Non-equilibrium Grain Research Articles

The influence of low-temperature annealing on the spectral acoustic characteristics of the 1515 sub-microcrystalline alloy

The acoustic characteristics of the 1515 aluminum sub-microcrystalline alloy that were obtained by equal channel angular pressing (ECAP) are investigated. It was found that the action of short-term (5, 10, and 15 min) low-temperature (T = 0.4 Tmelt) annealing substantially increased both the attenuation coefficient and the velocity of longitudinal elastic waves. The observed effects may be explained by the structures of the nonequilibrium grain boundaries and their evolution during annealing.

The formation of a multipeak relief on the surface on nanostructured nickel and field electron emission from it

Studies of the relief formed on the surface of nanostructured nickel with nonequilibrium grain boundaries formed upon ion-beam sputtering are presented. It has been shown that the relief on the nano-structured nickel surface differs substantially from the relief formed on the surface of the coarse-grained nickel with equilibrium grain boundaries. In particular, a multipeak relief with submicron dimensionality of the peaks forms on the surface of nanostructured nickel under certain conditions. This relief is advantageous for the field electron emission. For the sample with multipeak relief, a CVC of field electron emission is measured.

Investigation of non-equilibrium grain boundary segregation on CGHAZ in reactor pressure vessel steel

In order to explore the reasons for coarse grained embrittlement, post-weld heat treatment (PWHT) with different times was conducted on the coarse grained heat affected zone (CGHAZ) of A533B steel. The results reveal that the impact toughness of the CGHAZ at room temperature decreases with the prolongation of PWHT time from 0.5 to 4 h. As the PWHT time increases to 4 h, the fracture feature of the CGHAZ presents intergranular cleavage ruptures, and the impact toughness exhibits the minimum value. After PWHT for 4 h, the area of the intergranular cleavage fractures decreases, and the impact toughness gradually increases. Further research reveals that there is phosphorus grain boundary segregation in the CGHAZ. The theory of non-equilibrium grain boundary segregation (NGS) is proposed to explain the phenomenon. It is found that the PWHT time with minimum impact toughness is consistent with critical time of phosphorus NGS.

Mechanism of hot ductility loss in C–Mn steels based on nonequilibrium grain boundary segregation of impurities

Abstract

Description of grain-boundary diffusion in nanostructured materials for thin-film diffusion source

Using asymptotic solution methods of differential equations, the grain-boundary diffusion at transition (between regimes C and B) stages of annealing has been described for nano- and submicrocrystalline materials characterized by nonequilibrium grain boundaries and, thus, by a strong coordinate dependence of the diffusion coefficient near them. As a continuation of [1], the solution of the diffusion problem has been obtained for the condition of thin-film (instantaneous) exhaustible diffusion source frequently used in experiments. Based on the performed numerical calculations and qualitative estimates, expressions have been suggested for the average layer concentration and the region of their applicability has been established. The features of grain-boundary diffusion connected with allowance for near-boundary regions and type of the diffusion source have been treated analytically. Using the literature data the depth of penetration of diffusant into the near-boundary zone has been estimated for some materials.

Anneal hardening of a nanostructured Cu–Al alloy processed by high-pressure torsion and rolling

A Cu–7wt% Al alloy was processed by high-pressure torsion and rolling to obtain nanostructure and then annealed at a series of different temperatures. Annealing at 250°C for 120min produced higher strength, which is attributed to the formation of nanoscale annealing twins. It is proposed that the high densities of stacking faults and nonequilibrium grain boundaries, combined with the high residual shear stresses in the deformed samples, contribute to the development of nanoscale annealing twins during the recrystallization process.

Influence of Surface Nanocrystallization on Ti Ion Implantation of Pure Iron

In order to increase the depth or concentration of Ti ion implantation of pure iron, the surface mechanical attrition treatment (SMAT), which can fabricate a nanometer-grained surface layer without porosity and contamination in a pure iron plate, was used before ion implantation. Ti ion was implanted into the SMA treated sample and coarse-grained counterpart by using a metal vapor vacuum arc source implanter. The changing of depth and concentration of Ti was studied in a function of implantation time. By optical microscopy, transmission electron microscopy and X-ray diffraction, the grain size of the nano structured surface was studied. Micro-hardness, friction and wear behavior of nano surface layers were studied. By energy dispersive X-ray spectroscopy and Auger electron spectroscopy, the chemical composition and concentration of Ti ion in the surface implantation layer were studied. Experimental results showed that the concentration of Ti increased dramatically compared with untreated coarse-grained samples, which is attributed to the existence of higher density of defects (supersaturated vacancies, dislocations, non-equilibrium grain boundaries etc.) and compression stress field in the SMA treated nanocrystallined surface layer. The interaction between the defects and the implanted solute atoms leads to the increment of solid solubility. But the implantation depth showed inconspicuous change. It is shown that the ion range is just relevant to the energy and mass of the ion, dose of injection, the mass and density of target material.

Effects of Strain Energy and Grain Size on Corrosion Resistance of Ultrafine Grained Fe-20%Cr Steels with Extremely low C and N Fabricated by ECAP

Effect of strain energy and grain size on corrosion resistance of ultrafine grained (UFG) Fe-20%Cr steels with extremely low C and N fabricated by equal channel angular pressing (ECAP) was investigated. UFG structures of initial grain size of 144 nm exhibited the typical three-stage softening comprising recovery, recrystallization, and grain growth. Potentiodynamic polarization measurements were carried out with a conventional three-electrode cell to evaluate pitting potential. Pitting potential in 1000 mol·m−3NaCl solution was nobler in UFG state, but pitting potential started to decrease monotonously at lower temperature compared to hardness. The degradation of corrosion resistance in the early stage of annealing is attributed to stability change of passivation by recovery of dislocation structures inside grains and in nonequilibrium grain boundaries. We therefore conclude that nobler potentials of UFG states were realized by not only grain size reduction but also defective deformation-induced UFG.

Grain-size-dependent non-monotonic lattice parameter variation in nanocrystalline W: The role of non-equilibrium grain boundary structure

The grain size dependency of lattice parameter during high-energy ball milling of W has been investigated. The lattice parameter varies non-monotonically with grain size during milling with a lattice contraction initially followed by an expansion. The lattice parameters were calculated in view of the non-equilibrium grain boundary structure that evolved during milling using excess free volume and the interfacial stresses at the grain boundaries. The calculated lattice parameters closely match the experimentally observed values.

Non-equilibrium phosphorus grain boundary segregation and its effect on embrittlement in a niobium-stabilized interstitial-free steel

Grain boundary segregation of phosphorus in an Nb-stabilized and P-strengthened interstitial-free steel was investigated using Auger electron spectroscopy after the steel specimens were quenched from 880°C and aged at 680°C for different times. The segregation initially increased with increasing aging time until reaching a maximum value and then decreased gradually to its equilibrium value with further increasing aging time, demonstrating a non-equilibrium segregation kinetic characteristic with a critical time of around 30min. The ductile-to-brittle transition temperatures (DBTTs) of the aged specimens were determined through fracture appearance measurements. There was a linear relationship between DBTT and phosphorus boundary concentration (Cp, at%): DBTT(°C)=4.39Cp−64.06, suggesting that a P-strengthened IF steel should avoid slow cooling or staying in the intermediate temperature range so as to avoid apparent phosphorus grain boundary segregation and in turn to avoid embrittling.

Micromechanical modelling of nanocrystalline and ultrafine grained metals: A short overview

An overview of micromechanical models of strength and deformation behaviour of nanostructured and ultrafine grained metallic materials is presented. Composite models of nanomaterials, polycrystal plasticity based models, grain boundary sliding, the effect of non-equilibrium grain boundaries and nanoscale properties are discussed and compared. The examples of incorporation of peculiar nanocrystalline effects (like large content of amorphous or semi-amorphous grain boundary phase, partial dislocation GB emission/glide/GB absorption based deformation mechanism, diffusion deformation, etc.) into the continuum mechanical approach are given. The possibilities of using micromechanical models to explore the ways of the improving the properties of nanocrystalline materials by modifying their structures (e.g., dispersion strengthening, creating non-equilibrium grain boundaries, varying the grain size distributions and gradients) are discussed.

Non-equilibrium grain boundaries with excess energy in graphene

Exploring an approach to grain boundary (GB) mesoscopic structure in 3D polycrystals, we develop a model description for equilibrium and non-equilibrium states of GBs in graphene. Non-equilibrium states of GBs in graphene are analyzed for the first time. On the base of this model, dependence of total energy for symmetric GBs on the misorientation angle, E(θ), in graphene was calculated. It was found that for the same average misorientation angle the energy in a non-equilibrium state of a GB in graphene can significantly exceed the energy in the equilibrium state. In graphene, the GB energy depends to a greater extent on the deviation from the equilibrium state than on the misorientation angle. It is proposed to account for the presence of non-equilibrium GBs in the explanations of unusual physical properties of graphene.

Precipitation sequence of η phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy during artificial aging

The precipitation sequence of η(MgZn2) phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 °C for various times from 5 min to 6 h. High resolution transmission electron microscopy (HRTEM) observations and energy dispersive X-ray spectroscopy (EDX) analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution (SSS)→vacancy-rich clusters (VRC)→GP II zones→η′→η. Based on the theory of non-equilibrium grain boundary segregation (NGS) and non-equilibrium grain boundary co-segregation (NGCS), the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP II zones, η′ phase as well as η phase.

Nanostructured titanium alloys: New developments and application prospects

The work presents the results of recent studies of nanostructured titanium and its alloy of the grade VT-6 (Ti-6Al-4V), in which we managed, using the methods of severe plastic deformation (SPD), to obtain ultra-fine-grained (UFG) states and form other nanostructured elements as nanodispersed particles of secondary phases, nonequilibrium grain boundaries, and dislocation substructures. Special attention was paid to equal-channel angular pressing (ECAP) and its modifications in combination with thermal and thermomechanical treatments, which enable one to obtain UFG alloys in the form of elongated rods. The peculiarities of grinding the microstructure in titanium and binary alloy VT-6 in dependence on the deformation technological setup have been established. The principles of nanostructuring to enhance strength, plasticity, and fatigue life have been suggested. The results of studies of the mechanical properties of the UFG VT-6 alloy have demonstrated the high efficiency of its application in the manufacture of articles with enhanced service parameters on the example of compressor blades of a gas-turbine engine (GTE). The possibilities of scaling the ECAP process to implement the commercial manufacture of the new material have been examined.

Grain boundary segregations in nanocrystalline alloys

The dependence of grain boundary segregations and alloy composition in the bulk of the grain on the grain size was considered. At moderate temperatures, there is a critical grain size for which the integrated degree of alloy decomposition reaches its maximum; this size increases as the average (over the sample) impurity concentration decreases. In complex alloys, the type of the component that segregates on the boundaries depends on the grain size. For segregation kinetics, an approximate analytical solution was obtained based on the assumption that the grain boundary is an ideal drain for the impurity. The equilibrium grain size exists below the critical temperature and its temperature dependence is nonmonotonic in the general case. A segregation mechanism of stabilization of nonequilibrium grain boundaries in nanocrystalline alloys obtained by severe plastic deformation was suggested.

Investigation of relaxation processes in nanocrystalline cobalt produced by severe plastic deformation

It has been shown that the dependence of the width of X-ray diffraction lines of plastically deformed cobalt on the annealing temperature is described by the exponential function. Characteristic temperature regions corresponding to the processes of recovery and recrystallization have been established. It has been shown that the values of the activation energy of recrystallization determined from the experimental data are comparable with the activation energy of the grain-boundary diffusion in metals. The activation energy for the recovery region is considerably lower than the activation energy of migration of nonequilibrium grain boundaries in nanocrystalline metals. The X-ray diffraction data have been confirmed by the investigations of the microstructure and microhardness.

Elementary model of severe plastic deformation by KoBo process

Self-consistent model of generation, interaction, and annihilation of point defects in the gradient of oscillating stresses is presented. This model describes the recently suggested method of severe plastic deformation by combination of pressure and oscillating rotations of the die along the billet axis (KoBo process). Model provides the existence of distinct zone of reduced viscosity with sharply increased concentration of point defects. This zone provides the high extrusion velocity. Presented model confirms that the Severe Plastic Deformation (SPD) in KoBo may be treated as non-equilibrium phase transition of abrupt drop of viscosity in rather well defined spatial zone. In this very zone, an intensive lateral rotational movement proceeds together with generation of point defects which in self-organized manner make rotation possible by the decrease of viscosity. The special properties of material under KoBo version of SPD can be described without using the concepts of nonequilibrium grain boundaries, ballistic jumps and amorphization. The model can be extended to include different SPD processes.

Corrosion behaviour of Al 1050 severely deformed by rotary swaging

In this study, corrosion behaviour of ultrafine-grained (UFG) commercial pure aluminium Al 1050 processed by rotary swaging (RS) was examined using potentiodynamic polarization and weight loss immersion test in 3.5% NaCl solution. Corrosion behaviour of UFG Al 1050 was compared with that of coarse grained (CG) as-received material. The results showed that ultrafine grain refinement by RS led to marked improvement of the corrosion resistance. The improvement in corrosion resistance is profited from the denser and stable passive film due to more grain boundaries, larger fraction of non-equilibrium grain boundaries and residual stress of the UFG pure aluminium. The weight loss tests revealed low corrosion rate values of RS material compared to CG as-received material. Scanning electron microscopy (SEM) analysis revealed a higher number of rectangular shallow pits (more close to patches of general dissolution); larger pits size was observed in the as-received compared to RS materials.

Non-equilibrium grain boundaries in titanium nanostructured by severe plastic deformation: Computational study of sources of material strengthening

A computational model of ultrafine grained (UFG) or nanostructured titanium (Ti), based on a finite element (FE) unit cell model of the material and a dislocation density based model of plastic deformation has been developed. FE simulations of tensile deformation of UFG Ti with different fractions and properties of the grain boundary (GB) phase have been carried out. The effect of different degrees of deviation from the equilibrium state of the grain boundaries (GBs) on the mechanical behaviour of nanostructured Ti have been investigated using the combined composite/dislocation dynamics based model. In particular, the effects of different diffusion coefficients in the GB phase, of a high initial dislocation density in the grain boundaries, as well as of atomic scale precipitates are investigated for affecting the deformation behaviour of UFG or nanostructured Ti.

Compressive deformation behavior of Mg–Zn–Ca alloy at elevated temperature

The compressive deformation behavior at elevated temperature of the as-extruded and as-ECAPed Mg–5.3Zn–0.6Ca (wt%) alloys was investigated in the current study. The flow stress in the as-ECAPed alloy was lower than that of the as-extruded alloy when the compressive temperature was above 200°C, due to the grain boundary softening effect. The strain rate sensitivity (SRS) was remarkably enhanced with the decrease of grain size or the increase of test temperature, and the corresponding m-value was calculated as 0.06–0.13, which indicated that the deformation was mainly dominated by the climb-controlled dislocation creep. Furthermore, the grain boundary sliding (GBS) mechanism might also be activated, which contributed to the higher SRS in the as-ECAPed alloy. The true deformation activation energy (Q') modified by threshold stress was calculated as 92–98 and 70–76kJ/mol in the as-extruded and as-ECAPed alloy, respectively, which was close to the activation energy for grain boundary diffusion, and the lower Q'-value in the as-ECAPed alloy might be derived from the non-equilibrium grain boundaries.