Plastic Phase Transformation Research Articles

In situ HEXRD experimental study and prediction of microstructures and internal stresses during heat treatment of carburized and carbonitrided low-alloyed steels

Thermochemical surface treatments of steels, such as carburizing and carbonitriding are used to improve mechanical properties, especially resistance to fatigue. Carbonitriding can achieve excellent properties, but the origins remain to be understood, as it has been less studied than carburizing. Fundamental understanding has to be established regarding the formation of microstructures and internal stresses as well as their couplings, during the cooling which follows the enrichment treatment.For this purpose, an original experimental method is introduced to follow in situ by High-Energy X-Ray Diffraction the phase transformation kinetics as well as the evolutions of the internal stresses during cooling, inside laboratory scale, 3 mm-thick samples with C and/or N composition gradients. The usual trends are confirmed regarding carburizing: the carbon-enriched case is the last to transform and the surface ends up with compression residual stresses. Conversely, for carbonitriding, unusual evolutions of microstructure and internal stresses are observed. The presence of nitrogen reduces the hardenability in the enriched case. This modifies the chronology of the phase transformations and this leads to tensile residual stresses at the surface.A coupled thermal, mechanical and metallurgical model predicting the phase transformation kinetics and the evolutions of internal stresses is set up. It achieves quantitative predictions and it shows that, in the studied cases, the phase transformation plasticity strains are the main origin of the residual stresses.

Improving interfacial strength at room and elevated temperatures in ultrasonic welding of pure titanium: Interlayer-induced phase transformation and concentrated plastic deformation

Ultrasonic welding (USW) of Titanium (Ti) sheets presents several challenges, notably crack formation due to sliding friction. To overcome this problem, the present study applied USW to pure α-Ti sheets both with and without different interlayer metals (Al, Ni, and Fe). Al interlayer improved the strength at room temperature significantly to cause base metal fracture (1700 N) by inducing concentrated plastic deformation, facilitating bonding with Ti while minimizing damage. However, its strength decreased significantly at 150–300 °C, suggesting the limitation of using Al interlayer at elevated temperatures. Conversely, the Ni and Fe interlayers led to a two-phase strength development. This enhancement was due to β-phase transformation, which reduces the interfacial defects and generates a more pronounced β-stabilization effect. The Ni interlayer, which has a higher β-transus point (765 °C) and lower molybdenum equivalency, required a higher welding energy (1800 J) and longer diffusion time into Ti, resulting in a gradual increase in strength due to a slower β-Ti transformation. On the other hand, the Fe interlayer with a lower β-transus point relative to Ti (595 °C) achieved peak strength at a lower welding energy of 1200 J. Both Fe and Ni interlayers displayed only a slight decrease in strength (1500–1600 N) at 300 °C with base metal fracture, revealing better joint performance at higher temperatures. These insights suggest a strategical interlayer selection based on the operation temperature, where Al interlayers were advantageous for low-energy welding at room temperature application, Fe and Ni interlayers offer sufficient strength at elevated temperatures by facilitating the β-Ti formation.

Multiphysics Study of Thermal Profiles and Residual Stress in Welding.

One of the effects of welding is residual stress. Welding involves complex tests concerning differences in values of the mechanical parameters of its regions as an effect of residual stress. Such multiphysics characteristics of welding pose a challenge in predicting residual stress. In the present study, a thermo-mechanical constitutive model considering phase transformation and transformation plasticity is implemented in the numerical model in ABAQUS user subroutines. In order to consider phase evolution in welding, the metallurgical parameters for Leblond's phase equation were obtained from the calibration of DH36 steel with a CCT diagram. In addition, the effects of welding speed on thermal profiles and residual stress generation were investigated. Analysis has suggested that the width of the heat-affected zone (HAZ) decreases with an increase in welding speed, and the phase fraction is significantly affected by this kind of parameter. Such phase transformation has led to the generation of a compressive stress in the fusion zone (FZ) and HAZ. The volume difference between coexisting phases produces a compressive stress in cooling, and its magnitude was increased with martensite increasing.

Wire Arc Additive Manufacturing: A Study of Process Parameters Using Multiphysics Simulations

In this work, analyses focus on understanding the effects of the scanning pattern and speed on the thermal profile, phase transformation, and residual stress generation in the WAAM deposition. An FE numerical model is constructed that takes into account the phase evolution and transformation plasticity using the ABAQUS user subroutine, UMAT. The results show that the scanning pattern significantly affects the heat accumulation and the cooling rate during the AM deposition, and, eventually, the generation of residual stresses. According to the simulation results, the highest residual stress is generated in the case of the out–in scanning, while the alternate pattern leads to the lowest residual stress. The influence of the scanning speed on the thermal profiles and residual stress are also examined. The analyses show that an increase in the scan speed leads to a decrease in the peak temperature and an increase in the cooling rate, which result in an increase in the martensite volume fraction of the deposition.

Prediction and experimental verification of phase transformation plasticity

A finite element model was established to predict phase transformation plasticity (TP). Based on the simulated results, the factors that influence TP were revealed. A formula for predicting the TP coefficient K was proposed based on the identified influence factors, and the coefficients of the formula were determined by fitting with the experimental data from the published literature. The accuracy of the proposed formula was verified by measuring the TP coefficients of ferrite/pearlite, bainitic, and martensitic transformations of 42CrMo using Gleeble-3500. The predicted TP coefficient K values using the proposed formula were found to be closer to the experimental results compared to the formulas proposed by Leblond, Desalos, and Abrassart, particularly for the ferrite/pearlite transformation. Additionally, the normalized function f(φ) which depicts the evolution of TP with the transformed fraction φ during phase transformation, was discussed based on the finite element simulation. The predicted function f(φ) was verified with experimental results conducted by Liu. The simulation results demonstrated that the expression of f(φ) is influenced by the yield stress of the parent and product phases, which explains the existence of several expressions for f(φ) in the literature.

A multi-featured shape memory alloy constitutive model incorporating tension–compression asymmetric interpolation

The mechanical response of shape memory alloys is heavily governed by phase transformation and plasticity. These inelastic mechanisms present added complexities in the form of asymmetries with respect to the stress state (tension–compression asymmetry) and the direction (forward or reverse) of transformation, leading to the formation of internal loops. With this in mind, a constitutive model is developed incorporating phase transformation and yield plasticity. The model represents the tension–compression asymmetry through a novel approach by interpolating the maximum transformation strain, the Young’s modulus of martensite and a plasticity factor in the deviatoric plane of the transformation strain direction. Internal loops are also considered by scaling the transformation hardening and the hysteresis width based on memory points. The interaction between the processes of reorientation and phase transformation is also taken into account. The model is validated with several proportional and non-proportional loading tests with considerable success. The capability of the model to simulate complex loading scenarios is demonstrated by simulating the manufacturing and loading of a stent structure.

An investigation of the welding speed on residual stress of EH36 steel with an account of phase transformation and transformation plasticity

An investigation of the welding speed on residual stress of EH36 steel with an account of phase transformation and transformation plasticity

Residual stress and distortion in thick-plate weld joint of AF1410 steel: finite element simulations and experimental studies

The quantification of the residual stress and distortion in the thick-plate weld joint of AF1410 steel has great significance, which is the basis for the safety and process design of the component. Therefore, this paper systematically investigated the welding residual stress and distortion distribution in the 10 mm thick welded plate for AF1410 steel by tungsten inert gas welding. The residual stress measurements were carried out by the X-ray diffraction method and advanced contour method, and the distortion was determined by coordinate measuring machine. The developed thermo-metallurgical-mechanical finite element model was applied to predict the residual stress and distortion in multi-pass welding considering solid-state phase transformation (SSPT) and transformation plasticity. The measured residual stress and distortion distribution were in good agreement with the finite element simulation results. The comparison results revealed that the residual stresses in the upper and lower parts of the welding plate were small, and there was apparent stress concentration in the center of the welding joint. SSPT and the subsequent thermal cycle had a significant influence on the magnitude of final residual stress. The accurate finite element model and the understanding of stress formation mechanism are helpful to control the residual stress and distortion distribution of aviation parts reasonably.

Orientation-dependent shock compression behavior of non-porous/porous NiTi shape memory alloy: An atomic scale study

Crystal orientation has a strong influence on the mechanical behavior of a material, especially at high-strain-rate impact loading. In this study, the effects of different crystallographic orientations on the non-porous/porous NiTi shape memory alloy (SMA) under high-strain-rate impact loading are studied by using molecular dynamic methods. The structural phase transformation process, energy propagation trajectory, defect dislocation evolution, and shock wavefront changes of the NiTi SMA under shock compression are comparatively analyzed. The simulation results show that for three typical crystal orientations, [100], [110], and [111], the phase transformation structures extend in the form of disordered dots, "X"-shaped meshes, and line segments, respectively. Both phase transformation and phase transformation-induced plasticity are more likely to occur in the [110] NiTi SMA; thus, more energy is dissipated, and the propagation speed of the shock wave is the lowest. In addition, the magnitude of the shock wave propagation velocity in the three crystal orientations is of the following order: [111] crystal orientation > [100] crystal orientation > [110] crystal orientation. Finally, the presence of voids affects the propagation patterns of the wavefront, and the crystal orientation directly affects the shape of the shock wavefront and the steepness of the shock wavefront for porous samples. In terms of the crystal orientation and voids, the [110] crystal orientation and voids uniformly arranged along a straight line can improve the buffer performance of the single-crystal NiTi SMA under high-strain-rate impact loading.

Effect of Transformation Plasticity on Gear Distortion and Residual Stresses in Carburizing Quenching Simulation

This paper addresses the effect of gear steel on distortion and residual stresses due to phase transformation in carburizing and quenching. In particular, the martensitic and bainitic phase transformation expansion and transformation plasticity properties of two automotive gearbox steels (20CrMnTiH and 20MnCr5) and their physical parameters are measured by experiments of transformation plasticity properties. Numerical simulations of the actual carburizing and quenching process of the gearbox spline helical gears were carried out in combination with the thermal and mechanical properties with temperature variations calculated by the material design software JMAT-Pro. In particular, the phase transformation properties of the two materials and their influence on the distortion and residual stresses after carburizing and quenching were verified by experiments of transformation plasticity and numerical simulations. A reliable basis is provided for predicting the distortion mechanism of gear steels in carburizing and quenching.

Tailoring the strength and ductility of Al0.25CoCrFeNi high entropy alloy through cryo-rolling and annealing

FCC-structured Al0.25CoCrFeNi high entropy alloy (Al0.25 alloy for short) was melted and cryo-rolled. The influences of annealing temperature (800 °C, 1000 °C, 1200 °C) on microstructural evolution and mechanical properties were investigated. The results suggested the synchronous enhancements in strength and ductility were obtained in the cryo-rolled Al0.25 alloy annealed at 1000 °C with fully recrystallization and B2 phase precipitation. TEM studying on the deformation behavior of this sample revealed that a laminated HCP phase was transformed from FCC phase during the tensile deformation at ambient temperature, which served as the media of twinning. The outstanding improvements in mechanical properties benefited from the contributions of twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP). In addition, the order of recrystallization and the precipitation/dissolution of B2 phase for the cryo-rolled Al0.25 alloy during annealing process was tried to be clarified, which provided the guideline to tailor the mechanical properties of this alloy.

Influence of filler metal on residual stress in multi-pass repair welding of thick P91 steel pipe

Based on the thermal-elastic-plastic theory, a three-dimensional finite element (FE) model for residual stress in multi-layer and multi-pass repair welding of P91 steel pipe with wall thickness of 60 mm is established, which considers the sequential coupling among thermal field and microstructure and mechanical properties. The arc heat input is described as a double-ellipsoid heat source, and the solid-state phase transformation (SSPT) concerning austenite and martensite during welding of P91 steel is modeled through allowing for the changes of material volume and yield stress as well as phase transformation plasticity in the finite element analysis. Using the established model, the residual stresses in repair welding of P91 steel pipe with similar and dissimilar filler metals are calculated through SYSWELD software, which are also compared with experimental results to validate the accuracy of the numerical model. The distribution features of residual stresses under the two conditions are comparatively investigated, and the influence of different filler metals on them is also analyzed. When using similar filler metal, the stress components and equivalent stress have strong fluctuations on the weld surface due to martensitic transformation, and there exists relatively wide compressive stress region on the weld surface. At the fusion zone (FZ) near the weld surface, a serious stress concentration appears. The tensile stress peak can reach 813 MPa, close to the yield stress of untempered martensite. While using Ni-based alloy as filler metal, the peak values of tensile stress components and equivalent stresses in the HAZ of weldment surface are higher than those for the former owing to lack of relaxation by phase transformation. In the interior of weldment, the region with high stress in the FZ is much smaller, and the welding stress in the HAZ is also lower compared with those for the former.

Tailoring phase transformation strengthening and plasticity of nanostructured high entropy alloys.

Metastable high entropy alloys (HEAs) have attracted extensive attention due to their excellent combination of high strength and great plasticity. In this work, we utilize constraining effects to tailor phase transformation strengthening and plasticity of nanostructured HEA ([double bond, length as m-dash]FeCoCrNi) thin films prepared by the magnetron sputtering technique via HEA/Cu and HEA/Ni nanotwinned nanolaminates (NTNLs). It is uncovered that the HEA/Cu NTNLs without phase transformation exhibit the fashion of "smaller is stronger" and HEA layers become more favorable to detwin at a smaller layer thickness (h). By contrast, the HEA/Ni NTNLs manifest an ultra-high peak hardness plateau accompanied by the FCC-to-HCP phase transformation in HEA layers at large h≥ 25 nm, whereas they manifest size-dependent hardness when detwinning occurs at smaller h. This unusual plastic deformation behavior of HEA/X (X = Cu, Ni) NTNLs was rationalized by partial-based mechanisms. These findings open a new avenue to achieve superior mechanical properties of HEAs particularly at the nanoscale.

Characterization of the subsurface properties of metastable austenitic stainless steel AISI 347 manufactured in a two-step turning process

At high mechanical loads and below martensite deformation temperature, metastable austenitic stainless steels undergo a deformation-induced phase transformation from γ-austenite to ε- and/or α’-martensite. During cryogenic turning, this can be exploited in order to realize subsurface hardening, thus avoiding a separate hardening process. An increase of the cutting edge radius, the chamfer angle and the feed rate leads to higher passive forces, consequently a more pronounced phase transformation and ultimately to a higher microhardness. However, increasing these input variables also results in a significant increase in surface roughness.In order to eliminate this conflict of objectives between subsurface properties and surface topography, a two-step turning process is proposed. In the first process step a pronounced phase transformation and high plastic deformation of retained austenite by means of heavily chamfered tools, very high feed rates and precooling are realized in the workpiece subsurface. In the second process step, the pronounced roughness peaks are removed, while maintaining the desired subsurface properties achieved in the first step and even increasing the phase fraction of deformation-induced α’-martensite at and near the surface.In the presented study, the surface and subsurface of workpieces manufactured applying this approach were examined. The residual stresses and the phase fraction of γ-austenite, deformation-induced ε-martensite and α’-martensite after the first and second process step were measured by means of x-ray diffraction. Furthermore, the microhardness was measured in order to quantify the mechanical properties of the hardened subsurface. With the two-step turning process it was possible to generate 87 vol.-% of α’-martensite, while the workpieces manufactured in a one-step cryogenic turning process had a maximum phase fraction of 9 vol.-% α’-martensite. Consequently, the microhardness in the subsurface zone of the workpieces was higher, while also ensuring a good surface topography.

Hall-Petch and inverse Hall-Petch relations in high-entropy CoNiFeAlxCu1-x alloys

Significant theoretical efforts have been made to understand the Hall-Petch and inverse Hall-Petch relations of nanocrystalline pure metals, metallic glasses and binary alloy systems. However, only a few studies have investigated the Hall-Petch or inverse Hall-Petch relations in high-entropy alloys. In this work, phase stability of single-crystalline CoNiFeAlxCu1-x and uniaxial compression of polycrystalline CoNiFeAlxCu1-x are investigated by molecular dynamics simulation. Calculations of cohesive energies indicate that FCC structured CoNiFeAlxCu1-x is more stable at low Al concentrations (x ≤ 0.4) and BCC structured CoNiFeAlxCu1-x is more stable for high Al concentrations (x > 0.4). Based on the phase stability, FCC structured polycrystalline CoNiFeAl0·3Cu0.7 and BCC structured polycrystalline CoNiFeAl0·7Cu0.3 are constructed to perform uniaxial compression. Hall-Petch and inverse Hall-Petch relations are observed in both FCC and BCC structured polycrystalline CoNiFeAlxCu1-x. The microstructural evolutions of polycrystalline CoNiFeAlxCu1-x reveal that the dominant deformation mechanisms in the Hall-Petch regime of FCC structures are dislocation slip and deformation twinning due to relatively low stacking fault energy and that of BCC structures is phase transformation plasticity. For the inverse Hall-Petch relation, the dominant deformation mechanisms for both FCC and BCC HEAs are the rotation of grains and migration of grain boundaries. It indicates that FCC and BCC HEAs exhibit similar Hall-Petch and inverse Hall-Petch relations with the conventional polycrystalline materials, but its grain size exponent and gradient are quite different from those of pure metals.

A three-dimensional phenomenological model describing cyclic behavior of shape memory alloys

Nowadays, shape memory alloys, in particular Nickel–Titanium alloys (NiTi), are widely adopted in many fields (biomedical, aerospace, automotive, civil ...) for producing devices exploiting pseudo-elasticity or shape memory effect. These devices are often subjected to cyclic loadings and sometimes to large deformations that modify the material response inducing functional fatigue and/or plastic strains. Accordingly, the device effectiveness is limited or even completely compromised. In this paper, a model able to take into account both the phenomena is presented. Aiming to propose this model for the design and assessment of cyclically loaded devices by finite element simulations, a phenomenological approach was used, introducing internal variables able to describe the accumulation of inelastic strains due to fatigue and plasticity and assuming that their evolution law affects also the phase transformation domain amplitude. Two limit functions was introduced for defining the phase transformation domain and plastic region. The discrete formulation of the model is also presented. Peculiar attention is posed on the material model parameters and the procedure to calibrate them. Initially, a driver code was written in MATLAB for verifying the algorithm correctness and effectiveness; successively, the model was implemented by a user subroutine in a commercial finite element code. The results of several numerical tests are reported in order to show the model functionality and stability under different loading conditions. Finally, the model capability of reproducing 1D and 3D experimental tests is shown by comparison of numerical results with experimental tests published in the literature.

Phase transformation assisted twinning in a face-centered-cubic FeCrNiCoAl0.36 high entropy alloy

The FeNiCoCr-based high entropy alloys (HEAs) exhibit excellent mechanical properties, such as twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP) that can reach a remarkable combination of strength and ductility. In the present work, the face-centered-cubic (FCC) single-crystal FeNiCoCrAl0.36 HEAs were studied, using the density functional theory (DFT) combined with the phonon calculation to estimate the stacking fault energies, temperature-dependent phase stabilities of different structures. And the kinetic Monte Carlo (kMC) was used to predict the substructures evolution based on the transition state energies obtained from DFT calculations. We proposed two different formation paths of nano-twins in this Al-composited HEA and found that short-range hexagonal-close-packed (HCP)-stacking could occur in this HEA. The DFT calculations suggest that this HEA has negative stacking fault energy (SFE), HCP formation energy, and twin-formation energy at 0 K. Phonon calculations indicate that at the finite temperature, the competing FCC/HCP phase stability and propensity for twinning make it possible to form HCP-like twin boundaries. The kMC simulations suggest that under deformation, TWINs could form within the HCP laths which differs from the study of others. With the great agreement of results from kMC simulations and experiments, this twin-hcp laminated substructure formation path offers a new concept of designing TWIP HEAs containing tunable twin structures with HCP and TWIN lamellae structures, which could result in better mechanical properties of HEAs.

The FEM simulation and experiment of quenching distortion of a U-shape sample and the sensitivity analysis of material properties

Heat treatment is a standard post-forming process for parts. This work investigates simulating heat treatment process of two steel components and compares this to the equivalent experimental treated parts. Firstly, considering the influence of phase transformation and transformation plasticity, a Finite Element Method (FEM) model based on a U-shape sample is employed to simulate the quenching process. Additionally, the equivalent quenching deformation experiment of U-shape samples of two steel were done for verifying the calculation model. Finally, the sensitivity analyses of the material properties (including transformation related material parameters) on the distortion during the quenching processes are performed. Research results shows that the numerical simulation results of the quenching distortion of U-shape samples were in good agreement with the experimental results. The phase transformation related parameters, especially for transformation plasticity coefficient, are very important on the calculation on quenching distortion, and the phase transformation temperature and quenching temperature are the most sensitive input parameter on the calculation of quenching distortion for 0Cr13Ni4Mo and 316 steels, respectively. In summary, the accurate determination of the material parameters is very important to achieve accurate deformation prediction.

A Three-Dimensional Dynamic Constitutive Model and Its Finite Element Implementation for NiTi Alloy Based on Irreversible Thermodynamics

In the present work, the governing equations based on theory of irreversible thermodynamics is deduced by introducing two internal variables to characterize the phase transformation and finite plastic deformation evolution for NiTi shape memory alloy. Thus a three-dimensional dynamic constitutive model is established by summarizing the governing equations for phase transformation and plastic deformation under high strain rate loading conditions. By adopting a stress compensation algorithm to update the stress tensor, the phenomenological-based constitutive model is embedded into ABAQUS finite element software as user material subroutine with FORTRAN code. Thus the numerical simulation of dynamic responses of NiTi alloy is successfully implemented. The numerical simulation results are in good agreement with the experimental data, validating the feasibility of this proposed model. Comparison between the simulation results and the experimental data indicates that the proposed model can well describe not only the different deformation stages of NiTi alloy but also its constitutive behavior subjected to different high strain rates.

Metastability alloy design

Abstract