Strain-free Grains Research Articles

Delineating the role of dynamic and static recrystallization during hot working of nickel-based superalloy (IN600)

In the presented research, the dynamic working of nickel-based (IN600) superalloy was performed and the micro-mechanisms affecting the deformation behaviour were closely monitored, particularly the role of static and dynamic recrystallization. Hot deformation flow stress response indicated an increase in the true stress value with an increase in strain rate and decrease in temperature. The constitutive calculations indicated that deformation progressed mainly due to dislocation climb motion. The hot workability map indicated a peak power dissipation efficiency (PDE) of 0.39 in two working domains. The microstructural evolution in the hot deformed specimens depicted a consistent increase in dynamic softening at temperature ≥ 1000 °C, whereas, a steady decline in the extent of dynamic softening is observed with increase in strain rate from 0.01 s−1 to 1 s−1. The mechanism for microstructural evolution in this range (0.01–1 s−1) was identified as discontinuous dynamic recrystallization (DDRX). Moreover, at the highest strain rate of 10 s−1, a sudden surge in the recrystallization fraction was observed, which was attributed to the combined effect of DDRX and static recrystallization (SRX). Additionally, this study shows that the presence of twin boundaries (TBs) contributes to dynamic softening by serving as sites for nucleation of strain-free grains.

Microstructural studies of CuCrFeNi2Mn0.5 high entropy alloy during cold rolling

A study was conducted to determine the impact of cold rolling on the evolution of the microstructure of the CuCrFeNi2Mn0.5 high entropy alloy. Cold rolling resulted in the development of crystallographic textures and microstructure evolution. It was observed that the main deformation mechanism during cold rolling of the mentioned alloy is the movement of dislocations and the formation of mechanical twins, which lead to the formation of shear bands by increasing the amount of deformation. Microstructural studies suggest that the decrease in the microhardness of cold-rolled samples is caused by the formation of strain-free grains on the shear bands. The development of strain-free grains is aided by the rotation of subgrains on the shear bands.

Mechanism of dynamic recrystallization of a FeCrAl alloy during hot compression

FeCrAl alloys have the opportunity to replace current Zr-based cladding materials in light water reactors, and dynamic recrystallization (DRX) plays an important role on microstructural controlling of FeCrAl alloys during hot or warm deformation, such as forging, hot rolling, extrusion. In this research, dynamic recrystallization mechanism of FeCrAl alloy has been systematically investigated by microstructure characterization after isothermal compression. Results show that deformation strain rate and temperature could significantly influence microstructure and DRX mechanism of FeCrAl alloy. When deformed at 0.1s−1, prior grains become long strip with serrated boundaries and are refined by geometric dynamic recrystallization (GDRX). Prior grains are not separated by high angle grain boundaries (HAGB) even deformed at 1200 °C, because strain is too small to impingement of HAGB. Whereas, when deformed at 10s−1 strain rate, there are lots of sub-boundaries with about 2° misorientation within grains, and several strain-free grains are found located at boundaries when deformed at 900 °C or 1000 °C, while the prior grains are divided by HAGB and equiaxed grains are formed at 1100 °C. It was found that mechanism of DRX changes from discontinuous dynamic recrystallization (DDRX) to continuous dynamic recrystallization (CDRX) with increasing temperature from 900 °C to 1100 °C. The phenomenon results from the dislocation movement behaviors changes with increasing temperature. This research will provide meaningful guidance for industrial application of FeCrAl alloys.

Thermal annealing of DC sputtered Nb3Sn and V3Si thin films for superconducting radio-frequency cavities

Nb 3 Sn and V3Si thin films are promising candidates for the next generation of superconducting radio-frequency (SRF) cavities. However, sputtered films often suffer from stoichiometry and strain issues. This exploratory study investigates the structural and chemical effects of thermal annealing, both in−situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si films with varying thicknesses, deposited on Nb or Cu substrates. Building upon our initial studies [Howard et al., Proceedings of the SRF’21, East Lansing, MI (JACoW, 2021), p. 82.], we provide fundamental insights into recrystallization, phase changes, and the issues of stoichiometry and strain. Through annealing at 950 °C, we have successfully enabled the recrystallization of 100 nm thin Nb3Sn films on Nb substrates, yielding stoichiometric and strain-free grains. For 2 μm thick films, elevated annealing temperatures led to the removal of internal strain and a slight increase in grain size. Moreover, annealing enabled a phase transformation from an unstable to a stable structure in V3Si films, while we observed significant Sn loss in 2 μm thick Nb3Sn films after high-temperature annealing. Similarly, annealing films atop Cu substrates resulted in notable Sn and Si loss due to the generation of Cu–Sn and Cu–Si phases, followed by evaporation. These results encourage us to refine our process to obtain high-quality sputtered films for SRF use.

Quartz cleavage fracturing and subsequent recrystallization along the damage zone recording fast stress unloading

Strings of recrystallized grains along cleavage planes in host quartz crystals within pseudotachylyte-bearing breccias from the Silvretta basal thrust, Central European Alps, and within shocked gneisses from the Vredefort impact structure, South Africa, are compared and contrasted. The aim is to obtain the characteristic deformation and stress history during microfabric evolution. Strings of recrystallized grains occur in sets parallel to r- and z-rhombohedral planes of the host quartz in both localities and along basal planes in Vredefort gneisses. In Silvretta fault rocks, they exclusively occur in quartz clasts within tensional domains associated with the propagating pseudotachylyte-related fault tip, indicating that cleavage occurred simultaneously with pseudotachylyte generation. Cleavage of quartz in Vredefort gneisses is related to shock during impact. Quartz cleavage fracturing along planes of minimum free surface energy is suggested to require fast unloading from high transient stresses, as realized in both geological settings: unloading from >400 MPa within minutes during faulting and from <20 GPa within milliseconds during impact. An additional influence of thermal shock caused by frictional heating is likely. Strain-free grains grew in situ along the damage zone surrounding the cleavage fractures at quasi-isostatic stress conditions after deformation and temperatures to allow for static recrystallization of quartz.

The Casting and Hot Forging of Low-Carbon Copper-Bearing Steel and Its Substructural Characterization

The casting of metal alloys followed by hot forging is a widely used manufacturing technology to produce a homogeneous microstructure. The combination of mechanical and thermal energy envisages the microstructural properties of metal alloys. In the present investigation, a metal alloy of composition 0.05C-1.52Cu-1.51Mn (in weight %) was cast in an induction furnace using a zirconia crucible. The melt pool was monitored using optical emission spectroscopy (OES) to maintain the desired composition. The as-cast block was then subjected to forging under a pneumatic hammer of 0.5 t capacity so that any casting defects were eliminated. The as-cast block was reheated to a temperature of 1050 °C and held at that temperature for 6 h to homogenize, followed by hammering with a 50% strain using a pneumatic hammer. The microhardness was calculated using a Vickers microhardness testing apparatus. The microstructure characterization of the processed alloy was carried out using an optical microscope, electron backscattered diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDXA), and a transmission electron microscope (TEM). The sample for optical microscopy was cut using a diamond cutter grinding machine and surface polishing was carried out using emery paper. Further, mechanical polishing was performed to prepare the samples for EBSD using a TEGRAPOL polishing machine. The EBSD apparatus was operated at a 20 kV accelerating voltage, 25 mm from the gun, and with a 60 µ aperture size. HKL Technology Channel 5 Software was used for the post-processing of EBSD maps. The procedure of standard polishing for OES and TEM sample preparation was followed. Recrystallization envisages equiaxed grain formation in hot forging; hence, the strain-free grains were observed in the strained matrix. The lower distribution of recrystallized grains indicated that the driving force for recrystallization was not abundant enough to generate a fully recrystallized microstructure. The fractional distribution of the misorientation angle between 15 and 60° confirms the formation of grain boundaries (having a misorientation angle greater than 15°) and dislocations/subgrain/substructures (having a misorientation angle less than 15°). The fraction of misorientation angle distribution was higher between the angles 0.5 and 6.5°; afterwards, it decreased for higher angles. The substructure was observed in the vicinity of grain boundaries. The softening process released certain strains, but still, the dislocation was observed to be deposited mostly in the vicinity of grain boundaries and at the grain interior. The fine precipitates of the microalloying element copper were observed in the range of size in nanometers. However, the densities of these precipitates were limited and most of these precipitates were deposited at the grain interior. The microhardness of 210.8 Hv and mean subgrain size of 1.61 µ were observed the enhanced microhardness was due to the limited recrystallized grains and accumulation of dislocations/subgrain/substructures.

Development of creep resistant high yttria 18Cr ferritic ODS steel through hot powder forging route

The present work reports the creep behaviour of a high yttria Fe-18Cr-2W-0.57Ti-1Y2O3 (all compositions in wt.%) ferritic oxide dispersion strengthened (ODS) steel produced by powder forging. The creep tests were carried out in a temperature range of 873 K to1023K and a stress range of 150 MPa to 425 MPa. Creep data is analysed using the power law creep equation. A large value of stress exponents (n ∼ 10 at 873 K, ∼ 18 at 923 K and 8.5 at 973 K) and a high activation energy (Q ∼581 kJ/mol) are obtained. These high values are attributed to threshold stress (σth) generated due to attractive interaction between dislocation and oxide particles. Threshold stress for different creep conditions was determined. True stress exponent values of 4 to 5 were obtained after accounting for the threshold stress. Electron back scattered diffraction (EBSD) revealed an equiaxed strain free grain structure in the powder forged ODS steel. Transmission electron microscopy (TEM) of creep tested samples indicates that microstructure remains stable after creep with no significant coarsening of nano size oxide particles. Dislocation pinning by the oxide particles was observed in creep tested samples. The observed values of true stress exponent suggest dislocation climb over the nano oxide particles to be the creep mechanism operating in the ODS steel. Samples sections close to creep fracture surfaces were investigated by EBSD. Void formation at triple junctions followed by crack propagation normal to the loading direction was observed. All the samples exhibited mixed mode (cleavage + ductile dimple) failure. Conventional ODS steel compositions contain up to 0.35% yttria. A significant improvement in creep life as well as an increase in activation energy for creep was observed because of high (1%) yttria content used in the ODS steel studied.

Role of Grain Size and Recrystallization Texture in the Corrosion Behavior of Pure Iron in Acidic Medium

This work investigates the role of grain size and recrystallization texture in the corrosion behavior of pure iron in 0.1 M sulfuric acid solution. Annealing heat treatment was applied to obtain samples with different average grain sizes (26, 53 and 87 µm). Optical microscopy, X-ray diffraction and electron backscatter diffraction techniques were used to characterize the microstructure. The EBSD data analysis showed ferrite phase with no inclusions and very low geometrically necessary dislocation density, indicating strain-free grains constituting all samples. The crystallographic texture analysis of the samples revealed that the 26 µm grain size sample had a high volume fraction of {111} oriented grains parallel to the sample surface, while other samples exhibited nearly random crystallographic texture. The electrochemical results from potentiodynamic polarization and electrochemical impedance spectroscopy showed a decrease in corrosion resistance from 87 µm to 53 µm grain size sample and then an increase for the 26 µm grain size sample. This increase was attributed to the dominant effect of recrystallization texture on the corrosion behavior of the sample. The cathodic hydrogen evolution reaction kinetics was found to play a decisive role in the corrosion behavior of iron.

Enhanced thermo-mechanical processing through stress-induced martensitic transformation and its effect on grain refinement in metastable β titanium alloys

Stress-induced transformations in retained metastable phase are known deformation mechanisms that provide additional plasticity in several alloy systems. In the present study on metastable β Ti-10V-2Fe-3Al (Ti-1023) and Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloys, stress-induced martensitic transformation (SIMT) has been effectively demonstrated to result in achieving a noticeable cold reduction of ∼43% and ∼45%, respectively. A significant fraction of stress-induced α″ martensites provide preferential nucleation sites for new strain-free grains formation upon recrystallization annealing. The thermomechanical treatment resulted in grain refinement by ∼93% and ∼80%, respectively, for Ti-1023 and Ti-5553 alloys. Electron Probe Micro Analyzer (EPMA) confirms the role of alloying element (such as Fe, Mo, V, Cr and Al) diffusivities resulting in altering the stability of the β phase as a function of recrystallization annealing time. Further, the effect of grain size on the SIMT ability based on microhardness variations has been deduced.

On the critical strain of thermal-mechanical processing to tailor grain boundary character distribution in INCOLOY 925 alloy

Through a series of thermal-mechanical processing (TMP) routes with various ratios of cold rolling and subsequent high-temperature annealing, the samples with different critical strain states were prepared to explore the evolution of grain boundary character distribution (GBCD) in INCOLOY 925 during grain boundary engineering (GBE). The GBE microstructure was characterized using the electron backscatter diffraction to study the influence of TMP parameters on the evolution of GBCD and the penetration of random high angle grain boundaries. The results show that the different states of pre-strain correspond to the variational formation mechanism of grain boundary network (GBN). The strain induced boundary migration is the predominant mechanism with the rolling reduction lower than critical strain. With the slight increase of strain, the recrystallization nucleation can be initiated to generate the strain-free grains at a certain annealing temperature. Moderate strain is needed to ensure adequate growth for twin related domains (TRDs). Besides, with the increase of strain and temperature, the dominant mechanism of GBCD gradually transfers from the new twinning mechanism to the Σ3 regeneration mechanism. Specifically, the TMP sample with 10% rolling reduction followed by annealing at 1075 °C for 10min shows relatively higher grain size difference ratio and average grains numbers in the TRDs, indicating that a larger initial grain size can increase critical strain and delay the nucleation of strain-free grains during TMP treatment. The moderately higher grain size shows an opposite effect on the formation of GBN. Finally, an estimation model of critical strain, as a function of initial grain size and annealing temperature, was constructed based on the artificial neural network, and the predicted results show a high consistency with the actual microstructure evolution.

Effect of thermomechanical processing via rotary swaging on grain boundary character distribution and intergranular corrosion in 304 austenitic stainless steel

In the present work, the thermomechanical processing consisting of rotary swaging deformation and annealing treatment was performed to optimize the grain boundary character distribution (GBCD) of 304 austenitic stainless steel. To systematically investigate the effect of GBCD evolution on intergranular corrosion (IGC), double loop electrochemical potentiokinetic reactivation and electrolytic oxalic acid etch tests were employed. The experimental results show that the fraction of low-Σ coincident site lattice (CSL) boundaries increased from 58.1% to 74.0% for the specimen swaged to 0.06 true strain and then annealed at 1050 °C for 5 min duration. By characterizing the evolution of GBCD as a function of strain level in terms of low-Σ CSL boundaries fraction, average twin-related domain (TRD) size, average number of grain per TRD and fractal dimension of the maximum random boundary connectivity, the grain boundary engineering (GBE) microstructure was realized by the occurrence of prolific multiple twinning events during strain-induced boundary migration while static recrystallization has a detrimental effect on optimizing GBCD for the prolific of new strain-free grains with random boundaries. The IGC resistance of the GBE-treated 304 austenitic stainless steel is enhanced by inhibiting the nucleation and propagation of IGC cracks, resulting from the increase in the fraction of low-Σ CSL boundaries, especially Σ3 boundaries and the disruption of random boundary network connectivity.

Microstructural engineering of medium entropy NiCo(CrAl) alloy for enhanced room and high-temperature mechanical properties

This work demonstrates the development of a strong and ductile medium entropy alloy by employing conventional alloying and thermomechanical processing to induce partial recrystallization (PR) and precipitation strengthening in the microstructure. First, the cold working of homogenized alloy resulted in a highly deformed microstructure. On annealing at 700 °C, B2 ordered precipitates heterogeneously nucleate on the highly misoriented sites. These B2 promotes particle stimulated nucleation (PSN) of new recrystallized strain-free grains. The migration of recrystallized grain boundaries leads to discontinuous precipitation of L12 ordered regions in highly dense lamellae structures. We also observe near-spherical L12 precipitates in the non-recrystallized grains. Atomic-scale compositional analysis reveals a significant amount of Ni confined to the Phase Boundary (PB) regions between B2 and L12 precipitates, indicating Ni as a rate-controlling element for coarsening the microstructure. On 20 h of annealing, the alloy comprises a composite microstructure of recrystallized and non-recrystallized grains, B2 particles at the grain boundaries (GBs), and coherent L12 precipitates inside the grains. The microstructure results in a 0.2% yield stress (YS) value of ∼1060 MPa with ∼25% elongation at ambient temperature and retains up to ∼910 MPa at 670 °C. Examination of deformed microstructure reveals excessive twinning, formation of stacking faults, shearing of L12 precipitates, and accumulation of dislocations at around the B2 precipitates and GBs attributed to high strain hardening of the alloy.

Effect of partial recrystallization on microstructure and tensile properties of NiFeCoCrMn high-entropy alloy

To obtain a balance between strength and ductility in NiFeCoCrMn high-entropy alloy, the degree of dislocation strengthening was tuned via partial recrystallization during traditional thermomechanical processing (cold rolling and recrystallization). The tensile properties in each state were then examined. Significant improvements in uniform elongation and work hardening rate, with decrease in yield strength and ultimate tensile strength, are associated with increase in the recrystallized fraction, i.e., reductions in the degree of strain hardening. In particular, recrystallized fractions of 37% and 74% are obtained by annealing at 650 °C for 10 min and 15 min, respectively, which results in yield strengths of 1003 MPa and 742 MPa and uniform elongations of 4% and 24%, respectively. The strengthening is due to the unrecrystallized grains with a high density of dislocations, whereas the ductility benefits from the presence of recrystallized strain-free grains.

A comprehensive study on solidification (hot) cracking in austenitic stainless steel welds from a microstructural approach

The present work evaluates solidification cracks in stainless steel welds that have been produced on a laboratory scale by carrying out a detailed EBSD (electron backscattered diffraction) investigation. The welding speed presented a near-linear relationship with the susceptibility to cracking. A wide range of microstructures were generated by varying the welding speed. The cracks were predominantly transgranular type and mainly propagated through grains with lower Taylor factors. These cracks were accompanied by a strong presence of a localized strain field (high local misorientation), which was observed by a higher presence of low angle grain boundaries (LAGBs). A correlation was found between the local in-grain misorientation and the crack sensitivity. With increased thermal tensile strains, certain LAGBs transformed into HAGBs to accommodate the strain and that resulted in the formation of strain-free grains. The elastic stored energy estimated from EBSD also showed an increase with the local misorientation.

Effects of annealing on microstructures and properties of cold-rolled GH3536 sheet

In this study, the effects of annealing on microstructures and the properties of GH3536 superalloy sheets were studied in detail by means of Electron Backscatter Diffraction (EBSD), Scanning Election Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results show that a series of phenomena happened after annealing at 950 °C. The recrystallization occurs when the alloy is annealed at 950 °C, and new strain-free grains replace the strained grains. There is a significant reduction in grain size, low-angle grain boundaries and dislocation density. Meanwhile, a mass of M 6 C carbides precipitate along grain boundaries after annealing at 950 °C. And a large number of annealing twins are also formed during this process. The reduction of dislocation density due to the recrystallization of the alloy leads to a noteworthy decrease in strength and an obvious increase in plasticity, which means excellent reprocessing ability are obtained after recrystallization. • The recrystallization temperature of GH3536 superalloy is obtained. • Amounts of annealing twins are formed in the stage of recrystallization. • Increased recrystallized fraction was linked to excellent reprocessing ability. • Higher ductility and lower strength are developed through annealing treatment.

Effects of build orientation and heat treatment on microstructure, mechanical and corrosion properties of Al6061 aluminium parts built by cold spray additive manufacturing process

Aluminium 6061 alloy (Al6061) is one of the widely used materials in the aluminium alloy family for structural applications owing to its high strength-to-weight ratio and high corrosion resistance. Al6061 components are traditionally manufactured using conventional processes such as casting, forging, rolling, and machining, which typically involved high scrap/waste generation. However, with the advent of advanced additive manufacturing (AM) processes, there is an opportunity to reconsider component design to near-net shapes with reduced material wastage. Although mechanical and corrosion properties of Al6061 fabricated with conventional processes is well understood, very little information is known about AMed Al6061. This study investigates the effect of build orientation and heat treatment on the mechanical and corrosion properties of Al6061 samples manufactured by cold spray additive manufacturing (CSAM) process. To the best of the authors’ knowledge, this work is the first to explore both the mechanical and corrosion properties of Al6061 alloy print-parts fabricated using this unique CSAM system. Due to the lack of metallurgical bonding between powder particles, as-deposited samples - regardless of their build orientation, exhibited lower strength and ductility compared to the wrought counterparts. Particle shape generated by cold spray process exhibits an influence on the strength of horizontally and vertically build deposits. Expectedly, heat treatment significantly improved the mechanical properties by inducing metallurgical bonding at prior particle boundaries. As for corrosion resistance, horizontally-built samples exhibited the highest pit depth and pit penetration rate compared to the other conditions, which suggested a higher susceptibility to pitting corrosion owing to more strain-hardened particle sites available for corrosive attack. Heat treatment on as-deposited samples produced strain-free grains; thus, eliminating areas prone to corrosive environmental attack, leading to improvement in pitting corrosion resistance. Investigating post-processing heat treatment is crucial to improve the mechanical properties of as-deposited samples to a level similar to the ones of wrought condition. This research work is essential for selecting the correct processing routes and their parameters to manufacture components exposed to corrosive environments.

Heterogeneous microstructure and deformation behavior of an automotive grade aluminum alloy

Aluminum alloy is today considered as a prime selection for manufacturing lightweight structural components in the automotive industry to increase fuel efficiency and reduce harmful emissions. The aim of this study was to identify the effect of microstructure and strain rate on the tensile deformation behavior of a high-pressure die-cast Silafont®-36 alloy, with special attention to strain hardening behavior and deformation mechanisms. The alloy consisted of randomly oriented primary α-Al phase and Al–Si eutectic structure in a form of heterogeneous microstructures, with Sr-modified Si particles exhibiting a coral-like fibrous network. The local misorientations in most primary α-Al grains was below 1°, suggesting strain-free grains along with some extent of near-boundary residual strains due to the thermal mismatch between aluminum and silicon. A superior strength-ductility combination was achieved, along with enhanced Young’s modulus and quality index owing to the unique heterogeneous microstructures. The cast alloy exhibited a smooth deformation characteristic with good coordination deformation and strong strain hardening capacity. Strain hardening exponents evaluated via the equations proposed by Ludwik, Hollomon, Swift, and Afrin et al., respectively, showed basically the absence of strain-rate effect from 1 × 10−5 to 1 × 10−2 s−1. During the tensile deformation, crack initiated from the sample surface and propagated through the alternate microconstituents of softer primary α-Al phase and harder eutectic structure.

Preferential grain growth and textural evolution of AZ31B Mg alloy during annealing after isothermal compression at 400 °C

Optimizing post-deformation cooling conditions can to a great extent control grain size and texture in the hot forming components, especially larger-size ones. Here, we investigated the effects of pre-strains and 400 °C isothermal annealing on the microstructural evolution of AZ31B magnesium alloy after isothermal compression. For the pre-strain exceeding ~6%, grains coarsen immediately without the need of an incubation period. The rapid coarsening mainly happens for a group of strain-free grains with grain orientation spread value lower than 0.5°. Two kinds of strain-free grains in the pre-strained microstructure are mainly responsible for the rapid increase in grain size. One is the undeformed grains at the pre-strain of 6%. The other is the dynamic recrystallized grains when the pre-strain is higher than 10%. Driving force for the preferential grain growth is ascribed to the difference in stored strain energy between deformed and strain-free grains. The final grain size after 900 s annealing decreases with the increase in pre-strain level. As both c-axes of undeformed grains and dynamic recrystallized grains are nearly parallel to the compression direction, the preferential growth of these grains results in an enhanced basal texture after annealing. These results demonstrate that larger deformation extent and instant cooling can effectively refine grains and weaken the texture of magnesium alloy during hot forming.

Assessing the potential of sparsely nucleated recrystallized grains to lead grain boundary engineering during extending annealing in Alloy 600H

Alloy 600H is subjected to a lower extent of deformation (7.5% and 10%) followed by annealing at a higher temperature (1273 K and 1373 K) for different time duration (5 min to 600 min) to explore whether sparsely nucleated recrystallized grains can promote grain boundary engineering (GBE). Since strain-induced boundary migration (SIBM) may also occur in the aforementioned deformation-annealing conditions, this study is expected to explicate the relative role of recrystallization vs. SIBM on GBE. It is observed that the sparsely nucleated grains formed during annealing (60 min at 1273 K) of the specimen subjected to critical strain (i.e., 10%) for recrystallization does not lead to a GBE microstructure following extended annealing treatment even up to 600 min. This is due to the dominant nucleation of new strain-free grains during the extended annealing treatment, rather than the growth of initially nucleated recrystallized grains. As a result, the product microstructure post extended annealing treatment yields lower fraction of Σ3n boundaries and lesser numbers of grains per twin-related domains (TRDs) with concomitant increase in random high angle boundaries (HAGBs) connectivity, as corroborated by its higher fractal dimension (Df) and lower J2/1-J3 value. In contrast, significant SIBM has happened in the specimens subjected to 7.5% deformation (i.e., lower than the critical strain for recrystallization) when annealed at 1373 K. During such SIBM, the existing TRDs in the as-received specimen have grown allowing the occurrence of multiple twinning behind the migrating grain boundaries. The evolution of a higher fraction of Σ3n boundaries, promoted via SIBM, and complete suppression of recrystallization in these specimens lead to the significant disruption in random HAGBs connectivity as substantiated by their lower Df and a higher J2/1-J3 value.

Micro-mechanism of evolution of microstructure and texture in Ni-Fe alloys

A comprehensive investigation was carried out to explore the micro-mechanisms associated with the evolution of deformation microstructure and texture in Ni-Fe alloys during rolling. X-ray diffraction, electron back-scatter diffraction (EBSD) and Mössbauer spectroscopy as well as simulation methods involving crystal plasticity and molecular dynamics simulations were used to explore the mechanism of evolution. Pure Ni, Ni20wt.%Fe and Ni40wt.%Fe were rolled to true strain 3.0, following two different strain paths. Unidirectionally rolled Ni20wt.%Fe and Ni40wt.%Fe alloys show copper type texture, similar to that of pure Ni. By contrast, the cross rolled samples show the development of α-fibre along with A {110}<111> and cube {100}<001> orientations. Both Ni20Fe and Ni40Fe exhibit extensive shear banding in UDR as well as CR conditions, unlike pure Ni, which has been attributed to the appearance of two short-range ordered phases, Ni3Fe and NiFe. In case of UDR, the nature of shear banding is different in Ni20Fe and Ni40Fe alloys, while after CR both the alloys show similar microstructures displaying higher fraction of shear banding compared to UDR. Simulation results have indicated that larger number of slip systems are activated in case of CR compared to the UDR samples that leads to the formation of a weaker texture in the former. Some strain free grains have been observed near the shear banded regions, indicating that an extended recovery mechanism could be operative in Ni20Fe and Ni40Fe. This phenomenon was more pronounced when the fraction of shear bands was more.