A commercial Niobium–Hafnium–Titanium (Nb-Hf-Ti) alloy (C-103), produced by vacuum arc remelting (VAR), showed clear signatures of internal oxidation viz. presence of monoclinic-HfO2 at all the grain ...boundaries. Minor cold rolling, to ∼10% reduction in thickness, initiated cold cracking. This was observed at some, but not all, of the high angle Nb–Nb grain boundaries containing coarse HfO2 2nd phase oxide precipitates. Closer microscopic examination revealed cold cracking at selective Nb-HfO2 interfaces. There appeared no clear mesoscopic experimental rationale behind this selective interfacial cold cracking. In order to understand this, density functional theory (DFT) calculations were performed on the metal-oxide interface models. They showed significant separation or decohesion energy difference (∼76%) with different atomistic bonding at the terminating layer of the Nb-HfO2 interface. In particular, the interface with one atomic layer of Nb bonded with oxygen of HfO2 had very low work of separation, which was about ∼8% lower than the Nb–Nb interface of the bulk Nb. In brief, the DFT simulations brought out the defining role of metal-oxide interface’s terminating layer on the interface decohesion. This is in agreement with the past theoretical research performed on several metal||non-metal interfaces. The present study extends this hypothesis to a practical example of selective cold cracking of a commercial Nb-Hf-Ti alloy.
•Nb-Hf-Ti alloy (C-103) showed signature of internal oxidation: formation of HfO2 at all the grain boundaries.•On ∼10% deformation, the alloy experienced cold cracking at selective grain boundaries (GBs) containing coarse 2nd phase HfO2.•Extensive microtexture measurements could not identify the rationale behind the selective grain or phase boundary cold cracking.•Density functional theory calculations showed that the … NbNb||NbOHfO … interface has the lowest work of separation for the cold cracking to be initiated.
The evolution of microstructure and crystallographic texture during thermo-mechanical processing and their effect on tensile behavior of Al-Cu-Li (AA2195) alloy were investigated in the present work. ...The initial hot rolled sheets were cold rolled by unidirectional rolling and cross-rolling. These sheets were annealed and imparted T6 (solution treatment + water quenching + aging) temper. Uni-directionally rolled samples exhibited typical copper texture which subsequently transformed to cube component after thermal treatment whereas, the cross-rolled sample exhibited brass texture which subsequently weakened upon thermal treatment. The difference in the precipitation characteristics resulted in transition from anisotropic to isotropic strength in cross rolled samples in heat treated condition. The strength anisotropy has been explained based on the average precipitation strengthening factor and accommodation tensor.
Al-Cu-Li alloy (AA2195) was produced using vacuum induction melting (VIM) furnace under dynamic argon atmosphere. The as-cast billets were homogenized using a two-step homogenization cycle. The hot ...deformation behavior of homogenized and forged AA2195 alloy was studied by hot isothermal compression in a thermo-mechanical simulator. The contour maps of efficiency of power dissipation and instability maps have been generated within the temperature range of 250–450 °C and strain rate range of 10−3–102 s−1. Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of these maps and complementary microstructural analysis of the deformed specimens was carried out. Results indicate four distinct deformation domains within the range of experimental conditions examined. Out of these four domains, the optimum temperature and strain rate range for obtaining a completely reconstituted microstructure is T: 400 °C-450 °C andε˙: 10−2-10−1.5 s−1 andε˙: 10−0.5-101 s−1. Instability mechanisms in the material are attributed to localized plastic flow and cracking. A constitutive equation that describes the flow stress of AA2195 alloy as a function of strain rate and deformation temperature was also established.
•AA2195 alloy was processed by vacuum induction melting.•Hot deformation behavior of AA2195 in cast and homogenized condition was studied by hot isothermal compression testing.•Power dissipation/instability maps were plotted in temperature (250–450 °C) and strain rate (10−3–102 s−1) ranges.•Optimum T and ε˙ ranges to obtain reconstituted microstructure are T: 400–450 °C andε˙: 10−2-10−1.5 s−1 andε˙: 10−0.5-101 s−1.•Constitutive equation that describes the flow stress of AA2195 alloy as a function of ε˙ and T was established.
Multi-axial forging of Al was carried out at room temperature in a confined channel die as a means of achieving severe plastic deformation (SPD). The microstructures of the 3, 6 and 9 pass samples ...were quantified using electron back scattered diffraction to obtain the distribution of boundary spacing and fraction of high angle boundaries. Subsequent compression tests at room temperature were carried out to determine their strength and strain rate sensitivity. Softening was observed beyond 6 passes both in terms of the pressing stress as well as the subsequent compression tests. Beyond 6 passes the fraction of high angle boundaries increased and the geometrically necessary dislocations (GND) density decreased. It was estimated that during SPD, Al had a higher hardening contribution due to GND and subgrain walls than that due to Hall–Petch type grain boundary strengthening. It was shown that as the fraction of high angle boundaries increased with increasing deformation, the dislocation strengthening term decreased and the Hall–Petch term increased, resulting in a net softening effect.
•Scale growth on Fe-Cr alloys obeys the diffusion-controlled parabolic kinetics.•Cross-sectional EBSD, Raman spectroscopy and XRD reveal the type of oxides with Cr.•Chromic oxide displays higher ...hardness and increased compressive (Raman) stresses.•The oxide at metal-oxide interface exhibits an increase in fracture toughness with Cr.
The present study involved a range of iron-chromium (0–20 wt% Cr) alloys of similar crystallographic texture. The oxidation kinetics reduced remarkably when Cr content was ≥12 wt%. This coincided with the reduction in the number (from three to two) of clearly distinguishable oxide layers. Though the outer hematite (Fe2O3) layer remained, the inner chromite (FeCr2O4, spinel structure) transformed to chromic oxide ((Fe,Cr)2O3, corundum structure) and the columnar coarse grained magnetite (Fe3O4) middle layer disappeared. The chromic oxide displayed higher hardness and increased compressive residual (Raman) stresses. It also showed significantly higher fracture toughness in micro-cantilever bending tests. A clear picture of oxidation resistance, in high-Cr alloys, through phase transformation and associated mechanical integrity of the inner oxide layer thus emerged.
This study involved fully pearlitic wires of seven different diameters (5.5–1.6 mm). All samples were laboratory annealed to re-austenitize and were then air-cooled to reform the pearlite structure. ...Morphological alignment of the pearlite, along the wire axis, improved significantly, 32% to 93%, as the wire diameter decreased. This improvement coincided with increases in the ferrite fiber texture, and falls in the axial residual stresses. In all the wires, the majority of the pearlite lamellae appeared to align, in a 2-D analysis, with minimum elastic stiffness (EMin under simple compression) for the ferrite (α). This correlation increased from 80% to 98% with decrease in wire diameter and fall in axial residual stresses. 3-D microstructures by serial sectioning, 3-D rotations seeking EMin and observations on coarse pearlite, indicated that {011}α and α were, respectively, the pearlite interface (or habit plane) and growth direction.
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Isothermal sintering behavior of pure molybdenum (Mo) and mechanically alloyed Mo–TZM (Mo–0.6Ti–0.2Zr–0.02C) has been investigated in the temperature range 1000–1800
°C. A linear relationship has ...been found to exist between logarithms of increment in density and time. Although the volume diffusion has been found to be the dominant sintering mechanism, a significant contribution from grain boundary diffusion is also identified. Both the diffusion coefficients (
D
v
) obtained from shrinkage data and the grain boundary mobility (
M
b
) during grain growth are found to be lower for Mo–TZM due to the presence of carbides in the microstructure. The grain boundary migration is restricted due to the presence of carbides and porosities in the microstructure.
In this study, we demonstrate the microstructure dependency of erosion behaviour of laser clad, detonation sprayed and atmospheric plasma sprayed chromium carbide based coatings. The final chromium ...carbide content in all the coatings was a strong function of rapid solidification rate associated with the processes. In the laser clad coating majority of the chromium carbides re-solidified while in the thermally sprayed coatings chromium carbide re-solidification was hindered to a large extent. Hence, the final chromium carbide content in the thermally sprayed coating decreased with increased extent of particle melting during spraying. Decarburisation and oxidation during thermal spraying lead to the formation of chromium carbides with lower carbon content and chromium oxide(s). Laser clad and detonation sprayed coatings, with higher chromium carbide content, showed lower erosion rates and exhibited fewer brittle erosion events. Embrittlement due to excessive dissolution of chromium carbides into the matrix and poor splat bonding were found to be the reasons for higher erosion rate of the plasma sprayed coating. Scanning electron microscopy and quantification of single erodent impact events clearly established ductile material removal in the laser clad and detonation sprayed coating and brittle material removal in the plasma sprayed coating as the dominant mechanism(s). A good agreement was found between solid particle erosion testing and nano impact testing results.
•Erosion behavior of various chromium carbide based coatings has been compared.•Microstructure and carbide content strongly depend on the processing technique.•Erosion mechanism has been established by quantification of single erodent impacts.•Good agreement between erosion and nano impact testing results was found.
•The magnetization obtained from FMR is found to be 2600 G.•Observation of large negative anisotropy field (HU).•The stress induced in the film is the primary cause for such a large negative HU.•The ...presence of this stress in YIG film was confirmed using multiple GIXRD.•Systematic FMR and dc magnetic studies in the temperature range 4.2 ≤ T ≤ 300 K.
Magnetic properties and FMR of pulsed laser deposited Y3Fe5O12 (YIG) thin film on Gd3Ga5O12 (1 1 1) substrate have been investigated in the temperature range 4.2 ≤ T ≤ 300 K. The effective saturation magnetization (4πMeff), obtained from Ferromagnetic Resonance at room temperature, is found to be 2600 G, which is higher than SQUID measured 4πMS value of 1770 G. This implies that the value of 4πMeff can only be accounted by considering a negative anisotropy field (HU) of around 830 Oe at room temperature. Such an anisotropy is attributed to a compressive stress (−1.70 × 1010 dyne/cm2) induced in the film closer to the film-air interface. The presence of this stress in YIG film was confirmed using multiple {hkl} stress measurements with grazing incidence X-ray diffraction performed at different depth of penetration on the YIG film.
•Sensitization was dependent on grain fragmentation due to cold/warm rolling.•Depth of attack after DL-EPR test established by white light interferrometry.•Near boundary gradient zone established by ...electron backscatterd diffraction.•Near boundary gradient zone correlated with depth of attack.
The dependence of degree of sensitization on partially deformed (rolled and then sensitized) microstructure of 304L stainless steel was investigated. Structures with visible grain fragmentation, or clear reductions in grain size, showed a poor resistance to sensitization. However, non-fragmented deformed grains with clear presence of near boundary orientation gradients provided an improved resistance. Bulk statistical data were further corroborated through direct observations from electron backscattered diffraction (EBSD) and white light interferometry (WLI). EBSD quantified the near boundary gradient zones (NBGZ) while WLI characterized localized corrosion. Scaling between grain average depth of attack and dimension of NBGZ was established.
