Search

The effect of inter-lamellar (IL) spacing on the microstructure and texture of severely cold-rolled and annealed pearlite was studied. For this purpose, the behavior of fine IL spacing starting material (FILSM) (~ 190 nm) was compared with a coarser IL spacing starting material (CILSM) (~ 275 nm). The materials were severely cold-rolled to 95 pct reduction in thickness and annealed at 973 K (700 °C) for different time intervals. Despite similarities in microstructural evolution featured by progressive alignment of the pearlitic colonies along the rolling direction (RD), bending and kink morphologies, fragmentation of cementite, and deformation-induced nanoscale structure, the FILSM showed a smaller nanoscale IL spacing (~ 45 nm) than CILSM (~ 60 nm) after severe cold-rolling. Both FILSM and CILSM showed similar deformation texture featured by the presence of typical ND (// [removed]) and RD (// [removed]) fibers. Annealing of the severely cold-rolled materials resulted in an ultrafine microduplex structure. However, annealed FILSM consistently showed lower ferrite and spheroidized cementite sizes than those in the annealed CILSM. Irrespective of the starting IL spacing, continuous recrystallization was found to be the softening mechanism, which was amply corroborated by the texture evolution during annealing. Therefore, although the starting IL spacing considerably affected the microstructure, the texture evolution remained largely unaffected. The tensile behavior of the microduplex structured FILSM was further analyzed considering Hall–Petch (H–P) and Orowan–Ashby (O–A) strengthening mechanisms.

The effect of moderate cryo-rolling (∼65 % reduction) and annealing on microstructure and mechanical properties of a cobalt-free (FCC + B2) AlCrFe2Ni2 high entropy alloy (HEA) was investigated. The cryo-rolled HEA showed a nanocrystalline structure with profuse nano-twins and stacking faults. The lamellar structure gradually transformed into a complete microduplex structure with increasing annealing temperature. The remarkably heterogeneous microstructure at the intermediate annealing temperature consisted of a mix of lamellar and microduplex domains and showed strength-ductility synergy superior to other cobalt-free HEAs. The present results established the efficacy of moderate cryo-rolling in tailoring heterogeneous microstructure and tensile properties of HEAs. © 2022 Elsevier B.V.

Microstructure and texture formations were investigated in a coarse-grained starting material (average grain size ~100 µm) of equiatomic CoCrNi MEA after severe warm-rolling and annealing. The results were compared with a fine-grained starting material (FGSM with average grain size ~7 µm) to highlight the effect of starting grain size. Progressive microstructural evolution accompanied by deformation bands, shear bands, and nano-twins formation resulted in an ultrafine microstructure in the heavily warm-rolled CGSM. The hardness of CGSM after 90% warm-rolling was similar to FGSM, indicating a rather comparable ultrafine microstructure. The deformation texture of the CGSM was rather similar to the FGSM featured by a weak brass component. Remarkably, the annealed grain sizes of the CGSM and FGSM series of specimens were rather similar. These observations amply corroborated with comparable deformation microstructure in the two materials. Both CGSM and FGSM showed weak recrystallization texture featured by the retention of deformation texture components. The results indicated rather limited effect of the starting grain size on microstructure and texture formation in warm-rolled and annealed CoCrNi MEA. Further, a remarkable strength-ductility balance could be obtained in annealed CGSM due to ultrafine microstructure and solid solution strengthening. © 2022 Elsevier B.V.

The evolution of microstructure and recrystallization texture was investigated in a severely deformed (90% cold-rolled) and annealed (FCC + BCC/B2) AlCrFe2Ni2 high entropy alloy (HEA) with an exclusive focus on the FCC phase. Cold-rolling resulted in pronounced bending and folding of the lamellae, progressive alignment of the phases along the RD, large misorientation gradient in the FCC phase, and finally ultrafine structure formation. The FCC phase showed a predominantly brass-type (B) texture with orientations spreading from the G ({011}<100>) to B ({011}<112>), indicating its low stacking fault energy (SFE). The prior cold-rolling deformation strongly influenced the annealing behavior, including lamellar to microduplex structure, grain boundary character evolution, and faster attainment of equilibrium phase fractions. The 90% cold-rolled and annealed material revealed the presence of a remarkably strong B component, unlike the weak recrystallization texture of typical FCC single-phase low SFE HEAs. The origin of the strong B recrystallization component was attributed to a combined effect of the high misorientation gradient in the FCC phase, increased boundary mobility at high annealing temperatures, and the presence of the BCC/B2 phase inhibiting grain growth and thus preserving the recrystallized B grains. The tensile properties of the HEA were evaluated to highlight the outstanding cost-performance advantage of the alloy. © 2022 Elsevier Ltd

The microstructure and properties of cobalt-free cost-effective AlCrFe2Ni2 high entropy alloy (HEA) in the as-cast condition and after thermo-mechanical processing by severe cold-rolling and annealing were investigated in the present work. The as-cast HEA showed a heterogeneous microstructure consisting of relatively coarse lamellar and much finer intertwined regions. The coarse regions consisted of eutectic mixture of FCC and ordered B2 (along with minor BCC) phases. The FCC phase was enriched in Fe and Cr, while the B2 phase was found enriched in Ni and Al but depleted in Cr. The BCC/B2 phase in the as-cast material showed phase separation due to spinodal decomposition to two different B2 phases in the fine regions. The overall FCC phase and BCC/B2 phase fractions were ∼60% and 40%, respectively. Despite the complex microstructure, the presence of a high fraction of the ductile FCC phase rendered remarkable workability, allowing heavy cold-rolling up to 90% reduction in thickness. Heavy deformation resulted in the development of intriguing microstructural features such as folding and bending of the lamellae, local shearing, and finally, deformation-induced nanocrystallization of the FCC phase. However, the B2 phase retained the ordered structure even after 90% cold-rolling. Annealing at 800 °C resulted in the formation of an ultrafine microduplex structure with significant resistance to grain growth even up to an annealing temperature of 1200 °C. A high yield (∼880 MPa) and tensile strength (̴ 1100 MPa) coupled with appreciable elongation (∼10%) could be achieved after annealing at 800 °C. The tensile properties obtained were superior to the other cobalt-free HEAs, which indicate the promising application of the cobalt-free cost-effective AlCrFe2Ni2 HEA as an advanced structural material. © 2021 Elsevier B.V.

The effect of cross-rolling on microstructure and texture evolution in equiatomic CoCrNi medium entropy alloy (MEA) was investigated in the present work. For this purpose, the MEA was warm-rolled to 90% reduction in thickness at 400 °C by unidirectional and different cross-rolling routes. The heavily deformed specimens were further annealed at temperatures varying from 700 °C to 1200 °C. The development of fine-scale microstructures and profuse shear band formation was confirmed in all the processed materials. Compared to unidirectional processed material, the different cross-rolled materials showed greater propensity for forming intersecting shear bands. Higher hardness in the different cross-rolled materials was consistent with finer microstructure and abundant shear bands. Unidirectional processed material showed weak brass ({110}[removed]) component, whereas the different cross-rolled material showed rotated brass components. Upon annealing, the different cross-rolled materials showed smaller grain sizes than the unidirectional processed material due to more abundant potential nucleation sites. The recrystallization texture of the different processed materials showed retention of the deformation texture components. High fractions of random components indicated weak recrystallization texture, contributed by the absence of preferential nucleation and growth and profuse annealing twin formation. © 2022 Elsevier Ltd

Two new quaternary tellurides, Cs19Ln19Mn10Te48 (Ln = Pr and Gd), were synthesized by the reactive molten-flux method at 1273 K using an excess of CsCl as a reactive flux. The single-crystal X-ray diffraction studies reveal that these compounds are isostructural and crystallize in a new structure type in the space group C32h-C2/m of the monoclinic crystal system with two formula units. There are forty-nine unique crystallographic sites in the unit cell of the structure: ten Cs sites, ten Ln sites, five Mn sites, and twenty-four Te sites. All atoms have a site symmetry of.m. except for Cs(10) and Ln(10), which have a site symmetry of 2/m. The unique crystal structure of Cs19Ln19Mn10Te48 consists of complex layers of 2∞[Ln19Mn10Te48]19- that are separated by filling of Cs+ cations between the layers. The Ln atoms are bonded to six Te atoms that form a distorted octahedral geometry around the central Ln atom, whereas Mn atoms are coordinated to four Te atoms in a distorted tetrahedral fashion. The building blocks of these structures (LnTe6 octahedra and MnTe4 tetrahedra) are fused in a complex fashion to create 2∞[Ln19Mn10Te48]19- layers. These structures do not contain any homoatomic bonding and hence, can be easily charge-balanced as (Cs1+)19(Ln3+)19(Mn2+)10(Te2-)48 as per the Zintl-Klemm concept. The optical absorption study on finely ground single crystals of the Gd-compound reveals a direct bandgap of about 0.5(1) eV. The DFT studies also suggest both systems to be semiconductors with bandgaps of ∼0.5 eV consistent with the experimental value. © The Royal Society of Chemistry.

High entropy alloys (HEAs) are an emerging class of metallic materials with immense potential for advanced structural applications. Considerable effort has been directed towards further improving their properties by thermo-mechanical processing involving deformation and annealing treatments. Herein, we have presented a brief yet critical review of the recrystallization behavior of these novel materials. The present work underlines the critical aspects of recrystallization behavior in single and multiphase HEAs processed by different thermo-mechanical treatments, emphasizing the impact on microstructure, texture, and mechanical properties. The challenges, opportunities, and main observations are finally summarized.

The effect of severe warm-rolling on the microstructure and texture development in FCC equiatomic CoCrFeMnNi HEA was investigated in the present work. For this purpose, the HEA was warm-rolled to 90% reduction in thickness at 600 °C and annealed for 1 h at temperatures up to 1200 °C. To highlight the effect of warm-rolling, a critical comparison was made with similarly deformed and annealed cold-rolled HEA. The warm-rolled HEA showed an ultrafine lamellar microstructure, which was, however, significantly coarser than the cold-rolled HEA. The significantly coarser microstructure in the warm-rolled HEA could be attributed to the dynamic annihilation of dislocations during deformation. Warm-rolled HEA showed a pure metal or copper type texture instead of a predominantly brass type texture in the cold-rolled HEA. The stark differences in the deformation texture could be attributed to the increase in the SFE at the temperature of warm-rolling, which promoted more homogeneous deformation by dislocation slip over twin mediated deformation and extensive shear band formation. The lower stored energy and coarser deformation structure of the warm-rolled HEA resulted in higher recrystallization temperature, and consistently larger recrystallized grain size than the cold-rolled HEA. Annealing also resulted in the weakening of the recrystallization texture owing to the absence of strong preferential nucleation or growth. The HEA warm-rolled and annealed at 750 °C resulted in a fine-grained, completely recrystallized microstructure with the optimum strength-ductility combination. The present results revealed that warm-rolling could be effectively used as a processing route for tailoring microstructure and properties of CoCrFeMnNi HEA.

The single crystals of Cs3FeCl5 were synthesized at 973 K using the sealed tube solid-state molten flux method using CsCl as a reactive flux. The polycrystalline sample of Cs3FeCl5 was obtained by the stoichiometric reaction of CsCl and FeCl2 powders at 823 K by the sealed tube solid-state method. The crystal structure of Cs3FeCl5 was determined by single-crystal X-ray diffraction study at 298 (2) K. This ternary halide crystallizes in the body-centered tetragonal crystal system in I4/mcm space group with cell constants of a = b = 9.279 (1) Å and c = 14.824 (3) Å with four formula units per cell. The asymmetric unit of Cs3FeCl5 contains five crystallographically independent atomic sites: Cs1 (site symmetry: m.2 m), Cs2 (422), Fe1 (4¯2 m), Cl1 (..m), and Cl2 (4/m..). The structure of Cs3FeCl5 consists of isolated distorted FeCl42− units that are separated by the Cs+ cations. Charge balance in this closed-shell compound can be achieved by 3 × Cs+, 1 × Fe2+, and 5 × Cl−. Bond valence sum (BVS) calculation also support this assignment of formal oxidation states of elements in Cs3FeCl5 structure. The electronic structure calculation for Cs3FeCl5 performed within density functional theoretical framework predicts a band gap of 3.5 eV, which is in good agreement with the experimental band gap of 3.71 (2) eV, that was estimated from the UV–vis absorption edge study of polycrystalline Cs3FeCl5.

High-entropy alloys (HEAs) have shown unique combinations of strength and ductility along with other properties after optimized thermomechanical processing (TMP). In this study, the hot deformation of an AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) was studied by using a Gleeble 3800® thermomechanical simulator at 800 °C, 1000 °C and 1200 °C deformation temperature under different strain rates of 10−4, 10−2, and 1 s−1. The influence of strain rate and temperature on the evolution of the microstructure was discussed. The apparent activation energy (Q), stress exponent (n) and Zener–Holloman parameter (Z) were determined from the true stress–strain curves to develop the constitutive equation. The activation energy was determined to be 336 kJ/mol, and the stress exponent (n) was 2.5. Microstructures, characterized using electron-backscattered diffraction (EBSD), were analyzed as a function of (Z) values and interpreted in terms of solute diffusion and the interactions between dynamic softening (dynamic recovery and dynamic recrystallization) and hardening (dynamic precipitation) phenomena. It was found that the hot deformation behavior of EHEAs was controlled by pipe diffusion for the selected process parameters. The pipe diffusion coefficient of Ni increased with increasing strain rate at constant deformation temperature, whereas the lattice diffusion coefficient remained constant for all the examined strain rates. Dynamic recovery (DRV) was the dominant softening mechanism at lower temperatures irrespective of strain rates, while dynamic precipitation played a critical role in the inhibition of dynamic recrystallization (DRX) under these conditions. At higher temperatures and for all the strain rates, microstructure evolution was controlled by the DRX mechanism.

Possibilities of tailoring microstructure and mechanical properties of complex intermetallic containing CoCrFeNi2.1Nbx (x = 0.2, 0.4) high entropy alloys (HEAs) using thermo-mechanical processing were investigated in this work. For this purpose, the homogenized HEAs containing complex Laves phases distributed in the FCC matrix were successfully severely cold-rolled up to 90% reduction in thickness. The cold-rolled HEAs showed a nano lamellar microstructure with fragmented Laves phase particles arranged preferentially along the rolling direction. The microstructure of the Nb0.4 HEA revealed intense deformation zones surrounding the hard Laves phase particles, owing to their higher volume fraction and larger size as compared to those in the Nb0.2 HEA. Upon annealing at 800 °C, the lamellar deformed FCC phase in the two HEAs was transformed into an equiaxed nano-grained structure having fine dispersion of nano precipitates. However, the Nb0.4 showed a decidedly lower recrystallized grain size than the Nb0.2, resulting from the smaller deformed grain size and particle stimulated nucleation. The annealed Nb0.2 showed remarkable strength-ductility combination featured by tensile strength ∼1080 MPa and ductility ∼21%, originating from the combined effect of a high volume of ductile FCC matrix, remarkable grain refinement, and precipitation hardening. In stark contrast, the annealed Nb0.4 showed complete absence of tensile elongation, likely due to a low volume fraction of the ductile FCC matrix and a high volume fraction of the intermetallic phases. The present study emphasized the critical role of microstructural tailoring for achieving outstanding strength-ductility synergy in complex intermetallic containing CoCrFeNi2.1Nbx HEAs.

High entropy or complex concentrated alloys (HEAs/CCAs) were originally conceived as single phase alloys having simple crystal structures [1]. Some of the recent research in these alloys has been directed at increasing the strength of ductile (FCC) single phase alloys by adding one or more complex intermetallic phases for their potential applications as structural materials. It widely expanded the field of HEAs to include non-equiatomic concentrated alloys with multiple phases [2, 3]. The addition of Nb in the single phase HEAs has been considered very promising for improving high temperature strength properties [4]. However, with the increase in Nb-content, the amount of Laves phase in these alloys increases and thereby resulting in a loss of ductility. In order to restore the ductility in the base alloy and also to maximize the benefits of Nb addition, we have designed and developed novel complex intermetallic containing CoCrFeNi2.1Nbx (x=0.2, 0.4) HEAs by arc melting. In this paper, we present the microstructure and mechanical properties of CoCrFeNi2.1Nbx HEAs in the as-cast and annealed conditions.

The evolution of microstructure and texture was investigated in a severely warm-rolled ultrafine pearlitic steel. The steel was 95% warm-rolled at 600°C and annealed at 700°C for different time intervals. The spheroidisation of cementite initiated after 30% reduction and completed beyond 70% reduction. The 95% warm-rolled steel showed elongated as well as ultrafine equiaxed ferrite grains. Texture inhomogeneities were evidenced by the presence of Goss component ({011} <100>) on the surface originating due to surface shear and diffuse texture at the interior. Formation of equiaxed microstructure after annealing through continuous recrystallisation resulted in the retention of the surface Goss component. However, the Goss component was destroyed beyond annealing for 180 minutes due to the abnormal growth of other grains.

The plastic flow behavior of materials is normally determined on the basis of uniaxial tensile/compressive tests. However, the use of this technique is not straightforward in case of coatings or when the volume of the material available for testing is very small. Indentation technique (especially with a spherical ball or spherical tip as the indenter) is a good alternative for determining the plastic flow behavior of coatings and thin films and also to understand the behavior of individual phases in multi-phase alloys. While the indentation technique is simple, fast and relatively inexpensive, it poses a significant challenge in terms of data analysis compared to uniaxial testing. The primary origin of such a challenge is linked to the fact that the stress fields underneath the indent are not uniform and the measured response is an aggregate of the complex tri-axial stress field. In spite of this challenge, the pioneering work of David Tabor presented a simple methodology to relate the spherical indentation and uniaxial response of bulk materials. Tabor first demonstrated that the hardness (H) and corresponding contact radius (a) at a given load using a spherical indenter of radius R, can be converted to uniaxial plastic flow stress-strain data using the conversion factor C (called constraint factor) for converting the hardness to equivalent uniaxial flow stress and the second conversion factor q (called Tabor coefficient) for converting a/R, which was taken as a measure of indentation strain, to uniaxial plastic equivalent strain. Given the fact that indentation occurs under multiaxial stress state, it is rather surprising that one can easily obtain the uniaxial plastic flow stress-strain from hardness data using the two conversion factors mentioned above. However, repeated indentation experiments on a wide range of metallic materials have demonstrated the validity of the approach. Of the two conversion factors, factor C has a sound theoretical basis while on the o

Introduction of axial chirality in bioactive 3-indolyl furanoids has been achieved by systematic alteration of functional groups around the stereogenic axis, keeping in mind that atropisomerically pure analogues may possess different binding affinities and selectivities towards a target protein. The kinetics of racemization of axially chiral 3-indolyl furanoids have been studied through chiral HPLC analysis, electronic circular dichroism (ECD) spectroscopy, and computational modeling. The results identify the configurational parameters for optically pure 3-indolyl furanoids to exist as stable and isolable atropisomeric form.

The effect of warm-rolling and subsequent annealing on the evolution of microstructure and texture was systematically investigated in a lamellar (L12+B2) AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA). The EHEA was warm-rolled at 400°C up to 90% reduction in thickness and annealed at different temperatures. Ultra-fine lamellar microstructure evolved after heavy deformation with the characteristic textures of the two phases. Annealing treatments resulted in the gradual development of an ultrafine micro-duplex structure with increasing annealing temperature, even though the alloy showed significant resistance to grain growth. The annealing process was featured by concomitant microstructural and textural changes.

The influence of severe warm-rolling and subsequent annealing on the microstructure and mechanical properties in an AlCoCrFeNi2.1 high entropy alloy consisting of a lamellar arrangement of ordered L12 and B2 phases was studied. For this purpose, the as-cast EHEA was severely warm-rolled up to 90% reduction in thickness at selected temperatures of 400 °C, 600 °C, and 750 °C. The severely warm-rolled EHEAs were further annealed at temperatures ranging from 800 °C to 1200 °C for 1 h. Warm-rolling resulted in a unique disordering behavior of the L12 depending on the temperature of warm-rolling. However, the B2 phase maintained the ordered structure irrespective of the warm-rolling temperature. The EHEA warm-rolled at 400 °C and 600 °C showed a predominantly lamellar microstructure, while the EHEA warm-rolled at 750 °C showed a novel heterogeneous microstructure featured by a mixture of lamellar and non-lamellar regions. Annealing of the 90% warm-rolled materials resulted in the formation of predominantly duplex microstructures consisting of disordered FCC and B2 phases. The EHEA warm-rolled at 400 °C and 600 °C showed high strength but rather limited ductility, which could be improved upon annealing. Remarkably, the EHEA warm-rolled at 750 °C showed the most outstanding properties featured by exceptional strength-ductility combination owing to the novel heterogeneous microstructure. Annealing of the heterogeneous EHEA resulted in the deterioration of the strength-ductility balance owing to the annihilation of the heterogeneous microstructure, being replaced by a microduplex structure.

[Abstract Not Available]

Microstructure and tensile behavior of CoCrFeNi2.1Nbx (x = 0.2, 0.4) high entropy alloys were investigated in the as-cast and aged (800 °C/24 h) conditions. Increasing Nb addition increased strength but reduced ductility due to the increased volume fraction of Laves phase. On aging, fine acicular precipitates formed throughout the matrix in the CoCrFeNi2.1Nb0.4 alloy, but only at the dendrite boundaries in the CoCrFeNi2.1Nb0.2 alloy. While both the aged alloys showed increase in strength due to the presence of the fine precipitates, the favorable microstructure of the CoCrFeNi2.1Nb0.2 ensured a larger mean free path for gliding dislocations, thus rendering superior strength-ductility synergy.

Nano-lamellar (L12 + B2) AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was processed by cryo-rolling and annealing. The EHEA developed a novel hierarchical microstructure featured by fine lamellar regions consisting of FCC lamellae filled with ultrafine FCC grains (average size ∼200-250 nm) and B2 lamellae, and coarse non-lamellar regions consisting of ultrafine FCC (average size ∼200-250 nm), few coarse recrystallized FCC grains and rather coarse unrecrystallized B2 phase (∼2.5 μm). This complex and hierarchical microstructure originated from differences in strain-partitioning amongst the constituent phases, affecting the driving force for recrystallization. The hierarchical microstructure of the cryo-rolled and annealed material resulted in simultaneous enhancement in strength (Yield Strength/YS: 1437 ± 26 MPa, Ultimate Tensile Strength/UTS: 1562 ± 33 MPa) and ductility (elongation to failure/ef ∼ 14 ± 1%) as compared to the as-cast as well as cold-rolled and annealed materials. The present study for the first time demonstrated that cryo-deformation and annealing could be a novel microstructural design strategy for overcoming strength-ductility trade off in multiphase high entropy alloys.

The effect of strain path on microstructure, texture and hardness properties of AlCoCrFeNi2.1 eutectic high entropy alloy containing ordered FCC (L12) and ordered BCC (B2) was investigated. The EHEA was cryo-rolled using UCR, MSCR (during which the samples were rotated by 90° around the ND between each pass) and TSCR(45°) (in which the samples were deformed by unidirectional rolling to half of the total strain and then diagonally rolled for rest half of the strain). The UCR processed material showed a rather heterogeneous microstructure. The textures of the L12/FCC and B2 phases in the MSCR processed material agreed with the cross-rolling texture of the corresponding single phase materials, while the texture of the two phases in the TSCR(45°) processed materials appeared rather weak. Upon annealing at 800 °C, the UCR processed materials showed a novel heterogeneous microstructure, while the MSCR and TSCR(45°) processed materials revealed microduplex structure. The heterogeneous microstructure was replaced by the usual microduplex structure at higher annealing temperatures. The annealing texture of the L12/FCC phase showed the presence of α-fiber (ND//<011>) components while the B2 phase showed strong ND-fiber (ND//<111>) components. The UCR processed material with novel heterogeneous microstructure showed much greater hardness as compared to the MSCR and TSCR(45°) processed materials. The present results indicate that strain path exerted significant influence in controlling microstructure, texture and hardness properties of EHEA.

In the present work, the effect of strain-path on the microstructure, texture and hardness properties of AlCoCrFeNi2.1 eutectic high entropy alloy was investigated. The EHEA was first cryo-rolled at 77K upto 90% thickness reduction using three different cross-rolling routes namely unidirectional cryo-rolling (UCR), multistep cross-rolling (MSCR) and two-step cross-rolling (TSCR(45º)) followed by annealing at three different temperatures. The UCR processed material showed heterogeneous microstructure as compared to MSCR and TSCR(45º)) processed materials. The deformation texture of L12/FCC in MSCR processed material agrees quite well the texture of cryo-rolled FCC materials whereas the texture of both the phases in TSCR(45º) processed material appears relatively weak. Upon recrystallization at 800ºC, the UCR processed material showed a rather novel heterogeneous microstructure whereas MSCR and TSCR(45º)) processed materials indicate ultra-fine micro-duplex structure. At higher annealing temperature, the micro-duplex structures remain stable in all the three processed materials. The annealing texture of L12/FCC phase showed presence of strong α-fibre (ND//<110>) whereas B2 phase revealed intense ND-fibre (ND//<111>) which is usual recrystallization texture found in BCC material. The UCR processed material showed much higher hardness as compared to MSCR and TSCR(45º)) processed materials which is attributed to its novel heterogeneous microstructure. Hence, it can be concluded that strain-path results in significant influence in controlling microstructure, texture and hardness properties of EHEA.

Measuring the uniaxial stress–strain response from indentation testing has been of great interest to the materials community ever since the seminal work on spherical indentation by David Tabor. In this regard, spherical indentation is the primary choice due to the ability to access a wide range of strains in a single test. While indentation testing is fairly simple to perform, the conversion factors required to calculate the uniaxial flow stress from hardness, which is commonly referred to as constraint factor and uniaxial strain from indentation contact radius and ball radius, which is called the Tabor coefficient, are not necessarily constant and most of the prior work involves assumptions about one of these conversion factors to calculate the other. In this work, we present a finite element analysis-based approach to independently determine the constraint factor and Tabor coefficient in the fully plastic indentation regime which is a pre-requisite for this analysis. The criteria to determine whether fully plastic indentation regime is satisfied has also been presented. The proposed approach has been validated by comparing the uniaxial stress–strain response from spherical indentation tests on OFHC copper and the data obtained by conventional uniaxial testing. Excellent agreement has been found between the two approaches which can be readily applied for measuring the uniaxial stress–strain response of coatings which is otherwise difficult to determine.

Uniaxial compression behavior of porous copper samples having a wide range of initial porosity prepared by Spark Plasma Sintering (SPS) process is presented. Simple analytical model is developed for evolution of porosity and flow stress during uniaxial compression based on the Gurson model which was initially developed for low porosity solids (<10%) having simple pore geometry. A unique linear relationship was observed for normalized porosity (porosity/initial porosity) with strain and normalized flow stress (flow stress/flow stress of matrix). The model predictions are in good agreement with the experimental results over a wide range of porosity (2–25%) and strain (0–0.26). Furthermore, a new model for strain hardening rate is developed which explains the significant strain hardening observed in the uniaxial compression tests. The observed strain hardening rate of the porous samples depends not only on the strain hardening rate of the matrix material but also on the extent of pore closure during compression. While the relative contribution of the strain hardening rate of the matrix largely dominates the observed response, the contribution due to pore closure is significant for samples with high initial porosity, especially at higher values of strain.

CoCrFeMnNi High Entropy Alloy is a single phase solid solution with FCC crystal structure which has shown very novel properties in its bulk form. Present study investigates the stability of these alloys in thin film form deposited by pulsed laser deposition. For this purpose depositions are done on two different substrates namely glass and platinized silicon at three different temperatures (room temperature, 2500C and 4000C). XRD studies have shown that the required phased formation is observed on films deposited at room temperature and no phase formation is observed on films deposited at 4000C and 2500C. Energy dispersive x-ray analysis (EDAX) analysis has shown that the composition of the bulk target materials is maintained in the thin films as well. The thickness of the deposited films was found to be around 100 nm which was studied by Ellipsometry. Reexamination of the films after 3 months of deposition by SEM analysis have shown a new phase formation and pores on the films. however, no extra phase could be detected within the limits of x-ray diffraction studies. To analyze the phase segregation if any, the samples were subjected to heat treatment studies. These samples were heat treated at 1500C for 1 hour to enhance the new phase observed under microstructural studies. however no enhancement of the phase was observed under x-ray diffraction studies. Suspecting the formation of metal oxide (mainly Mn and Cr) oxidation studies were attempted. An oxygen annealing was performed at a steady state of 50 mTorr of oxygen ambience at 3000C. But no oxide peaks or any new phase peaks was noticed in XRD analysis. however, EDAX analysis has shown that the new phase formed is mainly due to the formation of manganese oxide. Magnetic studies carried out using MFM studies before and after oxidation of the films have showed that films are paramagnetic which may be attributed to nano grain size of the film.VSM studies carried out on oxidized and heat treated samples have shown t

The effect of cold-rolling strain on the evolution of deformation and grain growth textures in an equiatomic HfZrTiTaNb refractory high entropy alloy (RHEA) was investigated. For this purpose, the development of microstructure and texture was compared in 50% and 90% cold-rolled materials. The development of deformation microstructure was accompanied by the formation of extensive deformation heterogeneities. The 90% cold-rolled material showed ultrafine lamellar regions as well as regions severely fragmented by the fine scale deformation heterogeneities. The 50% cold-rolled material showed dominant ND-fiber (ND//<111>), while the 90% cold-rolled material showed presence of both RD (RD//<110>) and ND-fiber components. Profuse deformation heterogeneities presumably weakened the deformation texture considerably. The annealing texture of the 50% and 90% cold-rolled material showed stronger {111}<110> component. Comparison of the annealing texture of the 50% and 90% cold-rolled materials with other BCC HEA phases indicated much weaker texture. The annealing texture of the RHEA was not significantly strengthened by increasing cold-rolling strain, presumably due to the high density of deformation heterogeneities in the cold-rolled microstructure. Thus, deformation heterogeneities significantly influenced the formation of cold-rolling and annealing textures of the equiatomic HfZrTiTaNb RHEA.

[Abstract Not Available]

A CoCrFeMnNi high-entropy alloy (HEA) showed elongation to failure similar to 320% at T = 1023 K and a strain rate. epsilon = 10(-4) s(-1). Strain hardening and texture weakening occurred at low. e, whereas flow softening and texture strengthening were observed at high. epsilon (> 3 x 10(-2) s(-1)) experiments. The strain rate sensitivity (m) decreased from 0.5 to similar to 0.25 with increasing. epsilon. Deformation with m similar to 0.5 and deformation-enhanced grain growth at low. e indicated superplasticity associated with grain boundary sliding. The grain boundary diffusion coefficient diminished by a factor of similar to 4 in the HEA. Concurrent nucleation, growth and cavity interlinkage caused premature failure compared with conventional superplastic alloys.

The possibility of microstructural refinement and improvement of mechanical properties by severe cold-rolling was investigated in an AlCoCrFeNi2.1 lamellar eutectic high entropy alloy (EHEA). The as-cast alloy revealed fine scale eutectic mixture of L1(2) (ordered FCC) and B2 (ordered BCC) phases. During severe cold-rolling up to 90% reduction in thickness the B2 phase maintained the ordered structure, while the L1(2) phase showed the evolution of a nanocrystalline structure and progressive disordering. Annealing of the severely cold-rolled material resulted in the formation of duplex microstructures composed of two different phases with equiaxed morphologies and significant resistance to grain growth up to 1200 degrees C. Annealing at 1000 degrees C resulted in an optimum strength-ductility balance with the tensile strength of 1175 MPa and the total elongation of 23%. The present results showed that severe cold-rolling and annealing can impart very attractive mechanical properties in complex EHEAs.

Constitutive modeling of the ingot breakdown process of large size ingots of high strength steel was carried out through comprehensive thermomechanical processing using Gleeble 3800® thermomechanical simulator, finite element modeling (FEM), optical and electron back scatter diffraction (EBSD). For this purpose, hot compression tests in the range of 1473 K to 1323 K (1200 °C to 1050 °C) and strain rates of 0.25 to 2 s−1 were carried out. The stress-strain curves describing the deformation behavior of the dendritic microstructure of the cast ingot were analyzed in terms of the Arrhenius and Hansel-Spittel models which were implemented in Forge NxT 1.0® FEM software. The results indicated that the Arrhenius model was more reliable in predicting microstructure evolution of the as-cast structure during ingot breakdown, particularly the occurrence of dynamic recrystallization (DRX) process which was a vital parameter in estimating the optimum loads for forming of large size components. The accuracy and reliability of both models were compared in terms of correlation coefficient (R) and the average absolute relative error (ARRE).

Hot deformation behavior of an equiatomic CoCrFeMnNi high entropy alloy was investigated. Hot compression tests were carried out at various temperatures ranging from 800°C to 1000°C at different strain rates from 0.001s-1 to 1s-1. Stress-strain curves indicated softening due to dynamic recrystallization (DRX) under most conditions. Flow stress analysis was carried out by using Arrhenius type hyperbolic-sine relationship, and a linear dependence of flow stress on Zener-Hollomon parameter (Z) was found. Based on the flow stress analysis, a constitutive equation was formulated for describing interdependency between deformation temperature, strain rate, flow stress and strain. The estimated apparent activation energy (Q) ∼350 kJmol-1 for the hot deformation was approximately similar to the activation energy for diffusion of the slowest diffusing element Ni in this alloy. Microstructural observations were carried out by electron backscatter diffraction (EBSD). The EBSD analysis indicated that the DRX grains nucleated along initial grain boundaries resulting in necklace like structures. The fraction and size of DRX grains showed a strong dependence on Z. Textures of deformed and DRX grains were found to be similar to each other, but rather weak. This feature could be attributed to the nucleation of dynamic recrystallization without any preferential orientation selection.

The effect of strain path on microstructure and texture formation in an equiatomic CoCrFeMnNi high entropy alloy was investigated. For this purpose, the alloy was cold-rolled to 90% reduction in thickness (total equivalent strain (epsilon(eq)) similar to 2.65) using unidirectional cold-rolling (UCR) and different cross-rolling routes. In the MSCCR route, the samples were rotated by 90 degrees around the ND in between each pass. In the TSCCR(90 degrees) and TSCCR(45 degrees) routes, the samples were deformed by unidirectional rolling to half of the epsilon(eq) (similar to 1.3) in the first step and the rest half of the strain (epsilon(eq) similar to 1.3) was imparted after rotation around the ND by 90 degrees and 45 degrees, respectively. The development of deformation microstructures in all the processing routes were characterized by fine lamellar type structure intercepted by inclined shear bands. The TSCCR(45 degrees) processed material appeared to be most prone to shear band formation. The UCR processed material showed brass type texture, while the textures of different cross-rolled materials were featured by different rotated brass components. Upon annealing, the UCR processed material showed the lowest grain size while the highest grain size was observed in the TSCCR(45 degrees) processed material. This indicated greater available nucleation sites in the UCR processed material as compared to the TSCCR(45 degrees) processed material. The differences in annealed grain size was presumably higher due to substructure destabilization and misorientation build-up diminishing the number of potential nuclei in the TSCCR(45 degrees) processed material. The annealing textures of the differently processed materials were featured by the presence of a-fiber (ND//< 110 >) and absence of preferential nucleation and growth.

The development of microstructure and mechanical properties was investigated in a heavily cold-rolled and annealed AlCoCrFeNi2.1 high-entropy alloy. The as-cast alloy having a eutectic morphology consisting of alternate bands of ordered L1(2) and B2 phases was 90% cold-rolled. The deformed microstructure showed profuse shear banding and disordering of the L12, but no transformation of the B2 phase. A duplex microstructure consisting of ultrafine equiaxed grains (similar to 0.60 mu m) of disordered face centered cubic and B2 was observed after annealing at 800 degrees C. The annealed material showed remarkable strength-ductility combination having ultimate tensile strength similar to 1.2 GPa and elongation to failure similar to 12%.

The evolution of microstructure and texture after heavy cold-rolling and annealing was investigated in FCC equiatomic medium to high entropy alloys. For this purpose medium entropy ternary CoFeNi and quaternary CoCrFeNi, and high entropy quinary CoCrFeMnNi alloys were cold-rolled to 90% reduction in thickness and annealed at different temperatures. The ternary alloy showed the development of lamellar microstructure and pure metal type texture after heavy cold-rolling. In contrast, the quaternary and quinary alloys showed heavily fragmented microstructure and brass dominated texture. After annealing, the ternary alloy showed abnormal grain growth at lower annealing temperatures and strengthening of the cube component ({001}<100>) with increasing annealing temperature. In contrast, the quinary and quaternary alloys showed more homogenous grain growth and only minor changes in the volume fraction of different texture components during annealing. The observed differences in the microstructure and texture formation during cold-rolling could be understood considering the differences in the stacking fault energy of the three alloys. On the other hand, the evolution of microstructure and texture of the quarternary and quinary alloys was affected by the sluggish diffusion behavior.