Functional properties of nanocrystalline, submicrocrystalline and polygonized Ti–Ni alloys processed by cold rolling and post-deformation annealing

Abstract

Abstract. Thermomechanical processing consisting of cold rolling (e=0.3-2.0) and post-deformation annealing (300-450oC, 1h) was applied to binary Ti-Ni alloys to produce nanocrystalline structures (NS) or polygonized dislocation substructures (PDS), or their mixture. The evolution of the material structure and properties was studied using TEM, X-ray, microhardness, calorimetry and tensile testing techniques. Recovery stress and strain of the 50.26at%Ni alloy and superelastic strain of the Ti-50.6at%Ni alloy were measured under static and fatigue conditions. It was found that higher true yield stress of NS alloys not only increases the recovery stress potential, but, since it is combined with a relatively low transformation yield stress; it increases the completely recoverable strain. NS alloys generate recovery stresses that are twice as high as those of PDS alloys (1200 MPa), completely recoverable strains that are 10% greater (up to 6% in tension), and they demonstrate a higher cyclic stability of functional properties. This improvement comes with the cost of a lower NS alloy fatigue damage tolerance, aggravated by the presence of microcracks caused by cold working. Binary Ti-Ni alloys, processed by annealing of an intermediately cold-worked (e=0.75…1) alloy and containing mixed nanocrystalline structure and polygonized dislocation substructure, allow a high fatigue life combined with relatively high and cyclically stable functional properties.

Formation of Nanostructure in Titanium Nickelide as a Result of Quazi-Continuous Isothermal Deformation

“…% Ni удалось достигнуть рекордных значений реактивного напряже-ния (1420 МПа) и полностью обратимой деформации (8 %). Наиболее высокий комплекс усталостных функ-циональных свойств в условиях термомеханического и сверхупругого циклирования был получен в случае формирования смешанной НС + НК структуры [5].…”

“…% Ni удалось достигнуть рекордных значений реактивного напряже-ния (1420 МПа) и полностью обратимой деформации (8 %). Наиболее высокий комплекс усталостных функ-циональных свойств в условиях термомеханического и сверхупругого циклирования был получен в случае формирования смешанной НС + НК структуры [5].Указанные результаты были получены в прокатан-ной проволоке сечением менее 1 мм 2 . В то же время задача получения объемных наноструктурных СПФ Ti -Ni оставалась нерешенной, поскольку для этого требуется обеспечить непрерывную изотермическую деформацию при 300 -400 °С [6].…”

Gradation of Nanostructures in Cold-Rolled and Annealed Ti–Ni Shape Memory Alloys

“…The structure formation features observed by transmission electron microscopy in cold-rolled and then annealed Ti-Ni alloys can be classified as a function of deformation intensity [11][12][13]17]. In the range of moderate true strains (up to e = 0.3), a well-developed dislocation substructure forms.…”

“…During PDA, the above-mentioned structures undergo specific structural changes: in the highly dislocated substructure, recovery, polygonization (which includes subgrain formation and growth), and finally recrystallization occur; the grains of the deformation-induced nanocrystalline structure grow, the amorphous phase crystallizes, and the crystallized nano-sized grains grow alongside their deformation-induced counterparts [11][12][13][15][16][17]. Note that individual groups of nano-sized subgrains can still be found in the latter case [5].…”

“…The lowangle boundaries (i.e., subgrain boundaries, subboundaries) are directly inherited by growing martensite crystals and serve as partially permeable barriers for dislocation slip, while high-angle boundaries (grain boundaries) are non-permeable barriers for both the martensite crystals and dislocations [22]. As concerned with the functional properties, an outstanding combination of high recovery stress and completely recoverable strain can be obtained in the case of B2-austenite structure consisting of nano-sized grains [11,12,17], while the best functional fatigue properties can be observed in the case of a mixed B2-austenite structure containing nano-sized grains and nanosized subgrains [17]. The nanostructure containing exclusively nano-sized subgrains exhibits the lowest performance from the static and fatigue functional properties points of view [11,12,17].…”