The effect of grain boundary character distribution on the high temperature deformation behavior of Ni–16Cr–9Fe alloys

The objective of this work was to test the Thaveeprungsriporn model for the dependence of creep rate on the coincident site lattice (CSL) fraction. The model attributed the large reduction in creep rate in alloys with a high population of CSL boundaries to the greater difficulty of extrinsic grain boundary dislocation (EGBD) absorption at coincident site lattice boundaries (CSLBs) vs. high angle boundaries (HABs). Ease of EGBD absorption was assessed by measuring the annihilation rates of EGBDs in both CSL-related and HABs following an anneal at 360 °C. Results showed that EGBDs are annihilated at HABs at a rate that is on average three times that at CSLBs, implying a grain boundary diffusion coefficient in CSLBs that is 12 times lower than that in HABs. The expectation that a reduction in EGBD absorption would lead to greater matrix hardening was investigated using nano-hardness measurements. Results showed that the hardness in the vicinity of CSLBs is greater than that near HABs, and the grain-averaged hardness increases with the fraction of contiguous CSLBs. Further, strain hardening is greater in CSL-enhanced samples than in reference, solution annealed samples. These results taken together substantiate the hypothesis that CSLBs impede dislocation absorption into the grain boundary, thereby increasing lattice hardening and internal stress in the sample, resulting in a reduced creep rate.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

Influence of recrystallization and environment on tensile behavior of cold-rolled Ni3Al（Zr） alloys

1 Introduction Intermetallic compounds have long been the subjects of considerable interest for high temperature applications. In particular, Ni3Al is one of the more promising intermetallic compounds due to its anomalous temperature dependence of the yie…     

Coarsening of Ni3Ge in binary Ni–Ge alloys: microstructures and volume fraction dependence of kinetics

Coarsening of Ni₃Ge precipitates in binary Ni–Ge alloys containing 12.15, 13.01 and 14.03 at.% Ge, aged at 724 °C (equilibrium volume fractions, f_e, equaling 0.022, 0.105 and 0.202, respectively) was investigated using transmission electron microscopy and magnetic analysis. The rate constants for the kinetics of particle growth and depletion of supersaturation depend anomalously on f_e, i.e. they both decrease slightly as f_e increases. Average values of the chemical diffusion coefficient and the Ni₃Ge/matrix interfacial free energy, derived from analysis of the data, are in reasonable agreement with previously reported values. Alignment of the Ni₃Ge precipitates parallel to cube directions is strong, but coalescence into plate shapes is never observed. Precipitates of the solid solution γ phase nucleated in large, coherent, concave-cuboidal Ni₃Ge precipitates. This behavior is expected, considering the phase boundary between the two-phase and Ni₃Ge regions in a recently published phase diagram.     

On the effect of strain on peritectic reactions and transformations in Fe–Ni and Fe–Cu binary alloys

Abstract

Differential thermal analysis (DTA) experiments conducted on Fe–Ni and Fe–Cu alloys showed undercooling below the equilibrium peritectic temperatures, T_P . The intervals between the observed liquidus and peritectic temperatures were on average 11°C and 8°C larger than the intervals obtained from equilibrium phase diagrams of Fe–Ni and Fe–Cu respectively. The transformation from δ-Fe to γ-Fe during the peritectic reaction is associated with density change and strain build up at the δ-Fe/γ-Fe interface. Thermodynamic calculations showed that by introducing the strain energy at the δ-Fe/γ-Fe interface, T_P dropped 9 K below its equilibrium value and the increase in the liquidus-to-peritectic temperature interval was in reasonable agreement with the experimental observations. The growth rate of γ-Fe during a peritectic transformation was calculated based on the strain-induced undercooling in T_P and the results showed partial agreement with observations obtained from CSLM directional solidification experiments conducted earlier on Fe–Ni alloys.     

Effect of swaging on microstructure and tensile properties of W–Ni–Fe alloys

The objective of this study was to investigate the effect of swaging on the microstructure and tensile properties of high density two phase alloys 90W–7Ni–3Fe and 93W–4.9Ni–2.1Fe. Samples were liquid phase sintered under hydrogen and argon at 1480 °C for 30 min and then 15% cold rotary swaged. Measurement of microstructural parameters in the sintered and swaged samples showed that swaging slightly increased tungsten grain size in the longitudinal direction and slightly decreased tungsten grain size in the transverse direction. Swaging increased the contiguity values in both longitudinal and transverse directions. Swaging led to more severe deformations at the edges than at the center of the specimens. Solidus and liquidus temperatures of the nickel-based binder phase in the sintered and swaged samples were determined by differential scanning calorimetry measurements. An increase in tensile strength with a reduction in ductility was observed due to strain hardening by swaging.     

Influence of Fe on Microstructure and Mechanical Properties in Pdoped Ni–Cr–Fe Alloys

The influence of Fe on the microstructure and mechanical properties of P-doped Ni–Cr–Fe alloys has been investigated.Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe.In addition, the precipitation of M_(23)C_6 was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.     

Simulation of the coarsening behavior of intermetallic precipitates in Ni75AlxV25-x alloys

The coarsening behavior of L12 and D022 in Ni75AlxV25-x （x,at.%） alloys including coherent strain was investigated using the microscopic phase-field model. The simulation results indicate that the shape transition and spatial correlation of L12 and D022 are caused by the morphological-dependent anisotropic elastic interactions in the system. The coarsening process of the particles is by means of neighbor particles impingement and aggregation into larger ones. For the strain-induced interactions between the precipitates,the LSW theory is altered for the coarsening behavior of L12 and D022. In addition,the simulation reveals that the growth and coarsening of D022 present two obvious stages at lower Al concentration regions and proceed simultaneously at high Al concentration regions. The growth and coarsening processes of L12 at the same regions is reverse to those of D022.     

Effect of deformation temperature and strain rate on semi-solid deformation behavior of spray-formed Al-70 %Si alloys

Spray-formed Al-70%Si(mass fraction) alloys were deformed by compression in the semi-solid state.The effects of the deformation temperature, strain rate and the microstructure were studied. Two strain rates(1 s-1and 0.1 s-1) and six deformation temperatures (600 ℃, 720 ℃ , 780 ℃, 900 ℃, 1 000 ℃ and 1 100 ℃) were chosen. The stress-strain curve exhibits a peak at low strain and then decreases to a plateau before it starts to increase again as the strain increases. The stress required for deformation at lower strain rate and at higher deformation temperatures is less than those at higher strain rate and at lower deformation temperatures. Four mechanisms of semisolid deformation can be used to explain the different behaviors of the stress-strain curves under different conditions.     

Superdislocation core structure in L12 Ni3Al,Ni3Ge and Fe3Ge: Peierls–Nabarro analysis starting from ab-initio GSF energetics calculations

Superdislocation core structures in L1₂ Ni₃Ge, Fe₃Ge and Ni₃Al were determined on the basis of ab-initio generalized stacking fault (GSF) energetics calculations. Superdislocation dissociation schemes, partials separation and core widths were calculated within the modified Peierls–Nabarro (PN) model with ab-initio GSF parameterization. Calculated superdislocation core structure parameters were found to be in agreement with available experimental data. The superdislocation core with type I (fourfold) dissociation was found to be unstable in Fe₃Ge but stable and energetically preferred in Ni₃Ge and Ni₃Al. These results allows us to suggest a mechanism for the octahedral glide being inactive in Fe₃Ge which is found to be different in several aspects with those discussed so far in the literature.     

Analysis of surface and bulk behavior in Ni–Pd alloys

The most salient features of the surface structure and bulk behavior of Ni–Pd alloys have been studied using the Bozzolo–Ferrante–Smith (BFS) method for alloys. Large-scale atomistic simulations were performed to investigate surface segregation profiles as a function of temperature, crystal face, and composition. Pd enrichment of the first layer was observed in (1 1 1) and (1 0 0) surfaces, and enrichment of the top two layers occurred for (1 1 0) surfaces. In all cases, the segregation profile shows alternated planes enriched and depleted in Pd. In addition, the phase structure of bulk Ni–Pd alloys as a function of temperature and composition was studied. A weak ordering tendency was observed at low temperatures, which helps to explain the compositional oscillations in the segregation profiles. Finally, based on atom-by-atom static energy calculations, a comprehensive explanation for the observed surface and bulk features will be presented in terms of competing chemical and strain energy effects.     

Effect of microstructure on tensile properties and fracture behavior of intermetallic Ti2AlNb alloys

The tensile properties and fracture behaviors of Ti-22Al-27Nb and Ti-22Al-20Nb-7Ta alloys were investigated in the temperature range of 25-800℃ Three typical microstructures were obtained by ifferent thermomechanical processing techniques.The results indicate that the duplex microstructure has an optimum combination of tensile yield strength and ductility both at room and elevated temperatures.Adding Ta to Ti2AlNb alloy can improve the yield strength,especially at high temperature while retain a good ductility.The study on crack initiation and propagation in dedformed microstructure of Ti2AlNb alloys indicates that microstructure has ikmportant effect on the tensile fracture mechanism of the alloys.The cracks initiate within primary O/α2 grains along O/B2 boundaries or O phase laths in B2 matrix,and propagate along primary B2 grain boundaries for the duplex microstructure.The fracture mode is transgranular with ductile dimples for the duplex and the equiaxed microstructures,but intergranular for the lath microstructure.     

Effects of carbon addition and aging on the shape memory effect of Fe–Mn–Si–Cr–Ni alloys

The effects of carbon content and aging treatment on the microstructures and shape recovery ratio of Fe–Mn–Si–Cr–Ni alloys were studied. It was found that the carbon content possessed significant effects on the shape recovery ratio of Fe–Mn–Si–Cr–Ni alloys. The shape memory effect of alloys containing different carbon amount could be improved through aging.     

Martensitic transformation behavior of Ti–Ni–Ag alloys

Microstructures and martensitic transformation behavior of Ti–Ni–Ag alloys prepared by arc melting were investigated by means of scanning electron microscopy (SEM), electron probe micro analysis (EPMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermal cycling tests under constant load. Ti–Ni–Ag alloys consisted of Ti–Ni–Ag matrices, Ti₂Ni and TiAg phases. Ti–Ni–Ag matrices contained 0.27–0.52 at.% of solute Ag atoms depending on alloy compositions. The B2–B19′ transformation occurred in Ti–50.1Ni–0.7Ag, Ti–49.2Ni–0.9Ag, Ti–49.2Ni–0.6Ag and Ti–49.0Ni–0.7Ag alloys, while the B2-R-B19′ transformation did in Ti–47.5Ni–1.3Ag and Ti–44.4Ni–1.1Ag alloys. Thermo-mechanical treatment separated the B2-R from the R–B19′ transformation clearly and improved shape recovery by increasing the critical stress for slip deformation in a Ti–50.0Ni–0.7Ag alloy.     

Effects of Si on deformation behavior and cavitation of coarse-grained Al–4.5Mg alloys exhibiting large elongation

Deformation behavior and cavitation characteristics of Al–4.5Mg–0.09Si and Al–4.5Mg–0.2Si alloys containing Mg₂Si particles have been investigated at 613–693 K. Tensile elongations above 350% were obtained in both alloys. Determination of the strain rate sensitivity and activation energy showed that in both alloys viscous dislocation glide creep deformation mechanism was operative and was not influenced by Si content. However, the level of cavitation was increased by the large addition of Si because Mg₂Si particles promoted more extensive cavity nucleation. Cavitation reduced the post forming tensile properties of both alloys. Investigation of the spatial distribution of cavitation revealed that the intragranular particles were more likely to nucleate cavities than the intergranular particles.     

Influence of cooling rate and composition on orientation of primary carbides of Fe–Cr–C hardfacing alloys

Abstract

Four Fe–Cr–C hardfacing alloys with carbon contents of 3˙34–6˙5% were studied. The orientation of primary carbides in the microstructures of hardfacing layers produced by arc surfacing was investigated under controlled cooling conditions. Carbon content and cooling conditions were found to play an important role in determining overlayer microstructures. Increasing carbon content or decreasing Cr/C ratio increased the tendency for primary carbides to be oriented perpendicular to the surface of the overlayers, and the carbides in the microstructure became more compact. Under water cooling conditions, the primary carbides were preferentially oriented perpendicular to the surface, which would be expected to improve wear resistance. At lower cooling rates, primary carbides were oriented randomly.     

High-strain-rate superplasticity of the Al–Zn–Mg–Cu alloys with Fe and Ni additions

During high-strain-rate superplastic deformation, superplasticity indices, and the microstructure of two Al–Zn–Mg–Cu–Zr alloys with additions of nickel and iron, which contain equal volume fractions of eutectic particles of Al₃Ni or Al₉FeNi, have been compared. It has been shown that the alloys exhibit superplasticity with 300–800% elongations at the strain rates of 1 × 10^–2–1 × 10^–1 s^–1. The differences in the kinetics of alloy recrystallization in the course of heating and deformation at different temperatures and rates of the superplastic deformation, which are related to the various parameters of the particles of the eutectic phases, have been found. At strain rates higher than 4 × 10^–2, in the alloy with Fe and Ni, a partially nonrecrystallized structure is retained up to material failure and, in the alloy with Ni, a completely recrystallized structure is formed at rates of up to 1 × 10^–1 s^–1.     

Electrochemical behavior of nanocrystalline Ni–P alloys containing tin and tungsten

Autocatalytic ternary Ni-Sn-P, Ni-W-P and quaternary Ni-W-Sn-P films were prepared using an alkaline bath. Plain Ni-P films were also prepared for comparison. Corrosion resistance of the films was evaluated in 3.5% sodium chloride solution in non-deaerated condition by potentiodynamic polarization and electrochemical impedance spectroscopy methods. Deposits were also immersed in 3.5% sodium chloride solution for 7 days. All the coatings attained stable equilibrium potential within 30 minutes in NaCl medium. Lower corrosion current density values were obtained for ternary Ni-Sn-P coatings compared to the plain Ni-P coatings. Ternary Ni-W-P and quaternary Ni-W-Sn-P alloys did not show improved corrosion resistance compared to the ternary Ni-Sn-P coatings. Similar behavior of these coatings was further confirmed by the electrochemical impedance studies. After the potentiodynamic polarization test deposits were examined by scanning electron microscope. It was found that more corrosion occurred for the quaternary deposit compared to other deposits. Energy dispersive analysis of X-ray results indicated that more amount of Fe present on NiWP and NiWSnP coated samples. Similar behavior was confirmed from the optical images of the surfaces obtained for the deposits after the immersion test. The article is published in the original.