Isothermal Deformation Research Articles

Hot deformation behavior of Cu/Al laminated composites under interface constraint effect

In order to understand the hot deformation behavior of novel Cu/Al laminated composites and evaluate the effect of interface on deformation, isothermal compression tests were conducted on the Cu/Al composites with and without a bonding interface and monometallic Al. Results show that after hot compression, there is a obvious relative sliding between Cu and Al layers of the Cu/Al composites without a bonding interface. As for the Cu/Al composites with a bonding interface, soft Al layer trends to flow synchronously with hard Cu layer under the interface constraint effect. And further microstructure examinations indicate the cooperative deformation capability of Cu/Al composites increases with increasing stain rate and decreasing deformation temperature. Strain hardening exponent, calculated based on the true stress-true strain data, also proves the effect of deformation temperature and strain rate on the cooperative deformation behavior. Meanwhile, because softening mechanism of Al layer plays a dominant role at high deformation temperature, the interface constraint effect was weaken and the flow stress of Cu/Al composites was similar to that of monometallic Al. Due to the unique composites structure, the dynamic softening mechanism of Al layer in Cu/Al composites is dynamic recrystallization during isothermal deformation. Additionally, the deformation activation energy of Cu/Al composites was calculated to be 186.43 KJ mol−1.

Effect of multipass deformation at elevated temperatures on the flow behavior and microstructural evolution in Ti-6Al-4V

Simulated multipass deformation experiments were carried out via torsion testing on a Ti-6Al-4V alloy in the two-phase region under both isothermal and continuous cooling conditions. Flow softening was observed during the isothermal multipass deformation tests as indicated by the evolution of the mean flow stress (MFS). The MFS values increased with interpass time from 2 s to 32 s. Using BSE-SEM characterization techniques, dynamic phase transformation (alpha to beta) and coarsening of the alpha phase took place. The quantitative results indicate that the alpha phase transforms into beta during straining, but it retransforms statically into alpha by amounts that increase with interpass time. The flow softening observed is the net result of softening by dynamic transformation and hardening by reverse transformation.

Effect of Deformation Temperature on Dynamic Recrystallization and CSL Grain Boundary Distribution of Fe-36%Ni Invar Alloy

This paper investigated the effect of deformation temperatures on microstructure evolution and grain boundary character distribution (GBCD) of Fe-36%Ni Invar alloy during hot compression tests. The isothermal hot deformation tests were carried out in a temperature range of 850-1100 °C and a strain rate of 0.01 s−1. The grain boundary character distribution was based on the Σ3 n (n = 1, 2, 3,…) coincidence site lattice (CSL) boundaries. The experimental results show that the extent of dynamic recrystallization significantly expanded at the strain rate of 0.01 s−1 with the increase in temperature and this phenomenon was consistent with the evolution tendency of Σ3 n boundaries. To some degree, the increasing proportion of typical low-ΣCSL boundaries, especially the increasing trend of Σ3, positively promoted the wider process of dynamic recrystallization.

Microstructural evolution of a Ni-Fe-Cr-base superalloy during non-isothermal two-stage hot deformation

Generally, the hot deformation behaviors and microstructural evolution of alloys are studied by isothermal hot compressive experiments. However, in the practice industrial production, the temperature in metallic parts is variant in hot forming process. In this study, the deformation mechanism and microstructural evolution of a solid-solution Ni-Fe-Cr-base superalloy during the non-isothermal two-stage hot compressive deformation are investigated. Here, the deformation temperature of the first stage is 1040 °C, while those of the second stage are 950, 980, and 1010 °C. Meanwhile, the dynamic recrystallization (DRX) behaviors and nucleation mechanisms are analyzed. It is found that the temperature of the second stage (namely the final deformation temperature) has a great impact on the microstructures. During the non-isothermal two-stage hot deformation, the DRX volume fraction increases as the deformation degree of the first stage or the final deformation temperature is increased, or the strain rate is decreased. Compared with the isothermal hot deformation, DRX behaviors are more obvious during the non-isothermal two-stage hot deformation. In addition, DRX nucleation mechanisms mainly consist of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) during the non-isothermal two-stage hot deformation. Furthermore, DDRX plays the primary role.

Austenite Grain Growth Kinetics after Isothermal Deformation in Microalloyed Steels with Varying Nb Concentrations

Austenite Grain Growth Kinetics after Isothermal Deformation in Microalloyed Steels with Varying Nb Concentrations

Properties and Structure of X30MnAlSi26-4-3 High Strength Steel Subjected to Dynamic Compression Processes

Abstract The paper presents the results of investigation on X30MnAlSi26-4-3 austenitic steel subjected to dynamic compression using the split Hopkinson pressure bar. The strain rate was 3700 s−1. The compression test was also carried out without the use of breaking rings and then true strain was about 0.3. The split Hopkinson pressure bar test take only few milliseconds to complete during which time it is impossible to transfer the excess heat out of the specimen, therefore the test must be carried out in adiabatic conditions and so the increase of the temperature caused by the work of plastic deformation had to be calculated. The stepping load method was used in order to evaluate the effect of adiabatic heating on the properties of steel which allowed to maintain the isothermal deformation conditions. The paper presents the comparison of results obtained during deformation under adiabatic and isothermal conditions in correlation to structure changes occurring in course of dynamic compression.

Effects of temperature and strain rate on the dynamic recrystallization of a medium-high-carbon high-silicon bainitic steel during hot deformation

Isothermal compressive deformation tests for 0.58C–1.89Mn–1.57Si–0.88Cr (wt.%) bainitic steel were conducted on a Gleeble 1500D thermo-simulator machine at temperatures ranging from 950 °C to 1150 °C and strain rates ranging from 0.005 s−1 to 5 s−1. The flow characteristics, particularly dynamic recrystallization (DRX) behavior, were investigated. The results showed that the deformation temperature and strain rate have significant effects on the DRX behavior of the studied steel. The grain boundaries through local bulging provide the nucleation cites for DRX. The high temperature and low strain rate can speed up the progress of DRX and the growth of recrystallized grains. The hot deformation activation energy is estimated to be 317.389 kJ/mol, and the critical conditions for initiating DRX are identified by using the Zener-Hollomon parameter. Based on the microstructure observation and the data of flow curves, the DRX models of grain size and kinetics are developed; these models yield satisfactory precision and reliability.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ПАРАМЕТРОВ ИЗОТЕРМИЧЕСКОЙ ДЕФОРМАЦИИ И ТЕРМИЧЕСКОЙ ОБРАБОТКИ НА СТРУКТУРУ И МЕХАНИЧЕСКИЕ СВОЙСТВА ШТАМПОВКИ ИЗ СПЛАВА ВТ6ч.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ПАРАМЕТРОВ ИЗОТЕРМИЧЕСКОЙ ДЕФОРМАЦИИ И ТЕРМИЧЕСКОЙ ОБРАБОТКИ НА СТРУКТУРУ И МЕХАНИЧЕСКИЕ СВОЙСТВА ШТАМПОВКИ ИЗ СПЛАВА ВТ6ч.

Microscale modeling of creep deformation and rupture in Nickel-based superalloy IN 617 at high temperature

This manuscript presents the computational modeling and analysis of creep deformation and failure of Nickel-based superalloy, Inconel 617 (IN 617), operating at high temperature. Crystal plasticity finite element (CPFE) approach, considering isothermal and large deformation conditions at the microstructural scale has been extended for creep deformation and rupture modeling of IN 617 at 950 °C. In order to accurately capture the creep strains that accumulate particularly at relatively low stress levels, a dislocation climb model has been incorporated into the CPFE framework. In addition, a cohesive zone (CZ) model is adopted to capture intergranular creep damage, and incorporated into the CPFE framework. The CPFE and the CZ models work in tandem to describe the viscoplastic deformation as well as progressive failure in the material microstructure. The calibration of dislocation climb and CZ parameters is performed based on experimental data. The microstructure model is validated using independent creep experiments performed at various stress levels. Microstructural analysis of the stress and damage distributions as well as their time-dependent evolution is carried out to provide insight into the dominant microscale deformation and failure mechanisms. Creep life predictions are performed to describe rupture life as a function of load amplitude at high temperature.

Pseudo-creep in Shape Memory Alloy Wires and Sheets

Interruption of loading during reorientation and isothermal pseudoelasticity in shape memory alloys with a strain arrest (i.e., holding strain constant) results in a time-dependent evolution in stress or with a stress arrest (i.e., holding stress constant) results in a time-dependent evolution in strain. This phenomenon, which we term as pseudo-creep, is similar to what was reported in the literature three decades ago for some traditional metallic materials undergoing plastic deformation. In a previous communication, we reported strain arrest of isothermal pseudoelastic loading, isothermal pseudoelastic unloading, and reorientation in NiTi wires as well as a rate-independent phase diagram. In this paper, we provide experimental results of the pseudo-creep phenomenon during stress arrest of isothermal pseudoelasticity and reorientation in NiTi wires as well as strain arrest of isothermal pseudoelasticity and reorientation in NiTi sheets. Stress arrest in NiTi wires accompanied by strain accumulation or recovery is studied using the technique of multi-video extensometry. The experimental results were used to estimate the amount of mechanical energy needed to evolve the wire from one microstructural state to another during isothermal pseudoelastic deformation and the difference in energies between the initial and the final rest state between which the aforementioned evolution has occurred.

Manufacturing of Axisymmetric Components out of Superalloys and Hard-to-Deform Steels by Roll Forming

Heat-resistant alloys are the basic material of gas turbine engine (GTE) design. Fine-grained structure in these alloys can be formed by isothermal forging and then different axisymmetric GTE components as wheels, shafts, rings can be superplastic roll formed. Examples of the superplastic and isothermal deformation use for manufacturing components out of superalloys and steels for critical applications are given. The possibility of roll forming parts as rings with a diameter up to 800 mm and as flange - cone with a diameter up to 600 mm out of superalloys (Inconel 718, EK79, EP741NP), accordingly, on SRZHD-800 and modified PNC-600 mills were showed. The macrostructure investigations of the components after the roll forming showed that the homogeneous structure was formed. The microstructure at the flange portion was fine-grained and at membrane zone was coarse-grained. Cone part was roll formed at isothermal condition from pre-stamped chromium martensitic steel sheet. Manufacturing technology of roll forming was tested by computer and physical simulation. Service properties of components were obtained by subsequent heat treatment. The effectiveness of the technology associated with increased service properties of components and decreases the labor content by automated equipment.

Texture Evolution during Isothermal, Isostrain and Isobaric Loading of Polycrystalline Shape Memory NiTi.

In situ neutron diffraction was used to provide insight into martensite variant microstructures during isothermal, isobaric, and isostrain loading in shape memory NiTi. Results show variant microstructures were equivalent for the corresponding strain and more importantly, the reversibility and equivalency was immediately evident in variant microstructures that were first formed isobarically but then reoriented to a near random self-accommodated microstructure following isothermal deformation. Variant microstructures formed isothermally were not significantly affected by a subsequent thermal cycle under constant strain. In all loading cases considered, the resulting variant microstructure correlated with strain and did not correlate with stress. Based on the ability to select a variant microstructure for a given strain despite thermomechanical loading history, the results demonstrated here can be obtained by following any sequence of thermomechanical loading paths over multiple cycles. Thus for training shape memory alloys (repeating thermomechanical cycling to obtain the desired variant microstructure), optimal paths can be selected so as to minimize the number of training cycles required thereby increasing the overall stability and fatigue life of these alloys in actuator or medical applications.

Kinetics of static recrystallization in the coarse-grained Fe–40 at.%Al–Zr–B alloy

The aim of works was to describe mathematically the kinetics of static recrystallization of the alloy type Fe–40at.%Al–Zr–B (with 24.6 Al–0.01 B–0.18 Zr–0.01 C in wt.%) with the coarse-grained structure. The microstructure of the laboratory castings made of this extremely brittle alloy was homogenized by hot rolling of the material in the protective capsules and by the long-term annealing at 1200°C. An initial microstructure with average grain size 0.77±0.27mm was obtained. Based on the isothermal plastic deformation tests and EBSD analysis, the static recrystallization kinetics of the prepared coarse-grained B2 iron aluminide after strain 0.20 was mathematically described. Recrystallized fraction depends on deformation/annealing temperature (900–1100°C) as well as on annealing time. The activation energy of static recrystallization was calculated as 255kJmol−1. Competition between dynamic recovery and static recrystallization was proved after strain 0.35 and annealing temperature 1100°C. Static recrystallization starts relatively easily in the studied alloy, but a very long-term annealing is quite necessary for the complete course of recrystallization. The mean size of recrystallized grains falls with the decreasing annealing temperature (0.47±0.15mm for temperature 1100°C, and 0.22±0.04mm for 900°C). Even at a temperature of 1200°C the annealing after deformation should last approx. 1min for obtaining the fully recrystallized microstructure. That is why the standard hot forming technologies should be combined by an interpass annealing in order to refine sufficiently the coarse grains.

Hot Ductility and Compression Deformation Behavior of TRIP980 at Elevated Temperatures

The hot ductility tests of a kind of 980 MPa class Fe-0.31C (wt pct) TRIP steel (TRIP980) with the addition of Ti/V/Nb were conducted on a Gleeble-3500 thermomechanical simulator in the temperatures ranging from 873 K to 1573 K (600 °C to 1300 °C) at a constant strain rate of 0.001 s−1. It is found that the hot ductility trough ranges from 873 K to 1123 K (600 °C to 850 °C). The recommended straightening temperatures are from 1173 K to 1523 K (900 °C to 1250 °C). The isothermal hot compression deformation behavior was also studied by means of Gleeble-3500 in the temperatures ranging from 1173 K to 1373 K (900 °C to 1100 °C) at strain rates ranging from 0.01 s−1 to 10 s−1. The results show that the peak stress decreases with the increasing temperature and the decreasing strain rate. The deformation activation energy of the test steel is 436.7 kJ/mol. The hot deformation equation of the steel has been established, and the processing maps have been developed on the basis of experimental data and the principle of dynamic materials model (DMM). By analyzing the processing maps of strains of 0.5, 0.7, and 0.9, it is found that dynamic recrystallization occurs in the peak power dissipation efficiency domain, which is the optimal area of hot working. Finally, the factors influencing hot ductility and thermal activation energy of the test steel were investigated by means of microscopic analysis. It indicates that the additional microalloying elements play important roles both in the loss of hot ductility and in the enormous increase of deformation activation energy for the TRIP980 steel.

Modelling the material resistance to cutting

In the numerical simulation of machining processes, it is very widespread to use empirical dependences of the yield point of the machined material on strain, strain rate and temperature, which are known as constitutive equations.To take of interactions between strain, strain rate and temperature into consideration as well as for describe conditions of the transition from the hardening to the softening of the material to be deformed, it is suggested that the specific deformation work in integral and differential form should be applied as characteristics of the machined material's resistance to cutting. This paper presents results of the investigations into the development of how to mathematically model the dependences of flow curves as well as of the material's yield point on temperature increase for adiabatic and isothermal deformation conditions during cutting. How temperature affects the specific deformation work and the yield point of the material to be deformed is obtained here by integrating the deduced differential equation. The layout of the deformation zones shows that it is necessary and effective to take account of the interactions regarding the models of the material resistance to deformation during cutting. The analyses also revealed that the material to be deformed in cutting hardens under almost adiabatic conditions. The material softens under isothermal conditions, which is related to the fact that deformation takes place at high temperatures in the area of the plastic contact between chip and wedge.

Structure formation in high-strength nitrogen-bearing steel on hot deformation

The resistance to deformation of nitrogen-bearing Cr–Ni–Mn steel at 800–1200°C is investigated by means of the Gleeble 3800 system. By analysis of the deformation diagrams—in particular, determination of the threshold strain for dynamic recrystallization—the temperature and strain corresponding to the onset of dynamic recrystallization are established as a function of the strain rate, and optimal temperatures for hot stamping, forging, and rolling are recommended for industrial conditions. With true strain e = 0.9, the dynamic recrystallization in the steel at strain rates of 10–2–2 s–1 occurs at temperatures no lower than 900°C. Metallographic data confirm the experimental results and show that the structure formation in the steel on isothermal deformation at different rates is different above 900°C. With increase in temperature and decrease in strain rate, relaxation processes are more developed. At a strain rate of 0.01 s–1 (stamping on a press), dynamic recrystallization begins at around e = 0.1 (relative reduction around 10%) in the range 1100–1200°C. Strain of around 20 and 30%, respectively, is required with decrease in temperature to 1000 and 900°C. With increase in strain rate to 0.1 s–1 (forging), dynamic recrystallization begins with around 20% strain above 1100°C, 28% at 1000, and 35% at 900°C. At a strain rate of 1–2 s–1 (rolling), dynamic recrystallization begins at around 30% strain in the range 1000–1100°C. In that case, the threshold strain is 36% at both 900 and 1200°C.

Evaluation of Friction Properties of Magnesium Alloy during Hot Forging by Ring Compression Test

This paper deals with friction properties and deformation resistance during hot forming of Mg-Al-Ca-Mn series magnesium alloys. Friction coefficients between dies and magnesium alloys were obtained by ring compression tests that used graphite, PTFE, and an oil lubricant in a hot-forging process. Hot forging was performed under various conditions to clarify the effects of types of lubricants and slide motion of the press machines on friction properties. Two types of slide motion, a constant velocity motion and a pulse motion were selected in the ring-compression test. It was found that graphite with an oil lubricant effectively eliminated die sticking in hot forming of magnesium alloys. The isothermal deformation resistances were derived using friction coefficients obtained by ring-compression tests as well as finite-element simulations. The predicted stress strain curves with temperature were examined with the stress-strain relationship obtained in experiments using a servo press and demonstrated the effectiveness of the proposed method.

Constitutive modeling of Ti-6Al-4V at a wide range of temperatures and strain rates

Abstract Constitutive equations, as extremely useful computational/numerical tools in describing metals’ thermomechanical behavior, strongly depend on crystal structure, chemical composition and the range of applied strain, strain rate and temperature. Therefore, utilizing one constitutive equation for a wide range of field variables (strain, temperature, and strain rate) is less likely to accurately capture the behavior observed in experiments. It is particularly challenging to derive an inclusive constitutive model for a multiphase alloy such as Ti-6Al-4V , considering its phase transformation (HCP-BCC crystal structure change), variation in reported chemical compositions and a spectrum of different microstructural morphologies. In this work, a microstructures-based constitutive relation based on the Voyiadjis-Abed (VA) model is presented to describe the thermoviscoplastic behavior of Ti64 Alloy by additively decomposing the coupling effect of temperatures and strain rates on yielding and strain hardening. The present modeling is compared with the empirical relation of Johnson-Cook (JC) at a wide range of temperatures (up to 1000 K) and strain rates (up to 6000 s −1 ). The isothermal and adiabatic deformation behavior of the alloy are predicted and compared using three different sets of experimental results. The findings of this work suggest that the VA model is more capable in predicting the thermomechanical behavior of the alloy, with at most a 5% deviation compared to experimental results.

Influence of maximum thermal cycle temperature on the two-way shape memory effect in TiNi Alloy

The influence of the maximum thermal cycle temperature on the value and temperatures of the two-way shape memory effect (TWSME) was studied in Ti50Ni50 alloy subjected to a preliminary deformation by two ways: through isothermal deformation up to 12% in the martensite state, or via cooling and heating under a constant stress of 80 MPa. Additionally, the samples were subjected to one or 10 thermal cycles within the temperature range of 440–300 K to change the dislocation structure formed during preliminary deformation. It was found that values of the TWSME and martensitic transformation temperatures varied in three temperature ranges on an increase in maximum thermal cycle temperature. In the first temperature range from 440–490 K, a variation in TWSME parameters was observed in the samples that were not subjected to a preliminary 10 thermal cycles. In the second temperature range from 490–640 K, a redistribution of the dislocations resulted in a decrease in the value of TWSME at a low rate, and hardly increased the Ms temperature. In the third temperature range from 640–700 K, a recrystallization led to a significant decrease in the value of TWSME and a strong increase in Ms temperature.

Поведение вторых фаз в магниевом сплаве МА14 при всесторонней изотермической ковке и последующей изотермической прокатке

Methods of transmission and scanning electron microscopy were used to investigate an influence of the processing, involving severe isothermal deformation with stage-by-stage decreasing temperature in the range of 200-400 °C, realized by multidirectional forging and subsequent rolling, on distribution and size of the second phases in a commercial rod of MA14 (Mg-6Zn-0,6Zr) alloy. It is established that forging at 400 and 3000C with a total true strain of е ~10 practically eliminates the string type structure of coarse excess phases peculiar to the pressed semi-finished products. Besides it considerably reduces the size of these particles with no changes in their volume fraction, preserving at a level of 3-4%. Decreasing the temperature of forging up to 2000C, below the solvus point of the main strengthening -phase (MgZn2), intensifies decomposition of the supersaturated by Zn magnesium solid solution. The processes of excess phase refinement and solid solution decomposition with precipitation of near 8% of phases were continued at rolling under the same temperature conditions. They resulted in anisotropic character of phase distribution in the form of secondary string structure of the coarsest excess phases elongated in the rolling direction. Besides, the strong anisotropy in size and densities of -phase plate-shape precipitates, as in interparticle spacing, determined by direct measurements in long and transverse sheet sections, was formed. It was concluded that the last factor is the main reason for the yield strength anisotropy that observed under ambient temperature tension in the ultrafine-grained alloy sheet processed via mentioned complex treatment.