A. N. Uksusnikov

Effect of Heat Treatment on the Structural and Phase Transformations and Mechanical Properties of TiNi Alloy Subjected to Severe Plastic Deformation by Torsion

Effect of annealing on the structural and phase transformations and mechanical properties of large-size samples of the Ti49.4Ni50.6 alloy preliminary subjected to severe plastic deformation by torsion under a high pressure (HPT) is studied. The study was performed by transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and measurements of mechanical properties. The annealing was found to result in the nanocrystallization of initial samples amorphized by HPT. In this alloy, the high-strength uniform nanostructured state is formed the size of nanocrystalline grains of which and the mechanical properties depend on the temperature and time of annealing.

Application of Severe Plastic Deformation by Torsion to Form Amorphous and Nanocrystalline States in Large-Size TiNi Alloy Sample

Results of investigations of the initial structure of large-size Ti49.4Ni50.6 samples subjected to severe plastic deformation by torsion under a high pressure (HPT) are reported. The study was performed using transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and measurements of mechanical properties. Under an applied pressure of 6 GPa, the alloy was found to undergo a martensitic B2 → B19′ transformation. Even after HPT using a single revolution of anvils, the granular structure of titanium nickelide is refined so that there is formed a nanocrystalline state of B2 austenite (i.e., the reverse martensitic B19′ → B2 transformation occurs) and amorphization of the alloy begins. The HPT with a high number of revolutions leads to the almost complete amorphization of the alloy, which is explained by a high degree of shear deformation. In this case, all nanocrystalline inclusions in the amorphous matrix have an ordered B2 structure.

Formation of nanocrystalline structure in the amorphous Ti50Ni25Cu25 alloy upon severe thermomechanical treatment and the size effect of the thermoelastic martensitic B2 ↔ B19 transformation

The results of the comparative analysis of the Ti50Ni25Cu25-alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain-induced amorphization even with the retention, even after 5–15 rev, of a large number of highly dispersed nanocrystals less than 3–4 nm in size with a distorted B2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low-temperature nanocrystallization during subsequent annealing starting from 250–300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B2-type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2-austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.

Specific features of cast high-entropy AlCrFeCoNiCu alloys produced by ultrarapid quenching from the melt

Results of studying structural and phase transformations that occur in the cast high-entropy equiatomic AlCrFeCoNiCu alloy after ultrarapid quenching from the melt in an inert atmosphere (RQM) and various isothermal treatments are presented for the first time. The investigations have been performed using analytical, transmission and scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction structure and phase analyses, as well as measurements of the nanohardness, microhardness, and elastic moduli. It has been found that an ultrafine-grained structure is formed in this alloy during RQM. Already during quenching and, especially, during subsequent annealing, the alloy undergoes decomposition, which is accompanied by the precipitation in the bcc (B2) matrix of some nanosized phases, predominantly of equiaxed morphology, both atomically ordered (B2) and disordered (A2), with various chemical compositions. All nanophases are multicomponent solid solutions and are enriched in a few elements, which leads to a pronounced nanomodulation of the elemental and phase compositions over the alloy bulk, identified, in particular, from the presence of satellites in the vicinity of some reflections in selected-area electron diffraction patterns.

Phase and structural transformations in the Ti49.5Ni50.5 alloy with a shape-memory effect during torsion under high pressure

Results of investigations of structural and phase transformations that occur in the titanium-nickelide-based alloy Ti49.5Ni50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X-ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19′ transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19′ phase is retained in the alloy. The fine structure of the B19′ martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30–50 nm have a B2-type structure and, therefore, the reverse martensitic B19′ → B2 transformation is realized in the alloy at room temperature after unloading.

Microstructure features of high-entropy equiatomic cast AlCrFeCoNiCu alloys

The structural and phase transformations that take place in the cast high-entropy equiatomic alloy AlCrFeCoNiCu after solidification, homogenizing heat treatment, and cooling have been studied. Analytical transmission microscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, and X-ray diffraction analysis were used to conduct the studies. The elastic modulus, nano-, and microhardness have been measured. The alloy decomposition has been found to occur with the precipitation of no less than six nanoscale phases with different morphologies, structures (A2, B2, L12), and chemical compositions. All the nanophases are multicomponent solid solutions enriched with several elements, which indicates the pronounced elemental and phase nanomodulation over the alloy volume.

Effect of severe plastic deformation on the structure and properties of the Ni2.16Mn0.84Ga alloy

The effect of severe plastic deformation by torsion (high-pressure torsion or HPT) on the crystalline structure and behavior of electrical resistivity, thermoelectric power, thermal expansion, and magnetic properties of the rapidly quenched Ni2.16Mn0.84Ga alloy has been investigated. It has been shown that after HPT treatment of the rapidly quenched submicrocrystalline alloy, a mixture of amorphous and nanocrystalline phases is formed. Electrical properties of the alloy have been explained in terms of the Mott two-band model. Specific features of the magnetic properties of the alloy are related to the coexistence of ferro-and antiferromagnetic interactions in an atomically disordered state of the alloy.

Structure and thermoelastic martensitic transformations in ternary Ni–Ti–Hf alloys with a high-temperature shape memory effect

The effect of alloying by 12–20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi–NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic B2 ↔ B19’ martensitic transformations, which occur in the temperature range 320–600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the B2 and B19’ phases are measured, and the microstructure of the B19’ martensite is analyzed.

Amorphization of bulk TiNi-based alloys by severe plastic deformation under high pressure torsion

The structure of bulk samples of Ti49.4Ni50.6 alloy after severe plastic deformation by torsion (SPDT) under high pressure have been studied by transmission and scanning electron microscopies. It is found that SPDT by five to seven turns led to almost complete alloy amorphization.

Structure and phase transformations in TiNiFe ternary alloys subjected to plastic deformation by high-pressure torsion and subsequent heat treatment

The results of a complex study of ternary TiNiFe alloys with a low-temperature shape-memory effect subjected to megaplastic deformation by high-pressure torsion (HPT) with subsequent heat treatment are presented. Investigations have been performed using X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. It has been established that, at moderate degrees of reduction, the plastic deformation in the Ti50Ni49Fe1 alloy induces a B2 ↔ B19′ thermoelastic martensitic transformation and the formation of a developed banded dislocation and twin structure in the B19′ martensite; in the Ti50Ni47Fe3 alloy, a mainly analogous dislocation substructure is formed, but in the B2 austenite. The megaplastic deformation by HPT at room temperature leads to the amorphization of the Ti50Ni49Fe1 alloy and to the high-angle nanofragmentation of the Ti50Ni47Fe3 alloy. Specific features of the evolution of the structure and martensitic transformations in the TiNiFe ternary alloys after plastic deformation and heat treatment have been established. It has been found that the heat treatment of both alloys after HPT at temperatures of 553–773 K results in the formation of a nanocrystalline or mixed nano-submicro-crystalline structure.