Details Of Phase Transformations Research Articles

Cuboid-like nanostructure strengthened equiatomic Ti–Zr–Nb–Ta medium entropy alloy

This study investigates the effect of annealing treatment on the phase transformation and mechanical properties of the equiatomic TiZrNbTa MEA from room temperature to 1200 °C. After annealing at 1200 °C for 24h, the single solid-solution body-centred cubic (BCC) phase in the as-cast Ta25Zr25Nb25Ti25 transformed into an extremely high number density (~103/μm2) of Ta–Nb-rich BCC nanocuboidal phase (28 ± 10 nm square, BCC1) and a nanostrip-like Zr-rich BCC phase (3 ± 2 nm thick, BCC2). The phase separation from BCC to BCC1 and BCC2 arises from two primary reasons: (i) the high positive mixing enthalpy of both Ta–Zr and Nb–Zr (strong tendency to separation between each pair), and (ii) the 3–4 orders of magnitude higher mobility of Zr than Ta, Nb and Ti in these MEAs (kinetically driven). Detailed CALPHAD simulations of phase formation in this MEA agreed with experiments and provided insightful phase transformation details. The calculated diffusion distance of Zr (~4.1 nm) from the CALPHAD data corresponds to the measured Zr-rich nanostrip thickness (3 ± 2 nm). The nanocuboidal BCC1-BCC2 structure exhibited 112<111>-type of twinning deformation under compression at room temperature. The Ta25Zr25Nb25Ti25 MEA retained yield strength of ~410 MPa at 1000 °C and ~210 MPa at 1200 °C. The phase transformation during cooling after annealing and the microstructural evolution during compression at temperatures from 600 °C to 1200 °C were characterized and discussed in detail.

Comment on "Tunable thermal quenching properties of Na3Sc2(PO4)3:Eu2+ phosphors tailored by phase transformation details" by Z. Liu et al., Dalton Trans., 2020, 49, 3615.

Liu et al. (Dalton Trans., 2020, 49, 3615-3621) reported that emission of the Na3Sc2(PO4)3:Eu2+ phosphor showed tunable thermal quenching properties which were ascribed to the thermal event associated with polymorphic phase transformations. In this comment, I argue that the abnormal negative thermal quenching of the Na3Sc2(PO4)3:Eu2+ phosphor could be a pitfall caused by a diminishment in the optical path lengths of the spectrofluorometer originating from a continuous increase in the volume of the phosphor due to lattice thermal expansion and phase transformation at elevated temperatures.

Tunable thermal quenching properties of Na3Sc2(PO4)3:Eu2+ phosphors tailored by phase transformation details.

Luminescence thermal quenching of phosphors is one of the prominent problems in restricting their application in high-power LED devices. In the current work, tunable luminescence thermal quenching behaviors, including abnormal negative thermal quenching and normal thermal quenching, were demonstrated for Na3Sc2(PO4)3:Eu2+ phosphors tailored by phase transformation details. A series of ionic substitution schemes were employed to synthesize α-, β- and γ-Na3Sc2(PO4)3:Eu2+ phosphors, which show discrepant phase transformations during heating. The thermal event associated with phase transformation is perceived to be responsible for the observations of abnormal negative thermal quenching performances, in view of the suppression of non-radiative paths by consuming thermal energy during the phenomenon of phase transformation. Our results provide an insight into the relationship between the thermal quenching behaviors and phase transformation details of the Na3Sc2(PO4)3:Eu2+ phosphor and offer a useful clue to exploit high thermal stability phosphors.

Atomistic simulations of PdTi high-temperature shape-memory alloys

Martensitic phase transformations in palladium-titanium high-temperature shape-memory alloys (HTSMA) are studied using molecular dynamics simulations. On the basis of the second nearest-neighbor modified embedded-atom method formalism, an interatomic potential for the binary palladium-titanium system is determined by improving the unary descriptions of pure palladium. The developed interatomic potential accurately reproduces physical properties at the equiatomic composition and the resultant temperature- and stress-induced phase transformations between B2 austenite and B19 martensite structures. Subsequent large-scale molecular dynamics simulations demonstrate that the developed potential can be successfully utilized to investigate atomic details of phase transformations in nanocrystalline palladium-titanium alloys.

Dehydrocyclization of n-octane over boron- and barium-doped V-Mg-O catalysts: influence of n-octane/oxygen ratios

Boron- and barium-doped vanadium–magnesium oxide catalysts (BVMgO and BaVMgO) were synthesized by the wet impregnation method and were used for the oxidative dehydrogenation of n-octane at different n-octane/O2 molar ratios. The catalysts were characterized by ICP-OES, in situ XRD, 51V MAS NMR, SEM, EDX and TGA-DSC. The catalytic tests were carried out in a continuous flow fixed bed reactor. The incorporation of boron and barium as dopants into the VMgO system resulted in catalysts with some differences in properties, such as the details of phase transformations, porosity and degree of hydration. The catalytic performance was affected by the strength of the oxidative environment. Moreover, incorporation of barium enhanced the catalytic performance of VMgO, while the addition of boron adversely affected the performance of VMgO over all n-octane/O2 molar ratios with regard to both activity and oxidative dehydrogenation selectivity.

Synthesis of core–shell heterostructured Cu/Cu2O nanowires monitored by in situ XRD as efficient visible-light photocatalysts

Core–shell heterostructured Cu/Cu2O nanowires with a high aspect ratio were synthesized from Cu foam using a novel oxidation/reduction process. In situ XRD was used as an efficient tool to acquire phase transformation details during the temperature-programmed oxidation of Cu foam and the subsequent reduction process. Based on knowledge of the crucial phase transformation, optimal synthesis conditions for producing high-quality CuO and core–shell Cu/Cu2O nanowires were determined. In favor of efficient charge separation induced by the special core–shell heterostructure and the advanced three-dimensional spatial configuration, Cu/Cu2O nanowires exhibited superior visible-light activity in the degradation of methylene blue. The present study illustrates a novel strategy for fabricating efficiently core–shell heterostructured nanowires and provides the potential for developing their applications in electronic devices, for environmental remediation and in solar energy utilization fields.

Solidification behavior in three-phase region of AISI 304 stainless steel

Phase transformation details in the three-phase region of AISI 304 stainless steel were revealed by quenching. At the initial stage of solidification, microstructure consisting of thin lathy ferrite and austenite forms directly from the melt. When forming lathy ferrite in the two-phase microstructure, Cr concentration at the solid/liquid interface remains almost unchanged and large numbers of Ni solutes are rejected into the liquid. When austenite forms, both Cr and Ni solutes are rejected into the liquid. When this process repeats several times, the concentration of Ni in the liquid becomes higher and Cr solute is slightly enriched. Thus, the retained liquid is transformed into austenite after forming the two-phase microstructure due to high concentration of austenite-stabilizing element. This phase transformation pattern was confirmed by consideration of the difference in Gibbs free energy.

Attrition of precipitated iron Fischer-Tropsch catalysts

A precipitated, doubly promoted, iron oxide catalyst was studied to elucidate phenomena that may lead to catalyst attrition during slurry phase Fischer-Tropsch synthesis. The catalyst was examined by electron microscopy (SEM and TEM), electron and X-ray diffraction, sedigraph particle size analysis and BET surface area measurements. The catalyst undergoes attrition both at the micro- as well as the nano-length scales. On the micro-scale, this involves a breakup of ca. 30 μm spherical agglomerates into ca. 1 μm particles, a process that can be initiated even by the mixing that occurs in the sedigraph analyzer. On the nano-scale, we find that exposure of the catalyst to CO at increasing temperatures transforms single crystals of α-Fe 2O 3 into Fe 3O 4 and ultimately to Fe-carbide, with rounded particles as small as ca. 20 nm. The details of phase transformations and the resulting crystallite morphology and size distribution could play a major role in influencing the overall attrition resistance of precipitated iron oxide catalysts. In this paper, we describe the effects of the CO activation temperature on catalyst structure at the micro- and nano-scales.

Phase transformations in Mo under simultaneous implantation of metal and gas ions

Phase transformations of molybdenum under ion implantation with metal and gas ions have been studied by means of transmission electron microscopy. It is shown that, as a result of ion mixing of the surface-adsorbed gas elements of the ion source, thin (⩽ 100 nm) solid layers of the interstitial phases may be formed in the ion-implanted layers. The mechanisms of formation of the structure found are discussed by investigating the crystallographic details of phase transformations.