Onset Of Phase Transformation Research Articles

Stacking faults along the {111} planes seed pressure-induced phase transformation in single crystal silicon

We explore the onset of phase transformation, at the nanoscale, in single-crystal diamond-cubic silicon (dc-Si) subjected to pressures of 13 GPa using a diamond anvil cell with a methanol-ethanol pressure medium. Transmission electron microscopy reveals two distinct structural features along {111} planes: (1) thin bands of defective dc-Si and (2) thicker bands of body-centered cubic silicon (bc8), surrounded by defective dc-Si. We propose that these features are consistent with shear bands that have been formed by slip along the low energy {111} planes and have a range of thicknesses depending on how much plastic deformation has occurred. The presence of bc8-Si within the thicker bands can be explained by localized regions of high pressure or energy at their center facilitating phase transformation to the metastable metallic β-Sn phase, which in turn, transforms to bc8 on pressure release. Our observations reveal that phase formation in silicon can be shear-activated, the transformation is not nucleation-limited, and its sluggish nature may be due to the slow growth of the metallic phase.

Revealing the Pressure-Induced Phase Transformation of Xenotime TbPO4 via In Situ Photoluminescence Spectroscopy.

The pressure-induced phase transformations of certain rare earth (RE) orthophosphates have attracted broad interest from geoscience to structural ceramics. Studying these transformations has required in situ Raman spectroscopy or synchrotron X-ray diffraction (XRD), each of which suffers from poor signal or limited accessibility, respectively. This study exploits the photoluminescence (PL) of Tb3+ ions and the unique sensitivity of PL to the local bonding environment to interrogate the symmetry-reducing xenotime-monazite phase transformation of TbPO4. At pressures consistent with the XRD-based phase transformation onset pressure of 8.7(6) GPa, PL spectra show new peaks emerging as well as trend changes in the centroids and intensity ratios of certain PL bands. Furthermore, PL spectra of recovered samples show transformation is irreversible. Hysteresis in certain PL band intensity ratios also reveals the stress history in TbPO4. This in situ PL approach can be applied to probe pressure-induced transformations and crystal field distortions in other RE-based oxide compounds.

Laser-driven ramp-compression experiments on the national ignition facility

This report details the analyses and related uncertainties in measuring longitudinal-stress–density paths in indirect laser-driven ramp equation-of-state (EOS) experiments [Smith et al., Nat. Astron. 2(6), 452–458 (2018); Smith et al., Nature 511(7509), 330–333 (2014); Fratanduono et al., Science 372(6546), 1063–1068 (2021); and Fratanduono et al., Phys. Rev. Lett. 124(1), 015701 (2020)]. Experiments were conducted at the National Ignition Facility (NIF) located at the Lawrence Livermore National Laboratory. The NIF can deliver up to 2 MJ of laser energy over 30 ns and provide the necessary laser power and control to ramp compress materials to TPa pressures (1 TPa = 10 × 106 atmospheres). These data provide low-temperature solid-state EOS data relevant to the extreme conditions found in the deep interiors of giant planets. In these experiments, multi-stepped samples with thicknesses in the range of 40–120 µm experience an initial shock compression followed by a time-dependent ramp compression to peak pressure. Interface velocity measurements from each thickness combine to place a constraint on the Lagrangian sound speed as a function of particle velocity, which in turn allows for the determination of a continuous stress–density path to high levels of compressibility. In this report, we present a detailed description of the experimental techniques and measurement uncertainties and describe how these uncertainties combine to place a final uncertainty in both stress and density. We address the effects of time-dependent deformation and the sensitivity of ramp EOS techniques to the onset of phase transformations.

Mechanical Behavior of Shape-Memory Alloy Triply Periodic Minimal Surface Foam Based on Schwarz Primitive

This work investigated the effective structural and functional behavior of shape-memory alloy (SMA)-based triply periodic minimal surface foams based on the Schwarz primitive (P-foams) using finite-element analysis (FEA) and numerical homogenization methods. The effect of relative density and applied temperature on the homogenized mechanical behavior of the SMA foam, including its superelasticity, and the evolution of the effective martensite volume fraction with the applied load was investigated considering axial and shear loading cases. In contrast to dense SMA, the effective martensite volume fraction in the considered foam was found to vary exponentially with the strain in the case of monotonic loading, asymptotically approaching 1 as the strain increased indefinitely. Moreover, the effective superelasticity of the SMA P-foam was found to be facilitated by decreased temperature and relative density. The onset of phase transformation for the P-foam under various loading scenarios was shown to be well approximated using an extended Hill loading surface.

Effective phase transformation behavior of NiTi triply periodic minimal surface architectures

The effective behavior of shape memory alloy triply periodic minimal surface (TPMS) structures is investigated using finite element analysis and numerical homogenization. The TPMS unit cells considered are primitive, IW-P, gyroid, and diamond subjected to different loading conditions. Under uniaxial displacement-driven loading, the results show a dramatic increase in effective stress and martensite volume fraction with increased relative density of the TPMS unit cell. Comparison among the four types of TPMS unit cells shows that diamond has superior mechanical performance for the loading cases considered. Based on numerical homogenization results, the onset and subsequent thresholds of phase transformation are determined for all four unit cells considering multiaxial loading. At lower relative density, the loading surfaces corresponding to the onset of phase transformation were reasonably well represented by either von Mises or Hill’s criteria. The observed fit with the von Mises model degenerated with increased effective martensite volume fraction, while a proper fit was maintained with Hill’s criterion. Subsequent loading surfaces, corresponding to monotonically increasing martensite volume fraction, show a nonlinear hardening behavior, which seems to follow a similar trend regardless of geometry. The loading surfaces were found to reach asymptotic states with distinctly different features compared to their initial shapes.

Shockwave impact on the stability of anatase titania nanoparticles

The stability of anatase titania nanoparticles was investigated under exposure to repeated loading of shockwaves produced in a diaphragm-driven high-pressure shock tube. The titania samples were exposed to 20 shocks with pressure, temperature, and steady exposure time in the range of 20.5 – 26 bar, 1399 – 2101 K, and 1.74 – 1.83 ms, respectively. The crystal structure and surface morphology were analyzed before and after the shockwave treatment using various characterization techniques. After repeated shock exposure, there was reduction in crystallite size by ~ 40%, modification in the surface charge, and an increase in the hydroxyl groups in titania. The onset of phase transformation from anatase to rutile was observed after 20 shocks. This work gives insights into the structural changes induced by shockwaves in anatase nanoparticles which can help improve their performance as a catalytic support in heterogeneous catalysis.

Effect of crystallite size on the low-temperature solid-solid phase transformations in the WO3 system

Ferroelectric nanostructured ε phase tungsten trioxide (ε-WO3) is emerging as an important functional material with unique gas sensing and optoelectronic properties. However, ε-WO3 phase is typically unstable in the bulk at room temperature (RT) and it has only been produced at ambient conditions by rapid solidification processes, such as Flame Spray Pyrolysis (FSP). This work aims to assess the overall stability of the ε phase at the nanoscale by studying its transformation from the stable γ phase to upon cooling. In-situ Raman spectroscopy has explored the effect of crystallite size on the solid-solid phase transformation from the symmetric γ (monoclinic) phase into the acentric ε (monoclinic) phase. The phase transformation onset temperature (Tonset) ranges from −43 °C for bulk state to −150 °C for crystallites of 68 nm in size and varies linearly with the inverse of the crystallite size radius (1r).

Research on falling film dehumidification performance of microencapsulated phase change materials slurry

In the process of liquid desiccant dehumidification, the temperature-rise of the desiccant solution caused by moisture absorption will seriously affect the dehumidification capacity and performance. It is expected that the temperature rise of solution can be restrained by adding microencapsulated phase change materials (MicroPCMs) into the liquid desiccant solution. In this paper, based on computational fluid dynamics (CFD) method, two-dimensional models of flat plate and corrugated plate falling film dehumidification of the microencapsulated phase change material slurry (MPCMS) were developed and validated. The dynamic boundary method was used to determine the interfacial mass transfer coefficient. The influences of several factors on the dehumidification performance, including the concentration of MicroPCMs, the inlet temperature of the MPCMS and the falling film plate configuration, were investigated. The results show that, adding MicroPCMs into the liquid desiccant solution is beneficial to the dehumidification performance improvement. The inlet temperature of MPCMS is a very critical variable. When the inlet temperature of the slurry is in the phase transformation temperature range of MicroPCMs, with the adding of the microcapsule, the temperature rise in dehumidification process can be restrained, and the moisture removal performance can be improved significantly. When the MPCMS inlet temperature is at the phase transformation onset point, and the concentration of MicroPCMs is 5%, the water condensation rate is 14.2% higher than that of the base solution. Moreover, it’s not that the more microcapsules added, the larger mass transfer coefficient is. There is an optimum concentration of MicroPCMs to achieve the maximum interfacial mass transfer coefficient. The corrugated plate configuration is more conducive to the dehumidification enhancement by the MPCMS.

Thermomechanical and electrical response of a superelastic NiTi shape memory alloy cable

Superelastic shape memory alloys are a unique class of smart materials that can recover up to 6% strains. Due to their appealing properties such as high energy dissipation and corrosion resistance, several researchers have assessed the use of such materials in numerous applications ranging from biomedical to civil engineering. This article investigates the thermomechanical and electrical response of a NiTi shape memory alloy cable with a complex configuration subjected to cyclic loading of various strain amplitudes ranging from 3% to 7%. The cable consists of several multi-layered strands with a cumulative outer diameter of 5.5 mm. The thermomechanical results indicate a good correlation of the change in cable temperature with the applied strains and maximum stresses. The temperature history can be used to map the degradation pattern of the shape memory alloy cable. In addition, due to the complex geometry of the cable, the global electrical resistance change does not predict the strain/stress state of the cable; however, it can be used to predict the onset of phase transformations.

Fabrication and characterization of a novel bionic manipulator using a laser processed NiTi shape memory alloy

The thermo-mechanical properties of NiTi shape memory alloys (SMAs) have sparked significant research efforts seeking to exploit their bionic capabilities. Currently, the performance capabilities of NiTi-based devices have been inhibited by the retention of only one thermo-mechanical response in the as-received material, namely the shape memory effect (SME) or superelasticity (SE), which mainly depend on the transformation temperatures of the base material. In this work, a novel monolithic bionic manipulator was developed using a NiTi SMA by laser processing, which included both the shape memory and superelastic effects in a single Ni-rich monolithic structure. The device actuation and bending were achieved by resistive heating, which activates the SME of different laser processed regions. Each laser processed region has unique phase transformation onset temperatures and thermo-mechanical recovery characteristics thus providing distinct actuation characteristics. Additionally, the functional fatigue of the part was determined for rehabilitation training of patients including the fingers pairing and grabbing modes.

The effect of cold work on the transformation kinetics and texture of a zirconium alloy during fast thermal cycling

The effect of cold-work on the transformation kinetics and texture evolution after fast temperature cycling were studied in a dilute zirconium alloy. It was found that cold-work delays the onset of phase transformation and helps randomise the texture after the transformation. Samples of Zircaloy-4 in two conditions, cold-rolled to 70% reduction and fully recrystallised, were heated above the β-transus at a fast rate of 100 °C s−1 using resistive heating and without constraint. Electrical resistivity was used to measure the phase fraction during heating and electron back-scatter diffraction was used to measure the texture before and after the thermal cycle. Whereas previous work on titanium suggested that cold-work leads to texture strengthening after transformation, these new experiments show that, when the heating rate is fast enough, recrystallisation is incomplete on heating, which slows down the start of transformation, leading to a random α-texture after cooling. The texture in the recrystallised material after β heat-treatment is non-random but different from the start, a consequence of both a stronger β-texture after grain growth and stronger variant selection on cooling. This variant selection can be mostly explained by α-nucleation at special β-grain boundaries. These results demonstrate that prior deformation has a strong effect on the textures produced after β-heat treatment when heating rates are fast. This has implications for the anisotropy of nuclear cladding components after loss-of-coolant (LOCA) and reactivity-initiated accidents (RIA) but also more widely for the manufacturing of zirconium and titanium using welding or additive layer manufacturing where the heating rates are high.

Growth, morphology and crystal structure of electrodeposited Bi2Se3 films: Influence of the substrate

In this study, we investigated the growth, morphology and crystal structure of electrodeposited Bi2Se3 films on n-Si, Au and Ru substrates. The range of potentials at which films with good quality and stoichiometry can be grown have been identified. Scanning electron micrographs of the early stages of nucleation and growth suggests the film formation on all three substrates follow the Volmer-Weber growth mode. In the case of Si substrate, a pure orthorhombic phase of Bi2Se3 could be grown which is influenced by the epitaxy of the Si substrate. The Faradaic efficiency of film growth on Si substrate was found to be around 90%. However, on Au and Ru substrates, growth of mainly the rhombohedral phase could be achieved having relatively lower Faradaic efficiencies of 68% and 78%, respectively. The Bi2Se3 films grown on Au at more negative potentials were found to exhibit improvements in the rhombohedral crystal phase. Low temperature annealing was found to transform the orthorhombic or mixed phase crystal structure to that of pure rhombohedral. The onset of phase transformation in the mixed phase Bi2Se3 films (on Au) was found to be around 125 °C, determined using in-situ Raman spectroscopy. Aside from the temperature, the duration of heat treatment is identified to play a crucial role in this phase transformation. Based on our findings on room temperature deposition, we conclude that the growth of pure orthorhombic phase is favored on Si, while on Au and Ru, growth of the rhombohedral or mixed phase of Bi2Se3 could be achieved.

Ultra-fine grained β-type TNZT ELI alloy with high strength and low elastic modulus

This work deals with the improvement of mechanical properties of medical alloy Ti-34Nb-7Zr-4.6Ta ELI (TNZT) by severe plastic deformation (SPD) techniques at temperatures below the temperature of the onset of phase transformations in order to maintain a low modulus of elasticity. For the formation of a nanostructured state, high pressure torsion (HPT) at room temperature and equal channel angular pressing (ECAP) at 200 °C, that is, below the onset of diffusion ω-transformation, were used. It is shown that HPT leads to the formation of the UFG structure in the alloy with the grain size of about 110 nm without changing in the alloy phase composition. The ECAP processing allowed forming a subgrain structure with an average size of grains/subgrains of 500 nm and an ultimate tensile strength of 745 MPa, which exceeded the ultimate tensile strength after the standard aging in the (α+β) area (686 MPa). The issues of the effect of SPD on changes in the structure, phase transformations in the alloy and ways of maintaining a low modulus of elasticity in this alloy are discussed.

In situ Raman spectroscopy of pressure‐induced phase transformations in polycrystalline TbPO4, DyPO4, and GdxDy(1−x)PO4

AbstractXenotime DyPO4 and GdxDy(1−x)PO4 (x = 0.4, 0.5, 0.6) (tetragonal I41amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman‐active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7‐8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO4 occurred at a pressure of 15.3 GPa. DyPO4 underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (GdxDy(1−x)PO4) were in the range 10‐12 GPa. The GdxDy(1−x)PO4 solid solutions yielded partially reversible phase transformations, retaining some of the high‐pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO4 decreased the transformation pressure relative to pure DyPO4. The ability to modify the phase transformation pressures of xenotime rare‐earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.

Texture Inheritance on Phase Transition in Low-Carbon, Low-Alloy Pipe Steel after Thermomechanical Controlled Processing

Orientation microscopy (electron back scatter diffraction, EBSD) is used to investigate the structural and textural states of low-carbon, low-alloy pipe steel (resembling 06Г2MБ steel) after thermomechanical controlled processing (TMCP): heating to 1000°C with subsequent quenching in water; isothermal quenching with holding at 300°C; and slow cooling in the furnace. The heat treatment is associated with double phase recrystallization: α → γ → aht, where aht is martensite, bainite, or ferrite. The texture obtained after TMCP is mainly formed by two strong scattered orientations from {112}〈110〉and two weaker scattered orientations close to {110}〈223〉. Despite the double phase recrystallization, the main crystallographic orientations of the bainite after TMCP and after isothermal quenching are the same. That indicates structural and textural inheritance in the material. The structures obtained after other thermal treatments of the structure (both martensite and ferrite) also include complex multicomponent textures, which are nevertheless distinct. Some of the main textural components of martensite and ferrite are the same as bainitic components. All the structures after heat treatment have a similar spectrum of large-angle boundaries, with strongly expressed boundaries of the coincidence site lattices (CSL): Σ3, Σ11, Σ25b, Σ33c, and Σ41c. The orientations forming the texture of all the structures obtained are related to the main orientation of the deformed austenite grains formed on hot rolling in TMCP, in accordance with orientation relations intermediate between the Kurdjumov–Sachs and Nishiyama–Wasserman types. In all cases, the orientation relationship of the textural components of the initial material and the structure obtained by heat treatment may be explained in terms of the onset of phase transformations (both shear and diffusional transition) at crystallographically determined boundaries (including special boundaries) similar to the CSL boundaries Σ3 and Σ11.

Strain engineered pyrochlore at high pressure

Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor the intrinsic and extrinsic strain in pyrochlore, Dy2Ti2O7 and Dy2Zr2O7. Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were used to characterize atomic order over short-, medium-, and long-range spatial scales, respectively, under ambient conditions. Raman spectroscopy and X-ray diffraction were further employed to interrogate the material in situ at high pressure. High-pressure behavior is found to depend on the species and concentration of defects in the sample at ambient conditions. Overall, we show that defects can be engineered to lower the phase transformation onset pressure by ~50% in the ordered pyrochlore Dy2Ti2O7, and lower the phase transformation completion pressure by ~20% in the disordered pyrochlore Dy2Zr2O7. These improvements are achieved without significantly sacrificing mechanical integrity, as characterized by bulk modulus.

Structural Effects of Lanthanide Dopants on Alumina

Lanthanide (Ln3+) doping in alumina has shown great promise for stabilizing and promoting desirable phase formation to achieve optimized physical and chemical properties. However, doping alumina with Ln elements is generally accompanied by formation of new phases (i.e. LnAlO3, Ln2O3), and therefore inclusion of Ln-doping mechanisms for phase stabilization of the alumina lattice is indispensable. In this study, Ln-doping (400 ppm) of the alumina lattice crucially delays the onset of phase transformation and enables phase population control, which is achieved without the formation of new phases. The delay in phase transition (θ → α), and alteration of powder morphology, particle dimensions, and composition ratios between α- and θ-alumina phases are studied using a combination of solid state nuclear magnetic resonance, electron microscopy, digital scanning calorimetry, and high resolution X-ray diffraction with refinement fitting. Loading alumina with a sparse concentration of Ln-dopants suggests that the dopants reside in the vacant octahedral locations within the alumina lattice, where complete conversion into the thermodynamically stable α-domain is shown in dysprosium (Dy)- and lutetium (Lu)-doped alumina. This study opens up the potential to control the structure and phase composition of Ln-doped alumina for emerging applications.

Analysis of Bainite Onset During Cooling Following Prior Deformation at Different Temperatures

We apply method for calculating the austenite to bainite phase transformation onset for any cooling path for a steel of composition 0.09C, 0.2Si, 1.0Mn, 0.03Al, 1.1Cr, 0.18Mo, 0.026Ti, 0.0018B (wt. %) following deformation at different temperatures. The method is parameterized by constant cooling rate experiments to obtain the CCT diagram of this steel following deformation either above recrystallization limit temperature (RLT) or below the no-recrystallization temperature (Tnr). Using the CCT diagrams, we have performed analytical/numerical analysis of the transformation onset, which is based on the conversion of CCT to ideal TTT transformation diagram by the inversion of Scheil’s additivity rule. After the conversion, the transformation onset can be calculated for any cooling path by applying Scheil’s additivity rule. The analysis also provides information on the thermal activation parameters of the transformation onset. The discussion of the results is also provided.

Challenge of Industrial High-load One-point Hardness and of Depth Sensing Modulus

The physics of industrial single-point force indentation hardness measurements (Vickers, Knoop, Brinell, Rockwell, Shore, Leeb, and others) is compared with the depth-sensing nano, micro, and macro instrumental hardness technique that provides several further mechanical parameters, when using the correct force/depth curves exponent 3/2 on the depth of the loading curves. Only the latter reveal phase change onset with transition energy, and temperaturedependent activation energy, which provides important information for applications of all types of solids, but is not considered in the ISO or ASTM standards. Furthermore, the high-load one-point techniques leave the inevitably even stronger and more diverse consecutive phase-transformations undetected, so that the properties of pristine materials are not obtained. But materials are mostly not (continuously) applied under so high load, which must lead to severe misinterpretations. The dilemma of ISO or ASTM standards violating the basic energy law, the dimensional law, and denying the occurrence of phase changes under load is demonstrated with the physics of depth-sensing indentations. Transformation of iterated ISO-hardness and finite element simulated hardness to physical hardness is exemplified. The one-point techniques remain important for industry, but they must be complemented by physical hardness with detection of the phase transformation onset sequences for the reliability of their results. The elastic modulus EISO from unloading curves as hitherto unduly called modulus has nothing in common with unidirectional Young's modulus according to Hook's law, because the skew tip faces collect contributions from all crystal faces including shear moduli, while iteration fit is to Young's modulus of a standard. Unphysical and also physically corrected multidirectional indentation moduli mixtures of mostly anisotropic materials and there from deduced mechanical parameters have no physical basis and none of these should be used any more. A possible solution of this dilemma might be the use of indentation-Ephys and bulk moduli K from hydrostatic compression measurements. The reasons for obeying physical laws in the mechanics of materials are stressed.

Influence of porosity on nonlinear mechanical properties of unpoled porous Pb(Zr0.95Ti0.05)O3 ceramics under uniaxial compression

A series of uniaxial compression experiments on unpoled porous lead zirconate titanate Pb(Zr0.95Ti0.05)O3 (PZT95/5) ceramics were performed to investigate the influence of porosity on the mechanical properties, domain switching and phase transformation via non-contact digital image correlation optical technique to measure the full-field strain. Four unpoled PZT95/5 ceramics were fabricated with a range of porosity levels via systematic additions of pore formers. The nonlinear compressive mechanical behaviors of unpoled PZT95/5 ceramics were measured and evaluated as a function of porosity. The nonlinear deformation mechanism of unpoled PZT95/5 ceramics was attributed to domain switching and ferroelectric (FE) to antiferroelectric (AFE) phase transformation processes, which was verified by analyzing the anomalous behavior between axial strain and radial strain. Normalized critical stresses for the onset of domain switching as well as the onset and exhaustion of phase transformation resulted in a very similar linear decrease with increasing porosity. We suggest that a critical macroscopic volumetric strain is needed for the onset of phase transformation that is nearly independent of porosity.