Ultrafine-grained State Research Articles

Contribution of structural parameters to the strength and electrical conductivity of Cu-0.5%Cr alloy subjected to ECAP and cold rolling

The effect of cold rolling with a reduction rate of 95% at room temperature and subsequent aging on the evolution of the microstructure, strength properties and electrical conductivity of the Cu-0.5Cr alloy (wt.%), which is widely used in the electrical industry in the form of strips and ribbons, has been studied. The cold rolling and aging of the alloy was carried out in coarse-grained and ultrafine-grained states after equal-channel angular pressing. The regularities found were used to assess the contribution of various structural parameters to the level of strength properties as well as the electrical conductivity of the alloy in the obtained states.

The effect of neutron irradiation on the impact toughness of austenitic stainless steel in ultrafine-grained state

Austenitic stainless steel in conventional and ultrafine-grained states is studied by impact testing in an unirradiated condition and after neutron irradiation to 12 dpa at 350°C. We show that grain refinement provided higher resistance of the steel to the onset of evolving macrodefects (such as cracks) in irradiated state. Impact toughness of the coarse-grained steel notably degraded after irradiation, while the ultrafine-grained steel demonstrated better resistance of impact toughness to irradiation effect. This finding highlights the promising potential of ultrafine-grained steels to be used as advanced radiation resistant materials with enhanced mechanical performance.

Tensile and Corrosion Properties of Anodized Ultrafine-Grained Ti–13Nb–13Zr Biomedical Alloy Obtained by High-Pressure Torsion

Severe plastic deformation (SPD) is a popular group of techniques applied to achieve the nanostructuring of the metallic biomaterials and improvement of their mechanical characteristics. One of the most commonly used SPD methods is the high-pressure torsion (HPT) technique which enables the obtainment of the microstructure with small grains and high strength. In the present study, the influence of the plastic deformation and surface modification treatment on the tensile and corrosion properties of the Ti–13Nb–13Zr (wt%) alloy is investigated. In that purpose, the coarse-grained (CG) Ti–13Nb–13Zr (TNZ) alloy was subjected to the HPT processing by applying a pressure of 4.1 GPa with a rotational speed of 0.2 rpm and 5 revolutions at room temperature to obtain the ultrafine-grained (UFG) microstructure. The alloy microstructure before and after HPT processing was analysed using the scanning electron microscopy (SEM) and the X-ray diffraction (XRD). The homogeneity of the UFG TNZ alloy was determined by microhardness testing and microscopic observations. The nanotubular oxide layer on the surface of the TNZ alloy, both in CG and UFG condition, was formed by electrochemical anodization in 1 M H3PO4 + NaF electrolyte for 90 min. SEM analysis was used to characterise the morphology of the anodized surfaces, while energy dispersive spectroscopy was applied to determine the chemical composition of the nanostructured layers formed at the alloy surfaces. Mechanical properties of the TNZ alloy, before and after HPT processing and electrochemical anodization, were determined by tensile testing. After tensile testing, the fractographic analysis was conducted to identify the fracture mechanisms. The potentiodynamic polarization technique was used to determine the corrosion resistance of the alloy before and after plastic deformation and surface modification treatment. The obtained results showed that the alloy is reasonably homogeneous after the HPT processing. The XRD analyses reviled the presence of α′ and β phases in the CG TNZ alloy microstructure, while the additional ω phase was detected in the microstructure of the UFG TNZ alloy. The HPT obtained alloy exhibits higher hardness and improved tensile properties than the alloy in the as-received CG condition, while the electrochemical anodization leads to a decrease of its mechanical properties. Both CG and UFG alloys show excellent corrosion stability in Ringer’s solution. Moreover, electrochemical anodization leads to a decrease or an increase of the corrosion resistance of these materials, depending on the morphology of the formed nanotubular surface layers. The results indicate that the anodized CG TNZ alloy is characterized by a lower modulus of elasticity and better corrosion resistance properties than the anodized UFG TNZ alloy.

Effect of rotary swaging on structure and properties of low-carbon steel

The structural transformation and mechanical properties of a low-carbon 0.2% C steel and a 0.09% C-Mn-V steel in coarse- and ultrafine-grained (UFG) states were studied. The UFG structure with structural elements about 210-375 nm in size was obtained by rotary swaging (RS) and equal-channel angular pressing (ECAP). ECAP was used to compare the influence of the industrial deformation and SPD methods on the micro structure and properties of low-carbon steels. It was shown that an increase in true strain ratio and a decrease in final temperature of RS improve strength properties and reduce plasticity of these steels. In both steels after RS, σ UTS within the range from 800 up to 920 MPa with elongation in the range of 15-17% at the final deformation temperature of 400 °C was obtained. It was found that the strength of 0.2% C steel after ECAP was on the same level with that steel after RS. The strength properties of low- carbon 0.09% C-Mn-V steel after ECAP were significantly higher than those after RS.

Effect of Hydrogen on the Deformation Behavior and Localization of Plastic Deformation of the Ultrafine-Grained Zr–1Nb Alloy

In this paper, comparison studies of the hydrogen effect on the structural and phase state, deformation behavior, and mechanical properties of the fine- (average grain size 4 µm) and ultrafine-grained (average element size 0.3 and 0.4 µm) Zr–1wt.%Nb (hereinafter Zr–1Nb) alloy under tension at temperatures in the range of 293–873 K were conducted. The formation of an ultrafine-grained structure is established to increase the strength characteristics of the Zr–1Nb alloy by a factor of 1.5–2 with a simultaneous reduction of its resistance to the localization of plastic deformation at the macro level and the value of deformation to failure. The presence of hydrogen in the Zr–1Nb alloy in the form of a solid solution and hydride precipitates increases its resistance to the localization of plastic deformation at the macro level if the alloy has an ultrafine-grained structure and decreases if the structure of the alloy is fine-grained. In the studied temperature range, the Zr–1Nb alloy in the ultrafine-grained state has a higher resistance to hydrogen embrittlement than the alloy in the fine-grained state.

Excellent superplastic properties achieved in Mg–4Y–3RE alloy in high strain rate regime

This study is focused on high strain rate superplasticity of commercial magnesium alloy Mg–4Y–3RE (WE43) in ultrafine-grained condition prepared by equal channel angular pressing (ECAP). Attaining microstructure with the average grain size of ~340 nm and high density of secondary phase particles provided a possibility to significantly decrease temperature during the deformation at strain rates of 10−2 s−1 and 10−1 s−1 in comparison with previous studies. Consequently, extent of negative effect of grain growth was partially suppressed and the investigated alloy did exhibit exceptionally high deformability with a maximum elongation of ~1230% at strain rate of 10−2 s−1 and at two temperatures: 350 °C and 400 °C. Increase in the strain rate to 10−1 s−1 resulted in maximum elongation of ~1000% at the temperature of 400 °C. The microstructural analysis after the deformation showed that the microstructure was fine-grained even after large deformation and plasticity-controlled growth of cavities did occur only at the temperature of 450 °C. The optimization of ECAP processing resulted in very weak texture and, therefore, the deformability should be more isotropic in comparison with strongly textured materials usually investigated in this regard.

Microstructural and Mechanical Stability of a Ti-50.8 at.% Ni Shape Memory Alloy Achieved by Thermal Cycling with a Large Number of Cycles

The influence of thermal cycling (TC) with a large number of cycles on the microstructure, the parameters of martensitic transformations (MTs), and the mechanical properties of a Ti-50.8 at.% Ni shape-memory alloy in coarse-grained (CG) and ultrafine-grained (UFG) states was investigated. The effect of microstructural and mechanical stability was found in both coarse-grained and ultrafine-grained states starting from the 100th cycle of martensitic transformations. In addition, an unusual temperature change was observed in martensitic transformations occurring with the formation of an intermediate R phase.

Влияние ультрамелкозернистого состояния на теплофизические свойства сплавов Zr - 1 мас. % Nb и Ti - 45 мас. % Nb и процессы диссипации и накопления энергии при деформировании

The results of studying the thermal conductivity and specific heat capacity of Zr-1 wt.% Nb and Ti-45 wt.% Nb in coarse-grained and ultrafine-grained states are presented. The influence of the utrafine-grained state on the thermo physical properties and the processes of dissipation and accumulation energy during deformation of the alloys are estimated. It is shown that formation of the ultrafine-grained state in the Zr-1 wt.% Nb alloy by abc -pressing and subsequent rolling leads to a decrease in its thermal conductivity and specific heat capacity by 10 and 20%, respectively, due to substructural hardening under severe plastic deformation. It is found that thermal conductivity and specific heat capacity of the ultrafine Ti-45 wt.% Nb alloy increase by 7.5 and 5%, respectively, due to dispersion hardening of the ω phase by nanoparticles and formation of a new α phase. It is established that the ultrafine-grained structure has a significant influence on the regularities of the dissipation and accumulation energy during plastic deformation, which in turn depend on their thermo-physical characteristics and on the structural and phase state.

Влияние длительности термического воздействия на микроструктуру и механические свойства ультрамелкозернистых биоинертных сплавов Zr - 1 мас. % Nb и Ti - 45 мас. % Nb

The results for thermal stability of the microstructure and mechanical properties of ultrafine-grained alloys Zr-1 wt. % Nb and Ti-45 wt. % Nb alloys after long-term thermal annealing at temperature of 400°C are presented. It is shown that in the Zr-1 wt.%Nb alloy in the ultrafine-grained state, increase in the annealing duration from 5 to 360 hours leads to growth of the structural elements (grains, sub-grains, fragments) of the matrix phase a - Zr and the particles β-Nb. This fact is due to the processes of recrystallization. It leads to softening of the alloy and decrease of microhardness and yield stress. It was also found that annealing up to 360 hours does not affect the size of the structural elements of the matrix β-phase in the Ti-45 wt. % Nb alloy, but contributes to the noticeable growth of the α-phase and ω-phase grains. According to our results, softening of the Ti-45 wt. % Nb alloy and decrease in its mechanical characteristics are associated with the processes of return at the grain boundaries, as well as with growth of the nanoscale grains of α-phase and ω-phase and decrease in their contribution to dispersion hardening.

Effect of Cd and SPD on structure, physical, mechanical, and operational properties of alloy of Cu-Cr-Zr

Abstract An increase in the service life of electrical products from copper and its alloys is directly related to an increase in the wear resistance of materials. Structural refinement and alloying with cadmium are known to have a positive effect on the strength characteristics and wear resistance of copper, which makes it possible, with a Cd content of 1% by weight, to increase the wear resistance of copper several times, but cadmium is considered an environmentally unsafe element. In this regard, the paper presents the results of studies of a widely used Cu-Cr-Zr alloy system in the ultrafine-grained (UFG) state, micro-alloyed with cadmium (0.2%, weight), in order to improve physical, mechanical, and operational properties, as well as environmental safety. Severe plastic deformation, providing structure refinement to ~150 nm, and microalloying with cadmium of a Cu-Cr-Zr system alloy, after a complete processing cycle, provides a tensile strength of 570±10 MPa and 67% electrical conductivity. At the same time, the abrasion resistance increases by 12 and 35% relative to the industrial systems Cu-Cd and Cu-Cr-Zr, respectively. The obtained characteristics are very promising for improving the operational properties of continuous welding tips, collector plates, and contact wires operating under conditions of intense wear.

The microstructure and mechanical characteristics of the alloy Ti-50.7 at.%Ni with different deformation processing during multiple martensitic transformations

The influence of thermal cycling in the temperature range of B2-B19’ martensitic transformations on the TiNi alloy structure and properties are studied. Different states are considered, namely the initial coarse-grained (CG) state, the ultrafine-grained (UFG) state after ECAP, the state after ECAP and cold upsetting by 30%. It is shown that, as a result of thermal cycling, microhardness and strength grow in all the three states. According to the data from TEM analysis, the increment of the dislocation density is observed, resulting from thermal cycling.

Structure and Mechanical Properties of AISI1045 in the Helical Rolling–Pressing Process

The structure and mechanical properties of steel AISI 1045 subjected to three passes of the combined process “helical rolling–pressing” at a temperature of 500 °C were investigated. During the helical rolling–pressing process, the structure was substantially crushed to ultrafine-grained state, as the structure of steel AISI 1045 with an average grain size of 25 µm after deformation was decreased by 25 times, the average grain size was 1 µm. The values of tensile strength and yield strength increase in three passes from 619 to 925 MPa and from 545 to 867 MPa, respectively. The relative contraction after rupture varies from 42 to 35%; but the change is not as significant as in usual helical rolling. The combined helical rolling–pressing process is an effective way to form an ultrafine-grained structure and improve the mechanical properties of AISI 1045 steel.

Thermal Stability of Titanium Alloy VT8M-1 with Ultrafine-Grained Structure

The paper considers the effect of a long-duration heating at a service temperature of 450oC on VT8M-1 with a coarse-grained (CG) and ultrafine-grained (UFG) microstructure. A duplex ultrafine-grained microstructure, composed of equiaxed grains of primary α-phase and an ultrafine constituent of α- and β-phases, was processed by thermal treatment and further rotary swaging. This type of a microstructure demonstrates a best combination of strength and ductility at room temperature in comparison with the CG structure. A thermal stability of an UFG state was studied at 450°C for 50, 100, 200, 300, 400, and 500 hours. The evolution of the alloy microstructure against the duration of heating was considered by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM). No increase in the alloy structural elements and strength decrease resulting from a long-term annealing (up to 500 hours) at T=450°C have been observed. This proves a high thermal stability of the UFG structure and mechanical properties of VT8M-1 processed via rotary swaging.

Regular Features of Stage Formation in the Stress-strain Curves and Microstructure in the Zone of Fracture of Coarse-Grained and Ultrafine-Grained Titanium and Zirconium Alloys

The stage-like character of the stress-strain curves and microstructure in the fracture zone of coarse-grained and ultrafine-grained specimens of the VT1-0 and Zr–1Nb alloy specimens under quasistatic loading are investigated using the temperature curves obtained by the method of IR-imaging. New experimental data are obtained, which provide evidence on a considerable influence of the ultrafine-grained state on the evolution of plastic deformation and fracture in the VT1-0 and Zr–1Nb alloys.

Evolution of the Structural-Phase State of a Titanium Alloy of the System Ti–Al–V–Mo During Formation of an Ultrafine-Grained Structure Using Reversible Hydrogenaтion

Using methods of electron-microscopy and x-ray structural analysis, we have investigated peculiarities of the evolution of the structural-phase state of an alloy of the system Ti–Al–V–Мо during formation of an ultrafine-grained structure by a method combining hot plastic deformation and reversible hydrogenation. It has been shown that the presence in solid solution of 0.15 wt% hydrogen makes it possible to lower by more than a factor of 2 the magnitude of the plastic deformation needed to obtain an ultrafine-grained state in the alloy. We found that during deformation and subsequent degassing of hydrogen from the alloy, such processes as the phase transformations α′′→β→α and β→α′′ and a redistribution of the doping elements take place, facilitating the formation of a lamellar morphology in the grains and preservation of a high level of strength properties of the ultrafine-grained structure.

Microstructure and mechanical properties of the ultrafine-grained TiNi alloy after multiple martensitic transformations and subsequent aging

In this paper, we investigate the effect of preliminary thermal cycling in the temperature range of martensitic transformation on the aging processes in the ultrafine-grained Ti49.0Ni51.0 alloy, using as an example the examination of the microstructure, mechanical properties and fractography. Our studies show that preliminary thermal cycling leads to a significant increase in the density of dislocations that promote the intensification of the aging process and thereby enhance the strength and functional characteristics of the TiNi alloy in the ultrafine-grained state.

Mechanical behavior at elevated temperatures of the ultrafine-grained titanium alloy VT8M-1 processed by rotary swaging

This paper investigates the mechanical properties at elevated operation temperatures of the Ti alloy VT8M-1(Ti-5.3Al-4.0Mo-1.2Zr-1.3Sn-0.2Si) in the coarse-grained (CG) and ultrafine-grained (UFG) states. The duplex UFG structure was produced by rotary swaging at a temperature of 750 °C. The UFG state was studied in terms of thermal stability during annealing at temperatures 500, 550, 600, 700 °C for a period of 1 hour. It was demonstrated that the recrystallization processes in the UFG structure start after annealing at 550 °C and above. We performed tensile mechanical tests at such temperatures. In a range of 300-450 °C, the ultimate tensile strength of the UFG alloy was stably above that of the CG alloy. Mechanical tests at 500 and 550 °C revealed a more significant decrease in the ultimate tensile strength of the UFG alloy as compared to the CG alloy, conditioned by the intensification of the recovery and recrystallization processes at these temperatures.

Mechanical behavior and impact toughness of the ultrafine-grained VT8M-1 alloy

This paper reports on a study of the relationship between the microstructure, mechanical behavior and impact toughness of the two-phase Ti alloy VT8M-1 (Ti-5.7Al-3.8Mo-1.2Zr-1.3Sn) in the ultrafine-grained (UFG) state, which was produced by two different techniques – equal-channel angular pressing (ECAP) and rotary swaging (RS). It is shown that both processing techniques enable forming a UFG structure and enhanced mechanical properties in the alloy. The UFG structures formed in the billets are characterized by different shapes and sizes of the α- and β-phases. The relationship between the structure, strength, ductility and impact toughness of the alloy processed by RS and ECAP is discussed.

Effects of Grain Refinement and Predeformation Impact by Severe Plastic Deformation on Creep in P92 Martensitic Steel

Creep testing is conducted on an advanced martensitic‐ferritic creep‐resistant P92 steel (ASME Grade 92) to evaluate the effects of grain refinement and the predeformation impact after processing by severe plastic deformation (SPD), namely, by high‐pressure torsion and high‐pressure sliding. Constant‐load tensile creep tests are carried out in an argon atmosphere at 600 °C and under an applied stress ranging from 50 to 200 MPa. The results show that under the same creep loading conditions, the ultrafine‐grained (UFG) microstructure states after SPD processing exhibit higher minimum creep rate m and creep fracture plasticity , but significantly shorter creep lives in comparison with the coarse‐grained (as‐received) state of the steel. These distinct differences between the coarse‐grained and UFG states are explained by the different deformation mechanisms that operate; creep behavior in a coarse‐grained state is controlled by the intragranular climb of dislocations, while creep in UFG states can be interpreted as the synergistic action of the dynamic recovery of free dislocations at high‐angle grain boundaries and grain boundary‐mediated deformation processes.

Закономерности формирования стадийности деформационных кривых и микроструктуры в зоне разрушения крупно- и ультрамелкозернистых сплавов титана и циркония

Закономерности формирования стадийности деформационных кривых и микроструктуры в зоне разрушения крупно- и ультрамелкозернистых сплавов титана и циркония