Dislocation-free Grains Research Articles

Excellent strength-ductility combination of interstitial non-equiatomic middle-entropy alloy subjected to cold rotary swaging and post-deformation annealing

The effect of cold rotary swaging and subsequent annealing on the microstructure, texture and mechanical properties of a Fe49.5Mn30Co10Cr10C0.5 middle-entropy alloy was studied. Finite element modeling predicted inhomogeneous stress distribution and temperature gradient during deformation. Microstructure analysis indicated the development of deformation-induced γ→ε martensitic transformation during earlier steps of cold rotary swaging (20–40 % reduction). However, a single-phase face-centered cubic structure was attained after 60–90 % reduction due to reverse ε→γ transformation. Meanwhile, a twin-matrix microstructure was observed in the bar center, whereas an ultrafine microstructure was formed at the bar edge. Post-deformation annealing at 600 °C caused the onset of static recrystallization and thereby the formation of ultrafine dislocation-free grains. Apparently, after 60–90 % reduction, a ⟨100⟩- and ⟨111⟩-fiber texture gradient along the bar cross-section was developed. Besides, at the bar edge, a pronounced B/B‾ shear texture was obtained that transformed into a Cube texture after annealing at 700 °C. Compared to the as-swaged material (yield strength (YS) = 1250 MPa; ultimate tensile strength (UTS) = 1520 MPa; elongation to failure (EF) = 6.5 %), annealing at 500 °C resulted in additional strengthening (YS = 1655 MPa; UTS = 1660 MPa), but EF did not change noticeably. Yet, annealing at 600°С was accompanied by an essential increase in both EF (to 11 %) and YS (to 1500 MPa). The applied approach can be used for obtaining a material with an attractive strength-ductility combination due to overcoming the strength-ductility trade-off.

The Microstructural Evolution and Corrosion Behavior of Zn-Mg Alloys and Hybrids Processed Using High-Pressure Torsion.

Zinc (Zn) alloys, particularly those incorporating magnesium (Mg), have been explored as potential bioabsorbable metals. However, there is a continued need to enhance the corrosion characteristics of Zn-Mg alloys to fulfill the requirements for biodegradable implants. This work involves a corrosion behavior comparison between severe-plastic-deformation (SPD) processed cast Zn-Mg alloys and their hybrid counterparts, having equivalent nominal compositions. The SPD processing technique used was high-pressure torsion (HPT), and the corrosion behavior was studied as a function of the number of turns (1, 5, 15) for the Zn-3Mg (wt.%) alloy and hybrid and as a function of composition (Mg contents of 3, 10, 30 wt.%) for the hybrid after 15 turns. The results indicated that HPT led to multimodal grain size distributions of ultrafine Mg-rich grains containing MgZn2 and Mg2Zn11 nanoscale intermetallics in a matrix of coarser dislocation-free Zn-rich grains. A greater number of turns resulted in greater corrosion resistance because of the formation of the intermetallic phases. The HPT hybrid was more corrosion resistant than its alloy counterpart because it tended to form the intermetallics more readily than the alloy due to the inhomogeneous conditions of the materials before the HPT processing as well as the non-equilibrium conditions imposed during the HPT processing. The HPT hybrids with greater Mg contents were less corrosion resistant because the addition of Mg led to less noble behavior.

Influence of annealing on recrystallization and mechanical properties of Zr702 alloy obtained by accumulative roll bonding

The influence of annealing time on the recrystallization evolution and mechanical behavior of Zr702 alloy obtained by accumulative roll bonding (ARB) has been investigated. Results show that the bonding degree of the ARB interface gradually improved with the increase of annealing time from 5 to 20 min, which is because the atoms can obtain relatively more diffusion driving force with the increase of annealing time. In addition, almost complete recrystallization (average size of about 3.98 μm) and excellent comprehensive mechanical properties can be obtained as the annealing time is set to 20 min, and the evolution path of the microstructure with annealing time can be described as: (I) high density stacking dislocations; (II) formation of subgrain boundaries; (III) reduction of dislocation density and formation of recrystallization; (IV) growth of recrystallized grains. In this study, with the increase of annealing time, the release of deformation storage energy and the increase of recrystallized dislocation-free grains are very helpful to improve the ductility.

Влияние равноканального углового прессования на зеренную структуру и внутренние напряжения технически чистого никеля

The structure, phase composition, defects, amplitude of internal stresses and their sources in ultrafine grained technically pure nickel, obtained by equal-channel angular pressing deformation, have been studied by the method of transmission electron microscopy. At equal-channel angular pressing the samples were subjected to shear deformation by compression along two intersecting channels of equal diameter at an angle of 120° at temperature T = 400 °C without intermediate annealing. Number of passes n = 4. The grain structure was studied. It was found that all grains were anisotropic. According to the dislocation structure, the grains were classified into three types: 1) the smallest grains with no substructure (practically no dislocations) - dislocation-free grains, 2) larger grains containing chaotically distributed dislocations or a net substructure, and 3) the largest grains with a cellular or fragmented substructure. The average value of the scalar dislocation density in the grains of each type was calculated. It was found that equal-channel angular pressing resulted in the formation of nanometer-sized particles of secondary phases in ultrafine grained nickel localized inside, on grain boundaries and at grain junctions. The sources of internal stresses are revealed and their amplitude is determined. Determination of the amplitude of internal stresses consisted in determining the amplitude of the curvature-torsion of the crystal lattice by bending extinction contours

Structure and Strength of Isothermally Heat-Treated Medium Carbon Ti-V Microalloyed Steel

Isothermal transformation characteristics of a medium carbon Ti-V microalloyed steel were investigated using light microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and by uniaxial compressive testing. Samples austenitized on 1100 °C were isothermally treated in the range from 350 to 600 °C and subsequently water quenched. The final microstructure of the samples held at 350 °C consisted of bainitic sheaves and had compressive yield strength, approximately from 1000 MPa, which is attributed to high dislocation density of low bainite. At 400 and 450 °C, acicular ferrite became prevalent in the microstructure. It was also formed by a displacive mechanism, but the dislocation density was lower, leading to a decrease of compressive yield strength to approximately 700 MPa. The microstructure after the heat treatment at 500 °C consisted of coarse non-polygonal ferrite grains separated by pearlite colonies, principally dislocation free grains, so that the compressive YS reached a minimum value of about 700 MPa. The microstructure of the samples heat-treated at 550 and 600 °C consisted of pearlite and both grain boundary and intragranular ferrite, alongside with some martensite. After 600 s, austenite became stable and transformed to martensite after water quenching. Therefore, the presence of martensite increased the compressive YS to approx. 800 MPa.

Influence of Boundary Migration Induced Softening on the Steady State of Discontinuous Dynamic Recrystallization.

During discontinuous dynamic recrystallization (DDRX), new dislocation-free grains progressively replace the initially strain-hardened grains. Furthermore, the grain boundary migration associated with dislocation elimination partially opposes strain hardening, thus adding up to dynamic recovery. This effect, referred to as boundary migration induced softening (BMIS) is generally not accounted for by DDRX models, in particular by “mean-field” approaches. In this paper, BMIS is first defined and then analyzed in detail. The basic equations of a grain scale DDRX model, involving the classical Yoshie–Laasraoui–Jonas equation for strain hardening and dynamic recovery and including BMIS are described. A steady state condition equation is then used to derive the average dislocation density and the average grain size. It is then possible to assess the respective influences of BMIS and dynamic recovery on the strain rate sensitivity, the apparent activation energy, and the relationship between flow stress and average grain size (“Derby exponent”) of the material during steady state DDRX. Finally, the possible influence of BMIS on the estimation of grain boundary mobility and nucleation rate from experimental data is addressed.

Gradient layer of ultrafine equiaxed grains produced by ultrasonic energy accelerated dynamic recrystallization

Single site indentation experiments with ultrasonic and quasi-static indentations were performed on aluminum alloy sheet to study the effect of induced ultrasonic energy on microstructure evolution during indentation. Electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) techniques were employed to investigate the dislocation structures and grain refinement mechanisms of the material underneath the indents. Process simulation about the indentation under ultrasonic irradiation via the ANSYS LS-Dyna finite element code was conducted to analyze the stress wave and spin rate in the material for further explanation of the mechanisms. Results showed that, after 10 s of ultrasonic indentation, a 300 μm deep gradient ultrafine-grained layer was produced underneath the ultrasonic indent, where the top surface exhibited an equiaxed ultrafine dislocation-free grains layer of 60 μm in thickness with grain diameter ranged 200–300 nm. The hardness of the gradient ultrafine equiaxed grain layer was increased substantially. It is supposed that the ultrasonic irradiation effectively enhanced the dislocation multiplication, movement and vibration, which accelerated sub-grain rotations and strongly facilitated the dynamic recrystallization.

Sensitivity analysis of a phase field model for static recrystallization of deformed microstructures

Static recrystallization is a process whereby dislocation-free grains are nucleated in a deformed microstructure and then newly recrystallized grains grow and consume the previously existing grains. This paper describes a phase field model for static recrystallization, along with details of the implementation and simulation results. Recrystallized grains are seeded utilizing a probability-based method, including a hold time to allow the order parameters to adjust to seeded grains. The nominal simulation time is corrected to account for the nuclei hold and for the time required for a nucleus to grow from its critical size to the seeded size. Microstructural evolution was simulated for two- and three-dimensional systems and the fraction recrystallized was quantified via Avrami kinetics. The resulting Avrami time exponents were in agreement with the expected values for site-saturated nucleation. The variability in the Avrami parameters was quantified by simulating the recrystallization of the same underlying polycrystalline microstructure but using different seed locations. Additional simulations were performed to determine the influence of the deformed microstructure on recrystallization, specifically investigating the effects of the spatial distribution of the initial dislocation density within the microstructure as well as the morphologies of the polycrystalline microstructure. For the significantly deformed polycrystalline microstructures examined in this work, it is shown that microstructural evolution is primarily driven by stored energy in dislocations rather than grain boundary energy.

High-pressure torsion processing of Zn–3Mg alloy and its hybrid counterpart: A comparative study

Abstract Processing of metal hybrid nanocomposites have emerged in the recent years by means of solid-state reactions through severe plastic deformation techniques, resulting in heterogenous microstructures and mechanical properties. Despite the increased scientific interest in these unusual materials, little is known regarding their comparative attributes with respect to a homogeneous material having equivalent nominal composition. This work provides a direct comparison of the microstructure and the hardness evolution between a Zn–3Mg (wt.%) alloy and its hybrid counterpart, after high-pressure torsion (HPT) and after HPT followed by a post-deformation annealing (PDA) treatment. Experimental results indicate that both the alloy and the hybrid reach a similar level of grain refinement after HPT processing, however, grain growth follows different trends after PDA. The HPT-processed alloy exhibits clusters of Mg2Zn11 nanocrystalline domains that coalesce into coarser grains maintaining a unimodal GS distribution after PDA. On the other hand, the hybrid after PDA displays a multimodal GS distribution consisting of ultrafine Mg-rich grains containing MgZn2 and Mg2Zn11 nanoscale intermetallics, in a matrix of coarser dislocation-free Zn grains. These observations were supported by kernel average misorientation (KAM) maps and pole figures obtained through electron backscattered diffraction (EBSD) analysis.

Microstructure and microtexture evolution of dynamic recrystallization during hot deformation of a nickel-based superalloy

Effect of strain rate route on the microstructure and microtexture evolution during the dynamic recrystallization (DRX) of a nickel-based superalloy subjected to the isothermal compression tests was investigated using electron back-scatter diffraction. The evolutions of microtexture components and fiber textures and the role of Σ3 boundaries in microstructure on the pole density in the partially and fully DRX processes were explored. The α-fiber in Euler space is regarded as the compression microtexture. The locally organized substructures in the matrix grains are formed on certain {111} slip planes with the high Schmid factor. Although the dislocation-free DRX grains show the random orientated distribution, the weak recrystallization 〈001〉 fiber parallel to the compression axis (ND) is developed at the high strain in the fully DRX processes. Whereas the compression 〈101〉 fiber parallel to ND in the partially DRX processes is sharpened. Moreover, the instantaneous decrease of strain rate leads to the highest fraction of Σ3 grain boundaries, indicating the lowest pole density. Cube-Twin component adjacent to Cube component contributes partly to the formation of Σ3 grain boundaries. The findings provide insights into the microtexture characteristics and enhance the understanding of the orientation dependence of the mechanical behavior of nickel-based superalloys.

Microstructure and strengthening model of Al-3%Mg alloy in a heat treated state subjected to ECAP process

Purpose: The aim of this paper is to investigate the microstructure evolution of a heat treated Al-3%Mg aluminium alloy subjected to the ECAP (equal channel angular pressing process). Design/methodology/approach: Commercial Al-3%Mg alloy subjected to the solution treatment followed by an artificial aging was subjected to the 6 ECAP passes using a processing route Bc. Then the microstructure investigations using a light microscopy, scanning electron microscopy and transmission electron microscopy were carried out. Additionally the XRD technique was used to calculate lattice micro strain and dislocation density. Findings: The experimental results showed that the obtained microstructure is refined by mutual interactions of shear and microshear bands. The TEM investigation revealed typical for deformed aluminium alloys structure constituents such as dislocation-free grains, nonequilibrium grain boundaries, dislocation cell and (sub)grain structures. Research limitations/implications: The presented investigation results were carried out on samples, not on final products. Practical implications: Current research is moving towards to develop high strength material having a ultra-fine grained microstructure and increased mechanical strength. Originality/value: The paper focuses on the microstructure characterization of ECAP processed Al-3%Mg aluminium alloy. The relationship between the obtained microstructure and its contribution to the Yield strength is investigated.

Asymmetric cryorolling of AA6061 Al alloy: Strain distribution, texture and age hardening behavior

Asymmetric rolling combines direct compression and shear strain imposed by working rolls with different diameters or different rotations speeds. Processing at cryogenic temperatures may partially suppress dynamic recovery mechanisms, modifying the work-hardening rate and increasing the amount of accumulated strain. In this work, samples of AA6061 Al alloy were processed by asymmetric and conventional rolling, at room and cryogenic temperatures. The effects of asymmetric cryorolling were analyzed in terms of strain distribution, grain refinement, hardness, tensile strength and crystallographic texture after post-deformation heat treatments. Artificial aging after conventional rolling at room temperature proved to be more effective in increasing hardness and tensile strength, while the same heat treatment after asymmetric rolling at cryogenic temperature increased uniform elongation and reduced texture intensity. This behavior was associated with recovery and recrystallization phenomena after aging, which cause small dislocation-free grains to be generated inside deformation bands. This was observed directly by TEM and indirectly by texture analysis.

Kinetic study of static recrystallization in an Fe–Al–O ultra-fine-grained nanocomposite

A nearly abrupt coarsening of grains is observed in a newly developed Fe–Al–O ultra-fine-grained nanocomposite with a significant volume fraction (4%) of alumina nano-precipitates. The microstructure of the alloy was analysed in different states (as-received and annealed) by means of scanning electron microscopy, transmission electron microscopy (TEM) and hardness. The initial grain size 150–200 nm increases up to 50 μm during annealing 1000 °C/8 h and thereafter demonstrates saturation. A linear correlation between volume fraction of coarse grains and hardness was found. It was identified by TEM that alumina nano-precipitates stabilize the dislocation microstructure against recovery very effectively and the grain coarsening is due to fast growth of very few dislocation free grains. Thus, the observed grain coarsening has the attributes of static recrystallization.

Simulating recrystallization in titanium using the phase field method

Integrated computational materials engineering (ICME) links physics-based models to predict performance of materials based on their processing history. The recrystallization phase field model is developed and parameterized for commercially pure titanium. Stored energy and nucleation of dislocation-free grains are added into a phase field grain-growth model. A two-dimensional simulation of recrystallization in titanium at 800°C was performed; the recrystallized volume fraction was measured from the simulated microstructures. Fitting the recrystallized volume fraction to the Avramiequation gives the time exponent n as 1.8 and the annealing time to reach 50% recrystallization (t0.5) as 71 s. As expected, the microstructure evolves faster when driven by stored energy than when driven by grain boundary energy.

Texture evolution during hot deformation of Moly-TZM

Moly-TZM was deformed at constant strain rate of 1.0 s−1 to investigate the high strain rate deformation behaviour by micro structural and stress response change within a temperature range of 1400-1700 °C. To correlate the deformation behaviour with orientational change, recrystallization and recovery of the material, the microstructural investigation was undertaken using scanning electron microscopy (SEM) of electron back scattered diffraction (EBSD). Depending on the grain size and orientation spread recrystallized grains were identified and texture was calculated. Change in grain boundary characteristics with increasing temperature was determined by the misorientation angle distribution for the deformed and recrystallized grains. Subgrain coalescence and increase in subgrain size with increasing temperature was observed, indicating recrystallization not only occurred from the nucleation of the dislocation free grains in grain boundaries but also from the subgrain rotation and merging of the subgrains by annihilation of the low angle grain boundaries. Detailed studies on the evolution of texture of recrystallized grains showed continuous increase in <001> fiber texture in recrystallised grains, in contrast to a mixed fiber <001> +<111> for the deformed grains.

Dynamic stored energy gradient induced oriented growth of subgrains in rapidly hot-extruded tungsten heavy alloys

Highly oriented lath-like subgrains decorated by numerous dislocation tangles and dislocation-free irregular grains were observed to prevail in a heavily hot-extruded fine-grained 93W-4.9Ni-2.1Fe (wt.%) alloy. Except for such lath-like subgrains, there were also a considerable number of equiaxed strain-free grains visible in an extruded coarse-grained 93W-4.9Ni-2.1Fe alloy. A mechanism in terms of a new concept designated as dynamic stored energy gradient was proposed to illustrate the formation of the lath-like subgrains that were actually created in extremely short time (only around 0.3s) during hot extrusion. Due to the limited extrusion time, thermally activated diffusion of tungsten atoms was unlikely to play a decisive role in this process. It is found that there has been a significant driving force gradient at the growth front of the subgrains, which is believed to substantially promote directional annihilation of edge dislocation dipoles to develop the lath-shaped configuration of the subgrains in the extruded alloys.

Tailoring Dynamic Recrystallization in Ni- and Fe-Base High Temperature Alloys

Theory for describing the conditions leading to dynamic recrystallization in FCC metals is introduced. The approach also describes stress-strain curves when this process occurs, and is unique in incorporating the effects of strain rate and temperature employing only physical parameters as input. The novelty of the approach stems from incorporating an incubation period in the equations describing the progress of dislocation density with strain; beyond such incubation dislocation free grains form. The energy barrier to ignite grain growth is expressed as a function of the strain energy stored in the material and a statistical entropy contribution due to the degrees of freedom available to a dislocation for annihilation. The incubation strain is obtained by performing an energy balance between the stored energy on the subgrain boundaries, the slip energy for boundary migration and the interfacial energy required for grain nucleation. The application of this work to Fe and Ni multicomponent alloys has lead to transition maps in temperature-strain rate space indicating the conditions for dynamic recrystallization occurrence.

Modelling Dynamic Recrystallization in FCC Metals Employing Thermostatistics

Theory for describing the conditions leading to dynamic recrystallization in FCC metals is introduced. The approach also describes stress-strain curves when this process occurs, and is unique in incorporating the effects of strain rate and temperature employing only physical parameters as input. The novelty of the approach stems from incorporating an incubation period in the equations describing the progress of dislocation density with strain; beyond such incubation dislocation free grains form. The energy barrier to ignite grain growth is expressed as a function of the strain energy stored on the material and a statistical entropy contribution due to the degrees of freedom available to a dislocation for annihilation. The incubation strain is obtained by performing an energy balance between the stored energy on the subgrain boundaries, the slip energy of boundary migration and the interfacial energy required for grain nucleation. The application of this work to pure Cu and Ni has lead to transition maps in temperature-strain rate space indicating the conditions for dynamic recrystallization occurrence.

Effect of Substructure Characteristics on the Dislocation Annihilation Process during Post-Deformation Annealing

Two distinct substructures were produced in a Ni-30Fe austenitic model alloy by different thermomechanical processing routes. The first substructure largely displayed organized, banded subgrain arrangements with alternating misorientations, resulting from the deformation at a strain just before the initiation of dynamic recrystallization (DRX). By contrast, the second substructure was more random in character and exhibited complex subgrain/cell arrangements characterized by local accumulation of misorientations, formed through DRX. During the post-deformation annealing, the latter substructure revealed a rapid disintegration of dislocation boundaries leading to the formation of dislocation-free grains within a short holding time, though the former largely preserved its characteristics till becoming replaced by growing statically recrystallized grains.

Plastic deformation experiments to high strain on mantle transition zone minerals wadsleyite and ringwoodite in the rotational Drickamer apparatus

We report the results of plastic deformation experiments on polycrystalline wadsleyite and ringwoodite performed at 15–23GPa and 1300–2100K conducted using the rotational Drickamer apparatus (RDA). Wadsleyite was synthesized from fine-grained (∼2μm) San Carlos olivine in a Kawai-type multianvil apparatus; the average grain size of the resulting wadsleyite was 1.2μm. The initial water content of the undeformed wadsleyite was 24,000–26,000H/106 Si but the final water content is variable and less than the initial water content. Ringwoodite was synthesized from wadsleyite in situ in the RDA. Both strain and stress were measured in situ using a synchrotron x-ray facility. Determinations of strains and strain rates were made from x-ray radiographs of the sample, using a Mo foil strain marker in the sample assembly. The state of stress was determined from the observed d-spacing of multiple lattice planes as a function of azimuth angle. Samples were deformed at various strain rates at around 10−4–10−5s−1. Deformation mechanisms were inferred from the stress exponent and the microstructures. We determined the stress exponent n for wadsleyite to be 6±3, suggesting dislocation creep was the dominant deformation mechanism in wadsleyite. At grain sizes of ∼1μm, our samples were still deforming primarily by dislocation creep. However, small dislocation-free grains are also observed suggesting that diffusion creep may operate in some parts of our samples.