Low-temperature Phase Transformation Research Articles

Quantitative Analysis of Ferrate(VI) and Its Degradation Products in Electrochemically Produced Potassium Ferrate for Waste Water Treatment

Potassium ferrate(VI) (K2FeO4) as a particularly strong oxidant represents an effective and environmentally friendly waste water treatment material. When produced by anodic oxidation in highly alkaline aqueous solution, the K2FeO4 product is separated and sealed in inert plastic bags with the retention of some liquid phase with high pH. This method proved to be excellent for long-term storage at moderately low temperature (5 °C) for industrial applications. It is still imperative to check the ferrate(VI) content of the product whenever it is to be used. Fe-57 Mössbauer spectroscopy is an excellent tool for checking the ratio of ferrate(VI) to the degradation product iron(III) in a sample. For this purpose, normally the spectral areas of the corresponding subspectra are considered; however, this approximation neglects the possible differences in the corresponding Mössbauer–Lamb factors. In this work, we have successfully determined the Mössbauer–Lamb factors for the ferrate(VI) and for the most common iron(III) degradation products observed. We have found superparamagnetic behavior and low-temperature phase transformation for another iron(III) degradation product that made the determination of the Mössbauer–Lamb factors impossible in that case. The identities of a total of three different iron(III) degradation products have been confirmed.

Effect of Mo and Cr on the Microstructure and Properties of Low-Alloy Wear-Resistant Steels.

Low-alloy wear-resistant steel often requires the addition of trace alloy elements to enhance its performance while also considering the cost-effectiveness of production. In order to comparatively analyze the strengthening mechanisms of Mo and Cr elements and further explore economically feasible production processes, we designed two types of low-alloy wear-resistant steels, based on C-Mn series wear-resistant steels, with individually added Mo and Cr elements, comparing and investigating the roles of the alloying elements Mo and Cr in low-alloy wear-resistant steels. Utilizing JMatPro software to calculate Continuous Cooling Transformation (CCT) curves, conducting thermal simulation quenching experiments using a Gleeble-3800 thermal simulator, and employing equipment such as a metallographic microscope, transmission electron microscope, and tensile testing machine, this study comparatively investigated the influence of Mo and Cr on the microstructural transformation and mechanical properties of low-alloy wear-resistant steels under different cooling rates. The results indicate that the addition of the Mo element in low-alloy wear-resistant steel can effectively suppress the transformation of ferrite and pearlite, reduce the martensitic transformation temperature, and lower the critical cooling rate for complete martensitic transformation, thereby promoting martensitic transformation. Adding Cr elements can reduce the austenite transformation zone, decrease the rate of austenite formation, and promote the occurrence of low-temperature phase transformation. Additionally, Mo has a better effect on improving the toughness of low-temperature impact, and Cr has a more significant improvement in strength and hardness. The critical cooling rates of C-Mn-Mo steel and C-Mn-Cr steel for complete martensitic transition are 13 °C/s and 24 °C/s, respectively. With the increase in the cooling rate, the martensitic tissues of the two experimental steels gradually refined, and the characteristics of the slats gradually appeared. In comparison, the C-Mn-Mo steel displays a higher dislocation density, accompanied by dislocation entanglement phenomena, and contains a small amount of residual austenite, while granular ε-carbides are clearly precipitated in the C-Mn-Cr steel. The C-Mn-Mo steel achieves its best performance at a cooling rate of 25 °C/s, whereas the C-Mn-Cr steel only needs to increase the cooling rate to 35 °C/s to attain a similar comprehensive performance to the C-Mn-Mo steel.

Low-temperature behavior of heat capacity and pho-toluminescence of a binuclear pivalate complex [Eu2(bath)2(piv)6

The low-temperature behavior of the heat capacity and photoluminescence of the previously structurally characterized binuclear pivalate complex [Eu2(bath)2(piv)6] (monoclinic, I2/a) (1), where bath = 4,7-diphenyl-1,10-phenanthroline, piv = (CH3)3CCO2-. Using the adiabatic calorimetry method, the temperature dependence of the heat capacity was measured in the temperature range 5.96–302.88 K and the thermodynamic functions Cp0(T), S0(T), Ф0(Т) and Н0(Т) - Н0(0) were calculated. The absence of low-temperature phase transformations of complex 1 was shown. It was established that in the temperature range 98-295 K, complex 1 demonstrates high temperature stability of the integral intensity of photoluminescence of the transitions of the Eu3+ ion 5D0-7Fj (j = 0.6).

Preparation and mechanical properties of ultra-high strength superalloy fibers

GH4169 superalloy fibers with ultra-high strength and ultra-fine grain (UFG) were fabricated by cold deformation and low temperature annealing, which provides a simple and effective method for preparation of UFG materials. In this study, the fibers were cold-drawn for 13 times and then cold-rolled 25 passes. Finally the annealing experiment is carried out on the cold-rolled samples. The fiber area was reduced by 97% and the grain size reaches 66 nm after cold deformation. In the subsequent annealing 450 ℃ for 30 min, the grain size was further reduced to 42 nm and the tensile strength of the fiber increases to about 2.05 GPa. The increase of the superalloy fiber energy storage and decrease of the curvature of the grain boundary lead to the low-temperature phase transformation. By controlling the annealing condition, these precipitation nanoparticles keep stable, the superalloy fibers are further refined and thus obtain higher strengthening superalloy fibers.

Laser-accelerated phase transformation in cesium lead iodide perovskite

<h2>Summary</h2> High-temperature phase CsPbI₃ has garnered increased attention due to its suitable band gap for photovoltaics, but its structural instability under ambient conditions limits its long-term application. While there have been studies on its structural stability under heat and moisture, insight on the influence of light absorption is limited. We demonstrate that, under ambient moisture, illumination by an above-band gap laser transforms high-temperature CsPbI₃ into its low-temperature phase at rates that are orders of magnitude larger than that of the moisture-induced phase transformation alone, likely due to additional surface vacancy creation and/or migration. Without moisture, laser illumination does not trigger the phase transformation, but introduces defects that lower the material's photoluminescence intensity and accelerate the high-temperature to low-temperature phase transformation when the sample is exposed to moisture. These results expand our understanding of the influence of light exposure on CsPbI₃ and highlight the interdependencies at play when subjecting CsPbI₃ to combined environmental stimuli.

A novel observation of Widmanstätten ferrite and pearlite by thermo-mechanical processing of interstitial-free steel

ABSTRACT In the present work, interstitial-free steel is rapidly cooled from the austenitic state, followed by isothermal holding at 630°C for 5 minutes in a thermo-mechanical simulator (Gleeble 3800®). The heat-treatment provides massive ferrite, Widmanstätten ferrite and pearlite, as the triplex-phase mixture. Massive ferrite, as the early decomposed product of austenite, arises by rapid cooling. Subsequent formation of Widmanstätten ferrite and pearlite during the isothermal holding is elusive in steel. The phase transformations account for a dramatic improvement in tensile strength by two-fold with a negligible reduction in elongation. Morphologically, massive ferrite is granular and contains recovered sub-grain boundaries inside the phase. The pearlite nucleates within a lath-shaped grain, unconventionally. Due to the low-temperature phase transformation (630°C) and a ppm level carbon in the alloy, the observed pearlitic structure is either underdeveloped or fragmented, which promotes cracks against the stronger Widmanstätten ferrite at an advancing front of the crack tip.

The Toughness of High-Strength Steel Weld Metals

Low-temperature phase transformation (LTPT) welding consumables are a new class of welding wires developed to mitigate hydrogen-induced cracking in the welding of high-strength steels without preheating or postweld heat treatment. LTPT weld metals have a high strength, but their toughness needs further investigation. LTPT weld metals predominately contain a martensite microstructure, which is necessary to achieve high strength; however, martensitic weld metals containing oxide inclusions have relatively poor toughness. Three welding processes — gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and hot wire GTAW — were investigated. Optical microscopy, scanning electron microscopes, and transmission electron microscopes were employed for characterization. The role of the shielding gas in the formation of oxide inclusions in LTPT weld metals was investigated. The formation of oxide inclusions in the weld metals was related to the CO2 in the shielding gas. When 100% Ar or a pure inert shielding gas mixture was used for all three welding processes, oxide inclusions were greatly reduced, and the weld metal toughness improved considerably, matching the base metal toughness. The mechanism by which inclusions promote fracture propagation in the weld metal was proposed.

The effect of processing parameters and material properties on residual forces induced in Laser Powder Bed Fusion (L-PBF)

During the L-PBF process stresses are developed due to the large temperature gradients induced by the laser beam and the fast cooling rates which occur while the part is being built. In this study, the development of forces induced layer by layer from the cycles of thermal gradient during L-PBF are measured in-situ using a Force Transducer Device (FTD) for several processing parameters, including laser power and scanning strategy in various metal powders. Three scan strategies were studied: Stripe, Meander and Chessboard, and the results showed that Meander developed the lowest residual forces. All of the processing parameters had a strong influence on the development of residual strains in L-PBF; and, it was found that the laser power had relatively little effect on the residual forces above a minimum threshold and that above a critical value the point distance had little effect on the residual forces. Four materials were investigated: Ti–6Al–4V, Inconel 625, Inconel 718 and maraging steel. Maraging steel exhibited much lower residual forces than the other materials, which could be due to the low temperature phase transformation in this material. Evidence for this mechanism in the maraging steel was provided by changing the inert atmosphere in the L-PBF chamber. In addition, changing the inert gas atmosphere from argon to nitrogen for the maraging steel samples had an effect on the development of forces which changed the direction of the part deformation when it was released at the end of the build process.

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).

Control of residual stress in metal additive manufacturing by low-temperature solid-state phase transformation: An experimental and numerical study

Directed energy deposition (DED) technology commonly known as additive manufacturing (AM), is a rapid prototyping method for fabricating metallic functional components with minimum finishing operations. However, excessive tensile residual stress (RS) is often a potential source of structural failure due to the fact that it can cause surface/subsurface cracking of DED parts. In this work, the RS properties in DED specimens made of the low-temperature transformation (LTT) pre-alloyed powders was investigated. A DED model coupling the finite element (FE) and thermo-mechanical-metallurgical (TMM) analysis was constructed, and was experimentally validated by thermographic system and X-ray diffraction (XRD) stress measurements. An additional set of RS prediction for the DED specimen, using increased inter-track idle time was developed to reveal the effects of thermal history on TMM evolution behavior and RS distribution. The combination of experiment and predictive simulation provided an insight into the non-linear thermo-metallurgical evolution and the formation mechanism of compressive RS during DED of LTT alloy, and showed the great potentiality of LTT powder in minimizing the RS in DED parts by the austenite-to-martensite transformation on cooling.

Mesoporous ZnO nanosheets with rich surface oxygen vacancies for UV-activated methane gas sensing at room temperature

Photoactivation is a promising approach for room-temperature gas sensing with metal oxide semiconductors. For room-temperature methane (CH4) gas sensors, it is highly desirable to develop sensing materials with high photocatalytic CH4 oxidation activity so as to improve their UV-activated CH4 sensing performance. Herein, we reported that mesoporous ZnO nanosheets with rich surface oxygen vacancies (VO) exhibited enhanced CH4-sensing properties under UV illumination at room temperature. The VO-enriched mesoporous ZnO nanosheets were fabricated via a low-temperature thermal phase transformation route. Under UV irradiation, the VO-rich ZnO nanosheets demonstrated a response of 62 % to 0.1 % CH4, about 4.7 times higher than that of VO-deficient commercial ZnO nanoparticles. The improved CH4 sensing capacity of the porous ZnO nanosheets under UV illumination was correlated to their enhanced photocatalytic oxidation activity of CH4 induced by surface VO. The in-situelectron paramagnetic resonance and O2 temperature programmed desorption analysis indicated that the generation of Zn+/O− active centers and active oxygen species could be promoted by the rich surface VO on mesoporous ZnO nanosheets. Furthermore, the density functional theory calculations suggested that VO could improve the surface charge density of ZnO and lower the Gibbs free energy for the CH4 activation on the ZnO surface.

Grain-orientation-dependent phase transformation kinetics in austenitic stainless steel under low-temperature uniaxial loading

Deformation-induced martensites generally follow the principle on the selection of their variants in intrinsic crystallographic orientations with regard to the parent grains, which should be significantly affected by the cooperative rotation of the matrix. In this paper, the microstructural changes related to the deformation-induced transformation from metastable γ austenite to ε and α′ martensites in 304 austenitic stainless steel upon uniaxial tensile loading at 180 K was investigated by employing in-situ synchrotron-based high-energy X-ray diffraction technique. The detailed information on low-temperature phase transformation kinetics was analyzed in terms of the grain rotation and the change in phase volume, stress partitioning, and dislocation density, which were further compared with experimental observations for the room temperature deformed specimen almost without stress-induced martensite. The elastic strain measured in the newly formed α′ martensite was quite low (only ~200 μɛ) upon tensile loading due to stress relaxation, evidencing the role of α′ martensite nucleation in strain accommodation. The minor statistical evolution of texture for all constituent phases, in combination with the martensitic variant selection principle, enables us to reveal the complex interactions of deformation and transformation, finding that the ε martensite firstly originated in the [0 0 1]//LD-oriented grains of γ matrix, while α′ martensite was initially formed in the [1 1 1]//LD-oriented γ grains. Furthermore, the interplay of phases enhanced the grain rotation toward [0 0 1] during deformation at 180 K, which could be elucidated by the influence of transformed martensites on the specific selection of slip systems.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C system alloys. Part 2. Metallographic studies

Original metallographic studies of annealed medium carbon steel (steel 45, steel 40X and steel 35XGA) are carried out in order to reveal leakages in pearlite of these steels of low-temperature carbide transformation of peritectoid type, in which solid solutions of ferrite and cement form a solid solution of wide area of homogeneity based on ε-carbide Fe2C. Most of the inclusions of pearlite cement 45 steel are almost entirely covered with a light grey “foam” shell of nanoglobular crystals ε-сarbide Fe2C. In the process of crystallization of ε-сarbide Fe2C on the cement plates of pearlite steel 45 three morphological types of structure of ε-сarbide Fe2C can be distinguished: “foam” globular shell, granular “outgrowths” and wrapping of particles of decomposed and partially dissolved cement plates. Chromium, which is a part of steel 40X, abruptly inhibits peritectoid transformation. On many cement plates pearlite surface is smooth. In areas where the concentration of chromium is low, the process of formation of ε-сarbide Fe2C is active, with the formation of individual sites with light gray “foam” shell of nanoglobular crystals ε-сarbide Fe2C. It can be expected that in high-alloy chrome steels, the peritectic transformation can be completely blocked through the stabilization of chrome cement or its transformation into thermodynamically stable high chrome carbides. In 30ХГСA steel a sharp intensification of the process of peritectoid transformation of solid solutions of ferrite and cement which are part of pearlite is fixed. The reason for acceleration of the disintegration process of pearlite cement into individual fragments and intensification of release of ε-сarbide Fe2C in the form of column-shaped crystals between the plates of pearlite cement is silicon and manganese, which are part of steel 30ХГСА. As a result of the acceleration of martensite decomposition, the morphology of the released crystals of ε-сarbide Fe2C has changed from “foam” nanocrystals of ε-сarbide Fe2C, typical for steel 45 and steel 40X, to granular. There was also intensive fragmentation or disintegration of cement plates with the appearance of plane-parallel boundaries between the fragments and the formation of large longitudinal flat inclusions of ε-сarbide Fe2C above 100 nm, whose axis is mainly perpendicular to the axis of the cement plate. On the basis of the performed experiments it is possible to consider as proved the presence of low-temperature carbide peritectoid phase transformation in the Fe-C alloy system as a result of interaction of solid solutions of ferrite and cement at 3820C with formation of solid solution on the basis of ε-сarbide Fe2C with wide area of homogeneity. The influence of the chemical composition of steel on the peritectoid transformation between ferrite and cement slurries opens up additional possibilities for regulating the microstructure of pearlite, such as the degree of dispersion of pearlite, which has a determining influence on a number of performance characteristics of steel, such as wear resistance, yield strength and others.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C-system alloys. Part 1. Basics of theory

In the present work low-temperature carbide phase transformations in the system of Fe-C alloys based on ε-сarbide Fe2C with consideration of identification of θ-Fe3C cement as a solid solution were studied. It has been proved that the θ-Fe3C cement slurry is colourfastonide, and the ε-Fe2C carbide slurry is bertollide. When tempering hardened steels, ε-сarbide Fe2C is emitted in the structure of hardened martensite, which is absent in the phase diagram of iron-carbon system alloys. It is believed that ε-сarbide Fe2C is not a stable phase, and since it is metastable, it is formed only in quenched steels under non-equilibrium conditions. The isolation and dissolution of ε-сarbide Fe2C is a classic phase transformation and the absence of this transformation on the diagram is not caused by the metastable phase of ε-сarbide Fe2C, but by the incomplete iron-carbon diagram. The martensite decomposition phase transformation is based on the formation of carbon enriched zones. The processes of carbon segregation on dislocation structures and grid planes of martensite create zones with excess energy. Beginning approximately with temperature 100 °С in structure of martensite begins to allocate ε-сarbide Fe2C, finishing a stage of two-phase segregational disintegration of martensite. At rather small concentrations of carbon in cluster zones the fastest and most effective way of relaxation of redundant energy in these zones, as well as in the tetragonal lattice of martensite is the formation of phases with low value of work of nucleation, first of all ε-сarbide Fe2C and α-Fe(C) or ferrite. The main stages of phase transformations in the peritectoid reaction of martensite decomposition are considered. It is proposed to introduce the peritectoid transformation horizontal at 382 °C and the peritectic transformation horizontal of cement at 1650 °C into the Fe-C alloy state diagram.

Electrochemical Corrosion Behavior of X65 Pipeline Steel Joints Welded by Low Temperature Phase Transformation Electrodes

To meet the corrosion challenge of pipeline steel under complex mixed cases, the corrosion behavior of X65 steel joints welded by employing three low-temperature phase transformation welding wires was investigated. The low temperature phase transformation electrodes welded joints were exposed to saturated CO2 aqueous solution containing H2S of different pH values for electrochemical corrosion test. The Potentiodynamic polarization curve, Electrochemical Impedance Spectroscopy (EIS) and corrosion morphology were analyzed. The results showed that the Cr and Ni element within joints can enhance the corrosion resistance. The corrosion resistance of the welded joint is improved as the pH of etching liquid increases. The corrosion mechanism was further revealed.

A Combination of Keyhole GTAW with a Trapezoidal Interlayer: A New Insight into Armour Steel Welding

The ballistic performance of armour steel welds using austenitic filler materials is poor on account of the disparity in the mechanical properties of the weld and base metals. Consequently, a novel Keyhole Gas Tungsten Arc Welding process with a trapezoidal AISI309 austenitic stainless steel interlayer was developed to tailor chemical composition and microstructure by controlling the solidification sequence. Results show that the dilution rate in the weld metal region can reach up to 43.5% by placing a specially designed interlayer in between the base metal, providing a major scope for microstructure modification. Detailed weld analysis was undertaken by X-ray diffraction, optical and secondary and transmission electron microscopy, energy dispersive spectroscopy and electron back-scattering diffraction. The results from Vickers hardness indents and Charpy impact toughness testing at −40 °C show that the properties of the weld metal region are comparable to that of the base metal. This is ascribed to the weld metal comprising a two phase microstructure of martensite and retained austenite, which contribute to improvements in strength and toughness, respectively. Furthermore, the tailored chemical composition, microstructure and low temperature phase transformation in the weld metal may reduce the tendency toward both solidification cracking and hydrogen assisted cold cracking.

Low-temperature synthesis of α-alumina nanosheets on microfibrous-structured Al-fibers for Pd-catalyzed CO oxidative coupling to dimethyl oxalate

We reported the low-temperature synthesis of α-Al2O3 nanosheets on the microfibrous-structured Al-fibers at 800 ºC. The boehmite (AlOOH) nanosheets were initially formed on the Al-fibers through in situ endogenous growth. Then the AlOOH/Al-fibers was transformed to the α-Al2O3/Al-fibers composite at 800 °C by a single heating step. The low-temperature phase transformation was tentatively attributed to the Al metal in the AlOOH/Al-fibers. Palladium was then dispersed on the α-Al2O3/Al-fibers composite, and the resulting Pd/α-Al2O3/Al-fibers catalyst was examined in the strongly exothermic CO oxidative coupling to dimethyl oxalate (DMO) reaction. High CO conversion of 58% and DMO selectivity of 95% were obtained and maintained for at least 150 h using a feedgas of CH3ONO / CO / N2 (1 / 1.4 / 7.6, mole) at 150 ºC with a gas hourly space velocity of 20,000 mL g―1 h―1. Computational fluid dynamics calculations and experimental results indicated that the Pd/α-Al2O3/Al-fibers catalyst remarkably decreased the hot-spot temperature of catalyst bed due to its enhanced thermal conductivity.

Probing long-range ordering in nickel-base alloys with proton irradiation

Twelve commercial-grade austenitic alloys based on the Ni-Cr-Mo-Fe quaternary system were irradiated using 2 MeV protons at 360 °C to a damage level of 2.5 displacements per atom (dpa). Long-range ordered (Pt2Mo-type) precipitation under proton irradiation was observed, for the first time, in alloys C22, 625, 625P, 625D, 725, and 690. No relevant short-range ordering was observed. These irradiation-enhanced long-range ordered precipitates are coherent with the matrix despite their irregular shape. Of the potential influences on long-range ordering (Ni:Cr, and Ni:(Cr + Mo) ratios, Mo, and iron concentration), Fe content was the strongest by far. The volume fraction of LRO decreases with increasing Fe content by virtue of its role as a stabilizer of the disordered FCC phase, thus reducing the energy savings from ordering. The observed effects of Fe on long-range ordering show qualitative agreement with predictions from thermodynamic modeling. Although solid state diffusion kinetics dominate long-range ordering under purely thermal conditions, ordering under irradiation here (∼ 2.5dpa) is controlled by the thermodynamic driving force. Proton irradiation thus offers a unique approach for studying the low temperature phase transformation in a thermodynamically favored, but kinetically constrained condition.

The Properties and Residual Stress of Argon arc Cladding Metal by Low Temperature Martensitic Transformation Powder

Martensitic Fe-based metal was achieved by argon arc surface cladding using self-developed low-temperature phase transformation (LTT) powders. Residual stress (RS), retained austenite (RA) content, hardness and wear resistance of the cladding layer under different powder compositions were tested. Meanwhile, the phase composition, microstructure and wear mechanism of the cladding metal were also analyzed. The results show that under suitable process parameters, the cladding layer and the substrate are metallurgical bonding without crack and porosity. The microstructure of the cladding layer is mainly lath martensite with less RA distributed on. Due to the volume expansion during phase transformation from austenite to martensite at about 200°C degree, large compressive RS is produced within the cladding metals, which has effectively compensated the tensile RS result from thermal shrinkage. The compressive RS reaches maximum of -361 MPa, and the average RA in the cladding layer is 10.85% for powder with composition of 10% Cr and 8% Ni. The hardness of the cladding layer is 2.6 times higher than that of the matrix material, which could reach up to 557.2 HV. The wear resistance of the surface cladding layer is nearly 57 times higher than that of the base material, which has the most excellent comprehensive performance.

Water steam modified crystallization and microstructure of mesoporous TiO2 nanofibers

High surface area and high crystallinity structure of TiO2 have been concentrated for the synergistic promotion of performance in the previous studies. As high temperature heat treatment can lead the opposite development of crystallinity and surface area, high surface area TiO2 with high crystallinity at a lower temperature has been focused. In the present work, crystallized and mesoporous TiO2 nanofibers (NFs) with a high surface area were obtained by a simple water steam treatment at the low temperature of 150°C without any template agents. We demonstrated that water steam could promote not only the removal of organic component but also the crystallization and phase transformation of TiO2 NFs. The mechanisms of the low-temperature phase transformation and the formation of mesoporous of TiO2 NFs in water steam atmosphere have been analyzed. The change of the surface area and pore size distribution of the mesoporous structures with increasing temperatures were also studied. Furthermore, photocatalytic properties of the mesoporous TiO2 NFs were characterized by the photodegradation of methyl orange. It suggested that the mesoporous TiO2 NFs heat-treated at 550°C exhibited the highest photocatalytic activity for the synergistic effect of the crystallinity and surface area. In the future, such a facile, scalable, cost-effective approach can be used for the fabrication of mesoporous TiO2 NFs.