Temperature-compensated Strain Rate Research Articles

Peculiarities of DRX in a Highly-Alloyed Austenitic Stainless Steel.

The features of discontinuous dynamic recrystallization (DRX) in a highly-alloyed austenitic stainless steel were studied at temperatures of 800 °C to 1100 °C. Hot deformation accompanied by DRX was characterized by an activation energy of 415 kJ/mol. The frequency of the sequential DRX cycles depended on the deformation conditions; and the largest fraction of DRX grains with small grain orientation spread below 1° was observed at a temperature of around 1000 °C and a strain rate of about 10−3 s−1. The following power law relationships were obtained for DRX grain size (DDRX) and dislocation density (ρ) vs. temperature-compensated strain rate (Z) or peak flow stress (σP): DDRX ~ Z−0.25, ρ ~ Z0.1, σP ~ DDRX−0.9, σP ~ ρ1.4. The latter, i.e., σP ~ ρ1.4, was valid in the flow stress range below 300 MPa and changed to σP ~ ρ0.5 on increasing the stress. The obtained dependencies suggest a unique power law function between the dislocation density and the DRX grain size with an exponent of −0.5.

Grain Orientation Spread in Dynamically Recrystallized Austenitic Steel

Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10-3-10-1 s-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.

Investigation of Flow-Formability of an AZ31 Magnesium Alloy

Flow-formability of a Ca-added AZ31 magnesium alloy tube is investigated. The flow-forming process is conducted at various temperatures (100–500 °C), thickness reductions (30–85%), and feed rates (0.1–0.56 mm/rev). Inner and outer surfaces of the tubes are heated by means of a thermal element embedded inside the mandrel and a radiation element, respectively. The formed tubes are visually inspected for the occurrence of cracking and fractures. Microstructures and tensile properties of the samples are analyzed by optical microscopy and tensile test, respectively. It is shown that deformation above 200 °C is required for sound processing with the occurrence of dynamic recrystallization (DRX). Up to 200 °C, the twinning-induced shear banding is the dominant phenomenon in microstructural evolution and responsible for the early strain localization and subsequent fracture. By increasing the temperature, the maximum achievable thickness reduction increases. However, at about 300 °C, the maximum thickness reduction reaches a limit value of about 76%. A twist in the deformed part of the tube occurs at greater thickness reductions. A simple analytical model is presented to analyze the occurrence of the twist phenomenon. Accordingly, a flow-formability map is proposed for the alloy. The DRX grain size is shown to follow a power law with the temperature compensated strain rate known as the Zener–Hollomon parameter. While the grain size is not affected by the feed rate, dimensional accuracy is deteriorated at feed rates over 0.2 mm/rev due to the diametral growth of the workpiece. Based on the tensile test results, by increasing the deformation temperature, the tensile strength increases and the ductility decreases, so that the sample processed at 500 °C shows a brittle fracture. The impacts of temperature on the strength and ductility are attributed to the combined effects of microstructural and texture evolutions during the flow-forming process.

High-temperature creep deformation in FeCrAl-oxide dispersion strengthened alloy cladding

The FeCrAl-oxide dispersion strengthened (ODS) alloy is the promising cladding material for the accident-tolerant fuel (ATF) of the light water reactors (LWR) to suppress the oxidation reaction with hot steam by Al2O3 formation. Ring-creep tests for FeCrAl-ODS alloy cladding were carried out at 973 K and 1273 K to evaluate the deformation mechanism at the sever accident. The temperature-compensated strain rate by Fe-diffusion coefficient versus the normalized stress by shear modulus was constructed. The dislocation detachment stress from the dispersoid was derived by considering the dislocation-dispersoid elastic interaction and the dislocation relaxation effect by climb motion. When the applied stress exceeds the dislocation detachment stress, dislocations overcome the dispersoids with the reduced values of the stress exponent, which is attributed to the dislocation climb motion. When the stress is lower than the dislocation detachment stress, large size of cavities exists at the grain boundaries. Grain boundary sliding (GBS) is dominant factor for the low strain rate creep deformation at 1273 K, where dislocation climb also plays a critical role for release of the stress concentration induced by GBS. Based on those findings, new constitutive equations for creep deformation were constructed, which is applicable to low stress, low strain rate and high temperature conditions encountered at the reactor sever accident.

Constitutive modelling of hot deformation behaviour of a CoCrFeMnNi high-entropy alloy

ABSTRACT Models describing the constitutive flow behaviour of a metallic material are desired for appropriate process design and realization of defect-free components. In this study, constitutive equations based on the hyperbolic-sinusoidal Arrhenius-type model have been developed to define the hot deformation characteristics of a CoCrFeMnNi high-entropy alloy. The experimental true stress-true strain data were generated over a wide temperature (1023–1423 K) and strain rates (10−3–10 s−1) ranges. The impact of strain rate and temperature on deformation behaviour was further characterized through a temperature compensated strain rate parameter, i.e. Zener-Hollomon parameter. Additionally, a mathematical relation was employed to express the influence of various material constants on true-strain ranging from 0.2 to 0.75. Typical third order polynomial relations were found to be appropriate to fit the true-strain dependency of these material constants. The accuracy of the developed constitutive equations was evaluated by using the average absolute relative error (AARE) and correlation coefficient (R); the obtained values were 7.63% and 0.9858, respectively, suggesting reasonable predictions. These results demonstrate that the developed constitutive equations can predict the flow stress behaviour of the alloy with a good accuracy over a wide range of temperature and strain rate conditions and for large strains.

Grain sizes and dislocation densities in fcc-metallic materials processed by warm to hot working

Our just received results on the deformation microstructures evolved in a medium-Mn austenitic steel during large strain warm to hot rolling were analyzed with a reference to other alloys with face centred cubic lattices and relatively low stacking fault energies. The structural changes were characterized by the development of dynamic recovery (DRV) and dynamic recrystallization (DRX). The deformation grain size decreased while the dislocation density increased with a decrease in deformation temperature. Both the grain size and the dislocation density could be expressed by power law functions of temperature-compensated strain rate, i.e., Zener-Hollomon parameter, with exponents of -0.33 and 0.2, respectively, in the case of discontinuous DRX. On the other hand, the exponents of -0.1 and 0.06 were obtained for the grain size and dislocation density dependencies on the temperature-compensated strain rate, respectively, under conditions of warm deformation accompanied by DRV and continuous DRX. Therefore, a power law function could be obtained between the grain size and the dislocation density with a grain size exponent of -0.6, irrespective of the mechanisms of microstructure evolution in a wide range of deformation conditions.

Characterization of Hot Deformation Behavior of Nanosized Al₂O₃ Reinforced Al6061 Composites.

The hot deformation behavior of Al6061/Nano-Al₂O₃ composites were investigated at temperatures of 300 to 500 °C and strain rates of 0.001∼1/s using compression tests. The composite fabricated by the infiltration method consisted of an Al matrix and Al₂O₃ particles with a mean size of 200 nm. Interestingly, the true stress-true strain curves under all compressive conditions showed a peak stress at the initial stages of deformation, in which the peak stress increased with decreasing temperature and faster strain rate. The Z parameter, which is known as the temperature-compensated strain rate showed a linear relationship with the flow stress. The hot deformation mechanism is believed to occur through dynamic recrystallization, where fine equiaxed grains and dislocations were observed at the deformed specimens. A processing map was applied to evaluate the hot workability and flow instability region to determine the optimal deformation conditions of the composite.

On Regularities of Grain Refinement through Large Strain Deformation

The recent studies on grain refinement in austenitic stainless steels during large strain deformations are critically reviewed. The paper is focused on the mechanism of structural changes that is responsible for the development of submicrocrystalline structures that can be interpreted as continuous dynamic recrystallization developing under conditions of warm working. The final grain size that is attainable by large strain warm working can be expressed by a power law function of temperature compensated strain rate with an exponent of about -0.15. The development of submicrocrystalline structures is assisted by the deformation microbanding and dynamic recovery, which are characterized by opposite temperature dependencies. The grain refinement kinetics, therefore, are characterized by a weak temperature dependence for a wide range of warm working conditions.

Effect of temperature–strain-rate conditions of deformation on structure formation in commercially pure copper deformed in Bridgman anvils

Commercially pure copper (99.9 wt % Cu) subjected to high-pressure torsion at room temperature is investigated. A relation between the copper structure and temperature–strain-rate conditions has been established. It has been shown that impurity dragging prevents grain growth during postdynamic recrystallization. This has allowed us to determine the conditions under which either the hardening of the deformed material, which is accompanied by a continuous increase in the hardness and structural refinement, or the dynamic recrystallization resulting in the stabilization of the hardness and the average grain size predominantly occurs in commercial-purity copper compared to high-purity copper (99.99 wt %). It has been shown that, under the hardening conditions, the structure of the investigated copper is determined by the true strain, whereas under dynamic recrystallization conditions, by temperature–compensated strain rate.

Some aspects of hot axisymmetric upsetting pressure prediction

In this research work, the pressure–deformation curves of hot upsetting of AA 6015 aluminium alloy were measured and compared to curves calculated on the basis of torsion test results. Such tests, performed in the temperature range from 300 to 500°C at strain rates varying from 0.042 to 1.4 s−1, yielded a constitutive equation correlating peak flow stress with temperature compensated strain rate. This equation was used in the calculation of hot upset forming pressures of the same material by means of a relationship derived from the slab method. The pressures predicted by the proposed method, are in good agreement with the experimental results over a wide strain range. Accuracy limits and discrepancies between predicted and experimental data have been analysed and are discussed in detail.

Fracture mechanics of idealised bituminous mixes

The durability of asphalt pavements is strongly impaired by cracks, caused primarily by traffic loads and environmental effects. In this work, fracture behaviour of idealised asphalt mixes is investigated. Experiments on idealised asphalt mixes under pure-tension mode (mode I cracking) were performed and fracture parameters were evaluated. In these three-point bend fracture tests, the test variables were temperature and load rate. The test data were stored in an asphalt materials database and special-purpose tools were implemented to analyse and handle the laboratory data automatically. Fracture mechanism maps were constructed, showing the conditions associated with ductile, brittle and ductile–brittle transition regimes of behaviour. The mechanism maps show the failure response of the material in terms of the stress intensity factor, strain energy release rate and J-integral as a function of the temperature-compensated crack mouth opening strain rate. Fracture behaviour of asphalt mix specimens was simulated by cohesive zone model in conjunction with a novel material constitutive model for asphalt mixes. The finite element model agrees well with the experimental results and provides insights into fracture response of the notched asphalt mix beam specimens.

Constitutive Analysis and Microstructure Evolution of the High-Temperature Deformation Behavior of an Advanced Intermetallic Multi-Phase γ-TiAl-Based Alloy

In the present study the high-temperature deformation behavior of a caste and subsequently HIPed β-solidifying γ-TiAl-based alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at. %), termed TNM alloy, is investigated. At room temperature this alloy consists of ordered γ-TiAl, α2-Ti3Al and βo-TiAl phases. By increasing the temperature, α2 and βo disorder to α and β, respectively. In order to get a better understanding of dynamic recovery and recrystallization processes during thermomechanical processing, isothermal compression tests on TNM specimens are carried out on a Gleeble®3500 simulator. These tests are conducted at temperatures ranging from 1100 °C to 1250 °C (in the α/α2+β/βo+γ phase field region) applying strain rates in the range of 0.005 s-1 to 0.5 s-1 up to a true strain of 0.9. The evolution of microstructure along with the dynamically recrystallized grain size during hot deformation is examined by scanning electron microscopy (SEM). The flow softening behavior after reaching the peak stress in the true stress-true strain curve is attributed to dynamic recrystallization. By using the Zener-Hollomon parameter as a temperature-compensated strain rate the dependence of flow stress on temperature and strain rate is shown to follow a hyperbolic-sine Arrhenius-type relationship.

The flow behavior modeling of AZ61 magnesium alloy at elevated temperatures considering the effects of strain rate and grain size

In this study, the hot deformation behavior of a magnesium alloy was investigated under various process conditions. Tensile testing experiments were performed to determine the effects of temperature, strain, and strain rate on the flow stress of the material. A new constitutive model was established to characterize the dynamic recrystallization of the magnesium alloy at elevated temperatures. The critical strain was evaluated based on the temperature-compensated strain rate to consider the work softening. The amount of high temperature softening due to dynamic recovery and dynamic recrystallization was formulated as a function of strain, strain rate, and temperature. It was demonstrated that the proposed model is able to predict the flow softening as well as the growing strain hardening of the material very accurately. The failure characteristics were also studied at different temperatures and strain rates. Finally, the grain size effect on the flow behavior of the material was discussed.

Regularities of Grain Refinement in an Austenitic Stainless Steel during Multiple Warm Working

The structural changes that are related to the new fine grain development in a chromium-nickel austenitic stainless steel subjected to warm working by means of multiple forging and multiple rolling were studied. The multiple warm working to a total strain of 2 at temperatures of 500-900C resulted in the development of submicrocrystalline structures with mean grain sizes of 300-850 nm, depending on processing conditions. The new fine grains resulted mainly from a kind of continuous reactions, which can be referred to as continuous dynamic recrystallization. Namely, the new grains resulted from a progressive evolution of strain-induced grain boundaries, the number and misorientation of which gradually increased during deformation. In contrast to hot working accompanied by discontinuous dynamic recrystallization, when the dynamic grain size can be expressed by a power law function of temperature compensated strain rate as D ~ Z-0.4, much weaker temperature/strain rate dependence of D ~ Z-0.1 was obtained for the warm working.

High Temperature Deformation and Dynamic Recrystallization Behavior of Alloy718

Ni base superalloy is one of the representative heat resistant materials for high temperature environment in various industrial applications. As for cast and wrought material, thermo-mechanical processing during hot forging has a significant role to control and optimize crystal grain structure in order to attain required critical properties. The essential deformation and dynamic recystallization behavior of Ni base superalloy, Alloy718 during hot forging were revealed through hot compression tests and quantitative relations among various parameters that were available for numerical calculation were derived. Dynamically recrystallized grain diameter depended on temperature, strain rate and was independent of initial grain diameter and strain. And the grain diameter was expressed as a function of the temperature compensated strain rate, i.e., Zener­Hollomon parameter quantitatively. Avrami-type equation was available for comprehensive and quantitative expression of dynamic recrystallization transition tendency. And the fraction of dynamic recrystallization was a function of the given strain and the strain for 50% recrystallization which depended on the initial grain diameter and Zener­Hollomon parameter. [doi:10.2320/matertrans.M2012202]

Modeling Mode I Fracture of Bitumen Films

AbstractThe fracture behavior of thin films of bitumen in double cantilever beam (DCB) specimens was investigated over a wide range of temperature and loading rate conditions using finite-element analysis. The model includes a phenomenological model for the mechanical behavior of bitumen, implemented into a special-purpose finite-element user material subroutine, combined with a cohesive zone model (CZM) for simulating the fracture process. The finite-element model is validated against experimental results from laboratory tests of DCB specimens by comparing measured and predicted load–line deflection histories and fracture energy release rates. Computer simulation results agreed well with experimental data of DCB joints containing bitumen films in terms of peak stress, fracture toughness, and stress-strain history response. The predicted “normalized toughness,” G/2h, was found to increase in a power-law manner with effective temperature-compensated strain rate in the ductile region as previously observed ...

마그네슘 합금 판재의 변형률, 변형률 속도 및 온도 환경을 고려한 유동응력 모델에 대한 연구

The formability of magnesium alloy sheets at room temperature is generally low because of the inherently limited number of slip systems, but higher at temperatures over <TEX>$150^{\circ}C$</TEX>. Therefore, prior to the practical application of these materials, the forming limits should be evaluated as a function of the temperature and strain rate. This can be achieved experimentally by performing a series of tests or analytically by deriving the corresponding modeling approaches. However, before the formability analysis can be conducted, a model of flow stress, which includes the effects of strain, strain rate and temperature, should be carefully identified. In this paper, such procedure is carried out for Mg alloy AZ31 and the concept of flow stress surface is proposed. Experimental flow stresses at four temperature levels (<TEX>$150^{\circ}C$</TEX>, <TEX>$200^{\circ}C$</TEX>, <TEX>$250^{\circ}C$</TEX>, <TEX>$300^{\circ}C$</TEX>) each with the pre-assigned strain rate levels of <TEX>$0.01s^{-1}$</TEX>, <TEX>$0.1s^{-1}$</TEX> and <TEX>$1.0s^{-1}$</TEX> are collected in order to establish the relationships between these variables. The temperature-compensated strain rate parameter which combines, in a single variable, the effects of temperature and strain rate, is introduced to capture these relationships in a compact manner. This study shows that the proposed concept of flow stress surface is practically relevant for the evaluation of temperature and strain dependent formability.

Microstructural evolution of 2026 aluminum alloy during hot compression and subsequent heat treatment

2026 aluminum alloy was compressed in a temperature range of 300–450 °C and strain rate range of 0.01–10 s −1. The correlation between compression conditions and microstructural evolution after solution and aging heat treatment was investigated. It is found that the recrystallization and precipitation behavior after heat treatment are associated with the temperature compensated strain rate Z value during hot deformation. Under low Z parameter condition, a small quantity of fine recrystallized grains are formed, and the well formed subgrains with clean high-angle boundaries and coarse precipitates seem to be remained during heat treatment. Under high Z parameter condition, a large number of fine equiaxed recrystallized grains are produced, and a high dislocation density with poorly developed cellularity and considerable fine dynamic precipitates are replaced by the well formed subgrains and relatively coarse precipitates after heat treatment. The average recrystallized grain size after heat treatment decreases with increasing Z value and a quantitative relation between the average grain size and the Z value is obtained.

Hot deformation behavior of Al-Zn-Mg-Cu-Zr aluminum alloys during compression at elevated temperature

The hot compression tests of Al-Zn-Mg-Cu-Zr aluminum alloys (7056 alloy and 7150 alloy) were performed in a temperature range from 300 to 450 °C and at strain rate range from 0.01 to 10 s −1. The results show that the true stress-true strain curves exhibit a peak stress at a critical strain, then the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stresses depend on the temperature compensated strain rate, which can be represented by the Zener-Hollomon parameter Z in the hyperbolic-sine equation with hot deformation activation energy of 244.64 kJ/mol for 7056 alloy and 229.75 kJ/mol for 7150 alloy, respectively, while the peak stresses for the former are lower than those for the latter under the similar compression condition. The deformed microstructures consist of a great amount of precipitates within subgrains in the elongated grains at high Z value and exhibit well formed subgrains in the recrystallized grains at low Z value. The smaller subgrains and greater density of fine precipitates in 7150 alloy are responsible for the high peak stresses because of the substructural strengthening and precipitating hardening compared with 7056 alloy.

Flow stress modeling of AZ91 magnesium alloys at elevated temperature

The flow curve is constructed by flow stress data obtained from compression test conducted at different temperature and strain rate. Accurate flow stress model is crucial for investigating magnesium alloys deformation behaviour at the elevated temperatures. An analytical method, which reflects temperature, strain and strain rate effect by introducing temperature-compensated strain rate (Zener–Hollomon Parameter), is proposed in this study. This model has been applied on the experimental data and predicted flow stress curve match well with those measurements.