Phase Transformation Point Research Articles

A Comparative Study of Conventional, Dynamic Rotation and Heat-assisted Friction Stir Welding of Ti-6Al-4V Plates to Reduce Welding Defects

It is very easy to form bottom defects during friction stir welding (FSW) of medium-thick/thick Ti alloys, while there is a lack of in-depth research on its essential cause. In this study, conventional FSW (CFSW), dynamic rotation FSW (DRFSW) and back heating assisted FSW (BHAFSW) were conducted to 6mm Ti-6Al-4V plates. The combination of experimental examination and finite element modelling was utilized to deeply study the bottom defect formation mechanism. The results showed that bottom defects always existed in both CFSW and DRFSW joints, which was mainly attributed to the phase transformation difference at the joint bottom, as the result of the difference of the bottom temperature and the phase transformation point. A proper bottom temperature compensation of 150oC via BHAFSW was designed to avoid the phase transition difference and improve the material flow, which effectively eliminated the bottom defects. This is the first time to uncover the reason of the bottom defect generation during FSW, and thus defect-free Ti alloy medium-thick plate joints were achieved via BHAFSW. The BHAFSW joint achieved the 1022MPa in strength and 16% in ductility, with the equal strength to the base material (BM) but with very few losses of the ductility. The joint exhibited an impact toughness of 10.4J, 48.1% higher than the BM. This study uncovers the reason for the typical bottom defect and provides a new way to improve the quality of medium-thick/thick Ti alloy FSW joints.

Characteristics of residual stress distribution in wire-arc additive manufactured layers of low transformation temperature material

High heat input in wire and arc additive manufacturing (WAAM) tends to result in high tensile residual stress, which adversely affects performance and service reliability. However, low-transformation-temperature (LTT) materials are susceptible to compressive residual stresses due to their low phase-transformation point. Here, the LTT material 10Cr10Ni was used as the deposited material to fabricate a pipe structure, and the contour method was applied to measure the residual stress distribution in the cross-section. A thermomechanical coupling analysis model was developed to investigate the residual stress generation mechanism in 10Cr-10Ni(LTT) during the WAAM process. The simulated deformation and residual stress distributions agree well with the measurements. Through simulation analysis of the deposition of different layers, we found the key to the generation of compressive residual stresses in the LTT layers lies in whether there is a transformation from austenite to martensite during the final thermal cycle. When the deposition height exceeded 5.405 mm (approximately four layers), compressive residual stresses were only generated in the last four LTT layers and transformed into tensile residual stresses in the other layers. This results in a concept called the effective region, which can be used as a guide for reducing stress in WAAM using LTT materials.

Formation of white etching layer on rails due to coupled thermal and mechanical actions

The formation mechanism of the white etching layer (WEL) on the rail steel material has been confirmed via two different routes; thermal-induced WEL by phase transformation and mechanical-induced WEL by severe plastic deformation. In this study, a combination of thermal and mechanical processes was performed to examine the synergistic interaction of two formation mechanisms of the WEL on rails. An experiment was conducted on the rail steel material using a Gleeble thermo-mechanical simulation system. The results confirmed that the martensitic WEL is formed by phase transformation from rapid quenching at a temperature below the critical point of material phase transformation, under 550 °C, when high hydrostatic pressure is applied. The application of combined thermal and mechanical action induced martensite with the presence of retained austenite. The microstructure of the transition zone between the WEL and the base material contains partial and full dissolution of cementite, broken pearlite lamella, and dislocation density, confirming that both thermal and mechanical processes are involved in the phase transformation of the material.

Influence of fine content on undrained monotonic behavior of carbonate sand

The behavior of carbonate sand is different from that of siliceous sand in terms of high compressibility and crushability, specific particle shape, and physical properties. Thus, investigating the behavior of these soils is of great importance because of their prevalence in coastal areas. Nevertheless, a few studies have considered the presence of fines in the mixture with carbonate sand. The present study explores the influence of non-plastic fines on the undrained monotonic behavior of Bushehr carbonate sand. For this purpose, 30 undrained monotonic triaxial tests were performed on carbonate sand with silt ranges of 0% to 40% with two different void ratios. According to the results, the undrained shear strength of carbonate sand diminished with the addition of silt percentage up to 20%, and increased dramatically at 30% silt content. Also, the results of 40% silty sand revealed that this sample is stronger than samples with 10% and 20% silt content. By surveying the location of the phase transformation point in samples, it was observed that silt particles reduce the dilative phase. Also, confining pressure intensified the contractive phase while relative density had the opposite effect. The results showed that the effect of particle breakage is negligible in this study.

Effect of annealing temperature on the microstructures and impact toughness of Ti-6Al-4V-0.5Ni-0.5Nb titanium alloy plates

Numerous studies have clarified the importance of measuring the impact toughness of titanium alloy drill pipe, however, the influence of heat treatment parameters on the impact toughness of titanium alloy is seldom discussed. Herein, the influence of annealing temperature on notch impact toughness of Ti-6Al-4V-0.5Ni-0.5Nb alloy was investigated by Charpy impact test. Optical microscope and SEM characterized the impact specimens’ annealing microstructure and fracture surface. The results revealed that the primary α phase gradually decreases with increasing annealing temperature, and the impact toughness of the plate increases first and then decreases. The alloy fracture changes from quasi-cleavage fracture to ductile fracture, and with the temperature rising to the phase transformation point, the fracture of the alloy mainly turns into cleavage fracture.

Capturing the Turning Hook of Stress-Dilatancy Curve of Crushable Calcareous Sand

For crushable calcareous sand, the stress-dilatancy curve has a significant turning hook around the peak stress ratio, the hook contains the main features of the loading process, including the phase transformation point and the peak stress ratio point. However, more than half of this turning hook, i.e., the line after the peak stress ratio point, is usually ignored by known stress-dilatancy models. It is difficult to directly establish the stress-dilatancy model with such turning hook characteristics, since such turning hook demonstrates that the dilatancy is not a single-valued function of the stress ratio. Based on the first law of thermodynamic, we related dilatancy to breakage energy. Then, we mapped breakage energy from the stress-energy plane to the strain-energy plane to avoid the non-single-valued function problem. Then, the stress-dilatancy model was conveniently established. Compared with the other four existing stress-dilatancy models, the benefit of our modeling process is that it can easily capture the turning hook of the stress-dilatancy curve. Our model is also verified by simulating colloidal-silica-stabilized and MICP-stabilized calcareous sands, as well as three types of calcareous sands, respectively.

Janus MSiGeN4 (M = Zr and Hf) monolayers derived from centrosymmetric β-MA2Z4: A first-principles study

A two-dimensional (2D) MA2Z4 family with and phases has been attracting tremendous interest, the MoSi2N4 and WSi2N4 of which have been successfully fabricated ( Science 369, 670 (2020)). Janus monolayers have been achieved in many 2D families, so it is interesting to construct a Janus monolayer from the MA2Z4 family. In this work, Janus MSiGeN4 (M = Zr and Hf) monolayers are predicted from -MA2Z4, which exhibit dynamic, mechanical and thermal stabilities. It is found that they are indirect band-gap semiconductors by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). With biaxial strain from 0.90 to 1.10, the energy band gap shows a nonmonotonic behavior due to a change of conduction band minimum (CBM). A semiconductor to metal transition can be induced by both compressive and tensile strains, and the phase transformation point is about 0.96 for compressive strain and 1.10 for tensile strain. The tensile strain can change the positions of CBM and valence band maximum (VBM), and can also induce the weak Rashba-type spin splitting near CBM. For MSiGeN4 (M = Zr and Hf) monolayers, both an in-plane and out-of-plane piezoelectric response can be produced, when a uniaxial strain in the basal plane is applied, which reveals the potential as piezoelectric 2D materials. The high absorption coefficients in the visible light region suggest that MSiGeN4 (M = Zr and Hf) monolayers have potential photocatalytic applications. Our works provide an idea to achieve a Janus structure from the MA2Z4 family, and can hopefully inspire further research exploring Janus MA2Z4 monolayers.

The effect of Zr addition on the microstructure evolution and mechanical properties of hot-rolled TiAlNbZr alloy

The microstructure and mechanical properties of hot-rolled Ti–6Al–7Nb-xZr (x = 0, 5, 10, 15, 20 wt%) (TANZ) alloys were systematically studied. Herein, the effects of phase and microstructure evolution on the mechanical properties of hot-rolled TANZ alloy were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and statistical analysis. The addition of zirconium can change the β-phase stabilization ability of TANZ alloy and thus change its phase transformation point. Metastable phases (α′, α″, and metastable β phases) are retained during hot rolling at 850 °C and subsequent water quenching. Furthermore, the recovery of the initial α phase is promoted during hot rolling and holding. During the deformation process, the presence of both the initial α phase and the metastable phase (β transformation structure) is conducive for coordinated simultaneous deformation. The uniform deformation ability and the work-hardening behavior of TANZ alloys are significantly improved by increasing Zr content. The yield strength, true stress in tensile strength tests, and uniform elongation of the TAN15Z alloy are 848 MPa, 1160 MPa, and 9.5%, respectively. The corresponding values for the TAN20Z alloy are 810 MPa, 1331 MPa, and 11.6%, respectively. A significant improvement in the trade-off between strength and plasticity was achieved. Furthermore, the fracture mechanisms of the studied alloys were also discussed in depth.

The effect of hydrogen on the grain refinement and mechanisms for Ti6Al4V alloys during laser melting deposition

Titanium alloys have been widely used for AM (additive manufacturing) to manufacture complex parts duo to its excellent comprehensive properties. As one of the most important additive manufacturing methods, laser melting deposition (LMD) have been extensively used during the last few years. However, coarse columnar grains (CG) cannot be ignored generated in the LMD which will reduce the end-use performance of the alloy. In order to refine the microstructure, a well-designed THP is adopted. The powders with six kinds hydrogen content are used in a single-track experiment, and the optimum process parameters are obtained through deposit height, width, penetration depth as well as defects. Multi-layer deposition is used to analyze the evolution of the microstructure after forming. The results manifest that there is a number of acicular α′/α after solid solution treatment. With the increase of hydrogen concentration, the size of coarse CG gets smaller and finally turns to equiaxed grains at the bottom and middle area when hydrogen content is 1.0 wt%. Besides, the phase transformation point of deposition sample declines with the increase of hydrogen content and tends to be stable 900 °C when hydrogen content beyond 0.65 wt%.

Induction coil as sensor for contactless, continuous in-process determination of steel microstructure by means of Magnetic Induction Spectroscopy (MIS)

In hot forming and machining process chains heat treatment of steels is controlled by temperature and time. The process parameters significantly influence the material microstructure, which is important for the final component properties. The dependence between the predominant microstructure and the magnetic properties can be determined by means of Magnetic Induction Spectroscopy (MIS). In this paper, we propose to use the induction coil as an in-process sensor element to implement MIS in induction heating processes with subsequent cooling of steel sheets. This novel approach enables contactless, continuous in-process measurement of complex inductance and allows a non-destructive determination of steel microstructure. The novel measurement method can differentiate discrete microstructural states and determine phase transformation points during continuous heating and cooling, by analysing the inductance curve. The realized combination of sensor and actuator features enables both process monitoring and closed-loop control of production processes with integrated heat treatment.

Particle shape influence on the deformation resistance of carbonate sands under drained condition

The carbonate sand has high porosity and irregular particle shape compared with the quartz sand. Under dynamic loads, the carbonate sand perhaps experiences drained or partial drained shear, and the contribution of particle shape to its deformation resistance was still unrevealed. In this paper, particle shapes of five samples were first quantitively evaluated. Then, monotonic and cyclic tests under drained condition were carried out based on the simple shear apparatus. The particle size and shape were quantitively measured after the tests, and the particle breakage degree and its breakage mode were discussed. It is found that before phase transformation points (PT points), the irregular particles mobilize greater shear resistance. The particle breakage reduces the deformation resistance after the PT point due to its depression effect on the volume shear dilation. In cyclic shear, irregular particles have larger volume contraction due to the particle breakage, and the surface grinding is the major breakage mode. Under the same cyclic stress ratio (CSR), the irregular particles show smaller shear strain amplitudes, indicating that the irregular particles have stronger resistance to deformation. It is also found that the particle shape mainly influences the deformation resistance when the overall regularity (OR) is less than 0.8.

Investigation of the evolution of free carriers during femtosecond laser-induced ultrafast amorphization in nitrogen doped Ge2Sb2Te5 films

The femtosecond laser-driven ultrafast evolution of free carriers during the phase-change process of N-doped Ge2Sb2Te5 (N-GST) film was studied in this paper. And mechanism of initial absorption peak was calculated by two-temperature model. We found that the formation of Ge-N weaken the initial optical absorbance and decreased the concentration of free carriers, then improved the temperature of crystallization in N-GST. The doping(N) can reduce the grain size because of the easing of optical energy absorbance. At last, we inferred that, if the incident laser fluence is enough for phase transformation point, the transformation velocity of N-doped GST is faster than GST.

0.4% Electrostrain at Low Field in Lead-Free Bi-Based Relaxor Piezoceramics by La Doping

This work investigates the effects of lanthanum doping on the crystal structure, surface morphology, dielectric properties, ferroelectric properties, piezoelectric properties, and electromechanical strain properties of lead-free (Bi0.47Na0.47Ba0.06)1−xLaxTiO3 piezoelectric ceramics. All specimens were fabricated by a traditional solid-state reaction method. The results of crystal structure and dielectric properties categorize all specimens as relaxor materials. The results of dielectric properties and ferroelectric properties confirm the nonergodic-to-ergodic transformation as a function of La content. At the phase transition point, the material shows a high electric field-induced strain of 0.4% under an applied electric field of 5 kV/mm. The large electromechanical strain response at low applied field in the 0.02 mol La specimen may contribute to the two synergistic effects. First, the electric field induced the reversible ergodic-ferroelectric phase transition, which can generate the large strain. Second, the nonergodic and ergodic phases coexist at the phase transformation point, which can reduce the driving field by facilitating the growth of ferroelectric domains.

A Kinetic Model of Nonequilibrium Melting of Metal under Critical Heating by a Nanosecond Volumetric Heat Source

A kinetic model of nonequilibrium melting of a metal under conditions of critical heating by a nanosecond volumetric heat source is formulated. The proposed approach takes into account the characteristic features of the process: strong nonequilibrium of the system and thermodynamic fluctuations of the order parameter, which play a significant role in the vicinity of the critical phase transformation point, especially when the point of absolute instability of the system is reached. The curves of dependence of the model thermodynamic potential on the order parameter for various values of the superheat of a copper sample are calculated. During critical overheating, the minimum corresponding to the solid state disappears, turning into an inflection point. The solid state becomes completely unstable.

Investigation on the mechanical properties of a calcareous sand: the role of the initial fabric

This study examines the effect of the initial fabric on the coefficient of the lateral earth pressure at rest K0, the undrained monotonic loading strength and the liquefaction resistance of a calcareous sand from a reclamation land in Persian Gulf. K0 consolidation, undrained compression and extension and cyclic undrained loading triaxial tests are performed on laboratory samples reconstituted by air and water pluviation, dry and moist tamping and dry funnel deposition methods. Test results show that the air pluviation and moist tamping samples own the highest and the lowest K0 respectively. All the samples at medium loose and medium dense states show strain hardening when subjected to undrained monotonic loading. The air pluviation samples are more contractive than the other samples at the phase transformation points and the moist tamping samples exhibit an over-consolidated behavior. In the cyclic loading tests, the air pluviation and the moist tamping samples exhibit the lowest and the highest liquefaction resistance. It is shown that the evaluation and comparison of the cyclic undrained strength of various sands should take the sample preparation method into consideration scrupulously. The test results also indicate that although the mechanical property of soil sample is influenced by the fabric differences induced by sample preparation method, other factors such as homogeneity and stress history should not be neglected.

Experimental investigation of both the disturbed and undisturbed granitic saprolite soil

Granitic saprolites are abundant in many regions around the world and they are widely employed in various engineering projects. Behavior of such kinds of soils is not sufficiently understood and our knowledge about the fundamental behavior of saprolites is to a far less extent than the sedimentary soils. Owing to the limited knowledge, many problems occurred in the engineering applications that used saprolites as a construction material. This study presents results of a set of conventional drained and undrained triaxial compression shearing tests on the granitic saprolites in both the disturbed and undisturbed conditions. This study showed that the soil fabric was the paramount factor governing on the soil stress–strain response rather than the soil bonds. Under the drained shearing condition, the undisturbed soil revealed a strain-softening behavior with a dilative response at the low consolidation stress level, while a contractive response was present at the high consolidation stress levels. However, the disturbed soil showed a strain-hardening behavior with a contractive response, regardless of the consolidation stress levels during the drained shearing. The undrained response was complicated, as the soil showed both the contractive and dilative responses with phase transformation points, regardless of the soil condition (i.e., disturbed or undisturbed) and the consolidation stress levels. The bond effect of the intact soil persisted during the consolidation stage; however, its effect did not extend to the large strain-controlled shearing stage, where the tangential stiffness suddenly decreased at the axial strain of 1.5%.

Research of rheological properties and displacement of high-paraffin oil

One of the urgent problems in the extraction of hard-to-recover reserves (TRIZ) of oil is the formation of asphalt-resin-paraffin deposits (AFS) at oil fields, which entails a number of complications when extracting reserves from the subsoil. In solving the problems associated with the study of the mechanism of deposition of asphalt-resin-paraffin complexes on the downhole equipment or inside the reservoir, the leading role is played by laboratory studies with modeling of objects and conditions inherent in a particular field. In particular, it is necessary to prepare model solutions of hydrocarbons simulating downhole oil containing asphalt-resin-paraffin deposits. The purpose of this work was to establish by an experimental method the dependence of the rheological properties of model oil solutions on the amount of asphalt-resin-paraffin deposits dissolved in it, and to study the efficiency of displacing the prepared model oil solution from the bulk model of core with sodium laurine sulfate. The novelty of the work lies in comparing the effect of the mass content of asphalt-resin-paraffin deposits on the viscosity and density for light and heavy oils and in studying the effectiveness of sodium laurine sulfate when displacing paraffin oil. Several oil solutions were prepared with various mass proportions of ARPD in them, after which the temperature dependence of the viscosity and density of each solution was determined. It has been established that the presence of asphalt-resin-paraffin complexes more strongly affects the density when they are dissolved in light oil. As the mass concentration of paraffin deposits increases, their effect on density decreases for both the light oil sample and the heavy oil sample. The viscosity of the presence of paraffin is much more pronounced if they are dissolved in heavy oil than in light oil. There is a phase transformation point for asphalt-resin-paraffin complexes, which will need to be taken into account when setting up laboratory studies to study the mechanism of deposits of asphaltenes, resins, paraffins in the reservoir or downhole equipment. An experimental method was also used to study the displacement ability of sodium laurinsulfate on a bulk core model saturated with prepared model oil solutions, which are paraffinic oil. It was established that this surfactant has a greater oil displacement efficiency compared to water, in addition, based on the result, it follows that the oil displacement coefficient non-linearly depends on temperature. So, when oil is being displaced with paraffin deposits dissolved in it, there is an optimum temperature at which the maximum oil displacement coefficient is ensured.

Mineralogy, Micro-fabric and the Behavior of the Completely Decomposed Granite Soils

The main objective of this study is to investigate the impact of the micro-fabric and soil mineralogy on the overall macro-behavior of the completely decomposed granite soil through a set of drained and undrained triaxial shearing and isotropic compression tests on a medium-coarse grading completely decomposed granite soil. The mineral composition of the soil was a substantial factor governing the compressive behavior. The soil compressibility increased significantly in the case of existence crushable and weak minerals within the soil minerals like fragile feldspar, as well as the high content of fines, especially the plastic fines. The scanning electron microscopic photos indicated that the micro-fabric of the soil had a paramount impact on the compressive behavior. The mechanism of the volumetric change depended on the stress levels, the soil mineral composition and the grain morphology. In the low consolidated stress levels, the soils’ grains rearrangement was the prevailing mechanism of the volumetric change, particularly with the absence of weak and crushable minerals. On the other hand, at the high consolidated stress levels, particles’ crushing was the prevailing mechanism in the volumetric change. Both the mechanisms of volume change could occur simultaneously at the low stress levels in the case of presence crushable minerals in addition to micro-cracks in the soil grains. The soil showed an isotropic response after 250 kPa, as this stress level erased the induced anisotropy from the moist tamping preparation method. Under the drained shearing conditions, the soil showed a contractive response, while during the undrained shearing conditions, the soil exhibited both the contractive and dilative responses with phase transformation points. The studied soil showed a unique critical state line, irrespective of the drainage conditions and initial states, the critical state line was parallel to the isotropic compression line in the void ratio effective stress space. In the deviator effective mean stresses space, the studied soil approached a unique CSL with a critical stress ratio equal 1.5, corresponding to critical friction angle of 36.8°.

Hierarchically-structured large superelastic deformation in ferroelastic-ferroelectrics

Large superelastic deformation in ferroelastic-ferroelectrics (FMs) is a complex phenomenon involving multiple mechanisms operating simultaneously. Understanding how these mechanisms contribute corporately is critical to apply this useful property to the intrinsically brittle FMs, which can therefore display both excellent functional and mechanical performance. Here, we have directly observed and quantitatively analyzed in situ in a transmission electron microscope the three main mechanisms of twinning domain, phase transformation and mobile point defect contributing to extremely large superelastic deformation in single-crystal BaTiO3 (5.0% strain) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (10.1% strain). Our results reveal the hierarchical origin of large recoverable strain in “brittle” FMs.

SANDY SOILS DYNAMIC STRENGTH PARAMETERS ACCORDING TO TRIAXIAL TESTS

Correct determination of strength properties is always relevant for the prediction of the mechanical behavior of soils and their massifs. Under static loading conditions, the issue is prominent in domestic and foreign literature, but the determination of strength properties under dynamic loading causes theoretical and methodical issues. All developed types of tests, to date, describe particular cases of the reaction of a soil foundation under dynamic loading, a general scientific approach of determination of soil strength parameters in dynamic conditions has not been developed. The article describes the features of determining the strength and strength parameters of sand in static and dynamic conditions in triaxial tests. A review of current views on this topic is provided. On the basis of triaxial tests the method of determining the phase transformation point and the failure point is described. On the basis of the Coulomb-Mohr's strength theory and the critical state approach the strength parameters are determined. The test results concluded that the destruction of sand under both static and dynamic conditions occurs with the same ratio of normal, shearing and mean effective stresses and is described by a single critical state line (CSL), presented in the form of a bilinear equation. It has been experimentally proved that the strength of sand under static and dynamic conditions is different, the strength parameters are the same, that’s why the angle of internal friction (φ) and cohesion (C) of sand can be determined only under static conditions. At the same time to quantify the strength of soil, to assess the possibility of destruction of the soil requires a correct assessment of pore pressure, which implies conducting dynamic tests or modeling its accumulation.