leads to polygonization, while for the severely cold-worked one (e=1.9), to the material nanocrystallization (grains of 50–80 nm in size). Nanocrystallized alloy generates 30 % higher recovery stresses (up to 1450 MPa) and 10% higher completely recoverable strains (more than 8 %) as compared to the polygonized alloy, while having comparable mechanical properties in tension.

Role of the structure and texture in the realization of the recovery strain resource of the nanostructured Ti-50.26 at %Ni alloy

In this work, we have studied the nanostructure, the crystallographic texture, and the crystal lattice of the martensite of the Ti-50.26 at % Ni alloy subjected to a thermomechanical treatment, which includes cold rolling, warm (at 150°C) rolling, intermediate and post-deformation annealings (at 400°C) in different combinations. To calculate the resource of the recovery strain in the approximation of a polycrystal, we suggested and employed a method based on the sufficiently complete allowance for the orientation distribution function of the initial B2 austenite and on the assumption on the realization of the most favorable orientational variant of martensite in each grain. The calculated values of the resource of the recovery strain have been compared with the experimental data and have been analyzed along with the results of the determination of the recovery stresses and parameters of the loading-unloading diagram. Estimations have been made of the role of the structural and textural factors in the realization of the recovery strain of the nanostructured Ti-50.26 at % Ni alloy. To achieve the maximally high recovery strain, one should focus on obtaining a nanocrystalline structure in combination with a sharp texture, which ensures the maximum transformation deformation in the direction of tension.

Studies of Severe Plastic Deformation Conditions for Amorphous and Nanocrystalline Structures Formation in Ti-Ni Based Alloys

Structure formation in TiNi-based shape memory alloys depending on deformation temperature (-196 °C to 400 °C) and pressure (4 to 8 GPa) under conditions of high-pressure torsion (HPT) was studied using TEM and X-ray diffraction methods. The tendency to form an amorphous structure depends on relative positions of the deformation temperature and Ms temperature. Isothermal martensitic transformation is observed in the Ti – 48.5 % Ni alloy as a result of 10-year keeping at RT after HPT. Increasing of pressure suppresses the tendency to form an amorphous structure. The upper deformation temperature limits for amorphous and nanocrystalline structures formation are determined. The thermomechanical conditions of the equal-channel angular pressing for obtaining actual nanocrystalline structure are recommended.

Substructure and Nanocrystalline Structure Effects in Thermomechanically Treated Ti-Ni Alloys

Substructure and structure formation as well as functional properties of thermomechanically treated Ti-Ni wire have been studied using differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and mechanical. The low- temperature themomechanical treatment (LTMT) was carried out by rolling at room temperature in a true strain range e = 0.3 to 1.9. It was shown that severe plastic deformation (e=1.9) of Ti-50.0at.%Ni alloy results in partial amorphization and formation of nanocrystalline austenite structure during post-deformation annealings up to 400 °C. As a result, the fully recoverable strain and recovery stress become much higher than the values reachable after traditional LTMT (e=0.3 to 0.88) with post-deformation annealing which creates a poligonized dislocation substructure.