Fcc Layers Research Articles

High-temperature oxidation behavior of laser additively manufactured AlCrCoNiSi-based high-entropy alloys at 1100 ℃

High-temperature oxidation behavior of two laser additively manufactured AlCrCoNiSi-based high-entropy alloys (Al10 and Al15) are systematically studied in this work. These two alloys exhibit similar oxidation behaviors involving initial linear kinetics oxidation and subsequent parabolic kinetics oxidation at 1100 ℃. However, Al10 alloy has a superior oxidation resistance than Al15 alloy according to the oxidation rate constant. α-Al2O3 is the predominant phase of the oxide scales grown on both alloys. Cross-sectional views of the scales show that the scales consist of surface equiaxed grains and underlying columnar grains. For all oxidized alloys, a newly-formed fcc layer forms in the subsurface Al-depletion zone due to the preferential oxidation of aluminum. Al10 alloy exhibits a much thicker newly-formed fcc layer than Al15 alloy after the same oxidation time. This work ascribes the superior oxidation resistance of Al10 alloy to its thick newly-formed fcc layer, which suppresses the outward Al diffusion from the substrate to scale-metal interface by increasing the diffusion distance.

Enhancing the shear strength of the Au–Ge solder joint via forming a ductile face-centered cubic solid solution layer at the interface

The Au-28Ge (at%) eutectic alloy is a high-performance solder with a melting point of 361 °C. However, the brittle interfacial reaction products layer formed between the Au–Ge solder and substrate (Cu or Ni) is always the weak region. Hence, the joint fails at a relatively low shear strength (∼53 MPa). Here, the Cu–Ni alloys (Ni content is 20, 50, or 70 at%) are designed as the substrates to connect with the Au–Ge eutectic at 400 °C. A classic example is represented by the Au–Ge/Cu20Ni joint, which achieves a shear strength of above 100 MPa after soldering at 400 °C for 10–20 min. Microstructure characterization reveals that a face-centered cubic structured solid solution (FCC) layer forms at the Au–Ge/Cu20Ni joint. In contrast, it is mainly the NiGe or Ni2Ge layer in the Au–Ge/Cu50Ni and Au–Ge/Cu70Ni joints. On the one hand, compared to the brittle intermetallic compound products layer, the FCC layer shows adequate ductility, releasing the local strain incompatibility via pronounced local plastic deformation. On the other hand, the fine NiGe and Ni2Ge precipitation and fine grains make the FCC layer stronger than the solder matrix. Therefore, this products layer is no longer the weak region in the solder joint. As a result, the Au–Ge/Cu20Ni joint achieves outstanding shear strength. This study enriches our fundamental understanding of producing ductile phases to enhance the shear strength of the solder joints by designing alloy substrates.

Micro-abrasive wear study of a low-temperature plasma nitrided Inconel 625 superalloy

ABSTRACT In this study, the microabrasive wear of a plasma nitrided Inconel 625 superalloy was studied. Nitrogen supersaturation in the expanded FCC layer promoted hardening due to the higher compressive residual stresses. The results showed the formation of a nitrided layer consisting of nitrogen expanded FCC phase (γN) and CrN nitride, respectively. Friction coefficient, mechanical failure mode and critical loads for damaging the nitrided case, using the linear scratch test, were carried out at a linearly increased normal force. The scratch test results showed that the nitrided layer strongly decreased the apparent friction coefficient in comparison with the non-nitrided alloy. Tensile cracking was the prevalent mechanical failure mode of the nitrided layer. Microabrasion results showed that the nitriding layer improving the wear resistance. For the nitrided samples, wear coefficients were determined for the nitrided layer and the substrate, indicating a change in the wear volume loss rate with the test distance.

Recovery of Kink Boundaries During Post-Annealing of a Hot-Extruded Mg-Zn-Y Alloy

We have investigated post-annealing induced microscopic structural change of kink boundaries (KBs), which have been initially introduced by a hot-extrusion of an Mg-Zn-Y alloy containing long period stacking/order (LPSO) phase, based on scanning transmission electron microscopy (STEM) and atom-probe tomography (APT). STEM observations show that the KBs appear to be segmented into several submicron-scale, each of which is composed of arrays of extended basal (a-) dislocations that are geometrically necessary to account for the LPSO crystal rotations. Atomic-scale STEM/APT observations reveal significant reconstructions of these extended dislocation-core structures, which occur at the solute-enriched face-centered cubic (fcc) layers in the LPSO structures and forms local hexagonal close-packed (hcp) Mg region with distinct solute depletions. These recovery features indeed enhance the thermal stability of the KBs, and the corresponding driving force can be reasonably described according to disclinations, whose elastic energy is defined by crystal rotations and the relevant dipole distances.

Effect of Ti Addition on the Microstructure and High-Temperature Oxidation Property of AlCoCrFeNi High-Entropy Alloy

This study investigated the effect of Ti addition on microstructure and high-temperature oxidation property of AlCoCrFeNi high-entropy alloy. Ti content was controlled at 0 at% and 1 at%. The two alloys were found to have BCC single phase, and the average grain sizes of Ti0.0 and Ti1.0 were 47.3 µm and 49.7 µm, respectively, showing similarity. The EDS mapping of the inside of grains found that both alloys were characterized to be divided into Al-Ni element rich region and Cr–Fe element rich region. As a result of high-temperature oxidation test at 1100 °C, oxidation weight gains were measured at Ti0.0: 0.75 mg/cm2 and Ti1.0: 0.17 mg/cm2, respectively. The results indicate that Ti addition largely improved high temperature oxidation resistance of AlCoCrFeNi HEA. Ti1.0 alloy, in particular, showed remarkably more excellent 1100 °C high-temperature oxidation resistance than other previously reported major ones such as NiCrAl and FeCrAl. In the surface and cross-section observations after oxidation tests, both alloys were found to have Al2O3 oxides mostly. While the Ti0.0 material was observed to have Al2O3 spallation macroscopically, the Ti1.0 alloy showed Al2O3 spallation only in some local areas. In addition, a unique result was found in AlCoCrFeNiTix(x=0,1) alloy that BCC → FCC phase transformation was accelerated, and FCC phase layer was formed in the surficial area where Al element had been depleted due to high-temperature oxidation. Moreover, as Ti was added, the thickness of FCC layer induced by high-temperature oxidation decreased. Based on the results, it was also discussed on how to improve the high-temperature oxidation resistance of AlCoCrFeNiTix HEA.

Influences of laser surface melting on microstructure, mechanical properties and corrosion resistance of dual-phase Cr–Fe–Co–Ni–Al high entropy alloys

In this paper, a laser surface melting (LSM) method was utilized on CrFeCoNiAlx (x = 0.5, 0.6, 0.7) high entropy alloys (HEAs) with FCC + BCC structure, and the effects of LSM on their microstructure, mechanical properties and corrosion behavior were systematically investigated. It is intriguing that, a layer with BCC + B2 structure formed on the surface of CrFeCoNiAl0.6 and CrFeCoNiAl0.7 substrates after LSM, while a monolithic FCC layer formed on that of CrFeCoNiAl0.5 substrate under the same condition. LSM influences the mechanical properties of CrFeCoNiAlx HEAs to varying degrees including hardness and tensile properties decided by the phase formation in laser-treated surfaces and the substrates. In addition, compared to the substrates, obvious increase in self-corrosion potential and pitting potential were found in the laser-treated surface layers of CrFeCoNiAl0.6 and CrFeCoNiAl0.7. Interestingly, the corrosion resistance of laser-treated surface layers has positive correlation with Al content, while that of the substrates shows negative correlation with the concentration of Al, which is possibly resulted from the difference in corrosion mechanism. This work provides a new route for significantly improving surface properties of dual-phase HEAs, which would be very beneficial to their further application as advanced materials.

Thermodynamic origin of solute-enriched stacking-fault in dilute Mg-Zn-Y alloys

We have investigated thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Furthermore, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These spontaneous thermodynamic behaviors explain why the fault layers can be remarkably stabilized in the LPSO-forming ternary Mg alloys, and also support a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and has been confirmed for the present SESF by electron microscopy experiments.

Direct Observations of Precursor Short-Range Order Clusters of Solute Atoms in a LPSO-Forming Mg-Zn-Gd Ternary Alloy

We have investigated short-range order (SRO) clusters of solute atoms in a Mg97Zn1Gd2 alloy which forms a long-period stacking/order (LPSO) phase, based on aberration-corrected scanning transmission electron microscopy (STEM) observations and first-principles calculations. High-angle annular dark-field (HAADF) STEM provides directly the individual Gd atom positions through significant atomic-number dependent Z-contrast, and their averaged image-processing have immediately shown up the representative SRO-Gd configurations being in favor of the second-nearest networks in the hcp-Mg structure. Interestingly, Zn atoms seem to be essentially placed at the particular sites within the SRO-Gd configurations to form D019-like ZnGd3 clusters, a validity of which are verified by STEM image simulations and first-principles calculations. These D019-like ZnGd3 clusters in the hcp-Mg matrix are likely to be a precursor sub-structure to the L12-type Zn6Gd8 clusters that are embedded in the local fcc layers in the LPSO structure. In fact, we find that these D019-like ZnGd3 clusters seem to form a layered atmosphere prior to the LPSO formation, providing an important clue on how the LPSO phases nucleate and grow.

Low temperature plasma nitriding of a Co30Cr19Fe alloy for improving cavitation erosion resistance

Cobalt alloys are used when improved cavitation-erosion (CE) resistance is needed. Low temperature plasma nitriding - (LTPN) is known to greatly enhance the CE resistance of austenitic and duplex stainless steels, due to formation of a very hard, super-saturated fcc - phase, known as expanded austenite or S-phase. In this work, Low Temperature Plasma Nitriding of a non-standard Co-Cr alloy was carried out to explore the formation of an expanded S-phase hard layer and to assess its effect on the CE resistance of the Co-Cr alloy. The Co-Cr samples, containing α-fcc and ε-hcp solid solutions phases, were plasma nitrided at 350 °C and 400 °C for 20 h. The CE tests were carried out in a vibratory cavitation equipment according to ASTM G32-92. Microstructural and micromechanical characterization of the specimens indicated the formation of an expanded S-phase fcc layer, containing small amounts of CrN. Plasma nitriding at 400 °C and greater amounts of α-fcc volume fraction in the matrix led to thicker and harder (10.5 GPa) S-phase layers. The 400 °C nitrided samples exhibited higher CE resistances than the non-nitrided samples, with up to 267% greater incubation times and 5 times reduced wear rate. All Co30Cr19Fe samples showed higher CE resistances than AISI 304 and only the solution-treated sample showed lower CE resistance than Stellite 6. The results are discussed in terms of the mechanisms of material removal, during the initial stages of CE, which are controlled by plastic deformation, with formation of slip steps, grain boundaries protrusion and material removal from these protruded areas. Twin boundaries are preferably eroded. The increase in nitrogen content increases the elastic energy returned to the environment and decreases the amount of plastic energy absorbed by the alloy, at cavitation impact spots.

Thermodynamic Origin of Solute-Enriched Stacking-Fault in Dilute Mg-Zn-Y Alloys

We investigate thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. Segregation behaviors of solute Zn/Y atoms into the SESF are firstly estimated according to the Hillert's parallel tangent law, followed by the possible disorder-order phase transformation within the SESF using the multiple-sublattice model. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Besides, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These thermodynamic behaviors explain fairly well a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and also confirmed for the present SESF by electron microscopy experiments. Therefore, strong Zn-Y interactions even in dilute conditions play a key role to stabilize firmly the SESF in the Mg-Zn-Y alloys.

Proposed mechanism of HCP\u2009\u2192\u2009FCC phase transition in titianium through first principles calculation and experiments

By means of first principles calculation and experiments, a detailed mechanism is proposed to include the stages of slip, adjustment, and expansion for the HCP → FCC phase transformation with the prismatic relation of {{{}{10}bar{{1}}{0}{}}}_{{hcp}}{parallel }{{}{1}bar{{1}}{0}{{}}}_{{fcc}} and {{[}{0001}{]}}_{{hcp}}{parallel }{[}{001}{{]}}_{{fcc}} in titanium. It is revealed that the formation of four FCC layers is preferable after the slip of Shockley partial dislocations of 1/6 langle {1}bar{{2}}{10}rangle on {{}{10}bar{{1}}{0}{}} planes, and that the adjustment of interplanar spacing and the volume expansion are energetically favorable and could happen spontaneously without any energy barrier. It is also found that the transformed FCC lattice first follows the c/a ratio (1.583) of HCP and then becomes an ideal FCC structure (c/a = √2). The proposed mechanism could not only provide a deep understanding to the process of HCP → FCC prismatic transformation in titanium, but also clarify the controversy regarding volume expansion of HCP-FCC phase transition of titanium in the literature.

The potential energy landscape for crystallisation of a Lennard-Jones fluid

Crystallisation pathways are explored by direct analysis of the potential energy landscape for a system of Lennard-Jones particles with periodic boundary conditions. A database of minima and transition states linking liquid and crystalline states is constructed using discrete path sampling and the entire potential energy landscape from liquid to crystal is visualised. We demonstrate that there is a strong negative correlation between the number of atoms in the largest crystalline cluster and the potential energy. In common with previous results we find a strong bias towards the growth of FCC rather than HCP clusters, despite a very small potential energy difference. We characterise three types of perfect crystals with very similar energies: pure FCC, pure HCP, and combinations of FCC and HCP layers. There are also many slightly defective crystalline structures. The effect of the simulation box is analysed for a supercell containing 864 atoms. There are low barriers between some of the different crystalline structures via pathways involving sliding layers, and many different defective structures with FCC layers stacked at an angle to the periodic box. Finally, we compare a binary Lennard-Jones system and visualise the potential energy landscape from supercooled liquid to crystal.

Cooling rate dependence of polymorph selection during rapid solidification of liquid metal zinc

The polymorph selection during rapid solidification of zinc melt was investigated by molecular dynamics simulation. Several methods including g(r), energy, CNS, basic cluster and visualization were used to analyze the results. The results reveal that the cooling rate has no observable effect on the microstructure as T>Tc(Tc is the onset temperature of crystallization); and at the first stage of crystallization, although microstructures are different, the morphologies of nucleus are similar, which are composed of HCP and FCC layers; the polymorph selection of cooling rate finally takes place at the second stage of crystallization: at a high cooling rate, the rapid increase of FCC atoms leads to a FCC crystal mixed with less HCP structures; while at a low cooling rate, HCP atoms grow at the expense of FCC atoms, resulting in an almost perfect HCP phase. The results reveal that the cooling rate is one of the important factors for polymorph selection.

Stability and formation of long period stacking order structure in Mg-based ternary alloys

Long period stacking ordered (LPSO) structure plays an important role in determining the strength and ductility of Mg-based ternary alloys. In this work, we investigated the thermodynamic stability of 10H LPSO structures in comparison with those of 14H and 18R LPSO structures. A trimer model was proposed to predict the possibility of forming LPSO structure in Mg–XL–XS (XL=Rare earth, Ca, Sr and XS=Al, Zn, Cu, Ni) with first-principles calculations. The formation of LPSO structure was determined based on the intrinsic stacking fault energy, mix energy and segregation energy. An element which decreases the intrinsic stacking fault energy is essential for the formation of LPSO structure. Decreasing the stacking fault energy favors the formation of local FCC layers, which would attract solid solutes segregating from HCP to local FCC Mg and forming L12 cluster and finally the LPSO structure.

Influence of strain and polycrystalline ordering on magnetic properties of high moment rare earth metals and alloys

Despite being the most suitable candidates for solenoid pole pieces in state-of-the-art superconductor-based electromagnets, the intrinsic magnetic properties of heavy rare earth metals and their alloys have gained comparatively little attention. With the potential of integration in micro and nanoscale devices, thin films of Gd, Dy, Tb, DyGd and DyTb were plasma-sputtered and investigated for their in-plane magnetic properties, with an emphasis on magnetization versus temperature profiles. Based on crystal structure analysis of the polycrystalline rare earth films, which consist of a low magnetic moment fcc layer at the seed interface topped with a higher moment hcp layer, an experimental protocol is introduced which allows the direct magnetic analysis of the individual layers. In line with the general trend of heavy lanthanides, the saturation magnetization was found to drop with increasing unit cell size. In situ annealed rare earth films exceeded the saturation magnetization of a high-moment Fe65Co35 reference film in the cryogenic temperature regime, proving their potential for pole piece applications; however as-deposited rare earth films were found completely unsuitable. In agreement with theoretical predictions, sufficiently strained crystal phases of Tb and Dy did not exhibit an incommensurate magnetic order, unlike their single-crystal counterparts which have a helical phase. DyGd and DyTb alloys followed the trends of the elemental rare earth metals in terms of crystal structure and magnetic properties. Inter-rare-earth alloys hence present a desirable blend of saturation magnetization and operating temperature.

Polymorphic crystals selected in the nucleation stage

Molecular dynamics simulations are used to explore the atomic mechanism of formation of polymorphic crystals. Cooling the Lennard-Jones systems, we observe that the system almost always evolves into a polymorphic crystal with either fivefold-symmetric stacking faults or single-direction stacking faults. The detailed analysis reveals that such an evolution depends on the configuration of fcc/hcp concomitance in the nucleation stage. A defect-induced model is then introduced to illustrate these two evolution routes. Through calculating the formation energies of the defective critical nuclei, we find that the polymorphic crystals seem to be determined by their critical nuclei, in which the relatively lower formation energy ensures the preponderance of the fivefold-symmetric cluster. Before the nucleation, we observe that thermal fluctuations prefer hcp-like particles over fcc-like ones while in the nucleation and growth stage this preference reverses. Notably, an extended step rule of Ostwald is seemingly suitable to characterise the growth process because of the temporary hcp layers appearing among fcc layers in the growth stage. Although the crystalline cluster with single-direction stacking faults has higher growth rate and structural order than its competitor, the component (fcc and hcp) proportion of the final crystals is almost always constant regardless of the polymorphic type. Our finding renews the understanding of the polymorphism of crystals, and possibly draws more attention of people intending to control the polymorphic structures through nucleation.

Influence of the composition of titanium oxynitride layers on the fretting behavior of functionalized titanium substrates: PVD films versus surface laser treatments

In this work we compared the fretting behavior of pure titanium plates functionalized with titanium oxynitride surface layers, obtained by two methods: a Physical Vapor Deposition (PVD) method, reactive magnetron sputtering, and Surface Laser Treatments (SLT), under different mixtures of oxygen and nitrogen. The composition of the layers was determined by nuclear reaction analysis (NRA) and their structure was analyzed by Raman spectroscopy. PVD layers were TiN-like fcc layers, with an oxygen concentration going from 36 to 50at.%. Three SLT layers were studied. The first one was a TiN-like layer containing ~28at.% of oxygen. The second one was formed of different titanium oxide phases containing ~5at.% of nitrogen. The third one was a titanium dioxide layer with a negligible concentration of nitrogen.It was found that the steady friction coefficient was similar for all the layers and quite lower than that measured for uncoated Ti. The study of the fretting scars revealed a higher resistance of SLT layers to fretting wear, which can be due to the smooth layer-substrate interface. The detachment of coating particles was observed in some PVD layers.Finally, the transfer of matter between the first bodies was studied by micro-Raman spectroscopy and nuclear reaction techniques: NRA and Particle Induced X-ray Emission.

On chemical order and interfacial segregation in [formula omitted] (AlAg2) precipitates

A detailed study has been carried out on γ′ (AlAg2) precipitates in Al–Ag and Al–Ag–Cu alloys to reconcile the conflicting reports on chemical ordering and stacking faults in this phase. High-angle annular dark-field scanning transmission electron microscopy and convergent beam electron diffraction show no indication of chemical ordering on alternate basal planes of γ′ precipitates in alloys aged at 473K for 2–23h. Precipitates were visible as Ag-rich regions with 1–13 face-centred cubic (fcc)→hexagonal close-packed stacking faults, corresponding to γ′ platelets with thicknesses ranging from 0.69 to 6.44nm. There were no systematically absent thicknesses. Growth ledges with a riser height equal to the c-lattice parameter (0.46nm) were directly observed for the first time. Genuine stacking faults within the precipitates were extremely rare and only observed in thicker precipitates. In precipitates with 1–3 stacking faults there was also substantial Ag in the surrounding fcc layers of the matrix, indicating that Ag strongly segregated to the broad, planar precipitate–matrix interfaces. This segregation is responsible for previous reports of stacking faults in γ′ precipitates. The results indicate that the early stages of γ′ precipitate growth are interfacially controlled.

Formation of regular polyicosahedral and defected crystalline structures in growing Lennard-Jones clusters

Formation of solid structure and its subsequent enlargement have been analysed in simulated growth of Lennard-Jones (LJ) clusters from vapour. Four series of the Monte Carlo simulations, carried out for constant temperature T⁎=0.25, 0.30, 0.35 and 0.40 in reduced units, were started from LJ13 icosahedral cluster used as a seed in a supersaturated LJ vapour. In the first simulation stage, all the 504 simulated clusters (over 100 growth runs for every analysed temperature) achieved the size N≈1000. Structural analysis and visualisation showed that at this size many of the obtained clusters (i.e. 10%, 29%, 34% and 66%, respectively, for the above temperatures) were well ordered. They were found to have regular internal structure mainly of regular polyicosahedral and layered fcc/hcp structure. In the second simulation stage practically all the obtained clusters were grown until N=3300 was reached, while in the third stage the selected regular structures attain N=9800. Visualisation of the internal structure of the growing regular clusters from their solidification up to the final size N≈9800 revealed that (i) the layered fcc/hcp structure maintains layered character if fcc layers are relatively thin, and (ii) r-PIC clusters initially keep the structure during the growth, but its subsequent growth leads to the formation of disordered regions.

Evolutions of lamellar structure during melting and solidification of Fe9577 nanoparticle from molecular dynamics simulations

A structural evolution during solidification and melting processes of nanoparticle Fe9577 was investigated from MD simulations. A perfect lamellar structure, consisting alternately of fcc and hcp layers, was obtained from solidification process. A structural heredity of early embryo is proposed to explain the structural preference of solidification. Defects were found inside the solid core and play the same role as surface premelting on melting. hcp was found more stable than fcc in high temperature. The difference between melting and solidification points can be deduced coming fully from the overcoming of thermodynamic energy barrier, instead of kinetic delay of structural relaxation.