Fcc-hcp Transformation Research Articles

Enhanced deformation-induced FCC-HCP transformation of a metastable high-entropy alloy induced by a smaller grain size

This study reports the grain-size-dependent deformation-induced face-centered cubic (FCC)–hexagonal close-packed (HCP) transformation behavior of Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 (at. %) high-entropy alloy in the medium grain size range (10–20 μm), their mechanical properties, and deformation mechanisms. The materials annealed at 900 °C for 1 h (A900-1) and 10 h (A900-10) showed a fully FCC, recrystallized microstructure with a larger grain size in the latter owing to grain growth during annealing. The A900-1 sample exhibited a larger strength–ductility balance and a higher strain hardening rate than the A900-10 sample. The A900-1 sample, with a larger number of grain boundary nucleation sites of the deformation-induced HCP phase, exhibited a higher HCP transformation rate than the A900-10 sample in the early stage of deformation below 10 % strain, whereas both materials showed similar HCP transformation rates at strain levels higher than 30 %. The emission of deformation-induced HCP plates in both neighboring grains from the same grain boundary (A900-1) and the activation of the deformation-induced HCP phase in a single grain and only dislocations in the neighboring grains (A900-10) suggest the importance of grain boundary stress in the observed phenomena. The observed additional mechanisms other than dislocation slip and deformation-induced HCP phase, that is, reverse FCC-HCP transformation, HCP {101‾2} twinning, and kink banding, could contribute to deformation accommodation and stress relaxation, thereby leading to the excellent ductility of the investigated materials.

Microstructural evolution and duplex structure-enhanced high-temperature mechanical properties of Al-doped VCoNi medium entropy alloy

The microstructures and mechanical properties of 7 at% Al-added VCoNi medium entropy alloy are studied and compared with those of the undoped alloy at various temperatures. Doping Al promotes the formation of a microstructure composed of the FCC matrix and the BCC-L21 phase, which remains stable until the matrix gradually transforms into the HCP-к and tetragonal-σ phases after 600 °C. Compared with the VCoNi alloy, the differences involve that the Al addition suppresses the strain-induced FCC-HCP transformation at 25 °C, and delays the eutectoid reaction at 700 °C. Corresponding to the microstructure evolution, the doped alloy preserves higher strength and strain hardening capacity until 600 °C due to the high-content harder BCC phase but comparable values at 700 and 800 °C with the relatively less HCP phase. Besides, the added Al can also reduce the susceptibility to the serration flow. The results reveal that the (VCoNi)93Al7 alloy has high potentials for intermediate-temperature applications.

Microstructure evolution and deformation behavior during stretching of a compositionally inhomogeneous TWIP-TRIP cantor-like alloy by laser powder deposition

A TWIP-TRIP high-entropy alloy with inhomogeneous chemical composition was prepared by laser powder deposition (LPD). The microstructure and properties of the alloy were quite different from those of compositionally homogeneous alloy with the same composition produced via electric arc melting + hot forging (EAM-HF) technique. During stretching, deformation twinning occurred under low strain, while FCC-HCP transformation was observed under high strain. The yield strength, tensile strength, and elongation were 425 MPa, 715 MPa, and 83%, respectively. For comparison, a sample with the same chemical composition was fabricated by EAM-HF. While twinning was detected under high strain during stretching, no FCC-HCP transformation was observed. Its yield strength, tensile strength, and elongation were found to be 69.6%, 86.9%, and 90% of those of LPD sample, respectively. In order to discuss the evolution of hardening behavior, a constitutive model based on dislocation density evolution was established, taking the contribution from twinning and FCC-HCP transformation into account. Since deformation twinning could be generated at a moderate speed in a wide tensile strain range and FCC-HCP transformation occurred under high strains, the high strain hardening rate could be guaranteed under high strain, thus avoiding strain localization, which endowed the LPD sample with good plasticity. It was shown to be possible to adjust the mechanical behaviors of alloys by increasing their stacking fault energy span through LPD.

Martensitic fcc-hcp transformation pathway in solid krypton and xenon and its effect on their equations of state

The martensitic transformation is a fundamental physical phenomenon at the origin of important industrial applications. However, the underlying microscopic mechanism, which is of critical importance to explain the outstanding mechanical properties of martensitic materials, is still not fully understood. This is because for most martensitic materials the transformation is a fast process that makes in situ studies extremely challenging. Noble solids krypton and xenon undergo a progressive pressure induced fcc to hcp martensitic transition with a very wide coexistence domain. Here, we took advantage of this unique feature to study the detailed mechanism of the transformation by employing in situ X-ray diffraction and absorption. We evidenced a four stages mechanism where the lattice mismatch between the fcc and hcp forms plays a key role in the generation of strain. We also determined precisely the effect of the transformation on the compression behavior of these materials.

Tuning mechanical metastability in FeMnCo medium entropy alloys and a peek into deformable hexagonal close-packed martensite

We report here the compositional dependency of face-centered cubic (FCC) to hexagonal close-packed (HCP) martensitic transformation in FeMnCo medium entropy alloys (MEAs) and insights into the underlying transformation mechanisms. To this end, we designed MEAs with the same Fe-to-Mn ratio and explored the phase stability therein. Higher Co content was found to facilitate the FCC-HCP transformation kinetics. In situ electron backscatter diffraction studies underpinned an FCC-HCP-(new)FCC transformation chain and its underlying atomistic mechanisms were directly explored via aberration-corrected scanning transmission electron microscopy.

Atomistic analyses of HCP-FCC transformation and reorientation of Ti in Al-Ti multilayers

Atomistic simulations have been performed in this study to examine the roles of interface characteristics, layer thickness, and heating rate on microstructural transformation of Al-Ti multilayers. The individual layer thickness varies from 4 nm to 55 nm, and eight different configurations that have different crystallographic orientation relationships are examined. All the multilayers are heated to 300 K with a heating rate varying between 1.5 × 1013 K·s−1 and 3.0 × 1010 K·s−1. No microstructural changes occur for the multilayers having the traditional orientation relationship (OR) (configuration A) observed experimentally. Similarly, the multilayers having other configurations that are defined by different degrees of misorientations (configurations 1–6) from the traditional OR also do not show any microstructural change. The transformation in multilayers having incoherent and high energy interface (configuration B) is heavily influenced by layer thickness and heating rate. The Al layer does not show any transformation for all the considered configurations, layer thicknesses, and heating rates. However, the Ti layer has shown the formation of FCC Ti and reorientation of parent HCP Ti in multilayers with configuration B. The HCP to FCC structural transformation of Ti occurs by creating a series of successive prismatic stacking faults in the Ti layer, and results in a decrease of energy by 4–5 meV/atom for complete transformation and 1–3 meV/atom for partial transformation. The HCP-FCC transformation of Ti results in a volume change in the range of 0.06–0.08 Å3/atom. At larger layer thicknesses, reorientation in the Ti layer, which is equivalent to an effective rotation of parent Ti about [12¯10] direction by 90°, occurs. The reorientation occurs via the formation of an intermediate BCC Ti phase resulting from the coordinated displacement of atoms on alternating (0001) planes in [101¯0] direction, accompanied by almost 10% compression along [101¯0] direction. The intermediate BCC Ti transforms to reoriented HCP Ti by displacements of atoms on alternating (110) planes in [11¯0] direction, accompanied by almost 2% compression along [110] direction. Reoriented Ti is less strained as compared to parent HCP Ti because of the change in the c/a ratio.

Atomic structure evolutions and mechanisms of the crystallization pathway of liquid Al during rapid cooling

The solidification of pure aluminum has been studied by a large-scale molecular dynamic simulation. The potential energy, position D, height H, and width W of the first peak and valley of PDF curves, and the local structures were investigated. It was found that the FCC-crystallization ability of pure Al is so strong that still local crystal regions exist in the amorphized solid. As the temperature decreases, besides the counter-intuitive increase in Dp (D of the first peak), Hp increases monotonically; Wp, Dv, and Hv decrease monotonically; only Wv first decreases and then increases. They all change critically when phase transition happens. After the nucleation, orientation-disordered HCP-regions, as the grain boundaries or defects of FCC crystals, rapidly transform into FCC structures, and then the surviving HCP-regions regularize into few parallel layers or orientation-disordered HCP-regions. If parallel layers result in dislocation pinning, structural evolution terminates; otherwise, it continues. These findings will have a positive impact on the development of the solidification and nucleation theory.

Estimation of the chemical compositions and corresponding microstructures of AgInCd absorber under irradiation condition

Abstract AgInCd alloy is widely used as neutron absorber in nuclear reactors. However, the AgInCd control rods may fail during service due to the irradiation swelling. In the present study, a calculational method is proposed to calculate the composition change of the AgInCd absorber. Calculated results show that neutron fluence has significant impact on the chemical compositions. Ag and In contents gradually decrease while Cd and Sn conversely increases from the center to the rim of AgInCd absorber due to the depression of neutron flux. The composition change at the surface is higher almost two times than that at the center. Based on the calculated compositions, six simulated AgInCdSn alloys were prepared and examined. With the increase of Cd and Sn, the simulated AgInCdSn alloys transform from a single fcc phase into the mixed fcc and hcp phases, and finally into the single hcp phase. The atomic volume of the hcp phase is obviously larger than the fcc phase. The fcc-hcp transformation results in considerable volume swelling of the AgInCd absorber. Moreover, the lattice parameters of the fcc and hcp phases gradually increase with Cd and Sn contents, which also can induce small volume swelling.

Resolving the FCC/HCP interfaces of the [formula omitted] ([formula omitted]) precipitate phase in aluminium

The γ' (Ag2Al) phase in the Al–Ag alloy system has served as a textbook example for understanding phase transformations, precipitating hexagonal close-packed (HCP) crystals in the face-centred cubic (FCC) aluminium matrix. The γ' precipitates display fully coherent interfaces at their broad facets and semicoherent interfaces at their edges. Shockley partial dislocations are expected to decorate the semicoherent interface due to the FCC-HCP structural transformation. Determining the exact locations and core structures of interfacial dislocations, however, remains challenging. In this study, we used aberration-corrected scanning transmission electron microscopy and atomistic simulations to re-visit this classical system. We characterised and explained the Ag segregation at coherent interfaces in the early stage of precipitation. For semicoherent interfaces, interfacial dislocations and reconstructions were revealed by bridging advanced microstructure characterisation and atomistic simulations. In particular, we discovered a new FCC/HCP interfacial structure that displays a unique combination of Shockley partial, Lomer-Cottrell and Hirth dislocations that evolve from the known interfacial structure purely composed by Shockley partial dislocations. Our findings show that the FCC-HCP transformation is more complex than hitherto considered, due to the interplay between structure and composition confined at interfaces.

Experimental determination of the driving force of the fcc-hcp martensitic transformation and the stacking fault energy in high-Mn Fe-Mn-Cr steels

By the use of several experimental techniques the difference in the Gibbs free energy between fcc austenite and hcp martensite has been determined for a wide range of compositions of Fe-Mn-Cr alloys where the martensitic transition takes place. Martensitic transformation temperatures were determined by dilatometry measurements, while the lattice parameters of both phases and the volume change between them were determined by X-ray diffraction. Combining these results with dilatometry and differential scanning calorimetry measurements, the amount of martensite and the heat exchanged during transformation were obtained. These values allowed obtaining the enthalpy change associated to the fcc-hcp transformation and, following thermodynamic criteria, the Gibbs free energy change (ΔG), leading to absolute values in the range from 165 J/mol up to 240 J/mol for the analyzed compositions. These values are high enough to overcome the known resistances to the transformation, like the strain energy of the transformation and the surface energy. The measured parameters enable obtaining the stacking fault energy which is also discussed in the present framework.

Interstitial triggered grain boundary embrittlement of Al–X (X = H, N and O)

With the quick development of high-speed railway and the service of the series of CRH (China Rails High-speed) for almost a decade, one of the greatest challenges is the managements/maintenances of those trains in environmental conditions. It is critical to estimate the intergranular corrosion damage initiations and propagations, thus, to setup the relative database in order to support foundations for an interactive corrosion risk management. In this work, ∑301111¯1 coherent twin boundary (CTB) of FCC Al–X is utilized to reveal the interactions between interstitials (X = H, N and O) and CTB together with their embrittlement mechanism. The HCP-type fault layers within the CTB are same as those fault layers in growth fault, indicating the local FCC-HCP transformation at CTB and revealing the physical foundation for those criteria designing growth twins. The notable accumulation of bonding electrons of interstitial atoms occupying either T-site or O-site yields a dramatically reduction of Al–X and Al–Al bonding charge density and bonding strength, revealing the embrittlement mechanism of Al CTB.

Solidification dynamics and microstructure evolution in nanocrystalline cobalt

Solidification and microstructures of fast quenched cobalt (from 1950K to 600K) are studied using molecular dynamics simulations. Two types of microstructures, a nanocrystalline type consisting of multiple grains and a lamellar type are obtained. The lamellar type microstructure shows a dominant layering direction with fcc phases separated by stacking faults, coherent twin boundaries or hcp phases. The nanocrystalline type is dominant at fast quenching rates. Both microstructures are highly twinned. Twins can originate either from ab initio faults of several morphologies in the nuclei or from stacking accidents in 〈111〉 directions during growth. It is supported that fivefold twins form by a successive growth of the fcc subunits, but not by an equilateral layer-by-layer growth from a five-folded symmetric core that exists at the very beginning of nucleation. Annealing of the as-quenched samples above the phase transformation temperature leads to gradual growth of the fcc phase and local reversible fcc-hcp transformation facilitated by existing Shockley dislocations. Under uniaxial tensile loading, the nanocrystalline samples show an early initial yielding, followed by continuous work hardening, reaching an ultimate tensile strength of about 3000 MPa. The lamellar samples, in contrast, show a much more intermittent plastic flow. The strongest sample has a nanotwinned structure resembling TEM images of vapor deposited films of fcc metals. The ultimate tensile strength is over 5000 MPa, assisted by coherent twin boundaries serving as strong obstacles to dislocation motion.

Influence of deformation induced nanoscale twinning and FCC-HCP transformation on hardening and texture development in medium-entropy CrCoNi alloy

Texture evolution during room-temperature tensile testing of recrystallized equimolar CrCoNi was studied using electron backscatter diffraction and electron channeling contrast imaging on specimens from interrupted tests. Dominant deformation mechanisms included slip at low strains and deformation twinning at larger strains, which were accompanied by the development of a strong <111> texture parallel to the tensile axis. Highly deformed material also contained nanotwin/hcp lamellae, which have previously been hypothesized to act as potent barriers for non-coplanar dislocations. To examine this hypothesis, mean-field modeling was performed using the viscoplastic self-consistent framework with varying ratios for hardening by slip and twinning. In the optimal model, twinning produced approximately three times as much non-coplanar hardening as slip, which is larger than previous observations in other twinning-induced plasticity materials that do not form twin/hcp lamellae. Additional full-field elasto-viscoplastic simulations were performed using the fast Fourier transform (EVP-FFT) method to examine intragranular rotation and the effect of initial grain orientation on the deformation mode. Grains with initial orientations near <111> had the greatest propensity for deformation twinning while grains near <100> were more likely to deform by slip even at large strains. Excellent quantitative agreement was obtained between the experiments and EVP-FFT model.

Effect of the fcc-hcp martensitic transition on the equation of state of solid krypton up to 140 GPa

Solid krypton (Kr) undergoes a pressure-induced martensitic phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure. These two phases coexist in a very wide pressure domain inducing important modifications of the bulk properties of the resulting mixed phase system. Here, we report a detailed in situ x-ray diffraction and absorption study of the influence of the fcc-hcp phase transition on the compression behavior of solid krypton in an extended pressure domain up to 140 GPa. The onset of the hcp-fcc transformation was observed in this study at around 2.7 GPa and the coexistence of these two phases up to 140 GPa, the maximum investigated pressure. The appearance of the hcp phase is also evidenced by the pressure-induced broadening and splitting of the first peak in the XANES spectra. We demonstrate that the transition is driven by a continuous nucleation and intergrowth of nanometric hcp stacking faults that evolve in the fcc phase. These hcp stacking faults are unaffected by high-temperature annealing, suggesting that plastic deformation is not at their origin. The apparent small Gibbs free-energy differences between the two structures that decrease upon compression may explain the nucleation of hcp stacking faults and the large coexistence domain of fcc and hcp krypton. We observe a clear anomaly in the equation of state of the fcc solid at \ensuremath{\sim}20 GPa when the proportion of the hcp form reaches $\ensuremath{\sim}20%$. We demonstrate that this anomaly is related to the difference in stiffness between the fcc and hcp phases and propose two distinct equation of states for the low and high-pressure regimes.

Structure transformation of Ti films deposited on SiC single crystal substrates

This paper reports the structure transformation of Ti films deposited on SiC(0001) single crystal substrates using DC magnetron sputtering. Film thickness, sputter power and temperature of deposition were changed to investigate structure transformation of Ti films. The structure characterization of Ti film was performed by means of X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscope (HRTEM). The results showed that the Ti film grew epitaxially with a face centered cubic (fcc) structure even the thickness is up to about 50nm. High temperature and low sputter power are propitious to the formation of fcc-Ti. An interesting intermediate state of the fcc-hcp transformation was observed. This intermediate structure could help us to understand the mechanism of thickness-dependent fcc-hcp transformation in Ti thin films.

Service induced fcc→hcp martensitic transformation in a Co-based superalloy

Co-based superalloy, ECY768, applied on gas turbine vanes, shows the presence of cracks after service. EBSD studies revealed hcp transformation in the base material near cracked regions. This phase arises from a martensitic transformation of fcc matrix and bestows high fragility. The phase transformation is related to temperature and loading distribution that characterises components in service. However, at the service temperatures, the hcp transformation is not expected for ECY768. An in-house thermodynamic database was developed using the Calphad approach and thermodynamic calculations were applied to the complicated alloy composition for phase stability range evaluation. Moreover, a testing campaign was planned to artificially create this martensitic transformation and to comprehend the influence of plastic strain on fcc-hcp transformation. The transformation mechanisms were understood and some methods were developed for hcp-phase removal through refurbishment heat treatment.This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.

Homogeneous hydride formation path in α-Zr: Molecular dynamics simulations with the charge-optimized many-body potential

A path for homogeneous γ hydride formation in hcp α-Zr, from solid solution to the ζ and then the γ hydride, was demonstrated using molecular static calculations and molecular dynamic simulations with the charge-optimized many-body (COMB) potential. Hydrogen has limited solubility in α-Zr. Once the solubility limit is exceeded, the stability of solid solution gives way to that of coherent hydride phases such as the ζ hydride by planar precipitation of hydrogen. At finite temperatures, the ζ hydride goes through a partial hcp-fcc transformation via 13〈11¯00〉 slip on the basal plane, and transforms into a mixture of γ hydride and α-Zr. In the ζ hydride, slip on the basal plane is favored thermodynamically with negligible barrier, and is therefore feasible at finite temperatures without mechanical loading. The transformation process involves slips of three equivalent shear partials, in contrast to that proposed in the literature where only a single shear partial was involved. The adoption of multiple slip partials minimizes the macroscopic shape change of embedded hydride clusters and the shear strain accumulation in the matrix, and thus reduces the overall barrier needed for homogeneous γ hydride formation. This formation path requires finite temperatures for hydrogen diffusion without mechanical loading. Therefore, it should be effective at the cladding operating conditions.

Lattice distortion induced anomalous ferromagnetism and electronic structure in FCC Fe and Fe-TM (TM = Cr, Ni, Ta and Zr) alloys

Magnetic properties of BCC, FCC and HCP Fe and the effects of the ∑3<01¯1>{111} grain boundary (GB) and the alloying elements of Cr, Ni, Ta and Zr are investigated by first-principles calculations. The FCC Fe changes continuously from the non-magnetic state at low volumes to the ferromagnetic state at high volumes. It is observed that Σ3{111} type GBs in FCC Fe have a negative formation energy since the formation of Σ3 GB is attributed to the local FCC–HCP transformation. Moreover, consistent with the variation of equilibrium volume caused by TM solute atoms, the magnetic moment of FCC Fe70TM2 is decreased with alloying Ni while increased with alloying Cr, Ta and Zr. Due to the difference in the valence electrons and atomic radius among those solute atoms, the chemical and mechanical effects on the bond structure of Σ3{111} GB in Fe and Fe70TM2 are respectively characterized by deformation electron density and plots of spin alignments. It is understood that the variation of the spin state of Fe70TM20 is dominated by the electron redistributions, as illustrated by the spin-flipping and the change of the bond structure. This work provides an insight into the effect of lattice distortion on ferromagnetism of FCC Fe and Fe70TM2

Microstructure investigations of hcp phase CoPt thin films with high coercivity

CoPt films have been grown in the past with a high anisotropy in L11 or L10 phase, and a high coercivity is observed only in L10 CoPt films. Recently, we have grown CoPt films which exhibited a high coercivity without exhibiting an ordered phase. In this study, high resolution transmission electron microscopy (HRTEM) investigations have been carried out to understand the strong thickness and deposition pressure dependent magnetic properties. HRTEM studies revealed the formation of an initial growth layer in a metastable hexagonal (hcp) CoPt with high anisotropy. This phase is believed to be aided by the heteroepitaxial growth on Ru as well as the formation of Ru-doped CoPt phase. As the films grew thicker, transformation from hcp phase to an energetically favourable face-centered cubic (fcc) phase was observed. Stacking faults were found predominantly at the hcp-fcc transformation region of the CoPt film. The higher coercivity of thinner CoPt film is attributed to relatively less fcc fraction, less stacking faults, and to the isolated grain structure of these films compared to the thicker films.

Phase Transformation Kinetics: Advanced Modeling Strategies

Phase transformations in the solid state are often heterogeneous and can be described by concurring modes of nucleation, growth, and impingement. The classical Johnson–Mehl–Avrami–Kolmogorov-(JMAK-) model, although offering an easy-to-use description of the transformation kinetics, is limited to very specific cases of the transformation modes. Instead, a generalized modular model of phase transformation kinetics can be proposed that provides a flexible formalism adaptable to various modes of nucleation, growth, and impingement. Due to its large versatility, the modular model approach can be easily applied for characterization of phase transformation kinetics beyond the scope of classical JMAK(-type) modeling. Three different strategies recently employed for such advanced modeling are presented: (I) deliberate variation of the nucleation mode upon crystallization of an Fe-Ni-B metallic glass in order to determine separate activation energies for nucleation and growth, (II) incorporation of specific, dedicated modes for nucleation and growth kinetics for the allotropic hcp–fcc transformation in cobalt introducing driving-force-dependent rates of transformation, and (III) implementation of quantitative microstructural data for the description of the precipitation kinetics in a supersaturated CuCo alloy.