Victor Grishkov

Comparative analysis of inelastic strain recovery and plastic deformation in a Ti49.1Ni50.9 (at %) alloy under torsion and bending

The paper reports on an experimental study of inelastic strain recovery, due to superelasticity and shape-memory effects, and plastic deformation under torsion and bending in a binary TiNi alloy containing 50.9 at % of Ni. It is found that the material under torsion and bending displays an anomalously high superelasticity of 16.4% and 13.5% and total recovery strain of 24.6% and 16.4% which exceed the theoretical limit of recoverable strain equal to 10.3% in the alloy involved in a cubic B2 to monoclinic B19’ phase transformation. The correlation between the high recovery strains and the intensity and mechanisms of plastic deformation with increasing total strain is discussed.

Temperature dependence of inelastic strain recovery in TiNi-based alloys under torsion

The paper presents research results on the temperature dependence of inelastic strain recovery due to superelasticity and shape memory effects under torsion in coarse-grained and microcrystalline TiNi specimens containing 50.8 at % of Ni (Ti49.2Ni50.8 (at %)). It is found that under certain temperature and deformation conditions, the recovery strain exceeds the theoretical limit of recoverable strains or maximum lattice strain in B2↔R↔B19′ transformations. Using a special algorithm, the torsional strain is converted to its equivalent (by Mises) tensile strain. The results are compared with data obtained by other researchers.

Effect of hydrogen on superelasticity of the titanium nickelide-based alloy

The hydrogen effect on the inelastic properties and plastic strain development after electrolythical hydrogenation in physiological solution was investigated. This effect virtually results in a failure under torsion of Ti49.1Ni50.9 (atom per cent) alloy specimens with coarse-grained (CG) and submicrocrystalline (SMC) structures. It is shown that hydrogen embrittlement (HE) phenomenon occurs irrespective of the grain size in the studied specimens at approximately equal strain values. However, compared to the specimens with CG structure, those with SMC structure accumulate two to three times more hydrogen for the same hydrogenation time. It is found that hydrogen has a much smaller effect on the inelastic properties of specimens with SMC structure as compared to those with CG structure.

THE INCREASE OF THE MARTENSITIC DEFORMATION DURING SHAPE MEMORY EFFECT IN DEFORMED TiNi

The evolution of the ultrafine structure, obtained at thermal treatment below recrystallization, and its effect onto shape memory characteristics in NiTi alloy was under study. It was shown that low temperature annealing (< 0.5 Tmelt) of the hot rolled NiTi leads to the structure refinement accompanied by the growth in accumulated martensite deformation. It was suggested that this is possible due to the increase of the volume fraction of martensite crystals properly oriented in respect to external stress.

Structural-phase state of Ti–0.16Pd alloy after equal channel angular pressing

The paper presents research data demonstrating the influence of equal channel angular pressing (ECAP) at 650 K on the structural-phase state of a Ti–0.16Pd–0.14Fe alloy (wt %) with initially microcrystalline structure (〈d〉 = 10 µm). The data show that after equal channel angular pressing, the alloy reveals microinhomogeneities in its chemical and phase composition on grain scales. The central regions of grains are structured as a hexagonal α phase with a low content of Fe and Pd. Near the grain boundaries and junctions, the concentration of Fe and Pd is much higher, which provides the presence of orthorhombic α″ martensite and/or two-phase α + α″ state in these regions. At the grain boundaries and junctions, the material reveals bcc β precipitates (0.6–3.5 µm) containing Fe and Pd in amount of 6–14 and 0.9–1.9 wt %, respectively. When exposed to equal channel angular pressing, the alloy assumes submicrocrystalline structure (〈d〉 = 0.28 µm) and inherits features associated with grain boundary Fe and Pd seg...

Effect of isothermal loading temperature on reversible inelastic strain in coarse-grained Ti49.7Ni50.7

The paper presents experimental research results on the temperature dependence of reversible inelastic strain (superelasticity and shape memory effect) in coarse-grained Ti49.3Ni50.7 specimens (at %) under torsion. The experiments demonstrate that its value can measure up to ∼18% against minimum plastic strain accumulation in the material in martensitic and austenitic states with low martensite shear stresses.The paper presents experimental research results on the temperature dependence of reversible inelastic strain (superelasticity and shape memory effect) in coarse-grained Ti49.3Ni50.7 specimens (at %) under torsion. The experiments demonstrate that its value can measure up to ∼18% against minimum plastic strain accumulation in the material in martensitic and austenitic states with low martensite shear stresses.

Effect of warm rolling on the martensite transformation temperatures, shape memory effect, and superelasticity in Ti49.2Ni50.8 alloy

The paper presents research data demonstrating the effect of warm caliber rolling on the martensite transformation temperatures, shape memory effect, and superelasticity in Ti49.2Ni50.8 (at %). The experimental values of inelastic strain in coarse-grained and microcrystalline Ti49.2Ni50.8 (at %) specimens exceed the theoretical limit of recoverable strain or maximum lattice strain for TiNi-based alloys.

Effect of recrystallization annealing on the inelastic properties of TiNi alloy under bending

The data reported here show that recrystallization annealing at 973 K transforms the initial nanocrystalline structure of Ti49.1Ni50.9 (at %) into a microcrystalline structure with an average grain size of ≤11.4 µm and changes the sequence of martensite transformations from B2→R→B19′ to B2→B19′. Also dependences of the superelasticity effect (SE), shape memory effect (SME), summary reversible inelastic strain (SIS = SE + SME), and plastic strain on total strain for the alloy exposed to bending in isothermal (295 K) loading-unloading cycles with post-unloading heating are presented.

The effect of warm deformation by abc-pressing method on mechanical properties of titanium nickelide

Исследовано влияние температуры на предел прочности (σuts) и удлинение до разрушения (δр) при растяжении образцов никелида титана (50.2 ат.% Ni) с исходной крупнозернистой структурой и структурами после abc-прессования при 873К (сплав 1, истинная деформация е = 2.2) и после прессования с понижением температуры в последовательности 873К→673К (сплав 2, е = 4.2). Размер зёрен в образцах сплава 1 (2-40 мкм) меньше, чем в исходных образцах (20-70 мкм). Образцы сплава 2 имели субмикрокристаллическую структуру (зёрна/субзёрна 100-700 нм). Показано, что при охлаждении и нагреве в исходных образцах реализуется мартенситные превращения (МП) В2←→В19ʹ (фазы с кубической и моноклинной структурами, соответственно), а в сплавах 1 и 2 МП протекают в последовательности В2→R→В19ʹ→В2 (R – мартенситная фаза с ромбоэдрической структурой). Завершение МП В19ʹ→В2 в этих образцах происходит при температурах 340-355 К. Растяжение образцов проводили при 293К, 343К и 473К (или 523К), то есть в мартенситной фазе В19ʹ, предпереходной области температур и в В2 фазе. Зависимости «напряжение-деформация», полученные при этих температурах, для всех образцов качественно подобны. В процессе растяжения при 293К и 343К наблюдается площадка псевдотекучести, связанная с переориентацией кристаллитов фазы В19ʹ или формированием деформационной мартенситной фазы В19ʹ. Генерации мартенситной фазы В19ʹ в процессе растяжения при 473К (или 523К) не наблюдается. Показано, что σuts повышается после abc-прессования. Максимум σuts наблюдается в процессе растяжения сплавов 1 и 2 при 293К и 343К (в мартенситной фазе В19ʹ). Наиболее пластичными являются образцы сплава 1 (δр около 80% при 293-343 К). При повышении температуры деформирования до 473К (или 523К) σuts и δр существенно понижаются (в ~ 2 раза). Ключевые слова: тёплая деформация, никелид титана, механические свойства

The influence of warm deformation on the structure and martensitic transformations in TiNi-based alloys

The regularities of microstructure formation in binary TiNi alloys with a Ni content of 50.2 and 50.8 at.% under warm abc pressing and isothermal (723 K) multipass caliber rolling had been studied. Microstructure composed of submicrocrystalline grains and subgrains was found after final abc pressing at 723 K with a total true strain e = 8.44. Three stages of grain structure evolution were revealed in the TiNi alloy with 50.8 at.% Ni subjected to warm rolling. At the first stage with low strains, the average grain size 〈d〉 increased due to collective dynamic recrystallization. The average grain size 〈d〉 decreased steeply due to discontinuous and continuous dynamic recrystallization at the second and third rolling stage. Microstructure composed of micro- and submicrocrystalline grains was observed after rolling with e = 2.0. The sequences and temperatures of martensite transformations from a cubic B2 phase to rhombohedral R and monoclinic B19′ martensite phases were studied in depend on the strain accumulat...