Hcp-fcc Phase Transition Research Articles

Dynamic mechanical behavior of ultra-high specific strength lightweight Ti61Al16Cr10Nb8V5 multi-principal element alloy

This study investigated the dynamic mechanical property and microstructure evolution of the Ti61Al16Cr10Nb8V5 (Ti61) lightweight multi-principal element alloy(MPEA) with a density of 4.83 g/cm3. After melting and rolling, the Ti61 alloy was composed of a BCC matrix phase and dispersed B2 phase, and also contained a small amount of fine needle-like FCC phase and spherical HCP phase. Mechanical testing of Ti61 alloy revealed that the ultimate tensile strength is about 1223 MPa, and the tensile elongation is about 13.2 %, with a specific strength of around 253 MPa*cm3/g. Dynamic compression testing of Ti61 alloy showed a significant strain rate strengthening effect and excellent fracture strain greater than 50 %. Calculation and microstructural observations indicated that at low strain rates ranging from 10−3/s to 10/s, the strain rate sensitivity of Ti61 alloy was 0.0087, with the main plastic deformation mechanism of dislocation nucleation, dislocation slip, and stacking faults. At high strain rates ranging from 500/s to 5000/s, the strain rate sensitivity raises to 0.0963 and the main deformation mechanism of the Ti61 alloy involved higher-density dislocation nucleation, dislocation slip, stacking faults and twinning. At strain rates of 5000/s, HCP-FCC phase transition accompanied by FCC microtwins and BCC-α2HCP phase transition occurred. Furthermore, dynamic recrystallization also occurred in Ti61 alloy. The HCP-FCC has the orientation of (0001)HCP//(11−1)FCC, [1−210]HCP//[011]FCC. The BCC-α2HCP has the orientation of {101}BCC//{0001}HCP and [111]BCC//[2−1−10]HCP. Twining and phase transition strengthen the yield stress of Ti61 alloy, while dynamic recrystallization reduced the flow stress of Ti61 alloy. Ti61 alloy has high strength, high compression fracture strain, and ultra-high specific strength, which is a new LWMPEA with great application potential in military armor.

Tri-functional carbon nanocages coated CoNi electrocatalyst with micro twinning structure for high-performance electrochemical devices

The twinning structure could lead to unexpected effects on electrocatalytic performance; however, research on relevant mechanisms is still rare and there is still a lack of an effective and convenient strategy to introduce twinning structures into nanoparticles. In this work, inspired by the in-situ fcc-hcp phase transition in metals and benefiting from the features of Prussian blue analogues (PBAs), N-doped carbon cages-coated CoNi nanoparticles, which contain multi-active sites including alloyed nanoparticles, micro twinning structures and single atom sites could be effectively fabricated by pyrolyzing of CoNi-based PBAs. The catalyst exhibits an obvious enhancement in oxygen evolution/reduction reaction and hydrogen evolution reaction under alkaline conditions. Theoretical calculations verify that the micro-twinning structures originating from the fcc-hcp phase transition promote electrocatalytic activities significantly due to the optimized adsorption energy towards different intermediates. Moreover, the synergic effect between the multi-active sites may further optimize the electronic structures and lead to the deviation from the Scaling Effect. The water-splitting device and Zinc-Air battery are also assembled based on this micro- twinning electrocatalyst, which yields a maximum power density of 95 mW cm−2 and a low voltage of 1.76 V to reach 10 mA cm−2 current density, respectively. In all, this work contributes a simple and universal strategy to introduce both the multi-active sites and the twinning structure into transition metal-based catalysts, which improves the multi-catalytic activity towards different reactions significantly.

Significant strength enhancement of high-entropy alloy via phase engineering and lattice distortion

The extraordinary mechanical properties of high-entropy alloys (HEAs) hold great promise for their applications under extreme conditions (high pressure, high temperature, etc.). However, investigations on the mechanical properties of HEAs under extreme conditions (especially at high pressures) are extremely scarce, preventing their future applications. In this study, significant strength enhancement of Cantor (CoCrFeMnNi) HEA is achieved via phase engineering by using high-pressure techniques. The pressure-induced fcc-hcp phase transition of Cantor HEA leads to a strength increase of 65 % across the phase transition. Consequently, the hcp-Cantor HEA behaves as strongly as typical superhard ceramic B4C and surpasses corundum at moderate pressures. Moreover, the hcp-Cantor HEA quenched to ambient pressure is 47 % harder than the pristine fcc phase. In addition, by substituting the Mn atom with the Pd atom to introduce a severe lattice distortion in Cantor HEA, the CoCrFePdNi HEA turns out to be almost twice harder than Cantor HEAs at ambient pressure. The excellent mechanical properties of HEAs promise their potential applications at high pressures. These results also demonstrate that phase engineering and the introduction of severe lattice distortion represent two promising methods of developing high-strength HEAs in the future.

In Situ Atomic-Scale Quantitative Evidence of Plastic Activities Resulting in Reparable Deformation in Ultrasmall-Sized Ag Nanocrystals.

Permanent structural changes in pure metals that are caused by plastic activity are normally irreparable after unloading. Because of the lack of experimental evidence, it is unclear whether the plastic activity can be repaired as the size of the pure metals decreases to several nanometers; it is also unclear how the metals accommodate the plastic deformation. In this study, the in situ atomic-scale loading and unloading of ∼2 nm Ag nanocrystals was investigated, and three modes of plastic deformation were observed: (i) the phase transition from the face-centered cubic (fcc) phase to the hexagonal close-packed (hcp) phase, (ii) stacking faults, and (iii) deformation twin nucleation. We show that all three modes resulted in structural changes that were reparable, and their generation and restoration during loading and unloading were observed in situ. We discovered that the deformation modes of nanosized metals can be predicted from the ratio of the energy barriers of the fcc-hcp phase transition (ΔγH) and the deformation twin nucleation (ΔγT), which differ from those of the theoretical modes of relatively large-sized metals. The proposed ΔγH/ΔγT criterion provides insights into the deformation mechanism of nanometals.

Chemical inhomogeneity–induced profuse nanotwinning and phase transformation in AuCu nanowires

Nanosized metals usually exhibit ultrahigh strength but suffer from low homogeneous plasticity. The origin of a strength–ductility trade-off has been well studied for pure metals, but not for random solid solution (RSS) alloys. How RSS alloys accommodate plasticity and whether they can achieve synergy between high strength and superplasticity has remained unresolved. Here, we show that face-centered cubic (FCC) RSS AuCu alloy nanowires (NWs) exhibit superplasticity of ~260% and ultrahigh strength of ~6 GPa, overcoming the trade-off between strength and ductility. These excellent properties originate from profuse hexagonal close-packed (HCP) phase generation (2H and 4H phases), recurrence of reversible FCC-HCP phase transition, and zigzag-like nanotwin generation, which has rarely been reported before. Such a mechanism stems from the inherent chemical inhomogeneity, which leads to widely distributed and overlapping energy barriers for the concurrent activation of multiple plasticity mechanisms. This naturally implies a similar deformation behavior for other highly concentrated solid-solution alloys with multiple principal elements, such as high/medium-entropy alloys. Our findings shed light on the effect of chemical inhomogeneity on the plastic deformation mechanism of solid-solution alloys.

Temperature and Crystalline Orientation-Dependent Plastic Deformation of FeNiCrCoMn High-Entropy Alloy by Molecular Dynamics Simulation

The effect of the crystallographic direction and temperature on the mechanical properties of an FeNiCrCoMn high-entropy alloy (HEA) is explored by molecular dynamics simulations. The calculated static properties are in agreement with the respective experimental/early theoretical results. The calculated compressive yield stress along the <010> direction of a single crystal/polycrystal is the same in order of magnitude as the experimental results. The yield stress and Young’s modulus of the single crystal show strong anisotropy. Unlike the single crystal, the polycrystal behaves as an isotropic and has strong ductility. It is found that the dislocations produced in the plastic deformation process of the HEA are mainly 1/6<112> Shockley dislocations. The dislocations produced under normal stress loads are far more than that in the shearing process. FCC transformation into HCP does not occur almost until yield stress appears. The yield stress, yield strain, and Young’s modulus reduce gradually with increasing temperature. The modulus of the single/double crystal under compressive and tensile loads presents an obvious asymmetry, while there is only a small difference in the polycrystal. The strain point is found to be the same for stress yielding, FCC-HCP phase transition, and dislocation density, varying from slow to fast with strain at the considered temperature.

Study on Phase Transformation Orientation Relationship of HCP-FCC during Rolling of High Purity Titanium

High purity titanium (Ti) thin strip was prepared by rolling with large deformation and was characterized by the means of Transmission Electron Microscopy (TEM), selected area diffraction (SAED) pattern, high-resolution (HRTEM) analysis, as well as Transmission Kikuchi Diffraction (TKD). It is found that there are face-centered cubic (FCC) Ti laths formed within the matrix of hexagonal close packing (HCP) Ti. This shows that the HCP-FCC phase transition occurred during the rolling, and a specific orientation relationship (OR) between HCP phase and FCC phase obeys ⟨0001⟩α// ⟨001⟩FCC and {100}α//{110}FCC. The ORs of HCP-FCC phase transition are deeply studied by TKD pole figure and phase transformation matrix. It is found that the derived results via pole figure and transformation matrix are equivalent, and are consistent with TEM-SAED analysis results, which proves that these two methods can effectively characterize the ORs of HCP-FCC phase transition and predict possible FCC phase variants.

A Short Review on the Effect of Cr on the fcc–hcp Phase Transition in Fe–Mn-Based Alloys

The effect of Cr on the fcc–hcp martensitic transformation in the Fe–Mn–Cr system has been discussed considering different aspects: (a) the relative phase stabilities, (b) the magnetic order of the fcc phase, (c) the structural parameters and volume change between fcc and hcp, (d) the driving force of the martensitic transformation and relevant thermodynamics quantities, (e) the thermal cycling behavior, and (f) the pseudoelastic effect. Particularly, in this work it has been found that when Cr content increases, the effect of cycling on the energy barrier decreases. This may be explained by a small volume change, which could lead to a slighter introduction of plastic deformation during thermal cycling through the martensitic transition.

Superior strength-ductility CoCrNi medium-entropy alloy wire

High strength-ductility is the prerequisite requirement for the widely used steel wires in engineering applications. Traditional strengthening strategies on these wires are suffocated inevitably by strength-ductility trade-off dilemma. Recent emerging medium-entropy alloys (MEAs) usually exhibits excellent ductility but relatively lower strength. In this paper, a novel CoCrNi MEA wire with superior mechanical properties was successfully fabricated by heavily drawing process. The yield strength, ultimate tensile strength and elongation could reach 1.5 GPa, 1.8 GPa and 37.4% at liquid-nitrogen temperature, respectively. In-depth microstructure characterization indicates this superior strength-ductility derives from the synergy of dislocations, high-density twins and clear FCC-HCP phase transition.

Superior Strength-Ductility Cocrni Medium-Entropy Alloy Wire

High strength-ductility is the prerequisite requirement for the widely used steel wires in engineering applications. Traditional strengthening strategies on these wires are suffocated inevitably by strength-ductility trade-off dilemma. Recent emerging medium-entropy alloys (MEAs) usually exhibits excellent ductility but relatively lower strength. In this paper, a novel CoCrNi MEA wire with superior mechanical properties was successfully fabricated by heavily drawing process. The yield strength, ultimate tensile strength and elongation could reach 1.5 GPa, 1.8 GPa and 37.4% at liquid-nitrogen temperature, respectively. In-depth microstructure characterization indicates this superior strength-ductility derives from the synergy of dislocations, high-density twins and clear FCC-HCP phase transition.

Structural studies of plastically deformed cobalt

This paper is devoted to the study of the fcc-hcp phase transition delay during cooling in nanocrystalline cobalt obtained by severe plastic deformation. Using the methods of X-ray and electron microscopy, it was revealed that this delay in the phase transition is due to the small size of the crystallites and the stressed state of the crystal lattice.

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.

Carbonaceous nickel oxide composite films with homogenously dispersed metallic Ni nanoparticles by a facile sol–gel method

A composite of Ni/C/NiO thin films, in which the metallic nickel nanoparticles (MNNs) are homogeneously dispersed, has been prepared by a facile one-step sol–gel technique as a solar-selective absorber (SSA). Comprehensive analysis of the as-prepared composite films has been carried out employing various characterization techniques. The results reveal that both metallic Ni and NiO nanoparticles are surrounded by the resident carbon and the MNNs emerge from the NiO precursor reduced by the resident carbon. The size of the homogeneously dispersed MNNs increases with the annealing temperature from ~4 to ~7 nm. A HCP–FCC phase transition of MNNs was found at about 400–450 °C, while the contents of Ni and Ni2+ are almost equal at the same time. The solar-selective absorptance properties of Ni/C/NiO composite films were studied by measuring the reflectance in the UV-Vis and infrared wavelength range. The results indicate that the smaller MNNs and the thinner films attenuate the absorptance but enhance the emittance. The one-layer Ni/C/NiO composite film on copper substrate annealed at 450 °C is preferred in the present work, exhibiting an absorptance of about 0.86 and an emittance of about 0.11, and capable of a good candidate as SSA for the low–middle temperature applications.

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.

Thermal Stability of Co-Pt and Co-Au Core-Shell Structured Nanoparticles: Insights from Molecular Dynamics Simulations.

Co-Pt and Co-Au core-shell nanoparticles were heated by molecular dynamics simulations to investigate their thermal stability. Two core structures, that is, hcp Co and fcc Co, have been addressed. The results demonstrate that the hcp-fcc phase transition happens in the hcp-Co-core/fcc-Pt-shell nanoparticle, while it is absent in the hcp-Co-core/fcc-Au-shell one. The stacking faults appear in both Pt and Au shells despite different structures of the Co core. The Co core and Pt shell concurrently melt and present an identical melting point in both Co-Pt core-shell nanoparticles. However, typical two-stage melting occurs in both Co-Au core-shell nanoparticles. Furthermore, the Au shell in the hcp-Co-core/fcc-Au-shell nanoparticle exhibits a lower melting point than that in the fcc-Co-core/fcc-Au-shell one, while the melting points are closely equal for both hcp and fcc Co cores. All of these observations suggest that their thermal stability strongly depends on the structure of the core and the element of the shell.

First Principles Theory of the hcp-fcc Phase Transition in Cobalt

Identifying the forces that drive a phase transition is always challenging. The hcp-fcc phase transition that occurs in cobalt at ~700 K has not yet been fully understood, although early theoretical studies have suggested that magnetism plays a main role in the stabilization of the fcc phase at high temperatures. Here, we perform a first principles study of the free energies of these two phases, which we break into contributions arising from the vibration of the lattice, electronic and magnetic systems and volume expansion. Our analysis of the energy of the phases shows that magnetic effects alone cannot drive the fcc-hcp transition in Co and that the largest contribution to the stabilization of the fcc phase comes from the vibration of the ionic lattice. By including all the contributions to the free energy considered here we obtain a theoretical transition temperature of 825 K.

Calculated pressure induced fcc-hcp phase transition in aluminum

Calculated pressure induced fcc-hcp phase transition in aluminum

In situ X-ray diffraction measurements of the fcc–hcp phase transition boundary of an Fe–Ni alloy in an internally heated diamond anvil cell

The phase transition boundary between the face-centered cubic (fcc) structure and hexagonal close-packed (hcp) structure in an Fe–Ni alloy was determined at pressures from 25 to 107 GPa by using an internally resistive-heated diamond anvil cell (DAC), combined with in situ synchrotron X-ray diffraction measurements. The fcc–hcp phase transition boundary in Fe–9.7 wt% Ni is located at slightly lower temperatures than that in pure Fe, confirming the previous understanding that the addition of Ni expands the stability field of the fcc phase. The dP/dT slope of the boundary was determined to be 0.0426 GPa/K, which is slightly larger than that of pure Fe. The pressure interval of the two-phase region is about 6 GPa at a constant temperature, implying that the previous estimates by laser-heated DAC experiments of 10–20 GPa were overestimated. The two-phase region of fcc + hcp would be limited to a pressure of about 120 GPa even in Fe–15 wt%Ni, excluding the possibility of the existence of the fcc phase in the inner core if the simple linear extrapolation of the two-phase region is applied. The pressure and temperature dependences of the c/a axial ratio of the hcp phase in Fe–9.7 wt% Ni are generally consistent with those in pure Fe, suggesting that Ni has minor effects on the c/a ratio.

Melting of Fe–Ni–Si and Fe–Ni–S alloys at megabar pressures: implications for the core–mantle boundary temperature

High pressure melting behavior of three Fe-alloys containing 5 wt% Ni and (1) 10 wt% Si, (2) 15 wt% Si or (3) 12 wt% S was investigated up to megabar pressures by in situ X-ray diffraction and laser-heated diamond anvil cell techniques. We observe a decrease in melting temperature with increasing Si content over the entire investigated pressure range. This trend is used to discuss the melting curve of pure Fe. Moreover, our measurements of eutectic melting in the Fe–Fe3S system show a change in slope around 50 GPa concomitant with the fcc–hcp phase transition in pure solid iron. Extrapolations of our melting curve up to the core–mantle boundary pressure yield values of 3,600–3,750 K for the freezing temperature of plausible outer core compositions.

To the theory of isotope effect in the thermodynamics of “Classical” crystals

Based on general expressions for quantum contributions to the free energy of a statistical system, an approach is developed making it possible to carry out a complete theoretical analysis of the equilibrium thermodynamic characteristics of argon-type crystals as a function of the isotopic mass. The theory is based on independently calculated (in fact, without fitting parameters) one-particle and two-particle distribution functions of the classical crystal and permits one to quantitatively describe the data on the lattice parameters of isotope crystals obtained from numerical experiments. It is pointed out that real experiments are needed to study the isotope effect in crystals of noble gases, especially in view of the fact that, according to the literature data, xenon undergoes an fcc-hcp phase transition under increased pressure.