V. G. Pushin

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.

On the nature of anomalously high plasticity of high-strength titanium nickelide alloys with shape-memory effects: I. Initial structure and mechanical properties

This work presents the results of studies of the Ti49.4Ni50.6 alloy of enhanced purity with shapememory effects in an ordinary coarse-grained state with an average grain size of 20–30 μm or in a submicrocrystalline state with an average grain size of 0.2–0.3 μm. In this alloy the initial structure, phase composition, martensitic transformations, mechanical properties, and character of fracture have been investigated in a wide temperature range. It has been shown that upon cooling and mechanical tests at room temperature, the alloy exhibits highly reversible thermoelastic martensitic transformations. It has been established that the alloy exhibits high values of the strength and plastic properties and strain-hardening coefficients.

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.

Nanocrystallization Induced by Severe Plastic Deformation of Amorphous Alloys

The paper describes the influence of severe plastic deformation (SPD) on the crystallization of the amorphous rapidly quenched Ti-Ni and Nd-Fe-B alloys. It has been revealed that the SPD by high pressure torsion (HPT) at room temperature leads to formation of nanocrystals in these initially amorphous alloys. The subsequent annealing of these HPT-processed alloys leads to formation of homogeneous nanocrystalline structure. The SPD processing of the amorphous alloys can be used as a novel method for producing bulk nanocrystalline materials.

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.

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.

Crystal-structural features of pretransition phenomena and thermoelastic martensitic transformations in alloys of nonferrous metals

The unusual and practically important phenomena and physical properties that are caused by reversible martensitic transformations occurring mainly in alloys and compounds of nonferrous metals have been analyzed. The main types of martensitic transformations, their crystal-geometrical features and structure-morphological characteristics, the nature and structural mechanisms of reversibility, and the phenomenon of thermoelasticity have been considered. Structural mechanisms of atomic rearrangements upon martensitic transformations have been discussed, and premartensitic phenomena and their relation to martensitic transformations have been analyzed.

Mechanical Behavior and Stress-Induced Martensitic Transformation in Nanocrystalline Ti49.4Ni50.6 Alloy

Amorphous-nanocrystalline Ti49.4Ni50.6 alloy in the shape of a disc 20 mm in diameter has been successfully produced using high pressure torsion (HPT). Application of HPT and annealing at temperatures of 300–550°C resulted in formation of a nanocrystalline (NC) structure with the grain size (D) about 20–300 nm. The HPT samples after annealing at Т = 400°C with the D= 20 nm possess high yield stress and high ultimate tensile strength (more than 2000 MPa). There is an area of strain-induced transformation B2-B19’ on the tensile curve of the samples with the grain size D =20 nm. The stress of martensitic transformation (σm) of samples is 450 MPa, which is three times higher than σm in the initial coarse-grained state (σm ≈ 160 MPa). The HPT samples after annealing at Т = 550°C with the D= 300 nm possess high ductility (δ>60 %) and high ultimate tensile strength (about 1000 MPa).

Effect of severe plastic deformation by torsion on the properties and structure of the Ni54Mn21Ga25 and Ni54Mn20Fe1Ga25 alloys

The effect of one-percent substitution of iron for manganese on the physical (magnetic, electrical, thermal, and galvanomagnetic) properties and the crystal structure of the Ni54Mn21Ga25 alloy has been investigated. It has been demonstrated that the deviation of the alloy composition from the stoichiometric composition Ni50Mn25Ga25 leads to the formation of a mixed ferromagnetic-antiferromagnetic state. The atomic disordering and nanostructuring of the alloys under investigation due to the severe plastic deformation by torsion in Bridgman anvils to sizes of 10–20 nm result in the suppression of reversible magnetically controlled shape memory effects.

On the nature of anomalously high plasticity of high-strength titanium nickelide alloys with shape-memory effects: II. Mechanisms of plastic deformation upon isothermal loading

Mechanisms of plastic deformation have been studied in detail in the process of isothermal loading at room temperature in a high-purity shape-memory alloy of composition Ti49.4Ni50.6. The alloy was studied in two initial states: usual coarse-grained (with an average grain size of 20–30 μm) and submicrocrystalline (with an average grain size of 0.2–0.3 μm). It has been shown that during tensile tests there occurs a mechanically induced martensitic transformation in the alloy at stresses corresponding to stages I, II, and III in the tensile curve and then elastic and plastic deformation of B19′ martensite is observed at stages IV, V, and VI, respectively. Optical metallography in situ and electron microscopy have been used to study microstructural features and mechanisms of plastic deformation of the alloy up to its failure.