Occurrence Of Phase Transformation Research Articles

Shear behavior of loess: The role of drainage condition

This technical note investigates the shear behavior of loess using a specifically designed triaxial test scheme, in which five different drainage conditions of loess were controlled by a strain increment ratio (dεv/dεa) in the shear stage. It was established that with an increase in the strain increment ratio, the behavior of the loess samples changed from strain-softening to strain-hardening behaviors in the triaxial test. When dεv/dεa was less than zero, a strong contractive strain-softening behavior was revealed with a large collapsibility index. In the shear stage of a partially drained condition, the loess sample yielded a ‘limited flow’ behavior, which was marked by an occurrence of phase transformation from contractive to dilative response in the q-p’ diagram. More importantly, a unique relationship between the strain increment ratio and stress ratio (q/p’) was established for the test materials. The experimental results suggest a pressing need to thoroughly consider the drainage conditions of loess and the associated impacts on shear.

A Study on Microstructure and Phase Transformation in the Weld Fusion Zone of TIG-Welded IN939 with IN625 and IN718 as Filler Metal

Microstructure and phase transformations were investigated in the weld fusion zone (WFZ) of tungsten inert gas (TIG)-welded IN939 nickel-based superalloy with IN718 and IN625 alloys used as filler metal. The optical and field-emission scanning electron microscopy (FESEM) and also energy dispersive spectroscopy (EDS) were utilized for the microstructural study of the WFZ. The microstructural examination showed that the Nb and Ti elements were microsegregated in the inter-dendrite region, causing a significant concentration gradient between the core and the inter-dendrite region within the WFZ. Observations indicated the formation of the (Nb, Ti) C carbide phases in the solidification process. With a thickness of about 0.2 μm, some plate-like δ phase was observed in the WFZ of both samples upon heat treatment at 750 °C. The results of X-ray diffraction analysis (XRD) confirmed the presence of the δ phase in the WFZ. However, the δ phase disappeared at temperatures beyond 1000 °C. The occurrence of phase transformation during the solidification process was identified by differential thermal analysis (DTA). The WFZ was seen to solidify in the temperature range of 1370°C to 1270°C.

(Invited) Realizing the Electro(photo)Chemical Behavior in Liquid Electrolyte through in-Situ Methodology

Metal oxide of spinel family has performed great potential toward oxygen evolution reaction (OER), but the fundamental mechanism of spinel oxides is still far from being completely understood especially for the role of metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface condition (morphology and defects), it’s a great challenge to have a fair assessment of electrocatalytic performance of spinel system. Herein, we demonstrated a series of MFe2O4 (M = Fe, Co, Ni, Zn) family with a well-controlled morphology to achieve comprehensive study of electrocatalytic activity toward OER, in which most of the divalent metals (M2+) of MFe2O4 were located at octahedral sites. By utilizing several analyses, we could conclude a universal rule toward the spinel metal oxide for oxygen evolution. The activities for OER in spinel metal oxide system were determined by the occurrence of phase transformation rather than electronic state of metal ions, and this structural transformation could work well in both crystallographic sites (Td and Oh sites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship which depended on the atomic arrangement in the interface of spinel and metal oxyhydroxide, while trivalent metal ions were likely to be hosts lattices. The metal oxyhydroxide was formed during redox occurrence rather than OER, while the redox occurrence with remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believed that the approaching strategies and information obtained in present study can offer a guide to design a promising electrocatalytic system toward oxygen evolution reaction and other fields.

Effect of edge hone radius on chip formation and its microstructural characterization in hard milling of AISI H13 steel

Chip formation as a key factor during the machining process can affect cutting stability and machined surface integrity, specifically the formation of saw-tooth chip. The saw-tooth chip formation with respect to edge hone radius during hard milling of AISI H13 steel (50 ± 1 HRC) was investigated in this paper involving chip morphology, metallurgical and mechanical property evolution, chemical composition distribution, and the mechanism of saw-tooth chip formation. The chip microstructural characterization was determined by means of optical microscopy, scanning electronic microscopy (SEM), a nano-tester, and X-ray diffraction (XRD). First, the effect of edge hone radius on chip morphology and evolution of chip color has been investigated. The results highlight that saw-tooth pitch decreases while the degree of segmentation is opposite with edge hone radius and the chip back surface color evolves gradually from purple to blue. Secondly, larger edge hone radius induces a thicker white layer generated on chip back surface and a higher value of nano-hardness identified inside an adiabatic shear band (ASB) as well as on chip back surface. In addition, the occurrence of phase transformation which occurs on chip back surface is observed which can be semi-quantified by the chip back surface color. Furthermore, the detected carbon content almost keep unvaried both in adiabatic shear band and on chip back surface, showing no impact on nano-hardness variation. Finally, it is indicated that the mechanism of saw-tooth chip formation in hard milling of AISI H13 steel is mainly attributed to adiabatic shearing accompanying with cyclic crack formation which facilitates the degree of segmentation. This research is a basic study for tool edge geometry preparation as well as controlling hard milling process.

Influence of dissolved hydrogen on the early stage corrosion behavior of zirconium alloys in simulated light water reactor coolant conditions

The dissolved hydrogen concentration in the water phase represents a key factor for corrosion characteristic of zirconium alloy. A zirconium alloy was oxidized for 100d at two different concentrations, 2.49 and 4.15mg/kg. In situ Raman peaks of the specific oxide phases changed, indicating the occurrence of phase transformation. Moreover, the result of transmission electron microscopy was compared with in situ analysis of the oxide structure formed on the zirconium alloy. The phase stability of the oxide could be influenced by the dissolved hydrogen, and it the phase transformation from tetragonal to monoclinic is influenced by dissolved hydrogen.

Microstructural evolution and mechanical properties of electron beam welded dissimilar titanium alloy joints

The aim of this study was to analyze the microstructural changes of two dissimilar Ti-6Al-4V/Ti17 and Ti-6Al-4V/BT9 joints welded via electron beam welding (EBW), and establish the relationship between microstructure and properties. The results showed that the occurrence of phase transformation was mainly dependent on the peak heating temperature during EBW, and despite the same welding parameters the microstructures of two dissimilar joints were different except in the heat-affected zone (HAZ) on the Ti-6Al-4V side. Both dissimilar joints exhibited a higher strength compared with Ti-6Al-4V base metal (BM), and the strength of Ti-6Al-4V/Ti17 joint was slightly higher than that of Ti-6Al-4V/BT9 joint due to the presence of finer martensite. Fatigue life of dissimilar joints was almost the same as that of Ti-6Al-4V BM within the experimental scatter. Fatigue failure of both dissimilar joints occurred in the HAZ, and fatigue crack basically initiated from the specimen surface.

Synthesis, characterization and thermal stability of CeO2 stabilized ZrO2 ultra fine nanoparticles via a sol-gel route

CeO2 stabilized ZrO2 ultra fine nanoparticles were successfully synthesized via a simple and effective sol-gel synthetic approach by using zirconylchloride octahydrate, cerium nitrate hexahydrate, and citric acid as starting materials. A series of techniques, including X-ray diffraction (XRD), thermogravimetry (TG), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and N2-sorption analysis, were used to characterize the structure and morphology of the as-prepared samples. XRD studies indicate that the as-synthesized sample is of well crystallized tetragonal phase of CeO2 stabilized ZrO2 with high purity. TEM images show that the as-synthesized sample is composed of a large number of fine dispersive nanoparticles with an average size about 10 nm. The as-synthesized tetragonal CeO2 stabilized ZrO2 sample was heated at different temperatures in order to evaluate its thermal stability. The exprimental results reveal that the as-synthesized tetragonal CeO2 stabilized ZrO2 sample exhibits excellent stability without the occurrence of phase transformation.

In Situ Structural Evolution of Steel-Based MMC by High Energy X-Ray Diffraction and Comparison with Micromechanical Approach

AbstractIn situ high energy X-ray diffraction synchrotron was used to provide direct analysis of the transformation sequences in steel-based matrix composite (MMC) reinforced with TiC particles elaborated by powder metallurgy. Evolution of the phase fractions of the matrix and TiC particles as well as the mean cell parameters of each phase were determined by Rietveld refinement from high energy X-ray diffraction (ID15B, ESRF, Grenoble, France). In addition some peaks were further analysed in order to obtain the X-Ray strain during the cooling step. Non-linear strain evolutions of each phase are evidenced which are either associated with differences in the coefficient of thermal expansion (CTE) between matrix and TiC particle or to the occurrence of phase transformation. Micromechanical calculations were performed using the finite element method to estimate the stress state in each phase and outline the effects of differences in CTE and those of volume change associated with the matrix phase transformation. The calculated results led to a final compressive hydrostatic stress in the TiC reinforcement and tensile hydrostatic stress in the matrix area around the TiC particles. Besides, the tendencies measured from in situ synchrotron diffraction (mean cell parameters) matched well with the numerical estimates.

<i>In Situ</i> Structural Evolution of Steel-Based MMC by High Energy X-Ray Diffraction and Comparison with Micromechanical Approach

In situ high energy X-ray diffraction synchrotron was used to provide direct analysis of the transformation sequences in steel-based matrix composite (MMC) reinforced with TiC particles. Evolution of the phase fractions of the matrix and TiC particles as well as the mean cell parameters of each phase were determined by Rietveld refinement from high energy X-ray diffraction (ID15B, ESRF, Grenoble, France). In addition, some peaks were further analysed in order to obtain the X-ray strain during the cooling step. Non-linear strain evolutions of each phase are evidenced, which are either associated with differences in the coefficient of thermal expansion (CTE) between matrix and TiC particle or to the occurrence of phase transformation. Micromechanical calculations were performed through the finite element method to estimate the stress state in each phase and outline the effects of differences in CTE and of volume change associated with the matrix phase transformation. The calculated results led to a final compressive hydrostatic stress in the TiC reinforcement and tensile hydrostatic stress in the matrix area around the TiC particles. Besides, the tendencies measured from in situ synchrotron diffraction (mean cell parameters) matched with the numerical estimates.

A study on phase transformation of monocrystalline silicon due to ultra-precision polishing by molecular dynamics simulation

A three-dimensional molecular dynamics (MD) simulation is conducted to investigate the material removal mechanism of monocrystalline silicon by mechanical polishing at atomistic scale with diamond abrasives. By monitoring relative positions of atoms in the monocrystalline silicon specimen, the microstructure transformation of monocrystalline silicon is clearly identified and analyzed. The phase transformation is accomplished under extreme conditions with high temperature and huge hydrostatic pressure, and as a result the silicon microstructure transforms from the four-coordinated diamond cubic structure (Si-I) to the six-coordinated body-centered tetragonal structure (β-silicon). The values of local pressure and temperature are consistent with previous experimental results. In addition, the force between the diamond abrasive and specimen indicates the occurrence of phase transformation in the specimen. The potential energy of each atom is also calculated, which provides us an effective approach to analyze the energy variation of atoms in the mechanism of material deformation and the formation of machined surface after ultra-precision polishing.

Numerical Simulation of Welding Residual Stress Considering Phase Transformation Effects

The welding processes of steel materials are often accompanied by the occurrence of phase transformation. Volume change caused by phase transformation will affect the history of stress and strain. In this article, taking the welding of Q345 as an example, the effects of solid-state phase transformation on the residual stress were investigated by numerical simulation. The values of thermal strain at different temperatures were set to make the volume change caused by phase transformation equivalent as thermal strain. The simulation contained two cases both considering phase transformation and not. The results show that in both two cases the longitudinal stress distribution in the weld zone has almost the same trend. But in the case without considering phase transformation, there is large longitudinal tensile stress concentrating in the weld and HAZ zone and the maximum value is up to 427MPa in the weld. For transverse stress, phase transformation not only changes the value of the stress, but also alters the sign of the stress in the middle of the weld zone. Experiment was also carried out to measure the residual stress by X-ray diffraction. The result considering phase transformation matched much better with the experimental data. It can be concluded that phase transformation in the process of welding has a significant effect on the residual stress and can not be ignored in the numerical simulation of welding.

The oxidation behavior of a Ti 50Cu 28Ni 15Sn 7 bulk metallic glass at 400–500 °C

The oxidation behavior of a Ti 50Cu 28Ni 15Sn 7 bulk metallic glass (TC4-BMG) was studied over the temperature range of 400–500 °C in dry air. The oxidation kinetics generally followed the parabolic-rate law, and the oxidation rates increased with increasing temperature. The scales formed on the TC4-BMG consisted mostly of TiO 2 (rutile structure) intermixed with small amounts of CuO particles. It was found that the scaling rate of the amorphous alloy was slightly faster than that of the crystalline counterpart. The amorphous substrate transferred into the three crystalline phases of Ti 3Sn, Ti 3Cu 4, and Ti(Ni,Cu) during the oxidation at 400–500 °C, indicating the occurrence of phase transformation.

Microstructure of Cu60Zr20Ti20 bulk metallic glass rolled at different strain rates

The structural evolution of Cu60Zr20Ti20 bulk metallic glass during rolling at different strain rates and cryogenic temperature was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM). It is revealed that the deformation-induced transformation is strongly dependent on the strain rate. At the lowest experimental strain rate of 1.0×10−4 s−1, no phase transformation occurs until the highest deformation degree reaches 95%. In a strain rate range of 5.0×10−4−5.0×10−2 s−1, phase separation occurs in a high deformation degree. As the strain rate reaches 5.0×10−1 s−1, phase separation and nanocrystallization concur. The critical deformation degree for occurrence of phase transformation decreases with the strain rate increasing.

Air-Oxidation Behavior of a Cu60Hf25Ti15 Bulk Metallic Glass at 375–520 °C

The oxidation behavior of a Cu60Hf25Ti15 bulk metallic glass was studied over the temperature range of 375–520 °C in dry air. The oxidation kinetics of the amorphous alloy generally followed the parabolic law at all temperatures, with an oxidation rate increasing with temperature. The oxidation rates of the amorphous alloy were much higher than those of polycrystalline pure-Cu, implying that the additions of Hf and Ti accelerated the oxidation reaction. The composition of the scales formed on the amorphous alloy was strongly temperature-dependent, since they consisted mostly of Cu4O3 and CuO with minor amounts of HfO2 at T ≤ 450 °C, while mostly CuO with minor amounts of HfO2 and Cu2TiO3 were detected at higher temperatures. In addition, nanocrystalline Cu51Hf14 and Cu3Ti2 phases were detected on the substrate after oxidation at T ≥ 450 °C, indicating the occurrence of phase transformation.

Phase transformation in hot pressed cobalt and cobalt–diamond materials

The present paper describes the effect of oxygen and hard phase (diamond) content on the allotropic phase transformation in hot pressed cobalt. Investigations were carried out both on samples made of cobalt and cobalt–diamond materials. All specimens were obtained by hot pressing in graphite moulds. X-ray diffraction (XRD) was used in order to estimate the regular and hexagonal phase content of the cobalt. The XRD measurements were supplemented with microstructural observations by means of scanning electron microscopy and transmission electron microscopy. The obtained results provided evidence that occurrence of phase transformation in hot pressed cobalt upon cooling to room temperature strongly depends on the oxygen content, and it is hindered by a very fine grain structure and the cobalt oxides. The effect of residual stress produced in cobalt–diamond materials, which is the effect of the difference between linear expansion coefficients of metal matrix and diamond on the phase composition of the cobalt, proved to be of secondary importance.

Partial Hyperbolic-Tangent Friction Element Based Phenomenological Models for Shape Memory Alloy Pseudoelastic Hysteresis

A new mechanism-based phenomenological model, comprising linear and nonlinear springs and a nonlinear friction element, is presented for the pseudoelastic damping behavior of shape memory alloys. The use of a partial hyperbolic tangent friction element (a hybrid of an ideal and hyperbolic tangent friction element) is seen to increase accuracy in simulating experimental hysteresis behavior over earlier models. Comparisons are then made with existing models. Compared to the thermodynamic-based models, the present models have the benefit of not requiring calculation of austenite-martensite phase transformations. Unlike previously developed phenomenological models, the models presented herein have mechanical analogies that provide a strong physical basis, and clear relationships can be established between the unlocking of the friction element and the occurrence of phase transformation. These models are simpler and more intuitive than existing models.

Effects of Constitutive Parameters on Thickness of Phase Transformed Adiabatic Shear Band for Ductile Metal Based on Johnson-Cook and Gradient Plasticity Models

Gradient-dependent plasticity where a characteristic length is involved to consider the microstructural effect (interactions and interplaying among microstructures due to the heterogeneous texture) is introduced into Johnson-Cook model considering the effects of strain-hardening, thermal softening and strain rate sensitivity. Effects of initial static yield stress, strain-hardening coefficient and exponent, strain-rate and thermal-softening parameters on the occurrence of phase transformation and the thickness of phase transformed adiabatic shear band (ASB) in deformed ASB are numerically investigated. Higher initial static yield stress, strain-hardening coefficient, strain-rate parameter and lower strain-hardening exponent lead to earlier occurrence of phase transformation (lower plastic shear strain). Effect of thermal-softening parameter on plastic shear strain corresponding to the onset of phase transformation is not monotonous. Transformed ASB is located at the center of deformed ASB since the position has higher temperature exceeding the temperature of phase transformation. The thickness of transformed ASB increases with decreasing flow shear stress and the increasing tendency becomes slow. For the same flow shear stress, the thickness of transformed ASB is wider for higher initial static yield stress, strain-hardening coefficient and exponent, strain-rate and thermal-softening parameters. Compared with classical elastoplastic theory applicable to completely homogenous material, gradient-dependent plasticity considering the microstructural effect predicts that phase transformation occurs earlier and that the thickness of transformed ASB changes with flow shear stress.