Isothermal Uniaxial Compression Research Articles

A polycrystal plasticity-cellular automaton integrated modeling method for continuous dynamic recrystallization and its application to AA2196 alloy

Continuous dynamic recrystallization usually dominates the microstructural evolution in hot working of aluminum alloys, in which the high-angle grain boundaries of new grains mainly originate from the gradual increase in subgrain misorientation angles. In this work, an integrated computational method is proposed to simulate continuous dynamic recrystallization process of aluminum alloys by coupling three-dimensional cellular automaton and visco-plastic self-consistent models. The stress response, dislocation accumulation and recovery, and evolution of crystal orientations are computed in the context of polycrystal plasticity; the formation and rotation of subgrains, followed by stored energy and curvature-driven boundary migration, are captured and visualized by cellular automaton. The non-octahedral slip mode {110}<110> is additionally introduced to capture the 〈001〉 texture during hot compression. A universal cell topology deformation method is adopted to achieve an effective track of grain morphology evolution during plastic deformation. The proposed simulation framework is validated through simulating the isothermal uniaxial compression process of AA2196 alloy under different temperatures and strain rates. The orientation dependence of CDRX during compression is numerically reproduced by correlating the subgrain formation and rotation process with the activation state of slip systems. The simulated macroscopic flow stress, 3D microstructure and inherent microstructural characteristics such as subgrain size, subgrain boundaries and textures are in good agreement with the experimental results. The proposed method provides an effective and efficient tool for multi-scale simulation of hot forming process of aluminum alloys.

Thermal deformation behavior and microstructural evolution of the rapidly-solidified Al–Zn–Mg–Cu alloy in hot isostatic pressing state

Isothermal uniaxial compression experiments were performed in the temperature range of 340–460 °C and the deformation rate range of 0.001–1 s−1 to analyze the hot deformation behavior of the rapidly-solidified Al–Zn–Mg–Cu alloy in hot isostatic pressing state. A strain-compensated constitutive model was established to determine the flow stress in the alloy (based on the true stress-true strain data), and the average activation energy for the hot deformation was calculated as Q = ∼146 kJ/mol. The proposed model exhibited a high predictability with an average absolute relative error of 2.68% and a correlation coefficient of 0.99724. The processing map revealed that the alloy in the hot isostatic pressed state offers better workability than the spray-deposited alloy, and its optimal workable ranges at a strain of 0.78 are 370–390 °C/0.004–0.01 s−1 and 400–460 °C/0.005–0.06 s−1, respectively. The microstructural evolution shows that the main dynamic softening mechanism changes from dynamic recovery to continuous dynamic recrystallization with the increase in temperature and the decrease in strain rate.

Intragranular ferrite nucleation on MX carbonitrides and dislocations

The competing mechanisms of ferrite nucleation on (Ti,V)(C,N) MX carbonitrides and dislocations, as well as their dependence on deformation, are investigated experimentally by thermo-mechanical treatments and via simulation. The impact of recrystallization and the resulting austenite grain size on intragranular ferrite nucleation is evaluated. The austenite-to-ferrite transformation temperatures, affected by different microstructures, are examined by isothermal uniaxial single-hit compression tests on a dilatometer DIL 805 and compared to thermal treatments without deformation. Different resulting microstructures are analyzed by using optical light microscopy. The experimental data is used for the validation of thermo-kinetic simulations with the mean-field modeling software MatCalc using the implemented models for the austenite grain evolution, the on-particle nucleation of ferrite on the surface of MX particles, the dislocation density evolution, and recrystallization.

Modeling the hot deformation behavior of micro-alloyed steel in the single and two-phase fields

Continuous casting is a well-established process to produce steel slabs. However, the surfaces of the slabs may crack during processing. The occurrence of cracks is related to different phenomena, i.e., ferrite formation, presence of precipitates, and microstructural modifications during manufacturing processing. In the present work, we developed a physically based mesoscale model to describe the plastic deformation of micro-alloyed steel in the single and two-phase fields, as well as the microstructure evolution. We implemented a dislocation density-based constitutive model to calculate the strain hardening and the stress softening produced by dynamic recovery using the Kocks-Mecking (KM) formalism for the austenite and ferrite. We coupled the KM model with the Johnson-Mehl-Avrami-Kolmogorov model to consider the dynamic recrystallization of the austenitic phase. The nucleation and growth of the recrystallized grains, driven by the stored energy, compete with the annihilation of dislocations due to dynamic recovery. In the two-phase domain, the iso-work condition for the load partition is implemented. We calculated the ferrite volume fraction by fitting the data obtained using JmatPro software. Moreover, an Arrhenius-type equation correlates the yield stress to the Zener-Hollomon parameter. We validated the model with isothermal uniaxial compression tests of microalloyed steel over the temperature range of 650 °C-1100 °C and strain rates of 10−2 s−1 and 10−3 s−1 using a Gleeble® 3800 thermomechanical simulator.

Correction to: Optimizing Process Parameters of As-Homogenized Mg-Gd-Y-Zn-Zr Alloy in Isothermal Uniaxial Compression on the Basis of Processing Maps via Prasad Criterion and Murty Criterion

Correction to: Optimizing Process Parameters of As-Homogenized Mg-Gd-Y-Zn-Zr Alloy in Isothermal Uniaxial Compression on the Basis of Processing Maps via Prasad Criterion and Murty Criterion

A novel thermo‐mechanically coupled material model for glass above the glass transition temperature

AbstractThin glass products have a giant field of application in several engineering branches such as for example, electronics, medical equipment, and the automotive industry. The non‐isothermal glass molding is a novel replicative glass processing technology enabling the realization of a cost‐efficient production of surface shapes with high accuracy and complexity. However, the application of this technology to thin glass production still causes shape distortions, cracks, and surface defects in molded parts. Therefore, these glass‐forming processes should be simulated by means of the combination of experimental investigation and mechanical modeling of glass above the glass transition temperature at finite strains. Previous experimental studies have shown that the Maxwell model is a reasonable approach for an appropriate prediction of the material behavior. Based on this viscoelastic formulation, a thermo‐mechanically consistent material law is used enabling the prediction of rheological effects noticed during the experiments. More specifically, the relaxation behavior can be described by a stress‐dependent relaxation time and the dissipation generated is considered as well. Furthermore, isothermal uniaxial compression tests above the glass transition temperature are conducted for different strain rates and temperatures within the experimental investigation. Combining the experimental data with the simulation, a multi‐curve‐fitting leads to suitable material parameters with respect to distinct temperatures employing a nonlinear optimization.

Influence of strain rates and aging time on microstructure and hardness of integrally compressed 6082 aluminum alloy

In this study, an in-depth analysis of the microstructure and hardness of 6082 aluminum alloy under a novel solution-forging integrated process was conducted by performing a series of isothermal uniaxial compression tests at various strain rates and aging durations. The results indicated that the highest hardness was 125.0 ± 1.2 HV at peak aging with a forming strain rate of 10 s−1; a 16.1% improvement compared to the conventional T6 forming process. The deformation reduced the subsequent peak aging time from 8 h in the conventional process to 6 h. In addition, the composite phases, including short-range disordered B'/U2 phase and composite phase with multiple structures, were also observed under a strain rate of 10 s−1 with 6 h of aging. As the strain rate increased, the grain size increased, accompanied by an increase in dislocation density. For the integrated process, the deformation microstructure would inherit to the aging state and affect the precipitation kinetics during aging, including grain morphology, dislocation density, and the aging precipitated phase, especially for the formation of composite phase; these were the main reasons for the higher hardness after ageing when compared to the hardness of the alloy under traditional T6 state.

Hot Deformation Behavior and Processing Maps of an As-Cast Al-5Mg-3Zn-1Cu (wt%) Alloy.

One of the key issues limiting the application of Al-Mg-Zn-Cu alloys in the automotive industry is forming at a low cost. Isothermal uniaxial compression was accomplished in the range of 300-450 °C, 0.001-10 s-1 to study the hot deformation behavior of an as-cast Al-5.07Mg-3.01Zn-1.11Cu-0.01Ti alloy. Its rheological behavior presented characteristics of work-hardening followed by dynamic softening and its flow stress was accurately described by the proposed strain-compensated Arrhenius-type constitutive model. Three-dimensional processing maps were established. The instability was mainly concentrated in regions with high strain rates or low temperatures, with cracking being the main instability. A workable domain was determined as 385-450 °C, 0.001-0.26 s-1, in which dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred. As the temperature rose, the dominant dynamic softening mechanism shifted from DRV to DRX. The DRX mechanisms transformed from continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and particle-stimulated nucleation (PSN) at 350 °C, 0.1 s-1 to CDRX and DDRX at 450 °C, 0.01 s-1, and eventually to DDRX at 450 °C, 0.001 s-1. The eutectic T-Mg32(AlZnCu)49 phase facilitated DRX nucleation and did not trigger instability in the workable domain. This work demonstrates that the workability of as-cast Al-Mg-Zn-Cu alloys with low Zn/Mg ratios is sufficient for hot forming.

Special deformation mechanisms dominated by grain boundary sliding of extrusion–refined Ti–22Al–25Nb alloy during compression in single BCC phase field

The deformation behavior of Ti–22Al–25Nb alloys with refined initial grain size was investigated under isothermal uniaxial compression in the BCC (B2) single-phase field, where a high-temperature extrusion process was used to produce the test materials with finer initial grains. As compressed at low to moderate strain rates, the present material exhibits significant continuous flow softening. To study the deformation and softening mechanisms during the compression, the samples' microstructures were examined using EBSD. The analysis shows that this alloy has a considerable grain boundary sliding (GBS) mechanism in addition to the conventional mechanisms of dynamic recovery (DRV), dynamic recrystallization (DRX), and texture softening. For the first time, this study offers direct evidence of the presence of GBS in current alloys. And the dominant role of GBS in high-temperature deformation and softening of current alloys was identified by comparing the impacts of various mechanisms on the dislocation density distribution. Besides, the impact of GBS on texture evolution was also discussed.

Modified Zerilli-Armstrong and Khan-Huang-Liang constitutive models to predict hot deformation behavior in a powder metallurgy Ti-22Al-25Nb alloy

In order to establish constitutive models for a fine-grained Ti-22Al-25Nb alloy fabricated by spark plasma sintering under high vacuum, isothermal uniaxial compression tests are conducted at the different deformation conditions of 950–1070 °C and 0.001–1 s−1. Using the friction-corrected experimental data, the modified Zerilli-Armstrong model and the Khan-Huang-Liang model are developed in the α2 + β/B2 + O triple-phase and α2 + B2 two-phase fields, respectively. The determination coefficient (R2) and average absolute relative error (AARE) of the developed models are 0.9773 and 8.73%, 0.9831 and 12.36%, respectively. Based on the analyzing the reason of large deviation, the modified Zerilli-Armstrong and Khan-Huang-Liang models are improved. The prediction accuracy of the improved constitutive models is improved to 0.9896 and 6.14%, 0.9891 and 6.82%, respectively. This indicates that the improved versions of the modified ZA and KHL models are very appropriate for predicting the hot deformation behavior of a fine-grained Ti-22Al-25Nb alloy fabricated by spark plasma sintering.

Microscopic mechanism of nanoscale shear bands in an energetic molecular crystal (α-RDX): A first-order structural phase transition

Nanoscale shear bands formed in many energetic molecular crystals upon shock compression [including 1,3,5-trinitro-$s$-triazine (RDX)] are considered as a defect-free mechanism for formation and growth of hot spots which control detonation initiation. Using classical molecular dynamics, we predict the formation of similar nanoscale shear bands in the \ensuremath{\alpha}-RDX crystal subjected to quasistatic isothermal uniaxial compression indicating a common mechanism of shear strain localization under both shock and quasistatic conditions. In the framework of the Ginzburg-Landau phenomenology coupled with the coarse-grained (CG) Helmholtz free energy of the crystal from first principles, we explore the thermodynamics of stress-induced lattice transformations under quasistatic uniaxial load. We show that the shear banding exhibits a critical behavior associated with a first-order structural phase transition with bands of localized twinning strain as transient microstructure. Analysis of the CG Helmholtz free energy suggests that the stress-induced core softening of the effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands.

Stress-strain response properties of the KhN55MVTs nickel alloy under uniaxial compression and their mathematical modeling

The rheology of the KhN55MVTs alloy has been studied at finite quasi-static strains and at operational and elevated temperatures to develop methods for calculating the emergency conditions of heat-stressed equipment. Some basic features of the stress-strain response of the alloy and the physical mechanisms responsible for them were revealed. Constitutive models are proposed to describe the most significant and crucial effects in assessing the structural integrity. The rheological properties of the alloy were studied under isothermal uniaxial compression within a temperature range of 24 – 1150°C with log-strains up to 1.0 and deformation rate of 0.001 – 0.125 sec–1. The main deformation mechanisms have been revealed via optical microscopy. A constitutive model predicting the shape of stress-strain curves (a modification of the Bergström dislocation-based model of rigid-plasticity) is proposed which make it possible to obtain the rate and temperature dependencies of the mechanical properties. The dependence of maxima (peak values) of stresses on the deformation rate and temperature exhibits a non-monotonic character, while the yield stresses are weakly rate-dependent, and the linear slope is almost rate and temperature independent. The microstructure tests revealed the absence of a correlation between the softening stage and the onset of dynamic recrystallization process. No microcracks were found. Serrated flow and acoustic emission were observed within a temperature interval of 24 – 900°C probably attributed to dynamic aging (above 500°C), deformation twinning, and autowave effects during localization of the plastic strain. The proposed models of rheological effects differ from the existing dislocation models in a wider range of application — in terms of strain rates (10–8 – 1,0 sec–1) and temperatures (0 – 80% of the melting point). The rheological effects revealed in the experiments, analysis of their physical nature and constitutive description can be used in assessing failures of heat-stressed equipment and improving the methods for calculating emergency situations.

Constitutive Modeling of Hot Deformation of Carbon Steels in The Intercritical Zone

A previous constitutive modeling for single-phase steels is extended using the mixing law to predict the behavior of hot deformation in the dual phase ferritic-austenitic intercritical zone of Fe-C-Mn-Si alloys. Mixing law considers two phases instead one, so one phase formula was modified. The constant’s values used represents average values to the same conditions in austenitic and ferritic model. The amount of each phase is determined as function of temperature and chemical composition. The developed constitutive modeling is validated by comparing the theoretical stress-strain curves with experimental isothermal uniaxial compression tests of 1008 and 1035 carbon steels at different temperatures and strain rates. The compression tests were carried out in a dilatometer with the compression load at strain rate of 10-3, 10-2 and 10-1 s-1. A good agreement was obtained between the calculated and experimental results over different stages of deformation and hardening. Microstructural analysis was also carried out to relate the deformation results to the microstructure of the steels. Finally, a general constitutive equation has been proposed for hot deformation of steels in the intercritical zone.

Hot deformation constitutive model and processing maps of homogenized Al–5Mg–3Zn–1Cu alloy

Isothermal uniaxial compression experiments were performed in the range of 300–450 °C and 0.001 to 10 s −1 to clarify the hot deformation behavior of homogenized Al–5Mg–3Zn–1Cu alloy. The results revealed that the flow curves exhibited typical characteristics of DRV/DRX accompanied by the work-hardening, and the existence of three distinct stages of work-hardening, transition, and steady-state in each flow curve. A strain-compensated constitutive model for determining flow stress in this alloy was established with highly acceptable predictability. The dominant deformation mechanism of the alloy is dislocation climbing. Also, dynamic-material-model-based processing maps at various strains were constructed, and the unstable and workable domains were distinguished. Instability generally occurred in high strain rates and low temperatures zone with prevalent instability forms such as cracking and flow localization. The workable domain was 375–450 °C and 0.001–0.1 s −1 , in which the microstructure of the deformed alloy was characterized by dynamic recovery and multiple types of dynamic recrystallization. The dynamic precipitates were labeled as T-Mg 32 (AlZnCu) 49 phase. These intermittently distributed precipitates along the grain boundaries were stable in all temperature ranges, while the intragranular precipitates were inhibited at 300 °C. Temperature increment to 400 °C led to a large number of dispersed intragranular precipitates which redissolved into the matrix as temperature exceeded 450 °C.

Deformation mechanism of ZK61 magnesium alloy cylindrical parts with longitudinal inner ribs during hot backward flow forming

• Isothermal uniaxial compression was proposed as the physical simulation test of hot backward flow forming (HBFF) of thin-walled cylindrical parts with longitudinal inner ribs (CPLIRs). • 3D hot processing maps (HPMs) during HBFF of the Mg alloy were constructed as a function of temperature, strain rate and strain. • Finite Element simulation and 3D HPMs were integrated to analyze the formability of hot spin-forming. • Typical defects during HBFF of Mg alloy CPLIRs were identified, and forming quality can be qualified by the proposed evaluation indexes. Thin-walled cylindrical parts with longitudinal inner ribs (CPLIRs) made of magnesium alloys are one of the most promising lightweight components. Flow forming is an effective technology to realize the integrated manufacturing of thin-walled CPLIRs. However, due to the complicated material flow and narrow temperature window, defects like insufficient filling at the rib groove and concavity of the back of rib occur easily during hot backward flow forming (HBFF) of ZK61 alloy thin-walled CPLIRs. The isothermal uniaxial compression test was proposed as the physical simulation test of the HBFF of thin-walled CPLIRs, and a novel method for analyzing the formability of hot spin-forming was proposed on the basis of the combination of 3D hot processing maps (HPMs) and finite element (FE) simulation. The process experiment and FE simulation integrated with 3D HPMs were conducted to investigate the deformation characteristic and material flow, and formation mechanism of defects was revealed. Further, the influence of process parameters on the power dissipation efficiency of spun workpiece was discussed. The results show that safe deformation of the alloy is concentrated at a narrow region with high temperature and low strain rate; the large tangential flow at the cylindrical wall and radial flow at the inner rib contributed to the filling of rib groove; unsaturated inner rib and concavity of the outer wall at the back of rib are typical defects; the ZK61 alloy CPLIRs exhibits good formability under reasonable process parameters, which is beneficial for obtaining qualified forming quality and uniform recrystallized microstructure.

Constitutive Modeling of a Fine‐Grained Ti2AlNb‐Based Alloy Fabricated by Mechanical Alloying and Subsequent Spark Plasma Sintering

The flow stress evolution of a fine‐grained Ti2AlNb‐based alloy fabricated by mechanical alloying and subsequent spark plasma sintering is experimentally acquired for different deformation conditions. Isothermal uniaxial compression test is conducted in the deformation temperature range of 950–1070 °C and the strain rate range of 0.001–1 s−1. Based on the experimental flow stress with friction correction, the modified Johnson–Cook model and strain‐compensated Arrhenius‐type model are established in the α2 + β/B2 + O triple‐phase and α2 + B2 two‐phase fields. The average absolute relative error (AARE) and determination coefficient (R 2) are 9.78% and 0.9858 for the modified Johnson–Cook model, and 4.19% and 0.9927 for the strain‐compensated Arrhenius‐type model. Compared with the modified Johnson–Cook model, the strain‐compensated Arrhenius‐type model is more suitable for predicting the high‐temperature flow stress of a fine‐grained Ti2AlNb‐based alloy fabricated by mechanical alloying and subsequent spark plasma sintering.

High-Temperature Mechanical Behaviors of SiO2-Based Ceramic Core for Directional Solidification of Turbine Blades.

The high-temperature mechanical behaviors of SiO2-based ceramic cores for the directional solidification of turbine hollow blades were investigated. Isothermal uniaxial compression tests of ceramic core samples were conducted on a Gleeble-1500D mechanical simulator with an innovative auxiliary thermal system. The stress–strain results and macro- and micro- structures of SiO2-based ceramic cores were investigated experimentally. The microstructures were characterized by the scanning electron microscope (SEM). Based on the experimental data, a nonlinear constitutive model for high temperature compressive damage was established. The statistical results of Weibull moduli show that the stability of hot deformation increases with the increase of temperature. The fracture type of the SiO2-based core samples is brittle fracture, but when the temperature exceeds 1400 °C, the mechanical behavior exhibits thermo-viscoelastic and viscoplastic property. Under high-temperature (>1400 °C) and stress conditions, the strength of the ceramic core is weakened owing to the viscous slip of SiO2, which is initially melted at the temperature of 1400 °C. The comparison results between the predictions of nonlinear model and experimental values indicate that the model is applicable.

Assessment of constitutive models to predict high temperature flow behaviour of Ti-6Al-4V preform

ABSTRACT The application of hyperbolic sine equation and Artificial Neural Network(ANN) model to predict high temperature flow behaviour of Ti-6Al-4V preform is assessed for development of a constitutive equation to model ring rolling process. Uniaxial compression specimens were obtained from a specific location and direction, where the radial deformation is the major deformation location, to study the ring rolling process. The adiabatic temperature corrected data was used to find constants for hyperbolic sine equation and train the ANN model. The ANN model used normalised strain, strain rate and temperature as input variables and the output parameter was the flow stress. Comparison of the predicting capability of both models showed that the predictions of ANN were better. The hyperbolic sine equation was integrated in a user subroutine and finite element-based simulations of isothermal uniaxial compression were carried out for validation.

Hot deformation behaviour of 40CrNi steel and evaluation of different processing map construction methods

40CrNi steel is a low alloy steel with medium hardenability and has been widely used in the manufacturing of crankshafts, wind turbine forgings, etc. As forging is usually involved in the manufacturing of these components, optimization of the hot deformation process of 40CrNi steel is quite critical. Towards this, the prediction of flow stress and the determination of optimal processing parameter windows are of great importance. In this research, isothermal uniaxial hot compression tests were carried out for 40CrNi steel with true strain rates of 0.01, 0.1, 1, 10 and 30 s−1 and temperatures of 850–1200 °C. Both the strain compensated hyperbolic sine constitutive equation and the modified Johnson-Cook model were adopted to predict the flow stress curves, with average absolute relative errors of 0.034 and 0.061, respectively. Processing maps of the tested steel were constructed to determine the optimal deformation parameter windows, and the characteristics of the prior austenite microstructure generally correspond very well to the processing map established. Furthermore, different processing map construction methods were compared and it was found that power-dissipation efficiency maps strongly rely on the σ- ε˙ relationship. Assuming a certain type of constitutive equations leads to monotonic efficiency value patterns. Calculation methods based on experimental results are helpful to reflect the meaningful information carried by the experimental σ- ε˙ relationships.

Hot Deformation of 18 % Ni Maraging Steels: A Review

Abstract The published literature makes reference to the excellent hot workability of 18 % nickel maraging steels. However, production of large cross-sectioned products of 18 % nickel maraging steels by hot deformation can lead to different types of quality problems, such as (i) inadequate recrystallization and grain structure aligned in working direction, resulting in directionality of mechanical properties; (ii) thermal embrittlement that leads to poor values of ductility and fracture toughness; and (iii) sharp increase in flow stress with decreasing hot working temperature, necessitating the application of high loads for deformation at relatively low temperatures. To overcome these problems, primarily two sets of studies have been carried out by researchers. One set of studies was centered around the development of an optimum post–hot work solution treatment. Two-stage solution treatments, as opposed to standard single-stage solution treatment at 820°C, have been developed and are shown to lead to a fine-grained microstructure and an excellent combination of strength and fracture toughness with a high degree of isotropy. It has been documented that such heat treatments can also rejuvenate the quality of the products produced through a nonoptimum hot deformation schedule. The other set of studies was focused on establishing deformation processing maps using hot isothermal uniaxial compression testing. Different grades of 18 % nickel maraging steels have been the subject of investigation. Based on these studies, regimes of strain rate and temperature have been identified for optimum processing. Regimes in which flow localization occurs and hence need to be avoided have been identified. Processing regimes for obtaining fine-grained microstructures have been identified; regimes that lead to coarse grains, which need to be avoided, have been established. Constitutive equations have been developed to predict the deformation behavior as an aid to the design of the hot deformation process.