Phase Transformation Rate Research Articles

Indirect Phase Transformation of CuO to Cu2O on a Nanowire Surface.

The reduction of CuO nanowires (NWs) to Cu2O NWs undergoes an indirect phase transformation on the surface: from single crystalline CuO, to a disordered Cu2-δO phase, and then to crystalline Cu2O. A 9-12 nm disordered Cu2-δO is formed on the NW surface by exposing CuO NWs to CO at 1 Torr, 300 °C for 30 min. After 60 min, this layer decreases to 2-3 nm and is eliminated after 180 min. Energy dispersive X-ray spectroscopy using a scanning tunneling electron microscope and across a single NW reveals the disordered layer to be O-rich with respect to Cu2O with a maximum at. % Cu:O = 1.8. X-ray photoelectron spectroscopy shows adsorbed CO on the surface as evidence of the reduction reaction. Micro-Raman spectroscopy tracks the transformation in NWs as a function of reduction time. A CO enabled surface reduction reaction coupled to diffusion-limited transport of "nonlattice" O to the surface is proposed as a mechanism for Cu2-δO formation. The initial buildup of out-diffusing O to the surface appears to aid the formation of the disordered surface layer. The transformation follows Ostwald-Lussac's law which predicts formation of unstable phases over stable phases, when phase transformation rates are limited by kinetic or diffusional processes. The study provides a generalized approach for facile growth of few nanometer transient layers on multivalent, metal oxide NW surfaces.

Effect of vibration on behaviour of bifilms in A356 and A357 melts

The effect of applying vibration to a melt on the behaviour of bifilm defects in A356 and A357 melts was studied using a reduced pressure test technique. The results showed that vibrating a melt can have a dual effect on bifilms. This effect depends on the rate of phase transformations that occur in the oxide films. If the transformation occurs fast enough then the vibration would facilitate the formation of bonding between the layers of bifilm defects by causing the atmosphere of the defects to be consumed faster. Otherwise, the vibration might facilitate the diffusion of hydrogen into the atmosphere of the defects, and hence prevent or delay the formation of bonding between the oxide layers.

The evolution of phase transformation in Ni/Ni3Al laminated composite under high temperature treatments

In this study the impact of isothermal annealing on the phase transformation rate in laminated Ni/Ni2Al3 composite was investigated. The method of nickel-aluminide coatings of the required chemical composition fabrication was proposed.

Phase transformation behavior and kinetics of high Nb–TiAl alloy during continuous cooling

Phase transformation behavior and kinetics of Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at. %) alloy during continuous cooling under the cooling rates of 3 °C/min, 5 °C/min, 10 °C/min, 20 °C/min, 40 °C/min and water quenching (WQ) were investigated in this study. The phase composition and microstructure of the alloy were studied using X-ray diffraction (XRD) and optical microscopy (OM), respectively. The expansion and the volume fraction evolution of phases during continuous cooling were investigated by dilatometry (DIL), and the microhardness of the alloy suffered from cooling was also tested through micro-sclerometer. The results indicate that the phase transformation from parent α phase to γ and α2 take place during continuous cooling, the α→γ phase transformation occurs in the temperature range of 1150 °C–1310 °C while the α→α2 phase transformation takes place between 1100 °C and 1220 °C. The transformation temperature decreases with the cooling rate increasing, and the higher the cooling rate is, the higher the phase transformation rate is. The relative fraction of γ phase and α2 phase much depend on the cooling rate, and the lamellar colony sizes and interlamellar spacing of crystal structure decrease with the cooling rate increasing. The microhardness of high Nb–TiAl alloy increases with the cooling rate increasing due to the variations of phase composition and crystal structure.

Hydrothermal degradation of a 3Y-TZP translucent dental ceramic: A comparison of numerical predictions with experimental data after 2 years of aging

ObjectivesThe purpose of the study was to assess the hydrothermal resistance of a translucent zirconia with two clinical relevant surface textures by means of accelerated tests (LTD) and to compare predicted monoclinic fractions with experimental values measured after two years aging at 37°C. MethodsPolished (P) and ground (G) specimens were subjected to hydrothermal degradation by exposure to water steam at different temperatures and pressures. The t–m phase transformation was quantified by grazing incidence X-ray diffraction (GIXDR). The elastic modulus and hardness before- and after LTD were determined by nanoindentation. ResultsG specimens presented a better resistance to hydrothermal degradation than P samples. Activation energies of 89 and 98kJ/mol and b coefficients of 2.0×10−5 and 1.8×10−6 were calculated for P and G samples respectively. The coefficients were subsequently used to predict transformed monoclinic fractions at 37°C. A good correlation was found between the predicted values and the experimental data obtained after aging at 37°C during 2 years. Hydrothermal degradation led to a significant decrease of the elastic moduli and hardness in both groups. SignificanceThe dependency of the t–m phase transformation rate on temperature must be determined to accurately predict the hydrothermal behavior of the zirconia ceramics at oral temperatures. The current prevailing assumption, that 5h aging at 134°C corresponds to 15–20 years at 37°C, will underestimate the transformed fraction of the translucent ceramic at 37°C. In this case, the mechanical surface treatment influences the ceramic's transformability. While mild grinding could potentially retard the hydrothermal transformation, polishing after occlusal adjustment is recommended to prevent wear of the antagonist teeth and maintain structural strength.

冷却速率对轴承钢GCr15组织和性能的影响

本文采用光学显微镜、显微硬度等分析手段对某钢厂生产的GCr15轴承钢在不同冷却速度下的奥氏体相变规律进行了研究。结果表明：发生珠光体相变的临界冷却速度约为3℃/s，当温度降至约200℃时，发生马氏体转变。冷却速度小于5℃/s时，会产生较严重的网状碳化物，随冷却速度的提高，网状碳化物析出减少，珠光体相变温度降低。 The Austenite phase transformation law of GCr15 bearing steel with different processes is studied by means of optical microscope and hardness tester. The experimental results show that: the critical cooling rate of pearlite phase transformation is about 3˚C/s and the Martensite transfor-mation starts when the temperature drops at about 200˚C. There will be a serious precipitation of net carbide when the cooling rate is less than 5˚C/s, and with the increase of cooling rate, the pearlite phase transformation temperature becomes lower and the precipitation of net carbide decreases.

A kinetic model for two-step phase transformation of hydrothermally treated nanocrystalline anatase

A kinetic model that enables quantitative assessment of the contribution to the rate of phase transformation by dissolution–precipitation and by interface–nucleation has been developed. Results demonstrate that, under highly acidic, hydrothermal conditions, anatase phase transforms to rutile predominantly by dissolution–precipitation, presumably due to the comparatively high solubility of TiO2 at 250 °C and pH 1.0. In contrast, the phase transformation is dominated by interface–nucleation at pH 3.0, at which the solubility of TiO2 is substantially lower. Furthermore, kinetic data for the phase transformation at the intermediate pH of 2.2 were fit poorly by the interface–nucleation and dissolution–precipitation models individually but fit well using the new kinetic model. Generally speaking, interface–nucleation plays a critical role during the early stages of the transformation, regardless of pH, whereas dissolution–precipitation dominates the later stages of the transformation. The contribution to the rate of rutile production by dissolution–precipitation is the greatest under conditions of higher titania solubility. However, even under conditions of higher titania solubility, results are consistent with interface–nucleation playing a crucial role in producing the initial rutile crystallites, which subsequently grow by dissolution–precipitation. Transmission electron microscopy results are consistent with the results obtained by the new model. Thus, new insights into the mechanism of the anatase to rutile phase transformation under hydrothermal conditions are gained, enabling quantitative assessment of the contribution by interface–nucleation.

The microstructure of a single crystal superalloy after different aging heat treatments

To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated. Results show that for the experimental alloy, even the homogenization–solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ′ phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ′ cubes. For specimens with primary aging heat treated at 1120 °C, γ′ morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 °C, γ′ phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ′ phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ′ phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ′ phase. As a result, the γ′-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ′ size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then, high-temperature properties can be improved.

Semisolid microstructural evolution of fine grain in situ Mg2Sip/AM60B composite in mushy zone

The semisolid microstructural evolution of refined in situ Mg2Sip/AM60B composite in the mushy zone has been investigated. The results indicated that the adopted refining technique is effective for the composite. The non-dendritic microstructure with fine and spheroidal solid particles could be obtained after being partially remelted at the mushy zone for 30 minutes. Reheating temperature affects the size, shape factor and composition of the primary α-Mg particles and liquid fraction of the semisolid system. These effects result from phase transformation rate, which is determined by the atom diffusion ability. Rising the reheating temperature is beneficial for achieving small and spheroidal particles, but increases in the amount of liquid phase. The most appropriate reheating temperature range of the current composite is 600–605°C. The lower reheating temperature results in coarsening of primary α-Mg particles.

Low-temperature degradation in yttria-stabilized tetragonal zirconia polycrystal doped with small amounts of alumina: Effect of grain-boundary energy

Low temperature degradation (LTD) behavior was examined in 3mol% Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZP) doped with 0–1.1mol% Al2O3 sintered at 1500°C. 0.12mol% Al2O3-doped Y-TZP exhibited monolithic microstructure without dispersion of Al2O3 grain, while second-phase Al2O3 particles were observed in the Y-TZP sintered bodies doped with 0.54–1.1mol% Al2O3. In the Y-TZP doped with 0.12mol% Al2O3, neither amorphous nor second phase was present along the grain boundaries, but Y3+ and Al3+ ions segregated in the vicinity of the grain boundaries. The tetragonal-to-monoclinic (T→M) phase-transformation rate during the accelerated LTD test significantly decreased at 0.12mol% Al2O3, but the phase-transformation rate slightly increased with the increasing doping amount of Al2O3. The suppressed T→M phase-transformation in Al2O3-doped Y-TZP is explained in terms of decrease in grain-boundary free energy of tetragonal phase owing to Al3+ ions’ grain-boundary segregation.

Low temperature synthesis of nano alpha-alumina powder by two-step hydrolysis

The ultral fine alpha-alumina powder has been successfully synthesized via two-step hydrolysis of aluminum isopropoxide. Glucose and polyvinyl pyrrolidone were used as surfactants during the appropriate processing step. The alpha-alumina powder was used as seed particles. Several synthesis parameters, such as the amount of seeds, surfactants, and calcination temperature, were studied by X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Thermogravimetry-differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that glucose greatly lower the phase transformation temperature of alpha-alumina by impelling the gamma-alumina transformed to alpha-alumina directly, and the seed could improve the phase transformation rate of alpha-alumina, the polyvinylpyrrolidone have an effect on preventing excessive grain growth and agglomeration of alpha-alumina powder. Comparatively well dispersed alpha-alumina powder with particle size less than 50nm can be synthesized through this method after calcinations at 1000°C for 2h.

Phase transformations in YBa2Cu3O y (y ≤ 6.5) under low-temperature treatment

The influence of low-temperature annealing conditions on the properties and structure of YBa2Cu3O y (123) with y ≤ 6.5 has been investigated. It has been shown that, at T = 200°C, the transition to the orthorhombic phase and the separation into phases with different oxygen concentrations occur slowly. With an increase in the temperature, the rate of phase transformations increases significantly. The phase transition of tetragonal YBa2Cu3O6.3 to the orthorhombic phase due to the oxygen ordering leads to the appearance of superconductivity. There is a direct evidence that, at temperatures T ≤ 300°C in the absence of water vapor, the 123 compound is separated into phases with different oxygen concentrations. The phase separation is accompanied by the formation of an oxygen-enriched phase with a critical temperature T c ≈ 90 K (at T = 200°C) or T c ≈ 76 K (at T = 300°C). The presence of water vapor in the low-temperature annealing atmosphere significantly accelerates all the processes occurring in the system; however, the incorporation of water into the structure of the 123 compound leads to its transition into the 124-type phase.

The effects of heating/cooling rate on the phase transformations and thermal expansion coefficient of C–Mn as-cast steel at elevated temperatures

Abstract

Analytical modeling of activation procedure applied in α-alumina thermo-mechanical synthesis

The impact of the mechanical processing parameters on the alumina grain-size distribution affiliated characteristics and on the γ to α phase transformation rate was investigated. The moderation in the alumina samples behavior has been correlated to the granulometric and mineralogical changes induced by activation via an ultra-centrifugal mill. The assessment of the activation process variables influence on the final quality of the product parameters was conveyed in order to optimize the mechanical treatment of the alumina, which otherwise could be regarded as either energetically or economically unsustainable procedure. The Response Surface Method, Standard Score Analysis and Principal Component Analysis were applied as means of the mechanical activation optimization. The r2 values obtained by developed models were in range from 0.816 to 0.988. The established mathematical models were able to precisely predict the quality parameters in a broad range of processing parameters. The Standard Score Analysis emphasized that the optimal output sample was obtained using a sieve mesh of 120μm set of processing parameters (SS=0.96). Diverse comparison analyses disclosed that the optimal set of activation process parameters could reduce the negative effect of γ-alumina samples immanent properties on the final score, and furthermore to enhance the rate of γ to α transition which would improve energetic and economic sustainability of the alumina phase transformation procedure.

Monoclinic to Cubic Phase Transformation in Combustion Synthesized Gadolinium Oxide

Gadolinium oxide, cerium oxide, and 10 mol% gadolinia doped ceria ceramic powders have been synthesized using combustion technique. Though the cubic gadolinia phase is stable at room temperature, single phase monoclinic gadolinia was obtained as a result of combustion synthesis using fuel lean and stoichiometric precursor compositions. This powder was subjected to calcination treatment and ceria doping to study the stability of phases and the rate of phase transformation from monoclinic to cubic gadolinia. It was found that monoclinic gadolinia transforms to cubic gadolinia upon calcination at temperatures less than 1200oC. It was also found that rate of phase transformation is more for powder produced using fuel lean compositions; and the rate is enhanced upon ceria doping.

Effect of HCl Post-Treatment on Morphology of Hydrothermally Prepared Titanate Nanomaterials with Optical Limiting Properties

The effect of post-treatment HCl concentration on titanate-titania transformation was investigated. Titanate was prepared by the alkaline hydrothermal treatment of TiO2, and then transformed to titania/titanate upon subsequent washing with 0.10-0.55 mol∙L-1 HCl. The titanate almost completely transformed to rutile when treated with HCl concentrations>0.78 mol∙L-1. Accompanying this phase transformation, the morphology of the sample changed from nanotubes to nanosheets and nanoparticles. This was related to the rate of phase transformation, which depended on the HCl concentration. Titanate became resolved into detached TiO6 octahedra during HCl treatment, and titanate-titania transformation occurred via rearrangement of the TiO6 octahedra. The phase transformation and morphological evolution were studied by Xray diffraction, and transmission and scanning electron microscopies. The linear and nonlinear optical properties of the products were investigated using ultraviolet-visible absorption spectroscopy and the open aperture Z-scan technique, respectively. The minimum transmittance at Z=0 and maximum optical limiting effect were obtained when the HCl concentration was 0.55 mol∙L-1. Different nonlinear optical effects were exhibited by different morphologies.

A Comparative Study on Dwell Fatigue of Ti-6Al-2Sn-4Zr-xMo (x = 2 to 6) Alloys on a Microstructure-Normalized Basis

The dwell effects of Ti624x (x = 2 to 6) alloys, including dwell fatigue life debit, fracture mode and strain accumulation, were characterized and compared. With increasing Mo content, the dwell fatigue life debit decreases quickly, and dwell fatigue fracture exhibits a transition from subsurface to surface initiation. Accompanying these changes, the accumulated strain decreases, and the pattern of secondary cracks loses morphological features typical of dwell cracks. These variations in the fatigue behavior of Ti624x were attributed on the fundamental level to the dual effects of Mo: It decreases the β transus of titanium and, as a slow diffuser, reduces the rate of phase transformation from β to α. A higher Mo content encourages nucleation of multiple variants of α laths and promotes the transition from aligned colonies to basketweave microstructure during cooling after β forging. As a result both the grain size and microtexture intensity of α grains in the two-phase processed and heat treated microstructure are reduced. Smaller grain size of the alloys with higher Mo content produces smaller slip band spacing and reduces accumulated strain during dwell fatigue, thus reducing propensity for crack initiation. Microtexture was shown to be the direct cause of dwell sensitivity, and their relationship was described with the aid of a two-region redistribution model based on a previous two-element redistribution model proposed by Bache.

In Situ Observation of the Humidity Controlled Polymorphic Phase Transformation in Glycine Microcrystals

Glycine is a model crystal exhibiting three polymorphic phases and important functional properties such as piezoelectricity and ferroelectricity. We report here in situ observation of the irreversible transformation of the solution-grown glycine crystals from a β phase into a γ phase. The slow transformation process was monitored by piezoresponse force microscopy at room temperature. The process of β to γ conversion was entirely controlled by the variation of relative humidity in the sample chamber. The results show that the rate of phase transformation in glycine is humidity dependent with a threshold of about 25% RH. It is demonstrated that the phase boundary is highly rugged and the transformation front propagates inhomogeneously along the polar axis of the β phase. The mechanism of the phase transformation is discussed.

Study of the interaction of solutes with Σ5 (013) tilt grain boundaries in iron using density-functional theory

Substitutional alloying elements significantly affect the recrystallization and austenite-ferrite phase transformation rates in steels. The atomistic mechanisms of their interaction with the interfaces are still largely unexplored. Using density functional theory, we determine the segregation energies between commonly used alloying elements and the Σ5 (013) tilt grain boundary in bcc iron. We find a strong solute-grain boundary interaction for Nb, Mo, and Ti that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundary, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk.

Kinetics of the melt – Form II phase transition in isotactic random butene-1/ethylene copolymers

The kinetics of formation of the Form II mesophase from the melt has been investigated as a function of the concentration of ethylene chain defects in isotactic random butene-1/ethylene copolymers, using standard and fast scanning chip calorimetry. Presence of ethylene co-units in the butene-1 chain leads to a distinct reduction of the melt – Form II phase transformation rate which has been quantified by evaluation of the critical cooling rate to suppress ordering, and by isothermal analysis of half-times of Form II mesophase formation. For the first time, the temperature-dependence of the rate of Form II mesophase formation has been evaluated for butene-1/ethylene random copolymers and the butene-1 homopolymer. This study needs to be considered as a complementary addendum to former work about the Form II to Form I polymorphic transformation in isotactic random butene-1/ethylene copolymers.