D. V. Gunderov

Deformation twinning in nanocrystalline copper at room temperature and low strain rate

The grain-size effect on deformation twinning in nanocrystalline copper is studied. It has been reported that deformation twinning in coarse-grained copper occurs only under high strain rate and/or low-temperature conditions. Furthermore, reducing grain sizes has been shown to suppress deformation twinning. Here, we show that twinning becomes a major deformation mechanism in nanocrystalline copper during high-pressure torsion under a very slow strain rate and at room temperature. High-resolution transmission electron microscopy investigation of the twinning morphology suggests that many twins and stacking faults in nanocrystalline copper were formed through partial dislocation emissions from grain boundaries. This mechanism differs from the pole mechanism operating in coarse-grained copper.

Grain-size effect on the deformation mechanisms of nanostructured copper processed by high-pressure torsion

The unique nonuniform deformation characteristic of high-pressure torsion was used to produce nanostructures with systematically varying grain sizes in a copper disk, which allows us to study the grain-size effect on the deformation mechanisms in nanostructured copper using a single sample. The as-processed copper disk has 100–200 nm grains near its center and 10–20 nm grains at its periphery. High densities of full dislocations (2×1016/m2) were distributed nonuniformly in large grains, implying that dislocation slip is the dominant deformation mechanism. With increasing dislocation density, the dislocations accumulated and rearranged, forming elongated nanodomains. The originally formed nanodomains remain almost the same crystalline orientation as their parent large grains. Further deformation occurred mainly through partial dislocation emissions from nanodomain boundaries, resulting in high density of nanotwins and stacking faults in the nanodomains. The elongated nanodomains finally transformed into eq...

Controllable nanocrystallization in amorphous Nd9Fe85B6 via combined application of severe plastic deformation and thermal annealing

The control of nanocrystal formation in amorphous alloys is of particular importance for the development of advanced nanocrystalline materials. In the present study, the authors succeeded in controlling α-Fe and Nd2Fe14B nanocrystallization processes in amorphous Nd9Fe85B6 by a combination of severe plastic deformation at room temperature and subsequent thermal annealing. The α-Fe∕Nd2Fe14B nanocomposite magnets prepared by this approach possess homogeneously distributed nanocrystals with a small size, 15nm for α-Fe phase and 26nm for Nd2Fe14B, and therefore show enhanced magnetic properties as compared to those prepared by directly annealing amorphous Nd9Fe85B6.

Phase transformation induced by severe plastic deformation in the AISI 304L stainless steel

Abstract The phase transformation caused by high pressure torsion (HPT) of a cold rolled AISI 304L stainless steel was investigated. After cold rolling the steel has a microstructure of martensite α′ (bcc) with a magnetization saturation of 136.0 A m −2 kg −1 . The HPT processing with pressures of 2 and 5 GPa promoted the α′ (bcc)→e (hcp) partial transformation, as observed by X-ray diffraction. The magnetization saturation decreased by the increase of true deformation by HPT, which is in agreement to the α′ decrease.

Simultaneously increasing the magnetization and coercivity of bulk nanocomposite magnets via severe plastic deformation

In general, there is a trade-off between magnetization and coercivity in nanocomposite magnets. Here, we demonstrate a simultaneous enhancement of both the magnetization and coercivity in bulk α-Fe/Nd2Fe14B nanocomposite magnets prepared via a severe plastic deformation (SPD) compared with thermally annealed magnets. The enhanced magnetization results from a high fraction (>30%) of α-Fe phase induced by SPD, while the increase in coercivity from 4.6 to 7.2 kOe is attributed to an enhancement in domain wall pinning strength. This study shows an increase in energy product is possible in the nanocomposite magnets for a large inclusion of soft-magnetic phase.

Long-Length Ultrafine-Grained Titanium Rods Produced by ECAP-Conform

A new technique of continuous severe plastic deformation (SPD)-processing, i.e. ECAP (equal channel angular pressing)-Conform is applied for the first time to produce long-length rods of commercial purity Ti with ultrafine-grained structure. The paper reports on the results of investigation of the microstructure and mechanical properties of Ti rods processed by ECAPConform and the following wire drawing.

Nanocrystallization and magnetic properties of amorphous Nd9Fe85B6 subjected to high-pressure torsion deformation upon annealing

A high number density (∼1023 m−3) of α-Fe nanocrystals with a size below 10 nm has been induced in amorphous Nd9Fe85B6 by high-pressure torsion deformation (HPTD) at room temperature. The amorphous Nd9Fe85B6 subjected to HPTD presents a quite different crystallization behavior as compared with the nondeformed alloy. The growth activation energies Eg=0.9 eV for α-Fe nanocrystals and 0.5 eV for Nd2Fe14B nanocrystals are determined from the annealing time dependence of their size. The α-Fe/Nd2Fe14B nanocomposite magnets prepared by the combination of HPTD and subsequent thermal annealing show enhanced magnetic properties due to a small grain size as compared with the magnets prepared by directly annealing amorphous Nd9Fe85B6.

Structure evolution and changes in magnetic properties of severe plastic deformed Nd(Pr)–Fe–B alloys during annealing

Abstract Structural changes and hysteresis properties of Nd(Pr)–Fe–B alloys of various compositions were studied after severe torsion straining under high pressure and following annealing. It was shown that in all the alloys studied, severe plastic deformation leads to formation of ultrafine-grained non-equilibrium structure and, at extremely large strains, even to formation of an amorphous structure. Annealing resulted in crystallization of alloys, formation of stable magnetic and non-magnetic phases and in marked improvements in hard magnetic properties. Homogenization of alloys is responsible for an increase in coercivity of the deformed samples.

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.

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.