Pradeep konda Gokuldoss

CoCrFeMnNiV high entropy alloy (HEA) exhibits a high content of σ-phase (70 vol%) when produced by casting route. In the present work, a combination of mechanical alloying (MA) and spark plasma sintering (SPS) has been used to synthesize nanocrystalline CoCrFeMnNiV HEA where the formation of σ-phase has been avoided. Electron microscopy and atom probe tomography analysis indicated the formation of FCC structured HEA matrix along with (Cr,V) carbide (15 vol%) precipitation, without the presence of σ-phase in SPS processed alloy. Gibbs energy vs composition (G-x) diagrams of binary subsystems and possible carbides and oxides substantiate the absence of σ-phase during SPS of CoCrFeMnNiV alloy. Thus, the unsolicited contamination during MA-SPS route proves beneficial in suppressing the complex phase formation.

Formation of nano-twinned domains associated with ferro-elastic transformation in 2 mol.% ZrO2 alloyed YTaO4 has been investigated using a correlative microscopy approach. Transmission Kikuchi diffraction reveals the presence of two phases, monoclinic (M) and tetragonal (T) in the material sintered at 1485 °C for 3 hr. The microstructure comprises of alternate nano-twinned domains with an average spacing of 154.07 ± 42.52 nm. Crystallographic symmetry based spontaneous strain analyses support the observation of single domain state. Ab-initio calculations further indicate the propensity of stabilizing both M and T phases at room temperature as a function of ZrO2 alloying, without any change in chemical composition, as is evidenced by atom probe tomography.

Grade 300 maraging steel specimens were synthesized using the additive manufacturing technique laser powder bed fusion (LPBF). The influence of heterostructures, i.e. cellular/dendritic regions with micro-segregations, formed during LPBF processing on the microstructure evolution, particularly the formation of nano-precipitates were investigated. We applied aging and solution- aging post heat-treatments to allow the precipitation in presence and after elimination of the heterostructures, respectively. A model describing the precipitation mechanisms and sequence for both heat-treatment routes is proposed based on the three-dimensional elemental distribution at near-atomic scale and the quantification of nano-scale segregated regions performed by atom probe tomography (APT). We observed that the heterostructures favored the austenite retention in the as-built condition, while they promoted the martensite-to-austenite reversion in the aged condition. On the contrary, the retention and reversion processes are suppressed in the solution-aged condition. The formation of nanosized (Ni,Fe,Co)3(Ti,Mo,Al) and (Fe,Ni,Co)7(Mo)6 precipitates was confirmed for both post heat-treatment routes. However, the nano-precipitates were smaller in size and higher in number density in the solution-aged condition, as compared to the aged state. The current work demonstrates that the elimination of heterostructures in the solution-aged condition, especially Ni micro-segregations, led to a high supersaturation and a concomitant increase in number density of (Ni,Fe,Co)3(Ti,Mo,Al) precipitates. The corresponding Ni-depleted matrix inhibited the martensite-to-austenite reversion. In contrast, the partial retention of heterostructures and the presence of retained austenite during aging without prior solution treatment resulted in a reduced number density of (Ni,Fe,Co)3(Ti,Mo,Al) precipitates, which can potentially trigger the reversion transformation supported by the rejection of Ni in front of (Fe,Ni,Co)7(Mo)6 precipitates.

The work compares the peculiarities of the high-coercivity state formation in the Sm-Co-Fe-Cu-Zr high-temperature and high-energy permanent magnets (HTPM and HEPM) in the course of the heat treatment with the stepwise decreasing temperature from 830 to 400 °C. Two types of magnets with varying Fe concentration, i.e., Sm(Co0.88-xFexCu0.09Zr0.03)7 with x = 0–0.12 (the HTPMs) and Sm(Co0.91-xFexCu0.06 Zr0.03)7.5 with x = 0.24–0.33 (the HEPMs) were studied at different temperatures of heat treatment for phase formation by x-ray diffraction followed by magnetic property measurements. Microstructure characterization was performed using transmission electron microscopy, whereas the three-dimensional elemental distribution at near-atomic scale was obtained using atom probe tomography. In HEPMs, the main increase in coercivity and relaxation of stresses accompanied by intensive enrichment of the 1:5 phase in Cu are observed at high temperatures (Т ≈ 700 °C). In HTPMs, the coercivity monotonously increases in the entire temperature range of the slow cooling from 700 to 400 °C at a rate of 0.5 °C/s. At the temperature close to the Curie temperature (∼550 °C) of the Sm(Co,Cu)5-type phase, the anomaly of the coercivity increment has been observed. The interphase stresses grow and the elemental redistribution appears to be accelerated simultaneously. The non-uniform Cu distribution in the 1:5 phase can be described by the formation of Cu-rich interlayers at the interface of the Sm(Co,Cu)5 and Sm2(Co,Fe)17-type phases.

This work aims to characterize the influence of temperature and chloride solution concentration on the corrosion behavior of a newly developed high-entropy alloy system (Al2Cr5Cu5Fe53Ni35) in a simulated marine environment. A direct correlation was found between temperature and chloride concentration with localized corrosion resistance. The Point Defect Model approach is employed to analyze the influence of the temperature and chloride concentration upon the properties of the passive film formation over the alloy surface in seawater solutions. It turns out that the present system alloy exhibits a better local corrosion resistance than conventional martensitic stainless steel UNS S40300 when temperature is below 60 ∘C in similar electrolytic environment.

Laser beam directed energy deposition (DED-LB) differs from other metal additive manufacturing (AM) methods as it allows high building rates and manufacturing of multi-material components via in-situ alloying. This is especially compelling in combination with high-manganese steel (HMnS), as the mechanical properties can be influenced significantly by tailoring the chemistry-dependent stacking-fault energy (SFE). In DED, this can be used to design parts based on local deformation behavior. However, the increased affinity of HMnS to oxygen causes high amounts of oxide formation in the manufactured parts, ultimately deteriorating the mechanical properties due to premature cracking. To investigate the responsible mechanisms, two sets of processing parameters resulting in varying melt pool sizes were applied to produce X30Mn23 steel with up to 1 wt% in-situ alloyed Al. The melt pool was first modeled using a finite element method (FEM) approach and correlated with microstructure evolution (OM, SEM, EDS, EBSD, APT) and mechanical properties (tensile test). Interaction of the melt pool with the atmosphere and therefore oxide formation was successfully prevented by reducing the melt pool size, making the application of HMnS in DED feasible. The crack formation by oxides and its prevention are discussed in detail. Finally, the feasibility to manufacture HMnS with an in-situ alloying approach is critically evaluated.

The paper investigates the microstructure evolution and phase analysis of the Sm60Ni40 alloy. The arc melted sample retained the high temperature stable (>873 K) Sm7Ni3 and Sm3Ni2 phases along with the congruently melting SmNi phase. Annealing the as-cast sample at 903 K for 100 h stabilized the high temperature stable (873 K − 903 K) Sm3Ni2 phase. Rietveld refinement was performed to resolve the crystal structure of Sm3Ni2 phase and it was observed that Sm3Ni2 phase stabilizes in monoclinic crystal structure (space group: C2/m) with a lattice parameter of a = 13.49 Å, b = 3.75 Å, c = 9.68 Å and β = 106.6°. The Curie temperature of the Sm3Ni2 phase was determined to be ∼ 110 K. The high squareness ratio of 82 % along with high coercivity of 3 T indicates that the Sm3Ni2 phase possess spontaneous uniaxial magnetic anisotropy.

This paper presents methods and approaches that can be used for production of Sm-Co-Fe-Cu-Zr permanent magnets with working temperatures of up to 550 °C. It is shown that the content of Sm, Cu, and Fe significantly affects the coercivity (Hc) value at high operating temperatures. A decrease in the content of Fe, which replaces Co, and an increase in the content of Sm in Sm-Co-Fe-Cu-Zr alloys lead to a decrease in Hc value at room temperature, but significantly increase Hc at temperatures of about 500 °C. Increasing the Cu concentration enhances the Hc values at all operating temperatures. From analysis of the dependence of temperature coefficients of the coercivity on the concentrations of various constituent elements in this alloy, the optimum chemical composition that qualifies for high-temperature permanent magnet (HTPM) application were determined. 3D atom probe tomography analysis shows that the nanostructure of the HTPM is characterized by the formation of Sm2(Co,Fe)17 (2:17) cells relatively smaller in size along with the slightly thickened Sm(Co,Cu)5 (1:5) boundary phase compared to those of the high-energy permanent magnet compositions. An inhomogeneous distribution of Cu was also noticed in the 1:5 phase. At the boundary between 1:5 and 2:17 phases, an interface with lowered anisotropy constants has developed, which could be the reason for the observed high coercivity values.

The phase growth is investigated during the interdiffusion of an ultra-fine grained Ni (UFG) (grain size, d ∼ 320 nm) and coarse-grained Sn diffusion couple in the temperature range of 175–215 °C. The UFG Ni was produced through spark plasma sintering (SPS) of ball-milled Ni powder. Formation of only Ni3Sn4 intermetallic phase is observed, as confirmed by electron probe microanalysis (EPMA), micro-x-ray diffraction (XRD), and Transmission Kikuchi Diffraction (TKD) measurements. Using the CALPHAD approach, the driving forces for the formation of possible intermetallic phases in the Ni-Sn system were estimated and the highest value was found for Ni3Sn4, which matches the experimental observations. Time-dependent measurements at 200 °C revealed an acceleration in the growth (∼2 orders of magnitude) of Ni3Sn4 compared to the scenario prevalent in the coarse-grained (CG) Ni/Sn diffusion couple. This is reflected in the calculated integrated diffusivities for the UFG-Ni/Sn diffusion couple. The accelerated phase growth kinetics are attributed to the increased fraction of grain boundaries (GBs) in the UFG-Ni, which leads to an increased contribution of GB diffusion to the overall diffusion flux. The formation of Ni3Sn4 at GB was further ascertained using correlative microscopy strategy including transmission electron microscopy, transmission Kikuchi diffraction and scanning transmission electron microscopy.

Dispersion of nano Y2O3 (0.35 wt.%) in 18Cr ferritic steel (Fe–18Cr–2.33W–0.34Ti) was achieved by high energy ball milling of pre–alloyed powders after 6 h. The severe deformation induced nano-structuring during ball milling led to metastable solid solution formation, which gets stabilized during consolidation by upset forging and hot extrusion. Transmission electron microscopy, atom probe tomography and small angle X–ray scattering were combined to comprehensively characterize the crystal structure, morphology and the chemical composition of the dispersoids. Accordingly, the dispersoids of the type Y2Ti2O7 with cuboidal shape and Fd3¯m diamond cubic crystal structure having a lattice parameter of 1.01 nm were observed. The plastic deformation behavior of ODS steels at different operating temperatures was studied using the tensile test and the results were correlated with the size and morphology of the dispersoids.